Coastal El Niño triggers rapid marine silicate alteration on the seafloor.

Marine silicate alteration plays a key role in the global carbon and cation cycles, although the timeframe of this process in response to extreme weather events is poorly understood. Here we investigate surface sediments across the Peruvian margin before and after extreme rainfall and runoff (coastal El Niño) using Ge/Si ratios and laser-ablated solid and pore fluid Si isotopes (δ 30 Si). Pore fluids following the rainfall show elevated Ge/Si ratios (2.87 µmol mol -1 ) and δ 30 Si values (3.72‰), which we relate to rapid authigenic clay formation from reactive terrigenous minerals delivered by continental runoff. This study highlights the direct coupling of terrestrial erosion and associated marine sedimentary processes. We show that marine silicate alteration can be rapid and highly dynamic in response to local weather conditions, with a potential impact on marine alkalinity and CO 2 -cycling on short timescales of weeks to months, and thus element turnover on human time scales.