Marine Micropaleontology 74 (2010) 29–37

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The oxygen isotope composition of planktonic foraminifera from the Guaymas Basin,Gulf of California: Seasonal, annual, and interspecies variability

Katherine E. Wejnert a,⁎, Carol J. Pride b, Robert C. Thunell a,c

a Marine Science Program, University of South Carolina, Columbia, SC 29208, USAb Marine Sciences Program, Department of Natural Sciences and Mathematics, Savannah State University, Savannah, GA 31404, USAc Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA

⁎ Corresponding author. Tel.: +1 803 777 4526; fax:E-mail addresses: kwejnert@geol.sc.edu (K.E. Wejne

(C.J. Pride), thunell@geol.sc.edu (R.C. Thunell).

0377-8398/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.marmicro.2009.11.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 July 2009Received in revised form 18 November 2009Accepted 19 November 2009

Keywords:Guaymas BasinPlanktonic foraminiferaOxygen isotopesThermocline depthGulf of California

Sediment trap samples collected over a seven-year period (February 1991–October 1997) from GuaymasBasin in the Gulf of California were used to study the oxygen isotope composition of five species ofplanktonic foraminifera, Globigerinoides ruber (white), Globigerina bulloides, Neogloboquadrina dutertrei,Pulleniatina obliquiloculata, and Globorotalia menardii. The δ18O data were analyzed for temporal andinterspecies variability and were compared to local hydrography to evaluate the use of each species inreconstructing past oceanographic applications. The two surface dwelling species, G. ruber and G. bulloidesdisplayed the lowest δ18O values (∼0.0 to −5.0‰), while δ18O values for the thermocline dwellingN. dutertrei, P. obliquiloculata, and G. menardii were higher (∼0.0 to −2.0‰). The δ18O of G. ruber mostaccurately records measured sea surface temperatures (SSTs) throughout the year. G. bulloides δ18O tracksSSTs during the winter–spring upwelling period but for the remainder of the year records slightly colder,subsurface temperatures. The difference between the δ18O of the surface dwelling species, G. ruber andG. bulloides, and that of the thermocline dwelling species, N. dutertrei, P. obliquiloculata, and G. menardii, wasused to estimate the surface to thermocline temperature gradient. During the winter these δ18O differencesare small (∼0.50‰) reflecting a well-mixed water column. These interspecies δ18O differences increaseduring the summer (∼1.90‰) in response to the strong thermal stratification that exists at this time of year.

+1 803 777 3922.rt), pridec@savannahstate.edu

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Urey (1947) and Emiliani (1954, 1955) pioneered the use of thestable oxygen isotope composition of planktonic foraminifera toreconstruct paleotemperatures. Now the δ18O of planktonic forami-nifera is one of themost commonly used paleoceanographic tools (seeLea, 2003 for a review). Because different species occupy differentdepths in the water column, the δ18O of the planktonic foraminiferalcalcite can be used to reconstruct upper ocean thermal stratification(Fairbanks and Wiebe, 1980; Ravelo and Fairbanks, 1992; Mulitzaet al., 1997; Niebler et al., 1999). In particular, the difference in δ18Obetween surface dwelling species and thermocline species has beenused to estimate vertical temperature gradients within the watercolumn (Niebler et al., 1999; Tedesco et al., 2007; Steph et al., 2009).

However, the δ18O of foraminiferal calcite is not only controlled bythe physical/chemical properties of the water column (e.g. temper-ature, isotopic composition of the ambient water), but also by species-specific vital effects (Bemis et al., 1998; Erez, 2003), such ascalcification rate (Ortiz et al., 1996), symbiont photosynthesis

(Spero and Lea, 1993), and respiration (Wolf-Gladrow et al., 1999).Despite the advances made through culturing experiments tounderstand vital effects, it is important to study the relationshipbetween temperature and the oxygen isotope composition offoraminifera calcifying in the natural environment to properlyinterpret the sedimentary δ18O record.

In this study, we present the stable oxygen isotope results fromfive different species of planktonic foraminifera recovered fromsediment trap samples collected over a seven-year period (February1991–October 1997) in Guaymas Basin, Gulf of California. The δ18Odata are compared to hydrographic data to elucidate the relationshipbetween the oxygen isotope composition of foraminiferal calcite andenvironmental variables. The differences between the δ18O of speciesthat live in the thermocline versus the surface are used to reconstructupper ocean stratification and compared to available hydrographicdata.

