Methane-bearing,aqueous, saline solutions in the Skaergaard intrusion,east Greenland

Solutions of H₂O–NaCl–CH₄ occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 m diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH₄, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C–O–H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.     

Recognition of areas effected by petroleum seepage: Northern Gulf of Mexico continental slope

The presence of large amounts of gas and/or liquid hydrocarbon seepage in near surface sediments can produce distinct features including an irregular topography (on several scales, ranging from meters to kilometers); seismically transparent/chaotic sediments; oil staining; gas plumes; sediments containing elevated concentrations of extractable organic matter, organic carbon, and calcium carbonate; associated brine seepage and anoxic conditions; extensive bacterial mats; hydrate formation and decomposition; and dense chemoautotrophic communities. Although no single characteristic is always uniquely associated with seepage, the co-occurrence of several of these features is strongly suggestive of an area being exposed to non-indigenous upward migrating hydrocarbons.     

Mixed carbonate-siliciclastic infilling of a Neogene carbonate shelf-valley system: Tampa Bay, West-Central Florida

The shelf-valley system underlying Tampa Bay, Florida’s largest estuary, is situated in the middle of the Neogene carbonate Florida Platform. Compared to well-studied fluvially incised coastal plain valley systems, this shelf-valley system is unique in its karstic origin and its alternating carbonate-siliciclastic infill. A complex record of sea-level changes, paleo-fluvial variability and marine processes have controlled the timing and mechanisms of this ‘compound’ shelf-valley infill. A dense grid of high-resolution, single-channel seismic data were collected at the mouth of Tampa Bay, in an attempt to define this stratigraphy, determine the controls on deposition, and define the underlying structure of this shelf-valley system. The seismic data were correlated with nearby wells and boreholes for lithologic and age control. Sequence stratigraphic methods were incorporated in order to develop an integrated chronostratigraphy for the depositional infilling of the shelf-valley system. Five seismic sequences were identified. Sequence boundaries generally show erosional truncation and karstification, with downlap of overlying sequences. Structure contour and isopach maps indicate that the Tampa Bay shelf-valley system has remained in essentially the same location since its formation in the early Miocene, although the provenance of sedimentary infill has changed. This change is due to increasing amounts of siliciclastic material during the Neogene. Seismic facies interpretations indicate lower-energy, northward prograding deposition dominated by predominantly carbonate sediments within the lowest Sequence A. Higher energy, siliciclastic fluvio-deltaic deposition within sequences B and C originates to the east and northeast of the shelf-valley system related to a Pliocene pulse of sedimentation onto the Florida Platform. Finally, marine processes (longshore transport, ebb-tidal delta formation) dominate the upper two sequences (D and E), reworking these siliciclastic sediments into a spatially mixed carbonate-siliciclastic depositional setting.     

Organic geochemistry of sediments from chemosynthetic communities,Gulf of Mexico slope

We used a research submersible to obtain 33 sediment samples from chemosynthetic communities at 541–650 m water depths in the Green Canyon (GC) area of the Gulf of Mexico slope. Sediment samples from beneath an isolated mat of H₂S-oxidizing bacteria at GC 234 contain oil (mean = 5650 ppm) and C₁–C₅ hydrocarbons (mean = 12,979 ppm) that are altered by bacterial oxidation. Control cores away from the mat contain lower concentrations of oil (mean = 2966 ppm) and C₁–C₅ hydrocarbons (mean = 83.6 ppm). Bacterial oxidation of hydrocarbons depletes O₂ in sediments and triggers bacterial sulfate reduction to produce the H₂S required by the mats. Sediment samples from GC 185 (Bush Hill) contain high concentrations of oil (mean = 24,775 ppm) and C₁–C₅ hydrocarbons (mean = 11,037 ppm) that are altered by bacterial oxidation. Tube worm communities requiring H₂S occur at GC 185 where the sea floor has been greatly modified since the Pleistocene by accumulation of oil, thermogenic gas hydrates, and authigenic carbonate rock. Venting to the water column is suppressed by this sea-floor modification, enhancing bacterial activity in sediments. Sediments from an area with vesicomyid clams (GC 272) contain lower concentrations of oil altered by bacterial oxidation (mean = 1716 ppm) but C₁–C₅ concentrations are high (mean = 28,766 ppm). In contrast to other sampling areas, a sediment associated with the methanotrophic Seep Mytilid I (GC 233) is characterized by low concentration of oil (82 ppm) but biogenic methane (C₁) is present (8829 ppm).     

