Episodic preservation of pteropods in the eastern Arabian Sea: Monsoonal change, oxygen minimum zone intensity and aragonite compensation depth

Abstract

The preservational record of pteropod shells (aragonite) for the last 30 kyr has been studied in a core (SK17) recovered from the eastern Arabian Sea margin at off Goa coast (depth 840m, lat. 15°15’N, long. 72°44’E). The chronostratigraphy of the core established on the basis of high resolution stable isotope record of a planktic foraminifera (Globigerinoides ruber) and several AMS radiocarbon ages demonstrates millennial scale variation in δ ¹⁸O defining Younger Dryas and Heinrich like Events. Records of absolute abundance of pteropods (1g/dry wt >125 Mu m), abundance ratio of pteropod and planktic foraminifera, aragonite (wt %), organic carbon (OC) (wt %) and CaCO₃ (wt %) show major changes during these isotopic events. Aragonite maxima and higher number of well-preserved pteropod shells, are noticed during cold stadial periods. The study indicates a negative correlation between aragonite and OC % (productivity index). On the other hand, total CaCO₃ content (calcite and aragonite) is positively correlated with the aragonite. The variation patterns of pteropod shells and aragonite content in the sediment core are suggested to be controlled by the preservational conditions associated with the fluctuation in Aragonite Compensation Depth (ACD) and Oxygen Minimum Zone (OMZ) intensity. It is suggested that the high biological productivity during intensified summer monsoons in late Holocene and inter-stadial periods might have resulted in severe oxygen depletion (strong OMZ) leading to shallowing of the ACD. A weak summer monsoon and low productivity condition prevailing during cold stadial periods would have resulted in a weak OMZ and deepening of the ACD.