Atypical δ13C signature in Globigerina bulloides at the ODP site 723A (Arabian Sea): implications of environmental changes caused by upwelling

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 δ13C signatures of G. bulloides from Ocean Drilling Program (ODP) site 723A located in the intense upwelling area along the Oman Margin, Arabian Sea. The δ13C signature of G. bulloides from the Oman Margin during the last 19 ka ranges from −2.65‰ to −1.13‰. Strikingly, the δ13C 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 δ13C 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 δ13C 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 δ13C record of G. bulloides in the Arabian Sea. Significant ontogenic differences in both δ18O and δ13C occurred between large and small size classes of G. bulloides.