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• Olivine exposure results in significant Ni and Cr accumulation in Gammarus locusta. • Metal bioaccumulation was dependent on olivine grain size and concentration. • Chronic reproductive toxicity occurred at 10% w/w olivine. • Ecosystem impacts could limit the scale of coastal enhanced olivine weathering. Active atmospheric carbon dioxide removal (CDR) is needed at a gigaton scale in the next decades to keep global warming below 1.5 °C. Coastal enhanced silicate weathering (CESW) aims to increase natural ocean carbon sequestration via chemical weathering of finely ground olivine (Mg x Fe (1− x) SiO 4) rich rock dispersed in dynamic coastal environments. However, the environmental safety of the technique remains in question due to the high Ni and Cr content of olivine. Therefore, we investigated the short term bioaccumulation and chronic toxicity of olivine in the marine amphipod Gammarus locusta. Acute 24-h olivine exposure resulted in significant grain size dependent olivine ingestion and subsequent Ni and Cr accumulation in tissues. Thousands of small (mainly ≤ 10 µm) olivine grains were ingested by G. locusta , but their importance for trace metal bioaccumulation requires additional research. Most olivine grains were egested within 24 h. Chronic 35-day olivine (3–99 µm) exposure reduced amphipod survival, growth, and reproduction, likely as a result of metal induced oxidative stress and disturbance of major cation homeostasis. Amphipod reproduction was significantly reduced at olivine concentrations of 10% w/w and higher. In the context of ecological risk assessment, application of an arbitrary assessment factor of 100 to the highest no observed effect concentration of 1% w/w olivine yields a very low predicted no-effect concentration (PNEC) of 0.01% w/w olivine. This low PNEC value highlights the urgent need for additional marine olivine toxicity data to accurately assess the environmentally safe scale of coastal enhanced weathering for climate change mitigation. [Display omitted] [ABSTRACT FROM AUTHOR]