Combined chromium and cadmium isotopes in carbonates of the Bahama Banks – a test for the bioproductivity/surface water oxygenation link

Abstract:

This study contributes to the calibration of novel stable metal isotope proxies (Cd and Cr), applied to modern and ancient sedimentary archives deposited in shallow-water carbonate platforms. Cadmium stable isotope ratios of ¹¹⁴Cd/¹¹⁰Cd (ε¹¹⁴Cd) are proposed to be a useful tracer for investigating paleo-productivity in marine settings, including biogeochemical and physical processes that impacts distribution of marine bioactive cadmium and its micronutrient-like cycling. A new study combining Cr and Cd in black shales in Nanhua Basin suggests a link to surface water oxygenation and bioproductivity [1]. In this study, we analyzed samples from Ocean Drilling Program (ODP) Leg 166 Site 1003A and ooid sediment samples from Schooner Cays of Great Bahama Bank (GBB) for their Cd isotope composition to test if the linkages from black shales are convertible to carbonate deposits. The mean ε¹¹⁴Cd value for the GBB carbonates is 1.5 ± 1.1 ranging from -0.02 to 4.46 and reflects the cadmium isotopic composition of the surface seawater from which the carbonate precipitated, if assumed the cadmium is quantitatively removed by uptake of phytoplankton. The contribution of detrital Cd in our samples was estimated using Al concentrations and calculated as <0.0008% of total Cd, which indicates Al has a minimum of impact of the authigenic ε¹¹⁴Cd data with less than 0.001. Aragonite/Calcite ratios versus [Sr] in the platform carbonate show a negative linear relationship (R² = 0.76, p-value < 6·10^-7).

Overall, our data seems to suggest a diagenetic trend in the lowermost of the section suggesting that Cr isotopes vary with changes in mineralogy that are related to burial cements and diagenesis, which however seems not to affect cadmium isotopes to such degree as Cr in marine carbonates. This study suggests a combined model for Cr and Cd only in the relatively pristine upper part of the section. By modelling of Cd isotope fractionation or incorporation into marine carbonates, i.e., precipitation from surface seawater using the offset of ε¹¹⁴Cd value -4.5 ± 1.2 defined by the fractionation during uptake in calcium carbonate [2], we found that the average surface seawater ε¹¹⁴Cd of the modern GBB is 6.0 ± 1.1, which is compatible with the previously described ε¹¹⁴Cd value in surface seawater of 6.4 ± 1.4 from other locations [3].

[1] Frei et al. (2020) Chem. Geol., 552, 119785

[2] Horner et al. (2011) EPSL, 312(1), 243-253

[3] Ripperger et al. (2007) EPSL, 261(3), 670-684