A~30 kyr sub-centennial to millennial Indian summer monsoon variability record from the southern Andaman Sea, northeastern Indian Ocean

Abstract

This paper reports centennial-scale variations in the Indian summer monsoon (ISM) that induced stratification and resultant productivity changes in the southern Andaman Sea. The study is based on foraminifera assemblages and sediment geochemistry, along with statistical, spectral and wavelet analyses of faunal data of a well-dated sediment core. The Accelerator Mass Spectrometer (AMS) 14C dated record spans the past 30 kyr with an average temporal resolution varying between 38 and 150 years. The spectral and wavelet analyses of planktonic foraminiferal record reveal significant periodicities during the past 30 kyr suggesting varied forcing mechanisms on the ISM. The characteristic suborbital periodicities correspond mainly to solar irradiance variability. The varying productivity in the southern Andaman Sea over the northern region suggests control of surface salinity, which modulated stratification in the upper water column. The large north to south gradient in productivity during the Last Glacial Maximum (LGM; 23–19 ka) to Heinrich Stadial 1 (HS1; 16–15 ka) indicates the prevalence of strong winter monsoon and well-mixed upper water column in the south. A distinctive high productivity period was identified from the southern Andaman Sea during 16.0–11.2 ka with a peak during the Bølling/Allerød (B/A; 14.5–12.6 ka). On the other hand, a gradual decrease in productivity from the early Holocene to mid-Holocene reflects a response to strong stratification induced by intensified ISM. This enhanced ISM during the early to mid-Holocene resulted in low productivity in the entire basin, albeit with much reduced productivity in the northern part. However, during the marine isotope stage (MIS) 2, the southern Andaman Sea was relatively more productive than the northern Andaman Sea. We recognise three distinctive spatial and temporal changes in salinity structure in the Andaman Sea: 1) ‘normal’ salinity pattern (weak ISM) in which relatively less saline water in the northern part of the basin existed during the late Holocene; 2) ‘strong’ salinity pattern (strong ISM) with extensive freshwater tongue in the northern part existed during early to mid- Holocene; 3) ‘reverse’ salinity pattern with decreased north to south salinity gradient during LGM to early deglacial (19–15 ka), probably driven by the increased isolation of the Andaman basin during the sea-level lowstand, which would have restricted the mixing of northern surface waters with the more saline southern Bay of Bengal (BoB).