2. Regional setting

The Guaymas Basin, located in the central part of the Gulf ofCalifornia (Fig. 1), is a large evaporative basin (Roden, 1958; Bray andRobles, 1991) with a monsoon climate that is controlled by the NorthPacific high pressure center and associated continental lows (Schrader

Fig. 1.Map of the Gulf of California showing the sediment trap location in the Guaymas Basin at 27o53'N, 111o40'W and at 485 mwater depth. Summer currents are shown in A andWinter currents are shown in B.

30 K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

and Baumgartner, 1983; Badan-Dangon et al., 1991; Thunell, 1998). Inthe winter, the North Pacific high is strongest and a low-pressuresystem develops over Sonora, which generates strong northwesterlywinds that result in a net transport of surface water out of the Gulf(Badan-Dangon et al., 1991; Thunell, 1998). During this time of year,

Fig. 2. Temperature estimates derived from the oxygen isotope composition of the planktonbetween 1991–1997. Eq. (1) was used to derive estimates for G. bulloides, Eq. (2) for G. rutemperature derived from advanced very high resolution radiometer (AVHRR) data.

surface waters cool (Fig. 2) and the pycnocline weakens allowing theupwelling of cold, nutrient-rich waters (Roden, 1972; Bray andRobles, 1991). During the summer months, the North Pacific highmigrates northwards causing a reversal to southeasterly winds(Roden, 1958; Badan-Dangon et al., 1991). This allows equatorial

ic foraminifera from biweekly sediment trap samples from the Guaymas Basin collectedber (white) and Eq. (3) for N. dutertrei, G. menardii, and P. obliquiloculata. sea surface

31K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

Pacific surface waters to enter the Gulf, which terminates upwellingand warms the surface waters (Fig. 2). This results in a highlystratified water column with low primary production (Schrader andBaumgartner, 1983; Thunell et al., 1996; Thunell, 1998). In contrast,there is little seasonal variation in salinity in Guaymas Basin, with theupper 200 m showing almost no variation (Roden and Groves, 1959;Beron-Vera and Ripa, 2002;McConnell and Thunell, 2005). The annualaverage surface salinity is 35.10‰, with an average range of less than0.2‰.

Interannual variability in hydrographic conditions in the GuaymasBasin is dominated by the El Niño-Southern Oscillation (ENSO)(Baumgartner and Christensen, 1985; Thunell et al., 1999). During ElNiño, the North Equatorial Gyre intensifies, which confines theCalifornia Current north of the mouth of the Gulf. These conditionsallow the Costa Rica Current to bring tropical surface water well intothe Gulf (Bray and Robles, 1991; Thunell, 1998). This results inwarmer temperatures, diminished upwelling and lower productivityin the Guaymas Basin during El Niño years (Thunell, 1998; Thunellet al., 1999; Ziveri and Thunell, 2000). During this study, El Niñoconditions were present from early 1991 through the end of 1994 andbegan again at the end of the study period beginning in March 1997(Thunell, 1998; McConnell and Thunell, 2005).

3. Materials and methods

A moored, automated sediment trap was used to collect a timeseries of sediment flux samples from Guaymas Basin (27o53'N,111o40'W) from February 1991 to October 1997 (Fig. 1). The trapwas positioned approximately 200 m off the seafloor and at a waterdepth of 485 m. Two-week long samples were collected continuouslythroughout the study period, except for a four-month gap fromDecember 1992 through March 1993 due to a malfunction of thesediment trap.

