A Global Ocean Opal Ballasting–Silicate Relationship.

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 silicate concentrations, with high opal 'ballasting' in the silicate‐deplete tropical Atlantic Ocean, and low ballasting in the silicate‐rich Southern Ocean. A plausible, testable hypothesis is that opal ballasting varies because diatoms grow thicker frustules where silicate concentrations are higher, carrying less organic carbon per unit opal. The observed pattern does not fully emerge in an advanced ocean biogeochemical model when diatom silicification is represented using a single global parameterization as a function of silicate and iron. Our results suggest a need for improving understanding of currently modeled processes and/or considering additional parameterizations to capture the links between elemental cycles and future biological pump changes. Plain Language Summary: Opal, or hydrated silica, is taken up in the surface ocean by diatoms to construct their protective frustules. Another plankton type, coccolithophores, generate protective platelets from calcium carbonate. These two minerals, and thereby plankton types, play major roles in the global carbon cycle. The 'biological carbon pump' transfers carbon from the upper ocean to the ocean's depths, where it can stay for millenia. This process has influenced past atmospheric carbon dioxide concentrations and could also do so in the future. The transfer of carbon to the deep ocean is partially regulated by the amount of 'ballast' minerals in sinking particles, especially opal and calcium carbonate, which are denser and cause particles to sink faster and/or protect organic carbon from microbial consumption. We show that unlike calcium carbonate, opal's ballasting effect varies a great deal between different regions of the ocean. The variation in opal ballasting is well‐explained by the upper‐ocean concentration of silicate between these regions. This suggests a simple explanation: when silicate concentrations are high, diatoms grow thick frustules which actually results in lower carbon sinking per unit opal. Capturing this ballasting–silicate relationship in carbon cycle models may improve their ability to predict future biogeochemical cycles and climate. Key Points: Opal ballasting varies by more than a factor of six across ocean regions; calcium carbonate ballasting is uniformSilicate concentration predicts opal ballasting which suggests that the latter varies with diatom frustule thicknessThis emergent relationship's absence from a sophisticated biogeochemical model indicates it holds useful information for constraining models [ABSTRACT FROM AUTHOR]