STRATIGRAPHIC SIGNATURES OF LAKE-LEVEL CHANGES AT SWALLOW LAGOON, NORTH STRADBROKE ISLAND, AUSTRALIA

Lakes have a great capacity to hold records of climate archived in various sedimentary proxies. Coring is the primary method used to access the sedimentary archive layers to assess paleoclimates. These methods rely on proxies related to microfossils, macrofossils, isotopic composition and sedimentary changes that relate to specific environmental conditions. The use of proxies is constrained by preservation potential and ability of proxies to record environmental changes of interest. However, the recorded changes rarely reflect lake levels. Imaging the architecture of lake fill sediments provides a direct measurement of lake levels through the lifespan of the basin. In small, shallow lakes, ground penetrating radar (GPR) has been utilized to assess sediment thickness and paleoshorelines related former lake surfaces. While GPR alone can not answer the questions related to changes in paleoclimate, adding stratigraphic context to core locations and direct observation of lake levels can be a crucial component of understanding.

North Stradbroke Island, the second largest sand island in the world, hosts former dune complexes that have been proposed to exceed 800 kybp. The sedimentary architecture, former soil horizons and well sorted sands provide the necessary conditions for lacustrine environments coupled with the local climate in the subtropics which fosters stabilization of aeolian systems and persistence of lakes. Numerous lakes and wetlands on the island have been studied using the coring and proxy methods for reconstructing paleoclimate.

One lake, Swallow Lagoon, was recently cored and provided a 4.8 m continuous record. A sample at the base of the core was dated to 11.5 kybp. Swallow Lagoon in a small, roughly 3800 m² with a depth of > 2 m during the time of survey. Ongoing research is examining the sedimentary records for environmental proxies. To supplement these records, a series of high-resolution GPR profiles were acquired along the shore and over the lake. The data showed a lake-fill sequence in excess of the 4.8 m core collected. Changes to lake levels were observed as paleoshoreline complexes above and below the current level. Additionally, high-intensity reflections, interpreted as sand rich layers might show evidence of dune field reactivation or periods of drought with very low lake levels.