Marine barite morphology as an indicator of biogeochemical conditions within organic matter aggregates

Abstract

Marine barite is commonly used as a proxy to reconstruct past ocean productivity. Its distribution in the water column mirrors organic carbon fluxes since it precipitates within microenvironments in decomposing organic matter aggregates. Barite and barium proxies are therefore used to study various aspects of organic matter remineralization and the marine carbon cycle. Barite naturally occurs in a wide variety of crystal sizes and morphologies, but barite crystals that form in the ocean water column are dominantly 1–2 μm in length and have barrel-shaped morphologies. Here, we conducted a series of laboratory experiments to determine the physical and chemical conditions that yield barite crystals similar to marine barite. We found that barite saturation index, the presence and identity of organic compounds, and experiment duration all exert a strong influence on barite crystal size and morphology. Barrel-shaped, 1 μm length crystals resembling marine barite were produced in experiments with a barite saturation index of 2.5, soy phospholipid concentrations of ≥50 mg L−1, and experiment durations of ≤10 min. These findings help constrain the plausible biogeochemical conditions within the aggregate microenvironments in which marine barite precipitates. Relatively high experimental concentrations of phospholipids are consistent with the hypothesized involvement of extracellular polymeric substances in marine barite precipitation. Short experiment durations suggest that a favorable saturation state may be short-lived in marine organic matter aggregates. We present detailed mineralogical and crystallographic analyses of the crystals we synthesized to gain insight into barite crystal growth. This work deepens our understanding of the mechanisms behind marine barite precipitation and sheds light on microscale spatial and temporal dynamics within organic matter aggregates.