LASTING IMPRESSIONS OF CLIMATE FLUCTUATIONS AT BROWN LAKE (BUMMIERA), NORTH STRADBROKE ISLAND (MINJERRIBAH), AUSTRALIA

The sand islands off the coast of Australia are home to dune complexes exceeding ~800 kya. During times of lower sea level (~60-80 m), exposed shorelines and strong persistent winds created dune formations via aeolian deposition. The world’s second largest sand island, North Stradbroke Island (Minjerribah), is a noteworthy example to study paleoclimate because of the plethora of wetland systems. North Stradbroke Island (NSI) is located on the eastern boundary of Moreton Bay and hosts large, overlapping parabolic dunes. The sediment structure, former soil horizons, and well sorted sands –– coupled with a subtropical climate –– houses the necessary conditions for lacustrine environments to persist. Analyzing micro/macro fossil sequences from wetland sediment cores yields proxies for water availability, temperature, and the ecology of a specific area. However, proxies obtained from sediment cores are poor depictions of water level changes. The inclusion of ground penetrating radar (GPR) data is a useful addition to the stratigraphic context obtained from wetland sediment cores. The imagery provides direct evidence of past water level changes.

GPR has proven successful in evaluating paleoshoreline sequences of small, shallow lakes. Perched lake systems are unusual features of NSI and other sand islands. They are situated well above sea level and the regional water table and are characterized by shallow depths, small areas, and fluctuating basin size. Perched lake systems can dry during prolonged periods of dry climate. The largest perched lake on NSI is Brown Lake (Bummeria), which sits about ~25m above the regional water table.

On-going research has surveyed over 60 km of GPR data on NSI. At Brown Lake (Bummiera), two GPR frequencies 80 and 160 MHz have been used over water and land for a broader basin context. The major aims of this study are to map the extent of Brown Lake (Bummiera) to try and answer whether it has the capability to desiccate and identify past lake levels using geophysics and sequence stratigraphy. This will be done by connecting the GPR data, dated sediment cores, and known OSL dates. There is considerable concern over falling water levels at Brown Lake (Bummiera) during recent droughts. This study will provide better understanding of past water level changes and the interactions of Brown Lake (Bummiera) to the perched groundwater system that supports it.