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

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Meng Cheng1, Chao Li1, Thomas J Algeo2, Lian Zhou1, Xiao-Dan Liu3 and Lian-Jun Feng4, (1)China University of Geosciences Wuhan, Wuhan, China, (2)University of Cincinnati Main Campus, Cincinnati, OH, United States, (3)The Seventh Gold Detachment of China Armed Police Force, Yantai, China, (4)Institute of Geology and Geophysics, Chinese Academy of Science, Beijing, China
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.