Elemental Chemistry and Cyclone Reconstructions – Outlineby
VALERIU MURGULET
A DISSERTATION
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the
Department of Geological Sciences in the Graduate School of
The University of Alabama
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ABSTRACT
The study examines the question whether speleothems from Niue Island (19°00'S,
169°50'W), a large carbonate platform located at the edge of West Pacific Warm Pool, can serve
as archives of hydroclimate controlled by El-Niño/Southern Oscillation (ENSO) and of
catastrophic cyclones that frequent the island. Niue Island is heavily karstified, with modern and
fossil speleothems hosted by coastal and inland caves. The flank margin caves on Niue are
shown to be formed by the action of corrosive groundwaters on uplifted Pleistocene-age reef
carbonates in a tectonically active region.
The focus of this study is an actively growing stalagmite sampled from a flank margin
cave (Avaiki Cave) that contains about 146 years of deposition (2002-1856 AD). The stalagmite
consists of sub-annual couplets alternating between white porous calcite laminae deposited
during the austral summer and dark, compact calcite laminae deposited during the austral
relatively dry winter. High resolution (sub-annual) stable isotope and trace element profiles
accompanied by trace element X-ray mapping were used to test the validity of ENSO-controlled
hydroclimate and tropical cyclones archived in the stalagmite. The results show that interannual
variability in the stalagmite δ18O and δ13C time series agrees well with instrumental-derived
ENSO phases (El Niño and La Niña events during 1866-2002) and the sea level pressure
differential (Samoa-Fiji)-based SPCZ index (SPI) that controls the interdecadal hydroclimate
variability. Severe cyclones that directly impacted Niue Island over the last century are recorded
by abrupt, large increases in trace element concentration values of
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Mg, and S accompanied by stable isotope positive excursions bearing seawater-derived
signatures. Application of selected trace elements (i.e., Mg, Na, S, P) as proxies of severe storms
is a novel technique that can be successfully applied in carbonate coastal areas with flank-margin
caves impacted by severe cyclones. This study also demonstrates that sub-annual geochemical
cycles in trace element laminae, unresolved by analytical linear transects due their complex
distribution pattern, are successfully imaged by large area X-ray mapping of the stalagmite.
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DEDICATION
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ACKNOWLEDGMENTS
I would like to thank my advisor, Dr. Paul Aharon for the guidance and recommendations
to improve this dissertation. I would also like to thank my committee members, Dr. Fred Andrus,
Dr. Rona Donahoe, Dr. John Mylroie, Dr. Geoffrey Tick, for the support and recommendations
to improve this dissertation. I thank Dr. Michael Bersch for the training and the help provided to
obtain the trace element data and for the input on the trace element project. Fellow graduate
students, Joe Lambert and Michael Rasbury have helped me in numerous ways.
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CONTENTS
LIST OF TABLES.......................................................................................................................viii
ENSO EVENTS AND CATASTROPHIC CYCLONES IMPRINTED IN A TROPICAL STALAGMITE..................................................................................................... 39
TRACE ELEMENT (Mg, S, and P) CYCLYCITY DISRUPTED BY CATASTROPHIC CYCLONES IN A TROPICAL STALAGMITE.......................................... 81
CONCLUSIONS......................................................................................................................... 124
APPENDIX I. STALAGMITE δ18O AND δ13C GEOCHEMICAL DATA .............................. 129
APPENDIX II. Ca, Mg, P, AND S TRACE ELEMENT GEOCHEMICAL DATA................. 145
APPENDIX III. Mg AND Ca MEASUREMENTS OF THE ASM-1
STALAGMITE TOP LAYER .................................................................................................... 179
δ18O Oxygen isotopic composition
δ13C Carbon isotopic composition
Ca2+ Concentration of calcium in solution
CO2 Carbon dioxide
SST Sea Surface Temperature
ITCZ Intertropical Convergence Zone
ENSO El Niño-Southern Oscillation
IPO Interdecadal Pacific Oscillation
NAR Niue Annual Rainfall
DBT Depth below top
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LIST OF TABLES
Table 1.1. Chemical composition of Niue groundwater1, brackish2 and seawater3 samples. Calcite saturation indices (SIc) of groundwater and seawater end- members were obtained by inputting the average chemical compositions of the 19 groundwater samples and the 2 seawater samples, respectively into the Phreeqc geochemical software (Parkhurst and Appelo, 1999). Chemical composition of groundwater, brackish and seawater samples are from Wheeler (2000) ................................................................................................................................31
Table 1.2. Niue and Nauru groundwater/seawater theoretical mixing values calculated
using PHREEQC software (Parkhurst and Appelo, 1999) from the groundwater (0% South Pacific) and seawater (100% South Pacific) end-members.............................32
Table 1.3. Nauru samples and their corresponding seawater percent and saturation
indices of calcite are from Jankowski and Jacobs (1991). Normalization factors were calculated using the equation shown in Figure 1.6. Niue scaled values of saturation indices of calcite were obtained using the normalization factors shown here.........................................................................................................................33
Table 1.4. List of Ca2+, Ph and Dissolved Inorganic Carbon (DIC) of Niue vadose
groundwaters (from Aharon et al., 2006). The vadose groundwaters are labeled according to the sampling location: “AD”, Avaiki cave; and “PD”, Palaha cave (Fig. 1.2) ............................................................................................................................34
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LIST OF FIGURES
Figure 1.1 Location of Niue Island in the South Pacific (from Aharon et al., 2006) ................... 22
Figure 1.2. Toprographic map of Niue Island showing the location of flank margin caves along the Niue coastline. Note the numerous flank margin caves located on the leeward (west) side compared to their scarcity on windward (east) side of Niue. The position of the cross section (AB) in Figure 1.4 is also shown. Niue elevation data were obtained from ESRI world elevation and image data from GIS........................................................................ 23
Figure 1.3. Flank margin caves of Niue Island. A: Karst features (karenfelt) on the
windward site. B: Alofi terrace (Pleistocene-age) containing a multitude of flank-margin caves. C: Seaward entrance to the Avaiki cave. D: Palaha Cave. See Figure 1.2 for Avaiki and Palaha cave locations ............................................. 24
Figure 1.4. Cross section of Niue elevation and the groundwater-seawater
interface. Groundwater-Seawater interface data are from Jacobson and Hill (1980). ....................................................................................................................... 25
Figure 1.5. Calcite saturation index as a function of freshwater/seawater mixing at
Niue. Groundwater endmember is given by the 0% South Pacific Seawater, whereas the seawater endmember represents 100% South Pacific Seawater. Calcite saturation of zero value indicates saturation. Triangles indicate brackish water samples collected from the mixing zone of Niue (Wheeler, 2000) and pluses indicate Niue scaled mixed groundwater-seawater samples......................................................................................... 26
Figure 1.6. Normalization factor derived from scaling the Nauru
groundwater/seawater theoretical mixing values (0-30% South Pacific Seawater) (Table 1.2) to Niue groundwater/seawater theoretical mixing values (0-30% South Pacific Seawater) (Table 2). The equation was applied to scale the measured Nauru groundwater seawater mixing values to a Niue mixed groundwater-seawater samples scenario. The Niue scaled values are listed in Table 1.3. ........................................................................................... 27
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Figure 1.7. Calcite dissolution as a function of CO2 pressure at Niue. Triangles
indicate the vadose waters, while the square represents the groundwaters at Niue. The vadose waters are labeled according to the sampling location: “AD”, Avaiki cave; “PD”, Palaha cave (from Aharon et al., 2006). The Niue groundwater is labeled “P” and represents the average of Ca2+ and CO2 chemical composition of 19 groundwater samples (Ca2+ = 1.36 mM ± 0.17; CO2 = 0.44 mM ± 0.1). Equal proportions of vadose waters and groundwaters on the mixing lines are shown by the closed circles. See text for a discussion. ................................................................................................................ 28
Figure 1.7. Calcite saturation index plotted against the groundwater residence times for Niue and Nauru (this study), Bahamas, Yucatan Peninsula and Floridan aquifer. Calcite saturation indices of groundwaters from Bahamas, Yucatan Peninsula and Floridan aquifer are those reported by Smart et al., 1988; Back et al., 1986; Stoessell et al., 1989 and Wicks et al., 1995. Groundwater residence time calculations are given in the method section.................................................................................................................. 29
Figure 1.9. (A) Stages of flank margin cave development on a carbonate platform
in an active tectonic setting; 1, 2 and 3 represent past sea levels relative to the island and dashed lines represent past configurations of the groundwater lens. (B) Flank-margin cave formation at Niue; cave chambers are interconnected because of ceiling collapses ................................................30
Figure 2.1. Location of Niue Island in the West Pacific Warm Pool delimited by
the 28°C isotherm. The South Pacific “cyclone belt” is delineated by a rectangle, and the location of the South Pacific Convergence Zone (SPCZ) (Foland et al., 2002) is shown by an ellipsoid. Temperature gradient scale of the sea surface temperature is given below the figure. The figure is modified from NOAA (http://www.cdc.noaa.gov/map/clim/sst.shtml; accessed on December 10, 2008).......................................................................................62
Figure 2.2. Topographic map of Niue showing the location of Avaiki cave.................................63 Figure 2.3. Average monthly rainfall (mm) and temperature (°C) on Niue Island.
