Cadmium isotopes in Bahamas platform carbonates: A base for reconstruction of past surface water bioproductivity and their link with chromium isotopes

The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phyto-plankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fraction-ation leaving surface-waters enriched in isotopically heavy Cd. This first study focusses on tying the Cd isotopic record preserved in modern shallow platform carbonates of the Great Bahama Bank (GBB) to conditions in the upper water column, and provides a base for future studies aiming at reconstructing past bioproductivity levels in ancient ocean/basin surface waters. In addition, we compare delta Cd-114 values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core sam-ples yield [Cd] (21-188 mu g/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The delta Cd-114 values of these carbonates are mainly positively fractionated with an av-erage of 0.11 parts per thousand +/- 0.17 (2 sigma; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of delta Cd-114 values by relative abundances of the carbonate polymorphs aragonite and cal-cite in the studied profile. Likewise, delta Cd-114 values are not correlated to major and trace element (e.g. Ca, Mg, Mn and Sr) contents. We postulate that the burial and diagenetic processes of carbonate cannot modify the Cd isotope sig-nals. Using the experimental fractionation factor for Cd into calcite (-0.45 parts per thousand), calculated seawater delta Cd-114 of +0.56 +/- 0.17 parts per thousand is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that delta Cd-114 values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawa-ters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox. (C) 2021 Published by Elsevier B.V.