Ferromanganese crusts as archives of deep water Cd isotope compositions

The geochemistry of Cd in seawater has attracted significant attention owing to the nutrient‐like properties of this element. Recent culturing studies have demonstrated that Cd is a biologically important trace metal that plays a role in the sequestration of inorganic carbon. This conclusion is supported by recent isotope data for Cd dissolved in seawater and incorporated in cultured phytoplankton. These results show that plankton features isotopically light Cd while Cd‐depleted surface waters typically exhibit complimentary heavy Cd isotope compositions. Seawater samples from below 900 m depth display a uniform and intermediate isotope composition of ε 114/110 Cd = +3.3 ± 0.5. This study investigates whether ferromanganese (Fe‐Mn) crusts are robust archives of deep water Cd isotope compositions. To this end, Cd isotope data were obtained for the recent growth surfaces of 15 Fe‐Mn crusts from the Atlantic, Pacific, Indian, and Southern oceans and two USGS Fe‐Mn reference nodules using double spike multiple collector inductively coupled plasma mass spectrometry. The Fe‐Mn crusts yield a mean ε 114/110 Cd of +3.2 ± 0.4 (2 SE, n = 14). Data for all but one of the samples are identical, within the analytical uncertainty of ±1.1ε 114/110 Cd (2 SD), to the mean deep water Cd isotope value. This indicates that Fe‐Mn crusts record seawater Cd isotope compositions without significant isotope fractionation. A single sample from the Southern Ocean exhibits a light Cd isotope composition of ε 114/110 Cd = 0.2 ± 1.1. The origin of this signature is unclear, but it may reflect variations in deep water Cd isotope compositions related to differences in surface water Cd utilization or long‐term changes in seawater ε 114/110 Cd. The results suggest that time series analyses of Fe‐Mn crusts may be utilized to study changes in marine Cd utilization.