Holocene hydroclimate variability in south-eastern Australia; validation and application of cellulose oxygen isotopes at Lake Surprise, Victoria, Australia

Abstract

Understanding decadal-millennial scale patterns of climate variability are essential to provide context for future climate projections and to support climate mitigation actions. In south-eastern Australia, which has been vulnerable to climate extremes in recent decades, the relatively short (<125 years) instrumental climate record does not capture the full range of natural climate variability, undermining efforts to model and predict future droughts. In this context, there remains a need for decadal-scale palaeoclimate records which are well-dated and provide robust reconstructions of hydroclimate change based on a firm understanding of the contemporary proxy system. This thesis presents a new Holocene palaeoclimate record for south-eastern Australia using oxygen isotopes in aquatic cellulose, preserved in the sediments of Lake Surprise, Victoria. The palaeoclimate record is supported by, i. A combined monitoring and modelling program to quantify the link between climate, lake hydrology and the oxygen isotope composition of lake waters, with a particular focus on lake-groundwater interaction. Precipitation and lake waters were monitored for major ions and oxygen and hydrogen isotopes. These data were interpreted using the lake isotope mass balance modelling approach. ii. A robust chronology based on combined 210Pb, Pu and 14C analyses, as well as independent chronomarkers, with 14C measured on bulk sediment, plant macrofossils and pollen extracts. An age model was developed using Bayesian Plum modelling, and chronological biases with pollen radiocarbon dates were examined in detail. iii. Palaeoclimate validation-in-time by comparing the recent sedimentary record with climate data for the instrumental period. Correlations between instrumental precipitation-evaporation data with sedimentary oxygen isotope data provide greater confidence in the fidelity of the Holocene climate reconstruction. Changes in sediment carbonate concentration and carbon isotopes in organic matter appear to relate to changes in groundwater influence and aquatic primary productivity. The Holocene palaeoclimate record of lake water oxygen isotopes derived using cellulose oxygen isotope analysis agrees with previous studies from western Victoria, which indicate a relatively wet early-mid Holocene and a prolonged period of drier climate after ~4500 cal yr BP. It is suggested that the late Holocene drying was influenced by an increase in the frequency of El-Niño events. In contrast to previous studies, the Lake Surprise record indicates that the early Holocene wet period was not a singular event, but instead, three periods of wetter climate occurred from ~10850 – 10000 cal yr BP, 7400 – 7000 cal yr BP and 5600 – 4500 cal yr BP. These wet intervals are consistent with inferred periods of weaker and northward migration of the Southern Westerly Winds. By constraining the timing of major hydroclimate shifts in south-eastern Australia, the new record from Lake Surprise contributes to a deeper knowledge of the Holocene climate in the region, which can assist in modelling hydroclimate responses to global change. Furthermore, the modern monitoring, modelling and validation lay the foundation for replicating the Lake Surprise record to develop regional scale, quantitative records of hydrological change.