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

Clemens Vinzenz Ullmann,Philip A E Pogge Von Strandmann

doi:10.5194/bg-14-89-2017

Clemens Vinzenz Ullmann, Philip A E Pogge Von Strandmann

Open Access

https://doi.org/10.5194/bg-14-89-2017

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Abstract

Abstract. 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.

Highlights

The measurement of Mg and Sr concentrations in biogenic calcite as records of past environmental conditions and geochemical cycles has a long tradition (e.g. Odum, 1951; Turekian, 1955, Lowenstam, 1961)
While a positive co-variation of Mg / Ca ratios with ambient temperature in biogenic calcite agrees with experimental data from inorganic calcite precipitation experiments, Sr / Ca ratios in shell calcite that are positively linked with temperature (e.g. Lea et al, 1999; Stoll et al, 2002; Ullmann et al, 2013a) contradict an expected negative correla
Element / Ca ratios were screened for diagenesis using Mn / Ca ratios and only data from wellpreserved samples are further considered

Summary

Introduction

The measurement of Mg and Sr concentrations in biogenic calcite as records of past environmental conditions and geochemical cycles has a long tradition (e.g. Odum, 1951; Turekian, 1955, Lowenstam, 1961). A number of empirical studies have documented positive co-variation of Mg / Ca ratios and/or Sr / Ca ratios with temperature in biogenic calcite from belemnites (Rosales et al, 2004; McArthur et al, 2007; Li et al, 2012), bivalves (Klein et al, 1996; Freitas et al, 2006; Wanamaker et al, 2008; Tynan et al, 2017), brachiopods (Brand et al, 2013; Butler et al, 2015), coccoliths (Stoll et al, 2001), echinoderms (Pilkey and Hower, 1960), foraminifers (Lea et al, 1999; Elderfield and Ganssen, 2000; Lear et al, 2002; de Nooijer et al, 2014) and ostracods (Chivas et al, 1986; De Dekker et al, 1999). Pogge von Strandmann: Mg / Ca and Sr / Ca ratios in biogenic calcite tion of these two parameters (Kinsman and Holland, 1969; Rimstidt et al, 1998)

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