The Southern Ocean surface between Marine Isotope Stages 6 and 5d: Shape and timing of climate changes

Abstract

Summer sea surface temperature (SSST) and sea-ice extent, obtained at millennial to centennial resolution, are presented for seven cores recovered in the Atlantic and western Indian Antarctic Zone, from Termination II and the last interglacial s.s., i.e. Marine Isotope Substage (MIS) 5e. The diatom-based SSST estimates show that during the late MIS 6 intensified export of cold waters from the Weddell Sea resulted in SSSTs close to 0°C in the central and eastern Atlantic sector. Due to northward expansion of the winter sea-ice edge by 3–5° latitude the sea-ice seasonality was reduced. Such conditions have great potential for efficient year-round reduction in air–sea gas exchange and glacial CO 2 draw down. The warming at Termination II started at 132–131 ka, marked by rapid retreat of the sea-ice edge. The SSST increased within a few thousands of years to reach maximum interglacial summer temperatures ranging between 2.5 and 3.5°C, in the earliest MIS 5e (128–125 ka). Besides a distinct cold reversal around 129.5 ka, the warming was punctuated by short-lived temperature rebounds with amplitudes of 1–1.5°C, occurring at 200–300-yr frequency, thus in the range of the periodicity of solar input. Increased Chaetoceros spp. abundance suggests meltwater events as the primary cause of the coolings. The SSST optimum, characterised by strongly enhanced biogenic sedimentation rates, ended at ca. 125 ka, with the full re-establishment of the global conveyor belt and beginning of the Northern Hemisphere climatic optimum. At this time SSSTs decreased stepwise and stabilised on values slightly warmer than today, showing in the following period of MIS 5e only low amplitude oscillations. During MIS 5d a northward expansion of the sea-ice field took place, but SSSTs only slightly colder than modern ones and a weakened meridional temperature gradient indicate that the Southern Ocean did not fall in a true glacial mode. The tight correlation between Southern Ocean SSST and winter sea-ice records, and Vostok atmospheric temperature and CO 2 records support the idea that the Antarctic winter sea-ice and Southern Ocean temperature exert a primary control on the atmospheric–ocean CO 2 exchanges and hence on global climate. Reduction of the Antarctic sea-ice field and increase of SSST at the onset of Termination II could have been triggered by precessional changes influencing southern high latitude summer insolation. Positive feedback mechanisms, reduction of surface albedo and release of CO 2 into the atmosphere, and changes in global ocean circulation, affected by a collapse or strong reduction in North Atlantic Deep Water (NADW) production, could have induced further reinforcement of the Southern Ocean warming.