Cold-water corals as archives of seawater Zn and Cu isotopes

Abstract

Traditional carbonate sedimentary archives have proven challenging to exploit for Zn and Cu isotopes, due to the high concentrations of trace metals in potential contaminants (e.g., Fe Mn coatings) and their low concentrations in carbonate. Here, we present the first dataset of δ⁶⁶Zn_(JMC-Lyon) and δ⁶⁵Cu_(SRM 976) values for cold-water corals and address their potential as a seawater archive. Extensive cleaning experiments carried out on two corals with well-developed Fe Mn rich coatings demonstrate that thorough physical and chemical cleaning can effectively remove detrital and authigenic contaminants. Next, we present metal/Ca ratios and δ⁶⁶Zn and δ⁶⁵Cu values for a geographically diverse sample set of Holocene age cold-water corals. Comparing cold-water coral δ⁶⁶Zn values to estimated ambient seawater δ⁶⁶Zn values (where Δ⁶⁶Zn_(coral-sw) = δ⁶⁶Zn_(coral) – δ⁶⁶Zn_(seawater), we find Δ⁶⁶Zn_(coral-sw) = +0.03 ± 0.17‰ (1SD, n = 20). Hence, to a first order, cold-water corals record seawater Zn isotope compositions without fractionation. The average Holocene coral Cu isotope composition is +0.59 ± 0.23‰ (1SD, n = 15), similar to the mean of published deep seawater δ⁶⁵Cu values at +0.66 ± 0.09‰, but with considerable variability. Finally, δ⁶⁶Zn and δ⁶⁵Cu data are presented for a small subset of four glacial-age corals. These values overlap with the respective Holocene coral datasets, hinting at limited glacial-interglacial changes in oceanic Zn and Cu cycling.