The rainfall data are based on continuous instrumental recordings from 1906 to 2002, while the temperature data are based on continuous instrumental recordings from 1931 to 1998 (Data source: Niue Meteorological Service). A wet season from December to April and a dry season from May to November are characteristic to Niue climate....................................64
Figure 2.4. Time series of Niue air temperature and Annual Rainfall (NAR). Strong
El-Niño events that correlate with low rainfall are shown by red bars (http://www.ncdc.noaa.gov/oa/climate/research/1998/enso/10elnino.html, accessed on March 20, 2010). Note the low air temperature variability at Niue in the past seven decades (24.9 ± 0.3 °C) .................................................................65
Figure 2.5. Scanned image of the ASM-1 stalagmite. ASM-1 displays visible
growth laminae that consist of couplets of white porous calcite (WPC) and dark compact calcite (DCC). The image shows the micromilled trench sampling and the manually marked DCC layers that were used to determine the stalagmite chronology.................................................................................66
Figure 2.6. ASM-1 oxygen and carbon stable isotope profiles versus depth below
top (DBT), youngest samples are to the left. The stable isotopes time series were obtained by micromilling in 100 µm steps. This sampling protocol resulted in sub-annual resolution of the δ18O and δ13C records because the annual growth rate of the ASM-1 stalagmite is approximately 400 µm/yr. Red lines indicate the mean values of the δ18O and δ13C time series ..................................................................................................................................67
Figure 2.7. Hendy test results for the ASM-1 stalagmite: (a) δ18O and δ13C
variations along a single growth layer and (b) the relationship between δ18O and δ13C, R2=0.04. Note that the lack of covariance between δ18O and δ13C suggests isotopic equilibrium deposition. The Hendy test data (sampling as well as δ18O and δ13C measurements) were obtained by Michael Rasbury on a different ASM-1 slab than that used in this study .........................68
Figure 2.8. δ18O versus δ13C plot for the entire data set (R2 = 0.38).............................................69 Figure 2.9. Correlation between mean monthly δ18O in rainwater and amount of
rainfall in the South Pacific region. Rainfall data are from Samoa station (IAEA/WMO, 2001). The amount effect on the oxygen composition of rainfall in the South Pacific is evident through the statistically significant negative correlation between the δ18O and amount of rainfall ..........................................70
Figure 2.10. Time series of Niue annual rainfall (a) and Niue annual rainfall
amount reconstructed from the δ18O profile (b). The Niue rainfall amount reconstructed profile was obtained using the relation between the amount of annual rainfall and the δ18O of speleothem carbonate of Yadava and Ramesh (2005). The same profiles are also shown in standardized units ((Raw Datum – Mean)/Standard Deviation).See text for description of the equation used to reconstruct the rainfall amoun ................................................................71
Figure 2.11. Time series of Niue annual rainfall and Niue annual rainfall amount
reconstructed from the δ13C profile (a). The relationship between the amount of annual rainfall and the δ18O of speleothem carbonate of Yadava and Ramesh (2005) was modified to obtain the δ13C predicted Niue rainfall amount time series. The same profiles are also shown in standardized units ((Raw Datum – Mean)/Standard Deviation)(b).See text for description of the equation used to reconstruct the rainfall amount. ...........................72
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Figure 2.12. Comparison of stable isotopes (oxygen and carbon) and variabilities of, ENSO, South Pacific Convergence Zone (SPCZ), and Interdecadal Pacific Oscillation (IPO). ENSO is defined as the Southern Oscillation Index (SOI), and is calculated as Tahiti minus Darwin mean sea level pressure difference. SPCZ is defined as the South Pacific Index (SPI) and is calculated as normalized mean sea level pressure difference between Apia (Samoa) and Suva (Fiji). Interdecadal Pacific Oscillation (IPO) is calculated as sea surface temperature difference across Pacific basin (Folland et al., 2002). All units have been standardized ((Raw Datum – Mean)/Standard Deviation). Strong El Niño events are marked by light brown interrupted lines, whereas strong La Niña events are shown by black interrupted lines. Higher δ18O and δ13C values correspond to (i) low rainfall amounts, (ii) El Niño phase of the Southern Oscillation and (iii) positive phases of the IPO and SPCZ................................................................................73
Figure 2.13. Instrumental record of tropical cyclones that affected Niue. Blue bars
represent the cyclones that affected the northwestern coast of Niue. The severity of cyclones is shown numerically as follows: 1- Minor, 2 – Minor to Moderate, 3 – Moderate, 4 – Moderate to Severe, 5 – Severe. The questions mark indicates that the severity of the 1863 cyclone is not known. Sources: Niue Meteorological Service; Kreft (1986); Barker (2000).................................................................................................................................75
Figure 2.14. Tropical cyclone impacts on stable isotopic and trace element
compositions of the ASM-1 stalagmite. The occurrences of severe cyclones (“C” next to the red vertical lines) on Niue correspond to positive δ18O and δ13C values and abrupt increases in S/Ca and Mg/Ca concentration values. All units have been standardized ((Raw Datum – Mean)/Standard Deviation) ...............................................................................................76
Figure 3.1. Niue Island location in the South Pacific (Aharon et al., 2006)................................106 Figure 3.2. Topographic map of Niue showing the location of the Avaiki cave on
the northwestern coast. Niue elevation data were obtained from ESRI world elevation and image data for GIS ..........................................................................107
Figure 3.3. Scanned image of the ASM-1 stalagmite slab. The squares show the
locations of the thick section in Figures 3.4A, 3.7A, and 3.7B and the thin section in Figure 3.4A images. ASM-1 displays excellent visible growth laminae that consist of couplets of white porous calcite (WPC) and dark compact calcite (DCC). See text for explaining the nature of the physical (visible) stalagmite laminae.............................................................................................108
Figure 3.4. Thin section images showing optical growth laminae and calcite
crystal morphologies. A) Alternation from dark compact calcite (DCC) laminae to white porous calcite (WPC) laminae in ASM-1 viewed under
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plane-polarized light on a thick section. The corresponding location on the ASM-1 slabbed stalagmite is shown in Figure 3.3. B) Dark compact laminae viewed under plane-polarized light on a thin section; the corresponding location on the ASM-1 slabbed stalagmite is shown in Figure 3.3. C) Elongate (at least 4 mm long), columnar calcite crystals that are perpendicular to the stalagmite growth surface viewed under cross-polarized light. The photograph represents the corresponding B) image. Thin section photographs were obtained using a Nikon SMZ800 binocular microscope and the scale bars are 1 mm in all images....................................109
Figure 3.5. X-ray maps of Mg, P, S and Na (red = relatively high concentration,
blue = relatively low concentration). The Mg swath map is overlapped exactly where the electron microprobe mapping was conducted. Alternating relatively low and high concentration laminae patterns are observed throughout the stalagmite. Numbers 1 to 6 (also shown in analytical transects in Figure 3.7) represent outstanding increases in Mg, S and/or P concentrations. Example of detailed X-ray images of abrupt increases in Mg and S concentrations is shown in A). Example of outstanding discontinuous lamina of Mg is shown in B); analytical transects conducted according to vertical lines drawn (B) would not report the relatively high concentration of the Mg lamina. Vertical brown line on Mg swath map represents the location of the trace element profiles...............................110
Figure 3.6. High resolution X-ray elemental distribution maps of Mg, S and P (red
= relatively high concentration, blue = relatively low concentration; see box, lower left in Figure 3.5 for the map location). Trace element laminae (particularly Mg) are very complex and often discontinuous, with highly variable thicknesses. X-ray mapping technique also illustrates very well the enrichment of Mg (dark red) at the termination of Mg rich layers. Inverse correlation of Mg relatively high concentration layers and P relatively low concentration layers are interpreted as dry conditions in accordance with findings of Treble et al. (2005).............................................................111
Figure 3.7. Comparison of the Mg laminae (A) (red = relatively high
concentration, blue = relatively low concentration) with optical stalagmite laminae (B). Images were obtained from a thick section; the corresponding location on the ASM-1 slabbed stalagmite is given in Figure 3.3. Relatively high Mg concentration laminae are shown by red, while relatively low Mg concentration laminae are shown by blue colors. The maps illustrate that the optically DCC laminae are generally associated with the Mg rich laminae of the X-ray map. Scale is 1 mm in both images......................................................................................................................112
