Zn and Ni Isotope Systematics in the Black Sea, an Analogue for Past Ocean Anoxia

Abstract:

Organic rich sediments, indicators of low bottom water oxygen, are often enriched in trace metals. Their concentrations in such sediments are thus used to probe past Earth surface conditions. Enrichment mechanisms are difficult to distinguish from concentrations alone, however. A range of processes fractionate metal stable isotopes, offering the potential to unpick these controls.

The Black Sea is the classic modern analogue for past euxinia. Deep euxinic waters (dissolved sulfide ≤380mM) underlie a surface oxic layer and redoxcline at 80-250m. It is an ideal starting point to explore the behaviour of metal stable isotopes in low oxygen conditions. We report water column and sedimentary Zn and Ni data. A maximum in dissolved Zn (~3nM) occurs at the upper boundary of the redoxcline, probably reflecting cellular uptake above and sulfide co-precipitation below. Water column dissolved Ni concentrations remain within a narrow range throughout (9-12nM), with subtle variations in the redoxcline associated with Fe-Mn cycling and interaction with sulfide. Significant Zn and Ni isotopic variation (ca 1.5‰) is observed associated with these concentration variations. In the strongly sulfidic deep waters, Zn and Ni are homogeneous and isotopically heavy: [Zn] = 0.40 ± 0.05nM, δ⁶⁶Zn = 0.96 ± 0.05‰, [Ni] = 10.7 ± 0.7nM, δ⁵⁸Ni = 1.86 ± 0.16‰ (n=6, 1sd), suggesting preferential removal of light isotopes to the particulate phase.

Sediments underlying oxic waters show no authigenic enrichment and isotope ratios close to crustal values (δ⁶⁰Ni ~ 0.2‰, δ⁶⁶Zn ~ 0.3‰). In contrast, Ni and Zn are enriched in sediments underlying the euxinic portion of the water column (by factors of 2-3). Authigenic Ni isotope values are light (δ⁶⁰Ni_auth = 0.30 ± 0.21‰, n=13, 1sd) compared to the deep Black Sea water column, consistent with removal of light isotopes to the particulate phase. This may reflect sorption to Mn oxides, or co-precipitation with sulfide. Authigenic Zn isotope values in these sediments are also light (δ⁶⁶Zn_auth = 0.57 ± 0.07‰) compared to the deep Black Sea dissolved pool. Unlike Ni, however, removal of Zn is close to quantitative. This is seen in low Zn concentrations in deep waters and in the similarity of the euxinic sedimentary Zn isotopic composition to the likely dominant input, open ocean seawater (at ~0.5‰).