Tidewater cycle drives alpine glacial sediment plume geochemistry.

Sediments transported by glacial meltwaters are important sources of trace-metal micronutrients for coastal microbial communities, linking cryospheric processes with ocean fertilization and biogeochemical cycles. Tidewater glacier advance-retreat cycles drive sediment fluxes and influence fjord geochemistry. Here, we used a chemical extraction method to determine the iron and manganese fertilization potential of suspended sediment-plume and iceberg-laden particulate matter from two adjacent, yet geomorphologically distinct, fjords in Southcentral Alaska. We found that the glacier retreat status underpinned the fraction of labile trace-metals within fjord surface plumes, with distinctly lower lability of metals associated with recent and rapid retreat coincident with enhanced erosion and chemical weathering. Particle size did not affect chemical lability, resulting in a well-mixed particle assemblage transported to the coastal ocean. With global tidewater glacier retreat, these results provide implications for future ecosystem fertilization via cryospheric processes and the interpretation of fjord sediment archives.