Seasonal variation of planktonic foraminiferal isotopic composition from sediment traps in the South China Sea

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Abstract

The imprint of seasonal hydrographic changes on two widely distributed tropical/subtropical planktonic foraminifer species, Globigerinoides sacculifer (without sac) and Globigerinoides ruber (white variety), has been investigated using specimens collected in sediment traps deployed in the northern South China Sea (SCS; October 2000–October 2002). The faunal assemblage is dominated by G. ruber during the warm summer months, whereas G. sacculifer is consistently present year round at abundances of less than 20%. In general, the measured δ18O and δ13C isotopic compositions of G. ruber are more depleted than those of G. sacculifer, and the δ18O difference between seasons (summer vs. winter) is comparable to that of the local glacial–interglacial cycle (ca. 1‰). The lowest δ13C values are associated with relatively heavy δ18O values in January and March for both species and suggest higher nutrient levels in the surface water that are likely related to the prevailing winter monsoon. This speculation is supported by the increase in terrigenous input to the traps brought by the winter monsoon. The ratio between δ18O and δ13C in G. ruber is approximately two times that of G. sacculifer (with a δ18O/δ13C slope of −1.18 vs. −0.55) and differs from that in most biogenic carbonates (0.25–0.33) based on culture and field studies, which indicate enhanced kinetic fractionation at higher ambient carbonate ion concentrations [CO3=]. However, similar regression slopes of δ18O/δ13C (−0.52 to −0.6) are also derived for G. sacculifer collected from plankton tows. Factors other than the “carbonate ion effect” hence appear to be responsible for the negative relationship between δ18O and δ13C in this region.

Introduction

Paleoceanographic reconstructions rely on information from foraminiferal stable isotopic compositions since the pioneering study of Emiliani (1955). The oxygen isotopic composition (δ18O) of foraminiferal carbonate shells bears information on glacial–interglacial cyclostratigraphy and related hydrographic changes (e.g., Imbrie et al., 1992, Linsley, 1996, Bemis et al., 1998, Lea et al., 2000). The carbon isotopic composition (δ13C) of foraminiferal shells, on the other hand, provides insights into ocean circulation and surface water fertility with the added complications of species-specific “vital effects” (e.g., Berger, 1979, Broecker and Peng, 1982, Curry and Crowley, 1987, Charles and Fairbanks, 1990, Thunell et al., 1992, Spero, 1992, Spero and Lea, 2002). Furthermore, recent laboratory culture experiments on various planktonic foraminifera, including Orbulina universa, Globigerina bulloides, Globigerinoides sacculifer, and Globigerinoides ruber, indicate that both δ18O and δ13C values in those shells decrease as carbonate ion concentrations increase in the ambient sea water (Spero et al., 1997, Spero et al., 1999, Bijma et al., 1998). The carbonate ion effect has also been evaluated by field study (Russell and Spero, 2000).

Although foraminiferal isotope analyses from sediment cores have been widely applied to paleoceanographic reconstructions in the South China Sea (SCS; e.g., Wang et al., 1986, Wang et al., 1999, Wang and Chen, 1990, Tian et al., 2002), the isotopic compositions of their modern counterparts, i.e., foraminiferal shells from sediment traps and plankton tows, have scarcely been studied in this region. In this study, relative abundances and stable isotope measurements of two of the most popular species used to reconstruct late Quaternary tropical sea surface conditions, G. sacculifer (without sac) and G. ruber (white variety), have been studied in specimens collected in sediment traps and tows in the SCS to offer better constraints on the interpretation of downcore foraminiferal isotope records.

Section snippets

Materials and methods

Planktonic foraminifera were picked and identified from material collected in sediment traps deployed at locations M1, M2, and M3 in the northern SCS (Fig. 1). A description of the type of sediment trap used was given by Hung and Chung (1998), the procedures used to process the trap samples were as in Heussner et al. (1990). The original trap deployments were designed for radioisotope analyses, and only discrete but representative samples of different seasons from consecutive sediment trap

Faunal assemblage

Faunal abundance variations are commonly used as proxy indicators for changing sea-surface conditions. Prior to stable isotope analysis, the relative abundances of G. ruber and G. sacculifer in each individual collecting cup were calculated (Fig. 2). G. ruber is more abundant in warm months (August, September, and October) than during the regional cold season (December, January, and March), whereas G. sacculifer is present year round at relatively constant relative abundances of less than 20%.

Morphotypes of G. ruber

Wang (2000) showed that G. ruber in coretop samples has two distinct morphotypes, each with different δ18O and δ13C compositions. These two morphotypes were distinguished at an early stage of this study. Fig. 8 compares the stable isotope values (δ18O and δ13C) for the two morphotypes from the M2 trap. Because we only have three pairs of measurements for the two morphotypes, it is not sufficient to justify the observation of Wang (2000) that G. ruber (sensu lato) is characterized by heavier δ18

Conclusions

Seasonal signals of hydrographic change reflected in the stable isotopic composition of planktonic foraminifers have been studied based on shells obtained from sediment traps deployed in the northern SCS in the 2000 through 2002. The relative abundance of G. sacculifer in the faunal assemblage was found to be consistently <20%, whereas G. ruber showed strong seasonal variations with the highest percentages in the warmest months. Depleted δ13C values in both G. sacculifer and G. ruber,

Acknowledgements

The authors are grateful for the kind assistance of Prof. H. Spero and D. Winter at UC Davis for stable isotope analyses. Prof. L. Peterson at RSMAS/University of Miami is thanked for helping make the English language more readable. Laboratory assistance provided by T.-J. Lin, A.-P. Chiang, S.-F. Wu, and Y.-J. Hsieh is highly appreciated. Constructive comments from Dotty Pak, Stefan Mulitza, Ellen Thomas, and one anonymous reviewer improved the manuscript largely. This study was supported by

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