Ecological controls on the shell geochemistry of pink and white Globigerinoides ruber in the northern Gulf of Mexico: Implications for paleoceanographic reconstruction

Marine Micropaleontology 82–83 (2012) 28–37

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Research paper

Ecological controls on the shell geochemistry of pink and white Globigerinoides ruber in the northern Gulf of Mexico: Implications for paleoceanographic reconstruction Julie N. Richey a,⁎, Richard Z. Poore b, Benjamin P. Flower a, David J. Hollander a a b

University of South Florida, College of Marine Science, 140 Seventh Avenue South, Saint Petersburg, FL 33701, USA U.S. Geological Survey, St. Petersburg Coastal and Marine Science Center, 600 Forth Street North, Saint Petersburg, FL 33701, USA

a r t i c l e

i n f o

Article history: Received 1 December 2010 Received in revised form 28 August 2011 Accepted 19 October 2011 Available online 28 October 2011 Keywords: Globigerinoides ruber Mg/Ca Foraminiferal size Planktonic foraminifera Gulf of Mexico

a b s t r a c t We evaluate the relationship between foraminiferal test size and shell geochemistry (δ13C, δ18O, and Mg/Ca) for two of the most commonly used planktonic foraminifers for paleoceanographic reconstruction in the subtropical Atlantic Ocean: the pink and white varieties of Globigerinoides ruber. Geochemical analyses were performed on foraminifera from modern core-top samples of high-accumulation rate basins in the northern Gulf of Mexico. Mg/Ca analysis indicates a positive relationship with test size, increasing by 1.1 mmol/mol (~ 2.5 °C) from the smallest (150–212 μm) to largest (>500 μm) size fractions of G. ruber (pink), but with no significant relationship in G. ruber (white). In comparison, oxygen isotope data indicate a negative relationship with test size, decreasing by 0.6‰ across the size range of both pink and white G. ruber. The observed increase in Mg/Ca and decrease in δ 18O are consistent with an increase in calcification temperature of 0.7 °C per 100 μm increase in test size, suggesting differences in the seasonal and/or depth distribution among size fractions. Overall, these results stress the necessity for using a consistent size fraction in downcore paleoceanographic studies. In addition, we compare downcore records of δ18O and Mg/Ca from pink and white G. ruber in a decadal-resolution 1000-year sedimentary record from the Pigmy Basin. Based on this comparison we conclude that pink G. ruber is calcifying in warmer waters than co-occurring white G. ruber, suggesting differences in the relative seasonal distribution and depth habitat of the two varieties. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Improving understanding of the biological, chemical, and physical factors that contribute to uncertainty in foraminiferal geochemical proxies is critical to the interpretation of paleoceanographic records. The uncertainties inherent in Mg/Ca-based sea surface temperature (SST) estimates can exceed the environmental signal in some instances, especially in low-latitude records covering the past few millennia, when SST variability was likely small (b2 °C) relative to glacial–interglacial changes. Some sources of uncertainty in Mg/Ca-SST estimates include, but are not limited to, diagenesis (e.g. Boyle, 1983; Barker et al., 2003; Pena et al., 2005), salinity (e.g. Nürnberg et al., 1996; Ferguson et al., 2008; Kisakürek et al., 2008), shell heterogeneity (e.g. Eggins et al., 2004; Sadekov et al., 2008), and dissolution (e.g. Dekens et al., 2002). Each of these factors has the potential to overprint or diminish the Mg/Ca signal within downcore records, and the influence of these factors is often variable, and basin-specific. One issue that has been explored to some degree is the effect of foraminiferal test size on Mg/Ca (Elderfield et al., 2002; Ni et al., 2007). Elderfield et al. (2002) illustrated a positive correlation between Mg/ ⁎ Corresponding author. Tel.: + 1 727 553 1017; fax: +1 727 553 1189. E-mail addresses: [email protected] (J.N. Richey), [email protected] (R.Z. Poore), bfl[email protected] (B.P. Flower), [email protected] (D.J. Hollander). 0377-8398/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2011.10.002

Ca and test size in a number of species of planktonic foraminifera; however, it is difficult to quantify the relationship in that study due to the small number of individuals analyzed for each size fraction (~20), and insufficient temporal constraint in their sediment sample (their sample represented ~800 yrs). Ni et al. (2007) found no relationship between test size and Mg/Ca in the white variety of Globigerinoides ruber, but their sample also represented multiple centuries of deposition, with few replicate analyses. In this study we improve upon previous studies by presenting a temporally well-constrained data set to investigate the relationship between test size and test geochemistry (Mg/Ca, δ 18O, δ 13C) in the white and pink varieties of G. ruber. The zero-age dated core-top samples from the two high accumulation rate basins in the northern Gulf of Mexico represent the most recent few decades of deposition. We propose an internally consistent explanation of the changes in three geochemical parameters with size (δ 13C, δ 18O, and Mg/Ca), as well as for observed offsets in the geochemistry of the pink and white varieties of G. ruber. 2. Materials and methods The samples used for the size fraction study are from the core-top (top 0.5 cm) of the Pigmy Basin box core (PBBC-1; 27°11.61′N, 91°24.54′W; 2259 m water depth), and the Garrison Basin box core

