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Multidecadal rainfall variability in Mesoamerica over the Little Ice Age and Medieval time from a Mexican speleothem. Matthew Lachniet, University of Nevada Las Vegas Juan Pablo Bernal, Universidad Autónoma de México Yemane Asmerom, University of New Mexico - PowerPoint PPT Presentation
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Multidecadal rainfall variability in Mesoamerica over the Little Ice Age and Medieval time from a Mexican speleothem
Matthew Lachniet, University of Nevada Las VegasMatthew Lachniet, University of Nevada Las VegasJuan Pablo Bernal, Universidad Autónoma de MéxicoJuan Pablo Bernal, Universidad Autónoma de MéxicoYemane Asmerom, University of New MexicoYemane Asmerom, University of New MexicoVictor Polyak, University of New MexicoVictor Polyak, University of New MexicoWilliam P. Patterson, University of SaskatchewanWilliam P. Patterson, University of Saskatchewan
ENSO and the MCA
The El Niño/Southern Oscillation is the dominant source of interannual climate variability in the tropics and elsewhere
The Medieval Climatic Anomaly ca. 1200-700 yr BP (800-1300 A.D). was associated with temperature and hydroclimate anomalies in diverse locations
Paleoclimate data suggests a more La Niña-like climate state during the MCA
Modern studies link Western U.S. drought with La Niña
Mesoamerica also experienced severe droughts
Implicated in demise of Maya in late 9th century
MCA climate(low-latitude)
ENSO-sensitive records from the tropics are ambiguous about El Niño or La Niña forcing during Medieval time
Cool/La Niña
Warm/El Niño
Dry/El Niño
Low upwelling /El Niño
Cool/La Niña
Less runoff /La NiñaMore runoff /El Niño
Dry/La Niña
ENSO State
Objective and Hypothesis
Objective: to constrain past rainfall variability within the eastern Pacific
Intertropical Convergence Zone (ITCZ) and the southern portion of the North American Monsoon over the past two millennia
Hypothesis We hypothesize that hydroclimate in southwestern Mexico will
show sustained Medieval wet (e.g. La Niña-like) conditions.
Study Area
Southwestern Mexico is On the northern
fringe of the ITCZ
On the southern margin of the North American Monsoon
Western Hemisphere Warm Pool
Our study area is located next to the Western Hemisphere Warm Pool SSTs > 28.5oC promote deep convection and high specific humidity Is the moisture source for the southern part of the NAM
ENSO-sensitive climate
Correlation of Niño-3 SST with summer rainfall: El Niño = dry conditions La Niña = wet conditions
Wet and dry anomalies are -14.6% and +10.6 respectively (Higgins et al., 1999)
Correlation of August through October precipitation anomalies with the Niño-3 SST index. Negative values (light grey shading enclosed with dashed contours) indicate dry conditions during warm El Niño events, throughout most of southern Mexico and Central America. From Wang et al. 2006.
Study area
Juxtlahuaca Cave, Gro. SW Mex Water sample locations
Juxtlahuaca Cave
Veracruz
Modern Climate
Semi-arid seasonal climate: Mean annual
precipitation = 1163 mm/yr
PET = 2081 mm Boreal summer wet
season between June and October
MAT = 24.6oC
Colotlipa climate data
22
23
24
25
26
27
0 2 4 6 8 10 12 14
Month
Tem
pera
ture
o C
0
50
100
150
200
250
300
Prec
ipita
tion
(mm
)
Temperatura Media
Precipitacion
Seasonal amount effect
Veracruz data show a clear ‘amount’ effect
Mexico surface waters
dD = 8.4 * d 18O + 13.4R2 = 0.95
-80
-70-60
-50
-40-30
-20
-10
010
20
-12 -10 -8 -6 -4 -2 0
d 18O
dD
Surface water samples plot on the GMWL
Juxtlahuaca Cave and stalagmite JX-1
Methods Stable Isotopes at the Las Vegas Isotope Science Lab
(LVIS) at UNLV 1 mm sampling interval Corresponds to 2-3 yr resolution over MCA
U-series at the Radiogenic Isotope Lab at the University of New Mexico
U-series dating Improved chronology relative to
abstract with new 2010 dates
Very precise age dating: High U concentrations 4 to 8 ppm High initial δ234U (>740) Very low sensitivity to the
initial thorium correction Age errors <30 yr two σ.
Stalagmite JX-1 1115 mm tall Upper 430 mm consists
primarily of aragonite and is presented here
Hiatus between 620 and 364 yr BP, and no growth last 96 yr
630
730
830
930
1030
0 500 1000 1500 2000 2500Age (yr BP)
Heig
ht (m
m)
Likely hiatus @ 1080 mm
Black zone @ 639-672 mm
Tip
Stable Isotope Time SeriesDry Intervals in yr BP (yr AD)
970 -1060(1040-950)
884(1126)
1540 -1670(470-340)1195
(815)
1740 -1870(270-140)
1740-1690 (270 to 320)
2040-1840(-50 to 150)
1530-1245 (480 to 765)
1075 and 1150(935 and 860)
855-640 (1155-1370)
364-96 (1646 to 1914)
Wetter or drier Medieval Climate?(More La Niña- or El Niño–like?)
1) Neither the LIA nor the MCA are particularly unusual over the past two millennia
2) MCA relative to entire record: Drier 44% of the time Wetter 56% of the time Generally unremarkable MCA
3) MCA relative to most recent four centuries: Drier 15% of the time Wetter 85% of the time More La Niña-like climate
Climate and Culture
How did climate vary over Maya cultural periods?
