Barite Microcrystal Abundance, Size and Morphology in Eastern Pacific Water Column Profiles: Precipitation and Dissolution Dynamics

The accumulation rate of the mineral barite (BaSO₄) in marine sediments is one of the most promising proxies for export production, the organic carbon flux to the deep ocean. Barite is thought to precipitate in association with decomposing organic matter in the water column, but the processes of both barite formation and dissolution in the water column are poorly understood. Here, we investigate barite precipitation and dissolution in water column samples collected in Niskin bottles at 5 locations along the Fieberling-Guadalupe Seamount Chain in the Eastern Pacific. Filters were examined using Scanning Electron Microscopy (SEM) paired with Energy Dispersive X-Ray Spectroscopy (EDS) to characterize the shape, abundance and morphology of thousands of barite microcrystals. Barite microcrystals are abundant in the water column by depths of 100-150 m. Most barite crystals are small (1-3 μm), commonly with acicular or barrel shapes but also presenting as crystal aggregates and amorphous masses. Barite microcrystal abundance varies with depth (p < 0.001), with local barite maxima at multiple depths within the upper 1500 m of the water column. Individual barite microcrystals also display heterogeneity in characteristics such as area, circularity, and visible association with organic matter as a function of depth (p=0.005, p < 0.001, and p < 0.001 respectively). Our data suggest that water column barite is influenced by the complex interplay of precipitation and dissolution dynamics. This work also demonstrates the potential of quantitative visual analysis of modern barite microcrystals to advance our understanding of the export production proxy.