Hydrological controls on marine chemistry in the Cryogenian Nanhua Basin (South China)

Abstract

In order to explore Cryogenian seawater chemistry and its underlying controls, we present an integrated study of iron speciation, major and trace elements, and carbon isotopes of Mn‑carbonates and their host black shales in the Datangpo Formation from three drillcores (Gaodi, Xixibao and Changxingpo) in the Nanhua Basin, South China. A key aspect of this analysis was use of the B/Ga, Sr/Ba, and S/TOC proxies to evaluate paleosalinity variation in this marginal-marine basin. Our results indicate that salinity and redox conditions as well as cycling of Fe-Mn-Mo-C in the basin were hydrologically controlled by the relative fluxes of freshwater from riverine sources and seawater from open-ocean sources. Black shale deposition was associated with low-brackish salinity, probably due to reduced seawater exchange, which contributed to development of stagnant watermass circulation and euxinia in the basin. During such anoxic depositional intervals, Mo show gradient across the basin. In contrast, Mn‑carbonate deposition was associated with relatively higher salinity linked to enhanced exchange of the basinal watermass with the global ocean and to improved deep-water ventilation, which was due to sinking of higher-density ingressing open-ocean waters below lower-density basinal waters (as in the modern Baltic Sea). Enhanced decay of organic matter led to 12C-enriched dissolved inorganic carbon that was precipitated in Mn‑carbonates on the deep basin floor, leading to spatial gradient of inorganic carbon isotope. The shift from euxinic to ferruginous deep-water conditions during such intervals implies that ingressing oceanic waters were sulfate-poor relative to basinal waters. These depositional intervals were also characterized by operation of an active MnFe shuttle that enhanced accumulation of Fe2+, Mn2+, Mo and CO32− in the deep watermass. Our findings demonstrate that the seawater chemistry of the Cryogenian Nanhua Basin was significantly influenced by hydrological factors of both local and global character, and that similarities should be expected for all Cryogenian epicratonic seas, providing new insights into Cryogenian oceanic chemistry evolution and the contemporaneous formation of widespread Mn‑carbonate mines.