Holocene climate and ocean variability in the Sunda Strait region of the eastern Indian Ocean
Creators: name: Ellis, Bethany
Publisher: name: The Australian National University
Year: 2021

description: The Indian Ocean Dipole (IOD) is an ocean-atmosphere climate oscillation within the Indian Ocean basin and one of Australasia's key climate drivers that influences the distribution of rainfall across the region. Future projections of IOD activity suggests that extreme positive IOD events may become more frequent with greenhouse warming leading to an increase in occurrence of extreme climate and weather events in regions influenced by the IOD. However, the short duration of instrumental records and biases in model representations of the IOD make it difficult to confidently separate anthropogenic-related trends from natural variability. To better understand natural IOD variability, high-resolution reconstructions of the Indian Ocean sea surface temperature (SST) are needed to provide a comprehensive view of IOD upwelling activity prior to the Industrial Revolution. The southern Sunda Strait, Indonesia is a key region for measuring IOD activity, as the cold upwelling waters associated with a positive IOD event have a notable signature at this site that is captured by the geochemical changes in coral skeletal material. The use of X-Ray Fluorescence (XRF) Core Scanning for continuous high-resolution analysis of Sr/Ca in corals is investigated as a new method for analyzing coral skeletal material. Traditional methods of analyzing coral geochemistry involves extensive subsampling and wet chemistry techniques to obtain high precision elemental records. More recent developments of micro-beam techniques, such as laser ablation ICP-MS, have limitations in the length of core that can be scanned at any one time. However, XRF core scanning provides a fast, non-destructive method of analyzing long sections of coral cores whilst maintaining a high sampling resolution. Here we apply this method to two modern corals, from the Sunda Strait, Indonesia and One Tree Island, Great Barrier Reef, Australia. We find that XRF can provide a valuable method of quick and non-destructive screening of coral material, prior to more targeted analysis using traditional destructive geochemical analysis. Understanding spatial and temporal changes in the marine radiocarbon reservoir ages (dR) throughout the Holocene is important for reconstructing changes in global ocean circulation and investigating the impacts of ocean ventilation. Here, we present new dR values, calculated using paired U/Th and 14C dates, from modern and fossil coral samples collected from the Sunda Strait and Mentawai Islands in Indonesia. Throughout the Holocene dR values are found to experience large fluctuations within the southeast Indian Ocean. Along with previously published data from across the Indo-Pacific region, the large fluctuations in dR values during the mid-Holocene, are believed to reflect a combination of changes in the frequency and intensity of ocean upwelling, including IOD associated upwelling, and variations in ocean circulation. Some of the modern and fossil Porites corals have also been used reconstruct past SST and environmental conditions in the tropical eastern Indian Ocean over the past several hundred years. We have reconstructed 200 years of ~monthly resolved d18O record from a coral tsunami block that was deposited during the 1883 Krakatoa eruption. The coral temperature record has been combined with a modern coral record and historical ship logbook records from the region to provide a record of over 300 years of tropical Indian Ocean SSTs. This extended SST reconstruction reveals insights into the frequency and intensity of positive IOD events prior to anthropogenic climate change. In addition to the high resolution d18O record, annually resolved paired d18O and Sr/Ca records have provided insights into the hydroclimate of the Sunda Strait region over the past millennium.