Physical mixing effects on iron biogeochemical cycling: FeCycle experiment

Abstract

[1] The effects of physical processes on the distribution, speciation, and sources/sinks for Fe in a high-nutrient low-chlorophyll (HNLC) region were assessed during FeCycle, a mesoscale SF 6 tracer release during February 2003 (austral summer) to the SE of New Zealand. Physical mixing processes were prevalent during FeCycle with rapid patch growth (strain rate g = 0.17-0.20 d À1 ) from a circular shape (50 km 2 ) into a long filament of $400 km 2 by day 10. Slippage between layers saw the patch-head overlying noninfused waters while the tail was capped by adjacent surface waters resulting in a SF 6 maximum at depth. As the patch developed it entrained adjacent waters containing higher chlorophyll concentrations, but similar dissolved iron (DFe) levels, than the initial infused patch. DFe was low $60 pmol L À1 in surface waters during FeCycle and was dominated by organic complexation. Nighttime measurements of Fe(II) $20 pmol L À1 suggest the presence of Fe(II) organic complexes in the absence of an identifiable fast Fe(III) reduction process. Combining residence times and phytoplankton uptake fluxes for DFe it is cycled through the biota 140-280 times before leaving the winter mixed layer (WML). This strong Fe demand throughout the euphotic zone coupled with the low Fe:NO 3 À (11.9 mmol:mol) below the ferricline suggests that vertical diffusion of Fe is insufficient to relieve chronic iron limitation, indicating the importance of atmospheric inputs of Fe to this region.

Ocean acidification affects iron speciation during a coastal seawater mesocosm experiment

“…Hydrocarboxylic acids, such as glucaric acid, affect photoreduction of Fe(III) and may be released from phytoplankton (Kuma et al, 1992;Öztürk et al, 2004) while their direct effect on Fe(II) oxidation remains to be shown. However, organic Fe(II) complexation and consequential effects on the lifetime of Fe(II) in seawater have been suggested previously (Croot et al, 2001(Croot et al, , 2007Breitbarth et al, 2009). Our data suggest two possible mechanisms tying into Fe(II) cycling at different pH.…”

Iron redox dynamics in the surface waters of the Gulf of Aqaba, Red Sea

“…This problem was minimized by using black Teflon tubing for all solutions (including the waste line), and by placing the detector and the pump in a dark case and covering them with a dark cloth. On a few cruises, several samples were followed to complete decay and repeatedly analyzed to better constrain the blank values and test for the presence of more slowly-oxidized Fe(II) species, as was recently reported by Croot et al (2007) in the southwest Pacific. The blank values were found to be stable and Fe(II) oxidation rates remained unchanged throughout.…”

“…Consequently, a continuous supply of both energy and electrons is required to drive the redox cycle and maintain measurable Fe(II) concentrations. To date, Fe(II) concentrations ranging from a few picomolars to several nanomolars have been reported in a number of aquatic environments (Emmenegger et al, 2001;Shaked et al, 2002;Boyé et al, 2006;Croot et al, 2007;Hopkinson and Barbeau, 2007;Moffett et al, 2007;Ussher et al, 2007). In near surface waters, solar irradiance drives photochemical reduction of iron by direct photolysis and/or by indirect photolysis of chromophoric dissolved organic matter (CDOM) to form Fe(III)-reducing radicals (Barbeau, 2006).…”

“…Various organic ligands and colloids were shown to strongly influence Fe(II) oxidation kinetics, either by accelerating or decelerating the reaction (Rose and Waite, 2003b;Rijkenberg et al, 2006;Ussher et al, 2005). Several field studies have reported marked deviations in the rate of Fe(II) oxidation compared to laboratory rates, possibly as a result of organic complexation of the Fe(II) and/or the presence of photo-generated radicals (Emmenegger et al, 2001;Croot and Laan, 2002;Croot et al, 2005Croot et al, , 2007Hopkinson and Barbeau, 2007;Laglera and Van den Berg, 2007;Moffett et al, 2007;Roy et al, 2008). This extensive knowledge, coupled with recent analytical developments in real-time Fe(II) measurements, enables us to closely monitor and interpret the oxidation kinetics of Fe(II) in situ.…”

Mixed and mixing layer depths in the ocean surface boundary layer under conditions of diurnal stratification

“…Many of the parameterizations on energy dissipation and turbulence in the OSBL are based on the assumption that MLD = XLD, which is often not the case in a dynamic ocean [ Brainerd and Gregg , ; Croot et al , ; Cisewski et al , ; Noh and Lee , ; Stevens et al , ; Sutherland et al , ; Franks , ]. Observations of differences between the MLD and the XLD have been commented on in early microstructure experiments [ Shay and Gregg , ; Dewey and Moum , ]; however, there have been relatively few studies that have investigated the nature of this difference as it requires specialized instruments to measure turbulent properties and large data sets to quantitatively compare the MLD and XLD.…”