FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF₆ tracer experiment in unperturbed low iron waters

Abstract

[1] An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is $50-fold (i.e., 7-to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an ''fe'' ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an ''Fe'' ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).Citation: Boyd, P. W., et al. (2005), FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF 6 tracer experiment in unperturbed low iron waters, Global Biogeochem. Cycles, 19, GB4S20,

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

“…A mean value of 1.7±2.1 nmol m −2 d −1 is considered hereafter. These fluxes are low compared to those calculated during the Crozex and FeCycle cruises, which were 100 nmol m −2 d −1 (Planquette et al, 2007) and 7.58-75.8 nmol m −2 d −1 , respectively, assuming a 1-10% dissolution of the total flux given by Boyd et al (2005). The particulate iron flux (90-99% of the total flux) during KEOPS was estimated to be 12.6-31.7 nmol m −2 d −1 with a mean value of 22.2±13.5 nmol m −2 d −1 .…”

“…The calculation of F1 allows us to estimate the residence time (RT) of apparent particulate iron (RT=PFe stock/downward PFe flux, Boyd et al, 2005). It is equal to 1.7 days in surface waters.…”

Toward a Regional Classification to Provide a More Inclusive Examination of the Ocean Biogeochemistry of Iron-Binding Ligands

“…EPS are expected to be particularly abundant in waters characterized by high chlorophyll concentrations or on the base of sea-ice (Table 1). Carbohydrate exudation products tend to be linked to nutrient limitation and the stationary growth phase of cells (e.g., Myklestad, 1977;Liu et al, 2001), such as at the decline of a phytoplankton bloom when cells continue to fix carbon photosynthetically but have insufficient nutrients available for macromolecular synthesis (e.g., Boyd et al, 2005). To date, a clear link between phytoplankton Fe limitation and the specific production of exopolymeric substances is missing but putative proteomic evidence has been presented (Nunn et al, 2013).…”

Enhancement of the reactive iron pool by marine diatoms

“…Overall, these processes induced by the presence of diatoms that increased the Fe(II) production could play an important role in the retention and transformation of Fe from biological and lithogenic particulate Fe(III) in the dissolved phase (Boyd et al, 2005).…”