[M-IS02] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS02-05] Reconciling biomarker- and foraminifera-based upper ocean temperature records off Southwest Sumatra

*Maria Makarova¹、Mahyar Mohtadi²、Raul Tapia¹、Jeroen Groeneveld^2,3、A. Nele Meckler⁴、Kuo-Fang Huang⁵、Sze Ling Ho¹ (1.Institute of Oceanography, National Taiwan University, Taipei, Taiwan、2.MARUM-Center for Marine Environmental Sciences, University of Bremen, Germany、3.Department of Earth Sciences, Hamburg University, Germany、4.Bjerknes Centre for Climate Research and Department of Earth Science, University of Bergen, Bergen, Norway、5.Academia Sinica, Taipei, Taiwan)

キーワード：multiproxy temperature estimates, clumped isotopes , foraminifera, Sumatra, LGM, Holocene

The Indonesian seas link the Indian and Pacific Oceans, thus play an important role in modulating the ocean-atmosphere circulation in the tropics and globally. Climate variability of this region is complex due to the interactions between the Asian monsoon system, El Nino - Southern Oscillation (ENSO), and Indian Ocean Dipole (IOD) mode. Their effects on the hydrographic and thermal structure of the upper ocean are particularly evident offshore Sumatra-Java, where upwelling conditions during the SE monsoon (June-October) shoal the thermocline and produce the negative sea surface temperature anomaly off SW Sumatra and Java, which is modulated by ENSO and IOD on interannual scale. Previous studies used sedimentary records spanning the last 25,000 years to track changes in the thermal structure off SW Sumatra since the Last Glacial Maximum (19-23 ka) and infer changes in the ocean-atmospheric circulation under a colder climate of the LGM. Both foraminiferal (Mg/Ca of mixed layer and thermocline dwelling foraminifera) and organic (U^K’₃₇, TEX₈₆) proxies have been used to estimate the surface to the thermocline temperature gradient (Δt_s-t), but these proxies indicated opposite directions for the Δt_s-t change since the LGM. To shed more light on this, we generated new seawater temperature estimates using clumped isotopes (Δ₄₇), a recently developed paleothermometer independent of seawater chemistry. The measurements were performed on the tests of surface- (G. ruber, T. trilobus) and thermocline- (N. dutertrei, G. menardii) dwelling foraminifera from core GeoB 10038-4 located off Southwest Sumatra. Our new Δ₄₇ data show stronger warming in the mixed layer (~6°C) than in the thermocline (<2°C) since the LGM. Similar to Mg/Ca, the Δt_s-t estimates from clumped isotopes show an increase in Δt_s-t since the LGM. The spatial pattern of modern surface sediment data off Indonesia shows that Δt_s-t increases in the presence of upwelling, thus a smaller foraminiferal Δt_s-t during the LGM relative to the Holocene might indicate a deeper thermocline off SW Sumatra during the LGM. We further address the discrepancy between foraminiferal and organic proxies by considering ecological and non-thermal effects on the temperature proxies. Overall, our study adds to the large body of work on the glacial-interglacial hydrographic changes in the Indonesian seas and demonstrates the potential of the Δ₄₇ thermometry for paleoclimate reconstructions in this region.