RESUMO
The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phytoplankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fractionation 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 δ114Cd values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core samples yield [Cd] (21-188 µg/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The δ114Cd values of these carbonates are mainly positively fractionated with an average of 0.11 ± 0.17 (2σ; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of δ114Cd values by relative abundances of the carbonate polymorphs aragonite and calcite in the studied profile. Likewise, δ114Cd 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 signals. Using the experimental fractionation factor for Cd into calcite (-0.45), calculated seawater δ114Cd of +0.56 ± 0.17 is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that δ114Cd values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawaters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox.
Assuntos
Cádmio , Água , Bahamas , Cádmio/análise , Isótopos de Carbono , Carbonatos , Isótopos do Cromo/análise , IsótoposRESUMO
Great Bahama Bank (GBB) is the principal location of the formation and accumulation of ooids (concentrically coated, sand-size carbonate grains) in the world today, and as such has been the focus of studies on all aspects of ooids for more than half a century. Our view from a close look at this vast body of literature coupled with our continuing interests stresses that biological mechanisms (microbially mediated organomineralization) are very important in the formation of ooids, whereas the controlling factor for the distribution and size of ooid sand bodies is the physical energy. Mapping and coring studies of the modern ooid sand bodies on GBB provide insight into the rock record from different perspectives. An important consequence of the dual influence of ooid formation and distribution is that the geochemical signature of ooids is not in equilibrium with the seawater in which ooids form; therefore, extracting the paleophysical energy record from oolitic deposits is potentially more accurate than doing so for the paleochemical record.