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(PE07-2; 26°40.5′ N, 93°55.5′ W; 1570 m water depth). Each of these basins is an intraslope basin in the northern Gulf of Mexico and has a relatively high sediment accumulation rate (~ 40 cm/kyr) due to the influx of terrigenous material from the Mississippi River. For each of the box cores the sediment–water interface was recovered, and therefore the core-top samples incorporate the most recently deposited sediments. Radiocarbon dates from a mixed assemblage of planktonic foraminifera, for each of the core-top samples, reflect a modern age (b0 after reservoir correction; Richey et al., 2007, 2009). Size fractions were separated by sieving, and between 2 and 10 different aliquots of ≥60 foraminifera were analyzed for each size fraction of both pink and white G. ruber. Downcore analyses were performed on the 250–300 μm size fraction of pink and white G. ruber from the same sub-core of the Pigmy Basin box core, PBBC-1. The age model for PBBC-1, as well as the oxygen isotope and Mg/Ca records from the white variety of G. ruber, were previously published by Richey et al. (2007). Age control is based on seven accelerator mass spectrometer (AMS) radiocarbon dates. The sampling interval of 0.5 cm combined with the linear sedimentation rate of 43 cm/kyr yields a sampling resolution of ~12 yrs. For all elemental analyses, ≥60 foraminifera were picked from each size fraction of G. ruber (white and pink). Foraminifera were lightly crushed and underwent a cleaning process that includes multiple clay removal steps, an oxidative step to remove organic material, and an acid leaching step to remove adsorbed metals (Barker et al., 2003). A reductive cleaning step was not performed on these samples for the following reasons: (1) The Mn–Mg-rich contaminant phases removed by the reductive cleaning step are formed by processes below the sediment–water interface (Boyle, 1983), and (2) the reductive cleaning step may remove primary high-Mg calcite in core-top foraminifera that have not been subject to diagenetic alteration, thus artificially lowering Mg/Ca. Elemental analyses were performed on a Perkin Elmer Optima 4300 dual view inductively coupled plasma-optical emission spectrometer (ICP-OES) at the College of Marine Science, University of South Florida (CMS, USF). In all cases where there are replicate Mg/Ca analyses, replicates are based on separate aliquots of ≥60 foraminifera that have been crushed, cleaned, and analyzed separately. Oxygen and carbon isotope ratios were measured on a ThermoFinnigan Delta Plus XL light stable isotope ratio mass spectrometer (SIRMS) at the CMS, USF. The δ 18Ocalcite and δ 13Ccalcite are reported on the VPDB scale. 3. Results 3.1. Relationship between test size and carbon isotopic composition The positive relationship between δ 13C and foraminiferal test size has been well established in previous studies using a variety of different planktonic foraminifera species across a range of oceanographic settings (e.g. Berger et al., 1978; Curry and Matthews, 1981; Oppo and Fairbanks, 1989; Ravelo and Fairbanks, 1995; Elderfield et al., 2002). Berger et al. (1978) proposed that foraminiferal δ 13C increases with test size as a function of ontogeny, due to the fact that metabolic rates are highest in the early ontogenetic stages (i.e. smallest individuals), and therefore are the most depleted relative to equilibrium. As the foraminifer matures, the metabolic rate slows and the carbon isotopic composition of the test becomes less depleted relative to the δ 13C of seawater. It has also been proposed that the increase in δ 13C with increasing size can be attributed to symbiont photosynthesis (Oppo and Fairbanks, 1989; Spero and Lea, 1993). Several studies support the idea that increased symbiont photosynthesis leads to enrichment of foraminiferal δ 13C (Spero and DeNiro, 1987; Spero et al., 1991; Bijma et al., 1998). Symbionts preferentially utilize 12C during photosynthesis, leaving the carbon pool in the calcification microenvironment enriched with respect to 13C (Spero and Williams, 1988).

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Photosynthetic activity increases as a function of increasing light level (Spero and Lea, 1993) and the increase in symbiotic density with advancing ontogeny (Spero and Parker, 1985). A culture study in which Orbulina universa and Globigerinoides sacculifer were grown under variable light levels revealed that the δ 13C of the foraminifera was controlled by symbiont photosynthesis, rather than ontogeny. However, the observation of increasing δ 13C with increasing test size in non-symbiont bearing foraminifera (e.g. Ravelo and Fairbanks, 1995; Elderfield et al., 2002) suggests that symbiont photosynthesis is not the sole contributing factor to the δ 13C-size relationship. Our observation of the relationship between δ 13C and test size in white and pink G. ruber is consistent with previous studies. For G. ruber (pink), the δ 13C ranges from 0.5‰ in the smallest size class (150–212 μm), to 2.0‰ in the largest size class (425–500 μm). G. ruber (white) ranges from 0.3‰ in the smallest size class (150–212 μm) to 1.3‰ in the largest size class (355–425 μm) (Table 1) (Fig. 1A). The rate of change per increase in size slows as the foraminifera grow larger, which is consistent with a smaller fractionation with decreasing metabolic rate. The pink variety of G. ruber is consistently ~ 0.4‰ more enriched than the white variety. The overall range in δ 13C from the smallest to largest individuals exceeds the downcore δ 13C variability over the past 1500 yrs in both white and pink G. ruber (see Fig. S4 in supplemental materials), thus emphasizing the importance of using a narrow size range for all downcore paleoceanographic studies. 3.2. Relationship between test size and oxygen isotopic composition A number of studies have investigated the relationship between δ18O and size in planktonic foraminifera; however, unlike with δ13C, the observed relationship is not systematic. Elderfield et al. (2002) found that there was a general decrease in δ18O in three species of planktonic foraminifera (O. universa, Neogloboquadrina dutertrei, and white G. ruber), but 14 other planktonic species analyzed showed either the opposite relationship, or no significant relationship at all. Ravelo and Fairbanks (1992, 1995) observed a significant decrease in δ18O of G. ruber (white) with increasing test size, but no relationship in G. ruber (pink). Curry and Matthews (1981) also looked at the relationship between δ18O and test size in G. ruber (white), but found no significant relationship. In a culture study, Spero and Lea (1993) concluded that ontogeny did not have an effect on the shell δ18O of G. sacculifer, but they observed a significant depletion in δ18O with increasing light levels for the symbiont-bearing O. universa and G. sacculifer. Our results indicate that δ 18O decreases with increasing test size in both white and pink G. ruber (Table 1) (Fig. 1B). In the white variety, the δ 18O of foraminiferal calcite decreases from −1.1‰ in the smallest size class (150–212 μm) to − 1.6‰ in the largest size class

