Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Lannuzel, D, Bowie, AR, Remenyi, TA, Lam, P, Townsend, AT, Ibisanmi, E, Butler, E, Wagener, T, Schoemann, V
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2011
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Antarctic; Austral; Pacific; Southern Ocean; Antarc*; North Atlantic
• Online Access: https://doi.org/10.1016/j.dsr2.2011.05.027; http://ecite.utas.edu.au/75524

This paper presents iron (Fe) profiles in the upper 1000 m from nine short-term (transect) stations andthree long-term (process) stations occupied in the Australian sector of the Southern Ocean during theSAZ-Sense expedition in austral summer (JanuaryFebruary) 2007. Strong vertical and horizontalgradients in Fe concentrations were observed between the 18 sampled profiles (i.e. 0.090.63 nmol/ldissolved Fe (dFe)). Average dFe concentrations in surface gggwaters in the northern Sub-AntarcticZone (SAZ-N) West (station P1) were 0.2770.04 nmol/l. This is lower in the SAZ-N East region (stationP3 and around) where average dFe values in the mixed layer were 0.4870.10 nmol/l. The Polar Front(PF) station (P2) exhibited the lowest average surface Fe values (i.e., 0.2270.02 nmol/l). Ironconcentrations in deep waters down to 1000 m were more uniform (0.250.37 nmol/l dFe), which isin accordance with values reported elsewhere in remote waters of the Southern Ocean, but lower thanthose observed in the North Atlantic and North Pacific basins. A strong decoupling was observedbetween dFe and nutrient cycles at all stations. Particulate Fe levels were generally very low for all SAZstations (o0.081.38 nmol/l), with higher values observed at stations collected near Tasmania and inthe SAZ-N East region. The intrusion of subtropical waters, enriched with Fe from sediments or dustfurther north, is thought to mediate Fe input to the SAZ-N and STZ areas, while input from below wouldbe the main source of Fe in the PF region. We applied the tracer Fen (Fen [dFe]"RFe:P#[PO4 3 "], whereRFe:P is the algal uptake ratio) to estimate the degree to which the water masses were Fe limited. In thisstudy, Fen tended to be negative and decreased with increasing depths and latitude. Positive Fen values,indicating Fe sufficiency, were observed in the (near-)surface waters collected in the SAZ-N East andnear continental sources, where primary production was higher and ultimately limited by the lack ofmacro-nutrients, not Fe. Micro-organisms residing in the SAZ-N West and PF on the other handexperienced negative Fen, indicating a strong co-limitation by low silicic acid concentration and Fesupply (and light in the case of PF).