The Southern Ocean silicon trap: Data‐constrained estimates of regenerated silicic acid, trapping efficiencies, and global transport paths

Abstract

[1] We analyze an optimized model of the global silicon cycle embedded in a dataassimilated steady ocean circulation. Biological uptake is modeled by conditionally restoring silicic acid in the euphotic zone to observed concentrations where the modeled concentrations exceed the observational climatology. An equivalent linear model is formulated to which Green-function-based transport diagnostics are applied. We find that the models' opal export through 133 m depth is 166 6 24 Tmol Si/yr, with the Southern Ocean (SO) providing 70% of this export, 50% of which dissolves above 2000 m depth. The global-scale gradients of the opal dissolution rate are primarily meridional, while the global-scale gradients of phosphate remineralization are primarily vertical. The mean depth of the temperature-dependent silicic-acid regeneration reaches 2300 m in the SO, compared to 600 m for phosphate remineralization. Silicic acid is stripped out of the euphotic zone far more efficiently than phosphate, with only (34 6 5)% of the global silicic-acid inventory being preformed, compared to (61 6 7)% for phosphate. Subantarctic and tropical waters contribute most of the ocean's regenerated silicic acid, while Antarctic waters provide most of the preformed silicic acid. About half of the global silicic-acid inventory is trapped in transport paths connecting successive SO utilizations, with silicic acid last utilized in the SO having only a (5 6 2)% chance of being next utilized outside the SO. This trapping depletes subantarctic mode waters of silicic acid relative to phosphate, which has a (44 6 2)% probability of escaping successive SO utilization.

Inverse-model estimates of the ocean's coupled phosphorus, silicon, and iron cycles

“…We acknowledge that the minimum attained by sequentially optimizing groups of independent parameters is generally different from jointly optimizing all independent parameters, but computational and practical considerations demanded a sequential approach. We justify this a posteriori by the fact that we are able to achieve fits to the observed nutrient concentration fields with rms mismatches similar to those of other recent data-constrained models (e.g., Primeau et al, 2013;Holzer et al, 2014;. Given these considerations, we adopted the following strategy:…”

“…3.3. ) The redistribution operator B Si similarly injects silicic acid into the aphotic water column with the divergence of the opal flux, φ bSi , which attenuates because of temperature-dependent opal dissolution following Gnanadesikan (1999) and Holzer et al (2014). For each latitude and longitude, φ bSi is computed as the solution to…”

“…We use T E = 11, 481 K as Gnanadesikan (1999) and the same detrital fraction f dia for the opal export and diatom POP export. The parameter combination κ max Si /w Si has nearly the same value as determined by Holzer et al (2014), but was re-optimized here for a simple restoring-type model that takes subgrid topography into account (see Appendix B).…”

“…Silicic acid (Si(OH) 4 ) was considered because of the importance of diatoms in marine ecosystems (e.g., Nelson et al, 1995;Buesseler, 1998;Moore et al, 2004;Brzezinski et al, 2011b) and because the pronounced silicon trapping of the Southern Ocean (e.g., Holzer et al, 2014) might be sensitive to iron availability.…”

Barium-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation

“…In contrast, near-surface Si recycling efficiency is spatially variable (e.g. Holzer et al, 2014), which likely contributes to the regional differences in the preformed Si-isotopic compositions of northern-versus southern-sourced deep water masses (e.g. de Souza et al, 2014).…”