MULTI-TEMPORAL GEOMORPHIC EVOLUTION AND THE INFLUENCE OF CLIMATE ON A PERCHED DUNE LAKE ON NORTH STRADBROKE ISLAND, QUEENSLAND, AUSTRALIA

North Stradbroke Island (NSI), the world’s second largest sand island, is dominated by massive, vegetated, transgressive sand dunes. One of the unique features on this island is the formation of perched lakes, a water body that occurs well above the regional water table due to an impermeable layer located in the vadose zone. Brown Lake is the largest perched lake on NSI and is situated within a deflated dune hollow. After a switch from a predominantly dry climate that allowed for the deposition and migration of the massive dunes, a wetter climate promoted the growth of a thick vegetation cover that is still present today. Organic matter accumulated in the dune hollow with hill-wash and percolated downwards where it then became podzolized into an organic-rich, cemented, soil horizon. Brown Lake formed as a result of the accumulation of direct precipitation, surface runoff, and ground water input onto the underlying, relatively impermeable cemented sand layer. Little is known about how this unique hydrological feature has evolved over both recent time (~75 years) and throughout the Holocene. To address this, we have used a robust archive of aerial orthophotos dating back to 1958 to map changes in the Brown Lake surface area. The temporal resolution of this quasi-biennial dataset is improved through the use of high-resolution RapidEye (5 m) and PlanetScope (4 m) satellite imagery, which allowed for monthly measurements starting in August 2014 to present day. A grid of very-high resolution ground penetrating radar (GPR) data was acquired along the Brown Lake shoreline and water surface. Interpretation of radar reflections and facies in the GPR profiles provides insight into lake-level fluctuations that occurred well before the first aerial reconnaissance. Initial observations of the lake surface area curve show prominent peaks occurring in 1974 (52.1 ha) and 1990 (43.32 ha). Lake area has steadily declined since 2011 and reached its record low of 22.3 ha in February 2018. Stratigraphic signatures of potential lake-fill sequences are identified in the GPR profile data, indicating that the lake level may have been significantly higher than observed over the past 60 years. The modern lake level curve is compared to contemporary rainfall records and climate indices to determine a potential climate driver of lake level changes.