Evaluation of shallow-water carbonates as a seawater zinc isotope archive

Abstract

Carbonate zinc (Zn) isotopes have increasingly been used to track changes in seawater Zn isotopic composition. A large body of recent work, in particular studies conducted under the GEOTRACES program, have added significantly to our knowledge of how Zn behaves in the water column. However, our understanding of how water-column Zn isotopic signals become incorporated into carbonate sediments is at a much more nascent stage. To help solve this issue, we have measured the Zn isotopic composition of modern surficial and core-top carbonate sediments and seawater from the Bahamas, Panama, and the Persian Gulf. Whereas modern Bahamian seawater has a δ66Zn of 0.14 ± 0.12‰ (2σ), Bahamian carbonates show a large range of δ66Zn—from −0.56 to 1.11‰. The δ66Zn of carbonate mud is ∼0.3‰ higher than that of the Bahamian seawater, potentially due to Zn isotopic fractionation during precipitation. Carbonate ooids and peloids and certain skeletal and microbial aragonites, including gastropods, green algae, stromatolites and precipitates associated with cyanobacterial (Dichothrix) filaments, have δ66Zn indistinguishable, within analytical error, from that of modern seawater. This suggests that these carbonate phases may be promising archives for seawater δ66Zn.