ALBERT

Description: Investigations into Fe(II) cycling during two Southern Ocean mesoscale iron enrichment experiments, SOFeX and EIFeX, clearly show the importance of Fe(II) to iron speciation during these experiments. In both cases the added Fe(II) persisted significantly longer than its expected oxidation time indicating a significant Fe reduction process at work. During EIFeX diel studies showed a strong photochemically induced cycle in Fe(II) production in sunlit surface waters. Our results suggest that the photochemical cycling of iron may also be important in unfertilized waters of the Southern Ocean.

Description: Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (〉60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate 〈 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Description: 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).

Description: Hydrogen peroxide (H2O2) is an important oxidant for many bio-relevant trace metals and organic compounds and has potential as a tracer for mixing in near surface waters. In this study we combine H2O2 and bio-optical measurements with satellite data for a meridional transect from 46 degrees N to 26 degrees S in the eastern Atlantic in order to determine the key processes affecting its distribution. Surface H2O2 ranged from 21 - 123 nmol L-1, with maximum inventories (0 - 200 m) of 5.5 - 5.9 mmol m(-2) found at 30 degrees N and 25 degrees S. Analyses showed a strong positive correlation of surface H2O2 with daily irradiances and recent precipitation, though poor correlations with CDOM suggest sunlight is the limiting reactant for H2O2 formation. Vertical distributions of H2O2 were controlled by a combination of mixing processes and phytoplankton activity. The present study highlights processes controlling global H2O2 distributions and points towards the development of parameterization schemes for prediction via satellite data. Citation: Steigenberger, S., and P. L. Croot (2008), Identifying the processes controlling the distribution of H2O2 in surface waters along a meridional transect in the eastern Atlantic.

Description: Iron solubility measurements in the Mauritanian upwelling and the adjacent Open Ocean of the Tropical Atlantic show for all stations lower values in the surface mixed layer than at depth below the pycnocline. We attribute this distribution to a combination of loss terms, chiefly photo-oxidation of organic ligands in the surface, and supply terms, predominantly from the release of ligands from the decomposition of organic matter. Significant correlations with pH, oxygen and phosphate for all samples below the surface mixed layer indicate that biogenic remineralisation of organic matter results in the release of iron binding ligands into the dissolved phase. The comparison of the cFe(S)/PO(4)(3-) ratio with other published data from intermediate and deep waters in the Pacific suggests an enhanced release of iron chelators in the more productive Mauritanian upwelling zone. Citation: Schlosser, C., and P.L. Croot (2009), Controls on seawater Fe(III) solubility in the Mauritanian upwelling zone

Description: Surface active substances (SAS) in the water column were measured by voltammetry using the electrochemical probe o-nitrophenol (ONP) during EIFEX, a mesoscale open ocean iron enrichment experiment in the Southern Ocean. SAS levels were low throughout the experiment (〈0.005 – 0.03 mg L−1 Triton X-100 equivalents). Initially SAS was extremely low in the photic zone, but as the phytoplankton bloom developed concentrations markedly increased throughout the upper 100 m (∼0.02 mg L−1 Triton X-100 equivalents). Highest concentrations of SAS (〉0.02 mg L−1 Triton X-100 equivalents) were found at the end of the bloom particularly at density discontinuities where organic material may accumulate. Exudates from diatoms appeared to be the major source of SAS during EIFEX, either from direct extracellular release or in the action of being grazed upon by zooplankton.

Description: Volcanoes confront Earth scientists with new fundamental questions: Can airborne volcanic ash release nutrients on contact with seawater, thereby excite the marine primary productivity (MPP); and, most notably, can volcanoes through oceanic fertilization affect the global climate in a way that is so far poorly understood? Here we present results from biogeochemical experiments showing that 1) volcanic ash from subduction zone volcanoes rapidly release an array of nutrients (co-)limiting algal growth in vast oceanic areas, 2) at a speed much faster (minute-scale) than hitherto known and that marine phytoplankton from low-iron oceanic areas can swiftly, within days, utilize iron from volcanic sources. We further present satellite data possibly indicating an increase of the MPP due to the seaward deposition of volcanic particulate matter. Our study supports the hypothesis that oceanic (iron) fertilization with volcanic ash may play a vital role for the development of the global climate.

Description: 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 SF6 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 γ = 0.17–0.20 d−1) from a circular shape (50 km2) into a long filament of ∼400 km2 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 SF6 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−1 in surface waters during FeCycle and was dominated by organic complexation. Nighttime measurements of Fe(II) ∼20 pmol L−1 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:NO3 − (11.9 μmol: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.

Description: The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island-based AERONET measurements) at 500 nm is similar to 0.11 and Angstrom parameter (computed within spectral range 440-870 nm) is calculated to be similar to 0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship-based aerosol optical depth compares well to simultaneous island and near-coastal AERONET site AOD.