Prior to deployment, the sample cups were filled with a bufferedsodium azide solution to act as a poison. After collection, the sampleswere split with a rotary splitter, stored in buffered deionized water,and refrigerated until processed. The planktonic foraminifera wereextracted from the samples using the density separation techniquedescribed by Bé (1959) and were sorted into the following sizefractions: 125–150 µm, 150–180 µm, 180–212 µm, 212–250 µm, 250–300 µm, 300–355 µm, 355–425 µm, and 425–500 µm. Specimens ofGlobigerinoides ruber (white), Globigerina bulloides, Neogloboqua-drina dutertrei, Pulleniatina obliquiloculata, and Globorotalia menardiiwere removed, sonicated in a methanol bath for 1 min, and analyzedfor oxygen isotopes. A target size range was set for each species;however it was sometimes necessary to use specimens from smallerfractions (Table 1). Target size ranges were determined by consider-ing the ideal size range where ontogenetic effects are minimal and thesize range where species are most abundant (Ravelo and Fairbanks,1992; Elderfield et al., 2002; Peeters et al., 2002; Farmer et al., 2007;Steph et al., 2009). Approximately 60–80 µg of samples were used peranalysis. Eighty one of the nine hundred and thirty-four foraminiferalsamples were analyzed at the Woods Hole Oceanographic Institution

Table 1List of target size ranges and smallest size fraction used for each species.

Species Target size range forisotopic analyses

Smallest sizefraction used

Percent of samplesoutside target range

Globigerinoides ruber 250–300 µm 150–180 µm 50Globigerina bulloides 212–250 µm 125–150 µm 17Neogloboquadrinadutertrei

300–355 µm 150–180 µm 33

Pulleniatinaobliquiloculata

N425 µm 250–300 µm 0

Globorotalia menardii N425 µm 250–300 µm 20

on a Finnigan MAT 252 stable isotope mass spectrometer fitted with aKiel Carbonate Preparation Device. The remainder of the sampleswere analyzed at the University of South Carolina on either a VGOPTIMA or GV ISOPRIME stable isotope ratio mass spectrometer, bothof which are equipped with automated carbonate preparationsystems. The long-term standard reproducibility is 0.07‰ forδ18O and results are reported relative to Vienna Pee Dee Belemnite(V-PDB).

Both remotely sensed and in situ measurements were used todocument temporal variability in temperature and salinity in theGuaymas Basin. Sea surface temperature (SST) estimates werecalculated from advanced very high resolution radiometer (AVHRR)data obtained from the Physical Oceanography Distributed ActiveArchive Center (PODAAC) at the Jet Propulsion Laboratory. A record ofSSTs was derived from weekly composites generated by averagingdaytime grayscale values from four 18 km×18 km grid squares thatsurrounded the sediment trap site (Thunell, 1998; McConnell andThunell, 2005). The record covers the entire study period and has aprecision of ±0.4 °C (McConnell and Thunell, 2005).

Water column temperature and salinity data were measured withthree conductivity, temperature, and depth (CTD) casts made inGuaymas Basin during June 1990, February 1991, and August 1991.The CTD casts are used to assess the seasonal changes in temperatureand salinity and allow for the determination of calcification depths ofthe foraminifera by comparing the δ18O of the calcite with the δ18O ofthe ambient water, which can be calculated from temperature andsalinity. In addition to our CTD casts, Robinson (1973) generatedcomposite monthly temperature profiles of the upper 200 m at 28° N112° W.

4. Results

The oxygen isotope data measured on the five species ofplanktonic foraminifera are illustrated as time series in Fig. 3 andreveal two distinctive trends. First, each species is marked by seasonalchanges in δ18O that are fairly repeatable from year to year, althoughthe magnitude of the seasonal changes varies between species.Second, there is a clear ranking or ordering of δ18O values amongst thespecies with G. ruber and G. bulloides usually having lower values thanthe other three species. The range of the δ18O values and correlationwith SST for each species is listed in Table 2.

G. ruber lives in the surface mixed layer and occurs in subtropicalto tropical latitudes (Deuser, 1987; Ravelo and Fairbanks, 1992;Niebler et al., 1999). Using vertically stratified plankton tows from theSouthern California borderland basins, Field (2004) found thatG. ruber is most abundant at the base of the mixed layer and evenhad moderate abundances within the thermocline; however, incontrast to other species G. ruber has a low-slope response to adeepening isotherm, which makes G. ruber the most suitable recorderof near-surface temperatures when other species are living deeper(Field, 2004; Tedesco et al., 2007). Our δ18O data are consistent withthese findings; G. ruber had the lowest δ18O values of the five speciesanalyzed, ranging from−0.39 to−5.31‰with an average value overthe study period of −2.60‰.