Figure 3.8. Plots of calcium and trace element (Mg, P, S) profiles versus DBT
(depth below top), the youngest samples are those to the left. Sulphur shows episodic large increases in concentration values (peaks 1 to 6)
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which are accompanied, with one exception (DBT ~ 8mm), by concentration increases in Mg (yellow bars). Large increases in Mg concentrations which do not correspond with S peaks usually correlate with decreases in P concentrations. Troughs in Ca concentration values correspond with increases in Mg/S concentrations. The resolution of the analytical transects is approximately 50 μm. The minimum detection limits are 0.0173 wt% for magnesium, 0.0261 wt% for phosphorus, and 0.0256 wt% for sulphur ...................................................................................................113
Figure 3.9. Crossplots of Ca vs. P (a), S (b) and Mg (c) and trace element to Ca
ratios (d, e. f) of analytical transects from the ASM-1 stalagmite. With the exception of Ca vs. Mg plot that shows an excellent inverse correlation between them, all other plots reveal fields with different populations............................114
Figure 3.10. Mg swath map overlapped on the ASM-1 slab image. Counting of
the dark, thin laminae is shown by the black lines, while counting of Mg rich layers (bright layers in this image) is shown by the red lines. See Figure 3.11 for the chronologies of optical and Mg laminae plotted versus ASM-1 depth below top...................................................................................................115
Figure 3.11. ASM-1 stalagmite chronologies plotted against depth below top
(DBT). Chronologies were determined by counting the optical visible and Mg laminae (see Fig. 3.10 for the manually marked laminae). The total number of years obtained by counting visible laminae is 137, whereas that of Mg laminae is 125 which yields 9.1 % difference between the counting methods............................................................................................................................116
Figure 3.12. Comparison between six abrupt elemental increases (percent
increases) in S accompanied by Mg and the instrumental record of the tropical cyclones that affected Niue. The width of the percent increase bars (a), (b) represents the error age estimates based on optical laminae counting and Mg laminae counting. Percent increases in Mg and S concentrations were calculated by subtracting baseline concentration values from the elemental (S and Mg) concentration peaks. Blue bars (c) represent the cyclones that affected the (north) western coast of Niue. The severity of cyclones is shown numerically as follows: 1- Minor, 2 – Minor to Moderate, 3 – Moderate, 4 – Moderate to Severe, 5 – Severe. The questions mark indicates that the severity of the 1863 cyclone is not known. Sources: Niue Meteorological Service; Kreft (1986); Barker (2000)...............................................................................................................................117
Figure 3.13. Trace element (S/Ca, Mg/Ca, P/Ca) profiles against time plots.
Weight percent units have been standardized ((Raw Datum– Mean)/Standard Deviation). The standardized values are filtered with 7 pt (Mg/Ca and P/Ca), and 5 pt (S/Ca) running average to correspond to the annual ASM-1 stalagmite growth (approximately 400 μm/yr). The severe
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cyclones (blue arrows on the top X-axis) that struck the (north) western coast of Niue are shown because (i) this is the most affected area during cyclone occurrences and (ii) the ASM-1 stalagmite was collected from the Avaiki cave (Fig. 3.2) located on the west coast of Niue. Yellow bars show the correspondence between the timing of the severe cyclones and trace element (S and Mg) peaks. The width of the yellow bars represents the error age estimates based on optical laminae counting and Mg laminae counting. Gray bars show the correspondence between the 1863 cyclone whose severity is not known as well as the 1915 cyclone (gray arrows) and trace element concentration increases. S and Mg abrupt increases around 1885 are shown by a gray bar with a question mark because they might represent a cyclone that was not reported .............................................................118
Figure 3.14. I) Sketch of a Niue Island cross section (W - E) that shows the island
topographic features and the flank margin caves on its western coast. II) It was reported (Solomon and Forbes, 1999) that wave height during cyclone OFA (1990) was approximately 18 m. During severe cyclones, seawater can easily percolate/inundate the flank margin caves on the west coast. Sketches are not drawn to scale .......................................................................................119
Figure 3.15. Plot of Mg2+/Ca2+ and δ13C of Niue dripwater, Niue coastal
seawater and reconstructed dripwater geochemical composition of the 1990, 1959 and 1944 cyclone events mentioned in text. The 13C calculated values were obtained by applying the calcite-HCO3- enrichment factor of 1.0 ‰ determined by Romanek et al. (1992).................................120
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Appendix II. Ca, Mg, P, and S trace element geochemical data................................................. 146
Appendix III. Mg and Ca measurements of the ASM-1 stalagmite top layer ............................ 180
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INTRODUCTION
El Niño Southern Oscillation (ENSO) is an ocean atmosphere, climate phenomenon in
the tropical Pacific with worldwide impacts. It represents the most powerful source of
interannual climate variability and it is associated with shifts in patterns of storm and ocean
conditions, droughts, floods, and fires. During a typical El Niño event the easterly trade winds
across the tropical Pacific Ocean weaken or even reverse and the South Pacific Convergence
Zone (SPCZ), an area of cloudiness and precipitation (Kreft, 1986) shifts eastward. As a result of
the eastward shift in convection, islands of the southwest Pacific experience reduced rainfall
while those near the Central Pacific and further east receive enhanced rainfall (Terry, 2007).
In addition to the ENSO phenomenon, the tropical South Pacific region is also greatly
affected by tropical cyclones. The majority of tropical cyclones develop in the Southern
Hemisphere during the monsoon season from November to April because ocean surface
temperatures in the southwest Pacific attain their maxim at this time of year and, at the same
time, the SPCZ lies at its most southern position of the year (Terry, 2007).
A number of studies using various paleoclimate archives such as: coral reefs (Tudhope et
al., 2001; Cobb et al., 2003), planktonic foraminifera (Koutavas et al., 2002), ocean and lake
sediments (Rodbell et al., 1999), otoliths (Andrus et al., 2002), and geoarchaeological records
(Sandweiss et al., 1996) have been employed in order to improve understanding of ENSO
variability. These studies have unequivocally shown that ENSO have varied in strength and
frequency through time. However, some of these paleoclimate records used are
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discontinuous (Tudhope et al., 2001; Cobb et al., 2003) or lack a high resolution climatic signal
(Sandweiss et al., 1996). Therefore, long-term, continuous paleoclimate records are required to
“fill in the gaps” of other paleoclimate records and to provide detailed, high resolution records of
ENSO in order to predict with confidence how ENSO may change in the future.
In addition, paleoclimate reconstructions of tropical cyclones that severely affect the
islands of the South Pacific have been received little attention.
Studies of speleothems, secondary calcium carbonate deposits, as archives of climate
change play a significant part in the area of paleoclimate research (Fairchild et al., 2006) and
therefore, can potentially address some of the drawbacks that exist in the ENSO reconstruction
studies (i.e., long, continuous, and detailed paleoclimate records) and provide records of tropical
cyclone occurrences. The advantages of speleothems for paleoclimatic studies are (i) great
stability of climatic conditions (constant temperature and high humidity levels) within the cave
environment where they are deposited, (ii) long, continuous and high resolution (sub-annual)
records they can provide and (iii) multi proxy paleoclimate records speleothem archive: stable
oxygen and carbon isotopes (McDermott, 2004; Fairchild et al., 2006), trace elements (Fairchild
et al., 2006; Smith et al., 2009), lamina thickness (Brook et al., 1999; Rasbury and Aharon, 2006)
and calcite fabrics (Frisia et al., 2000).
Niue Island (19°00'S, 169°50'W) is a large carbonate platform located at the edge of the
West Pacific Warm Pool (WPWP) that offers an ideal opportunity to reconstruct El-
Nino/Southern Oscillation history . The climate on Niue is strongly influenced by ENSO, with
some of the largest rainfall anomalies (e.g., dry periods) occurring during ENSO events. Niue
Island is also within the Southwest Pacific’s “tropical cyclone belt” that incorporates the area
from 8°S to 25°S and from approximately 150°E (in the Coral Sea) to about 175°W (east of Fiji)
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(Barker, 2000) and has been subject to numerous tropical cyclones, some of them causing very
severe damage in infrastructure and environment due to their severe nature (strong winds,
torrential rains, saltspray, storm surges and waves). Furthermore, Niue is heavily karstified with
numerous water table and flank margin caves that contain a large number of both modern and
fossil speleothems. The flank margin caves are steeply inclined and formed at the seaward edge
of the groundwater lens, thus they are the most affected areas of the island during the severe
cyclones that strike Niue.