Spatio-temporal and interspecific variation in otolith trace-elemental fingerprints in a temperate estuarine fish assemblage

We tested whether estuarine fishes have site-specific differences in the concentrations of trace elements in their otoliths that can be used as ‘fingerprints’ to identify them to their estuary of origin. To evaluate the robustness of this approach, we tested whether elemental fingerprints were consistent among individuals of five species that were collected in 1996 from three temperate estuaries in southern California. We also tested whether elemental fingerprints were consistent between spring and autumn 1996 for three species in one of the sites, Carpinteria Marsh. The species evaluated comprised a mid-water-dwelling smelt (Atherinops affinis), two benthic gobies (Clevelandia ios and Ilypnus gilberti), and two flatfish (Paralichthys californicus and Hypsopsetta guttulata). The concentrations of six elements (Mn, Cu, Zn, Sr, Ba, and Pb) were determined in the otoliths using inductively coupled plasma-mass spectrometry (ICP-MS). Within estuaries, the five species exhibited strong variation in elemental concentration, indicating substantial interspecific differences in otolith environmental history. When the five fish species were considered separately, multivariate (MANOVA) and univariate (ANOVA) analyses of variance indicated that the elemental composition of otoliths differed significantly among the estuaries in four of the five species. Based on linear discriminant function analyses (DFA), differences were strong enough that trace element composition could be used to accurately assign fish to their site of origin [mean (range): 93.5% (74–100%)]. However, elemental signatures within Carpinteria Marsh were not consistent between spring and autumn 1996, and this was reflected in a substantial reduction in the accuracy of assigning fish to their true site of origin. When we compared site differences between fish species (site×species interactions), the elemental fingerprints were most similar between closely related species (e.g. the two gobies and the two flatfish) and most dissimilar between distantly related species, both phylogenetically and ecologically. Among the six elements analyzed, Sr and Ba exhibited the most inconsistent pattern among species, with significant differences in 80 and 70% of the pairwise species comparisons, respectively. The remaining four elements showed ≥70% consistency in the pattern of variation among sites for the different species. Thus, while otolith elemental fingerprinting can be a useful tool for inferring estuarine residency, such fingerprints may be temporally variable and species specific.     

Comparative study of radiation-induced damage in magnesium aluminate spinel by means of IL,CL and RBS/C techniques

A comparative study of damage accumulation in magnesium aluminate spinel (MgAl₂O₄) has been conducted using ionoluminescence (IL), cathodoluminescence (CL) and Rutherford Backscattering Spectrometry/channeling (RBS/C) techniques. MgAl₂O₄ single crystal and polycrystalline samples were irradiated with 320 keV Ar⁺ ions at fluencies ranging from 1 × 10¹² to 2 × 10¹⁶ cm^?2 in order to create various levels of radiation damage. RBS/C measurements provided quantitative data about damage concentration in the samples. These values were then compared to the luminescence measurements. The results obtained by IL and RBS/C methods demonstrate a two-step character of damage buildup process. The CL data analysis points to the three-step damage accumulation mechanism involving the first defect transformation at fluencies of about 10¹³ cm^?2 and second at about 10¹⁵ cm^?2. The rate of changes resulting from the formation of nonluminescent recombination centers is clearly nonlinear and cannot be described in terms of continuous accumulation of point defects. Both, IL and CL techniques, appear as new, complementary tools bringing new possibilities in the damage accumulation studies in single- and polycrystalline materials.     

Eutrophication erodes inter-basin variation in macrophytes and co-occurring invertebrates in a shallow lake: combining ecology and palaeoecology

Aquatic biodiversity is commonly linked with environmental variation in lake networks, but less is known about how local factors may influence within-lake biological heterogeneity. Using a combined ecological and multi-proxy palaeoecological approach we investigated long-term changes in the pathways and processes that underlie eutrophication and water depth effects on lake macrophyte and invertebrate communities across three basins in a shallow lake—Castle Lough, Northern Ireland, UK. Contemporary data allow us to assess how macrophyte assemblages vary in composition and heterogeneity according to basin-specific factors (e.g. variation in water depth), while palaeoecological data (macrophytes and co-occurring invertebrates) enable us to infer basin-specific impacts and susceptibilities to nutrient-enrichment. Results indicate that variability in water depth promotes assemblage variation amongst the lake basins, stimulating within-lake macrophyte assemblage heterogeneity and hence higher lake biodiversity. The palaeo-data indicate that eutrophication has acted as a strong homogenising agent of macrophyte and invertebrate diversities and abundances over time at the whole-lake scale. This novel finding strongly suggests that, as eutrophication advances, the influence of water depth on community heterogeneity is gradually eroded and that ultimately a limited set of eutrophication-tolerant species will become homogeneously distributed across the entire lake.     

Influence of industrial activity and pollution on the paleoclimate reconstruction from a eutrophic lake in lowland England,UK

Reliable estimates of Holocene temperatures are important for understanding past climate dynamics, the response of biota to climate change, and validating climate models. Chironomids in lake sediment cores are used widely to quantify past summer temperatures, for which high-latitude and/or high-altitude lakes, remote from human influence, are usually considered appropriate. Temperature inferences from lowland lakes are likely influenced by other variables, specifically eutrophication and industrial pollution, but their reliability has never been tested. We used a Norwegian chironomid-based transfer function (r ² = 0.91; RMSEP = 1.01 °C) to infer mean July air temperature over the last 200 years, using chironomid assemblages in a core collected from a polluted, nutrient-enriched lake at Speke Hall, Liverpool, England. The chironomid-inferred temperatures correlate significantly with the local instrumental temperature record and follow long-term national temperature trends. These results show that chironomids can be used to produce reliable estimates of past mean July air temperature, even when other variables have also influenced the composition of the chironomid community. These findings underline the value of chironomids as sensitive and reliable quantitative proxies for summer temperature.