Abstract
Despite intensive study of the early Cambrian Nanhua Basin (ECNB) in South China, its degree of connectedness to the open ocean remains uncertain, impeding analysis of contemporaneous oceanic redox evolution and its role in the Cambrian Explosion. We analyzed bulk-shale B/Ga, a newly proposed proxy for watermass salinity in ancient depositional systems, for three sections (Daotuo, Yuanjia, and Zhalagou) representing a range of water depths in the ECNB and spanning early Cambrian Stages 2 and 3. The B/Ga proxy records a salinity gradient in the basin, with increasing salinity from shallow- to deep-water areas, particularly during the deposition of black shales of the lower Jiumenchong/Niutitang/Zhalagou Formation. The gradient was probably due to the mixing of freshwater from adjacent landmasses with marine waters within a moderately restricted basin. All three study sections exhibit a monotonic rise in salinity over the ~9-Myr study interval, reflecting increased watermass exchange with the open ocean due to regional tectonic and/or global sea-level changes. The improved watermass exchange may have weakened water-column stratification and reduced deep-water anoxia in the ECNB, contributing the enhanced oceanic oxygenation along with global environmental controls and further modulating the expression of the Cambrian Explosion recorded in Lower Cambrian sections of South China. Our study thus highlights the need for comprehensive analyses of hydrographic dynamics in marginal-marine settings that are used to explore co-evolutionary relationships between early animals and environmental oxygen levels.