Table 1 Oxygen and carbon isotopic data versus size for G. ruber (pink and white). Data represent the mean value of n measurements, where n is the number of replicate measurements for each size fraction, and each measurement is based on ≥ 60 individual foraminifera tests. Std dev represents the standard deviation among replicate measurements for each size fraction.

G. ruber (pink)

G. ruber (white)

Size fraction (μm)

n

150–212 212–250 250–300 300–355 355–425 425–500 150–212 212–250 250–300 300–355 355–425

4 7 9 7 8 6 5 7 8 7 3

δ18O (per mille) −1.38 −1.58 −1.52 −1.66 −1.96 −2.09 − 1.17 − 1.22 − 1.41 − 1.39 − 1.70

δ18O std δ13C dev (per mille)

δ13C std dev

0.06 0.12 0.14 0.09 0.08 0.11 0.20 0.18 0.18 0.15 0.16

0.14 0.14 0.11 0.16 0.20 0.22 0.10 0.05 0.27 0.15 0.17

0.52 1.01 1.26 1.70 1.87 1.99 0.26 0.56 0.86 1.24 1.34

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White G. ruber; r2=0.87 slope= -0.25(±0.08)‰/100μm

-1.9 Offset = 0.27 ‰

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Fig. 1. A) The carbon isotopic composition is plotted against test size for pink (open circle) and white (square) G. ruber. A third-order polynomial is fit through both data sets (r2 = 0.99). B) The oxygen isotopic composition is plotted against test size for pink (open circle) and white (square) G. ruber. A linear least-squares regression line is fit through both data sets, and they have similar slopes (−0.26 ± 0.04‰ per 100 μm, and − 0.25 ± 0.08‰ per 100 μm, respectively). The mean offset between pink and white G. ruber in δ18O is 0.27‰. Error bars represent the standard deviation among all replicate measurements for each size fraction.

(355–425 μm), while pinks range from −1.4‰ in the smallest size class (150–212 μm) to −2.0‰ in the largest size class (425–500 μm). A leastsquares linear regression through the mean δ 18O values for each size fraction indicates a significant negative correlation between δ18O and size for both pink and white G. ruber (r2 = 0.93 and 0.87, respectively). The slopes of the relationships are nearly identical for pink and white G. ruber and indicate a 0.3‰ decrease in δ 18O per 100 μm increase in size. While there is overlap in the δ18O of pink and white G. ruber for some size fractions (e.g. 250–300 μm), there is a mean offset between their size-δ18O regressions in which the pink variety is depleted by ~0.27‰ relative to the white. 3.3. Relationship between foraminiferal test size and Mg/Ca In G. ruber (pink) there is a significant increase in Mg/Ca with increasing test size from 150 μm to > 500 μm (Table 2) (Fig. 2A). The overall range in Mg/Ca from the smallest size (150–212 μm) to the largest size (>500 μm) is 1.1 mmol/mol. This is equivalent to an SST Table 2 Mg/Ca data versus size for G. ruber (pink and white). Data represent the mean value of n measurements, where n is the number of replicate measurements for each size fraction. Std dev is the standard deviation among the replicate measurements. The SSTs listed are calibrated using the respective equations for G. ruber (pink) [Mg/ Ca = 0.38exp(0.09*SST)], and G. ruber (white) [Mg/Ca = 0.449exp(0.09*SST)] from Anand et al. (2003). Size fraction (μm) G. ruber (pink)

150–212 212–250 250–300 300–355 355–425 425–500 >500 G. ruber (white) 150–212 212–250 250–300 300–355 355–425

n 5 5 10 7 7 6 1 5 6 8 5 2

Mg/Ca (mmol/mol) Std dev SST (°C) 4.00 4.19 4.35 4.37 4.57 4.83 4.92 4.36 4.29 4.32 4.30 4.73

0.32 0.19 0.23 0.19 0.30 0.27 n/a 0.10 0.27 0.16 0.22 0.19

26.1 26.7 27.1 27.1 27.6 28.2 28.5 25.2 25.1 25.2 25.1 26.2

difference of 2.5 °C from the smallest to largest size fraction based on the Mg/Ca paleotemperature equation for G. ruber (pink) (from Anand et al., 2003): Mg=Ca ¼ 0:38 expð0:09⁎T Þ:

