Testing the salinity of Cambrian to Silurian epicratonic seas

Ancient epicratonic formations, which represent the bulk of pre-Jurassic sedimentary rocks, have been widely interpreted as marine deposits, but recently developed bulk-shale elemental proxies for paleo-watermass salinity (i.e., B/Ga, Sr/Ba, and S/TOC) have shown this inference to be frequently incorrect. Here, we use these proxies to test the salinity conditions of 22 representative shale and marl formations of early Cambrian to early Silurian age from five cratons (Laurentia, Avalonia, Baltica, Iran and South China) in the context of formation-specific paleogeographic and paleoclimatic reconstructions. Our dataset shows that around half of these formations were probably deposited under brackish or mixed brackish-marine conditions rather than fully marine conditions (as previously inferred), and that one of them represents a freshwater facies (previously interpreted as marine mainly on the basis of Cruziana traces). In most cases, the development of reduced-salinity conditions can be related to the coastal and/or humid climate belt setting in which the formation of interest was deposited. Our dataset also reveals systematically low Sr/Ba values (i.e., relative to modern brackish and marine facies), suggesting that seawater Sr concentrations were lower during the Early Paleozoic than in the modern. Our findings suggest that re-evaluation of the salinity characteristics of many ancient epicratonic shale and marl formations is necessary.