Pre-Classic Abandonment was wet
Maya hiatus was dry
Terminal Classic Drought was variable: 3 dry periods and two wet periods
Driest interval was ~1155 yr BP (855 AD), followed by a ‘double dip’ dry period at 1080 yr BP (930 AD)
We attribute to the Classic Maya Collapse in late 9th century
Time Spectra
7-8 yr ENSO band Decadal (9-14) Multidecadal (16-50) spectral
density Solar variability?
has 22 yr at 99% CI 11 yr at 80% CI lacks 88 and 210 yr
Possible forcings include A) Pacific Decadal Oscillation B) Atlantic Multidecadal
Oscillation C) Low-frequency ENSO
Wet conditions = calcite 3 thick layers of calcite are present in the stalagmite Begin in wet periods, and transition into dry periods Suggestive of secondary recrystallization without loss of
d18O signal during wet events Complicates the climate story, thus need for replication
New stalagmite
2010 field work Stalagmite JX-6 All aragonite Super-precise
dating No hiatuses
over past 2000 years
0
100
200
300
400
500
600
700
800
900
1000
1100
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Age
Dep
th (m
m)
Possible hiatus @ 985 mm
LIA
May
a Pe
riod
Was Medieval Climate locked into a La Niña-like climate state?
Our data suggest 1) large hydroclimate variability 2) generally wetter climate relative to LIA 3) but with persistent droughts
Most severe drought at 855 A.D. contemporaneous with Classic Maya Collapse and another during Maya hiatus
The MCA if interpreted in terms of ENSO: More La Niña-like climate than last 4 centuries BUT not unusual relative to past 2 millennia
Conclusions
MCA hydroclimate in Mexico: 1) Highly variable rainfall 2) A more La Niña-like climate 3) Similar in mean and variability to preceding
millennium
Evidence suggests that MCA climate in the eastern Pacific warm pool region was not locked into a La Niña-like climate state, but rather associated with high variability and a cooler temperature state
Agradecimentos a
NSF P2C2 ProgramNSF EAR/IF Program to fund LVIS Lab
Docents of Juxtlahuaca Cave UNLV
UNAM UNM
Optional Slides
Comparison to Temperature records
Wet then dry Mexico while SBB and Palmyra are cool
Supports La Niña-like climate
Conflicts with mean state of climate from the Panama and GOC records
CMC = Classic Maya Collapse PCA = pre-Classic abandonment Hiatus = Maya hiatus
Mexico
Sta. Barbara Basin
Gulf of California
Panama
Palmyra
Comparison to hydroclimate records
Most similar to the Galapagos Lake record (El Junco) and Peru margin lithics
Mexico
Galapagos
Peru Margin
Peru Margin
Pallcacocha, Ecuador
Is solar variability driving Mexico rainfall?
Perhaps
Similar to Total Solar Irradiance proxy of Bard
Dissimilar to residual 14C
But JX-1 d18O time series lacks solar spectra…
Ambiguous evidence for a solar control on Mexico rainfall
High-Res Late Holocene records
Hendy tests show no d18O enrichment along growth layers
Seven ‘Hendy’ tests for oxygen isotopes from 10 to 34 mm away from axis in individual growth bands support equilibrium calcite
Carbon/Oxygen Scatterplots
JX-1 d18O/d13C by mineralogy
Covariance in both aragonite and calcite layers
Common environmental forcing, fast degassing, or another process?
JX-1 Cross plot by mineralogy
-10.0
-9.5
-9.0
-8.5
-8.0
-7.5
-7.0
-6.5
-6.0-12.0 -10.0 -8.0 -6.0 -4.0 -2.0 0.0
d 13C
d18O
Calcite Layer 1
Calcite Layer 2
Calcite Layer 3
Calcite Layer 4
Aragonite Layer 1
Aragonite Layer 2
Aragonite Layer 3
Aragonite Layer 4
Aragonite Layer 5
Abstract We present high-resolution data for a precisely-dated aragonite/calcite stalagmite collected from
southwestern Mexico that documents pronounced variability in rainfall on multi-decadal to centennial time scales. The oxygen isotope and mineralogical variations in the stalagmite document alternating wet and dry periods over the interval between 1800 yr BP and present, with a hiatus between 840 and 550 yr BP. The climate of the region is currently semi-arid and rainfall variability is strongly linked to El Niño (dry) and La Niña (wet). The stalagmite record indicates relatively dry conditions over the modern and Little Ice Age (LIA) periods when aragonite with relatively high δ18O values was precipitated. During and prior to the Medieval Climatic Anomaly (MCA), δ18O values alternated rapidly between conditions much wetter and drier than today, indicating greater rainfall variability relative to the last several centuries. Three calcite layers likely formed during wet intervals when cave drip waters were initially under-saturated in aragonite, at approximately 1590-1546, 1164-1204, and 846 to 1050 yr BP, the youngest being coincident with a wet spell during the MCA. The interval between 1200 and 1590 yr BP was consistently dry, similar to the LIA and modern periods. The δ18O data display spectral power in multi-decadal bands, which suggest that Mexican rainfall on the northern fringe of the eastern Pacific ITCZ responds to multi-decadal scale forcing, possibly via the Pacific Decadal Oscillation and/or the Atlantic Multidecadal Oscillation. As a whole, wet conditions (La Niña-like) were more common during the MCA than the Little Ice Age, but that the driest interval during the entire record occurred 890-910 A.D. within the MCA. This dry period we correlate with the Terminal Classic Drought in Mesoamerica. Our data suggests that Medieval climate was not locked into a sustained La Niña state but rather fluctuated rapidly and drastically about a stable mean state.