The oceanic budgets of nickel and zinc isotopes: the importance of sulfidic environments as illustrated by the Black Sea

Abstract

Isotopic data collected to date as part of the GEOTRACES and other programmes show that the oceanic dissolved pool is isotopically heavy relative to the inputs for zinc (Zn) and nickel (Ni). All Zn sinks measured until recently, and the only output yet measured for Ni, are isotopically heavier than the dissolved pool. This would require either a non-steady-state ocean or other unidentified sinks. Recently, isotopically light Zn has been measured in organic carbon-rich sediments from productive upwelling margins, providing a potential resolution of this issue, at least for Zn. However, the origin of the isotopically light sedimentary Zn signal is uncertain. Cellular uptake of isotopically light Zn followed by transfer to sediment does not appear to be a quantitatively important process. Here, we present Zn and Ni isotope data for the water column and sediments of the Black Sea. These data demonstrate that isotopically light Zn and Ni are extracted from the water column, probably through an equilibrium fractionation between different dissolved species followed by sequestration of light Zn and Ni in sulfide species to particulates and the sediment. We suggest that a similar, non-quantitative, process, operating in porewaters, explains the Zn data from organic carbon-rich sediments.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes

“…Sulfides can also capture organic‐delivered Zn released during biomass decay within sedimentary porewaters, a process that would drive the signature toward lighter δ 66 Zn values typical of organic matter. Zinc sulfide formation can also be associated with a negative isotope fractionation (Vance et al., ). However, near‐quantitative Zn drawdown, due to much lower Zn concentration relative to that of H 2 S, as is typical in sulfide‐rich water columns and porewaters (e.g., Tankéré et al., ), is likely to mute isotope fractionation tied to aqueous Zn speciation or kinetic effects during sulfide precipitation (John, Kunzmann, Townsend, & Rosenberg, ; Vance et al., ).…”

Zinc Geochemical Cycling in a Phosphorus‐Rich Ocean During the Early Ediacaran

“…Recent experiments and natural studies have also reported the same fractionation direction, with Δ 66 Zn sulfide‐solution = ~−0.3‰ (Archer & Vance, ; Jamieson‐Hanes et al, ; Veeramani et al, ). This potential mechanism has been widely used to explain lower δ 66 Zn values in sphalerites (Veeramani et al, ) and organic‐rich shales (Little et al, ; Vance et al, ). If Zn precipitation in deep seawater is dominated by Zn sulfides, then the Zn isotope composition of the Zn sulfide fraction should be equal to or lighter than that of the water column.…”

“…Previous studies have suggested that the deep‐water Wuhe section may have been deposited under ferruginous conditions with intermittent euxinic conditions (Han & Fan, ; Sahoo et al, ), where the dissolved Zn could be scavenged from seawater by H 2 S that was reduced from dissolved sulfate. If the dissolved Zn was quantitatively precipitated as sulfide under sulfidic condition, the sediments would be isotopically similar to seawater‐derived Zn (Tang et al, ; Vance et al, ). For example, the euxinic sediments in the Black Sea record the Zn isotopic signature of the deep ocean, which has been attributed to the quantitative removal of Zn from the water column by Zn sulfide precipitation (Vance et al, ).…”

“…If the dissolved Zn was quantitatively precipitated as sulfide under sulfidic condition, the sediments would be isotopically similar to seawater‐derived Zn (Tang et al, ; Vance et al, ). For example, the euxinic sediments in the Black Sea record the Zn isotopic signature of the deep ocean, which has been attributed to the quantitative removal of Zn from the water column by Zn sulfide precipitation (Vance et al, ). Therefore, the highest δ 66 Zn value (0.45‰) of black shale (WH‐43) deposited in euxinic condition may proximately reflect Zn isotope signal of ambient seawater.…”

“…However, the redox conditions of the deep‐water environment would regulate Zn levels in the deep part of the water column. For example, euxinic conditions have caused lower dissolved Zn concentrations in the deep water of the Black Sea (Vance et al, ). In contrast, euxinic sediments in the Black Sea exhibit Zn/Al ratios that are two to nine times higher than those in oxic sediments.…”