Silicon isotope-based assessments of biogenic and dissolved silica content determination in marine sediments

Sedimentary biogenic silica (bSi) is a primary sink of oceanic silicon (Si), and its quantification is essential for constraining the marine Si budget. The traditional alkaline leaching method estimates bSi contents by extrapolating a regression of Si leachates at 2, 3, and 5 h, but this approach is biased by lithogenic Si (LSi) dissolution. We analyzed 59 marine sediment samples (0.2 to 57.0 wt% bSi) and used Si isotopes to re-assess this method. Samples with moderate bSi contents (5–20 wt%) show a marked decline in dissolution rates after 3–5 h of leaching. Si isotopes of the leachates reveal that the decline reflects either a shift from dominant bSi to LSi phase or changes in bSi species through time. Sediments with 20 wt% bSi display a stable Si dissolution rate throughout 8 h, likely due to unchanged dominant Si phases of lithogenic or biogenic Si. Additionally, we re-assessed the brucite co-precipitation method (MAGIC) for porewater dissolved Si (dSi) recovery. Mg/Si molar ratios