Therefore, the overall objective of this study was to evaluate whether speleothems from
Niue South Pacific are reliable archives for reconstructing paleoclimate in the South Pacific area,
particularly the ENSO variability and the occurrences of the tropical cyclones affecting the
region. The specific objectives of this study were:
1. To understand the geochemical processes in the groundwater seawater mixing zone
and to explain flank margin cave formation.
2. To examine if oxygen and carbon stable isotopes and trace elements in stalagmites
are reliable proxy tools for past climate variability.
3. To determine the factors controlling the stable isotopes and trace element
variations in stalagmites.
4. To determine whether stalagmites from Niue are suitable as paleoclimate archives.
The dissertation is divided into three chapters. The first chapter discusses the
geochemical processes that lead to formation of flank margin caves on Niue Island, an area
where both active tectonics and glacioeustasy have played an important part in the
hydrogeological history of the island. This work is focused on two primary areas of carbonate
dissolution at the Niue groundwater lens margin(s) thought to promote flank margin cave
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development: (i) the halocline, where groundwaters mix with seawater and (ii) the top of the
groundwater lens, where vadose waters mix with groundwaters. The former is tested using
geochemical models of groundwater seawater mixing, while the latter through a plot of calcite
solubility as a function of Ca2+ and CO2 concentrations known as a mixing corrosion diagram.
The results show that the groundwater seawater mixing promotes dissolution of calcite but is
limited to the sites where initial mixing takes place and the flank margin caves are likely formed
in the early stages of groundwater/seawater mixing. In addition, a possible link between
groundwater lens geochemistry and its movement through time due to uplifting events likely
exists because the groundwater lens has a relatively low stability time and an undersaturated
calcite saturation state compared to those on coastal carbonate aquifers in tectonically stable
areas.
The second chapter presents high resolution (sub-annual) oxygen and carbon stable
isotope records of a modern stalagmite, ASM-1, sampled in 2002 in active growing positions
from a flank margin cave. The stalagmite contains approximately 146 years of deposition record
(2002- 1856). The objectives of this study are to determine the factors controlling oxygen and
carbon stable isotope variability in the modern stalagmite and to test whether stalagmites from
Niue are reliable paleoclimate archives for detecting ENSO variability and the severe cyclones
passing Niue Island. Additional objectives of this study are related to location of Niue within the
South Pacific Convergence Zone (SPCZ) area of influence. Thus, this work also evaluates if
stalagmites from Niue archive SPCZ variability. Rainfall reconstructed profiles from 18O and
13C stalagmite generally agree with the instrumental record of Niue rainfall, confirming that the
rainfall amount is the dominant controlling factor of the oxygen and carbon isotopic composition
of the stalagmites from Niue. The results also show that the ASM-1 stalagmite 18O and 13C
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positive/negative anomalies are synchronous with negative/positive phases of the ENSO and
positive/negative phases of SPCZ, thus supporting the ability of stalagmite 18O to record past
interdecadal-decadal climatic variations in the South Pacific area. To distinguish between El
Niño and the potential imprints in the ASM-1 stalagmite of tropical cyclones, high resolution
trace element data accompanied this study. It was shown that both phenomena (i.e., El Niño and
tropical cyclones) are associated with stalagmite 18O and 13C heavier values, but the severe
tropical cyclones affecting Niue are also recorded in the ASM-1 stalagmite by large and abrupt
increases in elemental concentrations.
The aims of the third chapter are to determine the processes controlling trace element
variations in the stalagmite ASM-1, discuss whether annual cyclicity of trace elements represents
a feature of stalagmites from Niue, and to improve understanding of trace element variability in
speleothems. The investigation of the South Pacific tropical cyclone signature(s) in Niuean
stalagmites is also one of the objectives of this chapter. Large, X-ray elemental mapped areas of
the entire growth axis of stalagmite ASM-1 as well as high resolution (sub-annual) profiles of
Mg, S and P were used for this work. High resolution X-ray mapping of Mg, P and S of the
entire ASM-1 stalagmite growth axis reveals complex elemental distributions of Mg, P, and S
that can be traced along stalagmite layers in spite of their intricate and discontinuous nature.
There are also distinct alternating patterns of trace element concentration layers. Mg, P and S
show clear alternating relatively low and high concentration layers throughout the ASM-1
stalagmite and are consistent with stalagmite laminae deposition. Conspicuous inverse
correlations between Mg and P are also very well illustrated by the X-ray mapping technique and
it is interpreted as rainfall seasonality on Niue Island. Thus, the (inverse) correlation and the
alternating patterns of low and high concentration layers of trace elements and their
6
correspondence with the stalagmite laminae suggest that trace element (sub)annual cycles
represent a feature of the stalagmites from Niue. This study also shows that trace elements can be
used as proxies for detecting imprints of severe cyclones in speleothems by reporting abrupt,
large increases in trace element concentration values that coincide with stable isotope seawater
signatures and the occurrences of severe cyclones on Niue.
7
Abstract
The flank margin cave model, initially developed for the tectonically stable Bahamas,
calls for mixing of groundwater with seawater and vadose water with groundwater in carbonate
coastal areas. The carbonate-corrosive mixing zone leads to karst development and cave
formation. The flank margin cave model is extended here to the Pacific carbonate islands that are
located in tectonically active areas in order to explain the occurrence of coastal caves there. The
study focuses on the carbonate island of Niue (19°00'S, 169°50'W) in the tropical South Pacific
that possesses a single, unconfined, groundwater lens with an average thickness of approximately
100 m that floats on top of seawater. Carbonate speciation data based on 19 groundwater wells
indicate that groundwaters are undersaturated for calcite. The theoretical mixing line between the
Niue groundwaters and South Pacific seawater end-members show that calcite under saturation is
greatly enhanced in the initial phases of mixing (up to 30% seawater). Data from
groundwater/seawater mixing from the island of Nauru (0°32’S, 166°56’E), normalized to Niue,
show that the mixture is corrosive to calcite up to 8% seawater. Thus dissolution of calcite is
confined to the sites where initial fluid mixing occurs and the flank margin caves are likely
formed in the early stages of groundwater/seawater mixing. Vadose and groundwater mixing
does not enhance carbonate dissolution because Niue karst waters are either supersaturated
(vadose waters) or undersaturated (vadose waters and groundwaters) with respect to calcite. The
formation of flank-margin caves on Niue is primarily due to the corrosive nature of groundwaters
8
which likely results from the relative low stability time of groundwater lens in a tectonically
active region.
Water-rock interactions in the groundwater seawater mixing zones in carbonate coastal
aquifers promote changes in the chemical composition of the groundwater, the porosity and
permeability of the aquifers, and lead to karst development and cave formation. A particular type
of cave formed in coastal carbonates, flank margin cave, term coined by Mylroie and Carew
(1990), has been reported from various settings including Bahamas (Mylroie and Carew, 1990),
Puerto Rico (Frank et al., 1988), the Mariana Islands (Mylroie, 2001; Stafford et al., 2005), Niue,
South Pacific (Aharon et al., 2006) and New Zealand (Mylroie et al., 2008).
Extensive studies of geological settings and morphologies of flank margin caves (Mylroie
and Carew, 1990, 2003; Mylroie, 2001;), their hydrodynamic behavior (Raeisi and Mylroie,
1995) and the porosity enhancement in carbonate aquifers (Sanford and Konikow, 1989; Florea
et al., 2004) suggest that formation of flank margin caves on carbonate islands is primarily the
result of groundwater/seawater mixing in the discharging margin of groundwater lens. In
addition, the flank margin cave development is also enhanced among other geochemical
processes by the vadose waters mixing at the top of groundwater lens (Mylroie and Carew,
1990). However, speleological studies of karst development and cave formation on carbonate
islands have been not accompanied by geochemical models/studies and are based on the theory
of carbonate undersaturation by mixing (Plummer, 1975; Bögli, 1980) and the geochemical
studies in the groundwater/seawater mixing zones along the Yucatan Peninsula (Back et al.,
1986; Stoessell et al., 1989) and San Andros Island, Bahamas (Smart et al., 1988). Niue Island
(1900’S, 16950’W) represents an ideal environment for flank margin cave formation studies. It
9
is one of the largest carbonate platforms (259 km2) in the tropical South Pacific (Fig. 1.1) and is
heavily karstified with various features including wave-cut cliffs, cleft karren, large chasms,
pinnacle karrenfelds, water table and flank margin caves (Aharon et al., 2006). Numerous flank
margin caves which are steeply inclined and formed at the seaward edge of the groundwater lens
are present. Flank margin caves such as Avaiki and Palaha, located on Alofi Terrace (Fig. 1.2)
contain multiple levels that have been damaged due to roof collapse. The upper cave levels
contain large, fossils speleothems while the lowest chambers which are rounded, contain modern
speleothems. Figure 1.2 also shows the numerous flank margin caves that occur on the leeward
(west) side compared to those on the windward (east) side. The difference between the windward
and leeward karst is also illustrated in the Figure 1.3 where pinnacle karrenfelds rather than flank
margin caves are commonly present on the windward side of the island (Fig. 1.3).
