Appearance of SSFs in earliest Cambrian seas of South China linked to salinity conditions

The Ediacaran-Cambrian (E/C) transition in South China witnessed the evolution of early animals during an epoch of major oceanic environmental changes. Although the relationship of metazoan development to redox conditions has been investigated in many studies, the influence of watermass salinity has remained largely unexamined to date. Here, we report large salinity fluctuations in earliest Cambrian seas of South China based on shale elemental proxies (B/Ga, Sr/Ba, and S/TOC) in two sections (i.e., the platform Gunshi'ao and slope Huangye sections), and we examine their relationships to contemporaneous biotic and environmental evolution. Our results show persistently sub-fully-marine salinities on the Yangtze Platform, marked by long-term fluctuations between low-brackish (Units I, III) and high-brackish conditions (Units II, IV), likely due to variations in the relative influences of freshwater runoff from adjacent landmasses and seawater influx from the Nanhua Basin. Furthermore, we show that other environmental factors, e.g., redox, hydrographic restriction, and productivity, covaried strongly with and may have been largely controlled by watermass salinity variation. The low-brackish Units I and III were associated with ferruginous conditions, stronger restriction, and lower productivity, whereas the high-brackish Units II and IV were associated with close-to-euxinic conditions, weaker restriction, and higher productivity, suggesting that seawater influx provided both aqueous sulfate and nutrients to the Yangtze Platform. The appearance of early metazoan faunas was linked to salinity changes: a sponge-acritarch community was dominant under low-brackish conditions, whereas early Cambrian small shelly faunas (SSFs) appeared concurrently with a shift to high-brackish conditions. It is as yet unclear whether the temporal relationship of salinity and faunal changes was purely coincidental or whether it reflects exclusion of the small shelly faunas from the Yangtze Platform under low-brackish conditions. Fully marine conditions were not established until near the top of Unit IV, even in the slope Huangye section. The ultimate control on secular variation in salinity and other environmental parameters in the lower Cambrian of South China was likely eustatic sea level and its influence on the rate of watermass exchange across marginal sills of the Nanhua Basin. Our results highlight the importance of undertaking salinity analysis in epicratonic and marginal-marine settings in the context of exploring controls on early metazoan evolution.