Ba, Ra, Th, and U in marine mollusc shells and the potential of Ra-226/Ba dating of Holocene marine carbonate shells

摘要

The geochemistry of Ba, Ra, Th, and U and the potential of using 226 Ra[Ba ratios as an alternative dating method are explored in modern and Holocene marine mollusc shells. Five modern shells of the Antarctic scallop Adamussium colbecki collected from the present day beach and six radiocarbon dated specimens from Holocene beach terraces of the Ross Sea region (Antarctic) between 700 and 6100 calibrated yr BP old have been analysed by mass spectrometry. In clean shells Ra-226 concentrations and Ra-226/Ba ratios show a clear decrease with increasing age, suggesting the possibility of Ra-226 dating. Limiting factors for such dating are Ba and Ra-226 present in surface contaminants, and ingrowth of Ra-226 from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using Th-232. Sub-samples from the same shell with different proportions of contamination form a mixing line in a Ra-226/(BaTh)-Th-232/Ba graph, and the Ra-226/Ba of the pure shell can be derived from the intercept on the Ra-226/Ba axis. Contaminant corrected Ra-226/Ba ratios of late-Holocene C-14-dated samples fall close to that expected from simple Ra-226 excess decay from seawater Ra-226/Ba values. Ra-226 ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of postmortem U uptake into the shell makes a correction for Ra-226 ingrowth from secondary U Ra-226 difficult to achieve. In the A. colbecki shells, Ra ingrowth from such secondary U becomes significant only Ra-226 when ages exceed similar to2500 yr. In younger shells, Ra-226/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between Ra-226 and Ba in the ocean can be confirmed for that time scale, the Ra-226/Ba chronometer may enable the reconstruction of variability in sea surface C-14 reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer. Copyright (C) 2004 Elsevier Ltd. [References: 36]