Abstract
Abstract. Small-scale heterogeneity of biogenic carbonate elemental composition can be a significant source of error in the accurate use of element/Ca ratios as geochemical proxies. In this study ion microprobe (SIMS) profiles showed significant small-scale variability of Mg/Ca, Sr/Ca and Mn/Ca ratios in new shell calcite of the marine bivalves Pecten maximus and Mytilus edulis that was precipitated during a constant-temperature culturing experiment. Elevated Mg/Ca, Sr/Ca and Mn/Ca ratios were found to be associated with the deposition of elaborate shell features, i.e. a shell surface stria in P. maximus and surface shell disturbance marks in both species, the latter a common occurrence in bivalve shells. In both species the observed small-scale elemental heterogeneity most likely was not controlled by variable transport of ions to the extra-pallial fluid, but by factors such as the influence of shell organic content and/or crystal size and orientation, the latter reflecting conditions at the shell crystal-solution interface. In the mid and innermost regions of the P. maximus shell the lack of significant small-scale variation of Mg/Ca ratios, which is consistent with growth at constant temperature, suggest a potential application as a palaeotemperature proxy. Cross-growth band element/Ca ratio profiles in the interior of bivalve shells may provide more promising palaeo-environmental tools than sampling from the outer region of bivalve shells.
Highlights
The elemental composition of marine biogenic carbonates has been thought to provide a powerful tool to obtain information on Earth’s past climate and oceanographic conditions
6 Small-scale element heterogeneity and implications for the use of geochemical proxies in bivalves. It is clear from the ion microprobe elemental data collected in this study that, for both bivalve species investigated highly variable Mg/Ca, Sr/Ca and Mn/Ca ratios can occur within one structural layer of shell calcite precipitated at a single and constant seawater temperature and via deposition from the same marginal extra-pallial fluid (EPF), albeit a variably non-isolated EPF in P. maximus
Significant heterogeneity in Mg/Ca, Sr/Ca and Mn/Ca ratios within the new growth of shells of P. maximus and M. edulis deposited at a constant temperature of 20◦C has been determined using secondary ionisation mass spectrometry (SIMS)
Summary
The elemental composition of marine biogenic carbonates has been thought to provide a powerful tool to obtain information on Earth’s past climate and oceanographic conditions. In an initial preliminary assessment of the extent of any small-scale heterogeneity in Mg/Ca, Sr/Ca and Mn/Ca ratios in bivalve shell calcite, new shell material precipitated by Pecten maximus (king scallop) and Mytilus edulis (blue mussel) in a constant-temperature laboratory culturing experiment, during which other seawater parameters were monitored, has been analysed using the ion microprobe (SIMS) technique These two bivalve species, as well as closely related taxa, have been proposed previously as archives for palaeoceanographic studies (Krantz et al, 1988; Klein et al, 1996b; Hickson et al, 1999; Chauvaud et al, 2005; Gillikin et al, 2006; Thebault et al, 2007; Wanamaker et al, 2007) and are suitable materials for such an investigation of the extent of spatial variability in bivalve shell elemental concentrations. This methodological approach is especially valid for Mg/Ca ratios in bivalve calcite, which should be invariant within the new shell growth if a simple thermodynamic influence is the predominant control on this geochemical proxy in these archival materials, and may provide further insights into explaining the reported limited temperature control and large variability of bivalve calcite Mg/Ca ratios (Dodd, 1965; Klein et al, 1996b; Vander Putten et al, 2000; Lorrain et al, 2005; Freitas et al, 2006, 2008)
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