Assessing Shallow Water Carbonates as a Record of Seawater Nickel Isotopic Composition

Nickel plays a dynamic role in modern oceans as an essential component of seven enzymes that regulate the global C, N, and O cycles1. The Ni concentration and isotopic composition (60/58NiSW) of seawater appear to have changed drastically over the course of Earths history, and these shifts likely played a role in driving global biogeochemical events such as the Great Oxidation Event2,3. Further research on Nis role in the co-evolution of Earth and life would benefit from a continuous geologic record which preserves 60/58NiSW and [Ni]SW. Here, we present the first 60/58NiSW from modern marine carbonates, to begin assessment of carbonates as a potential proxy record. We analyzed a set of well-characterized, primary to diagenetically altered samples from the Great Bahama Bank. We found a relatively homogenous Ni isotopic composition in abiotic primary carbonates (1.310.28 2sd). Interestingly, while samples that had undergone meteoric diagenesis and dolomitized samples are isotopically lighter than the primary carbonates, we found that samples which had undergone marine burial diagenesis are isotopically similar to primary precipitates. The narrow range of 60/58Ni in primary carbonates, small offset from 60/58NiSW, and small shift between primary and altered carbonates are encouraging. Marine burial diagenesis does not yet appear to isotopically alter the primary values, suggesting the potential of these samples to preserve 60/58NiSW. Additional analyses will further assess the utility of shallow water carbonates as a faithful record of seawater Ni concentration and isotopic composition and further develop Ni isotopes as a proxy for the evolution of the Ni biogeochemical cycle. 1Ragsdale, S. (2009) J. Biol. Chem. 284, 18571-18575. 2Konhauser, K.O. et al., (2015) Astrobiology. 15, 804-815. 3Wang, S.J. and Wasylenki, L. (2019) Geochim. Cosmochim. Acta. 206, 137-150.