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 <5 wt% or > 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 <100 yield dSi recovery <90%, resulting in large isotope fractionation. Hence, an Mg/Si ratio of ≥300 is recommended to guarantee full recovery, and H 2 O 2 pretreatment is necessary to eliminate matrix effects induced by high Fe, Mn and DOC concentrations. Our findings imply that the traditional leaching method likely underestimates current bSi contents, especially in clay or/and organic-rich sediments with moderate bSi contents.