The purpose of this study is to employ the existing geochemical theory to investigate how
flank margin caves form on Niue Island, South Pacific, an area where both active tectonics and
glacioeustasy have played an important part in its hydrogeological history. Thus, this work is
focused on two primary areas of carbonate dissolution at the Niue groundwater lens margin(s)
thought to promote flank margin cave development: the halocline and the top of the groundwater
lens. The former will be tested using geochemical models of groundwater/seawater mixing,
while the latter through a plot of calcite solubility as a function of Ca2+ and CO2 concentrations
known as a mixing corrosion diagram. In addition, a possible link between groundwater lens
geochemistry and its movement through time due to uplifting events will also be examined.
Study Area
Niue is an uplifted former atoll located on the outer slope of the Tonga Trench forebulge.
The island consists mostly of middle and late Miocene to Plio-Pleistocene reef limestone, which
10
has been partially dolomitized and has a mean thickness exceeding 400 m from the top of a
volcanic seamount (Jacobson and Hill, 1980; Wheeler and Aharon, 1997). The island topography
is dominated by a shallow, flat bottomed, centrally located paleo-lagoon (Mutalau Lagoon), 35-
60 m above sea level, and is surrounded by the former atoll rim (Mutalau Reef), which rises at an
elevation of 60-96 m above sea level and is on average 1.2 km wide (Fig. 1.2). The reef falls
steeply to a narrow, wave cut cliff of late Pleistocene age (Alofi Terrace, ~25m above sea level)
that encircles the island (Wheeler and Aharon, 1997). A Holocene coral reef approx. 100 m wide
but locally discontinuous, fringes Niue at sea level.
There are no surface streams on Niue because the limestone surface is very porous (up to
44%) and fissured, which results in dry soil horizons within minutes of prolonged heavy rainfall.
Niue has a single, unconfined groundwater lens with a 70 m thickness beneath the Mutalau
Lagoon (Wheeler and Aharon, 1997) that thins to 0 m near the coast (Fig. 1.4). The vadose zone
is 30-95 m thick and the water table is found at a maximum elevation of 1.83 m above sea level
in the center of the island (Jacobson and Hill, 1980). Groundwater recharge occurs by
infiltrating rainfall through the vadose zone along vertical fissures and solution channels. Thus,
the groundwater recharge is entirely autogenetic. The groundwater flow is laterally toward the
coast, which is very common to carbonate platforms (Jacobson and Hill, 1980).
The carbonate island of Niue is classified as a “simple carbonate island” according to the
geomorphic description of Mylroie et al. (2001) because the carbonate cap extends at least 400
m below sea level, well below the depth of freshwater lens (~70 m below sea level) (Wheeler
and Aharon, 1997).
Geochemical Data Used in the Study
Vadose, groundwater and seawater samples are necessary to conduct a conclusive
geochemical study on flank margin cave formation on Niue. Vadose waters are dripwater
samples collected in August 2002 from the tip of stalactites in two adjacent flank-margin caves
(Avaiki and Palaha, Fig. 1.2) and their cation concentration values were previously reported in
Aharon et al. (2006) work. Because efforts to prevent CO2 degassing during dripwater collection
were made, it is assumed that the chemical composition of the dripwater samples is similar to
that of vadose waters before infiltrating to Niuean caves. Niue groundwater and South Pacific
seawater geochemical data are from the work of Wheeler (2000) and are reported in Table 1.1.
To test the geochemical model of groundwater/seawater mixing, cation and anion
geochemical data of Niue groundwater and South Pacific seawater samples from Wheeler (2000)
(Table 1.1) were used. Calcite saturation indices of groundwater and seawater end-members and
those resulting from their mixing were determined with PHREEQC, geochemical modeling
software that calculates speciation, phase distribution, and reaction pathways (Parkhurst and
Appelo, 1999). Calculated calcite saturation indices of Niue groundwater and South Pacific
seawater end-members and those of the fluid mixture are listed in Table 1.2.
To test the mixing corrosion principle, Ca2+ and CO2 concentration values of vadose
waters and groundwaters are required. Ca2+ concentration values of Niue vadose waters were
obtained from Aharon et al. (2006), while CO2 theoretical concentration values of Niue vadose
waters were calculated from the Niue vadose dissolved inorganic carbon (DIC) concentration
(Aharon et al., 2006) and pH values (Table 1.4) using the equation(s) of Langmuir (1997).
Similarly, Ca2+ concentration values of Niue groundwaters were obtained from Wheeler (2000)
(Table 1.1) while CO2 theoretical concentration values of Niue groundwaters were calculated
12
from the Niue groundwater DIC concentration and pH values (Wheeler, 2000) (Table 1.1) using
the equation(s) of Langmuir (1997).
Groundwater Residence Time Calculations
Groundwater residence times were estimated for the Nauru and Niue Islands and for the
Yucatan Peninsula. These estimates were based on the available hydrogeologic data for each of
the investigated areas and were calculated using the residence time equation of Danckwerts
(1953) below:
Q = flow for the system determined using Darcy’s law
Residence times for the Niue and Nauru islands were estimated using hydrogeologic data
published by Jacobson and Hill (1980) and Jacobson et al. (1997). The groundwater residence
time estimates for the Yucatan Peninsula were calculated using hydrogeologic data published by
Escolero et al. (2002), Gonzales et al. (2002) and Marin et al. (2001). It was assumed that
recharge is constant and evenly distributed throughout the investigated area.
Groundwater residence times for the Floridan aquifer and Bahamas Island were adopted
from Worthington (2007) and Vacher et al. (1990), respectively.
Results
Calcite dissolution as a function of groundwater and seawater mixing
The results of simple mixing of Niue groundwater and South Pacific seawater end-
members for a closed system are given in Figure 1.5. Positive values of saturation index indicate
that the fluid mixture is supersaturated with respect to calcite, negative values indicate
13
undersaturation and zero indicates equilibrium. Niue groundwater end-member (0% South
Pacific seawater) is undersaturated with respect to calcite with a calcite saturation index of - 0.60
and a CO2 partial pressure of 10-1.92 bars. The groundwater end-member represents the average
calcite saturation index (CSI) of 19 water samples from the inland interior. Calcite saturation
index plotted as a function of percentage South Pacific seawater shows that the mixing solution
is undersaturated with respect to calcite from 0 to 70% seawater. It is also shown that the
resulting mixture has the greatest corrosive capabilities in the initial phases and it takes
approximately 32% seawater mixing for the fluid mixture to return to the initial undersaturation
level (Fig. 1.5, hachured area). However, brackish water samples collected from the mixing zone
on Niue are supersaturated with respect to calcite and show departure from the theoretical mixing
line (Fig. 1.5).
The limited number of brackish water samples available from the mixing zone on Niue
(Fig. 1.5) is due to the fact that groundwater wells don’t access the mixing zone on Niue because
of its thick groundwater lens (Fig. 1.3). Thus, a geochemical set of data from the work of
Jankowski and Jacobs (1991) on Nauru, a carbonate island in the South Pacific (0°32’S,
166°56’E) with similar hydrogeologic settings to those of Niue, is also used to test the carbonate
dissolution in the groundwater seawater mixing zone. Nauru is a raised coral atoll with a
maximum elevation of 70 m asl, underlain by a volcanic seamount that is capped by dolomitised
limestone (Jankowski and Jacobs, 1991). The limestone on the island is also extremely karstified
to the depth of at least 55 m below sea level. It has a thin, freshwater lens (up to 7 m thick) that
floats on a brackish water mixing zone, which overlies seawater (Jankowski and Jacobs, 1991).
These geochemical data cover a greater detailed range in salinity, from that of
groundwater to that of South Pacific seawater. The Nauru groundwater is also undersaturated
14
with respect to calcite with a CSI of -1.09 and the pCO2 value prior to mixing is10-1.86 bars,
statistically indistinguishable from that of Niue. Given the approximately same groundwater
chemical composition of Niue and Nauru groundwaters, the geochemical data from the
groundwater/seawater mixing zone of Nauru were scaled to the plot of calcite saturation index as
a function of groundwater/seawater mixing on Niue (Fig. 1.5). Scaling was done in two steps:
first, a normalization equation (Fig. 1.6) was obtained from scaling Nauru groundwater seawater
theoretical mixing values to those of Niue (Table 1.2). Only the interval of 0 to 30% seawater
mixing values (Table 1.2) was used because the samples from the mixing zone (Fig. 1.5, plus
symbols) belong to this interval. This equation was then applied to each mixed water sample
from Nauru (Table 1.3) and converted to Niue mixing zone (Fig. 1.5, plus symblos). Niue scaled
water chemistry data also show departure from the calculated mixing curve with many of the
samples reaching the saturation state of calcite in the early phase of mixing (Fig. 1.5).
