Implications of sea level variability on the formation and evolution of subtropical Rainbow Beach patterned fen complexes, Queensland, Australia

Abstract

The Great Sandy National Park [K’gari (Fraser Island) and Cooloola] contains the largest subtropical patterned fen complexes in the world. These globally significant, groundwater-dependent ecosystems have been previously studied in relatively undisturbed areas on K’gari and were suggested to be resilient to changes in hydrology, sea level and wildfires. The Rainbow Beach patterned fens are under-studied systems thought to be formed in local perched aquifers. The palaeoenvironmental conditions required for the formation and continuation of these peatlands, and how they react to changes in hydroclimate, sea level and human activities are uncertain. We attempt to resolve this ambiguity using proxies for vegetation and environmental changes over the last ~12,770 cal yr BP from a sediment core located in the Rainbow Beach patterned fen complex. We infer the formation of an aquitard layer and Empodisma minus mire development at ~12,770 cal yr BP, with conditions conducive for patterning ~12,000–10,000 cal yr BP. Paludification occurred in the early Holocene, coincident with increased sea levels, which expanded the mire inland. Increased salt marsh taxa during this period coincides with decreased E. minus values, while further peatland development occurred ~4200 cal yr BP, suggesting that marine influences greatly effect these coastal peatlands. Evidence of vegetation thickening associated with post-European fire suppression was observed. Compared to those on K’gari, the Rainbow Beach complex appears to have initiated through different processes and show greater sensitivity to changes in sea levels. Therefore, subtropical patterned fens should be assessed independently to identify individual trajectories and sensitivities.