Cu and Zn Isotopic Evidence for the Magnitude of Organic Burial in the Mesoproterozoic Ocean

The rate of net primary production in the Proterozoic ocean was suggested to be no more than 10% of its modern value (Laakso and Schrag, 2019), however, in the Mesoproterozoic Xiamaling Formation, the export production values could reach 20%-150% of the present-day Equatorial Atlantic average values (Zhang et al., 2016). Here, we report Zn and Cu isotope data for black shales from the Xiamaling Formation to illustrate the biogeochemical cycling of Zn and Cu in the Mesoproterozoic ocean. The ⁶⁵Cu-enriched signature in the authigenic fraction is similar to that in bioauthigenic Cu of the modern marine sediments. The Zn isotope ratios of sediments deposited in euxinic conditions are commonly higher than those of clastic sediments, indicating light Zn sinks in the coeval ocean. Combined with previously reported Mo isotope data, the proportion of organic carbon to total carbon burial in the Mesoproterozoic was about as half as that at present, which is larger than the previous estimation-a quarter of today's value (e.g., Ozaki et al., 2019) and is evidenced by a wide distribution of black shales. The organic burial may be ascribed to the increasing phosphorus inputs from large igneous provinces and consequently high primary productivity, which has spurred the hypothesized atmosphere-ocean oxygenation at ~1.4 Ga.