OS14B-01 INVITED
Iron Distribution in the Surface and Oxygen Minimum Waters of the Tropical North Atlantic
Bergquist and Boyle (2006, GBC 20 GB1015) and Measures et al. (2008, GBC 22 GB1005) have reported that high iron levels (>1 nmol/kg) are found in the oxygen minimum zones (OMZ) of the western and central tropical North Atlantic. Although iron should be expected to peak in oxygen minimum waters due to decomposition from sinking biological debris, iron levels in the OMZ of the tropical North Atlantic (TNA) are significantly higher than would be expected from typical Fe:C ratios of marine organic matter. Two possibilities for these high Fe levels seem possible: (1) Fe may escape from reducing continental margin sediments on the African margin and diffuse/advect into the ocean interior, or (2) Fe:C ratios of biological debris in the TNA may be higher than elsewhere because of the supply of iron from North African dust transport into the surface waters. In order to better constrain these possibilities, we have occupied 27 stations in the TNA between 6-20 degN and 21-60 degW (August 2008) collecting data for hydrography (T,S) nutrients (O2, P, NO3, Si) and samples for Fe and Mn from the upper 1000 m (all stations) and full water column (5200 m, one station). We will present our preliminary measurements from this cruise to address the question of the extent to which the Fe distribution can be explained either by mixing relationships between end members or by in-situ injection of Fe from biological decay.
OS14B-02
Toxicity of Atmospheric Aerosols on Marine Phytoplankton
Atmospheric aerosol deposition is an important source of nutrients and trace metals to the open-ocean that can enhance ocean productivity and carbon sequestration and thus influence atmospheric carbon dioxide concentrations and climate. Using well-characterized aerosol samples in incubation experiments, we demonstrate, however, that the response of phytoplankton growth to aerosol additions depends on the specific aerosol chemistry and differs across phytoplankton species. Aerosol additions enhanced growth by releasing nitrogen and phosphorus, but not all aerosols stimulated growth. Toxic effects were observed with some aerosols. This could be due to high copper, consistent with direct copper addition experiments. Toxicity affected picoeukaryotes and Synechococcus, the second most abundant marine phytoplankter, but not Prochlorococcus. Anthropogenic emissions are increasing atmospheric copper deposition sharply and based on coupled atmosphere-ocean calculations, potentially altering patterns of marine primary production and community structure in high aerosol deposition, low chlorophyll areas.
OS14B-03
The isotopic composition of fixed nitrogen species in precipitation at Bermuda: Implications for the atmosphere and ocean
Recent estimates document an increasing input of anthropogenic fixed nitrogen to the world's oceans, so that it now contributes as much as a third of the oceans' external nitrogen supply. An important component of this input is from atmospheric nitrate, ammonium and organic nitrogen. Using wet-only deposition samples collected over a year in Bermuda, we consider the relationships between the isotopic composition of nitrate, total nitrogen, and reduced nitrogen species (reduced nitrogen (RN) ~ ammonium + dissolved organic nitrogen). During the cool season, when air chemistry at Bermuda is influenced directly by continental emissions from North America, the average δ15N of nitrate and RN are -6.8‰ and - 2.7‰ (vs. N2), respectively. Both nitrate and RN δ15N are significantly higher on average during the warm season (-1.9 and +1.5‰, respectively), when air arriving at Bermuda is typically from the south and southeast. The similarity in seasonality between nitrate and RN δ15N suggests that their sources are related. However, there are a number of additional hypotheses that arise: the maxima in RN δ15N observed in spring and late summer may be related to the typical maxima found in rainfall ammonium concentrations; marine emissions of NH3 could potentially play a role in the seasonal variability of ammonium concentrations and RN δ15N; the temporal similarity between nitrate and RN δ15N and the fact that RN δ15N is typically higher than that of nitrate is consistent with the idea that nitrate is potentially derived from RN. An additional observation of note is that the high oxygen isotopic composition of nitrate in Bermuda rain (δ18O = 60.3 to 86.5‰, Δ17O = 22.5 to 29.4‰ vs. VSMOW) suggests that alkyl nitrates do not make up a significant component of the deposited nitrate, since this formation pathway for nitrate is expected to be influenced by the very low oxygen isotopic composition of atmospheric oxygen and/or water vapor. Several implications for ocean biogeochemistry are also apparent from our results. The total nitrogen precipitation flux (~ 10 to 19 mmol N m-2 yr-1) is twice that of nitrate, and the annual average δ15N of TN is -2.3‰. The low δ15N of atmospherically (wet) deposited N may substantially reduce the rate of oceanic N2 fixation required to explain the low-δ15N of thermocline nitrate measured in a variety of regions. For example, using our δ15N of TN, the TN flux range above, and a N2 fixation rate estimate of 45 mmol N m-2 yr-1 (Hansell et al., Mar. Chem., 2004), atmospheric deposition can explain 25 to 50% of the previously documented decrease in nitrate δ15N in the thermocline of the Sargasso Sea. A low δ15N source from the atmosphere, primarily derived from anthropogenic sources, also has implications for interpreting changes in δ15N in the marine environment over time.
