This dataset contains oyster mortality records and the main physicochemical parameters monitored in the exposure mesocosms, the tanks holding 60 oysters per treatment (unamended control, dissolved alkalinity addition with NaOH and CaCl2 at +250 or +500 µmol/kg, and olivine-based coastal enhanced silicate weathering at +250 or +500 µmol/kg). Mortality is reported as the number of oysters alive, the interval and cumulative number of oysters found dead, and the relative cumulative mortality (a proportion of the initial 60 individuals per tank). Physicochemical parameters include water temperature, dissolved oxygen, conductivity, salinity, total chlorophyll a, suspended particulate matter and ammonia concentration. Measurements span 17 April to 10 July 2024, covering the alkalinization period and the subsequent 63-day chronic exposure. Water temperature, oxygen and conductivity/salinity were recorded with a multi-probe (Xylem Analytics Germany). Total chlorophyll a was measured by fluorescence on unfiltered seawater with a bbe fluorimeter. Suspended particulate matter was determined gravimetrically from filtered seawater. Ammonia was quantified fluorometrically with a microplate reader assay kit.