Calcite dissolution as a function of Ca2+ and CO2 (aq) concentrations of Niue karst groundwaters
The calcite solubility plot (Fig. 1.7) as a function of Ca2+ and CO2 concentrations at 25°C
is used (Langmuir, 1997) to determine whether the vadose waters and groundwaters on Niue are
geochemically distinctive and whether their mixing results in solutions that are corrosive to
carbonates. Groundwaters are undersaturated with respect to calcite and their Ca2+ (1.36 mM ±
0.17, n = 19) and CO2 (0.44 mM ± 0.1, n = 19) concentration values do not vary significantly
(Fig. 1.6). Vadose waters are found to be supersaturated or undersaturated with respect to calcite
and with varying Ca2+ and CO2 values. Mixing of calcite supersaturated vadose and calcite
undersaturated groundwater in equal proportions leads to solutions that are supersaturated,
undersaturated and/or at equilibrium with respect to calcite. For example, mixing of vadose
water PD-3 with the undersaturated groundwater in equal proportions results in a solution that is
15
supersaturated with calcite (Fig. 1.7). Vadose waters type PD-1 and AD-3 mixed with ground
water in equal proportions results in solutions that are saturated and undersaturated with respect
to calcite. Of great importance is that when AD-3 supersaturated vadose water mix with AD-2 or
PD-4 undersaturated vadose waters, a corrosive mixture forms even though these waters come
from the same settings (i.e., vadose zone). Mixing of vadose waters with different chemical
composition and saturation state would be possible through the cracks and pathways of the
vadose zone in the limestone bedrock.
Discussion
Groundwater seawater mixing at the Niue groundwater lens margin
Geochemical models of Plummer (1975) showed that mixing of seawater and/or saline
groundwater with calcium bicarbonate groundwater result in a mixture that is undersaturated
with respect to calcite even if both solutions are saturated/supersaturated with calcite prior to
mixing. Enhanced calcite undersaturation and carbonate dissolution has been largely reported in
the coastal mixing zones of the Atlantic-Caribbean region. The coastal groundwater/seawater
mixing zone of the Yucatan Peninsula is undersaturated with respect to calcite and aragonite over
a wide range of salinities and dissolution of these minerals is considered the major geochemical
process in cave development along the Yucatan coast (Back et al., 1979; Back et al., 1986;
Stoessell et al., 1989). Smart et al. (1988) and Whitaker and Smart (1997) also reported calcite
undersaturation in the modern groundwater/seawater mixing zone, in submarine cavities of the
Andros Island, Bahamas.
Water samples from Nauru normalized to Niue (Fig. 1.5) show that the groundwater
seawater mixture is corrosive to calcite up to 8% seawater. Although these samples are
undersaturated with respect to calcite, they are more supersaturated than the mixing line
16
calculations predict. A more supersaturated state than the theoretical calculations is also
observed for the both Niue brackish waters and Niue scaled samples with higher percentage of
seawater (Fig. 1.5). These supersaturated conditions of the water samples from the mixing zone
may be related to the calcite dissolution and then precipitation reactions that occur in the early
phases of carbonate groundwater and seawater mixing.
The results of geochemical modeling of water chemistry data from Yucatan (Back et al.,
1986; Stoessell et al., 1989), central Florida (Plummer, 1975), Bahamas (Smart et al., 1998) have
shown that mixing of groundwater with seawater (saline water) promotes an important reactive
geochemical zone that increases dissolution of carbonates. The theoretical model presented in
this study predicts a corrosive fluid mixture to calcite equal to 70% seawater on Niue and
therefore it generally agrees with the results from the Caribbean studies. However, the observed
Niue and Niue-scaled geochemical data show that the waters in the mixing zone become
practically saturated with calcite nearly 8% Pacific seawater (Fig. 1.5) and not at 70% seawater
as predicted. These results differ from those in Yucatan region where the experimental data
usually follow the theoretical mixing curves of groundwater and seawater end-members.
Bögli’s mixing model
The top of the groundwater lens is an area of active carbonate dissolution where two
geochemically distinct karst waters (i.e., vadose and groundwater) mix, thus enhancing flank
margin cave development (Mylroie and Carew, 1990). This process is known as mixing
corrosion and it was first introduced by Bogli (1964) to explain a cave formation mechanism.
According to Bögli’s model, mixing of two waters that are saturated with respect to
calcite with different CO2 concentrations results in renewed aggressiveness to calcite because
Ca2+ concentration in water in equilibrium with calcite is a parabolic function of the CO2
17
concentration (see calcite saturation curve, Fig. 1.7) and any mixing line between 2 waters in
equilibrium with calcite lies below the calcite saturation curve (Bögli, 1980).
Hence, in testing the mixing corrosion principle, a first step is to document whether karst
waters are in equilibrium with calcite and whether vadose waters and groundwaters have largely
different CO2 concentrations values. As shown in Figure 1.7, none of the water sample were in
equilibrium with calcite; the karst waters are either supersaturated (vadose) or undersaturated
(vadose and groundwater) with respect to calcite. Because CO2 concentrations of vadose waters
range from 0.06 to 0.83 mM and overlap with those of groundwaters (0.28 to 0.66 mM), renewed
dissolution capacity according to the Bögli mixing model does not occur.
However, notable observations can be drawn from this plot regarding Niue. Mixing of
calcite supersaturated vadose waters (i.e., AD-3 chemical composition type, Fig. 1.7) in
proportion of 40% and less with calcite undersaturated groundwaters (i.e., groundwater chemical
composition type, Fig. 1.7) in proportions of 60% or more will cause the former to lose their
supersaturation state and the formed mixture to be undersaturated with respect to calcite. In
addition, mixing of the supersaturated vadose with undersaturated vadose waters also results in a
corrosive mixture to calcite. In summary, mixing of vadose waters and groundwaters results in
corrosive mixtures to carbonates due to variation in calcite saturation states of karst waters rather
than the vadose waters being geochemically distinct from groundwaters. The distinct calcite
saturation states in which vadose waters are observed are likely explained by different flow rates
of waters in the vadose zone: long residence time of waters in limestone bedrock promotes
saturation/supersaturation with respect to calcite, whereas short residence time leads to
undersaturation with respect to calcite (Dreybrodt, 1981).
18
Flank Margin Cave Model of Raised Carbonate Platforms
It appears that mixing corrosion occurs in the coastal carbonate aquifers of Niue. The
groundwater/seawater mixing model indicates that the halocline represents an area of active
carbonate dissolution up to 8% South Pacific seawater mixing. The top of the groundwater lens
where the calcite undersaturated groundwater causes the vadose water to lose its calcite
supersaturated state and form a corrosive mixture is also an active area of carbonate dissolution
at the groundwater lens margin(s) on Niue. It is important to point out that the
groundwater/seawater mixture that is undersaturated with respect to calcite is found in the range
of 0-30 meters inland from the present Niue shoreline. Thus, the implication for the coastal
carbonate coastal areas of the two Pacific carbonate platforms is that dissolution of calcite is
probably limited to the sites where initial mixing takes place. Furthermore, the coastal caves
(flank margin caves) are also possibly formed in the early stages of groundwater/seawater
mixing.
Geochemical data of both Niue and Nauru groundwaters also indicate a possible link
between groundwater geochemistry and uplifting events known to cause the two former atolls to
become emergent (Wheeler and Aharon, 1997; Jankowski and Jacobson 1991). Groundwater
lens movement is controlled by sea level which in a tectonically active environment like Niue
Island is a function of both glacioeustasy and island uplifting (Wheeler and Aharon, 1997).
Assuming constant uplifting, groundwater lens on Niue and Nauru islands would have a
relatively low stability time and therefore, the groundwater would not have time to become
saturated/supersaturated with respect to calcite. Present calcite saturation indices of Niue (-0.60)
and Nauru (-1.09) groundwaters indicate that groundwaters on these two carbonate islands are
undersaturated with respect to calcite and have a relatively low residence time (Fig. 1.8).
19
On the other hand, groundwater lens movement on tectonically stable regions is a
function of glacioeustasy. The sea level changes caused by glacioeustasy will move the
groundwater lens up and downward through time causing constant overlapping of the same
phreatic zone (Mylroie and Carew, 2003). This would likely result in relatively higher stability
time of the lens, and calcite saturation indices of the groundwaters close to equilibrium. This is
confirmed by several studies in tectonically stable environments such as Bahamas, Yucatan
Peninsula and Floridan aquifer where groundwaters are at equilibrium or supersaturated with
respect to calcite (Fig. 1.8) (Back et al., 1986; Stoessell et al., 1989; Smart et al., 1988; Wicks et
al., 1995) and have a higher residence time compared to groundwaters from Niue and Nauru
(Fig. 1.8).
