Unravelling patterns of reworking in lowstand shelf deposits using amino acid racemisation and radiocarbon dating

Abstract

Amino acid racemisation calibrated by radiocarbon dating (liquid scintillation and accelerator mass spectrometry) has provided a chronostratigraphic framework in which to evaluate the later Quaternary evolution of mollusc-rich carbonate sediments on the outer continental shelf of New South Wales, Australia. Three vibracores (Cores 112/VC/130, 112/VC/121 and 112/VC/134) from the outer continental shelf in present water depths of 123, 139 and 150 m, respectively, each contain alternating successions of pedogenically-unmodified, fine-grained mixed quartz-carbonate sand, and densely-packed mollusc-dominated sediments, set in a matrix of carbonate sand. The extent of racemisation for several amino acids and radiocarbon dates on the shallow-water molluscs Pecten fumatus and Placamen placidium reveals the presence of sediments deposited during three successive glacial maxima (i.e. Stages 8, 6, and 2 of the Marine Oxygen Isotope record). Three chronostratigraphically distinct aminozones (Oxygen Isotope Stages 8, 6 and 2) are identified in Core 134. Samples in Cores 121 and 130 record only two aminozones collectively (Stages 6 and 2). Amino acid racemisation indicates that fossil molluscs with ‘background’ radiocarbon results are in fact significantly beyond the range of radiocarbon dating, by analogy with other deposits from southern Australia. A thin veneer of early Holocene fine calcareous sand occurs at the top of each core. All three cores reveal varying degrees of reworking of fossil molluscs, which is not immediately apparent on the basis of lithostratigraphy or biostratigraphy. Reworking of macrofossils is more pronounced closer to the inferred location of the palaeoshoreline of Pleistocene glacial maxima.