Modelling the speciation and biogeochemistry of iron in oceanic surface waters at the Bermuda Atlantic Time-series Study site

Abstract

By means of numerical modelling the cycling of iron between its various physical (dissolved, colloidal, particulate) and chemical (redox state and organic complexation) forms in the upper mixed layer of the ocean is analysed. Using the model an initial quantitative assessment is made of how this cycling influences iron uptake by phytoplankton and its loss via particulate export. The model is forced with observed dust deposition rates, mixed layer depths, and solar radiation at the site of the Bermuda Atlantic Timeseries Study (BATS). It contains an optimised ecosystem model which yields results close to the observational data from BATS.

Firstly, the results of a zero-dimensional model approach show that the mixed layer cycle strongly influences the cycling of iron between its various forms. This was mainly attributed to the light dependency of photoreductive processes and to the seasonality of primary production. The daily photochemical cycle is driven primarily by the production of superoxide and its amplitude depends on the concentration and speciation of dissolved copper. Model results are largely insensitive to the dominant form of dissolved iron introduced via dust deposition, and also to the form of iron that is taken up directly during algal growth. In the model solutions, the role of the colloidal pumping mechanism depends strongly on assumptions made regarding rates of colloid aggregation and photoreduction rate.

Secondly, a one-dimensional approach of the model is coupled with the General Ocean Turbulence Model (GOTM). The combined model was able to simulate the temporal patterns and vertical profiles of dissolved iron in the upper ocean at the BATS site reasonably well. Subsurface model profiles depended strongly on the parameter values chosen for loss processes affecting iron, colloidal aggregation and scavenging onto particles. Current estimates for these parameters result in depletion of dFe. A high stability constant of iron-binding organic ligands is required to reproduce the observed degree of organic complexation below the mixed layer. Solubility of atmospherically deposited iron higher than 2% leads to dissolved iron concentrations higher than observations. Despite neglecting ultraviolet radiation, the model produces diurnal variations and mean vertical profiles of dFe which are in good agreement with observations.

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