ð1Þ

A temperature range of 2.5 °C exceeds the expected amplitude of decadal- to centennial-scale SST variability expected for late Holocene records from the low latitudes. The increase in Mg/Ca with test size in pink G. ruber appears to be linear, with a positive slope of 0.3 ± 0.02 mmol/mol (0.7 °C ± 0.05 °C) per 100 μm increase in test size. The Mg/Ca values for the 250–300 μm and 300–355 μm size fractions are not significantly different from each other and have Mg/Ca values (mean = 4.3 ± 0.02 mmol/mol) that correspond to the modern summer-weighted (April–October) SST for the Gulf of Mexico (27.0 °C), when calibrated using Eq. (1). This exercise was repeated for a second depth interval (320–325 mm, 650 yrs B.P.) in the Pigmy Basin box core (PBBC-1) to test whether the relationship between test size and Mg/Ca is stationary through time. The interval ca. 650 yrs B.P. was 2 °C colder than the modern core-top in the downcore record, as indicated by the Mg/Ca-SST from the 250–300 μm size fraction of G. ruber (white) (Richey et al., 2007). Fig. 2B illustrates a nearly identical slope of the size-Mg/Ca relationship for both the modern core-top sample and for the sample at 650 yrs B.P. (slope= 0.3± 0.04 mmol/mol), indicating that the increase in Mg/Ca with test size is a robust relationship, even under different climatic conditions. The fact that each size fraction from the interval 650 yrs B.P. is consistently offset ~2 °C cooler than the corresponding size from the modern core-top SST further corroborates the 2 °C cooling observed in the downcore record. It is also worth noting that there were no G. ruber (pink) in the >500 μm size fraction for the 650 yrs B.P. sample, whereas there were more than 60 individuals in the >500 μm size fraction in the core-top sample. This supports the hypothesis that foraminifera grow larger under warmer conditions. The relationship between test size and Mg/Ca is not as clear in the white variety of G. ruber. In our samples, G. ruber (white) is not abundant enough in the >425 μm size fractions to make a Mg/Ca measurement.

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A

2000) signatures. In our Gulf of Mexico sample set, G. ruber s.s. is the dominant morphotype, and we don't observe significant shifts in the relative abundance of G. ruber s.s. versus s.l. among samples. There are many individual foraminifera that are considered “transitional”, and cannot be easily classified as either s.s. or s.l. The combined analysis of the two morphotypes in this study may be a contributing factor to the lack of a robust Mg/Ca-size relationship in white G. ruber.

5.5 G.ruber (Pink) G.ruber (White)

Mg/Ca (mmol/mol)

5.0

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4. Discussion of size fraction data 4.0

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G. ruber (pink) 0 yrs BP (core-top) G. ruber (pink) 650 yrs BP

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Size Fraction (μm) Fig. 2. A) Mg/Ca is plotted against test size for pink (open circles) and white (black squares) G. ruber. Error bars represent standard deviation among replicate measurements. A linear least-squares regression is plotted through the data for G. ruber (pink). There is no significant relationship in G. ruber (white), but the values for G. ruber (white) are not significantly different from the values for G. ruber (pink). The shaded box indicates the size range (250–355 μm) of G. ruber upon which the Anand et al. (2003) Mg/Ca-paleotemperature equations used in this study are based Mg/Ca = 0.38exp(0.09*T) for pink G. ruber and Mg/Ca = 0.449exp(0.09*T) for white G. ruber. B) Size versus Mg/Ca for G. ruber (pink) is plotted from two different depths in the Pigmy Basin box core. The slope of the relationship is identical in the two different samples, and there is a 2 °C offset, which is consistent with the downcore Mg/Ca that suggests that SST was 2 °C cooler 650 yrs B.P.

The four size fractions between 150 μm and 355 μm (150–212 μm, 212–250 μm, 250–300 μm and 300–355 μm) have the same Mg/Ca value of 4.32 mmol/mol (within the analytical error) (Fig. 2A). The Mg/Ca value of the largest size fraction, 355–425 μm, does increase significantly, by 0.41 mmol/mol, consistent with the increase in Mg/Ca for this size fraction in pink G. ruber. Each of the size fractions between 212 μm and 425 μm in G. ruber (white) falls, within 1σ error, on the trend line relating Mg/Ca to test size for G. ruber (pink). However, looking at the test size versus Mg/Ca data for just the G. ruber (white) data, there is no significant relationship between Mg/Ca and size. In this study we did not differentiate between the two morphotypes of white G. ruber, sensu stricto (s.s.) and sensu lato (s.l.). Studies have shown that the two morphotypes are genetically distinct (Kuroyanagi et al., 2008), and they have different depth distributions with distinctive Mg/Ca (Steinke et al., 2005) and isotopic (Wang,

