A.R. Flegal

Dissolved trace element cycles in the San Francisco Bay estuary

Dissolved trace element (copper, nickel, cadmium, zinc, cobalt, and iron) concentrations were measured in surface water samples collected from 27 stations in the San Francisco Bay and Sacramento—San Joaquin Delta during April, August and December of 1989. The trace element distributions were relatively similar for all three sampling periods, and evidenced two distinct biogeochemical regimes within the estuarine system. The two regimes were comprised of relatively typical trace element gradients in the northern reach and anthropogenically perturbed gradients in the southern reach of the estuary. These dichotomous trace element distributions were consistent with previous reports on the distributions of nutrients and some other constituents within the estuary. In the northern reach, trace element and dissolved phosphate concentrations were non-conservative. Simple estuarine mixing models indicated substantial internal sources of dissolved copper (46–150%), nickel (250–500%) and cadmium (630–780%) relative to riverine inputs in April and August, and sizable internal sinks for dissolved cobalt (> 99%) and iron (> 70%) during the same periods. Dissolved zinc fluxes varied temporally, with a relatively large (135%) internal source in April and a relatively small (29%) internal sink in August. Concentrations of many trace elements (copper, nickel, cadmium, zinc, and cobalt) in the southern reach were anomalously high relative to concentrations at comparable salinities in the northern reach. Mass balance calculations indicated that those excesses were primarily due to anthropogenic inputs (waste-water discharges and urban runoff) and diagenetic remobilization from benthic sediments. The magnitude of these excesses was amplified by the long hydraulic residence time of dissolved constituents within the South Bay. The influence of other factors was evident throughout the system. Notably, upwelling appeared to elevate substantially dissolved cadmium concentrations at the mouth of the estuary and authigenic flocculation appeared to dominate the cycling of dissolved iron in both the northern and southern reaches of the system. Biological scavenging, geochemical scavenging and diagenic remobilization were also found to be important in different parts of the estuary. Additional complementary information is required to quantify accurately these processes.

Trace metal distributions off the Antarctic Peninsula in the Weddell Sea

Dissolved trace metals (Ag, Al, Cd, Co, Cu, Fe, Ni, Pb, and Zn), inorganic nutrients (PO4, H4SiO4), and chlorophyll a were measured at 19 stations along a surface water transect from the Antarctic Peninsula into the Weddell Sea. The range of concentrations of metals and nutrients measured along the western rim of the Weddell Sea were consistent with previous results from the Southern Ocean. Metal levels measured in the Weddell Sea showed two distinct patterns: (1) metals (Al, Co, and Pb) that generally had lower levels in the Weddell Sea; and (2) dissolved constituents (Ag, Cd, Cu, Fe, Ni, Zn, PO4, H4SiO4) that showed an enrichment in the Weddell Sea, as compared to other ocean basins. While this dichotomy suggested that high metal concentrations may result from natural processes, the impact of anthropogenic processes on metal levels in Antarctic waters is also evident. A comparison of the stable lead isotopic composition reported for surface waters of the Weddell Sea and our Southern Hemisphere aerosol samples suggested that the cycling of Pb in those waters has been influenced by industrial Pb from South America. The importance of biological activity on the cycling of bioactive metals in the Weddell Sea was suggested by the inverse relationship between chl a concentrations and trace metal residence time. A strong linear relationship between Cd and PO4 was observed, as in other oceans. The Cd/PO4 ratio along the western rim of the Weddell Sea was consistent with previous ratios reported for the northern part of the Weddell Sea and the Antarctic Circumpolar Current. However, those ratios were significantly higher than the previously reported Cd/PO4 ratio for the southern part of the Weddell Sea, suggesting that Cd/PO4 ratios within the same oceanographic basin are susceptible to temporal and spatial variability. We also showed that the Ag/Cu ratio could potentially be used as a geochemical tracer of different water masses in the world ocean. The Ag/Cu ratio in the Weddell Sea was essentially the same as the ratio reported for the Pacific Ocean, suggesting that Weddell Sea surface waters may influence the composition of trace metals in subsurface waters of the Pacific.

Analytical intercomparison results from the 1990 Intergovernmental Oceanographic Commission open-ocean baseline survey for trace metals: Atlantic Ocean

“Dissolved” (< 0.4 μm filtered) and “total dissolvable” (unfiltered) trace element samples were collected using “clean” sampling techniques from four vertical profiles in the eastern Atlantic Ocean on the first IOC Trace Metals Baseline expedition. The analytical results obtained by 9 participating laboratories for Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, and Se on samples from station 4 in the northeast Atlantic have been evaluated with respect to accuracy and precision (intercomparability). The data variability among the reporting laboratories was expressed as 2 × SD for a given element and depth, and was comparable to the 95% confidence interval reported for the NASS seawater reference standards (representing analytical variability only). The discrepancies between reporting laboratories appear to be due to inaccuracies in standardization (analytical calibration), blank correction, and/or extraction efficiency corrections.Several of the sampling bottles used at this station were not adequately pre-cleaned (anomalous Pb results). The sample filtration process did not appear to have been a source of contamination for either dissolved or particulate trace elements. The trace metal profiles agree in general with previously reported profiles from the Atlantic Ocean. We conclude that the sampling and analytical methods we have employed for this effort, while still in need of improvement, are sufficient for obtaining accurate concentration data on most trace metals in the major water masses of the oceans, and to enable some evaluation of the biogeochemical cycling of the metals.

