Baseline Geochemical Data Research Articles

Petrography and geochemistry of the iron-rich rocks in the banded iron formation of the Chilpi Group, Central India: Implications on the level of oxygen in the Paleoproterozoic atmosphere before the “Proterozoic iron ore gap”

The Chilpi Group (2050–1850 Ma) in the Bastar Craton contains Banded Iron Formation (BIF) deposited immediately after the Great Oxidation Event but before the “Proterozoic iron ore gap”. Baseline geochemical data generated from this crucial period of Earth's history representing the transition from an initial high productivity period followed by productivity collapse, thereby delaying the evolution of biota, are interpreted in terms of the redox state of the ocean and atmospheric oxygen content. Presence of chamosite, greenalite and siderite in the ironstones of the Chilpi Group, identified during present analysis, provide valuable information regarding the redox state of the shallow sea. Geochemical analyses reveal high concentrations of Fe2O3total and SiO2 (average ~ 87.85 wt%) in iron-rich bands. Trace elements commonly enriched in detrital phases (e.g. Sc, Hf, Nb, Th and Zr) show good correlation with total REE concentration, but these have concentration below the cut-off limit defined for detrital sediments (except 3 samples). Higher concentration of these elements in greenalite/chamosite-rich samples indicates accumulation of these elements in greenalite/chamosite during primary precipitation. Most of the samples have low concentrations of Al2O3 (<5 wt%) and TiO2 (<0.5 wt%), but some chamosite-bearing samples show enrichments of Al2O3 (up to ~20 wt%). Post-Archean Australian Shale normalised rare earth elements with Y (REEY) patterns, showing a superchondritic Y/Ho ratio (average 32.15) and positive La, Gd and Y anomalies, indicate the preservation of seawater like signatures. Though a low positive Eu-anomaly, and Al/(Al + Fe + Mn) versus Fe/Ti plot suggest seawater signature with possible mixing of hydrothermal fluids and/or <10 % detrital components, preservation of seawater signatures and no involvement of high-temperature hydrothermal fluids are deduced from Eu/Sm versus Sm/Yb and Eu/Sm versus Y/Ho patterns. The Eu/Sm and Y/Ho ratios lower than the seawater and a mixing trend away from oceanic hydrothermal solution indicate possible mixing of freshwater with seawater for the observed REEY patterns. Redox-sensitive trace element ratios indicate a dysoxic to suboxic-anoxic condition in the depositional basin. Presence of oolitic textures and occurrence of chamosite support the shallow water (<60 m water depth) and anoxic condition in the depositional basin. The oxygen content of 10−3–10−5 times the present atmospheric level in the atmosphere is inferred for the period during the deposition of the BIF.

Mercury and other trace metals in the seasonal snowpack across the subarctic taiga-tundra ecotone, Northwest Territories, Canada

In Canada's Northwest Territories, mining for base metals and diamonds are vital economic activities which carry risks of adverse environmental impacts. To gather baseline geochemical data against which the impact of future mining activities may be measured, a survey of trace metal concentrations in snow was carried out in 2012 along a 285-km stretch of winter mining road crossing the taiga-tundra ecotone between latitudes 62.8 and 65.5° N. The distribution of 17 elements, including mercury (Hg), was measured and mapped. Results indicate that road traffic along the winter road has only a modest impact on the metal content of the nearby tundra-taiga snowpack, and that this impact is largely due to the mobilization of soil dust and associated elements. However, some enrichment of As, Pb, Sr and Zn in snow was detected near former gold mine sites, likely reflecting the windborne dispersion of contaminated soils. The Hg concentrations in snow across the study area were generally low (≤3.01 ng L−1), and did not covary with any other metals, which suggests atmospheric deposition from distant/diffuse sources. An analysis of air back-trajectories pointed to the most likely distant (>103 km) anthropogenic source regions being eastern Asia or Russia. Using Hg data from the present survey and another source, in combination with gridded maps of snowpack water equivalent, we calculated the potential flux of atmospherically-derived Hg that could be released by spring snowmelt into the Mackenzie River to be in the order of ∼195–404 kg a−1, which may represent a substantial fraction of the estimated total Hg discharge to the Beaufort Sea.