Unlike the positive test size-δ 13C relationship that has been observed for nearly all species of planktonic foraminifera investigated (e.g. Curry and Matthews, 1981; Ravelo and Fairbanks, 1995; Elderfield et al., 2002), the relationship reported between test size and the Mg/Ca and δ 18O parameters varies between positive, negative, and no relationship for a variety of planktonic foraminifera (e.g. Curry and Matthews, 1981; Elderfield et al., 2002; Ni et al., 2007). Spero and Lea (1993) concluded from analysis of individual chambers that ontogeny has no significant effect on shell δ 18O in two species of symbiont-bearing planktonic foraminifera (O. universa and G. sacculifer); therefore changes in metabolic rate are not likely to be the cause of δ 18O-size relationships. Likewise, in a study of intratest Mg/Ca variability in G. ruber, Sadekov et al. (2008) found no significant differences in the mean Mg/Ca values of the different chambers within a single test. This finding suggests that changing metabolic rates as a foraminifer grows does not have a significant effect on shell Mg/Ca. We hypothesize that changes in Mg/Ca and δ18O with test size observed for G. ruber in this study reflect differences in calcification temperature among the different size fractions. The increase in Mg/Ca and decrease in δ18O with increasing test size is consistent with larger individuals calcifying in warmer waters than smaller individuals. In a culture experiment, Spero and Lea (1993) demonstrated that the two symbiont-bearing foraminifers, O. universa and G. sacculifer, grow significantly larger under “high-light” culture conditions than “low-light” culture conditions. They also found that foraminiferal δ 18O decreased with increasing light levels, most likely as a result of increased symbiont photosynthesis. In the marine environment, the highest-light conditions are found at shallow depths in the water column or during the summer season, so it follows that size fractions in G. ruber may vary in their depth/seasonal distribution at our study site. We established that the increase in Mg/Ca with size was equivalent to a 0.7° ± 0.05 °C increase in calcification temperature per 100 μm increase in test size, with a total range of 2.5 °C among the size fractions of G. ruber (Section 3.3). The mean vertical temperature gradient in the upper 50 m of the water column in the region is ~ 4 °C (Fig. 3A), and the seasonal range in mixed layer (0–30 m) temperature is ~ 7 °C (Fig. 3B). Therefore, if we adopt the working hypothesis that the Mg/Ca change we observe with test size is a function of calcification temperature, it is reasonable to assume that the range in calcification temperatures observed could result from differences in depth and/or seasonal distribution of the different-size foraminifera. To assess whether the change in Mg/Ca-based calcification temperature with size is consistent with δ 18O-based calcification temperatures, we use the following paleotemperature equation to convert foraminiferal δ 18O measurements to temperature,   18 18 T ¼ 14:9−4:8⁎ δ Oc −δ Osw ;

ð2Þ

where T is temperature (°C), δ18Oc is the measured δ 18O of foraminiferal calcite, and δ 18Osw is the oxygen isotopic composition of seawater (converted from the VSMOW scale to VPDB by subtracting 0.27‰). Eq. (2) is the paleotemperature equation developed for O. universa under “high-light” culture conditions by Bemis et al. (1998) and has

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20 0

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Fig. 3. Vertical profiles and seasonal cycles of temperature and δ18Osw variability in the Gulf of Mexico. A) Summer-weighted (Apr.–Sept.) vertical temperature profile for the Gulf of Mexico. B) Seasonal cycle of mixed layer (upper 30 m) temperature for the Gulf of Mexico. Temperature data are from the Levitus (2003) data set. C) Summer-weighted (April– October) vertical profile of δ18Osw inferred from salinity data in the Gulf of Mexico. D) Seasonal cycle of mixed layer (upper 30 m) δ18Osw inferred from salinity in the Gulf of Mexico. The δ18Osw was calculated from salinity data from the Levitus (2003) data set, using the equation: δ18Osw = 0.558 * salinity − 19.26 (Duplessy et al., 1991).

been determined to be appropriate for use in G. ruber (Thunell et al., 1999; Spero et al., 2003). Since δ 18Oc is a function of both calcification temperature and the oxygen isotopic composition of ambient seawater, we can convert δ18Oc to calcification temperature in two different ways: 1) we assume a constant δ18Osw when calculating temperature from δ18Oc for each size fraction, or 2) we can vary the δ18Osw over a range similar to the range of δ 18Osw observed seasonally, and in the upper water column. In the first scenario, we assume a fixed δ 18Osw of 0.8‰. This is the 18 δ Osw value that corresponds to the mean annual mixed layer (0–30 m) salinity of 36.0 for the Gulf of Mexico, calculated using the equation: δ 18Osw = 0.558 ∗ salinity − 19.26, from Duplessy et al. (1991). This results in a 1.2 °C (±0.2 °C) increase in calcification temperature per 100 μm increase in test size for both pink and white G. ruber (Fig. 4). This magnitude of temperature increase with size is nearly twice as large as the temperature change with test size predicted from Mg/Ca in this study. However, if we adopt the working hypothesis that the increase in Mg/Ca with size is the result of larger size fractions having a shallow/summer weighted distribution relative to smaller size fractions, it follows that δ 18Osw should vary with size. We calculate calcification temperature again, but in this second scenario we assume a decrease in δ 18Osw of 0.06‰ with each larger test size (i.e. we assume a δ 18Osw of 0.8‰ for the 150–212 μm size fraction, decreasing to 0.5‰ for the 425–500 μm size fraction). This range of δ 18Osw is within with the range of δ 18Osw variability over the upper 50 m of the water column (Fig. 3C), or over the seasonal cycle (Fig. 3D) in the Gulf of Mexico. This is also consistent with the 0.3‰ decrease in δ 18Osw across the 150–425 μm size range if δ 18Osw is calculated from the Mg/Ca-derived temperature paired with the measured δ 18Oc (see Fig. Fig. S3 in supplementary materials). Varying the δ 18Osw in the paleotemperature equation for each size fraction

results in an estimated increase in calcification temperature with increasing size, which is equal to 0.7° ± 0.2 °C per 100 μm increase in test size (Fig. 4). This is equivalent to the increase in calcification temperature with size estimated by Mg/Ca in this study. Thus, an increase in calcification temperature with increasing test size in G. ruber provides an internally consistent explanation for the observed relationship between the two geochemical parameters (Mg/Ca and δ 18Oc) and test size.

