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.
Highlights
The oceanic cycling of metals and their isotopes, and the degree to which the inputs are partitioned into different sedimentary sinks as controlled by biology, redox, and the physical chemistry of seawater, has led to applications in Earth history via records from ancient sediments. [1,2]
The modern study focuses on the Peru Margin, as it offers the opportunity to investigate organic-rich sediments deposited at sites with a range of bottom-water oxygen (O2 ) and pore water sulphide (H2 S) abundance, sampling different sedimentary redox states
Separate analyses of the organic-pyrite fraction and of the hydrogen fluoride (HF)-digestible fraction extracted from bulk Peru Margin sediments better characterise the behaviour of Cu and its isotopes in a modern setting with variable redox conditions
Summary
The oceanic cycling of metals and their isotopes, and the degree to which the inputs are partitioned into different sedimentary sinks as controlled by biology, redox, and the physical chemistry of seawater, has led to applications in Earth history via records from ancient sediments. [1,2]. [1,2] One example of such a system is the transition element copper (Cu), whose isotope composition is universally heavy in seawater and all other surface Earth aqueous fluids [3,4]—heavier, for example, than the rocks of the upper continental crust. This prominent modern feature appears to result exclusively through its complexation by organic ligands, whose only known source is cyanobacterial cells [5]. Despite the dominant controls of these organic ligands on the modern Earth, a recent study of changes in the Cu isotope composition of organic-rich shales from across the Great Oxidation Event (GOE) attributes secular changes exclusively to the demise of Banded Iron Formations [6]
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