Therefore, geochemical processes that promote the development of flank margin caves on
Niue and Nauru carbonate platforms are likely controlled by tectonic uplifting events
superimposed on sea level changes caused by glacioeustasy. Thus, the groundwater lens on
tectonically active carbonate islands would have a relatively low stability time and an
undersaturated calcite saturation state compared to those on coastal carbonate aquifers in
tectonically stable areas.
A geotectonic model of flank margin cave formation on raised atolls can be viewed in
this way (Fig. 1.9): in the first stage of its emergence, a raised atoll develops a thin groundwater
lens and has a water table just below the surface (Fig. 1.9A, Stage 1; note the groundwater lens
configuration in dashed line). Assuming constant meteoric recharge, subsequent uplifting
movements will cause the island to become significantly emergent (Fig. 1.9A; Stages 2, 3) and
the groundwater discharge to increase due to an increase in the island area (Raeisi and Mylroie,
1995). A higher groundwater discharge will allow more calcite to dissolve (Sanford and
20
Konikow, 1989), thus increasing the porosity and/or permeability of the carbonate aquifer.
Therefore, a thicker groundwater lens is expected to form with time (Fig. 1.9A).
A summary of stages of flank margin cave development can be also seen in Figure 1.9A:
assuming episodic uplift movements, a thick groundwater lens and flank margin caves located at
multiple levels above the sea level are characteristic to the present sea level. Flank margin caves
located at relatively high elevation above sea level are relatively old, whereas those located at the
current sea level are relatively young. The caves at higher elevations represent episodes of past
uplifting events; while those located at sea level represent the most recent uplifting event.
Figure 1.9B shows flank margin cave development at Niue. Speleothem dating in the
Avaiki and Palaha caves (Fig. 1.2) confirms that lowermost cave level (1.5 m) contains
stalagmites that yield 230Th maximum ages of 3 ka, and the uppermost level cave level (15 m)
contains stalagmites that yield 230Th maximum ages of 43 ka (Aharon et al., 2006). Therefore,
the speleothem dating confirms that the caves at higher elevations represent episodes of past
uplifting events; while those located at sea level represent the most recent uplifting event.
Conclusions
Carbonate speciation data based on 19 groundwater wells indicate that groundwaters are
undersaturated for calcite and the theoretical mixing line between the groundwaters and South
Pacific seawater end members show that undersaturation is greatly enhanced by mixing with
seawater in the initial phases of mixing (up to 30% seawater). However, the groundwater
seawater mixture samples from Nauru normalized to Niue show that the mixture is corrosive to
calcite up to 8% seawater, suggesting that calcite dissolution and then precipitation reactions that
occur in the early phases of carbonate groundwater and seawater mixing are related to the more
supersaturated conditions. The percentage of South Pacific seawater in mixture that is
21
undersaturated with respect to calcite is found in the range of 0-30 meters inland from the present
Niue shoreline. Therefore, the dissolution of calcite is probably limited to the sites where initial
mixing takes place and the flank margin caves are likely formed in the early stages of
groundwater/seawater mixing.
The Bögli’s theoretical mixing corrosion model of two geochemically distinct karst
waters (i. e., vadose water and groundwater) that enhances carbonate dissolution is not applicable
to Niue because karst water are either supersaturated (vadose water) or undersaturated (vadose
water and groundwater). The Bögli’s model is however useful to calcite supersaturated vadose
waters which lose their supersaturation state when they are mixed with undersaturated
groundwaters.
Development of the flank margin caves that are found on multiple levels on Niue
carbonate platform is likely controlled by tectonic uplifting events because the groundwater lens
will exhibit a relatively low stability time and an undersaturated calcite saturation state compared
to those on coastal carbonate aquifers in tectonically stable areas.
22
Figure 1.1. Location of Niue Island in the South Pacific (from Aharon et al., 2006).
23
Figure 1.2. Toprographic map of Niue Island showing the location of flank margin caves along the Niue coastline. Note the numerous flank margin caves located on the leeward (west) side compared to their scarcity on windward (east) side of Niue. The position of the cross section (AB) in Figure 1.4 is also shown. Niue elevation data were obtained from ESRI world elevation and image data from GIS.
24
Figure 1.3. Flank margin caves of Niue Island. A: Karst features (karenfelt) on the windward site. B: Alofi terrace (Pleistocene-age) containing a multitude of flank-margin caves. C: Seaward entrance to the Avaiki cave. D: Palaha Cave. See Figure 1.2 for Avaiki and Palaha cave locations.
25
Figure 1.4. Cross section of Niue elevation and the groundwater-seawater interface. Groundwater-Seawater interface data are from Jacobson and Hill (1980).
26
Figure 1.5. Calcite saturation index as a function of freshwater/seawater mixing at Niue. Groundwater endmember is given by the 0% South Pacific Seawater, whereas the seawater endmember represents 100% South Pacific Seawater. Calcite saturation of zero value indicates saturation. Triangles indicate brackish water samples collected from the mixing zone of Niue (Wheeler, 2000) and pluses indicate Niue scaled mixed groundwater-seawater samples.
27
Figure 1.6. Normalization factor derived from scaling the Nauru groundwater/seawater theoretical mixing values (0-30% South Pacific Seawater) (Table 1.2) to Niue groundwater/seawater theoretical mixing values (0-30% South Pacific Seawater) (Table 2). The equation was applied to scale the measured Nauru groundwater seawater mixing values to a Niue mixed groundwater-seawater samples scenario. The Niue scaled values are listed in Table 1.3.
28
Figure 1.7. Calcite dissolution as a function of CO2 pressure at Niue. Triangles indicate the vadose waters, while the square represents the groundwaters at Niue. The vadose waters are labeled according to the sampling location: “AD”, Avaiki cave; “PD”, Palaha cave (from Aharon et al., 2006). The Niue groundwater is labeled “P” and represents the average of Ca2+ and CO2 chemical composition of 19 groundwater samples (Ca2+ = 1.36 mM ± 0.17; CO2 = 0.44 mM ± 0.1). Equal proportions of vadose waters and groundwaters on the mixing lines are shown by the closed circles. See text for a discussion.
29
Figure 1.8. Calcite saturation index plotted against the groundwater residence times for Niue and Nauru (this study), Bahamas, Yucatan Peninsula and Floridan aquifer. Calcite saturation indices of groundwaters from Bahamas, Yucatan Peninsula and Floridan aquifer are those reported by Smart et al., 1988; Back et al., 1986; Stoessell et al., 1989 and Wicks et al., 1995. Groundwater residence time calculations are given in the method section.
30
Figure 1.9. (A) Stages of flank margin cave development on a carbonate platform in an active tectonic setting; 1, 2 and 3 represent past sea levels relative to the island and dashed lines represent past configurations of the groundwater lens. (B) Flank-margin cave formation at Niue; cave chambers are interconnected because of ceiling collapses.
31
Table 1.1. Chemical composition of Niue groundwater1, brackish2 and seawater3 samples. Calcite saturation indices (SIc) of groundwater and seawater end-members were obtained by inputting the average chemical compositions of the 19 groundwater samples and the 2 seawater samples, respectively into the Phreeqc geochemical software (Parkhurst and Appelo, 1999). Chemical composition of groundwater, brackish and seawater samples are from Wheeler (2000).
32
Table 1.2. Niue and Nauru groundwater/seawater theoretical mixing values calculated using PHREEQC software (Parkhurst and Appelo, 1999) from the groundwater (0% South Pacific) and seawater (100% South Pacific) end-members.
33
Table 1.3. Nauru samples and their corresponding seawater percent and saturation indices of calcite are from Jankowski and Jacobs (1991). Normalization factors were calculated using the equation shown in Figure 1.6. Niue scaled values of saturation indices of calcite were obtained using the normalization factors shown here.
34
Table 1.4. List of Ca2+, Ph and Dissolved Inorganic Carbon (DIC) of Niue vadose groundwaters (from Aharon et al., 2006). The vadose groundwaters are labeled according to the sampling location: “AD”, Avaiki cave; and “PD”, Palaha cave (Fig. 1.2).
