No description available.

No description available.

No description available.

In the Autumn of 1989, Durham University took part in the BABEL Project, a collaboration of scientists from five nations recording wide angle and normal incidence seismic data in the Baltic Shield. Recording stations were set up along the Swedish coast of the Sea of Bothnia to record marine airgun shots at wide angle. Similar stations were operated by Finnish teams on the eastern coast, and by a German team on Aland. The data recorded are of high quality and high resolution in comparison to previous wide angle surveys in the region, with a shot spacing of 75 m. A large proportion (around fifty percent) of the project involved developing software for processing this data. The in-line data from Line 1, and also those recorded at two off-line stations, have been interpreted using Cerveny's Gaussian Beam forward modelling package BEAM87, the in-line model being further constrained by 2D gravity modelling. The resulting models are compared and contrasted with normal incidence data from the same line, other models derived from BABEL data in the Sea of Bothnia and older refraction lines in the vicinity. The models show a highly complex crust whose thickness varies between 50 and 60 km along the profile. The seismic velocity is high, increasing from 5.85 km s(^-l) near the surface to 7.4 km s(^-1) at the base of the crust. Lateral velocity variations are seen in the mid-upper crust while discontinuous reflectors and diffracting bodies are seen at 30 km depth. In the central/northern part of the line the crust thickens and there is a change in seismic velocity. Using other geophysical information from the region, two hypotheses are put forward for interpreting this part of the seismic model. The first is the presence of a large igneous intrusion, and the second is the existence of a shear zone or tectonic boundary cutting Line 1. Further work will be required to confirm either or both of these hypotheses.

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Methylmercury (MeHg) is the most toxic species of mercury (Hg), and is an important ecosystem contaminant. In wetlands on the Canadian Shield, in NW Ontario, MeHg concentrations in peat and peat porewater ranged from 0.3 to 53 ng $ rm g sp{-1}$ and $<$0.1 to $ rm 7.3 ng l sp{-1},$ respectively. The greatest concentrations of MeHg occurred just below the water table, emphasizing the importance of redox reactions in Hg methylation. Methylmercury partition coefficients between peat and peat porewater ranged from $1.6 times 10 sp3$ to $8.6 times 10 sp5.$ No significant correlations between MeHg and concentrations of $ rm H sp+, NH sb4 sp+, NO sb3 sp-, NO sb2 sp-,$ total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), $ rm SO sb4 sp{2-},$ and dissolved organic carbon (DOC) in the porewater of the wetlands were found. / Following shallow impoundment of a wetland, MeHg concentrations in the upper metre of peat porewater increased from $ rm 0.2 pm 0.2 ng l sp{-1}$ to $ rm 0.8 pm 0.8 ng l sp{-1}.$ Total mercury (T-Hg) and MeHg concentrations were determined in decomposing sedge, spruce needles, and Sphagnum moss, placed in a headwater wetland and the impounded wetland. The amount of T-Hg decreased in all tissues regardless of location. the amount of MeHg increased by as much as an order of magnitude in the tissues placed in the impounded wetland and wet areas (hollows and lawns) of the headwater wetland, but decreased in tissue placed in the dry areas (hummocks) of the headwater wetland. Therefore, it is during anaerobic decomposition of plant material that MeHg is produced in wetlands. / Incubations of peat were performed with addition of Hg, molybdate, $ rm SO sb4 sp{2-}, S sp{2-}, NH sb4NO sb3,$ pyruvate, and upland DOC. Methylmercury production was increased only after addition of $ rm SO sb4 sp{2-}$ and retarded only by $ rm NH sb4NO sb3.$ Although $ rm SO sb4 sp{2-}$ may not be required to methylate Hg, the increased availability of $ rm SO sb4 sp{2-}$ may influence the size and composition of the population of sulfate reducing bacteria in peat, thereby increasing the potential for Hg methylation.

No description available.

