Organic interfaces enhance strontium content of marine barite

The history of life and environmental change on Earth is told largely by physical and chemical indicators hosted within the sedimentary rock record. Barite (BaSO4) is one of the few minerals that is known to crystallize from seawater, and due to its relatively high chemical resiliency in nature, its trace element and isotope contents are frequently taken to reflect the composition of its parental fluid. Consequently, barite is often utilized to place constraints on the compositions of ancient waters in a variety of settings (e.g., bulk seawater, hydrothermal brines, sedimentary pore waters, and cold-water seeps) (1). The low barium concentrations that are prevalent in modern seas generally preclude BaSO4 mineralization (2); however, barite is known to form in association with suspended organic matter in open waters (3) and is a nearly ubiquitous, albeit minor, component of deep-water marine sediments, particularly underlying regions characterized by high biological activity (4). By dint of this correlation, the abundance of barite in sedimentary cores likely reflects the extent of biological productivity in ancient environments (5). The mechanisms by which organic matter promotes barite formation are not fully appreciated; however, it is clear that the presence of living organisms and/or decaying organic matter can establish a locally favorable environment for barite nucleation and growth. This local environment may also be necessary to explain the formation of strontium-enriched marine barites, which are expected to be less stable than more barium-rich compositions in seawater (2). In PNAS, Deng et al. (6) advance the understanding of marine barite formation by investigating the competitive adsorption of strontium and barium on organic interfaces and the influence of surface-directed nucleation on the strontium content of synthetic barite.