Changes in beach sediment mineralogy and grain size associated with pelagic Sargassum landings

Massive influxes of pelagic Sargassum spp. have caused significant environmental, social, and economic impacts across the tropical Atlantic. The accumulation of these macroalgae on beaches represents a novel source of carbonate material for tropical shorelines. Among the most abundant components are bryozoan skeletons, composed mainly of Mg-calcite, which can alter the sedimentological and mineralogical properties of beach sediments. This study examines the textural and mineralogical changes in beach sediments on the northern Mexican Caribbean coast, where the first massive pelagic Sargassum strandings occurred in late 2014 and have become recurring events since 2018. Surface sediments collected in 2016 and 2022 were compared with samples from 2006. Mean grain size was determined by laser particle size analysis, and mineral composition was quantified using X-ray diffraction (XRD). Results revealed significant coarsening of mean grain size associated with Sargassum influxes. From 2006 to 2022, the carbonates mineral composition shifted, with Mg-calcite increasing by 9.6 % and aragonite decreasing by 8.9 %. The enrichment in Mg-calcite is attributed to the high abundance of bryozoan epibionts colonizing up to 70 % of Sargassum fronds. These findings demonstrate that recurrent Sargassum beaching’s have rapidly modified both the grain-size distribution and the mineralogical maturity of beach sediments on the northern Mexican Caribbean coast.