Ostracods are ubiquitous micro-organisms, found in marine, estuarine, continental and hyper-saline waters. Identification of ostracodal assemblages, indicative of ecological facies, allows the palaeoenvironmental reconstruction of localities that have experienced changing aquatic conditions. This study discusses ostracod valves that have been extracted from core material from the Gulf of Carpentaria, Australia. Interpretation of palaeoenvironments is determined by species identification and facies delineation. Special attention has been given to dissolution and other diagenetic affects.
The Gulf of Carpentaria is an epicontinental sea (max. depth 70m) between Australia and Papua New Guinea, bordered to the east by Torres Strait (currently 12m deep) and to the west by the Arafura Sill (53m below present sea level). Throughout the Quaternary, during times of low sea-level, the Gulf has been separated from the open waters of the Indian and Pacific Oceans, forming Lake Carpentaria, perched above contemporaneous sea-level with outlet channels to the Arafura Sea. During 1997, six sediment cores were collected by the IMAGES III program from the Gulf of Carpentaria. The longest of these cores, MD972132, reaching a depth of 14.84m, was collected near the deepest part of the modern Gulf and is presented here.
Prior to the establishment of Recent marine conditions, the core may be broadly divided into two sections; a lower marine and an upper non-marine phase, The base of the core has returned dates of around 125ka by both thermal- and optically stimulated luminescence techniques (Chivas et al., in press). These dates were obtained from a barren quartzose unit, with evidence of subaerial exposure, overlain by a shallow-marine facies. This sequence suggests the sea-level rise associated with the Last Interglacial. The marine phase consists of assemblages varying from restricted estuarine to open shallow marine indicating oscillations of sea-level about the Arafura Sill.
The non-marine phase, first evidenced by a lake-shore facies at a
core depth of 920cm is predominantly composed of shallow, saline
lacustrine taxa. Drying of the lake is apparent, with iron-mottled
quartz, calcareous nodules and ostracods showing dissolution between
680-560cm. Saline lacustrine conditions return, briefly interrupted
by another incursion of marine waters at 490-380cm. The onset of the
south-east Asian monsoon is implicated by the freshening of lake
waters reaching a maximum just prior to the most recent marine
transgression, dated at around 9.7ka (conventional radiocarbon
years)(Chivas et al, in press).
Chivas, A.R., Garcia, A., van der Kaars, S., Couapel, M.J.J., Holt, S., Reeves, J.M., Wheeler, D.J., Switzer, A.D., Murray-Wallace, C.V., Banerjee, D., Wang, S.X., Pearson, G., Edgar, N.T., Beaufort, L., De Deckker, P., Lawson, E. and Cecil, C.B., (in press). Sea-level and environmental changes since the Last Interglacial in the Gulf of Carpentaria, Australia: An overview. Quaternary International.