Lynda S. Cutter

Behavior of dissolved antimony, arsenic, and selenium in the Atlantic Ocean

Abstract Vertical profiles for dissolved antimony, arsenic, and selenium were obtained at four stations in the eastern basins of the North and South Atlantic Ocean, and on a surface-water transect from 24 ° S to 31 ° N. Total dissolved selenium displays surface-water depletion and deep-water enrichment, with organic selenide (selenium in soluble peptides) being the predominant species in surface waters and selenate predominating in deep waters. Although the concentrations of total selenium in surface waters of the Northern and Southern Hemispheres are similar, the ratio of inorganic to organic selenium is strongly influenced by the intensity of upwelling. Total inorganic arsenic is depleted in the surface waters of all stations, and increases to relatively constant deep-water concentrations (c. 20 nmol/l). In contrast, total inorganic antimony shows surface-water maxima at two stations. Although the average surface-water arsenic and antimony concentrations in the Northern and Southern Hemispheres are identical, there is some evidence for the atmospheric deposition of antimony. Overall, the cycling of the metalloids in the Atlantic is dominated by in situ biotic reactions, and modified by inputs from upwelling and atmospheric deposition.

Antimony and Arsenic Biogeochemistry in the Western Atlantic Ocean

The subtropical to equatorial Atlantic Ocean provides a unique regime in which one can examine the biogeochemical cycles of antimony and arsenic. In particular, this region is strongly affected by inputs from the Amazon River and dust from North Africa at the surface, and horizontal transport at depth from highlatitude northern (e.g., North Atlantic Deep Water) and southern waters (e.g., Antarctic Bottom and Intermediate Waters). As a part of the 1996 Intergovernmental Oceanographic Commission’s Contaminant Baseline Survey, data for dissolved As(III+V), As(III), mono- and dimethyl arsenic, Sb(III+V), Sb(III), and monomethyl antimony were obtained at six vertical profile stations and 44 sites alongthe 11,000 km transect from Montevideo, Uruguay, to Bridgetown, Barbados. The arsenic results were similar to those in other oceans, with moderate surface depletion, deep-water enrichment, a predominance of arsenate (>85% As(V)), and methylated arsenic species and As(III) in surface waters that are likely a result of phytoplankton conversions to mitigate arsenate ‘‘stress’’ (toxicity). Perhaps the most significant discovery in the arsenic results was the extremely low concentrations in the Amazon Plume (as low as 9.8 nmol/l) that appear to extend for considerable distances offshore in the equatorial region. The very low concentration of inorganic arsenic in the Amazon River (2.8 nmol/l; about half those in most rivers) is probably the result of intense iron oxyhydroxide scavenging. Dissolved antimony was also primarily in the pentavalent state (>95% antimonate), but Sb(III) and monomethyl antimony were only detected in surface waters and displayed no correlations with biotic tracers such as nutrients and chlorophyll a. Unlike As(III+V)’s nutrient-type vertical profiles, Sb(III+V) displayed surface maxima and decreased into the deep waters, exhibitingthe behavior of a scavenged element with a strongatmospheric input. While surface water Sb had a slight correlation with dissolved Al, it is likely that atmospheric Sb is delivered with combustion byproducts and not from mineral aerosols. In the Amazon Plume, antimony concentrations dropped substantially, and an Amazon River sample had a concentration (0.25 nmol/l) that was less than one-fourth those found in other major rivers. Usingthese river data, and estimates of atmospheric fluxes based on

Increased selenium threat as a result of invasion of the exotic bivalve Potamocorbula amurensis into the San Francisco Bay-Delta

