Roland Wollast

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.

Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals

Abstract The influence of pH on the rate of dissolution of various carbonates (calcite, aragonite, witherite, magnesite and dolomite) has been investigated at 25°C using a continuous fluidized bed reactor. The general rate dependence on pH observed for the simple carbonates is very similar and is in agreement with the results observed for calcite and aragonite by L.N. Plummer and coworkers. However, the rate of dissolution of magnesite is approximately four orders of magnitude lower than calcite. For simple carbonates, the elementary steps involved in the dissolution reaction are: where M represents the metal ion which can be Ca, Mg and Ba. According to the stoichiometry of the three reaction steps and the thermodynamic constraints, the total forward and backward rates can be expressed as: R f =k 1 a H +k 2 a H 2 CO 3 ∗ +K 3 r b =k -1 a M 2 + a HCO 3 - +k -2 a HCO 3 - +k -3 a M 2 + a CO 3 2- The rate constants (k1, k2, k3 and k−3) determined with our experimental results for calcite, aragonite and witherite show that the dissolution rates are similar for these three minerals and that the nature of the cations does not play a significant role. The good agreement between the Ksp calculated from the measured k 3 k −3 ratio and the theromodynamic value suggests that our dissolution mechanism is coherent. The rate dependence on pH of the dissolution of dolomite obeys a fractional order at low pHs and confirms previously published observations therein. However, the two-step reaction mechanism proposed does not explain the fractional reaction order observed, which is likely due to a more complex surface reaction.

Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor

Abstract A continuous reactor based on the fluidized bed technique was developed in order to study the kinetics and the mechanisms of the initial stages of weathering of albite. Simultaneous determination of Si, Al and Na and the observed low concentrations of the dissolved elements which were always at levels below saturation with respect to possible secondary precipitates, indicate that formation of a residual layer of a few tens of angstroms occurred at the surface of the feldspar. The composition of this layer, enriched in Si and/or Al, is strongly dependent on the pH of the aqueous solution. The formation of the layer is followed by the establishment of a quasi-steady state during which the dissolution of albite tends to become stoichiometric.

Interactions of C, N, P and S Biogeochemical Cycles and Global Change

This is an up-to-date synthesis of the global biogeochemical cycles of C, N, P and S. Processes and models involving the flow of these elements in and between the ocean, atmosphere, biosphere and land are emphasized. Human-induced perturbations to the global cycles are discussed, and the role of these space scales of global change is considered, from the geological past to the present and the future. Feedback mechanisms that enhance or ameliorate global change are an important feature. The book is strongly interdisciplinary in scope.

Kinetics of the alteration of K-feldspar in buffered solutions at low temperature

Abstract A study has been made of the release of Si and Al to solution from the alteration of a potassic feldspar in solutions buffered at pH values between 4 and 10. Release of both Si and Al is consistent with diffusion from an altered layer, presumably formed by rapid initial hydration and exchange of H+ for K+. In a limited volume of solution diffusion ceases when the Al concentration in the external solution reaches a fixed value at each pH; this value is reasonably consistent with the solubility of Al(OH)3. The Si concentration tends to reach a maximum at each pH. The interpretation is made that the maximum in Si concentration corresponds to a balance between Si diffusion into the solution, and Si removed from the solution by reaction with Al(OH)3 to form a hydrated silicate. The calculated equilibrium value for the reaction Al(OH)3 + SiO2aq = Al-silicate is 5 ppm SiO2.

Mechanism of kaolinite dissolution at room temperature and pressure Part II: kinetic study

Abstract Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.

The Portugal coastal counter current off NW Spain: new insights on its biogeochemical variability

Abstract Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September–October) to the cessation (February–May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40° and 43°N have previously been undersampled. Novel information extracted from these observations relate to: 1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)—the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

General description of the Scheldt estuary

A general description of the Scheldt estuary,including the hydrology, the sediment transport, theproductivity and the biodegradation with respect totheir influence on the trace metal behaviour in theScheldt estuary, is given.The river basin can be divided in several sectionsaccording to their morphological, hydrodynamical andsedimentary properties. The zone from km 78 to 55,which corresponds roughly with the salinity zone from2 to 10 psu, is the zone of high turbidity, highsedimentation and of oxygen depletion, especially inthe summer period. That area is called the geochemicalfilter because the solid/dissolved distribution of thetrace metals is controlled by redox,adsorption/desorption, complexation andprecipitation/coprecipitation processes. Thesedimentation rate in that area is estimated at280 Mkg y-1.In the downstream estuary the phytoplankton activityincreases due to the restoration of oxygen and to themuch lower turbidity values. That area is called thebiological filter because incorporation of tracemetals by the plankton communities lowers the tracemetal concentrations during the productivity period,while transformation of metal species, especiallyobserved with mercury, occurs during that period too.

