Glacial-Interglacial Cycles over the Past 150,000 years of South-Eastern Arabian Sea from δ18O Record

Abstract

This study aims to reconstruct the surface water productivity and related monsoonal intensity along the southwestern continental margin of India, over the last 150,000 years by utilizing the temporal variations of δ18O and δ13C isotopes in the planktic foraminifers (Globigerinoides ruber) from a sediment core obtained from the continental slope off Kochi, south-eastern India. The chronostratigraphy of the core is established on the basis of high resolution stable isotope record of planktic foraminifera (Globigerinoides ruber) and a few AMS radiocarbon ages. Down-core records of δ18O and δ13C show millennial scale variations of primary productivity and monsoonal intensity during past 150 kyr. Boundaries of the Marine isotope stages (MISs) were delineated in the core based on the visual comparison of the δ18O with Martinson’s (D. M. Martinson, N.G. Pisias, J.D. Hays, J. Imbrie, T.C. Moor Jr, N.J. Shackleton, Age dating and the orbital theory of the ice ages: development of a high resolution 0 to 300000 year chronostratigraphy, Quaternary Research, 1987 27, p 1-29) and the SPECMAP data. From these studies it is inferred that the bottom of the core is about 150 kyrs, demarcating the paleoclimatic/paleoceanographic information covering MIS 1 to 6. Boundaries among MIS 6 and MIS 5, MIS 5 and MIS 4, MIS 4 and MIS 3, MIS 3 and MIS 2 and MIS 2 to MIS 1 (Holocene) have been demarcated at depths 11.47 m, 4.68 m, 4.03 m, 18 m, and 21 m respectively in the core studied. MIS 5 section is characterized by alternating warm and cold periods as observed in Polar Ice cores. The main findings of this work indicates that the core site has recorded higher productivity with increased fluvial input during the warm, humid interglacial periods and interstadials than the stadials during the glacial period. However, the temporal variability from 150 kyr to 130 kyr has been considered as the glacial period with cold and dry climate that corresponding to the MIS 6, MIS 4 within this an episode of extremely low terrigenous input with heavier δ18O values is noticed from 12.91 to 12.67 m. Similarly, a period of heavy δ18O values, with less surface runoff and with low δ13C values between 4.03 to 4.68 m bsf has been interpreted as MIS 4. Episodic fluctuation in SW monsoon induced productivity during MIS 3 has been observed in present study. The de-glacial event in MIS 2 marked in present core characterized by an abrupt increase in productivity during the mid-MIS 2.