Coral and speleothem reconstructions of early holocene ocean-atmosphere dynamics in southern Australasia

Abstract

Reconstructing the impacts of abrupt climate change on the Indo-Pacific Warm Pool, the Australasian monsoon, and El Nino-Southern Oscillation, and their teleconnections to higher latitudes, is crucial for understanding the role of the tropics in global climate change. The main objective of this thesis is to use geochemical tracers in skeletons of modern and fossil Porites corals from Alor, Indonesia, and a speleothem record from the island of Flores nearby, to explore the climatic response of southern Indonesia to the abrupt cold event 8,200 years ago originating in the North Atlantic region, referred to as the 8.2 ka event. The early Holocene interval before, during and after the 8.2ka event was investigated using 5-year resolution Sr/Ca and d18O records for eight fossil corals with U-series ages spanning 8.5ka to 7.8ka (thousand years before the present). Relative to today, fossil coral Sr/Ca-SSTs from 8.4 to 8.0 ka indicate slightly warmer SSTs and higher residual d18O (Dd18O) values suggesting drier conditions. These results compare well with other Australasian oceanic and atmospheric records, however the high-resolution Alor coral records also show gradual cooling of SSTs starting at 8.3ka and an abrupt 1.5-2C cooling at 8.0 ka associated with the 8.2ka event. Also, the speleothem record from Flores shows sharp increases in d18O, d13C, Mg/Ca and Sr/Ca around 8.4ka and 7.9ka, indicating drier/cooler conditions on land during the 8.2 ka event. High-resolution analysis of the Alor corals confirms these results. The lack of a clear antiphased interhemispheric increase in monsoon rainfall around Alor and Flores, as observed in South American records of the 8.2ka event, may be due to anomalously cool SSTs in the Indonesian maritime continent at that time. The interannual and seasonal characteristics of the El Nino-Southern Oscillation (ENSO) during the early Holocene and 8.2ka event were explored via frequency analysis, threshold analysis, and analysis of the annual cycles of SST and rainfall to gain information about changes in the ENSO frequency and magnitude during different background climate states. High-resolution analysis of coral Sr/Ca and d18O show that ENSO events were suppressed during the early Holocene, though not as much as indicated by other palaeo-ENSO records and modelling studies. The coral records also show that the frequency, duration and intensity of El Nino events was greater during the 8.2ka event compared to the early Holocene ENSO climate state. A synthesis of the causes and mechanisms driving the 8.2 ka event suggests that definitive conclusions cannot be drawn about the role of the Atlantic Meridional Overturning Circulation because such a mechanism cannot explain the long duration of the 8.2ka climate anomaly, nor the fact that many records show an onset of climatic deterioration well before the abrupt 8.2ka event in the North Atlantic. Therefore, the 8.2 ka event is probably best interpreted in terms of a recurrent cycle of climatic deteriorations whose impact was amplified around 8.2ka by massive meltwater discharge into the North Atlantic. Show more