Southern Hemisphere water mass transport to the Arabian Sea linked to Greenland climate variability during Heinrich Event 4

Several paleoceanographic and climate modeling studies have shown both oceanic and atmospheric teleconnections between climate in the tropics and the high latitudes on timescales ranging from decadal to multi-centennial. The last glacial period is characterized by millennial-scale abrupt warmings (interstadials) followed by rather gradual coolings to colder (stadials) Dansgaard-Oeschger (DO) events. The more pronounced of these stadial phases coincide with occurrences of ice-rafted debris in sediments from the mid-latitude Atlantic Ocean, referred to as Heinrich events (HE). Climate oscillations associated with DOs and HEs are also recorded in tropical climate archives around the Indian Ocean and on the Asian continent. However, forcing and response mechanisms of the Indo-Asian monsoon system and ocean-atmosphere exchange processes in conjunction with these millennial-scale oscillations are still poorly understood. Here, we present high-resolution geochemical and micropaleontological data from a sediment core located at 571 m water depth offshore Pakistan, representing the past 80,000 years at millennial-scale resolution.

Alkenone unsaturation-derived sea surface temperature (SST) estimates show overall variations between 23 and 28°C. Millennial scale SST changes of 2°C are modulated by longer-term SST fluctuations. Interstadial intervals are characterized by higher organic carbon (TOC) concentrations, whereas sediments with low TOC contents mark stadials. Productivity-related and anoxia-indicating proxies show abrupt shifts with a 50-60 year duration at climate transitions, such as interstadial inceptions. Inorganic data consistently indicate that enhanced fluxes of terrestrial-derived sediments are paralleled by productivity maxima, and are characterized by an increased fluvial contribution from the Indus River during interstadials. The hydrogen isotopic composition of terrigenous plant waxes indicates that stadials are dry phases whereas humid conditions seem to have prevailed during interstadials. Stadials are characterized by an increased contribution of aeolian dust. HEs are especially dry, indicating a dramatically weakened Indian summer monsoon and increased continental aridity.  

The stable oxygen isotope (δ¹⁸O) records of the surface-dwelling foraminifera G. ruber and of the thermocline-dwelling P. obliquiloculata both show a strong correspondence to Greenland ice core δ¹⁸O, whereas the δ¹⁸O signal of benthic foraminifera (U. peregrina and G. affinis) reflects patterns similar to those observed in Antarctic ice core records. Distinct shifts in benthic δ¹⁸O during stadials indicate frequent injections of oxygen-rich intermediate water masses of Southern Ocean origin into the Arabian Sea. The most pronounced oceanographic changes occur during the transition and the termination of HE 4, respectively. Mg/Ca ratios of G. affinis show a rapid increase (decrease) of bottom water temperatures during the onset (termination) of HE 4, which is in good agreement to modelling studies. The hydrogen isotopic composition of terrigenous plant waxes indicates that HE 4 is much drier than the surrounding DOs.

Overall, our results strengthen the notion that North Atlantic temperature changes and shifts in the hydrological cycle of the Indian monsoon system are closely coupled, with significant impacts on regional environmental conditions such as river discharge and ocean margin anoxia. These shifts were modulated by changes in the supply of water masses from the Southern Hemisphere.