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

The geochemistry of Ba, Ra, Th, and U and the potential of using ²²⁶Ra/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 ²²⁶Ra concentrations and ²²⁶Ra/Ba ratios show a clear decrease with increasing age, suggesting the possibility of ²²⁶Ra dating. Limiting factors for such dating are Ba and ²²⁶Ra present in surface contaminants, and ingrowth of ²²⁶Ra from U present within the shell. Surface contamination is difficult to clean off entirely, but moderate levels of residual contamination can be corrected using ²³²Th. Sub-samples from the same shell with different proportions of contamination form a mixing line in a ²²⁶Ra/Ba-²³²Th/Ba graph, and the ²²⁶Ra/Ba of the pure shell can be derived from the intercept on the ²²⁶Ra/Ba axis. Contaminant corrected ²²⁶Ra/Ba ratios of late-Holocene ¹⁴C-dated samples fall close to that expected from simple ²²⁶Ra excess decay from seawater ²²⁶Ra/Ba values. ²²⁶Ra ingrowth from U incorporated into the shell during the lifetime of the mollusc can be corrected for. However, the unknown timing of post mortem U uptake into the shell makes a correction for ²²⁶Ra ingrowth from secondary U difficult to achieve. In the A. colbecki shells, ²²⁶Ra ingrowth from such secondary U becomes significant only when ages exceed ∼2500 yr. In younger shells, ²²⁶Ra/Ba ratios corrected for surface contamination provide chronological information. If evidence for a constant oceanic relationship between ²²⁶Ra and Ba in the ocean can be confirmed for that time scale, the ²²⁶Ra/Ba chronometer may enable the reconstruction of variability in sea surface ¹⁴C reservoir ages from mollusc shells and allow its use as a paleoceanographic tracer.