ELEMENTAL PROXIES FOR SALINITY ANALYSIS OF ANCIENT SHALES AND MUDSTONES: UPDATE AND CASE STUDIES

Salinity, a fundamental property of watermasses that often controls biological and geochemical processes in aqueous systems, is often difficult to determine in paleodepositional systems. We recently investigated three elemental ratios as proxies for the salinity of watermasses in which muds and shales accumulate: boron/gallium (B/Ga), strontium/barium (Sr/Ba), and sulfur/total organic carbon (S/TOC) [Wei, W. and Algeo, T.J., 2019. Elemental proxies for paleosalinity analysis of ancient shales and mudrocks. Geochimica et Cosmochimica Acta]. We showed that, in modern systems, B/Ga was ~88 % effective at distinguishing between freshwater, brackish, and marine conditions (for values of <3, 3-6, and >6, respectively), and Sr/Ba was ~66 % effective (for values of <0.2, 0.2-0.5, and >0.5, respectively). B/Ga and Sr/Ba vary approximately linearly with watermass salinity, although the use of Sr/Ba as a salinity proxy is subject to certain constraints (i.e., absence of carbonate-hosted Sr and productivity-related Ba). S/TOC was ~91 % effective at distinguishing freshwater from brackish+marine conditions (with values of <0.1 and >0.1, respectively) because of the low sulfate concentrations that typify freshwater systems. However, the S/TOC proxy is not effective at distinguishing brackish from marine conditions owing to a strong dependence on organic production and decay processes when even moderate amounts of sulfate are present. We illustrate the application of these proxies with case studies of (1) the middle Eocene Bohai Bay Basin in NE China, where salinities ranged from freshwater to marine [Wei, W., et al., 2018. International Journal of Coal Geology, 200:1-17]; (2) the Early Jurassic Cleveland Basin, UK, marked by a shift from marine to low-brackish conditions during the Toarcian OAE [Remírez, M.N. and Algeo, T.J., 2020. Earth-Science Reviews, 201, 103072]; and (3) the Late Devonian Illinois Basin of eastern North America, which recorded an abrupt shift from low-brackish conditions (1-15 psu) in the Frasnian to high-brackish (15-30 psu) or marine conditions in the Famennian [Song, Y., et al., 2020. Geological Society of America Bulletin, in press].