Biogeochemical processes in a freshwater–seawater mixing zone in permeable sediments along the Coast of Southern Brazil

The southern portion of the Brazilian coast is dominated by coastal lagoons formed by sandy barrier spits with small inlets. This coastal configuration is a barrier to the surface flow of freshwater to the sea; thus, we suspect that a significant amount of freshwater flows through the permeable sands, beneath the barrier spits, where it mixes with seawater. We excavated an 18-mdeep well into the barrier spit which separates the Patos Lagoon from the South Atlantic. Using this well, we were able to sample interstitial waters from discrete layers, at 1-m intervals, which were analyzed for salinity, temperature, pH, nutrients (ammonium, nitrate, phosphate, and silicate), uranium, molybdenum, and barium. Similar analyses were made on surface water samples from the Patos Lagoon estuarine mixing zone. Results of well samples show a continuous increase in salinity with depth reaching 18 at the bottom. Ammonium and silicate are high, generally around 100 and 100–150 AM, respectively, throughout the subterranean profile. Phosphate shows a distinct maximum at about 6 m (ca. 25 AM), and nitrate is generally low in all well samples. Uranium and molybdenum exhibit a minimum in the well profile at about the same location where barium exhibits a maximum (greater than 2 AM). When results are compared to the surface lagoon–seawater mixing data, ammonium, phosphate, silicate, and barium in well samples of similar salinity show considerable enrichment, while a comparison of uranium and molybdenum data indicates significant depletion of these metals in most well samples. Based on these and other data, we deduce that the following processes are active: products of remineralization of organic detritus accumulated in lagoon sediments are advected through permeable sediments to the oceans; dissolution of biogenic solids and/or solid silicates mobilizes silicate; phosphate, uranium, and molybdenum are mobilized from phosphate-rich sediment layers; sulfate reducers remove uranium and perhaps molybdenum from solution throughout most of the well profile; barium is desorbed from solids in the subterranean mixing zone. These results demonstrate that freshwater discharged to the ocean through permeable sediments may have a significantly different composition than that discharged at the surface

Arsenic Geochemistry in a controlled Marine Ecosystem

309-313Arsenic added to controlled enclosures changed in chemical speciation as predicted from laboratory measurements. The reduction of As(V)and production of As(111) and DMA (dimethylarsinic acid) in the As(V) enriched enclosure resulted in As species in proportions very similar to those measured in productive coastal marine systems. Further, the rates of As(V) reduction were remarkably similar to those measured in phytoplankton cultures exposed to As(V). Arsenite in the As(111) enriched enclosure was oxidized at a rate similar to that calculated in the laboratory. Microbial activity and the presence of dissolved organic matter could have contributed to the oxidation. The results support earlier suggestions thatbiota mediate and control the rate of As(V) reduction in marine systems. It is suggested that the concentration of As(111) is probably controlled by both chemical oxidation and the biota, and is in equilibrium between them. However, the importance of microbial oxidation and dissolved organic matter catalysis should not be ignored and needs further study

Mineralogy of some Australian desert samples (Table 3)

The relative amounts of chlorite, montmorillonite, kaolinite and illite in the less than 2 micron size fraction of pelagic sediments are related to the sources and transport paths of solid phases from the continents to the oceans and to injections of volcanic materials to the marine environment. Three modes of entry of solid phases from the lands to the seas are considered: by glaciers, by rivers and by atmospheric winds. The compositions of the clay size fraction are also related to rates of accumulation of the non-biogenous phases

Controls on metal partitioning in contaminated sediments

Issued as final reportLouisiana State University (Baton Rouge, La.

