Strontium isotope variations in single ostracod valves from the Gulf of Carpentaria, Australia: A palaeoenvironmental indicator

Abstract

The Gulf of Carpentaria is a shallow (<70 m), epicontinental sea which was partially and, at times, fully isolated from the ocean, due to changing sea-levels in the Late Quaternary. In an attempt to elucidate the palaeoenvironment of this marine-lacustrine system and hence constrain the rates of sea-level changes, high-precision measurements (±0.0036%) of 87Sr86Sr ratios are reported for individual fossil ostracod valves. These valves were extracted from two piston cores taken from the deepest part of the Gulf of Carpentaria. Single valves weighing 20 μg–40 μg and containing a total of 10−8 g to 2 · 10−8 g of Sr have been analysed. At the bottom of the main core, in a depth interval equivalent to an age of ~36,000 to 40,000 years, valves from the ostracod species Cyprideis have 87Sr86Sr ratios of from 0.70955 ± 2 to 0.70967 ± 2 (ΔSr = 6.1 to 7.9). From ∼ 26,000 years to 22,000 years slightly higher but relatively constant 87Sr86Sr ratios of from 0.70998 ± 2 to 0.71016 ± 2 (ΔSr = 12.2 to 14.9) are present. The highest Sr ratios are found in ostracod valves of ∼ 13,000 years age, which have values from 0.71034 ± 2 to 0.71039 ± 2 (ΔSr = 17.2 to 18.0). All these values are significantly greater than that of contemporary seawater and are indicative of continental sources with high 87Sr86Sr ratios. Significantly lower 87Sr86Sr ratios of from 0.70918 ± 2 to 0.70961 ± 2 (ΔSr from 0.8 to 7.0) are found in ostracods younger than 12,000 years, and ratios identical to modern seawater (87Sr86Sr = 0.70912 ± 2, ΔSr = 0.0) are present in the upper 2 cm of core which is <5,000 years in age. The variations in 87Sr86Sr ratios in ostracod valves from the Carpentaria basin are consistent with a lacustrine environment from ∼ 36,000 years to 12,000 years, followed by a transgressive marine sequence which commenced in the Gulf of Carpentaria ∼ 12,000 years ago. This marine transgression was a result of sea-level rising above the ∼ −53 m (relative to present-day sea-level) Arafura Sill. Sr isotope ratios identical to that of modern seawater were, however, only obtained when the −12 m Torres Strait Sill was opened to the Coral Sea, which occurred ∼ 8,000 to ∼ 7,000 years ago. The marked decrease in Sr isotopic ratios at the bottom of the core may be due to flow of the Fly River (from New Guinea) prior to its diversion into the Coral Sea, or more probably a transitory input of marine water into the Carpentaria basin during a relatively high sea-level ∼ 40,000 years ago. It is suggested that the highest ASr values present ∼ 13,000 years ago reflect increased rainfall in the Gulf area of northern Australia, which is attributed to a southwards migration of the monsoonal rainfall pattern at that time. In contrast to some other chemical indicators of palaeoenvironments, the ASr results of this study provide definitive and unambiguous resolution of marine versus non-marine environments and can thus be used to resolve with great accuracy transgressive/regressive sequences.