Following the aggressive invasion of the bivalve, Potamocorbula amurensis, in the San Francisco Bay-Delta in 1986, selenium contamination in the benthic food web increased. Concentrations in this dominant (exotic) bivalve in North Bay were three times higher in 1995-1997 than in earlier studies, and 1990 concentrations in benthic predators (sturgeon and diving ducks) were also higher than in 1986. The contamination was widespread, varied seasonally and was greater in P. amurensis than in co-occurring and transplanted species. Selenium concentrations in the water column of the Bay were enriched relative to the Sacramento River but were not as high as observed in many contaminated aquatic environments. Total Se concentrations in the dissolved phase never exceeded 0.3 microg Se per l in 1995 and 1996; Se concentrations on particulate material ranged from 0.5 to 2.0 microg Se per g dry weight (dw) in the Bay. Nevertheless, concentrations in P. amurensis reached as high as 20 microg Se per g dw in October 1996. The enriched concentrations in bivalves (6-20 microg Se per g dw) were widespread throughout North San Francisco Bay in October 1995 and October 1996. Concentrations varied seasonally from 5 to 20 microg Se per g dw, and were highest during the periods of lowest river inflows and lowest after extended high river inflows. Transplanted bivalves (oysters, mussels or clams) were not effective indicators of either the degree of Se contamination in P. amurensis or the seasonal increases in contamination in the resident benthos. Se is a potent environmental toxin that threatens higher trophic level species because of its reproductive toxicity and efficient food web transfer. Bivalves concentrate selenium effectively because they bioaccumulate the element strongly and lose it slowly; and they are a direct link in the exposure of predaceous benthivore species. Biological invasions of estuaries are increasing worldwide. Changes in ecological structure and function are well known in response to invasions. This study shows that changes in processes such as cycling and effects of contaminants can accompany such invasions.

Metalloids in the high latitude North Atlantic Ocean : Sources and internal cycling

The concentrations and speciation of antimony, arsenic, and selenium were determined in horizontal transects and eight vertical profiles in the North Atlantic Ocean between 54°N to 68°N as a part of the 1993 IOC Contaminant Baseline Survey. Both arsenic and selenium display surface-water depletion and deep-water enrichment typical of nutrient-type elements, although selenium accumulates in deep waters to a much greater extent than arsenic. In contrast, antimony displays surface enrichment and a decrease with water mass age, consistent with scavenged behavior. Trivalent arsenic and antimony, and dimethyl arsinic acid, are restricted to the surface waters, but organic selenide is found in the youngest deep waters. A residence time of ca. 10 years for organic selenide is consistent with this behavior. Comparisons of allochthonous fluxes required to maintain the observed metalloid concentrations and atmospheric depositional fluxes measured on the islands of Bermuda and Ireland indicate that atmospheric deposition can be the dominant source of metalloids to the high latitude North Atlantic.

Biogeochemistry of arsenic and antimony in the North Pacific Ocean

The biogeochemical cycles of the metalloid elements arsenic and antimony were examined along a 15,000 km surface water transect and at 9 vertical profile stations in the western North Pacific Ocean as part of the 2002 IOC Contaminant Baseline Survey. Results show that the speciation of dissolved arsenic (As III, As V, and methylated As) was subtly controlled by the arsenate (AsV)/phosphate ratio. An additional fraction of presumed organic arsenic previously reported in coastal waters was also present (∼15% of the total As) in oceanic surface waters. Dissolved inorganic antimony displayed mildly scavenged behavior that was confirmed by correlations with aluminum, but atmospheric inputs that may be anthropogenic in origin also affected its concentrations. Monomethyl antimony, the predominant organic form of the element, behaved almost conservatively throughout the water column, radically changing the known biogeochemical cycle of antimony.

Aeolian contamination of Se and Ag in the North Pacific from Asian fossil fuel combustion.

Energy production from fossil fuels, and in particular the burning of coal in China, creates atmospheric contamination that is transported across the remote North Pacific with prevailing westerly winds. In recent years this pollution from within Asia has increased dramatically, as a consequence of vigorous economic growth and corresponding energy consumption. During the fourth Intergovernmental Oceanographic Commission baseline contaminant survey in the western Pacific Ocean from May to June, 2002, surface waters and aerosol samples were measured to investigate whether atmospheric deposition of trace elements to the surface North Pacific was altering trace element biogeochemical cycling. Results show a presumably anthropogenic enrichment of Ag and of Se, which is a known tracer of coal combustion, in the North Pacific atmosphere and surface waters. Additionally, a strong correlation was seen between dissolved Ag and Se concentrations in surface waters. This suggests that Ag should now also be considered a geochemical tracer for coal combustion, and provides further evidence that Ag exhibits a disturbed biogeochemical cycle as the result of atmospheric deposition to the North Pacific.