5. Comparison of downcore geochemical records for pink and white G. ruber The pink and white varieties of G. ruber are the two most abundant planktonic foraminifers in modern Gulf of Mexico sediments and, when combined, make up >45% of the total assemblage (Brunner, 1979; Kennett et al., 1985; Dowsett et al., 2003). Both live in tropical to sub-tropical surface waters, are morphologically very similar, and are confined to the euphotic zone by their photoautotropic dinoflagellate symbionts. Despite their similarities, we observe differences in their shell geochemistry, and downcore records. If the differences between pink and white G. ruber are the result of the two varieties having distinct seasonal/depth habitats, relative offsets in their Mg/Ca and δ 18O records may potentially be exploited in downcore records to make inferences about changing seasonality in the past. In this section, we compare near-modern core-top data with Gulf of Mexico climatologic data to make inferences about depth/seasonal habitat preferences for pink and white G. ruber. Comparison of the downcore oxygen isotopic and Mg/Ca records of pink and white G. ruber from a high-resolution sedimentary record spanning the past 1500 yrs provides additional insights into the differences between the two varieties of G. ruber.

J.N. Richey et al. / Marine Micropaleontology 82–83 (2012) 28–37

28

28

A. G. ruber (white)

27

27

26

26

25

25

24

24

23

23

22

22

33

B. G. ruber (pink)

Fig. 4. Conversion of δ18Oc to calcification temperature. A) δ18Oc data for G. ruber (white) is converted to calcification temperature using the paleotemperature equation for Orbulina universa “high-light” from Bemis et al. (1998): T = 16.5–4.80* (δ18Oc–δ18Osw). The δ18Oc data are converted to SST using a fixed δ18Osw value of 0.8‰ for each size fraction (open squares), and using a variable δ18Osw (solid squares). For the variable δ18Osw, a value of 0.8‰ is assigned to the smallest size fraction (150–212 μm) and the δ18Osw is decreased by 0.06‰ for each subsequent size fraction. B) The same calculation is performed for G. ruber (pink). When the δ18Osw is fixed at 0.8‰, the resulting change in calcification temperature with size is a 1.2 °C (± 0.2 °C) increase per 100 μm increase in size. When the variable δ18Osw is used, the result is a change in calcification temperature of 0.7 °C (±0.2 °C) per 100 μm increase in size.

A 29

G. ruber (white) G. ruber (pink) Summer (Apr-Nov)

27

25

SST (ºC)

Mean Annual

23

B 5.0 21

G. ruber (white) G. ruber (pink)

Mg/Ca (mmol/mol)

4.5

4.0

3.5

3.0

2.5

0

200

400

600

800 1000 Calendar Yrs B.P.

1200

1400

1600

Fig. 5. Downcore comparison of Mg/Ca records for white and pink G. ruber. A) Pink (open circles) and white (open squares) G. ruber Mg/Ca records are calibrated using their respective paleotemperature equations (Mg/Ca = 0.38exp[0.09*SST], and Mg/Ca = 0.449exp[0.09*SST] from Anand et al., 2003). The multi-decadal smoothed record (solid black curve) is overlaid on the raw downcore data. The mean summer (Apr.–Sept.) SST and mean annual SST for the Gulf of Mexico are indicated. B) Raw Mg/Ca records for pink and white G. ruber are plotted.

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Sediment trap data from 20 global sites indicate the optimum SST range for G. ruber (white) is 22–31 °C, while G. ruber (pink) has a slightly more limited ideal range of 23–30 °C (Žarić et al., 2005). Plankton tow data from the Sargasso Sea reveal that white and pink G. ruber are most commonly found in SSTs ranging from 18 to 26 °C, with highest concentrations observed between 23 and 27 °C. The pink variety is found at even warmer temperatures, up to 28 °C (Bé and Hamlin, 1967). Modern depth preferences of planktonic foraminifera such as G. ruber have been studied extensively in the Sargasso Sea, yet little work has focused on the Gulf of Mexico. Plankton tow results taken in November 1975 in the Sargasso Sea indicate that while G. ruber (white) is commonly present in the top 100 m, the pink variety is found at low concentrations at depths up to 200 m (Fairbanks et al., 1980). Monthly plankton tows also from the Sargasso Sea showed that white and pink G. ruber were most abundant in the top 10 m (Tolderlund and Bé, 1971). A single tow in April 1980 in the western Gulf of Mexico revealed that G. ruber (white) is present from the surface to 50 m water depth, while the less abundant pink G. ruber is found in the subsurface, from 25 to 50 m (Bé, 1982). From the limited sediment trap data available for the subtropical Atlantic Ocean, white and pink G. ruber appear to have distinct seasonal distributions. In the Sargasso Sea, white G. ruber is abundant in sediment traps throughout the year (with fluxes >50 tests m − 2 day− 1 during all months) (Deuser et al., 1981; Deuser, 1987; Deuser and Ross, 1989), suggesting that G. ruber (white) is representative of mean annual