Silver in the Eastern Atlantic Ocean

Vertical profiles of total silver concentrations in the northeast and southeast Atlantic are oceanographically consistent with total dissolved ( 20 pM). This nutrient-type distribution is further evidenced by the highly significant (p < 0.01, simple linear regression) correlation (R ≥ 0.916) of silver and silicate concentrations in both the North Pacific and eastern Atlantic.

Cadmium, copper and nickel distributions at four stations in the eastern central and south Atlantic

Abstract Distributions of cadmium, copper and nickel at four stations in the eastern part of the Atlantic Ocean from 30 ° S to 34 ° N are described based on analytical results from three laboratories. The Cd and Ni profiles show nutrient-like distributions with concentrations in the deep waters increasing from north to south. Copper profiles all show gradual increases from surface to bottom with the highest concentrations occurring near bottom on the most northerly station. Variations in the deep-water Cd and Ni concentrations can be understood in terms of mixing of southern source waters with high concentrations with lower concentration northern source waters. The deep-water Cu distributions indicate a significant near-bottom source to the northern end of the section. Cadmium vs. phosphate relationships show features that result from both regeneration and mixing. Higher Cd:PO 4 ratios are seen in the southern source waters than in the northern waters, thus discounting the suggestion that the inflection in the global Cd:PO 4 relationship at PO 4 ≈ 1.3 μM originates in the southern ocean. Differential regeneration of Cd and PO 4 is seen through the equatorial oxygen minimum.

Stable lead isotopic ratios trace thermohaline circulation in the subarctic North Atlantic

Abstract Vertical profiles of lead concentrations in the subarctic North Atlantic attest to the predominance of anthropogenic lead inputs to those waters, while variations in their lead isotopic ratios ( 204 Pb : 206 Pb : 207 Pb : 208 Pb ) show the multiplicity of those industrial lead inputs. Spatial gradients in the isotopic ratios are consistent with the thermohaline circulation of different water masses, which seemingly have relatively discrete isotopic signatures. These include characteristic 206 Pb / 207 Pb ratios of the North Atlantic Drift (1.183–1.187), Iceland–Scotland Overflow Water (1.173–1.176), Denmark Straits Overflow Water (1.179–1.182), and Labrador Sea Water (1.190–1.120). Based on parallels between these initial isotopic data and T–S measurements, it is proposed that stable lead isotopic compositions may be employed as complementary tracers of the mixing of source waters in the Nordic seas, as they overflow the Iceland–Scotland Ridge and Denmark Strait, mixing into the Labrador Sea to form North Atlantic Deep Water.

Isotopic evidence of contaminant lead in the South Atlantic troposphere and surface waters

Abstract The third Intergovernmental Oceanographic Commission (IOC) Baseline Contaminant cruise (May–June 1996) has established the first lead isotopic compositions in the surface water and the atmosphere of the Equatorial and South Atlantic Ocean. These ratios have evidenced both anthropogenic and natural origins of lead along the cruise transect (from 33°S to 10°N). The isotopic gradients tentatively have been, attributed to aeolian as well as surface-water advective inputs from a suite of rather local and remote sources to the Southern Hemisphere. Relatively low 206Pb/207Pb ratios (x±sd) were encountered within the South Equatorial Current between 17°S and 5°S (1.156±0.003). Those were bracketed by more radiogenic ratios at higher latitudes in the Southern Hemisphere (33°S to 23°S), within the Brazil Current and the Subtropical Gyre (1.163±0.003), and in the Northern Hemisphere (0° to 10°N) (1.165±0.005). The latter were comparable to ratios of surface water in the North Atlantic Equatorial Ocean (1.169±0.006), under a combined contaminant influence of both North American westerlies (1.19–1.20) and European easterlies (1.155–1.165). That predominance of contaminant lead contrasts with the measurable presence of natural lead in surface waters of the Equatorial Ocean, which are attributed to aeolian inputs of Saharan dust. The ratios of lead in surface waters at higher latitudes in the South Atlantic are characteristic of anthropogenic lead aerosols also detected in Antarctic ice, and could substantiate as well the hypothesized aerosol recycling of lead by sea-spray emission in the far Southern Hemisphere. The atmospheric lead isotopic compositions (206Pb/207Pb) in bulk depositions (1.171±0.006), precipitation (1.171±0.006) and aerosols (1.168±0.011) were, generally, more radiogenic than the surface waters (1.162±0.005). Beside a poor representation of a short term atmospheric sampling, this difference could reflect a recent evolution in atmospheric lead emissions, which have not yet been reflected in oceanic surface waters. It may also be due, in part, to advective lead inputs from surface oceanic circulation of the South Equatorial Current.

