Surface ocean nitrate-limitation in the aftermath of Marinoan snowball Earth: Evidence from the Ediacaran Doushantuo Formation in the western margin of the Yangtze Block, South China

Geochemical and paleontological studies suggest that a rapid rise in atmospheric O-2 level in the termination of the Marinoan global glaciation (635 Ma) might have triggered the diversification of eukaryotes and caused the deep ocean oxygenation. It is proposed that the Earth's surface redox condition was directly linked to the marine primary productivity, which represented the only major O-2 source before the evolution of land plants. The marine primary productivity was mainly controlled by the bioavailability of macronutrients, phosphorus (P) and nitrogen (N). Thus, reconstruction of ancient P and N cycles can provide direct constraints on marine organic matter production and the redox landscape of the ocean. Previous studies of the Ediacaran Doushantuo Formation in the Yangtze Block, South China, suggested that the Ediacaran marine N cycle was highly dynamic and was characterized by dramatic fluctuations in the intensity of denitrification. However, these studies were mainly focused on sections deposited in the offshore marine environment, and little is known about the N cycle in the nearshore regions. In this study, we report high-resolution organic carbon (delta C-13(org)) and nitrogen (delta N-15(TN)) isotopes of the Doushantuo Formation at the E-Shan section in the western margin of the Yangtze Block. The Doushantuo Formation at the E-Shan section is composed of alternating deposition of thin bedded sandstone and mudstone, and was deposited in a nearshore delta environment. The delta C-13(org) profile displays two prominent negative excursions, which are coincident with two positive excursions in delta N-15(TN). The positive excursion in delta N-15(TN) implies the enhanced denitrification, resulting in the loss of N (nitrate and ammonium). In addition, there are negative correlations between delta N-15(TN) and TOC content and between delta N-15(TN) and delta C-13(org). Such relationships suggest that the reduction of N supply would lower primary productivity and accordingly decrease the carbon isotope of dissolved inorganic carbon (delta C-13(DIC)) in the surface ocean. In addition, this interpretation also implies that N-fixation was not active in the nearshore region. We speculate that the muted N-fixation in the nearshore regions could be attributed to the Fe limitation in oxic shallow seawater and general Mo deficiency of the Ediacaran ocean. Thus, our study indicates that the surface ocean productivity and marine redox landscape were coupled in the Ediacaran ocean.