Thomas M. Church

The atmospheric input of trace species to the world ocean

Over the past decade it has become apparent that the atmosphere is a significant pathway for the transport of many natural and pollutant materials from the continents to the ocean. The atmospheric input of many of these species can have an impact (either positive or negative) on biological processes in the sea and on marine chemical cycling. For example, there is now evidence that the atmosphere may be an important transport path for such essential nutrients as iron and nitrogen in some regions. In this report we assess current data in this area, develop global scale estimates of the atmospheric fluxes of trace elements, mineral aerosol, nitrogen species, and synthetic organic compounds to the ocean; and compare the atmospheric input rates of these substances to their input via rivers. Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. Careful consideration was given to atmospheric transport processes as well as to removal mechanisms and the physical and physicochemical properties of aerosols and gases. Only annual values were calculated. On a global scale atmospheric inputs are generally equal to or greater than riverine inputs, and for most species atmospheric input to the ocean is significantly greater in the northern hemisphere than in the southern hemisphere. For dissolved trace metals in seawater, global atmospheric input dominates riverine input for Pb, Cd, and Zn, and the two transport paths are roughly equal for Cu, Ni, As, and Fe. Fluxes and basin-wide deposition of trace metals are generally a factor of 5-10 higher in the North Atlantic and North Pacific regions than in the South Atlantic and South Pacific. Global input of oxidized and reduced nitrogen species are roughly equal to each other, although the major fraction of oxidized nitrogen enters the ocean in the northern hemisphere, primarily as a result of pollution sources. Reduced nitrogen species are much more uniformly distributed, suggesting that the ocean itself may be a significant source. The global atmospheric input of such synthetic organic species as HCH,PCBs, DDT, and HCB completely dominates their input via rivers.

The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean

Five large rivers that discharge on the western North Atlantic continental shelf carry about 45% of the nitrogen (N) and 70% of the phosphorus (P) that others estimate to be the total flux of these elements from the entire North Atlantic watershed, including North, Central and South America, Europe, and Northwest Africa. We estimate that 61 · 109 moles y−1 of N and 20 · 109 moles y−1 of P from the large rivers are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial of river sediments that are deposited directly on the continental slope. The effective transport of active N and P from land to the shelf through the very large rivers is thus reduced to 292 · 109 moles y−1 of N and 13 · 109 moles y−1 of P.The remaining riverine fluxes from land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental shelf is inversely correlated with the log mean residence time of water in the system. This is consistent with numerous observations of nutrient retention and loss in temperate lakes. Denitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appears to be directly proportional to the log mean water residence time. In general, we estimate that estuarine processes retain and remove 30–65% of the total N and 10–55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to the shelf amounts to 172–335 · 109 moles y−1 of N and 11–19 · 109 moles y−1 of P. These values are similar to the effective contribution from the large rivers that discharge directly on the shelf.For the North Atlantic shelf as a whole, N fluxes from major rivers and estuaries exceed atmospheric deposition by a factor of 3.5–4.7, but this varies widely among regions of the shelf. For example, on the U.S. Atlantic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4–2.2. This deficit must be met by a flux of N from the deeper ocean. Burial of organic matter fixed on the shelf removes only a small fraction of the total N and P input (2–12% of N from land and atmosphere; 1–17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings is very small. The gross exchange of N and P between the shelf and the open ocean is much larger than inputs from land and, for the North Atlantic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atlantic continental shelf appears to remove some 700–950· 109 moles of N each year from the deep ocean and to transport somewhere between 18 and 30 · 109 moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are included in the total balance, the net flux of N to the shelf is reduced by 60 · 109 moles y−1 and the P flux to the ocean is increased by 20 · 109 moles y−1. These conclusions are quite tentative, however, because of large uncertainties in our estimates of some important terms in the shelf mass balance.

Atmospheric deposition of nutrients to the North Atlantic Basin

Atmospheric chemical models are used to estimate the deposition rate of various inorganic oxides of nitrogen (NOy), reduced nitrogen species (NHx) and mineral dust to the North Atlantic Ocean (NAO). The estimated deposition of NOy to the NAO (excluding the coastal ocean) and the Caribbean is 360 × 109 Moles-N m-2 yr-1 (5.0 Tg N); this is equivalent to about 13% of the estimated global emission rate (natural and anthropogenic) and a quarter of the emission rate from sources in North America and Europe. In the case of NHx, 258 Moles-N m-2 yr-1 (3.6 Tg N) are deposited to the NAO and the Caribbean; this is about 6% of the global continental emissions. There is relatively little data on the deposition rate of organic nitrogen species; nonetheless, this evidence suggests that concentrations and deposition rates are comparable to those for inorganic nitrogen.

