Kenneth S. Johnson

What controls dissolved iron concentrations in the world ocean

Dissolved ( value in the data set at a depth near 750 m, where variability is at a maximum. The minimum concentrations are found at stations in the remote central Pacific and the maximum values occur at stations adjacent to the continental margin. The major source of iron in the deep sea is generally aeolian deposition. Integrated (surface to 500 m) concentrations of iron at each station are only weakly correlated with the aeolian iron deposition flux, however. This contrasts with other elements such as lead that also have strong atmospheric sources. These observations lead us to conclude that the nutrient-like profile is maintained by a mechanism that reduces the scavenging rate of dissolved iron at concentrations less than 0.6 nmol kg- ’ This mechanism may be complexation by strong iron binding ligands, which have been found in both the Atlantic and Pacific at concentrations near 0.6 nM. This apparent solubility would act to diminish inter-ocean fractionation. It would allow a nutrient-like profile to develop before scavenging began to remove iron. In order to test the concept, we developed a numerical model to make quantitative predictions of dissolved iron concentrations from place to place. The dissolved iron source in the ocean interior is remineralization from sinking particulate organic matter. Scavenging removes dissolved iron only at concentrations greater than the apparent solubility. The only geographically variable parameter in the model is the export flux of carbon from the surface layer, which carries iron with it. The model generated dissolved iron profiles, based on measured or estimated values of the carbon export flux, are in remarkable agreement with the observed profiles at all stations from the North Atlantic through the Southern Ocean to the North Pacific. 0 1997 Elsevier Science B.V.

Continental-shelf sediment as a primary source of iron for coastal phytoplankton

The availability of iron, an essential nutrient, controls rates of phytoplankton primary productivity in the open-ocean, upwelling ecosystems of the equatorial Pacific. Upwelling injects large amounts of macronutrients into the euphotic zone of eastern boundary currents, such as the California Current System (CCS), where iron can become the limiting factor on productivity. Iron addition to samples from some areas of the CCS has been shown to increase rates of biomass production, but the processes that control iron availability in these systems remain poorly understood. Here we report measurements of dissolvable iron (that is, dissolved plus leachable iron at pH 3) in transects across the CCS in March of 1997 and 1998. We foundhigh concentrations of iron in 1997 during strong upwelling conditions. During the 1998 El Niño, the concentration of dissolvable iron in surface waters was low, even though that yearwas marked by high river flow and low offshore salinity. These results indicate that the primary source of iron in the CCS isresuspension of particles in the benthic boundary layer, followed by upwelling of this iron-rich water, rather than direct riverine input. This source of iron must be an essential but variable component of the high productivity found in upwelling ecosystems.

Geochemistry of barium in marine sediments : Implications for its use as a paleoproxy

Abstract Variations in the accumulation rate of barium in marine sediments are thought to be indicative of variations in marine biological productivity through time. However, the use of Ba as a proxy for paleoproductivity is partly dependent upon its being preserved in the sediment record in a predictable or consistent fashion. Arguments in favor of high Ba preservation are partly based on the assumption that sediment porewaters are generally at saturation with respect to pure barite. The idea is that because nondetrital sedimentary Ba predominantly exists as barite, porewater saturation would promote burial. We present sediment porewater, sediment solid phase, and benthic incubation chamber data suggesting that solid-phase Ba preservation may be compromised in some geochemical settings. We propose that under suboxic diagenetic conditions, characterized by low bottom water oxygen and high organic carbon respiration rates, Ba preservation may be reduced. Independent of the mechanism, if this assertion is true, then it becomes important to know when the Ba record is unreliable. We present evidence demonstrating that the sedimentary accumulation of authigenic U may serve as a proxy for when the Ba record is unreliable. We then provide an example from the Southern Ocean during the last glacial period where high authigenic U concentrations coincide with high Pa:Th ratios and high accumulation rates of biogenic opal, but we find low accumulation rates of sedimentary Ba. Thus, for the study sites presented here during the last glacial, we conclude that Ba is an unreliable productivity proxy.

Unicellular Cyanobacterial Distributions Broaden the Oceanic N2 Fixation Domain

Oceanic Nitrogen Fixation Nitrogen fixation in the oceans is important in sustaining global marine productivity and balances carbon dioxide export to the deep ocean. It was previously believed that marine nitrogen fixation was due to a single genus of filamentous cyanobacteria, Trichodesmium. The recent discovery of unicellular open-ocean cyanobacteria has raised the question of how they contribute to global ocean nitrogen fixation and how they compare in distribution and activity to Trichodesmium. Using data collected from the southwest Pacific Ocean, Moisander et al. (p. 1512, published online 25 February) show that the unicellular nitrogen-fixing cyanobacteria (UCYN-A and Crocosphaera watsonii) have distinct ecophysiologies and distinct oceanic distributions from each other, and from Trichodesmium. These data can be incorporated into models to retune estimates of the global rates of oceanic nitrogen fixation and carbon sequestration. Nitrogen fixation in the South Pacific Ocean is partitioned among several microbe species with distinct ecophysiologies. Nitrogen (N2)–fixing microorganisms (diazotrophs) are an important source of biologically available fixed N in terrestrial and aquatic ecosystems and control the productivity of oligotrophic ocean ecosystems. We found that two major groups of unicellular N2-fixing cyanobacteria (UCYN) have distinct spatial distributions that differ from those of Trichodesmium, the N2-fixing cyanobacterium previously considered to be the most important contributor to open-ocean N2 fixation. The distributions and activity of the two UCYN groups were separated as a function of depth, temperature, and water column density structure along an 8000-kilometer transect in the South Pacific Ocean. UCYN group A can be found at high abundances at substantially higher latitudes and deeper in subsurface ocean waters than Trichodesmium. These findings have implications for the geographic extent and magnitude of basin-scale oceanic N2 fixation rates.

