Marine silicate alteration affects long-term carbon cycling by combining lithogenic silicate (LSi) dissolution and secondary clay formation. However, the net H₂CO₃ consumption and production ability of marine silicate alteration remains highly uncertain. Critical factors such as silicate types and rates of marine silicate alteration have not yet been studied thoroughly. This study aims to identify silicate types and quantify rates of marine silicate alteration using sequential leaching, elemental abundance, Si isotopic signature and numerical modelling in Ulleung Basin drill cores, in which porewater alkalinity is up to 130 meq/L caused by active LSi dissolution. A decreased porewater dissolved Si (DSi) concentration and increased δ³⁰Si_porewater value found from 0-9 meter below seafloor (mbsf) are attributed to the formation of secondary clay, which is mainly smectite suggested by numerical modelling. Below the sulfate-methane transition zone (9-32 mbsf), increased DSi concentration and decreased δ³⁰Si_porewater value result from LSi dissolution, likely mica-group minerals and albite suggested by elemental abundance of separated LSi phase and modelling. Further, sensitivity tests using various organic matter degradation rates show that alkalinity, Mg and K concentrations in porewater are affected majorly by smectite formation rate, while mica and vermiculite dissolution rates are constant. The decreased contribution of LSi dissolution and an increased contribution of BSi dissolution from 32 to 218 mbsf are indicated by increased DSi concentration and δ³⁰Si_pw values and rate results output by modelling.