The alteration mineralogy, major and trace element chemistry, and Sr-Nd-Pb-Hf isotopic compositions of post-shield lavas from Mauna Kea, Kohala, and Hualalai on the island of Hawaii in the Pacific Ocean are used to constrain the origin and distribution of heterogeneities in the Hawaiian mantle plume. Ocean island basalts contain a variety of secondary minerals that must be removed by acid-leaching to achieve high-precision Pb isotopic compositions, a powerful geochemical tracer of variation in plume source composition. Post-shield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g. LaN/YbN=6.0-16.2) relative to shield stage tholeiites. Post-shield lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr=0.70343-0.70365; ¹⁴³Nd/¹⁴⁴Nd = 0.51292-0.51301;¹⁷⁶Hf/¹⁷⁷Hf= 0.28311-0.28314) and have Pb isotopic compositions(²⁰⁶Pb/²⁰⁴Pb = 17.89-18.44; ²⁰⁷Pb/²⁰⁴ 15.44-15.49;²⁰⁸Pb/²⁰⁴Pb= 37.68-38.01) that belong to their respective Kea or Loa side of the Pb-Pb boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to post-shield stage and trend to low ⁸⁷Sr/⁸⁶Sr towards compositions characteristic of rejuvenated stage lavas. Hualalai post shield lavas lie distinctly above the Hf-Nd Hawaiian array (ƐHf = +12 to +13; ƐNd = +5.5 to +6.5) and have some of the least radiogenic Pb isotopic compositions (e.g.²⁰⁶/²⁰⁴pb= 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona shows that lavas from these volcanoes become more radiogenic in Pb during the late stages of volcanism. The Sr-Nd-Pb-Hf isotope systematics of the post shield lavas cannot be explained by mixing between the Kea and Koolau end-members or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust and sediments in their source. More than one depleted component is sampled by the post-shield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The geochemistry of the post-shield lavas provide evidence for a bilaterally zoned plume, where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are also significant.

The alteration mineralogy, major and trace element chemistry, and Sr-Nd-Pb-Hf isotopic compositions of post-shield lavas from Mauna Kea, Kohala, and Hualalai on the island of Hawaii in the Pacific Ocean are used to constrain the origin and distribution of heterogeneities in the Hawaiian mantle plume. Ocean island basalts contain a variety of secondary minerals that must be removed by acid-leaching to achieve high-precision Pb isotopic compositions, a powerful geochemical tracer of variation in plume source composition. Post-shield lavas range from transitional/alkalic basalt to trachyte and are enriched in incompatible trace elements (e.g. LaN/YbN=6.0-16.2) relative to shield stage tholeiites. Post-shield lavas are characterized by a limited range of Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sr=0.70343-0.70365; ¹⁴³Nd/¹⁴⁴Nd = 0.51292-0.51301;¹⁷⁶Hf/¹⁷⁷Hf= 0.28311-0.28314) and have Pb isotopic compositions(²⁰⁶Pb/²⁰⁴Pb = 17.89-18.44; ²⁰⁷Pb/²⁰⁴ 15.44-15.49;²⁰⁸Pb/²⁰⁴Pb= 37.68-38.01) that belong to their respective Kea or Loa side of the Pb-Pb boundary. Mauna Kea lavas show a systematic shift to less radiogenic Pb isotopic compositions from the shield to post-shield stage and trend to low ⁸⁷Sr/⁸⁶Sr towards compositions characteristic of rejuvenated stage lavas. Hualalai post shield lavas lie distinctly above the Hf-Nd Hawaiian array (ƐHf = +12 to +13; ƐNd = +5.5 to +6.5) and have some of the least radiogenic Pb isotopic compositions (e.g.²⁰⁶/²⁰⁴pb= 17.89-18.01) of recent Hawaiian volcanoes. In contrast, comparison of Kohala with the adjacent Mahukona shows that lavas from these volcanoes become more radiogenic in Pb during the late stages of volcanism. The Sr-Nd-Pb-Hf isotope systematics of the post shield lavas cannot be explained by mixing between the Kea and Koolau end-members or by assimilation of Pacific lithosphere and are consistent with the presence of ancient recycled lower oceanic crust and sediments in their source. More than one depleted component is sampled by the post-shield lavas and these components are long-lived features of the Hawaiian plume that are present in both the Kea and Loa source regions. The geochemistry of the post-shield lavas provide evidence for a bilaterally zoned plume, where the compositional boundary between the Kea and Loa sources is complex and vertical components of heterogeneity are also significant.