A Delayed Noeproterozoic Oceanic Oxygenation: Evidence from the Mo Isotope of the Cryogenian Datangpo Formation

Abstract:

The onset of the Neoproterozoic oxygenation event (NOE) is usually considered to be at 750-800Ma, which was supposed to have triggered the subsequent oxygenation of the earth’s atmosphere-ocean system, thus removing the barrier for the emergence and rapid diversification of animals. However, the subsequent oceanic redox responses in the Cryogenian are poorly constrained. Here, we conducted an integrated Fe-S-C-Mo biogeochemical study on black shales of the Cryogenian Datangpo Formation (~660Ma, Nanhua Basin, South China). Iron speciation data indicate that these black shales were deposited under euxinic water conditions. Co-variation between Mo and TOC suggests an increasing isolation of the basin from open ocean during the deposition of the black shales. Correspondingly, the Datangpo black shales show higher δ⁹⁸Mo values (+0.97‰ to +1.12‰) for the lower part (0-10m) and lower δ⁹⁸Mo values (+0.44‰ to +0.53‰) for the upper part (10-20m) consistent with a global scale seawater δ⁹⁸Mo recorded in the lower part but only a basin scale seawater δ⁹⁸Mo recorded in the upper part. Accordingly, we estimate the seawater Mo isotope closed to +1.1‰ at ~660 Ma, which suggests substantial oceanic anoxia compared to modern oceans (+2.3‰). The seawater δ⁹⁸Mo reconstructed by the Datangpo black shales is exactly the same to previously reported seawater δ⁹⁸Mo at ~750 Ma and ~640 Ma, indicating a continuous oceanic anoxia throughout the Cryogenian although widespread oceanic oxygenation was suggested for the subsequent Ediacaran by multiple geochemical records. Thus, in light of previous studies, our findings indicate a delayed oceanic oxygenation relative to the onset of NOE, which may help to explain the first presence of metazoa in Cryogenian but rapid diversification occurred in Ediacaran.