The glacial-interglacial monsoon recorded by stalagmites from southwest Sulawesi, Indonesia
Date
2016
Authors
Kimbrough, Alena Kay
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Climate variability in the tropics, and monsoons in particular, are critical to transporting moisture and heat around the earth. Deep atmospheric convection over the Indo-Pacific Warm Pool (IPWP) supplies heat and energy to two of the largest atmospheric circulation systems, the meridional Hadley Cell and zonal Indo-Pacific Walker Circulation. These convection centers are key components of the Australasian monsoon. Thus, the IPWP plays a fundamental role in global atmospheric circulation and hydrology. Paleoclimate records of IPWP climate have provided insight into the behavior of these major convective systems; however, significant differences between the reconstructions reveal a non-uniform response to various climate forcings. Recent modeling studies indicate that IPWP hydroclimate and convection are exceptionally sensitive to sea level and the exposure of the Sunda and Sahul shelves over glacial-interglacial (G-I) cycles, but the proxy evidence has not consistently supported this finding. Our study site, in southwest Sulawesi, is located within the IPWP and maintains a climatology dominated by the Australian-Indonesian summer monsoon (AISM). This site is an excellent location for tracking changes in IPWP climate variability and AISM strength through time. Stalagmite geochemical records have the potential to serve as precise monitors of past climatic and environmental change and can be absolute-dated to resolve terrestrial climate change on decadal to orbital timescales. We present eight new multi-proxy stalagmite records from the Maros karst in southwest Sulawesi, which collectively span 386–127 ky BP. The antiquity of the material inspired the design of a U-Th dating procedure using ultraviolet and transparency imaging to select the cleanest material, as well as geochemical assessment of U-Th data to construct robust and optimized chronologies. We investigated different age-depth modeling approaches and identified the Bayesian accumulation algorithm (BACON), coupled with tie-point optimization, as the ideal age-modeling technique for this work. Trace elements (Mg/Ca and Sr/Ca) in the Sulawesi stalagmites were assessed and found to derive primarily from the limestone host rock. Strong co-variation between Mg/Ca and Sr/Ca in slow-growing stalagmites demonstrates their sensitivity to prior calcite precipitation (PCP), and thus hydrological changes, whereas Mg/Ca and Sr/Ca in faster growing stalagmites appear to be influenced by additional factors. Using the hydrologically sensitive Mg/Ca data for two highly resolved (40–180 year-resolution), slow-growing stalagmites, we were able to test and confirm the reliability of stalagmite δ18O as a proxy for rainfall amount at our site. Coupled stalagmite δ18O and Mg/Ca records are particularly useful for investigating G-I changes in IPWP hydroclimate. The stalagmite δ18O record reveals coherent climatic features over G-I transitions. We focus on AISM variability during three periods of rapid global warming, glacial termination TIV (~340 ky BP) and both phases of TIII (TIII ~248 ky BP and TIIIa ~217 ky BP). Stalagmite δ18O and Mg/Ca data show rapid increases in rainfall during glacial terminations and wet interglacials. Glacial terminations are each characterized by an abrupt ~3‰ decrease in stalagmite δ18O that coincides with sea level rise, flooding of the Sunda and Sahul shelves, and strengthening of the AISM. Importantly, the good agreement of the δ18O and Mg/Ca records over G-I transitions demonstrates that southwest Sulawesi stalagmite δ18O is a reliable indicator of rainfall amount. The results show that, on G-I timescales, the strength of the AISM is most sensitive to the impact of changing sea level on the regional distribution of land and shallow-ocean. The new data, alongside an existing 40 ky record for southwest Sulawesi, provide eight transitional stages associated with G-I sea level change and reveal strong evidence for a shelf flooding/exposure threshold of approximately -65 ±9 m. Considered together, both glacial inceptions and glacial terminations imply a sea level threshold driving the AISM between two primary modes of intensity (‘interglacial’ and ‘glacial’). Precession-scale (23 kyr) monsoon variability associated with boreal autumn insolation at the equator is superimposed on these massive sea level driven shifts in AISM strength, indicating an underlying sensitivity of the AISM to the Walker Circulation’s influence on IPWP convection. When compared to stalagmite δ18O records for China, we find that AISM strengthening at glacial terminations consistently coincides with the final stages of weak monsoon intervals in the East Asian summer monsoon record, a key feature of global deglaciation. This observation, coupled with the inferred -65 m sea level threshold driving AISM strength, suggests that reinvigoration of the AISM, and thus IPWP convection, plays an important role in achieving and sustaining interglacial conditions. Although still in its infancy, this work provides critical insight into IPWP behavior over multiple G-I cycles, prompting new questions and inspiring collaboration with the paleoclimate modeling community.
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Australian-Indonesian summer monsoon, Austral-Asian Monsoon, paleoclimate, proxies, speleothems, stalagmites, Sulawesi, Maros, Glacial-Interglacial, warm pool, Indo-Pacific Warm Pool, IPWP, AISM
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