The tropical eastern Indian Ocean is part of Earth´s largest warm pool and its surface and thermocline temperatures exert strong control on deep atmospheric convection and play a critical role for the development of basin-wide, zonal climate anomalies across the Indian Ocean. However, the nature, timing, and mechanisms of changes in the Walker circulation in the tropical Indian Ocean since the Last Glacial Maximum (LGM) are poorly constrained owing to a lack of suitable proxy records and proxy-model disagreements (Mohtadi et al., 2017). Here we reconstruct surface and thermocline temperature and hydrographic changes in the tropical eastern Indian Ocean based on high-resolution, planktic foraminiferal (G. ruber and P. obliquiloculata) d18O and Mg/Ca records for the last 22 ka from three sediment cores retrieved offshore west Sumatra along a latitudinal transect across the equator.

Sea surface temperatures at all three sites show a stepwise warming of ~3°C with an ‘Antarctic timing’ between 18 ka and 11 ka. The thermocline temperature variability is also consistent among the three core sites but distinctly different from the sea surface temperature variability. Thermocline temperatures show a major warming of 2–3°C between ~13 ka and ~10 ka, while differences between LGM and Holocene temperatures are rather small. The resulting surface-thermocline temperature gradient reveals not only a difference between LGM and Holocene thermocline depth levels, but also a major breakdown pointing at a rapid deepening of the thermocline at ~12 ka. This thermocline deepening might have been associated with a strengthened convective activity and Walker circulation, with its timing suggesting a connection to feedbacks related to inundation of the large Sahul Shelf during deglacial sea level rise.

Additional individual foraminiferal d18O and Mg/Ca analyses for several key time periods suggest large thermocline temperature variability of ~4–7°C on seasonal to interannual time scales during the last 22 ka. On these short time scales, thermocline cooling in the upwelling-influenced region offshore southwest Sumatra appears to be less common during the glacial as compared to the Holocene, indicating a relative dominance of negative over positive Indian Ocean Dipole events and a strengthened Walker circulation during the LGM. Leaf wax dD analyses will document whether or not local rainfall was enhanced during the glacial as compared to the Holocene, and allow unravelling how the Indian Ocean Walker circulation and rainfall anomalies were shaped by sea level rise and deglacial millennial-scale climate changes.

Mohtadi et al., 2017, Nature Communications 8, 1015, doi:10.1038/s41467-017-00855-3

tropical eastern Indian Ocean, surface and thermocline temperature reconstruction, Indian Ocean Walker circulation, last glacial-interglacial transition