Zinc isotopic evidence for enhanced oceanic primary productivity during termination of the late Ediacaran Shuram Excursion

The recovery from the largest negative carbon isotope excursion in Earth history, the late Ediacaran Shuram Excursion, is thought to have been related to enhanced primary productivity, but direct evidence supporting this hypothesis is lacking. Zinc isotopes in marine sediments are a promising proxy to constrain oceanic productivity as well as organic matter burial rates. To explore secular variation in oceanic productivity and its role in recovery from the Shuram Excursion, Zn isotopes, major- and trace-element, and iron speciation data were generated for black shales from Member IV of the Ediacaran Doushantuo Formation in two South China sections (Jiulongwan and ZK6305). Redox proxy data reveal predominantly euxinic bottom waters, and salinity proxy data indicate mostly high-brackish conditions (∼15–30 psu) linked to moderate basinal watermass restriction. These conditions preserved a record of contemporaneous seawater Zn isotopic compositions, revealing a positive δ⁶⁶Zn excursion in the upper part of the study sections (from +0.31 ± 0.02 ‰ to +0.70 ± 0.01 ‰ at Jiulongwan, and from +0.35 ± 0.03 ‰ to +0.62 ± 0.04 ‰ at ZK6305, 2SD) that record elevated oceanic productivity and enhanced organic matter burial. This development may have led to re-coupling of the carbonate and organic carbon isotope records by elevating the relative proportion of particulate organic carbon relative to dissolved organic carbon in the sediment. Enhanced burial of organic carbon may have also removed isotopically light carbon from the ocean, thus promoting recovery from peak low values in marine carbon isotope records. Our study highlights the critical role of oceanic productivity in attenuating perturbations of the marine carbon cycle.