Diatomaceous Ocean Weathering, a new gateway of dissolved silica to the oceans: new supportive evidence

Recently, the rare earth element (REE) composition of diatom frustules has innovated the oceanic circulation of REEs^{1, 2}: Diatom frustules are important carriers of REEs in water columns, where diatoms incorporate dissolved and solid REEs into their frustules (as REE-silicic acid complexes¹ and from solid REEs in silicate minerals³), and release REEs during frustule dissolution³; a portion of the REEs dissolved in deep water are scavenged by carbonate phases of particles¹. This leads to the radical “Ocean Weathering”, where diatoms decompose silicate minerals

 This radical picture challenges the conventional view consisting of “boundary exchange” and “reversible scavenging”⁴. We will report new supportive evidence for the new picture.

1) Self-consistent calculation of REE composition of diatom frustules

 Two assumptions are introduced. 1) Diatom frustules with a particular REE composition are unique carriers of REEs from surface to deep water, and REEs are released by dissolution of diatom frustules to the deep water proportionally to silicic acid dissolution; 2) the REEs released to the deep water are removed with a particular partitioning pattern of scavenging, irrespective of water depth. The composition of diatom frustules was obtained such that the average relative deviation of the partitioning of all REEs in the 2^nd assumption is minimized. The results of the calculation showed the presence of extra-input of REEs in the surface water, which was in a good balance with that scavenged (output) in a water column.

2) ²⁷Al-NMR measurement of sediment trap samples Broad Al peaks of 4 and 6 coordinate numbers were observed in the sediment trap samples collected in the Bering Sea. They were distinct from those of Al in clay and silicate minerals, but were rather similar to the reported spectra by an Al-doped diatom cultivation study. To explain this amount of Al (>0.1%) in diatom frustules, an extra supply of Al from silicate minerals is necessary.

1 Akagi, GCA (2013) 2 Akagi et al. GCA (2011) 3 Akagi et al. GJ (2014) 4 e.g., Rempfer et al. GCA (2011)