G. bulloides has a wide geographic distribution ranging from thepoles to the low latitudes (Niebler et al., 1999; Schmidt and Mulitza,2002). This species most commonly lives in the surface mixed layer(Fairbanks et al., 1982; Hemleben et al., 1989), but it also occurswithin the thermocline (Field, 2004). The δ18O values for G. bulloidesrange from 0.02 to −4.97‰ with a mean value of −1.49‰, which ishigher than that of G. ruber, but the range sizes of the two species arenearly identical.

The δ18O values for P. obliquiloculata,N. dutertrei, andG. menardii aresimilar and mostly fall between 0 to−2.0‰. These values are generallyhigher than those for G. ruber or G. bulloides and indicate that thesespecies are calcifying in colder, deeper waters. P. obliquiloculata and

Fig. 3. Oxygen isotope composition (in ‰) of the planktonic foraminifera G. ruber (white), G. bulloides, N. dutertrei, G. menardii, and P. obliquiloculata from biweekly sediment trapsamples collected from the Guaymas Basin between 1991–1997.

32 K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

N. dutertrei are subtropical to tropical species that have been reported tolive within the thermocline near the chlorophyll maximum (Bé, 1977;Fairbanks et al., 1982; Ravelo and Fairbanks, 1992; Niebler et al., 1999;Farmer et al., 2007). The δ18O values of P. obliquiloculata range from0.31 to −2.57‰ with a mean value of −0.95‰, while the values forN. dutertrei range from 0.37 to−2.04‰with a mean value of−0.55‰.G.menardii is considered a deeper dwelling species and ismost commonin subtropical to tropical latitudes (Bé, 1977; Oberhänsli et al., 1992;Niebler et al., 1999). The δ18O values ofG. menardii range from−0.01 to−2.27‰ with a mean value of −0.90‰.

5. Discussion

5.1. δ18O–temperature relationship

Urey (1947) first introduced the concept that the 18O/16O ratio incarbonates varies as a function of the temperature. Emiliani (1955)then applied this concept to oxygen isotopes in foraminiferalcarbonates, assuming that the signal was primarily driven bytemperature, with only a minor component due to changes in theisotopic composition of sea water. However, a little over a decadelater, Shackleton (1967) convincingly demonstrated that changes inthe δ18O of seawater could be much larger than previously proposedby Emiliani (1955) and thus accounted for a larger portion of thecarbonate δ18O signal. Since these pioneering studies, oxygen isotopesin planktonic foraminifera have become one of the most commonlyused proxies for reconstructing past ocean conditions.

The relationship between temperature and the δ18O of CaCO3 wasinitially determined by Epstein et al. (1953). Since then, there havebeen a number of studies that have either refined this initial

Table 2The mean, minimum, and maximum δ18O, corresponding temperature estimates, and coparenthesis.

Species δ18O

Minimum Maximum M

Globigerinoides ruber −5.31 (−3.92) −0.39 −Globigerina bulloides −4.97 (−3.66) 0.02 −Neogloboquadrina dutertrei −2.04 0.37 −Pulleniatina obliquiloculata −2.57 (−2.17) 0.31 −Globorotalia menardii −2.27 −0.01 −

“paleotemperature equation” or developed similar equations forspecific foraminiferal taxa (e.g., Duplessy et al., 1981; Erez and Luz,1983; Bouvier-Soumagnac and Duplessy, 1985; Bemis et al., 1998;Mulitza et al., 2003). For example, Bemis et al. (1998) demonstratedthat the fractionation of oxygen isotopes varies between species ofplanktonic foraminifera, thus necessitating the need for species-specific temperature equations. Because of this, we used the followingequations to estimate temperatures from planktonic foraminiferalδ18O

Tð�CÞ = 13:2−4:89*ðδc−δwÞ ð1Þ

Tð�CÞ = 14:9−4:80*ðδc−δwÞ ð2Þ

Tð�CÞ = 16:5−4:80*ðδc−δwÞ ð3Þ

where T is the calcification temperature, δc is the oxygen isotopiccomposition of the foraminiferal calcite and δw is the oxygen isotopiccomposition of the seawater, both in ‰ relative to the Vienna PeeDeeBelemnite (V-PDB) standard. The δw is scaled from standard meanocean water (SMOW) to PDB by subtracting 0.27‰ (Bemis et al.,1998). Bemis et al. (1998) developed Eq. (1) for G. bulloides andEq. (2) for Orbulina universa; however, Eq. (2) can also be used forG. ruber (Tedesco et al., 2007). Although Tedesco et al. (2007) appliedEq. (2) to G. ruber (pink), we use it for G. ruber (white). Bemis et al.(1998) developed Eq. (3) for O. universa under low light conditionsand we use it to estimate temperatures for N. dutertrei, G. menardii,and P. obliquiloculata.

