Extremely high alkalinity due to dissolution of Mg-rich phyllosilicate in the hemipelagic sediments of the Ulleung Basin (East/Japan Sea): stable Si isotopic evidence and reactive transport modeling

Marine silicate alteration includes the processes of lithogenic silicate (LSi) dissolution, clay formation, and biogenic silica dissolution. LSi dissolution consumes CO₂ and results in marine silicate weathering. Formation of cation-rich clay minerals produces CO₂, which is known as reverse weathering. The net effects on carbon cycling of both processes are poorly constrained as the responsible silicate phases and controlling factors are unclear. We investigate the coupling between LSi dissolution and clay formation by analyzing stable Si isotopic signatures (δ³⁰Si) of porewater and solid Si phases (reactive LSi, biogenic silica, and amorphous secondary Si phases) in two drill cores from the Ulleung Basin, East/Japan Sea. High porewater total alkalinity (up to 131 meq L⁻¹) was measured, indicating net marine silicate weathering. Based on the elemental composition (Si, K, and Al) as well as δ³⁰Si of the reactive LSi phase in sediments, phyllosilicates that are potentially mica group silicates are identified as the primary silicate group that sustains marine silicate weathering in the Ulleung Basin. Our reactive transport modeling supports such an inference and further reveals how early diagenetic reactions could affect the downcore occurrence and rates of LSi dissolution and clay formation. Predominant clay formation/reverse weathering in sulfate-reducing sediments is evident from the high δ³⁰Si values in porewater. In the shallow methanogenesis zone, net marine silicate weathering due to phyllosilicate dissolution explains the observed high total alkalinity and low δ³⁰Si in porewater. Nonetheless, we show that the rates of clay formation primarily control the level of porewater total alkalinity, even in the condition of net marine silicate weathering. Clay formation is strongly suppressed by the low porewater pH as a result of the active fermentation in the site with the highest rate of organic matter degradation. Deep in the methanogenesis zone, enrichment of dissolved aluminum from LSi dissolution counteracts the influence of pH on silicate alteration processes, thereby further limiting the extent of LSi dissolution, as supported by the downcore increasing of porewater δ³⁰Si. Our results demonstrate that δ³⁰Si in porewater reflects downcore variations in marine silicate alteration, with microbial processes and dissolved aluminum accumulation regulating the rates of LSi dissolution and clay formation.