Salinity reconstruction in Proterozoic depositional systems

Although elemental proxies (i.e., boron/gallium [B/Ga], strontium/barium [Sr/Ba], and sulfur/total organic carbon [S/TOC]) have been extensively used to evaluate paleo-watermass salinities in Phanerozoic shales and marls, their application to Precambrian-age formations has been limited. Here, we evaluate these proxies for five depositional systems ranging in age from late Paleoproterozoic to late Neoproterozoic. Our analysis shows that the range of B/Ga ratios encountered (∼2−9) matches that seen in Phanerozoic deposits and is consistent with a spectrum of salinity conditions ranging from freshwater (B/Ga < 3) to brackish (B/Ga 3−6) to marine (B/Ga 6−12). Furthermore, all B/Ga-based salinity assignments are consistent with existing paleosalinity inferences, including fully marine conditions for the ca. 650 Ma Areyonga-Aralka and ca. 1640 Ma Barney Creek formations in Australia, mixed brackish-marine conditions for the ca. 650 Ma Datangpo Formation in China and the ca. 1100 Ma El Mreiti Group shales in Mauritania, and low-brackish conditions for the ca. 1080 Ma Nonesuch Formation in North America. Our salinity assessment for the Nonesuch Formation is particularly significant in view of extended debate regarding the lacustrine versus marine character of this unit. The influences of diagenesis and clay-mineral assemblages on B/Ga-based salinity interpretations are generally minor to insignificant. The Sr/Ba proxy yielded systematically lower salinity estimates than the B/Ga proxy based on salinity facies thresholds established from modern aqueous systems, a pattern that we interpret to reflect reduced Sr concentrations in Proterozoic seawater. The S content and S/TOC ratios of all units are sufficiently high as to preclude freshwater conditions (incidentally indicating that Proterozoic seawater sulfate concentrations were higher than sometimes inferred). These findings demonstrate that elemental salinity proxies, especially B/Ga, are broadly applicable to shale formations of Proterozoic age, providing an important new tool for paleoenvironmental analysis that has the potential to reshape our understanding of life-environment co-evolution in deep-time systems.