Constraining global marine iron source and scavenging fluxes with
GEOTRACES dissolved iron measurements in an ocean biogeochemical model
Abstract
Iron is a key micronutrient controlling phytoplankton growth in vast
regions of the global ocean. Despite its importance, uncertainties
remain high regarding external iron source fluxes and internal cycling
on a global scale. In this study, we used a global dissolved iron
dataset, including GEOTRACES measurements, to constrain source and
scavenging fluxes in the marine iron component of a global ocean
biogeochemical model. Our model simulations tested three key
uncertainties: source inputs of atmospheric soluble iron deposition
(varying from 1.4 - 3.4 Gmol/yr), reductive sedimentary iron release (14
- 117 Gmol/yr), and compare a variable ligand parameterization to a
constant distribution. In each simulation, scavenging rates were
adjusted to reproduce the observed global mean iron inventory for
consistency. The apparent oxygen utilization term in the variable ligand
parameterization significantly improved the model-data misfit,
suggesting that heterotrophic bacteria are an important source of
ligands to the ocean. Model simulations containing high source fluxes of
atmospheric soluble iron deposition (3.4 Gmol/yr) and reductive
sedimentary iron release (114 Gmol/yr) further improved the model, which
then required high scavenging rates to maintain the observed iron
inventory in these high source scenarios. Our model-data analysis
suggests that the global marine iron cycle operates with high source
fluxes and high scavenging rates, resulting in relatively short surface
and global ocean mean residence times of 0.83 and 7.5 years,
respectively, which are on the low-end of previous model estimates.
Model biases and uncertainties remain high and are discussed to help
improve global marine iron cycle models.