Author Version: J. Geol. Soc. India, vol.87(3); 2016; 323-326 A comparison of Mg/Ca ratios in Globigerinoides ruber (white): sensu stricto versus a mixture of genotypes SUSHANT S. NAIK* CSIR-National Institute of Oceanography, Dona Paula, Goa, 403004 INDIA *For correspondence, email:sushant@nio.org; Tel: +91 832 2450657; Fax: +91 832 2450602 Abstract Mg/Ca ratios were measured in core AAS9/21 from the eastern Arabian Sea (EAS), on two sets of planktonic foraminifera, one with a mixture of Globigerinoides ruber genotypes (sensu stricto and sensu lato) and the second which contained only G. ruber sensu stricto, living in the top 30 m of the water column. During the Holocene, sea surface temperatures (SSTs) in set 1 are cooler in comparison to that of set 2. During the deglaciation, SSTs using both these sets are similar, probably due to increased upwelling during this period. The drawback in using undifferentiated G. ruber is that the relative proportion of each genotype varies through different time periods as seen in the EAS and may influence the temperature reconstructions. Keywords: Mg/Ca, Globigerinoides ruber, SST, Arabian Sea
INTRODUCTION Planktonic foraminiferal Magnesium/Calcium (Mg/Ca) thermometry is a proxy used in determining past changes in Sea Surface Temperature (SST) (Nürnberg et al., 1996). Mg 2+ is one of several divalent cations which may substitute for Ca 2+ during the formation of biogenic calcium carbonate. The incorporation of Mg into foraminiferal calcite is influenced by the temperature of the surrounding seawater during growth such that foraminiferal Mg/Ca ratios increase with increasing temperature (Barker et al., 2005). Several studies have used Mg/Ca reconstructed SSTs in deciphering past climate (for example: Elderfield and Ganssen 2000; Anand et al. 2008; Dahl and Oppo 2006). The planktonic foraminiferal species Globigerinoides ruber is most commonly used in reconstructing low-latitude ocean SST as it dwells in surface waters (eg. Saraswat et al., 2005; Govil and Naidu, 2010, Naik and Naidu, 2014). Conventionally, studies have selected planktonic foraminifera G. ruber without differentiation into morphotypes. In the case of G. ruber, two morphotypes can be differentiated based on taxonomic criteria, sensu stricto (s.s.) and sensu lato (s.l.) (Wang, 2000). G. ruber sensu stricto (s.s.) refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide high-arched aperture over the suture; G. ruber sensu lato (s.l.) refers to more compact tests with compressed chambers sitting asymmetrically over the previous sutures, with a round or medium-arched and relatively small aperture over the suture (Wang, 2000). These two morphotypes represent two groups with different depth habitats, with G. ruber s.s. living in the upper 30 m of the water column and G. ruber s.l. living at depths below 30 m (Wang, 2000). Thus, higher Mg/Ca ratios are obtained from G. ruber s.s. in comparison to G. ruber s.l. (Steinke et al., 2005). Therefore, the reconstructed SST will depend upon the proportion of morphotypes present at that time period. A recent study based on results from molecular phylogenetic analyses suggested that G. ruber lineage originated and diversified after the late Miocene (Aurahs et al., 2011). Results show that G. ruber s.s. and s.l. are not merely morphotypes but are genotypes belonging to two separate phylogenetic lineages. Futhermore, thy showed that the morphotype G. ruber s.l. (as per Wang, 2000) actually corresponds to the species G. elongatus (Aurahs et al., 2011). This study aims to understand the effect of variation in genotype proportions on reconstructed SSTs for the last 17 kyrs. SSTs were reconstructed by the conventional practise of using undifferentiated G. ruber (set 1), as well as by using G. ruber s.s only (set 2). MATERIALS AND METHODS This study utilises sediment core AAS9/21 which was recovered from the EAS (14 30'N and 72 39'E; wd = 1807 m) (Govil and Naidu, 2010). The sediment samples were wet sieved (150 µm sieve), dried and again sieved in a narrow size range of 300-355 µm. This fraction was sonicated in methanol for ~8 s at 40 Hz to clean the shells. The cleaned shells were oven dried at 50 C. G. ruber shells (~ 60) were crushed using pre-cleaned glass plates. Successive