The annual range of surface salinity in Guaymas Basin is only 0.2‰(35.1–35.3‰; Roden and Groves, 1959; Robles and Marinone, 1987).

rrelation with SST for each species. Where applicable values excluding 1992 are in

Temperature estimate (°C) SST correlation

ean±std dev Maximum Minimum R2 (p-value)

2.60±0.94 41.2 (34.5) 17.6 −0.74 (b0.001)1.49±0.97 38.3 (31.9) 13.9 −0.71 (b0.001)0.55±0.40 27.1 15.5 −0.26 (0.005)0.95±0.81 29.7 (27.7) 15.8 −0.70 (b0.001)0.90±0.53 28.2 17.4 −0.41 (b0.001)

33K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

Using the salinity: δ18O relationship for the eastern Pacific (Fairbankset al., 1997)

δw = 0:273*ðsalinityÞ−9:14‰ ð4Þ

we estimate that the mean annual range of salinities is equivalent toan δ18O change of only 0.05‰. Given the low variation in surfacesalinity, an average δw can be used to estimate temperature. Theaverage surface salinity in Guaymas Basin determined from the threeCTD casts is 35.1‰, which corresponds to a δw of 0.17‰ relative to V-PDB. Calcification temperatures were estimated for all five species andplotted as time series (Fig. 2). In general, the highest δ18O values(Fig. 3) and lowest calculated temperatures (Fig. 2) for all speciesoccur during winter upwelling, with opposite being true for thesummer.

G. ruber has a large seasonal range in oxygen isotope values of4.92‰. However, this range is exaggerated by a small number of verylow values occurring in the fall of 1991 and 1992. Without theseextreme values, virtually all of the data fall within a 4‰ range (0 to−4‰). For the five-year period of 1993 through 1997, the summermaximum SSTs estimated from G. ruber δ18O agree extremely wellwith the satellite temperatures (Fig. 4, Table 2).

The anomalously low G. ruber oxygen isotope ratios during thesummers of 1991 and 1992 and the winter of 1992 (Fig. 3) occurred inassociationwith strong El Niño conditions, which bringwarm, tropicalwaters into the Gulf. These low δ18O values yield summer SSTs thatreach 35 °C in 1991 and over 40 °C in 1992, both of which aresignificantly warmer than the satellite temperatures. Strong El Niñoconditions also occurred during the summer of 1997, but the δ18O of

Fig. 4. Comparison of sea surface temperatures derived from advanced very high resolutioG. ruber and G. bulloides.

G. ruber in 1997 is not as high as those of 1991 and 1992, and yieldssummer temperatures that are consistent with the remotely sensedtemperatures. One possible reason for the discrepancy between theproxies and satellite measured SSTs in 1991 and 1992 is the presenceof different morphotypes of G. ruber. Although the morphology ofG. ruber was not examined, it is possible that during El Niño there is agreater abundance of G. ruber sensu stricto (s.s.), which prefers a morestratified environment, and would yield lighter isotope values (Wang,2000; Steinke et al., 2005). Another factor which could possiblycontribute to the extremely light isotope values is the anomalouslyhigh volume of rain recorded in the nearby Río Yaqui Basin during thesummer–winter of 1992 (Nicholas and Battisti, 2008), which wouldlower the salinity in Guaymas Basin and therefore lower the isotopecomposition of the foraminifera.