Geochemical study of groundwater in the Sho river fan, Toyama Prefecture for heat usage by geothermal heat pump

Chemical and isotopic (D, 18O) compositions of 40 shallow groundwater, 14 observation well water and 2 river water samples in the Sho river fan, Toyama, northern part of central Japan, were analyzed to examine their water quality, origins and water flow in order to obtain baseline geochemical data for application of groundwater to geothermal heat pump (Geo-HP). Water temperatures of the groundwaters in the observation wells are almost constant at 15°C from surface to ca. 60 m in depth and increase up to 25°C at depth of 200 m with the thermal gradient of 6°C/100 m. This geothermal gradient is twice as high as that of the other areas in Japan, suggesting existence of high temperature geothermal sources beneath this region and a high potential for heat usage in winter. The isotopic results show that the groundwaters are a mixture of two big river waters (Sho and Oyabe rivers) and precipitation in the Sho river fan and that the contribution of infiltration from the Sho river has increased in recent 10 years. The Sho river has lower δ18O values than the Oyabe river. The groundwaters shallower than 80 m depth are of a Ca–HCO3 type. Deep groundwaters from observation wells were characterized by high in pH and HCO3 compared to the shallow groundwaters due to ion exchange with rocks containing clay minerals. The deep groundwaters are almost saturated or oversaturated with respect to calcite and dolomite, becoming confined due to the presence of an impermeable layer. In this paper, formation of calcite and iron hydroxide scale inside pipes of the Geo-HP was discussed from the chemical compositions of groundwater to examine whether the water is applicable to geothermal heat pump or not.

Major ion and isotope geochemistry of fluids and gases from coalbed methane and shallow groundwater wells in Alberta, Canada

The production of coalbed methane (CBM) represents a vital new source of natural gas supply in Western Canada. There are, however, concerns over potential negative environmental impacts on shallow groundwater resources in the hypothetical case that leakage of fluids and gases from CBM operations occurs. This paper compares major ion and isotope geochemistry data for produced fluids or gases from two major coal deposits in Western Canada (Mannville Formation and the Horseshoe Canyon/Belly River Group) with similar data collected for shallow groundwater in south-central Alberta. The objective was to generate comprehensive baseline geochemical data to determine the key geochemical characteristics and differences of produced fluids and gases from two coal deposits and shallow groundwater in Alberta and to find parameters that are suitable for identifying potential leakage of fluids or gases into shallow groundwater. Shallow groundwater had average total dissolved solids (TDS) of 1037 mg/L. Most samples belonged to the Na–HCO3–SO4 water type and average SO42- concentrations were 185 mg/L. The Horseshoe Canyon/Belly River Group swabbing fluids had average TDS of 5427 mg/L, a Na–HCO3 water type, and average SO42- concentrations of 47.7 mg/L. The produced fluids from the Mannville Formation had average TDS contents of 74,500 mg/L, negligible SO42- and a Na–Cl water type. Shallow groundwater and produced fluids from the Horseshoe Canyon Formation and the Mannville group had distinct δ2H and δ18O values and plotting δ18O values versus total dissolved solids was found to be an effective approach to distinguish the waters. Sulfur isotope data revealed the occurrence of bacterial (dissimilatory) SO4 reduction in some shallow groundwater samples and in the produced fluids from the Horseshoe Canyon/Belly River Group. Methane was found in several shallow groundwater samples and its average δ13C (−72.1 ± 6.8‰) and δ2H values (−297 ± 17‰) indicated a biogenic origin predominantly from CO2 reduction. Dissolved gases from the Horseshoe Canyon Formation fluids with average δ13C values of methane of −54.0 ± 4.1‰ and ethane of −36.5 ± 2.4‰ and traces of higher alkanes suggest a mixture of predominantly biogenic and some thermogenic gas. Dissolved hydrocarbon gas from the Mannville Formation had the highest average δ13C values of −49.4 ± 3.6‰ for methane, −28.8 ± 2.1‰ for ethane and −26.9 ± 1.1‰ for propane. The presence of higher alkanes suggests that the Mannville produced fluids contain an appreciable thermogenic gas component. The biogenic gas component in the Horseshoe Canyon and the Mannville Formation was mainly formed via acetate fermentation according to αCO2–CH4 values between 1.02 and 1.07. It is concluded that δ18O values of the fluids in concert with total dissolved solids, and the isotopic compositions of methane and ethane are sufficiently distinct in shallow groundwater and produced fluids from the Horseshoe Canyon and the Mannville Formations that they may serve as tracers for evaluating potential contamination of shallow groundwater with produced fluids or gases.

