Late Pleistocene surface and thermocline conditions of the eastern tropical Indian Ocean

Abstract

Surface and thermocline conditions of the eastern tropical Indian Ocean were reconstructed through the past glacial–interglacial cycle by using Mg/Ca and alkenone-paleothermometry, stable oxygen isotopes of calcite and seawater, and terrigenous fraction performed on sediment core GeoB 10038-4 off SW Sumatra (∼6°S, 103°E, 1819 m water depth). Results show that annual mean surface and thermocline temperatures varied differently and independently, and suggest that surface temperatures have been responding to southern high-latitude climate, whereas the more variable thermocline temperatures were remotely controlled by changes in the thermocline temperatures of the North Indian Ocean. Except for glacial terminations, salinity proxies indicate that changing intensities of the boreal summer monsoon did not considerably affect annual mean conditions off Sumatra during the past 133,000 years. Our results do not show a glacial–interglacial pattern in the thermocline conditions and reject a linear response of the tropical Indian Ocean thermocline to mid- and high-latitude climate change. Alkenone-based surface temperature estimates varied in line with the terrigenous fraction of the sediment and the East Asian winter monsoon proxy records at the precession band suggestive of monsoon (sea level) to be the dominant control on alkenone temperatures in the eastern tropical Indian Ocean on sub-orbital (glacial–interglacial) timescales.

Introduction

Continuous archives of past sea surface temperature (SST) changes in and around the Indonesian Archipelago show that millennial-scale variability of alkenone-based SST corresponds to northern high-latitude records (e.g. Kienast et al., 2001, Steinke et al., 2001), whereas Mg/Ca-based SST estimates track Antarctic temperatures (e.g. Rosenthal et al., 2003, Visser et al., 2003, Stott et al., 2004, Levi et al., 2007, Xu et al., 2008). Studies from the Timor Sea suggest that local paleoproductivity variations were modulated by the Australasian monsoon (AAM) and the Indonesian Throughflow (ITF) over the last 460 ka (Müller and Opdyke, 2000, Holbourn et al., 2005, Kawamura et al., 2006). It has been further proposed that synchronous changes in the SST and ice volume during the last two Terminations (T) were associated with substantial cooling and freshening of thermocline waters, i.e. the ITF (Xu et al., 2008). On the other hand, a recently published SST reconstruction from the Andaman Sea in the eastern Indian Ocean implies ∼3 °C cooler SSTs during the last glacial compared to today and a direct link between local hydrography and changes in the intensity of the Asian (Indian) monsoon system (Rashid et al., 2007). To this end, we do not know to what extent marine records from the eastern Indian Ocean reflect past changes in the Australasian, the East Asian, or the Indian monsoon.

Paleoceanographic studies in the eastern Indian Ocean between these two regions in the NW (Andaman Sea) and the SE (Timor Sea) are rare, and contradictory. Based on biogenic and terrigenous proxy-records obtained off SW Sumatra, Gingele et al. (2002) suggested that both the East Asian monsoon (EAM) and the AAM alternately dominated the regional oceanographic and atmospheric circulations over the past 80 ka. In a micropaleontological study, obvious and persistent upwelling conditions off SW Sumatra could not be identified for the last 35 ka (Murgese et al., 2008). In contrast, another recently published study suggests that enhanced marine paleoproductivity off SW Sumatra during the past 300 ka is directly related to strengthening of coastal upwelling during periods of increased boreal summer insolation and associated SE monsoon strength with a precessional cyclicity (Lückge et al., 2009). Regional thermocline temperature and salinity reconstructions are thus far missing and hence, past variation in thermocline conditions and its effect on regional SST and paleoproductivity are not known at all.

Here we present paired Mg/Ca and stable oxygen isotope (δ¹⁸O) records of planktonic foraminifera species from mixed layer, Globigerinoides ruber sensu stricto (s.s.), and from thermocline depth, Neogloboquadrina dutertrei, from a sediment core retrieved off SW Sumatra that spans the last glacial–interglacial cycle (0–133 ka). We further present seawater δ¹⁸O reconstructions at surface and thermocline depths, and alkenone-based SST estimates, in order to reconstruct past changes in regional hydrography and to better understand the environmental controls on different proxies.