| Summary: | Opal and calcium carbonate are thought to regulate the biological pump’s transfer of organic carbon to the deep ocean. A global sediment trap database exhibits large regional variations in the organic carbon flux associated with opal flux. These variations are well-explained by upper ocean silica concentrations, with high opal \textquoteleft ballasting’ in the silica-deplete tropical Atlantic Ocean, and low ballasting in the silica-rich Southern Ocean. A plausible, testable hypothesis is that opal ballasting is due to mineral protection, and varies because diatoms grow thicker frustules where silica concentrations are higher, protecting less organic carbon per unit opal. These patterns do not emerge in an advanced ocean biogeochemical model when opal ballasting is represented using a single global parameterization for diatoms, indicating the need for additional parameterization of the dependence of diatoms traits on silica concentration to capture the links between elemental cycles and future changes in the biological pump.
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