Symposium on ‘Geographical and geological influences on nutrition’ Factors controlling the distribution of selenium in the environment and their impact on health and nutrition

Se is essential to human and animal health but can be toxic in excess. An interest in its geochemistry has developed alongside a greater understanding of its function in a number of health conditions. Geology exerts a strong control on the Se status of the surface environment; low-Se rock-types (0.05-0.09 mg Se/kg) make up the majority of rocks occurring at the Earth's surface, which in turn account for the generally low levels of Se in most soils. However, there are exceptions such as associations with sulfide mineralisation and in some types of sedimentary rocks (e.g. black shales) in which contents of Se can be much higher. Baseline geochemical data now enable a comparison to be made between environmental and human Se status, although a direct link is only likely to be seen if the population is dependent on the local environment for sustenance. This situation is demonstrated with an example from the work of the British Geological Survey in the Se-deficiency belt of China. The recent fall in the daily dietary Se intake in the UK is discussed in the context of human Se status and declining use of North American wheat in bread making. Generally, US wheat has ten times more Se than UK wheat, attributed to the fact that soils from the wheat-growing belt of America are more enriched in Se to a similar order of magnitude. In agriculture effective biofortification of crops with Se-rich fertilisers must be demonstrably safe to the environment and monitored appropriately and baseline geochemical data will enable this process to be done with confidence.

Heavy-metal loadings related to urban contamination in the Kooloonbung Creek catchment, Port Macquarie, New South Wales

Urbanisation and industrial development lead to contamination of estuaries and streams with dispersed loadings of heavy metals and metalloids. Contributions of these elements also occur from natural sources. This study provides baseline geochemical data on the respective natural and anthropogenic inputs of Cu, Pb, Zn, Cd, As, Sb, Cr, Ni, Mn and S to estuarine, fluvial and wetland sediments, and adjacent soils, in the Kooloonbung Creek catchment that drains the Port Macquarie urban area in north coastal New South Wales. There have been anthropogenic additions of Cu, Pb, Zn and As from dispersed urban sources at Port Macquarie, but they are restricted to the local catchment and do not impact on the adjacent Hastings River estuary. The most contaminated sediments display enrichment factors up to 20 × for Cu and Pb, 9 × for Zn and 5 × for As relative to local background values. However, only one value (for Pb) exceeds National Water Quality Management Strategy interim sediment quality guideline (high) values. On the other hand, sediments and local soils are commonly strongly enriched in Cr, Ni and Mn, reflecting adjacent ultramafic and mafic rock substrate and lateritic regolith. Concentrations of Cr and Ni are commonly well above interim sediment quality guideline (high) values for sediments, but are in mineralogical forms that are not readily bioavailable. Sediment and soil quality guideline values consequently need to recognise natural enrichments and the mineralogical siting of heavy metals. Although dissolved concentrations of heavy metals in stream waters are commonly low, there is evidence for mobility of Cu, Zn, Fe and Al. Parts of the Kooloonbung Creek wetland area lie on sulfidic estuarine sediments (potential acid sulfate soils). Experimental oxidation of uncontaminated and contaminated sulfidic sediments leads to substantial dissolution of heavy metals under acid conditions, with subsequent aquatic mobility. The results warn about disturbance and oxidation of potential acid sulfate soils that have been contaminated by urban and natural heavy-metal sources.

