Significant seasonal contrast in the Arabian Sea during deglaciation: Evidence from oxygen isotopic analyses of individual planktic foraminifera

摘要

The variation of stable isotopes between individual shells of planktic foraminifera of a given species and size may provide short-term seasonal insight into past sea surface temperature (SST) and salinity (SSS). In this context, oxygen isotope analyses of individual Globigerinoides sacculifer were carried out from the Ocean Drilling Program Site 723A in the western Arabian Sea to unravel seasonal changes over the last 22 kyr. delta O-18 values of single shells of G. sacculifer range from of 0.54-2.09 parts per thousand at various discrete depths in the core which spans the last 22 kyr. Maximum inter-shell delta O-18 variability and relatively higher standard deviation is noticed from 20 to 10 kyr (deglaciation), whereas from 10 kyr onwards (Holocene) inter-shell delta O-18 variability decreases. Based on statistical inferences, we interpret this result as greater seasonal SST and SSS contrasts during the deglaciation compared to the Holocene. These winter-to-summer contrasts during the deglaciation can likely be explained by a net reduction in summer upwelling linked to changes in wind forcing and net evaporation-minus-precipitation that might, along with local causes, be related to deglacial shifts in the intertropical convergence zone (ITCZ). We note that a common winter-dominated SST signal in the Arabian Sea and in Greenland was pervasive during the last glacial period, strengthening evidence of a North Atlantic link with Indian Ocean SST. Thus, the present study has wider implications in understanding whether the forcing mechanism of tropical monsoon climate lies in high latitudes or in the tropics.