During the past few decades scientists have increasingly realized that the cycling of silica in the world’s oceans is entwined with other key biogeochemical cycles, including those of carbon and nitrogen. And even though this coupling means silicon plays an important role in primary production and the amount of carbon dioxide in the atmosphere, a large mismatch in our estimates of the amount of this nutrient entering the oceans versus the amount that’s being removed remains.
One of the least understood pathways in the marine silica budget is the burial of biologically derived silica along continental margins with high sedimentation rates. In contrast to standard assumptions that much of the biogenic silica dissolves following burial, researchers have recently discovered that some of this silica is being converted to clay. Although this conversion would help explain where some of the silica is going, current estimates of this process account for just one quarter of the “missing” material.
To better constrain this pathway, Rahman et al. used cosmogenic silica, which is produced naturally in the atmosphere by cosmic ray bombardment, to trace the fate of biogenic silica in coastal sediments. Despite the very low levels of biogenic silica found along continental margins, the team was able to determine the amount and type of biogenic silica stored in samples collected from four depositional settings, including subtropical and tropical deltas and temperate coastal zones.
The results indicate that traditional techniques consistently underestimate clay formation and hence the amount of biogenic silica buried in subtropical deltas and temperate coastal zones by a factor of 2–4. This discrepancy, argue the authors, is due to an underestimate of the amount of clay and other products produced during the first stages of chemical alteration and is consistent with observations of rapid clay formation in laboratory experiments.
Their data indicate the amount of biogenic silica stored in clays along continental margins ranges from 4.5 to 4.9 teramoles per year, which could account for the entire discrepancy in marine silica budgets. (Global Biogeochemical Cycles, https://doi.org/10.1002/2017GB005746, 2017)
—Terri Cook, Freelance Writer