Has anyone considered another attribute of pumice - that chemically it surely includes a lot of Mg2SiO4 - which absorbs CO2 as it weathers to bicarbonate? As such it will counter AGW by drawing down CO2, and will counter ocean acidification by adding alkalinity to the ocean. It will also add silicic acid to the ocean, the limiting nutrient for diatoms, whose fall to the depths is also an important CO2 vector. The key thing here is that pumice should remain on the ocean surface long enough for the weathering reaction to take place. Typically the reaction is quite slow but the constant abrasion of pumice against pumice under wave action should speed it up considerably. Biotic factors may also come into play, if the pumice pieces are colonised by algae for example.
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