Year

2011

Degree Name

Master of Science - Research

Department

School of Earth and Environmental Sciences

Abstract

The study presents a continuous palaeoclimatic record for northern Australia covering the period ~ 60-14 ka BP. Novel geochemical techniques are applied to more than 500 fossil ostracods extracted from the well established lacustrine sequence of the Gulf of Carpentaria. Located between Papua New Guinea and Australia, the Gulf of Carpentaria was disconnected from both Pacific and Indian Ocean when global sea level was 53 m (or lower) below present sea level with sedimentological and geochemical evidence suggesting the dominance of a lacustrine environment after MIS 4. During this time period, the lake extent underwent wide oscillation from a maximal covered area of 190 000 km2 to totally empty with periods of subaerial exposure in the centre of the basin. Although the environment of the gulf has been previously characterized for the last glacial cycle, palaeoclimatic inferences using the lake sequence have remained challenging due to episodes of marine water inflow and strong seasonality.

Methodological and analytical developments were used to measure Na/Ca, Mg/Ca, Sr/Ca, Ba/Ca, Mn/Ca, Fe/Ca, U/Ca ratios and δ18O, δ13C values on individual ostracod valves of typical weights 40-60μg. Inferences about the palaeolake water-level and salinity are drawn from the ostracod element-to-calcium ratios and stable-isotope values. The chemical evolution of the palaeowater, inferred from the ostracod relative concentrations Na/Mg/Sr/Ba, is compared to the chemical signature of the rivers draining the modern Carpentaria basin. The combined use of these techniques allows precise reconstruction of the basin‟s hydrological history.

Wide climatic variations were recorded by the ostracod chemical and isotopic composition and reflect the changing state of the Australian monsoon through time. The monsoon appears weaker/absent during the Last Glacial Maximum (LGM), although irregular precipitation patterns during this time provoked the oscillations of the palaeolake water-level. Immediately after the LGM, the monsoon progressively developed over northern Australia until 15 ka BP. This causes the rivers from the far south of the basin to reconnect with the palaeolake.

Finally, this work provides new tools for future studies using ostracod shell chemistry as proxy of environmental changes.

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