Uranium in rivers and estuaries of globally diverse, smaller watersheds

Data for uranium concentrations in 29 rivers and eight estuaries are presented. The river data expands the existing database on riverine uranium transport to include more smaller watersheds which collectively account for a large portion of material transport from the continent to the oceans. Riverine concentrations for these smaller watershed range from less than 50 to 660 pM. The results for these systems, when combined with previously published data on mostly larger rivers, do not change significantly the calculated global riverine flux and thus earlier estimates by Palmer and Edmond wPalmer, M.R., Edmond, J.M., 1993. Uranium in river water. Geochim. Cosmochim. Acta, 57, pp. 4947–4955x are substantiated. Uranium transport through eight diverse estuaries was studied to assess the importance of estuarine removal in the global marine uranium budget. Results indicate that uranium is conservatively transported in most systems studied. Results reported here for the Savannah estuary, however, indicate significant uranium removal. Our results suggest that uranium is removed in salt marsh estuaries at a rate of ca. 70 mmolrm2. This compares to a rate of 15 mmolrm2 for Delaware salt marshes wChurch, T.M., Sarin, M.M., Fleisher, M.Q., Ferchlman, T.G., 1996. Salt marshes: an important sink for dissolved uranium. Geochim. Cosmochim. Acta, 60, pp. 3879–3887x. We suggest that uranium removal to salt marsh sediments is due to anaerobic microbially mediated processes. We use these results to estimate the global significance of the salt marsh sink in the oceanic budget of uranium. We estimate that 2.7=107 mol of uranium are removed to salt marshes annually as compared to an annual global riverine input of 3–6=107 mol estimated by Palmer and Edmond wPalmer, M.R., Edmond, J.M., 1993. Uranium in river water. Geochim. Cosmochim. Acta, 57, pp. 4947–4955x. q2000 Elsevier Science B.V. All rights reserved

Inadequacy of NASQAN Data for Assessing Metal Trends in the Nation\u27s Rivers

Results of our analyses of dissolved Cd, Cu, Pb, and Zn in east coast North American rivers are considerably lower than those reported by the U.S. Geological Survey, National Stream Quality Accounting Network (NASQAN) for samples collected at similar locations during a similar time period. These results along with recent literature suggest that the NASQAN dissolved trace metal data are unreliable for the purpose of establishing water quality trends in the Nation\u27s rivers. Dissolved trace metal concentrations in east coast rivers are probably controlled more by river chemistry than by anthropogenic inputs. Trace metal concentrations on suspended particles may provide a better index of anthropogenic influences

Submarine groundwater discharge: a large, previously unrecognized source of dissolved iron to the South Atlantic Ocean

This paper reports the initial results of a study of groundwater and coastal waters of southern Brazil adjacent to a 240 km barrier spit separating the Patos Lagoon, the largest coastal lagoon in South America, from the South Atlantic Ocean. The objective of this research is to assess the chemical alteration of freshwater and freshwater–seawater mixtures advecting through coastal permeable sands, and the influence of the submarine discharge of these fluids (SGD) on the chemistry of coastal waters. Here we focus on dissolved iron in this system and use radium isotopic tracers to quantify SGD and cross-shelf fluxes. Iron concentrations in groundwaters vary between 0.6 and 180 μM. The influence of the submarine discharge of these fluids into the surf zone produces dissolved Fe concentrations as high as several micromolar in coastal surface waters. The offshore gradient of dissolved Fe, coupled with results for Ra isotopes, is used to quantify the SGD flux of dissolved Fe from this coastline. We estimate the SGD flux to be 2×106 mol day−1 and the cross-shelf flux to be 3.2×105 mol day−1. This latter flux is equal to about 10% of the soluble atmospheric Fe flux to the entire South Atlantic Ocean. We speculate on the importance of this previously unrecognized iron input to regional ocean production and on the potential significance of this source to understanding variations in glacial–interglacial ocean production

Submarine groundwater discharge of nutrients to the ocean along a coastal lagoon barrier, Southern Brazil

The Patos–Mirim Lagoon system along the southern coast of Brazil is linked to the coastal ocean by a narrow mouth and by groundwater transport through a Holocene barrier. Although other groundwater systems are apparently active in this region, the hydraulic head of the lagoon, the largest in South America, drives groundwater transport to the coast. Water levels in wells placed in the barrier respond to changing water level in the lagoon. The wells also provide a measure of the nutrient concentrations of groundwater flowing toward the ocean. Additionally, temporary well points were used to obtain nutrient samples in groundwater on the beach face of the barrier. These samples revealed a subterranean freshwater–seawater mixing zone over a ca. 240 km shoreline. Previously published results of radium isotopic analyses of groundwater and of surface water from cross-shelf transects were used to estimate a water flux of submarine groundwater discharge (SGD) to nearshore surface waters of 8.5×107 m3/day. Using this SGD and the nutrient concentrations in different compartments, nutrient fluxes between groundwater and surface water were estimated. Fluxes were computed using both average and median reservoir (i.e. groundwater and surface water) nutrient concentrations. The SGD total dissolved inorganic nitrogen, phosphate and silicate fluxes (2.42, 0.52, 5.92×106 mol day− 1, respectively) may represent as much as 55% (total N) to 10% (Si) of the nutrient fluxes to the adjacent shelf environment. Assuming nitrogen limitation, SGD may be capable of supporting a production rate of ca. 3000 g C m2 year− 1in the nearshore surf zone in this region