Sulfur speciation and associated trace metals (Fe, Cu) in the pore waters of Great Marsh, Delaware

Abstract The pore waters of sediments from a salt marsh along the Delaware estuary have been analyzed for sulfur species and associated trace metals. Since the sediment interface is usually in contact with the atmosphere, the sulfur species are dependent on the production of hydrogen sulfide by sulfate reduction and subsequent oxidation by diffusing oxygen. The most important species observed are hydrogen sulfide, polysulfide ions and thiosulfate. Secondary reactions of hydrogen sulfide and polysulfides with decomposing organic matter yield significant concentrations of both thiols and organic polysulfides. Upon isolation of the sediment from the atmosphere due to tidal inundation, bacterial sulfate reduction becomes the dominant process. This results in the reduction of the polysulfides in agreement with thermodynamic predictions, and suggests that the redox couple sulfide/polysulfide is a good redox indicator under such reducing environments. The concentrations of trace elements Cu and Fe in the pore waters are mainly controlled by sulfide formation. Calculations show that copper is strongly complexed probably with organo-sulfur ligands. Iron might be complexed as such sulfur species to a much lesser extent than copper.

Impact of anthropogenic combustion emissions on the fractional solubility of aerosol iron: Evidence from the Sargasso Sea

We report empirical estimates of the fractional solubility of aerosol iron over the Sargasso Sea during periods characterized by high concentrations of Saharan dust (summer 2003) and by low concentrations of aerosols in North American/maritime North Atlantic air masses (spring 2004 and early summer 2004). We observed a strong inverse relationship between the operational solubility of aerosol iron (defined using a flow-through deionized-water leaching protocol) and the total concentration of aerosol iron, whereby the operational solubility of aerosol iron was elevated when total aerosol iron loadings were low. This relationship is consistent with source-dependent differences in the solubility characteristics of our aerosol samples and can be described by a simple mixing model, wherein bulk aerosols represent a conservative mixture of two air mass end-members that carry different aerosol types: “Saharan air,” which contains a relatively high loading of aerosol iron (27.8 nmol Fe m−3) that has a low fractional solubility (0.44%), and “North American air,” which contains a relatively low concentration of aerosol iron (0.5 nmol Fe m−3) that has a high fractional solubility (19%). Historical data for aerosols collected on Bermuda indicate that the low iron loadings associated with North American air masses are typically accompanied by elevated V/Al, Fe/Al, and V/Mn mass ratios in the bulk aerosol, relative to Saharan dust, which are indicative of anthropogenic fuel-combustion products. The identification of similar compositional trends in our Sargasso Sea aerosol samples leads us to suggest that the elevated solubility of iron in the aerosols associated with North American air masses reflects the presence of anthropogenic combustion products, which contain iron that is readily soluble relative to iron in Saharan soil dust. We thus propose that the source-dependent composition of aerosol particles (specifically, the relative proportion of anthropogenic combustion products) is a primary determinant for the fractional solubility of aerosol iron over the Sargasso Sea. This hypothesis implies that anthropogenic combustion emissions could play a significant role in determining the atmospheric input of soluble iron to the surface ocean.

Seasonal iron cycling in the salt-marsh sedimentary environment: the importance of ligand complexes with Fe(II) and Fe(III) in the dissolution of Fe(III) minerals and pyrite, respectively

Abstract A biogeochemical cycle is proposed for the reactivity of iron in salt-marsh sediments. The main reactions of the iron cycle are: (1) solubilization of Fe(III) by organic ligands; (2) reduction of soluble Fe(III) to Fe(II) by these ligands, soluble reduced sulfur or solid phase reduced sulfur; (3) the oxidation of the resulting Fe(II) (complexed to organic chelates) by Fe(III) minerals; (4) the formation of iron sulfide minerals when dissolved sulfide is in excess. The cycle of iron solubilization will continue as long as bacteria and/or plants produce organic ligands. The cycle will stop when sulfate reduction rates are high and organic ligand production is low. At this point soluble hydrogen sulfide reacts with Fe(II) and Fe(III) to form sulfide minerals. Penetration of O 2 into the surface sediments will also oxidize Fe(II) to Fe(III) with subsequent formation of Fe(III) (oxy)hydroxide minerals. The reactions which represent the iron cycle indicate that the iron mineral system has substantial acid/base buffering capacity. The ligands responsible for the cycling of iron are weak field anionic ligands containing oxygen as the ligating atom. The electron transfer from Fe(II) complexes to Fe(III) minerals is discussed using the molecular orbital approach. An outer sphere electron transfer is possible. Laboratory evidence is presented for the reaction of Fe(III) complexes with pyrite over the pH range of 4–6.5. The Fe(II) production rate and pH decrease are consistent with field data from Great Marsh, Delaware. The direct oxidant for the oxidation of pyrite and other reduced sulfur compounds in salt-marsh sediments is Fe(III) rather than oxygen based on the cycle and data presented. Oxygen is not present in any pore waters sampled in this work. This is consistent with the microelectrode work of other researchers. Manganese oxides are not likely oxidants in salt-marsh sediments in Great Marsh, Delaware because they are not as abundant as Fe(III) minerals. The iron cycle presented may occur in other marine and freshwater sedimentary systems and in aquatic systems with oxic/anoxic interfaces.

