Getting a taste for Australian drought history Why are we concerned with Australian Drought? Australia is recognized as the driest inhabited continent with remarkable deviations in rainfall both spatially and temporally. With a simple satellite image, one can see these changes in precipitation where the deep green of the east coast rapidly transition to shades of brown moving inland. Situated in the middle of this transition, in the sub-tropics of Eastern Australia, lie some of Australia’s most fertile farm lands, largest and most biodiverse ecosystems and the third biggest urban area (Brisbane). Due to the rapid population expansion and agricultural pressures, climate change is likely to have dramatic effects on local water supply. A report submitted by The Bureau of Meteorology and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in 2016 states that Australia has experienced a mean temperature increase of 1oC since 1910. Naturally, drought is not an uncommon occurrence here. Since the settlement of Europeans, up to seven major droughts have been recorded. The most recent drought, dubbed the “Millennium Drought” starting in 2001 and which lasted till 2009 caused over $7 billion in losses in agriculture per year, major strains on power production, and created reductions in drinking water availability. However, the true responses of future climate scenarios are difficult to uncouple due to the lack of understanding of past climate variability and its complex interactions with human disturbances. Thus, understanding the past to inform future long-term changes in hydrological balances and frequency, scale, and intensity of droughts in the subtropics is critical for the development and security of Australia.
Unlocking eastern Australia’s past climate Roughly 100 km west of the east coast, Coalstoun Lakes National Parks may just hold the key to uncovering Australia’s climate history. Situated within an agricultural district, an unsuspecting volcanic remnant is extruded 150 m off the local basin floor. Despite the seemingly desiccated exterior of this landform, nestled tightly within the caldera lies an oasis of lush rainforest encircling two small lakes (~1 km2 each). For nearly ~600,000 years the creators have persisted, potentially making the Coalstoun Lakes containing the longest and continuous palaeoecological record for Queensland. Like two buckets, these lakes accumulate rainfall during the sub-tropical summers and slowly redistribute their contents to the local wildlife and aquifer. As the rains recede, drawing into the drier winter’s months, the water slowly become exhausted by either evaporation or groundwater discharge, unless replenished by the following year’s wet season. Along with the rain, over time sediments have been accumulating, filling the basin with exotic aeolian particulates, adjacent hillslopes sediments, and local in-lake production year after year. Stacked in sequential order, remnants of the sub-tropics complex environmental history is preserved through glacial and interglacial times. This makes Coalstoun Lakes an exciting and ideal site for reconstructing past climates over a long timescale.
Working towards answers The two main research areas for my thesis are: 1) evaluate the paleoclimate of the eastern subtropics of Australia by utilizing stable isotopes, and 2) create a modern hydrological model of the Coalstoun Lakes system using a mass balance approach. These research focuses will be accomplished utilizing an assortment of samples (ie: vegetation, a 4.5 m sediment core, two species of snail shells, and soil samples) which were collected by a variety of scientists over several years. In addition, future plans are in motion to extract two pairs of cores from both lakes approximate depth of 15 m, covering nearly 150,000 years of climate. My intended focus is on stable isotopes from sponge spicules but I have taken advantage of an available core (thanks to efforts from Mike Evans and Kevin Welsh) to look at some carbon isotopes also. To start my project off on the right foot, in mid-September I undertook a three week stint at the British Geological Survey’s Stable Isotopes Facility and the University of Nottingham under the advisement of Prof. Melanie Leng and Dr. Matthew Jones, respectively. The first two weeks at BGS were focused on preparing and weighing vegetation and sediments to be analyzed for organic carbon isotopes, along with any calcium carbonate containing layers for bulk oxygen isotopes. With any additional time between running samples, I began sampling on a snail shell. In short, snails consume moisture from rainfall which is incorporated in the structure of its aragonite based shell. As, the organism grows it retains the oxygen isotope allowing us to reconstruct the precipitation variability over its lifespan (~10 yrs). We selected one species of snail (biggenden banded snail) and began the intricate sampling by drilling 0.8 mm holes into the shell. In all, 200 samples were attained from the apex (oldest section of the snail) to the outer rim. Samples were carefully collected perpendicular to the growth bands simultaneously recording all flaws or cracks to aid in data interpretation. Results of the analysis will not only give us insight on the modern climate, but it will also provide us with an interesting perspective on the local biology and lifecycles of the Biggenden Band Snail that has yet been documented. During my final week, I spent acquiring information on hydrological and isotopic modeling at the University of Nottingham. I focused my attention on developing an experimental design and setup to aid in model selection. Specifically, I annotated manuscripts utilizing mass balance approaches help in my understanding of model sensitivity and complexity. This prompted discussions on future sampling procedures and instrument set up that will optimize our modeling efforts. My time at BGS and the University of Nottingham has provided a fantastic opportunity to work with an interdisciplinary group of scientist. In the future, I hope continued collaboration is possible to help guide my academic growth and cementing the techniques and skills needed for coupling of both hydrologic and isotopic disciplines. As the preliminary data are continued to be analyzed, we begin to piece together the intricate past of Australia’s drought history. With any luck, the implications of our work will directly address frequency, severity, and duration of drought hazards and future climate scenarios we may face.