Determinants of the bioavailability of iron from dust input: CDOM, reactive oxygen, ligands, and acidification
The residence time and bioavailability of iron, and hence the reaction of the marine biota to dust (one of the main sources of iron to the ocean) depend on a number of factors. These factors include ocean acidification, but also two components of dissolved organic matter in the ocean, namely colored dissolved organic matter (CDOM) that influences the redox-speciation of iron, and iron-binding ligands that prevent iron from precipitating onto particle surfaces.
The aim of this subproject is to synthesize our understanding of the coupling between CDOM, ligands, acidification and dust iron bioavailability into a global model that allows to quantify the effect on phytoplankton growth and the biogeochemical cycling of nutrients and carbon. Such a model allows to study the effect of changes in dust deposition and ocean acidification that are expected under ongoing climate change, and to better asses the magnitude of feedback processes in the climates that are mediated via iron biogeochemistry.
We have started by modelling the global distribution of Fe-binding organic ligands globally and compiling measurements of organic ligand concentrations for comparison. Model results so far show that a prognostic description of ligand production and degradation leads to a more realistic distribution of iron, especially in the mesopelagic, but tends to increase surface iron concentrations (Fig. 1, from Völker and Tagliabue, submitted 2014).
Fig. 1: Average vertical profile of dissolved iron in a model run with prognostic ligands (black) and with a uniform ligand concentration of 1 nmol/L (red).
We are now busy investigating the connection between the distribution of CDOM and other sources and sinks of reactive oxygen and the photochemical redox cycling of dissolved iron.
PI's: Christoph Völker, Dieter A. Wolf-Gladrow