This is the author's manuscript - under review for journal Biogeosciences and available online from the publisher via the DOI in this record. ; 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/Ca ratio as a temperature proxy has had considerable success. It is often hard to constrain, however, which parameter ultimately controls the concentration of these elements in calcite. Here, multiple Mg/Ca and Sr/Ca transects through a belemnite rostrum of Passaloteuthis bisulcata (Blainville, 1827) are used to isolate the effect of calcite precipitation rate on incorporation of Mg and Sr into the calcite. With increasing calcite precipitation rate Mg/Ca ratios decrease and Sr/Ca ratios in the rostrum increase. In the studied specimen this effect is found to be linear for both element ratios over a precipitation rate increase of ca. 150 %. Mg/Ca ratios and Sr/Ca ratios show a linear covariation with increasing relative precipitation rate, where a 100 % increase in precipitation rate leads to a (8.1 ± 0.9) % depletion in Mg and a (5.9 ± 0.7) % enrichment in Sr. The magnitude of the precipitation rate effect on Mg is (37 ± 4) % greater than that on Sr. Precipitation rate effects are well-defined in the rostrum of Passaloteuthis bisulcata but only account for a minor part of chemical heterogeneity. Biasing effects on palaeoenvironmental studies can be minimised by informed sampling, whereby the apex and apical line of the rostrum are avoided.