OS14B-04
Atmospheric Contributions to Excess Nitrate Development in the Subtropical North Atlantic: Atmospheric Viewpoint
Atmospheric deposition of high N:P material to the subtropical North Atlantic has more than doubled in the past century due to anthropogenic activity, and is increasingly thought to be an important source of essential nutrients to the oligotrophic subtropical gyre. However, the long-term fate of North Atlantic atmospheric nitrogen deposition is not well understood. This modeling study evaluated an observed pool of N in excess of Redfield ratios located in the main thermocline as a potential sink for atmospheric N. Modeled atmospheric deposition was added to a coupled ocean ecosystem and circulation model. Results suggest that nearly half of the atmospheric nitrogen entering the North Atlantic is transported to the main thermocline, contributing ~15% of the annual growth of excess N there. Transport mechanisms include differential remineralization of N and P in sinking biogenic particles and physical transport. If atmospheric nutrient inputs from the year 2000 were maintained for 50 years, the model suggests that nutrient deposition would contribute to an increase in excess N of more than 0.4 μM, or an additional 45% of the present signal. Quantifying the fate and important transport mechanisms of deposited atmospheric nutrients will improve our understanding of N cycle dynamics in the North Atlantic, as well as improve N2 fixation estimates based on mass-balance techniques.
OS14B-05 INVITED
Modeling of atmospheric transport and deposition of iron to the oceans
Today it is widely accepted that atmospheric transport and deposition of mineral dust from continental regions plays an important role in oceanic iron (Fe) supply. The major challenge for oceanic biogeochemists is the conversion of dust deposited fluxes of Fe to those that are bioavailable, i.e., fluxes that can directly affect marine ecosystem productivity. Formation of soluble Fe in ambient aerosols is very complex process. Iron solubility can be strongly influenced by properties of the overall system (e.g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and properties of the minerals (e.g. stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Dissolved Fe concentrations in ambient aerosols can be further controlled by cloud cycling of aerosols and complex photochemical/chemical cycling between highly insoluble ferric, Fe(III) and relatively more soluble ferrous, Fe(II) complexes. This talk will give an overview of prognostic and diagnostic modeling approaches for chemical evolution of Fe in ambient aerosols and fluxes of dissolved Fe to different parts of the global ocean. The relative importance of different sources and mechanisms in Fe solubilization during atmospheric transport of aerosols will also be addressed. Results taken from in-situ and laboratory experiments will be used to illustrate the existing uncertainty areas in the modeling field. Some insights will be shared on the need and direction of future research.
OS14B-06
The role and implementation of a two-step mechanism for hematite dissolution on aeolian input of soluble iron to the ocean
Recent observational research has provided growing evidence of increased iron solubilization with transport time. It has been shown that acid coating of mineral dust aerosols increases the rate of hematite dissolution and the solubility of ferric hydroxide, both by orders of magnitude. Acid coating of fresh dust particles results from adsorption of gaseous molecules of strong acids and from cloud processing in the atmosphere. The ensuing hematite dissolution time-scale in acid coating is estimated to be comparable to the average time of transport of fresh aerosols before going through cloud processing. A two-step mechanism is implemented in the GFDL Global Chemical Transport Model for hematite dissolution, which produces very low iron solubility near desert source region and high iron solubility in aged mineral dust aerosols, thus significantly increasing the deposition of bioavailable iron over the remote oceans. Utilizing this mechanism, a significant correlation was found between model calculated soluble iron deposition and in-situ measurements of net community production in the Southern Ocean. This result suggests that ocean biogeochemistry models should use variable iron solubility based on the two-step mechanism and calibrated by measurements of aerosol iron soluble fraction.
OS14B-07
The Impacts of Increasing Soluble Iron and Nitrogen Deposition on Ocean Biogeochemistry
We present results from sensitivity studies with the Biogeochemical Elemental Cycling (BEC) ocean model to increasing atmospheric inorganic N and soluble iron deposition since the pre-industrial era. Increasing soluble iron deposition results from increased atmospheric processing in the presence of anthropogenic pollutants and soluble iron from combustion sources. Increasing iron inputs raised nitrogen fixation by ~11% globally, with the majority of the increase in the sub-tropical north and south Pacific, and increased production and export in the high nutrient low chlorophyll (HNLC) regions. Increasing inorganic N inputs impacted coastal and open ocean regions close to major source areas in South and East Asia, North America and Europe. N2-fixation decreased by ~6% compared to the control because the diazotrophs (N2-fixers) were out-competed by diatoms and small phytoplankton. Net global nitrogen fixation, increased by ~5% compared to the control, in the simulation with increasing iron and nitrogen inputs. Global primary production increased by 2%, 0.4% and 2.4% under increasing Fe, inorganic N, and combined Fe and N inputs relative to the pre-industrial. This resulted in an increase in sinking POC export at 103m by 2.5%, 0.5%, and 3.2% leading to a reduction in atmospheric pCO2 by 1.8ppm, 0.5ppm and 2.2ppm under the three different cases relative to our control. Our results suggest that increasing combustion iron sources and aerosol Fe solubility along with atmospheric anthropogenic nitrogen deposition are significantly perturbing marine biogeochemical cycling and could partially explain the observed trend towards increased P limitation at Station Aloha in the sub-tropical North Pacific.
OS14B-08
Human perturbation to atmospheric phosphorus
A worldwide compilation of atmospheric total phosphorus (TP) and phosphate (PO4)concentration and deposition flux observations are combined with transport model simulations to derive the global distribution of concentrations and deposition fluxes of TP and PO4. Our results suggest that mineral aerosols are the dominant source of TP on a global scale (82 percent), with primary biogenic particles (12 percent) and combustion sources (5 percent) important in non dusty regions. Globally averaged anthropogenic inputs are estimated to be approximately 5 and 15 precent for TP and PO4, respectively and may contribute as much as 50 percent to the deposition over the oligotrophic ocean where productivity may be phosphorus limited. There is a net loss of TP from many (but not all) land ecosystems, and a net gain of TP by the oceans (560 Gg P yr-1). More measurements of atmospheric TP and PO4 will assist in reducing uncertainties in our understanding of the role that atmospheric phosphorus may play in global biogeochemistry.