Calcification depths and temperatures of planktonic foraminifera off southwest Hainan Island and their paleoceanographic implications

Abstract

Abstract The East Asian monsoon system influences the local oceanographic and climatic conditions of South China Sea (SCS), making the habitat depths of planktonic foraminifera (PF) specific for this region.The previous paleo-ocean reconstructions of SCS have been based on calcification habitat depths derived from other oceans, which may have affected the accuracy of interpretations. In this study, the calcification depths of Globigerinoides ruber, Globigerinoides sacculifer, and Neogloboquadrina dutertrei from sediment traps deployed at 1000 m depth off southwest Hainan Island from July 2012 to April 2013 were analyzed with regard to the stable isotopes of oxygen (δ18O) and carbon (δ13C) to calculate their regional apparent calcification depths (ACDs) with the overall aim to improve the reconstruction of the upper ocean thermal structure. ACDs and temperature estimated from foraminifera δ18O ranged from 0 to 42 m and 25°C for G. ruber, 30-65 m and 23.3°C for G. sacculifer, and 75-100 m and 20.3°C for N. dutertrei, respectively. The difference between the thermocline-species (N. dutertrei) and mixed-layer species (G. ruber and G. sacculifer) δ18O (∆δ18OTh–ML) and the temperature (∆T) shows water column stratification. The periods of lower and higher ∆T and ∆δ18O were in phase with the changes in regional overlying wind speed. Additionally, this study reveals that N. dutertrei lives and calcifies its shells within the base of the mixed-layer depth (MLD) and upper thermocline depth in SCS. Overall, the results from this study provide a new insight to understand the mixed-layer and thermocline depth in the northern SCS.