The effect of shell secretion rate on Mg / Ca and Sr / Ca ratios in biogenic calcite as observed in a belemnite rostrum

Isotopic ratios and concentrations of the alkaline earth metals Mg and Sr in biogenic calcite are of great importance as proxies for environmental parameters. In particular, the Mg&thinsp;/&thinsp;Ca ratio as a temperature proxy has had considerable success. It is often hard to determine, however, which parameter ultimately controls the concentration of these elements in calcite.<br><br> Here, multiple Mg&thinsp;/&thinsp;Ca and Sr&thinsp;/&thinsp;Ca transects through a belemnite rostrum of <i>Passaloteuthis bisulcata</i> (Blainville, 1827) are used to isolate the effect of calcite secretion rate on incorporation of Mg and Sr into the calcite. With increasing calcite secretion rate Mg&thinsp;/&thinsp;Ca ratios decrease and Sr&thinsp;/&thinsp;Ca ratios in the rostrum increase. In the studied specimen this effect is found to be linear for both element ratios over a calcite secretion rate increase of ca. 150 %. Mg&thinsp;/&thinsp;Ca ratios and Sr&thinsp;/&thinsp;Ca ratios show a linear co-variation with increasing relative growth rate, where a 100 % increase in growth rate leads to a (8.1 ± 0.9) % depletion in Mg and a (5.9 ± 0.7) % enrichment in Sr. The magnitude of the calcite secretion rate effect on Mg is (37 ± 4) % greater than that on Sr. These findings are qualitatively confirmed by a geochemical transect through a second rostrum of <i>Passaloteuthis</i> sp.<br><br> Growth rate effects are well defined in rostra of <i>Passaloteuthis</i>, but only account for a minor part of chemical heterogeneity. Biasing effects on palaeoenvironmental studies can be minimized by informed sampling, whereby the apex and apical line of the rostrum are avoided