noSmall-scale heterogeneity of biogenic carbonate\ud elemental composition can be a significant source of error in\ud the accurate use of element/Ca ratios as geochemical proxies.\ud In this study ion microprobe (SIMS) profiles showed significant\ud small-scale variability of Mg/Ca, Sr/Ca and Mn/Ca\ud ratios in new shell calcite of the marine bivalves Pecten\ud maximus and Mytilus edulis that was precipitated during a\ud constant-temperature culturing experiment. Elevated Mg/Ca,\ud Sr/Ca and Mn/Ca ratios were found to be associated with the\ud deposition of elaborate shell features, i.e. a shell surface stria\ud in P. maximus and surface shell disturbance marks in both\ud species, the latter a common occurrence in bivalve shells. In\ud both species the observed small-scale elemental heterogeneity\ud most likely was not controlled by variable transport of\ud ions to the extra-pallial fluid, but by factors such as the influence\ud of shell organic content and/or crystal size and orientation,\ud the latter reflecting conditions at the shell crystalsolution\ud interface. In the mid and innermost regions of the\ud P. maximus shell the lack of significant small-scale variation\ud of Mg/Ca ratios, which is consistent with growth at constant\ud temperature, suggest a potential application as a palaeotemperature\ud proxy. Cross-growth band element/Ca ratio profiles\ud in the interior of bivalve shells may provide more promising\ud palaeo-environmental tools than sampling from the outer\ud region of bivalve shells
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