Mercury speciation and transport via submarine groundwater discharge at a southern California coastal lagoon system.

We measured total mercury (Hg(T)) and monomethylmercury (MMHg) concentrations in coastal groundwater and seawater over a range of tidal conditions near Malibu Lagoon, California, and used (222)Rn-derived estimates of submarine groundwater discharge (SGD) to assess the flux of mercury species to nearshore seawater. We infer a groundwater-seawater mixing scenario based on salinity and temperature trends and suggest that increased groundwater discharge to the ocean during low tide transported mercury offshore. Unfiltered Hg(T) (U-Hg(T)) concentrations in groundwater (2.2-5.9 pM) and seawater (3.3-5.2 pM) decreased during a falling tide, with groundwater U-Hg(T) concentrations typically lower than seawater concentrations. Despite the low Hg(T) in groundwater, bioaccumulative MMHg was produced in onshore sediment as evidenced by elevated MMHg concentrations in groundwater (0.2-1 pM) relative to seawater (∼0.1 pM) throughout most of the tidal cycle. During low tide, groundwater appeared to transport MMHg to the coast, resulting in a 5-fold increase in seawater MMHg (from 0.1 to 0.5 pM). Similarly, filtered Hg(T) (F-Hg(T)) concentrations in seawater increased approximately 7-fold during low tide (from 0.5 to 3.6 pM). These elevated seawater F-Hg(T) concentrations exceeded those in filtered and unfiltered groundwater during low tide, but were similar to seawater U-Hg(T) concentrations, suggesting that enhanced SGD altered mercury partitioning and/or solubilization dynamics in coastal waters. Finally, we estimate that the SGD Hg(T) and MMHg fluxes to seawater were 0.41 and 0.15 nmol m(-2) d(-1), respectively - comparable in magnitude to atmospheric and benthic fluxes in similar environments.

Indirect reduction of hexavalent chromium by copper in the presence of superoxide

Indirect photoreduction of chromium(VI) was investigated in the presence of superoxide radical (O2−) produced by continuous radiolysis of water. Although there is no direct reaction between superoxide and chromium(VI), the presence of nanomolar concentrations of copper can result in Cr(VI) reduction through copper(I) produced during superoxide redox cycling. The reduction rate is a function of metal speciation and concentrations. Cr(VI) reduction rates increase with increasing copper concentrations. At chloride concentrations typical of estuaries and oceans (i.e., >0.01 M), reduction rates decrease because CuCl2− is much less reactive towards chromium(VI) than Cu+. At pH>6.5, Cr(VI) reduction rates decrease because CrO42− is much less reactive than HCrO4−. Experiments performed in natural water samples suggest that photo-initiated reduction of chromium(VI) by copper and superoxide is the most important mechanism of homogeneous, abiotic chromium(VI) reduction in atmospheric and surface waters with low ionic strength and pH. The reaction is relatively insignificant in marine or estuarine waters.

Role of oceanic circulation on contaminant lead distribution in the South Atlantic

Abstract Both the relatively high lead concentrations and their characteristic anthropogenic isotopic compositions attest to the widespread contamination of industrial lead in the western Equatorial and South Atlantic Ocean. Spatial gradients in those isotopic signatures evidence the conservative lateral transport of lead in oceanic water masses, while the discrete isotopic signatures in deep oceanic waters substantiate the complementary hypothesis that the release of lead from settling particles is relatively small on a decadal time-scale. Specifically, the relatively low radiogenic lead (e.g., 206Pb/207Pb=1.148±0.009) in the Lower-North Atlantic Deep Water (l-NADW) south of 10° North is primarily attributed to US industrial lead emitted in the Northern Hemisphere prior to 1965, and the more radiogenic lead (e.g., 206Pb/207Pb=1.180±0.006) in the Upper-North Atlantic Deep Water (u-NADW) is primarily attributed to subsequent industrial lead emissions in that hemisphere. In contrast, the relatively radiogenic lead (e.g., 206Pb/207Pb=1.186±0.007) in the Antarctic Bottom Water (AABW) seemingly reflects a mixture of natural and anthropogenic lead sources within the Southern Hemisphere; and its isotopic dissimilarity with that (e.g., 206Pb/207Pb=1.159±0.002) of Antarctic Intermediate Water (AAIW) and the AABW may be due to differences in either their aeolian or water-mass inputs.