Variability in oceanic dissolved iron is dominated by the colloidal fraction

Abstract

Oceanic surface and deep iron distribution and size fractionation were investigated on three cruises in the sub-tropical and tropical Atlantic Ocean. Detailed profiles and transects were collected and analyzed for “dissolved” Fe (DFe, 0.4 μm filtered) and “soluble” Fe (SFe, 0.02 μm filtered). The difference between DFe and SFe is inferred to be the “colloidal” fraction of Fe (CFe). SFe concentration distributions and profiles showed little variability in the Atlantic Ocean with slightly lower concentrations of SFe in the upper ocean than the relatively uniform concentrations observed in deep-water (≈0.3 to 0.4 nmol/kg). In contrast, variability in the Atlantic DFe was dominated by variability in CFe. DFe and CFe followed dust deposition trends, and observed surface maxima in DFe profiles were always due to maxima in the CFe fraction. Where dust deposition and surface DFe were low (i.e., the South Atlantic), the CFe fraction of DFe was low and frequently negligible in surface waters. Below the surface maxima in CFe and DFe, CFe always decreased to negligible levels at 30–80 m, remained low or negligible throughout the pycnocline, and increased with depth below the pycnocline. At a site located on the edge of the equatorial system (10°N), high DFe and CFe concentrations were associated with an oxygen minimum zone (OMZ) at depths of 130 to 1100 m. Deep-water DFe and CFe concentrations varied between water masses depending on the source, age, and path of the water masses. DFe in NADW decreased by 30% from the North Atlantic to the South Atlantic site with most of the decrease due to loss of CFe. At the South Atlantic site, NADW had higher DFe and a higher fraction of CFe than the Antarctic water masses.