Insights into Carbonate Formation through the Incorporation of Trace Metals into Ooids

Abstract:

Trace metal ratios are widely used as paleoclimate proxies for past ocean conditions. In particular Mg/Ca and Sr/Ca ratios in biogenic carbonates have been used as paleothermometers. Of course the use of these trace metal ratios as reliable climate proxies does not come without complications. As well as biologically mediated “vital effects”, there have also been other secondary controls on trace metal incorporation reported, including salinity, carbonate ion concentration and growth rate. Within this study a range of trace metal ratios and their isotopes have been measured for modern ooid samples forming under a range of environmental conditions. Since ooids are thought to form through inorganic precipitation (although microbial mediation may play a role), the “vital effects” seen in biogenic carbonates should be minimised or absent. Therefore, ooids should be expected to incorporate trace metals similarly to carbonate precipitated in experimental studies. Through studying modern ooids we can test this hypothesis, as well as looking at the factors that affect the incorporation of trace metals into calcium carbonates without the control of typical “vital effects”.The sample set includes both pure aragonite and pure calcite ooids, as well as samples with intermediate mineralogy as determined by in situ Raman spectroscopy. The distribution coefficients for purely aragonite or calcite ooids are offset from the reported inorganic precipitate values, with DSr being larger, while DMg has been found to be lower. The incorporation of Mg and Sr across the sample set is inversely correlated and does not seem to be explained by mineralogy alone. Here we explore alternative secondary factors contributing to the incorporation of these trace elements into oolitic carbonate. This will be accomplished by utilising stable Sr isotope fractionation during incorporation and using kinetic models and distribution coefficients to investigate the controls on Mg and Sr partitioning into calcium carbonate.