Distribution, composition and flux of particulate material over the European margin at 47°–50°N

In the framework of the Ocean Margin Exchange project, a multi-disciplinary study has been conducted at the shelf edge and slope of the Goban Spur in order to determine the spatial distribution, quantity and quality of particle flux, and delineate the transport mechanisms of the major organic and inorganic components. We present here a synthesis view of the major transport modes of both biogenic and lithogenic material being delivered to the open slope of the Goban Spur. We attempt to differentiate between the direct biogenic flux from the surface mixed layer and the advective component, both biogenic and lithogenic. Long-term moorings, instrumented with sediment traps, current meters and transmissometers have yielded samples and near-continuous recordings of hydrographic variables (current direction and speed, temperature and salinity) and light transmission for a period of 2.5 years. Numerous stations have been occupied for CTD casts with light transmission and collection of water samples. The sedimenting material has been analysed for a variety of marker compounds including phytoplankton pigments, isotopic, biomineral and trace metal composition and microscopical analyses. These samples are augmented by seasonal information on the distribution and composition of fine particles and marine snow in the water column. The slope shows well-developed bottom nepheloid layers always present and intermediate nepheloid layers intermittently present. Concentrations are mainly in the range 50–130 mg m−3 in nepheloid layers and 6–25 mg m−3 in clear water. A seasonal variability in the concentration at the clear water minimum is argued to be related to seasonal variations in vertical flux and aggregate break-up in transit during summer months. It is suggested that the winter sink for this seasonal change in particulate matter involves some re-aggregation and scavenging, and some conversion of particulate to dissolved organic matter. This may provide a slow seasonal pump of dissolved organic carbon to the deep ocean interior. Differences in trapped quantities at different water depths are interpreted as due to lateral flux from the continental margin. There is a major lateral input between 600 and 1050 m at an inner station and between 600 and 1440 m at an outer one. The transport is thought to be related to intermediate nepheloid layers, but those measured are too dilute to be able to supply the flux. Observed bottom nepheloid layers are highly concentrated very close to the bed (up to 5 g m−3), with a population of large aggregates. Some of these are capable of delivering the flux seen offshore during intermittent detachment of nepheloid layers into mid-water. Concentrated bottom nepheloid layers are also able to deliver large particles with unstable phytoplankton pigments to the deep sea floor in a few tens of days. Calculated CaCO3 fluxes are adjusted for dissolution, which is inferred from Ca/Al ratios to be occurring in the CaCO3-saturated upper water column where up to 80% of the CaCO3 resulting from primary production is dissolved.

Transport and transformation of trace metals in the scheldt estuary

Abstract Water samples collected along the Scheldt estuary during several cruises were analysed for fundamental physico-chemical parameters and for dissolved and particulate trace metals (Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn). Mean suspended matter concentration and upstream dissolved concentrations of Ni, Cr and Mn are increasing with river discharge, suggesting a resuspension of fluid mud and release of dissolved elements from interstitial waters. Dissolved and particulate Ni and Cr behave conservatively during estuarine mixing, while Mn is removed from the dissolved phase and Cd, Cu and Zn are on the contrary mobilized. The distribution coefficients, K D s, show at low salinities the following sequence Cd > Cu > Zn > Ni. The K D s of Cd, Cu and Zn are well correlated with dissolved oxygen content suggesting a redissolution of sulphide produced in the anoxic part of the estuary. The analysis of suspended matter in the upper Scheldt confirms the high contamination of this estuary for Cd, Cr, Cu, Pb and Zn. The long residence time of the water in the estuary, due to its small river discharge and large tidal influence and the important variations of the river input make it difficult to estimate metal fluxes to the North Sea, without the use of an elaborated coupled hydrodynamical/biogeochemical model.