The effect of shell secretion rate on Mg / Ca and Sr / Ca ratios in biogenic calcite as observed in a belemnite rostrum
Pogge von Strandmann, PAE
European Geosciences Union (EGU) / Copernicus Publications
Copyright © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License.
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 determine, 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 secretion rate on incorporation of Mg and Sr into the calcite. With increasing calcite secretion 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 calcite secretion rate increase of ca. 150 %. Mg / Ca ratios and Sr / 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 Passaloteuthis sp. Growth rate effects are well defined in rostra of Passaloteuthis, 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.
Analyses and Philip Pogge von Strandmann were funded by NERC research fellowship grant NE/I020571/2. Clemens Ullmann acknowledges funding from the Leopoldina – German National Academy of Sciences (grant no. LPDS 2014-08). Kate Littler is thanked for comments on an earlier version of this paper. The authors thank the Associate Editor David Gillikin, Adrian Immenhauser and one anonymous reviewer for constructive comments that helped to significantly improve the quality of the manuscript.
This is the final version of the article. Available from European Geosciences Union via the DOI in this record.
Vol. 14, pp. 89 - 97