Diagenetic processes in fjord sediments of Southern Iceland – A complex interplay of organic matter respiration and submarine silicate weathering

The chemical weathering of mafic magmatic rocks (e.g., basalt) is known to remove CO₂ from the atmosphere, transforming it into dissolved or solid inorganic carbon phases. Natural marine sediments contain a wide variety of organic and inorganic phases as well as microbial communities impacting the “submarine weathering engine”, e.g., increasing weathering potential by lowering ambient pH, or decreasing the CO₂ removal potential by forming authigenic clay minerals. Environments rich in reactive organic matter, mafic silicate minerals, and amorphous silica (e.g., ash, biogenic opal) reflect this natural complexity, and can serve as natural laboratories for understanding what controls submarine silicate weathering. Icelandic fjords with their high primary productivity and their mafic hinterland can serve as examples for these complex conditions. We present geochemical sediment and pore water data down to 5 m sediment depth from Hvalfjörður (SW Iceland) and Reyðarfjörður (SE Iceland) taken during the 2023 DEHEAT research cruise onboard RV Belgica. Our data show intense diagenesis that is both related to organic matter degradation and to submarine silicate weathering. The relatively uniform sedimentary material is fine-grained and particularly rich in iron, titanium and magnesium compared to average shale. Tentative sedimentation rates of about 0.5 cm/yr and organic carbon ranging between ~0.5 and 2.5 wt% with a dominantly marine origin based on TOC/TN ratios indicate an accumulation environment providing large amounts of highly reactive organic matter. Sulphate-methane transition zones are established at 75-100 cm sediment depth, but pore water alkalinity and DIC linearly increase to, and probably beyond, the deepest samples. Below the SMTZ, Ikaite crystals are found at various depths throughout the sediments of both fjords. Pore water profiles e.g. of dissolved silica and lithium show undulating downcore structures hinting both at silicate dissolution, but also at clay mineral formation. The data altogether provides insight into a complex interplay of dissolution and precipitation processes tied to the geology of the area, accumulation characteristics and the availability and respiration of organic matter.