Stable carbon isotope ratios of the planktonic foraminifera Globigerina bulloides in ODP Hole 117-723A from the Arabian Sea (Table 2)

Production of Globigerina bulloides generally increases during upwelling in the tropical ocean and, in particular, during southwest monsoon season in the Arabian Sea. We studied the d13C signatures of G. bulloides from Ocean Drilling Program (ODP) site 723A located in the intense upwelling area along the Oman Margin, Arabian Sea. The delta13C signature of G. bulloides from the Oman Margin during the last 19 ka ranges from -2.65? to -1.13?. Strikingly, the delta13C values of G. bulloides are about 1? lower than that of benthic foraminifera Uvigerina excellens at this site. Apparently, the vital effects induced carbon isotopic fractionation in G. bulloides results in abnormal delta13C values. During upwelling seasons, G. bulloides calcifies faster, perhaps because of high-nutrient availability. Faster calcification rates in this species require higher respiration, which involves more respired CO2 enriched with 12C accounting for nearly 10% of shell carbon. Therefore, the combination the foraminifera of faster calcification and involvement of respired CO2 would induce more negative delta13C in G. bulloides compared to other foraminifera. Apart from vital effects, the environmental changes caused by the variations of upwelling intensity during Holocene and last glacial maximum also affected the down-core delta13C record of G. bulloides in the Arabian Sea. Significant ontogenic differences in both delta18O and delta13C occurred between large and small size classes of G. bulloides.

球室孔虫（Globigerina bulloides）的壳体产量在热带海洋上升流期间普遍升高，尤以阿拉伯海西南季风季最为显著。我们针对阿拉伯海阿曼陆缘强上升流区的大洋钻探计划（Ocean Drilling Program, ODP）723A站位的球室孔虫的碳同位素δ13C信号展开了研究。该站位阿曼陆缘海域的球室孔虫在过去19千年间的δ13C值介于-2.65‰至-1.13‰之间。值得注意的是，该站位球室孔虫的δ13C值较底栖有孔虫优秀六玎虫（Uvigerina excellens）低约1‰。显然，球室孔虫因生命效应引发的碳同位素分馏作用，导致了其异常的δ13C值。在上升流季，球室孔虫的钙化速率更快，这或许得益于高营养盐的供给条件。该物种更快的钙化速率需要更强的呼吸作用，而呼吸作用会释放更多富集碳12（12C）的二氧化碳，这类二氧化碳约占壳体总碳量的10%。因此，钙化速率更快且利用呼吸来源二氧化碳的组合效应，使得球室孔虫的δ13C值相较于其他有孔虫更为偏负。除生命效应外，全新世与末次冰盛期上升流强度变化所引发的环境变迁，同样影响了阿拉伯海地区球室孔虫的钻孔岩芯δ13C记录。不同粒径级的球室孔虫之间，δ18O与δ13C均存在显著的个体发育差异。