deionized water and methanol washes were carried out to remove clays followed by coarse grained silicate removal under a binocular microscope (Barker et al., 2003). Rigorous oxidative steps were carried out using alkali buffered 1% H 2 O 2 in the clean laboratory to remove organic matter (Barker et al., 2003). The reductive cleaning step was not carried out (see Yu et al., 2007). Before dissolution, samples were rinsed twice with acid (0.001M HNO3) to remove any adsorbed contaminants. The first set of Mg/Ca ratio was obtained using undifferentiated G. ruber (s.s. and s.l.) from a size range of 300-355µm. Cleaned samples were dissolved in 200 µl 0.075M HNO3. Mg/Ca ratios were measured using a
Varian 820MS quadrupole ICP-MS at the State University of New York, Oswego (See Naik and Naidu, 2014). Precision for Mg/Ca is <1% (n=16), of a standard with Mg/Ca = 1.89 mmol/mol. The second set of samples, from the same depth intervals, were measured at the University of Southampton, UK, using G. ruber, sensu stricto only. Cleaned samples were dissolved in 100-300 µl 0.5M HNO3. Mg/Ca ratios were analysed on a Thermo Finnigan Element 2 ICP-MS. Long term precision for Mg/Ca is 3% (2σ) based on repeat measurements of in-house consistency standards. Mg/Ca does not covary with Al/Ca or Mn/Ca indicating there is no contamination from diagenetic coatings or silicates. RESULTS AND DISCUSSION The Mg/Ca ratios obtained using two different set of samples, show a good resemblance to each other, as seen from the regression values between them (r 2 = 0.65; n = 9). The first set of Mg/Ca ratios (300-355µm, mixture of G. ruber s.s. and s.l.) ranged from 4.05 to 4.94 mmol/mol and the second set (300-355µm, G. ruber s.s.) ranged from 3.97 to 5.13 mmol/mol. Mg/Ca values were further converted to Sea Surface Temperature using the equation; T = (1/0.09)*LN (Mg/Ca/0.449) (Anand et al., 2003). Reconstructed SSTs ranged from 26.3 to 28.5 C in set 1 and 26.1 to 28.9 C in set 2 (Table 1; Fig. 1). In set 1, the average SST during the deglacial period is 27.2 C and the Holocene SST is 27.8 C, i.e. a deglacial to Holocene shift of 1.4 C in SST. In set 2 the average SSTs are 27.1 C and 28.5 C respectively, i.e. a SST shift of 0.6 C. Though Mg/Ca and the derived SSTs from the two sets show large differences of up to 1 C within the Holocene (Fig1a and b). The temperature difference between the two sets is least during the deglaciation and is also observed in South China Sea cores (Wang, 2000). During the Holocene, Mg/Ca derived SSTs display warmer temperatures for the second set (s.s). The differences in Mg/Ca between the two sets during this period are beyond the analytical error limits. The proportions of G. ruber s.s. and s.l. were determined at discrete intervals in the core depending upon availability of foraminifera. All except two intervals in the core had enough amount of the required species of foraminifera. It was seen that the G. ruber s.s. morphotype dominates throughout the core and remained always above 55% (Fig. 1c). Overall the percentage of G. ruber s.s. displays an increasing trend beginning from ~17 kyr with lower values during the deglaciation. Records from the South China Sea also show increased proportions of G. ruber s.l. during stage 2 (~14-22 Cal kyr BP) which is interpreted as a result of the increased productivity due to mixing of the subsurface nutrient-rich water into the surface layer (Wang, 2000). Similarly, the eastern Arabian Sea also experienced increased vertical mixing or upwelling during the colder period in comparison to the Holocene (Singh et al., 2011). Hence, the decrease in SST difference between the two sets during the deglaciation are due to upwelling of cooler sub-surface waters. Similarly, larger SST differences between the two sets during the Holocene results from a lack of upwelling. As observed earlier, the deglacial to Holocene SST shift between the two sets shows a large difference of 0.8 C. Consequently, globally reconstructed SSTs based on mixed morphotypes/genotypes are incomparable as their proportions will differ regionally as well as temporally. Hence, in the case of G. ruber, it is best to employ the sensu stricto genotype as it the closest representative of sea surface conditions.