For the seven-year study period, the average maximum G. ruberδ18O value during winter upwelling was −0.82‰ (19.7 °C) and theaverage minimum summer non-upwelling value was −3.88‰(34.3 °C). These yield a mean G. ruber δ18O-based seasonal temper-ature range of 14.6 °C, which is slightly smaller than the observedmean range of 15.7 °C (16.3–32.0 °C). This difference is due largely tothe fact that G. ruber is rarely present during the coldest wintermonths and thus the δ18O temperature range does not fully representthe entire year. Overall, our results indicate that the δ18O of G. ruberprovides an excellent proxy for SST in Guaymas Basin.

G. bulloides, another species considered to be a surface dweller, hasa seasonal variation in δ18O of 4.99‰, very similar to that of G. ruber(Fig. 3). Like G. ruber, this range is extended by a small number ofsamples with very low values in fall 1992. Without these samples therange is approximately 4‰. The mean maximum δ18O for G. bulloides

n radiometer (AVHRR) data with oxygen isotope derived temperature estimates from

34 K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

during winter upwelling was −0.36‰ (15.8 °C) and the meanminimum during summer non-upwelling was −3.47‰ (30.9 °C),which results in an average seasonal range of 3.10‰ (15.1 °C). Thisestimated range is very close to the measured SST range of 15.7 °C,with the G. bulloides δ18O temperatures being slightly colder thanmeasured SSTs (Fig. 2).

Comparison of the G. ruber and G. bulloides δ18O temperature timeseries with the measured SST record for the seven-year study periodindicates that both species accurately track seasonal changes in SSTwith a few exceptions (Fig. 4, Table 2). As already mentioned, themost obvious departures occur during the 1991–1992 El Niño, withboth G. ruber and G. bulloides yielding temperatures that are 5 °C ormore higher than the satellite-derived temperatures. The fact thatboth species yield similar isotope results suggests that the data arerobust, but maybe affected by large changes in salinity. The frequentabsence of G. ruber from Guaymas Basin during the winter monthsprevents us from using this species to capture the full annualtemperature cycle. Conversely, G. bulloides is consistently presentthroughout the winter/spring and scarce during the summer. G. bul-loides appears to be dwelling near the surface during the winterupwelling period and migrating deeper during the warm season.Thus, in order to provide themost accurate and continuous time seriesof SST estimates, it is best to use G. ruber during the summer warmperiods and G. bulloides during the cold winter upwelling.

The thermocline or subsurface dwelling species, N. dutertrei,P. obliquiloculata, and G. menardii have narrower seasonal ranges inδ18O, 2.41‰, 2.26‰ and 2.88‰ respectively, than those of the twosurface dwelling species. The mean maximum δ18O for N. dutertrei,P. obliquiloculata, and G. menardii during winter upwelling are−0.11‰ (17.8 °C),−0.13‰ (18.0 °C),−0.20‰ (18.3 °C) respectivelyand the mean minimum during summer non-upwelling are −1.32‰(23.7 °C), −1.92‰ (26.5 °C), and −1.68‰ (25.4 °C) respectively.N. dutertrei, P. obliquiloculata, and G. menardii have mean seasonalranges of 1.21‰ (5.9 °C), 1.79‰ (8.5 °C) and 1.48‰ (7.1 °C), whichare much narrower than G. ruber, G. bulloides. The estimated winterupwelling temperatures for N. dutertrei, P. obliquiloculata, andG. menardii are close to the observed SSTs, but the summer non-upwelling temperatures are significantly colder than SSTs. N. duter-trei, P. obliquiloculata, and G. menardii appear to be calcifying near thesurface during the winter and deeper during the summer.

Fig. 5. Temperature profile from Guaymas Basin from Robinson (1973). Predicted calcificaG. menardii, and P. obliquiloculata estimated from monthly averages of oxygen isotopes. Err

5.2. Depth habitats and surface water stratification

Since different species of planktonic foraminifera live and calcify atdifferent depths, their oxygen isotope ratios can be used as a proxy forupper ocean stratification (Fairbanks et al., 1982; Ravelo andFairbanks, 1992; Mulitza et al., 1997; Niebler et al., 1999; Stephet al., 2009). We can use the calcification temperatures discussedabove to estimate the habitat depths for the five species used in thisstudy. A representative annual temperature cross-section for theupper 100 m of thewater column in Guaymas Basinwas derived usingmonthly CTD data (Robinson, 1973; Fig. 5). The data from Robinson(1973) were used because they represent a complete year of data;however comparison with the CTD casts and satellite data suggestthat Guaymas Basin was colder in 1973 than in the 1990s. Despite thetemperature difference, the overall temperature profile pattern is thesame. As a result of this temperature difference, species might live atslightly deeper depths than predicted here. For each species, averagemonthly temperatures were estimated using the oxygen isotope datafor the seven-year study period and calcification depths weredetermined from these averages (Fig. 5). The depth habitats foreach species vary seasonally in response to changes in localhydrographic conditions.

