The effect of precipitation rate on Mg/Ca and Sr/Ca ratios in biogenic calcite as observed in a belemnite rostrum
Ullmann, CV
Date: 2016
Journal
Biogeosciences Discussions
Publisher
Published by Copernicus Publications on behalf of the European Geosciences Union.
Publisher DOI
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 constrain, however, which parameter ultimately controls the concentration ...
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.
Camborne School of Mines
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