-2.2

sea-surface conditions in the sediment record. Data indicate that G. ruber (pink) exhibits peak abundances from April–October (with fluxes of >5 tests m − 2 day− 1), and fluxes drop to nearly 0 during the winter (December–April). A recent sediment trap study in the Gulf of Mexico (January–December 2008) (Tedesco et al., 2009) shows that G. ruber (white) is present throughout the year (although in much lower abundances than in corresponding core-top sediments). The flux of G. ruber (pink) in that sediment trap study is broadly consistent with the Sargasso Sea data, such that fluxes are low (near zero) during the coldest months (December–April), while maximum fluxes occur during the summer months (July–September). 5.1. Comparison of downcore white and pink G. ruber Mg/Ca records A Mg/Ca record was generated from the 250–300 μm size fraction of G. ruber (pink) for comparison with the previously published Mg/Ca record from white G. ruber (Richey et al., 2007) from the Pigmy Basin. The raw downcore Mg/Ca values, as well as the overall pattern of variability, are nearly identical (±0.23 mmol/mol) for both pink and white G. ruber records (Fig. 5B). Centennial scale features, such as a 1.1 mmol/mol increase in Mg/Ca from ca. 300 yrs B.P. to the 20th century, are present in both records. Additionally, the abrupt transition from elevated Mg/Ca values that occurs ca. 950 yrs B.P., as well as the three multi-decadal intervals of elevated Mg/Ca that occur between 1000 and 1400 yrs B.P., are observed in both the pink and white G. ruber records. The similarity in the raw Mg/Ca records of both white and pink

A G. ruber (white) G. ruber (pink)

-1.8

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-0.6 0

0.6

200

400

600 800 Calendar Yrs B.P.

1000

1200

1400

200

400

600 800 Calendar Yrs B.P.

1000

1200

1400

B

0.4 0.2 0 -0.2 0

Fig. 6. Downcore comparison of δ18O data for white and pink G. ruber. A) Downcore, raw δ18Oc is indicated by dashed lines, and a multi-decadal smoothed record (solid black curve) is overlaid on the raw downcore data. Shaded vertical bars indicate intervals of high salinity inferred from the white G. ruber δ18Osw record from the Pigmy Basin (Richey et al., 2007). B) A multi-decadal smoothed version of the difference between the δ18Oc of white and pink G. ruber (δ18Oc white minus δ18Oc pink) is plotted. The pooled standard deviation for replicate δ18Oc analyses (n = 20 for both pink and white G. ruber) is ± 0.1‰, therefore data that fall within the horizontal shaded bar (the ± 0.1‰ range) are intervals in which the pink and white δ18Oc data are not significantly different from each other.

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G. ruber verifies that the multi-decadal to centennial scale trends observed in the Pigmy Basin Mg/Ca-SST record are reproducible. Anand et al. (2003) determined that the temperature dependence (i.e. the exponential constant of the Mg/Ca paleotemperature equation) was equal to a 9% increase in Mg/Ca per 1 °C for nearly all species of planktonic foraminifers in their study. However, the preexponential constants were determined to be different among species. Therefore, using the respective Mg/Ca paleotemperature equations for pink and white G. ruber results in a downcore SST record in which the amplitude is the same, but the G. ruber (pink) is reflecting SSTs that are 1.8 °C warmer than the G. ruber (white) record (Fig. 5A). If the assumption is made that there is a real difference in the distribution coefficient for Mg/Ca between white and pink G. ruber, the results suggest that G. ruber (pink) is consistently calcifying at significantly warmer SSTs than G. ruber (white) over the past 1400 yrs. This may be attributed to a seasonal distribution for G. ruber (pink) that is weighted toward warmer months of the year, and/or G. ruber (pink) may be living at a shallower depth in the water column than G. ruber (white).

0

35

To test whether a mean 1.8 °C offset between the white and pink G. ruber downcore records can be attributed to differences in seasonal distribution, we compared the core-top Mg/Ca-SST of pink and white G. ruber with monthly instrumental data for the Gulf of Mexico. The mean annual SST for the Gulf of Mexico is 25.4 °C, while the mean summer SST (April–October) is 27.0 °C (Levitus, 2003). The downcore Mg/Ca records for pink and white G. ruber vary in step, with G. ruber (pink) consistently recording 1.8 °C warmer SSTs when their respective paleotemperature equations are used. Therefore, this interpretation of differing seasonal distribution leads to the inference that the climatic variations observed in this 1400-year record indicate mean state variability and not changes in seasonal amplitude. Alternatively, the 1.8 °C offset between white and pink G. ruber could be explained by a difference in mean depth habitat. The mean temperature gradient across the upper 50 m of the water column from April–October (when the upper 50 m of the water column in the northern Gulf of Mexico are not fully mixed) is ~4 °C (Levitus, 2003). Therefore, it is plausible that G. ruber (pink) could be calcifying at shallower depths on average, than G. ruber (white), thus producing

A

decreasing G. ruber abundance

% G. ruber (pink)

4

8

12

B

0

5 16

15

20

% G. ruber (white)

10 20

25 1.4

C

30

increasing salinity

1.2

1.0

0.8

0.6

0.4

0.2 0

200

400

600

800

1000

1200

1400

1600

Calendar Yrs BP Fig. 7. Comparison of G. ruber abundance with Gulf of Mexico salinity. A) % abundance for G. ruber (pink). B) % abundance for G. ruber (white). The % abundance data are based on faunal analysis of the > 150 μm total planktonic foraminiferal assemblage. C) δ18O seawater record for Pigmy Basin (Richey et al., 2007). δ18O seawater record in panel C was generated using paired Mg/Ca-δ18O measurements on the 250–300 μm size fraction of the G. ruber (white), and using Eqs. (1) and (2).

