Decoupling of iron and phosphate in the global ocean

Abstract

[1] We formulate a mechanistic model of the coupled oceanic iron and phosphorus cycles. The iron parameterization includes scavenging onto sinking particles, complexation with an organic ligand, and a prescribed aeolian source. Export production is limited by the availability of light, phosphate, and iron. We implement this biogeochemical scheme in a coarse resolution ocean general circulation model using scavenging rates and conditional stability constants guided by laboratory studies and a suite of box model sensitivity studies. The model is able to reproduce the broad regional patterns of iron and phosphorus. In particular, the high macronutrient concentrations of the Southern Ocean, tropical Pacific, and subarctic Pacific emerge from the explicit iron limitation of the model. In addition, the model also qualitatively reproduces the observed interbasin gradients of deep, dissolved iron with the lowest values in the Southern Ocean. The ubiquitous presence of significant amounts of free ligand is also explicitly captured. We define a tracer, Fe* which quantifies the degree to which a water mass is iron limited, relative to phosphorus. Surface waters in high-nutrient, lowchlorophyll regions have negative Fe* values, indicating Fe limitation. The extent of the decoupling of iron and phosphorus is determined by the availability and binding strength of the ligand relative to the scavenging by particulate. Global iron concentrations are sensitive to changes in scavenging rate and physical forcing. Decreasing the scavenging rate 40% results in $0.1 nM increase in dissolved iron in deep waters. Forcing the model with weaker wind stresses leads to a decrease in surface [PO 4 ] and [Fe] in the Southern Ocean due to a reduction in the upwelling strength.

Diagnosing oceanic nutrient deficiency

“…However, there is little evidence that nutrient availability is directly limiting microbial processes in the sub-surface. Rather, any understanding of the relative deficiencies of nutrients in the ocean interior can potentially inform our understanding of how patterns of nutrient limitation emerge in the near surface [38], where the nutrients resupplied through physical transport become depleted through the net production of new organic matter.…”

Modeling the coupling of ocean ecology and biogeochemistry

“…where r DOM i Remineralization rate of DOM for element i, here P, Fe, N; r POM i Remineralization rate of POM for element i, here P, Si, Fe, N; c scav scavenging rate for free iron; Fe 0 free iron, modeled as by Parekh et al [2005]; a solubility of iron dust in ocean water; F atmos atmospheric deposition of iron dust on surface of model ocean; z NO3 oxidation rate of NO 2 to NO 3 ; z NO2 oxidation rate of NH 4 to NO 2 (is photoinhibited).…”

Winners and losers: Ecological and biogeochemical changes in a warming ocean

“…Excretion and mortality transfer living organic material into sinking particulate and dissolved organic detritus, which are respired back to inorganic form. Iron chemistry includes explicit complexation with an organic ligand, scavenging by particles [Parekh et al, 2005], and representation of aeolian [Luo et al, 2008] and sedimentary [Elrod et al, 2004] sources.…”