Zinc isotope evidence for dynamic nutrient cycling regulation of organic carbon burial in the mid-Proterozoic ocean

Abstract

Episodic deposition of marine organic-rich sediments during the mid-Proterozoic (1.8–0.8 Ga) stands in contrast to the long‐term environmental stasis at that time. Existing conceptual models of these sediments highlight the key role of nutrient supply in regulating marine productivity and related organic carbon burial, yet direct geochemical evidence linking these events remains rare. Zinc (Zn) is a pivotal micronutrient for biological productivity, and zinc isotopic compositions (δ⁶⁶Zn) of seawater are highly sensitive to fluctuations in the marine carbon cycle. In this study, we present high-resolution zinc isotopes, together with major and trace element data from the ca. 1.4 Ga organic-rich sediments of the middle Velkerri Formation, McArthur Basin, northern Australia. Stratigraphic variations in δ⁶⁶Zn values of the middle Velkerri Formation exhibit two contrasting intervals corresponding to enrichments in sedimentary zinc concentrations and total organic carbon (TOC) content. The initial negative δ⁶⁶Zn excursion occurred in the context of enhanced continental weathering and consistent upwelling intensity, with terrigenous sources dominating the influx of isotopically light Zn into the ocean. The subsequent interval exhibits positive shift in δ⁶⁶Zn that is synchronous with the positive carbon isotope excursion and a decline in Zn/TOC ratios, indicating an ocean-wide drawdown of isotopically light zinc driven by enhanced organic carbon burial. Increased sequestration of organic carbon may have promoted the pulse of atmospheric oxygenation during the mid-Proterozoic, providing a further backdrop for early biological evolution. The results of this study suggest that dynamic nutrient cycling in the mid-Proterozoic ocean can trigger a set of feedbacks that temporarily caused the carbon cycle to deviate from its long‐term stability.