Molybdenum isotope evidence from restricted-basin mudstones for an intermediate extent of oxygenation in the late Ediacaran ocean

Abstract

The Mo isotope composition of late Ediacaran global seawater and the global extent of ocean oxygenation are still debated due to the complex controls on sedimentary Mo isotope compositions and the rarity with which sediments directly capture global seawater Mo isotope compositions. Deep-water sulfidic sediments from modern severely restricted basins like the Black Sea have the best chance of capturing global seawater Mo isotope compositions. However, few studies have focused on sedimentary Mo isotope variations and their causes in late Ediacaran restricted basins. In this study, we present Mo isotope data for a new section from the Chengkou sub-basin of South China. The sedimentary rocks, mostly organic-rich mudstones, in this restricted basin have authigenic δ98Mo values from −0.01‰ to 1.27‰. Comparison of δ98Mo values with redox-sensitive elemental abundances reveals variation in the extent of Mn-oxide shuttling, related to enhanced hydrothermal activity and the inflow of open-ocean seawater, as the main cause of sedimentary Mo isotope variation in the Chengkou sub-basin. An unambiguous negative correlation between δ98Mo and Mn content points to a greater importance of Mn-oxide shuttling than Fe-oxide shuttling on the sedimentary Mo isotope signatures. Variations in the intensity of Mn-oxide shuttling were likely linked to changes in Mn inputs and the extent of basin restriction.A compilation of late Ediacaran sedimentary Mo isotope data suggests overall limited Mo isotope variation in (semi) restricted late Ediacaran basins, but local factors causing the variation are different and may result in the heaviest Mo isotopic signal in these settings being slightly lighter than that of open seawater. Using the observed negative linear relationships between δ98Mo values and the shuttling-sensitive proxies (V/TOC and Mn), the Mo isotope compositions recorded in the Chengkou sub-basin yield a reliable estimate of 1.3–1.6‰ for the δ98Mo of late Ediacaran global seawater. This value is slightly higher than the δ98Mo estimated for global seawater in the Cryogenian and early-middle Ediacaran, suggesting the extent of ocean oxygenation during the late Ediacaran Period was more similar to the oceans during Mesozoic Oceanic Anoxic Events than to the modern ocean.