Major and trace element concentrations and Ni isotope compositions of carbonates from the Great Bahama Bank and carbonate diagenesis model parameters
Description
We aimed to evaluate the fidelity of shallow-water carbonates as records of the Ni isotope composition of surface seawater. We first must know how Ni isotopes initially partition upon incorporation into carbonates and how early marine diagenesis (i.e., meteoric or marine diagenesis, dolomitization) alters the primary Ni signature. We measured the Ni isotope compositions and major and trace element concentrations of primary and diagenetically altered shallow-water carbonates from the Great Bahama Bank, a modern carbonate platform. The carbonates representing primary deposition are from short cores (collection described in Hardisty et al. 2017) and carbonates representing different types of diagenesis are from ODP cores, Clino and Unda (collection described in Eberli et al. 1997). Major and trace element concentrations were measured using ICP-MS. Ni isotope compositions were measured using either a Neptune or Nu III MC-ICP-MS, and instrumental mass fractionation was corrected using the double-spike technique as described in Siebert et al. (2001). We also modeled a simple diagenetic process (aragonite to calcite neomorphism under an open system), following Banner and Hanson (1990), to determine how the Ni isotope composition of diagenetically altered carbonate changes with diagenetic indicators (e.g., C isotope compositions). Model parameters are provided. We found that primary carbonates are fractionated from coexisting seawater (1.7‰, based on similar latitude and similar depth seawater; Lemaitre et al. 2022) by ~0.4‰. Only carbonates that experienced marine diagenesis, either with minimal alteration or alteration under sediment-buffered conditions, appear to preserve the primary carbonate Ni isotope signature. Our modeling supports this interpretation. Such carbonates hold promise as records of the Ni isotope composition of seawater.