Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR SR3 section): Meridional and seasonal trends

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 55; no. 8; pp. 911 - 925
• Main Authors: Sedwick, P.N., Bowie, A.R., Trull, T.W.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-08-2008; Elsevier; Pergamon Press Inc
• Subjects: Antarctic Zone; Earth, ocean, space; Exact sciences and technology; External geophysics; Iron; Marine; Measurement; Oceanography; Oceans; Physical and chemical properties of sea water; Physics of the oceans; Polar waters; Sea ice; Seawater; Southern Ocean; Antarctic Ocean; Australia; PSE, Australia, Tasmania; PS, Antarctic Ocean; PSW, South Atlantic, Subtropical Front; Iron; Antarctic Zone; Polar waters; Southern Ocean; Sea ice; time variations; biogeochemical cycle; Latitudinal gradient; Subpolar zone; Iron,Southern Ocean,Antarctic Zone,Sea ice,Polar waters; subtropical zone; biogeochemistry; Iron cycle; nutrients; Concentration measurement; Mixed layer; Depletion; marine environment; temperate zone; Depth profile; Ice edge; dissolved materials; spatial variations; seasonal variations; Concentration distribution
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2008.03.011

We report measurements of dissolved iron (dFe, <0.4
μm) in seawater collected from the upper 300
m of the water column along the CLIVAR SR3 section south of Tasmania in March 1998 (between 42°S and 54°S) and November–December 2001 (between 47°S and 66°S). Results from both cruises indicate a general north-to-south decrease in mixed-layer dFe concentrations, from values as high as 0.76
nM in the Subtropical Front to uniformly low concentrations (<0.1
nM) between the Polar Front and the Antarctic continental shelf. Samples collected from the seasonal sea-ice zone in November–December 2001 provide no evidence of significant dFe inputs from the melting pack ice, which may explain the absence of pronounced ice-edge algal blooms in this sector of the Southern Ocean, as implied by satellite ocean-color images. Our data also allow us to infer changes in the dFe concentration of surface waters during the growing season. South of the Polar Front, a comparison of near-surface with subsurface (150
m depth) dFe concentrations in November–December 2001 suggests a net seasonal biological uptake of at least ∼0.14–0.18
nM dFe, of which ∼0.05–0.12
nM is depleted early in the growing season (before mid December). A comparison of our spring 2001 and fall 1998 data indicates a barely discernible seasonal depletion of dFe (∼0.03
nM) within the Polar Frontal Zone. Further north, most of our iron profiles do not exhibit near-surface depletions, and mixed-layer dFe concentrations are sometimes higher in samples from fall 1998 compared to spring 2001; here, the near-surface dFe distributions appear to be dominated by time-varying inputs of aerosol iron or advection of iron-rich subtropical waters from the north.