Developing standards for dissolved iron in seawater

In nearly a dozen open- ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters. However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well- known dissolved iron (Fe) concentrations similar to open- ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002]. The Sampling and Analysis of Fe (SAFe) cruise was staged from Honolulu, Hawaii, to San Diego, Calif., between 15 October and 8 November 2004 to collect data and samples that were later used to provide this reference material. Here we provide a brief report on the cruise results, which have produced a tenfold improvement in the variability of iron measurements, and announce the availability of the SAFe dissolved Fe in seawater standards.

Iron photochemistry in seawater from the equatorial Pacific

Abstract The photochemistry of iron in surface waters, and its implications to iron bioavailability, was examined on two cruises to the equatorial Pacific. Decktop incubations were performed with equatorial seawater to which iron was added in various chemical forms. Results showed clear diurnal patterns in measurable iron levels, with the highest levels occurring midday. These results are consistent with a model of iron cycling involving the photo-reductive dissolution of colloidal iron and its subsequent oxidation and biological uptake of dissolved iron(III). Model calculations were based on independently determined rate constants. We suggest that photochemical reactions may have a significant impact on iron availability to phytoplankton in the open ocean.

Phosphorus regeneration in continental margin sediments

Benthic incubation chambers have been deployed in a variety of geochemical environments along the California Continental Margin. These include both high and low oxygen environments and sites where the rate of organic matter oxidation on the seafloor (Cox) ranges from < 1 mmol m−2 day−1 to more than 7 mmol m−2 day−1 through a depth range of 100–3500 m. This range in the rate of organic matter oxidation along with variations in the concentration of bottom water oxygen allow us to elucidate the diagenetic conditions under which P regeneration may be decoupled from organic matter cycling. Under conditions where bottom water oxygen concentration is low (<50 μM), and the rate of organic matter oxidation is also low (< 1 mmol m−2 day−1), P regeneration may be less than that expected from the decay of organic debris and, in some cases, there is a flux of phosphate into the sediments. At stations where bottom water oxygen is low, and the degradation rate of organic material is greater than 1 mmol m−2 day−1, phosphate may be released at a rate exceeding the production expected from the oxidation of organic matter. At stations having high bottom water oxygen concentrations, rates of organic matter decomposition < ∼7 mmol m−2 day−1, and where benthic irrigation is not significant, P regeneration is consistent with that expected from the decomposition of organic debris. In addition, our data indicate that high benthic iron fluxes are observed in regions exhibiting a decoupling between organic matter and phosphate, whereas low to zero iron fluxes are observed in regions where P regeneration is either consistent with or less than that expected from the decomposition of organic material. These results support previous work suggesting a coupling between iron cycling and phosphate cycling in suboxic environments. Data presented here show that this coupling may result in either preferential phosphate burial or release relative to organic material in suboxic environments.

Trace metal concentrations in the Ross Sea and their relationship with nutrients and phytoplankton growth

Abstract Dissolved and particulate trace metal concentrations (dissolved Fe, Zn, Cd, Co, Cu and Ni; particulate Fe, Mn and Al) were measured along two transects in the Ross Sea during austral summer of 1990. Total Fe concentrations in southern Ross Sea and inshore waters were elevated >3.5 times that of northern waters. Dissolved Zn, Cd and Co concentrations were lower by factors of 4.5, 3.5 and 1.6 in southern surface waters relative to northern waters. Dissolved Cu and Ni concentrations were similar in both areas. Elevated Fe concentrations coincided with areas of increased productivity, phytoplankton biomass and nutrient drawdown, indicating that Fe is an important factor controlling the location of phytoplankton blooms in the Ross Sea. Particulate concentrations of Fe, Mn and Al indicate two possible sources of iron to the Ross Sea, resuspension of continental shelf sediments and iron incorporated in annual sea ice and released with meltwaters.

Chemical and biological interactions in the Rose Garden hydrothermal vent field, Galapagos spreading center

The concentrations of a suite of redox reactive chemicals were measured in the Rose Garden hydrothermal vent field of the Galapagos spreading center. Sulfide, silicate, oxygen and temperature distributions were measured in situ with a submersible chemical analyser. In addition, 15 chemical species were measured in discrete samples. Variability in the slope of the temperature-silicate plots indicates that heat is lost from these relatively low temperatures (<15°C) solutions by conduction to the solid phase. Consumption of oxygen, sulfide and nitrate from the hydrothermal solution as it flows past the vent animals is apparent from the distributions measured in situ and in the discrete samples. The fraction of sulfide and nitrate removed from the solution by consumption appears to have increased between 1979–1985. Sulfide and oxygen appear to be consumed under different conditions: sulfide is removed primarily from the warmest solutions, and oxygen is consumed only from the cold seawater. This separation may be driven primarily by the increased gradients of each chemical under these conditions. There is no evidence for the consumption of significant amounts of manganese(II) by the vent organisms. The analysis of other data sets from this vent field indicate no significant consumption of methane by the vent organisms, as well.

In Situ Measurements of Chemical Distributions in a Deep-Sea Hydrothermal Vent Field

Large changes in the concentration of sulfide around a hydrothermal vent in the Gal�pagos Rift provide direct evidence for the consumption of sulfide by the organisms of the vent community. These changes were detected with a new chemical analyzer capable of measuring silicate, sulfide, oxygen, and temperature on the sea floor at depths of 2500 meters. More than 10,000 measurements showed systematic variations in the sulfide and oxygen concentrations due to biogenic oxidation of sulfide in the hydrothermal solutions. Silicate concentration was highly correlated with temperature, but different trends were observed at different locations.