Environmental geochemistry at the global scale

Land degradation and pollution caused by population pressure and economic development pose a threat to the sustainability of the earth's surface, especially in tropical regions where a long history of chemical weathering has made the surface environment particularly fragile. Systematic baseline geochemical data provide a means of monitoring the state of the environment and identifying problem areas. Regional surveys have already been carried out in some countries, and with increased national and international funding they can be extended to cover the rest of the land surface of the globe. Preparations have been made, under the auspices of the International Union of Geological Surveys (IUGS) and the International Association of Geochemistry and Cosmochemistry (IAGC) for the establishment of just such an integrated global database.

Environmental geochemistry at the global scale

Land degradation and pollution caused by population pressure and economic development pose a threat to the sustainability of the Earth’s surface, especially in tropical regions where a long history of chemical weathering has made the surface environment particularly fragile. Systematic baseline geochemical data provide a means of monitoring the state of the environment and identifying problem areas. Regional surveys have already been carried out in some countries, and with increased national and international funding they can be extended to cover the rest of the land surface of the globe. Preparations have been made, under the auspices of the IUGS, for the establishment of just such an integrated global database.

Identifying the Sources of Stray Methane by Using Geochemical and Isotopic Fingerprinting

Abstract Under certain conditions, the uncontrolled migration of natural gas in the subsurface can result in an explosion, with the potential for significant property damage and loss of life. In western Pennsylvania, there are numerous possible sources of stray natural gas, including producing and abandoned wells, transportation pipelines, subsurface storage reservoirs, coal mines, and municipal waste landfills. The landfill gases are strictly of microbial origin, whereas the others are largely thermogenic in origin. Baseline geochemical data from gases generated in municipal landfills and those produced from gas wells and coals in western Pennsylvania help to identify sources of stray methane while investigating gas migration incidents. Landfill gases exhibit C1/(C2 + C3) ratios > 3.8 × 103, δ13CO2 values of −19.7 to + 17.4 permil, δDCH4 values of −197 to −353 permil, and δ 13CH4 values of −33.2 to −57.4 permil. Most δ13CO2 values are positive, whereas most methane, carbon, and hydrogen are isotopically light. Exceptions to these generalizations can be traced to isotope fractionation due to methane oxidation in soil environments. 14C activity in the landfill methanes ranges from 125 to 145% modern. Thermogenic methane in the subsurface rocks of the region have δ13C and δD values of approximately −27 to −55 permil and −150 to −303 permil, respectively. Chemical and isotopic analyses, combined with a thorough site investigation, provide a comprehensive approach for differentiating microbial from thermogenic gases and thermogenic gases from different sources. We used such an approach to confirm the offsite migration of landfill methane from a municipal landfill and the migration of stray methane from an abandoned gas pool to the soils adjacent to a commercial office building in western Pennsylvania.

Sample Design and Analysis for Regional Geochemical Studies

AbstractThe compilation of baseline geochemical data prior to the development of large‐scale energy projects is necessary in order to assess possible impacts the development may have on the local environment. Most studies are site specific and difficult to integrate where several independent studies are being carried out on adjacent projects. Methods are presented, hierarchial analysis of variance complemented with kriging, to carry out preliminary geochemical baseline determinations over relatively large regions. These efficient, low‐cost methods provide data on natural compositional trends with a known degree of reliability. Soil molybdenum from the Piceance Basin of Colorado and Uinta Basin of Utah are used to illustrate the methods. Sample densities ranging from one per 62 km2 to one per 0.5 km2 are evaluated with respect to effectiveness in presenting the distribution of soil molybdenum of these areas. A sample density of one per 47 km2 was determined to be adequate by analysis of variance, for low resolution mapping of a portion of the eastern Uinta basin. Maps generated by geostatitical analysis followed by kriging provide a visual representation of soil molybdenum and the associated estimation variance. A density of one sample per 62 km2 was demonstrated not to be adequate for low resolution mapping of soil molybdenum in the Piceance basin. A higher sampling density of one per 0.66 km2 was utilized in mapping soil molybdeuum of oil shale Tract C‐a and vicinity. Analysis of variance demonstrates this is an adequate density for mapping at higher resolution. Geostatistical analysis and kriging were used to generate an estimate map and variance estimate for Tract C‐a and vicinity.