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a downcore Mg/Ca-SST record that is weighted 1.8 °C warmer. To achieve a 1.8 °C offset, there would need to be a 20-meter difference in the mean depth habitats of pink and white G. ruber (e.g. mean depth habitats of 10 and 30 meters, respectively). Our conclusion from this exercise is that it is possible to explain a consistent 1.8 °C offset by either differences in preferred season or depth habitat. 5.2. Comparison of downcore white and pink G. ruber δ 18O data The δ 18O of foraminiferal calcite is a function of both calcification temperature and the oxygen isotopic composition of ambient seawater. Unlike the raw Mg/Ca records of pink and white G. ruber, the δ 18Oc records from the Pigmy Basin do not covary (Fig. 6A). The major difference between the two δ 18Oc records occurs when there are excursions to more enriched δ 18Oc in the white G. ruber record (Fig. 6B). During these excursions of increased δ 18Oc in white G. ruber (ca. 1050, 500, 300 yrs BP, etc.), the pink G. ruber δ 18Oc record actually records a depletion in δ 18Oc (Fig. 6A). When the δ 18Oc record of G. ruber (white) is converted to a δ 18Ow record by removing the temperature effect on the δ 18Oc record (using Eq. (2)), those excursions to more enriched δ 18Oc are dominated by increases in the δ 18Ow. The first δ 18Ow excursion occurs during a warm period (as recorded by the Mg/Ca-SST of both pink and white G. ruber), while the second excursion occurs during an SST minimum in the Mg/Ca records. Interestingly the % abundance (based on faunal analysis of the >150 μm total planktonic foraminiferal assemblage) of both pink and white G. ruber drop to their lowest over the 1400-year record (to b5% and b10%, respectively) during inferred high-salinity excursions (Fig. 7). This is consistent with other studies in the Gulf of Mexico that show large drops in G. ruber abundance during high-salinity events (e.g., LoDico et al., 2006). 6. Conclusions In this study, we present a detailed assessment of the relationship between foraminiferal test size and geochemical parameters commonly used for paleoceanographic reconstruction (i.e. δ13C, δ18O, and Mg/Ca) in the pink and white varieties of G. ruber. A systematic relationship between foraminiferal test size and either δ18O or Mg/Ca has not been demonstrated in previous studies. However, this study finds significant relationships between shell geochemistry and size in pink and white G. ruber. In pink G. ruber, δ 18O decreases and Mg/Ca increases with increasing test size. For white G. ruber, δ18O decreases and Mg/Ca shows no significant change with increasing test size. We hypothesize that these relationships are the result of larger individuals calcifying at higher temperatures than smaller individuals. An increase in calcification temperature of 0.7 °C per 100 μm increase in size is found when both Mg/Ca and δ18O data are converted to temperature using their respective paleotemperature equations, providing an internally consistent explanation for these observations. The observed increase in calcification temperature with increasing test size is likely the result of differences in the seasonal and/or depth distribution of different size fractions (i.e. white and pink G. ruber grow larger during the summer and/or at depths closest to the surface). The overall range of calcification temperature (~2.5 °C) over the entire set of size fractions is reasonable, given the seasonal range of temperature and/or the thermal gradients in the mixed layer for the northern Gulf of Mexico. We also present a downcore comparison of decadal resolution δ 18O and Mg/Ca records for white and pink G. ruber from the Pigmy Basin. Results indicate an offset in the raw δ 18O of pink and white G. ruber, while the raw Mg/Ca of the two records is identical within analytical error. Application of the δ 18O-paleotemperature Eq. (2) from Bemis et al. (1998) indicates that the average δ 18O depletion of 0.27‰ in G. ruber (pink) relative to the white variety corresponds to G. ruber (pink) having a calcification temperature that is ~ 1.2 ± 0.4 °C warmer than G. ruber (white). Although the raw Mg/Ca values

are the same for both pink and white G. ruber, applying their respective Mg/Ca-paleotemperature equations results in the pink variety being offset by 1.8 ± 0.8 °C warmer than the white variety. This supports the hypothesis that pink G. ruber is consistently calcifying in warmer waters than white G. ruber, either due to a more summerweighted seasonal distribution or to a shallower depth habitat. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.marmicro.2011.10.002. Acknowledgments We thank Ethan Goddard for assistance with elemental and isotopic measurements. We also thank David Hastings, Terry Quinn, and Deb Willard for helpful comments and critiques of this manuscript. This work was supported in part by the USGS and the National Science Foundation under grants OCE-0318361 and OCE-0903017. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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