CONCLUSIONS The Mg/Ca values and hence reconstructed SSTs using G. ruber s.s. are seen to be higher during the Holocene whereas during the deglaciation, SSTs obtained using s.s. and s.l. are similar. This probably is a result of stronger upwelling during the deglaciation in comparison to the Holocene. The reconstructed SSTs tend to be warmer or cooler depending upon the relative proportions of G. ruber s.s. or s.l. respectively, which is seen to be variable through a time period. Therefore, future studies should focus on a single morphotype/genotype of a foraminiferal species, in order to obtain more precise Mg/Ca ratios for improved SST reconstructions. Acknowledgements: I thank Jimin Yu of LDEO and Miguel Martinez-Boti of the University of Southampton for the analytical help. This work was funded by the INDOUSSTF fellowship and a DST Fast Track Project to SN. This is National Institute of Oceanography contribution no... REFERENCES: 1. ANAND, P., ELDERFIELD, H. and CONTE, M. H. (2003) Calibration of Mg/Ca Thermometry in Planktonic Foraminifera from a Sediment Trap Time Series. Paleoceanogr., v. 18 (2), 1050. doi: 10.1029/2002PA000846. 2. ANAND, P., KROON, D., SINGH, A. D., GANESHRAM, R. S., GANNSSEN, G. and ELDERFIELD, H. (2008) Coupled Sea Surface Temperature-Seawater δ 18 O Reconstructions in the Arabian Sea at the Millennial Scale for the Last 35 Ka. Paleoceanogr., v. 23, PA4207. doi:10.1029/2007pa001564. 3. AURAHS, R., TREIS, Y., DARLING, K., and KUCERA M. (2011) A revised taxonomic and phylogenetic concept for the planktonic foraminifer species Globigerinoides ruber based on molecular and morphometric evidence. Mar. Micropal., v. 79, pp 1-14. doi: 10.1016/j.marmicro.2010.12.001. 4. BARKER, S., GREAVES, M. and ELDERFIELD, H. (2003) A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry. Geochem. Geophys. Geosyst., v. 4 (9), 8407. doi:10.1029/2003gc000559. 5. BARKER, S., CACHO, I., BENWAY, H. and TACHIKAWA, K. (2005) Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: a methodological overview and data compilation for the Last Glacial Maximum. Quat. Sci. Rev., v. 24, pp 821-834. 6. DAHL, K. A. and OPPO, D. W. (2006) Sea surface temperature pattern reconstructions in the Arabian Sea. Paleoceanogr., v.21, PA1014. doi:10.1029/2005pa001162. 7. ELDERFIELD, H. and GANSSEN, G. (2000) Past temperature and δ 18 O of surface ocean waters inferred from foraminiferal Mg/Ca ratios. Nature., v. 405, pp. 442-445. 8. GOVIL, P. and NAIDU, P. D. (2010) Evaporation-Precipitation Changes in the Eastern Arabian Sea for the Last 68 Ka: Implications on Monsoon Variability. Paleoceanogr., v. 25, PA1210. doi:10.1029/2008pa001687.
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Table 1: Downcore data from Core AAS9/21. Age (kyr) Mg/Ca-set1 Mg/Ca-set2 SST-set1 SST-set2 % of sensu stricto 3.67 4.73 4.30 28.01 26.96 less sample 6.26 5.13 4.94 28.93 28.49 71 7.38 4.89 4.79 28.39 28.16 72 8.49 5.02 4.72 28.67 27.98 89 11.22 4.91 4.54 28.44 27.56 56 12.25 4.42 4.57 27.27 27.64 66 13.28 4.60 4.42 27.71 27.27 63 14.38 4.51 4.62 27.48 27.76 less sample 17.21 3.97 4.05 26.07 26.28 62
Figure Legend: Fig. 1. a) Mg/Ca ratios measured from a size range of 300-355µm and a mixture of G. ruber s.s. and s.l (black line), size range of 300-355µm and G. ruber s.s. (blue line), b) Mg/Ca- reconstructed SSTs, and c) percentage of sensu stricto shown as circles. The red line signifies increasing trend.