The geochemistry of salt marshes: Sedimentary ion diffusion, sulfate reduction, and pyritization

A series of seasonal cores was taken in a high marsh near the terminus of Delaware Bay, U.S.A. A seasonal harmonic diffusion model was successfully fit to the concentration profiles of chloride ion in the salt marsh pore waters yielding a calculated sedimentary diffusion coefficient. Virtually all other chemical reactions within salt marsh sediments are directly linked to the rate and stoichiometry of organic decomposition. The rich organic input from the grass Spartina alterniflora is oxidized anaerobically through the process of sulfate reduction. Over 90% of this net decomposition of organic matter takes place in the uppermost 20 cm. The model for sulfate reduction proposed yields an internally consistent set of both pore water (HCO−3, NH+4, HPO2−4, HS−, SO2−4) and solid phase (FeS2) distribution profiles for these sediments. Steady state assumptions and the use of mean annual constants can be employed to model the net rates of diagenetic processes in salt marshes. The pore water concentrations of sulfate ion as well as those ions released by sulfate reduction (HCO−3, NH+4, HPO2−4, HS−) are modeled by a system composed of an upper zone, where extensive reconsumption of these metabolite ions occurs, and a lower zone where steady state production and no ion reconsumption occurs. A major product of the sulfate reduction is pyrite, whose accumulation rate is greatest between 7 and 9 cm depth, where it equals the net rate of sulfate reduction. Above this zone little pyrite accumulates due to extensive reoxidation. Below 9 cm the rate of pyritization is controlled by the rate of sulfidation of a refractory iron phase.

Marine barite saturation

Abstract Theoretical calculations of barium sulphate saturation in the sea water column have been compared to the barium concentrations in the eastern Pacific Ocean and in pore waters filtered from sediments containing marine barite. There is good agreement between the calculated saturation value, 44–49 μg/kg barium at 4–5 km depth and 1°C, and the barium concentrations of the corresponding pore waters filtered from barite-bearing sediments. However, the barium concentration of the entire sea water column in the eastern Pacific and probably most of the worlds oceans seems to fall below the calculated barite saturation curve. Thus the observed increase of barium concentration with depth in the Pacific is not the result of simple chemical equilibrium between solid barite and sea water of differing temperature and pressure. The barium concentration in pore waters of deep-sea sediments containing barite is, however, limited by saturation of marine barite which appears to form by authigenic precipitation within the sediment.

Inorganic and organic sulfur cycling in salt-marsh pore waters.

Sulfur species in pore waters of the Great Marsh, Delaware, were analyzed seasonally by polarographic methods. The species determined (and their concentrations in micromoles per liter) included inorganic sulfides (≤3360), polysulfides (≤326), thiosulfate (≤104), tetrathionate (≤302), organic thiols (≤2411), and organic disulfides (≤139). Anticipated were bisulfide increases with depth due to sulfate reduction and subsurface sulfate excesses and pH minima, the result of a seasonal redox cycle. Unanticipated was the pervasive presence of thiols (for example, glutathione), particularly during periods of biological production. Salt marshes appear to be unique among marine systems in producing high concentrations of thiols. Polysulfides, thiosulfate, and tetrathionate also exhibited seasonal subsurface maxima. These results suggest a dynamic seasonal cycling of sulfur in salt marshes involving abiological and biological reactions and dissolved and solid sulfur species. The chemosynthetic turnover of pyrite to organic sulfur is a likely pathway for this sulfur cycling. Thus, material, chemical, and energy cycles in wetlands appear to be optimally synergistic.

Sulfur enrichment of humic substances in a Delaware salt marsh sediment core

Abstract Humic sulfur, operationally defined as the sulfur extracted with humic substances in 0. l N NaOH solution, comprises up to 51% of the total sulfur inventory in a sediment core taken from a Delaware Spartina alterniflora marsh. Pyrite sulfur is the next largest fraction, except at the near-surface sediments, where zerovalent sulfur concentrations are significant. X-ray photoelectron spectroscopy indicates that the humic sulfur consists of sulfoxides or sulfones and, in a more reduced state, organic sulfides and/or organic polysulfides. A subsurface decrease in the humic acid C:S atomic ratio to 56 ± 2 suggests that the upper 4 cm of marsh sediment is the locus for humic sulfur formation. S. alterniflora detritus and microbial biomass cannot fully account for observed sulfur enrichment of humic C:S atomic ratios. Therefore, the enrichment of humic substances by sulfur is probably via reaction of reduced sulfur compounds with organic matter. A humic sulfur formation rate of 10.6 μmol S · cm−3 · a−1 is calculated for the surface sediments and leads to an areal production of 18 μmol S · cm−2 · a−1 of humic sulfur. Humic sulfur formation and preservation is enhanced by the limited availability of iron for the rapid precipitation of iron sulfide minerals and the apparent resistance of organic sulfur compounds towards reoxidation to sulfate, especially in the upper 9 cm of marsh sediment where inorganic sulfur compounds are rapidly oxidized.