Arsenic bioaccessibility and speciation in clams and seaweed from a contaminated marine environment

The bioaccessible concentration and speciation of arsenic (soluble in a gastrointestinal medium and available for absorption into the bloodstream) were determined in softshell clams (Mya arenaria), harvested by local residents until 2005, and in seaweed (Fucus sp.) from an arsenic-contaminated marine site in Seal Harbour, Nova Scotia, Canada. Bioaccessibility extractions to simulate the human gastrointestinal environment (pH 1.5 and glycine for 1h followed by pancreatin, bile extract and pH adjustment to 7 for an additional 4h) and speciation of arsenic in extracts (HPLC-HG-AAS to target inorganic arsenic species) and whole samples (XANES) were carried out. Total arsenic for the clams from the contaminated area ranged from 218 to 228 ppm wet weight, with a bioaccessible fraction of 34-46%, and the major bioaccessible species of arsenic were inorganic. The seaweed from the contaminated area contained 27-43 ppm wet weight total arsenic, with the bioaccessible fraction ranging from 63% to 81%, and inorganic arsenic was also predominant. The predominantly inorganic nature of arsenic in the whole samples was confirmed by XANES. In concurrence with the closure of the area for clam harvesting, the clams and seaweed from Seal Harbour should probably not be used for human consumption.     

Biological effects of gold mine tailings on the intertidal marine environment in Nova Scotia,Canada

From 1861 to the 1940s, gold was produced from 64 mining districts in Nova Scotia, where mercury amalgamation was the dominant method for the extraction of gold from ore until the 1880s. As a result, wastes (tailings) from the milling process were contaminated by mercury and were high in naturally occurring arsenic. In 2004 and 2005, sediments, water and mollusc tissues were collected from 29 sampling stations at nine former gold mining areas along the Atlantic coastline and were analysed for arsenic and mercury. The resulting data were compared with environmental quality guidelines. Samples indicated high potential risk of adverse effects in the intertidal environments of Seal Harbour, Wine Harbour and Harrigan Cove. Arsenic in Seal Harbour was bioavailable, resulting in high concentrations of arsenic in soft-shell clam tissues. Mercury concentrations in tissues were below guidelines. This paper presents results of the sampling programs and implications of these findings.     

Lead and strontium isotopes in rocks of the Absaroka volcanic field,Wyoming

The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr⁸⁷/Sr⁸⁶, 0.7042 to 0.7090; Pb²⁰⁶/Pb²⁰⁴, 16.31 to 17.30; Pb²⁰⁸/Pb²⁰⁴, 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.Publication authorized by the Director, U.S. Geological Survey.     

Primitive and contaminated basalts from the Southern Rocky Mountains,U.S.A.

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K₂O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K₂O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K₂O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts.Contamination causes lead isotope ratios, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, to become less radiogenic, but it has little or no effect on ⁸⁷Sr/⁸⁶Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks.Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios.The primitive basalts have: ²⁰⁶Pb/²⁰⁴Pb 18.09–18.34, ²⁰⁷Pb/²⁰⁴Pb 15.5, ²⁰⁸Pb/²⁰⁴Pb 37.6–37.9, ⁸⁷Sr/⁸⁶Sr 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of ²⁰⁶Pb/²⁰⁴Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas ²⁰⁸Pb/²⁰⁴Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. ⁸⁷Sr/⁸⁶Sr appears to be equal to or a little greater than those of the oceanic localities. These ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are distinctly less radiogenic and ⁸⁷Sr/⁸⁶Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.Publication authorized by the Director, U.S. Geological Survey     

U-Th-Pb systematics in hydrothermally altered granites from the Granite Mountains,Wyoming

U-Th-Pb systematics were investigated in 15 samples representing two types of deuterically altered Archean granite (albitized and silicified-epidotized granite) from the Granite Mountains, Wyoming. The loss of K-feldspar during both types of deuteric alteration was accompanied by an extreme reduction of Pb content from roughly 40 ppm to less than 12 ppm in the most altered samples. Nine of the 15 samples yield anomalously young whole-rock Pb-Pb and Th-Pb ages compared to concordia ages for zircons and to whole-rock Pb-Pb and Th-Pb ages for samples of unaltered granite. The young ages are interpreted to be the result of radiogenic Pb loss during a middle Proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite. The loss of radiogenic Pb from the whole-rock systems of the deuterically altered granites is most likely due to the absence of microcline. In many granitic rocks, potassium feldspar tends to act as a receptor for Pb that has been mobilized, and this effect may account for the closed-system behavior of Pb in whole-rock samples of the unaltered granite. It may also account for the apparent gain of radiogenic Pb during the Proterozoic by one sample which was collected at the edge of an alteration zone. The deuterically altered granites may have also lost U during the Proterozoic, but, except for two samples, the dominant U loss occurred relatively recently, probably during Tertiary uplift and erosion.The low common-Pb contents and rather high U and Th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the U-Th-Pb system because such rocks should have a high percentage of radiogenic Pb. Unfortunately, open-system behavior in response to post-crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution.