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.