Palaeoenvironmental change in the Gulf of Carpentaria (Australia) since the last interglacial based on Ostracoda

Abstract

Throughout the last glacial cycle, the region between Australia and New Guinea, which is today known as the Gulf of Carpentaria, has oscillated from open shallow marine conditions to a large, land-locked freshwater lake, including periods of subaerial exposure. Ostracod faunal assemblages and variations in the valve morphology, preserved in the sediments of the gulf, record these changes. A 14.8 m long core (MD972132) extracted from near the centre of the modern Gulf of Carpentaria, spanning the last 130 ka BP provides the basis of this study. Environmental facies were determined by R- and Q-mode cluster analysis of the ostracod assemblage data, including 72 species, and comparison with modern analogues from both the Gulf of Carpentaria and Southeast Asian region. Using these methods, six clearly distinct biofacies were identified:(i)open shallow marine facies, including bairdiids, pectocytherinids and cytherettids;(ii)shallow marine facies, dominated by Cytherella and Hemikrithe;(iii)marginal marine facies, including Xestoleberis and Praemunita;(iv)tidal channel facies, dominated by Loxoconcha;(v)estuarine assemblage, comprising Venericythere and Leptocythere;(vi)non-marine faciesa.brackish lagoon/lake facies dominated by Cyprideis and Leptocythere;b.freshwater facies, including Ilyocypris, with Cyprinotus and Cypretta. In addition, morphological variation of valves of the more "plastic" species, in particular Cyprideis australiensis and Leptocythere hartmanni, was observed and related to environmental variables, including salinity and solute composition. These two species occur through more than half of the length of the core and show significant variability. C. australiensis with irregular-shaped sieve pores is associated with heavily reticulated valves of L. hartmanni, around 90-70 ka BP, indicating increased salinity and carbonate-limited environments. Conversely, C. australiensis with round sieve pores occurs with finely reticulated valves of L. hartmanni from around 50 ka BP, in fresher, carbonate-rich environments. The preservation of valves was also noted so as to infer post-depositional effects, revealing shoreline features, channel activity and subaerial exposure. Combined ostracod biofacies and morphological analysis reveals a fluctuating marine environment through Marine Isotope Stage (MIS) 5, terminating in a restricted lagoon around 90 ka BP, followed by an extensive period of subaerial exposure in this region of the gulf. MIS 3 marks a return of marine conditions and an increase in fluvial activity. Lake Carpentaria then appears as a permanent feature, at least to the current 60 m depth contour, from around 40 ka BP, with the freshest water occurring around 16 cal. ka BP, prior to the most recent marine transgression, at 10.8 cal. ka BP. Although the presence of Lake Carpentaria had previously been identified, this study extends the knowledge of the region through the last glacial cycle and provides greater detail of the sequential biofacies.