Copper and its Isotopes in Organic-Rich Sediments: From the Modern Peru Margin to Archean Shales

Abstract

The cycling of copper (Cu) and its isotopes in the modern ocean is controlled by the interplay of biology, redox settings, and organic complexation. To help build a robust understanding of Cu cycling in the modern ocean and investigate the potential processes controlling its behavior in the geological past, this study presents Cu abundance and isotope data from modern Peru Margin sediments as well as from a suite of ancient, mostly organic-rich, shales. Analyses of an organic-pyrite fraction extracted from bulk modern sediments suggest that sulphidation is the main control on authigenic Cu enrichments in this setting. This organic-pyrite fraction contains, in most cases, >50% of the bulk Cu reservoir. This is in contrast to ancient samples, for which a hydrogen fluoride (HF)-dissolvable fraction dominates the total Cu reservoir. With <20% of Cu found in the organic-pyrite fraction of most ancient sediments, interpretation of the associated Cu isotope composition is challenging, as primary signatures may be masked by secondary processes. But the Cu isotope composition of the organic-pyrite fraction in ancient sediments hints at the potential importance of a significant Cu(I) reservoir in ancient seawater, perhaps suggesting that the ancient ocean was characterized by different redox conditions and a different Cu isotope composition to that of the modern ocean.

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…Authigenic Cu in modern organic-rich sediments is generally similar in isotopic composition to Fe-Mn sediments and suspended particulate material collected from the South Atlantic, all at about +0.3‰ (Little et al, 2017b;Ciscato et al, 2019). This homogeneity in authigenic Cu isotope compositions has been suggested to reflect an equilibrium isotope fractionation in the aqueous phase between organically complexed Cu and inorganic Cu 2+ , with the latter approximately 0.4‰ lighter than ligand-bound Cu, followed by near quantitative scavenging of inorganic Cu 2+ by (any type of) particulate material (Little et al, 2017b;.…”

“…Despite the strong reactivity of Cu toward sulfide, and unlike Zn (Section 4.3.4) and Mo (Section 7.2.3), Cu in euxinic sediments from the Black Sea and Cariaco Basin does not record open marine δ 65 Cu (Little et al., 2017). In fact, authigenic Cu in modern organic‐rich sediments (from both euxinic basins and continental margin settings) is generally similar in isotopic composition to Fe–Mn sediments and to suspended particulate material collected from the South Atlantic, all at about +0.3‰ (Ciscato et al., 2019; Little et al., 2018; Little et al., 2017). The homogeneity in authigenic sedimentary Cu isotope compositions has been suggested to reflect an equilibrium isotope fractionation in the aqueous phase between organically complexed Cu and inorganic Cu 2+ , with the latter ∼0.4‰ lighter than ligand‐bound Cu, followed by near quantitative scavenging of inorganic Cu 2+ by particulate material of any type (Little et al., 2018; Little et al., 2017).…”

“…However, organic ligands play a key role in the cycling of Cu and Cu isotopes, suggesting potential for the use of Cu isotopes in tracing the evolution of organic complexation on geological timescales. Additionally, careful sequential extraction procedures, such as those described by Ciscato et al (2019), may allow for the direct probing of past seawater properties, such as redox state.…”

“…At present, there are few applications of Cu isotopes to study ancient biogeochemical cycles. Interestingly however, there are two black shale records that indicate a shift from UCC-like to heavy Cu isotope values across the Great Oxidation Event (GOE, ~2.4 Ga; Chi Fru et al, 2016;Ciscato et al, 2019). Chi Fru et al (2016) interpreted this shift as reflecting the onset of oxidative weathering and waning of iron formation deposition, with the latter process driving pre-GOE seawater isotopically light due to the preferential scavenging of heavy Cu isotopes to Fe oxides.…”

Re-assessing copper and nickel enrichments as paleo-productivity proxies

“…It is also well known that the degradation of OM in the first stages of diagenesis releases Cu and Ni in a soluble form in interstitial waters, and these metals can then diffuse back to the water column (Chester, 1990;Widerlund, 1996;Charriau et al, 2010;Marchand et al, 2016;Ciscato et al, 2019), or be incorporated into authigenic phases, depending on specific conditions of early diagenesis (François, 1988;Huerta-Diaz and Morse, 1990, 1992Berner et al, 2013).…”