35
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ENSO EVENTS AND CATASTROPHIC CYCLONES IMPRINTED IN A TROPICAL STALAGMITE
Abstract
Niue Island (19°00'S, 169°50'W), a large carbonate platform located at the edge of West
Pacific Warm Pool offers an ideal opportunity to reconstruct El Nino Southern Oscillation
(ENSO) history by means of speleothems. Rainfall on Niue is primarily controlled by ENSO and
South Pacific Convergence Zone (SPCZ) variability. Here we present a high resolution (sub-
annual) δ18O and δ18C profiles from a stalagmite sampled in 2002 in active growing position
from a flank margin cave on Niue Island in order to test whether oxygen and carbon isotope
composition of Niue stalagmites are reliable proxy tools for detecting rainfall and ENSO
variability. The stalagmite contains approximately 146 years of deposition and consists of sub-
annual couplets alternating between white porous calcite laminae deposited during the austral
summer and dark, compact calcite laminae deposited during the austral dry winter. Comparison
of the stalagmite δ18O with Niue annual rainfall generally shows an inverse relationship between
oxygen isotope ratios and the amount of precipitation, with 18O depleted values reflecting higher
rainfall. Conversely, 18O enriched values usually correspond with low rainfall. Variability in the
stalagmite δ18O and δ13C time series also coincides with the ENSO phases (El Niño and La Niña
events) (1866-2002), the sea level pressure (Samoa-Fiji) based SPCZ index (SPI) and the sea-
surface temperature based Interdecadal Pacific Oscillation Index (IPO) (1871-2002).
40
Thus, stalagmite δ18O and/or δ13C represent a proxy tool for the amount of rainfall on Niue and
ENSO variability as well as SPCZ and IPO oscillations. In addition, this study shows that multi
proxy (stable isotope and trace element geochemistry) approach is very useful to distinguish
between distinct phenomena (i.e., El Niño and tropical cyclones in this case).
Introduction
El Niño Southern Oscillation (ENSO) is an ocean atmosphere, climate phenomenon in
the tropical Pacific with worldwide impacts. It represents the most powerful source of
interannual climate variability and it is causes shift patterns of storm and ocean conditions,
droughts, floods, and fires. Consequently, various paleoclimate archives such as: coral reefs
(Tudhope et al., 2001; Cobb et al., 2003), planktonic foraminifera (Koutavas et al., 2002), ocean
and lake sediments (Rodbell et al., 1999), otholits (Andrus et al., 2002), and geoarchaeological
records (Sandweiss et al., 1996) have been employed in order to improve understanding of
ENSO variability.
These studies have unequivocally shown that ENSO have varied in strength and
frequency through time. However, some of these paleoclimate records used are discontinuous
(Tudhope et al., 2001; Cobb et al., 2003) or lack a high resolution climatic signal (Sandweiss et
al., 1996). In addition, the instrumental data records cover only the last 150 years, a time span
relatively short to predict with confidence how ENSO may change in the future. Therefore, long-
term, continuous paleoclimate records are required to “fill in the gaps” of other paleoclimate
records and to provide detailed, high resolution records of ENSO in order to predict with
confidence how ENSO may change in the future.
Studies of speleothems, secondary calcium carbonate deposits, as archives of climate
change play a significant part in the area of paleoclimate research (Fairchild et al., 2006) and
41
therefore, can potentially address some of the drawbacks that exist in the ENSO reconstruction
studies (i.e., long, continuous, and detailed paleoclimate records). The advantages of
speleothems for paleoclimatic studies are (i) great stability of climatic conditions (constant
temperature and high humidity levels) within the cave environment where they are deposited, (ii)
long, continuous and high resolution (sub-annual) records they can provide and (iii) multi proxy
paleoclimate records speleothem archive: stable oxygen and carbon isotopes (McDermott, 2004;
Fairchild et al., 2006), trace elements (Fairchild et al., 2006; Smith et al., 2009), lamina thickness
(Brook et al., 1999; Rasbury and Aharon, 2006) and calcite fabrics (Frisia et al., 2000).
Niue Island (19°00'S, 169°50'W) (Fig. 2.1) is a large carbonate platform located at the
edge of the West Pacific Warm Pool (WPWP) that offers an ideal opportunity to reconstruct El-
Nino/Southern Oscillation history. Niue is heavily karstified with numerous water table and
flank margin caves that contain a large number of both modern and fossil speleothems. Niue
climate is strongly influenced by ENSO, with some of the largest rainfall anomalies (e.g., dry
periods) occurring during ENSO events. Furthermore, previously employed layer thickness
proxy of speleothems revealed that ENSO phenomenon is archived in stalagmites from Niue
(Rasbury and Aharon, 2006).
Niue Island is also located within the Southwest Pacific’s “tropical cyclone belt” that
incorporates the area from 8°S to 25°S and from approximately 150°E (in the Coral Sea) to about
175°W (east of Fiji) (Barker, 2000) (Fig. 2.1). Thus, Niue has been subject to numerous tropical
cyclones, some of them causing severe damage in infrastructure and environment due to their
severe nature (strong winds, torrential rains, saltspray, storm surges and waves). Because tropical
cyclones (also known as hurricanes in North Atlantic or typhoons in North Pacific) are among
the most destructive and frequent natural hazards that impact almost all tropical islands of South
42
Pacific, a thorough study needs to account for their possible signature(s) in Niuean speleothems
as well.
Therefore, the primary objective of this study is to employ a multi-proxy approach (e.g.,
oxygen and carbon stable isotopes and trace element variations) to test whether stalagmites from
Niue are reliable paleoclimate archives for detecting ENSO variability and the severe cyclones
passing Niue Island. Additional objectives of this study are related to location of Niue within the
South Pacific Convergence Zone (SPCZ) area of influence. Thus, this work will also evaluate if
stalagmites from Niue archive SPCZ variability.
Study Area
Niue (19°00'S, 169°50'W) is an uplifted former atoll located in the tropical South Pacific
(Fig. 2.1).The island consists mostly of middle and late Miocene to Plio-Pleistocene reef
limestone, which has been partially dolomitized and has a mean thickness exceeding 400 m from
the top of a volcanic seamount (Jacobson and Hill, 1980; Wheeler and Aharon, 1997). The island
topography is dominated by a shallow, flat bottomed, centrally located paleo-lagoon (Mutalau
Lagoon), 35 m above sea level, and is surrounded by the former atoll rim (Mutalau Reef), which
rises at an elevation of 60-96 m above sea level and is on average 1.2 km wide (Fig. 2.2). The
reef falls steeply to a narrow, wave-cut cliff of late Pleistocene age (Alofi Terrace, ~25m above
sea level) that encircles the island (Fig. 2.2). A Holocene coral reef approx. 100 m wide but
locally discontinuous, fringes Niue at sea level.
The stalagmite used in this study was collected from a flank margin cave (i.e., Avaiki)
located on Alofi Terrace (Fig. 2.2) that contains multiple cave levels damaged due to roof
collapse. The upper cave levels contain large, fossils speleothems while the lowest chambers
which are rounded, contain actively forming speleothems.
43
Climate and Weather of Niue
Niue has two seasons, a wet austral summer monsoon season and a dry austral winter
season. The wet season is from December to April, with monthly mean precipitation of 244 mm
and average air temperature of 26°C (Fig. 2.3). The dry season is from May to November, with
monthly mean precipitation of 118 mm and average air temperature of 24°C (Fig. 2.3). The
distinct seasonality of the rainfall at Niue is due to the seasonal migration of the South Pacific
Convergence Zone (SPCZ), a band of low-level convergence, cloudiness, precipitation and high
surface temperature gradients (Kreft, 1986). SPCZ migrates from its northeast position near the
equator in mid-winter (July) to a more southwest position, near Niue by mid-summer (January)
(Terry, 2007).
Like most South Pacific islands, Southeast trade winds (35-50 km/h) are the predominant
winds on Niue, affecting the eastern coastline of the island during the dry season (austral winter).
Stronger winds (65-90 km/h), usually from northwest sector affect the western coastline of Niue
during the wet season and coincide with tropical storms in the area that are accompanied by
(heavy) precipitation (Terry, 2007). Thus, the eastern coastline of the island facing the trade
winds is affected by sea spray carried by the trade winds, while the leeward side (western
coastline of the island) is sheltered during the austral winter. In contrast, the leeward side of
island is battered by rainfall deluges mixed with sea spray and storm surges mostly during severe
cyclone events (summer monsoon).
El Nino/Southern Oscillation and Tropical Cyclones in the South Pacific and their Influences on the Climate of Niue
The islands of the South Pacific have small land masses with very quick response to
climatological changes. El Nino/Southern Oscillation (ENSO) is a major controlling mechanism
of the South Pacific climate and, consequently, of the Niue climate. Under normal conditions,
44
high pressure in the Eastern Pacific and low pressure in the Western Pacific creates a pressure
gradient. During a typical El Niño event the easterly trade winds across the tropical Pacific
Ocean weaken or even reverse and the South Pacific Convergence Zone shifts eastward. As a
result of th