As expected, G. ruber and G. bulloides are the shallowest dwellers(Fig. 5). G. ruber most likely calcifies at the surface throughout theyear despite the large seasonal temperature range (∼16–32 °C),although the nearly isothermal conditions in the upper water columnduring the winter make it difficult to precisely determine the depth ofcalcification. G. bulloides displays more variability in its depthpreference. During the winter upwelling period G. bulloides lives atthe surface and then migrates to ∼40 m during the summer whensurface waters become highly stratified. When temperatures cool inthe fall, this species returns to the surface. Our results show thatG. bulloides can calcify over a large range of temperatures (∼16–28 °C), although it is only present sporadically in Guaymas Basinduring the warmest months of the year (Fuqua et al., 2009). This isconsistent with the previous findings of Lombard et al. (2009) that G.bulloides growth rate rapidly decreases above 25 °C.

Both plankton tow and stable isotope data indicate that N.dutertrei, P. obliquiloculata, and G. menardii are subsurface dwellers(Bé, 1977; Fairbanks et al., 1982; Ravelo and Fairbanks, 1992; Niebler

tion depths for the planktonic foraminifera G. ruber (white), G. bulloides, N. dutertrei,or bars represent one standard error.

Fig. 6. Comparison of the temperature difference between 0 and 40 mwater depth (data from Robinson, 1973) and the estimated temperature difference between shallow dwellingspecies, G. ruber and G. bulloides, and deep dwelling species, N. dutertrei, G. menardii, and P. obliquiloculata based on oxygen isotope composition.

35K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

et al., 1999; Field, 2004; Farmer et al., 2007). Our δ18O data for thesethree species yield temperatures that are similar to near-surfacetemperatures for the fall through the spring (October to May) (Fig. 5).However, during this period, the upper 40–60 m of the water columnis well-mixed and nearly isothermal, making it difficult to assignspecific depth habitats to these species. In contrast, our data clearlyshow that all three of these species live at ∼40–60 m depth during theearly summer and then migrate downward to ∼100 m depth in thelate summer/early fall, when SST is highest and thermal stratificationis greatest (Fig. 5). N. dutertrei, P. obliquiloculata, and G. menardii alloccupy narrower annual temperature ranges as shown by the δ18Odata (∼18–22 °C, 18–26 °C, 19–24 °C respectively) than G. ruber andG. bulloides. These narrow temperature ranges suggest that thesespecies have a strong temperature preference and are altering theirdepth habitat to accommodate this preference or that these speciesrestrict their distribution to the thermocline and as result recordnarrower temperature ranges. Although G. menardii has previously

Fig. 7. Comparison of the Δ δ18O(thermocline–shallow ) and opal flux recor

been recorded as a deeper dwelling (subthermocline) species (Bé,1977; Oberhänsli et al., 1992; Niebler et al., 1999), it appears to beliving in the thermocline in Guaymas Basin, which is consistent withthe observations of Spero et al. (2003).

The difference in δ18O between shallow and intermediate dwellingspecies reflects the vertical temperature gradient in the upper watercolumn (Ravelo and Fairbanks, 1992; Spero et al., 2003). AlthoughN. dutertrei, P. obliquiloculata and G. menardii vary their depth ofcalcification throughout the year, they occupy a narrow range oftemperatures (18–22 °C, 18–26 °C, 19–24 °C respectively), which isindicative of their preference for remaining within the thermocline(Ravelo and Fairbanks, 1992; Field, 2004; Tedesco et al., 2007).Therefore the δ18O difference (Δ δ18O) between the surface dwellingspecies (G. ruber and G. bulloides) and the deep dwelling species(N. dutertrei, P. obliquiloculata, and G. menardii) can be used to estimatethe temperature gradient between the surface and thermocline (Speroet al., 2003; Farmer et al., 2007; Tedesco et al., 2007; Steph et al., 2009).

d from Guaymas Basin and the Southern Oscillation Index (SOI).

