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Abstract
This study provides a high resolution, well dated multiproxy record of hydroclimatic and environmental change in the Sydney Region of eastern Australia over the last 18,000 years. A lack of moisture-sensitive records from the Australian region has hampered understanding of climate dynamics, especially in the mid-latitude regions between tropical and temperate zones. Significant controls on the Australian climate include heat redistribution from the tropics via ocean currents and teleconnection to the Antarctic moderated by the Southern Hemisphere westerlies. The unique peat forming environments of the Eastern Highlands of Australia are well placed to show a sensitive response to the interplay between these two major climate controls that contribute to overall moisture availability in the region. A meta-analysis of sediment accumulation rates revealed that these environments show a significant response to climatic amelioration around the Late Pleistocene-Holocene boundary with a doubling of accumulation rates after low values during the LGM and postglacial period up to a Holocene average of 5cm/century. Within the Holocene, sediment accumulation rates show an overall upwards trend that is punctuated by a decrease in accumulation rates between 9,000 and 8,000 cal. BP and again at 1,000 cal. BP.
The three sites investigated here (Goochs Crater, Hanging Rock Swamp and Queens Swamp) were chosen to be representative of the contemporary variation observed across the moisture-dependent peat forming environments of the Sydney region, determined using a range of palaeoenvironmental proxies with a particular focus on hydroclimatically sensitive indicators (Humification, ITRAX geochemistry), C/N ratios, grainsize and charcoal analysis). Comprehensive AMS radiocarbon (14C) dating was undertaken on multiple organic fractions from the same depth intervals to identify an optimal target for future dating and to quantify the inbuilt age and other sources of error from less suitable fractions, demonstrating plant macrofossils as an ideal target, with macrocharcoal being consistently older by 200 years on average while pollen usually returned the oldest 14C age.
The results proxy of techniques and robust age-depth models show that postglacial climatic amelioration in the region was interrupted by an abrupt return to cold LGM-like conditions between 16,250 and 11,250 cal. BP and again by cool, dry conditions late in the ACR at 12,800 cal. BP. The Sydney region also experienced significant aridity after an abrupt drying event at 9960 ± 40 cal. BP that persisted until 8,000 cal. BP, refuting the ‘early Holocene climatic optimum’ proposed elsewhere. Moisture variability then increases throughout the Holocene, with Br-derived easterly rainfall levels fluctuating on a 500 year period for the remainder of the record with maximum levels reached around 5,000 cal. BP. Contemporary conditions were then mostly maintained through till present, although subtle climate shifts appear to contribute to a region-wide increase in peat formation at around 3,000 cal. BP and possible decadal-scale hydrological variability in the last 1000 years that is attributed to ENSO cycles.