Wagener Thibaut

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  • Preprint
    Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)
    ( 2011-01) Lannuzel, Delphine ; Bowie, Andrew R. ; Remenyi, Tomas A. ; Lam, Phoebe J. ; Townsend, Ashley T. ; Ibisanmi, Enitan ; Butler, Edward ; Wagener, Thibaut ; Schoemann, Veronique
    This paper presents iron (Fe) profiles in the upper 1000 m from nine short-term (transect) stations and three long-term (process) stations occupied in the Australian sector of the Southern Ocean during the SAZ-Sense expedition in austral summer (January–February) 2007. Strong vertical and horizontal gradients in Fe concentrations were observed between the 18 sampled profiles (i.e. 0.09–0.63 nmol/l dissolved Fe (dFe)). Average dFe concentrations in surface waters in the northern Sub-Antarctic Zone (SAZ-N) West (station P1) were 0.27±0.04 nmol/l. This is lower than in the SAZ-N East region (station P3 and around) where average dFe values in the mixed layer were 0.48±0.10 nmol/l. The Polar Front (PF) station (P2) exhibited the lowest average surface Fe values (i.e. 0.22±0.02 nmol/l). Iron concentrations in deep waters down to 1000 m were more uniform (0.25–0.37 nmol/l dFe), which is in accordance with values reported elsewhere in remote waters of the Southern Ocean, but lower than those observed in the North Atlantic and North Pacific basins. A strong decoupling was observed between dFe and nutrient cycles at all stations. Particulate Fe levels were generally very low for all SAZ stations (<0.08 – 1.38 nmol/l), with higher values observed at stations collected near Tasmania and in the SAZ-N East region. The intrusion of subtropical waters, enriched with Fe from sediments or dust further north, is thought to mediate Fe input to the SAZ-N and STZ areas, while input from below would be the main source of Fe in the PF region. We applied the tracer Fe* (Fe*= [dFe]-RFe:P × [PO4 3-], where RFe:P is the algal uptake ratio) to estimate the degree to which the water masses were Fe limited. In this study, Fe* tended to be negative and decreased with increasing depths and latitude. Positive Fe* values, indicating Fe sufficiency, were observed in the (near-)surface waters collected in the SAZ-N East and near continental sources, where primary production was higher and ultimately limited by the lack of macro-nutrients, not Fe. Micro-organisms residing in the SAZ-N West and PF on the other hand experienced negative Fe*, indicating a strong co-limitation by low silicic acid concentration and Fe supply (and light in the case of PF).
  • Article
    Biogeochemical iron budgets of the Southern Ocean south of Australia : decoupling of iron and nutrient cycles in the subantarctic zone by the summertime supply
    (American Geophysical Union, 2009-12-31) Bowie, Andrew R. ; Lannuzel, Delphine ; Remenyi, Tomas A. ; Wagener, Thibaut ; Lam, Phoebe J. ; Boyd, Philip W. ; Guieu, Cecile ; Townsend, Ashley T. ; Trull, Thomas W.
    Climate change is projected to significantly alter the delivery (stratification, boundary currents, aridification of landmasses, glacial melt) of iron to the Southern Ocean. We report the most comprehensive suite of biogeochemical iron budgets to date for three contrasting sites in subantarctic and polar frontal waters south of Australia. Distinct regional environments were responsible for differences in the mode and strength of iron supply mechanisms, with higher iron stocks and fluxes observed in surface northern subantarctic waters, where atmospheric iron fluxes were greater. Subsurface waters southeast of Tasmania were also enriched with particulate iron, manganese and aluminum, indicative of a strong advective source from shelf sediments. Subantarctic phytoplankton blooms are thus driven by both seasonal iron supply from southward advection of subtropical waters and by wind-blown dust deposition, resulting in a strong decoupling of iron and nutrient cycles. We discuss the broader global significance our iron budgets for other ocean regions sensitive to climate-driven changes in iron supply.