36 K.E. Wejnert et al. / Marine Micropaleontology 74 (2010) 29–37

Using the average monthly calcification temperatures for thedifferent species (Fig. 5), we estimate the monthly average surface tothermocline difference for the entire seven-year study period (Fig. 6).The maximum temperature difference (7.9 °C, 2.09‰) occurs duringthe summer and reflects the strong thermal stratification at this timeof year (Fig. 6). During the winter the thermal gradient decreases to−1.2 °C (0.37‰), reflecting the fact that the water column hasbecome well-mixed as indicated by the nearly vertical isotherms(Fig. 5). The δ18O-based surface to thermocline temperature differ-ence agrees fairly well with the measured difference between 0 and40 m (Fig. 6). As expected, both show a larger gradient duringsummer non-upwelling and a smaller gradient during winter/springupwelling. However, the estimated thermal gradient shows a longerperiod of strong thermal stratification than the observed thermalgradient. This discrepancy could be due to the fact that the estimatedthermal gradient is derived from 7 years of data while the observed isa snapshot of 1 year.

A similar Δ δ18O(thermocline–shallow) record was generated for theentire time series in order to evaluate interannual variability in thesurface to thermocline temperature gradient between 1991 and 1997(Fig. 7). Furthermore, we compare this temperature gradient recordwith the opal flux record for the same Guaymas Basin sediment trapsamples and the Southern Oscillation Index (SOI). Thunell et al.(1994) have previously shown that opal fluxes are a good proxy forprimary production in the Gulf of California. During non-El Niñoconditions, such as in 1995 through early 1997, Guaymas Basin ischaracterized by the lowest Δδ18O(thermocline–shallow) and high opalfluxes in the winter/spring, which reflect a small temperaturegradient due to strong upwelling and increased productivity. Incontrast, the winter/spring upwelling period during El Niño years,such as early 1991 though 1994, is characterized by higher Δδ18O(thermocline–shallow) and lower opal fluxes with the exception ofthe early winter in 1994, during which El Niño conditions temporarilyabated. This indicates that upwelling and primary production inGuaymas Basin are reduced during El Niño. The summer/fall ischaracterized by high Δ δ18O(thermocline–shallow ) and low opal fluxesand while there is interannual variation, it does not appear to covarywith SOI.

6. Conclusions

Sediment trap samples collected over a seven-year period(February 1991–October 1997) from the Guaymas Basin in the Gulfof California were used to study seasonal to interannual variability inthe oxygen isotope composition of five species of planktonicforaminifera (G. ruber (white), G. bulloides, N. dutertrei, P. obliquilo-culata, and G. menardii) and its relationship to changes in localhydrography. Oxygen isotope ratios were lowest for G. ruber and G.bulloides (∼0.0 to −5.0‰), which live in the surface mixed layer andwere higher for the thermocline dwelling species N. dutertrei, P.obliquiloculata, and G. menardii (∼0.0 to −2.0‰).

Temperature estimates from the oxygen isotopes reveal that G.ruber most accurately records SST throughout the year; however,there are frequently not enough specimens to analyze during thewinter. G. bulloides records SST only during winter upwelling, whileduring summer non-upwelling it reflects colder temperatures. Asopposed to G. ruber, G. bulloides is most abundant during the winterand scarce during the summer. A combination of these two speciescan be used to generate a continuous time series of SST estimates.

Calcification depths for each of the five species were determinedby comparing temperature estimates with a temperature profile ofGuaymas Basin. N. dutertrei, P. obliquiloculata, and G. menardii tend tostay within a preferred temperature range and varied their depthseasonally to remain within the thermocline. As a result the differencein the δ18O of these species and the surface dwellers, G. ruber and

G. bulloides, can be used to estimate the thermocline to surfacetemperature gradient.

Acknowledgements

We thank E. Tappa for coordinating the sample collection and foroverseeing the stable isotope analyses. This research was supported,in part, by the U.S. National Science Foundation.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.marmicro.2009.11.002.

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