No abstract

Abstract. Development of unconventional energy resources such as shale gas and coalbed methane has generated some public concern with regard to the protection of groundwater and surface water resources from leakage of stray gas from the deep subsurface. In terms of environmental impact to and risk assessment of shallow groundwater resources, the ultimate challenge is to distinguish (a) natural in situ production of biogenic methane, (b) biogenic or thermogenic methane migration into shallow aquifers due to natural causes, and (c) thermogenic methane migration from deep sources due to human activities associated with the exploitation of conventional or unconventional oil and gas resources. This study combines aqueous and gas (dissolved and free) geochemical and isotope data from 372 groundwater samples obtained from 186 monitoring wells of the provincial Groundwater Observation Well Network (GOWN) in Alberta (Canada), a province with a long record of conventional and unconventional hydrocarbon exploration. We investigated whether methane occurring in shallow groundwater formed in situ, or whether it migrated into the shallow aquifers from elsewhere in the stratigraphic column. It was found that methane is ubiquitous in groundwater in Alberta and is predominantly of biogenic origin. The highest concentrations of biogenic methane ( > 0.01 mM or > 0.2 mg L −1 ), characterized by δ 13 C CH 4 values < −55 ‰, occurred in anoxic Na-Cl, Na-HCO 3 , and Na-HCO 3 -Cl type groundwaters with negligible concentrations of nitrate and sulfate suggesting that methane was formed in situ under methanogenic conditions for 39.1 % of the samples. In only a few cases (3.7 %) was methane of biogenic origin found in more oxidizing shallow aquifer portions suggesting limited upward migration from deeper methanogenic aquifers. Of the samples, 14.1 % contained methane with δ 13 C CH 4 values > −54 ‰, potentially suggesting a thermogenic origin, but aqueous and isotope geochemistry data revealed that the elevated δ 13 C CH 4 values were caused by microbial oxidation of biogenic methane or postsampling degradation of low CH 4 content samples rather than migration of deep thermogenic gas. A significant number of samples (39.2 %) contained methane with predominantly biogenic C isotope ratios (δ 13 C CH 4 < −55 ‰) accompanied by elevated concentrations of ethane and sometimes trace concentrations of propane. These gases, observed in 28.1 % of the samples, bearing both biogenic (δ 13 C) and thermogenic (presence of C 3 ) characteristics, are most likely derived from shallow coal seams that are prevalent in the Cretaceous Horseshoe Canyon and neighboring formations in which some of the groundwater wells are completed. The remaining 3.7 % of samples were not assigned because of conflicting parameters in the data sets or between replicates samples. Hence, despite quite variable gas concentrations and a wide range of δ 13 C CH 4 values in baseline groundwater samples, we found no conclusive evidence for deep thermogenic gas migration into shallow aqu...

Studies have shown the importance of submarine canyons as conduits of land-derived organic carbon beyond the coastal shelf into the deep-sea where a single obvious river source can be identified. When there is more than one river source, identifying which rivers contribute to canyon sediment organic matter is technically challenging. Here, we compare two contrasting submarine canyons: the Hokitika Canyon, a long, narrow, and gently sloping canyon on the west coast of New Zealand; and the Kaikōura Canyon, a high productivity, short, steep canyon close to shore on the east coast of New Zealand. Both canyons have multiple potential river sources, so we applied a compound specific stable isotope (CSSI) tracking technique to identify and apportion the contribution from each river at locations along the length of each canyon axis. We found that land-derived organic matter contributed between 74 and 100% of the total organic matter in the sediment of the Hokitika Canyon as far as 200 km from shore and to depths of 2000 m. However, less than 50% of the land-derived organic carbon came from the largest river closest to the canyon head. We hypothesize that longshore drift transported much of the sediment from that river past the Hokitika Canyon, while river inflows farther up-current supplied the bulk of the land-derived organic carbon. In contrast, land-derived organic matter contributed less than 50% of the total organic matter in Kaikōura Canyon sediments with land-derived organic sediment contribution decreasing steeply to less than 15% at about 24 km from shore in 1500 m water depth. Most of the land-derived organic matter (ca. 80%) came from the river with the largest suspended sediment yield, despite another (smaller) river discharging closer to the canyon head. We hypothesize that this difference in carbon source is partly due to

North Atlantic climate archives provide evidence for increased storm activity during the Little Ice Age (150 to 600 calibrated years (cal years) B.P.) and centered at 1700 and 3000 cal years B.P., typically in centennial-scale sedimentary records. Meteorological (tropical versus extratropical storms) and climate forcings of this signal remain poorly understood, although variability in the North Atlantic Oscillation (NAO) or Atlantic Meridional Overturning Circulation (AMOC) are frequently hypothesized to be involved. Here we present records of late Holocene storminess and coastal temperature change from a Bermudian submarine cave that is hydrographically circulated with the coastal ocean. Thermal variability in the cave is documented by stable oxygen isotope values of cave benthic foraminifera, which document a close linkage between regional temperature change and NAO phasing during the late Holocene. However, erosion of terrestrial sediment into the submarine cave provides a "storminess signal" that correlates with higher-latitude storminess archives and broader North Atlantic cooling events. Understanding the driver of this storminess signal will require higher-resolution storm records to disentangle the contribution of tropical versus extratropical cyclones and a better understanding of cyclone activity during hemispheric cooling periods. Most importantly, however, the signal in Bermuda appears more closely correlated with proxy-based evidence for subtle AMOC reductions than NAO phasing.

[1] Stable-isotope ratios of two modern Saxidomus gigantea specimens from Namu, British Columbia, are presented to show intraspecimen and interspecimen isotopic variation. Isotopic profiles (d 13 C shell , d 18O shell ) were generated along the axis of maximum growth. The profiles show that analogous seasonal variation is recorded in d 18O shell ; however, significant variability is recorded in d 13 C shell . We suggest this is caused by differences in metabolic activity between individuals. Intrashell variability along a growth horizon shows good reproducibility in d 18 O shell profiles generated from any portion of the shell can be used to evaluate seasonal fluctuations and may be excellent to evaluate types and rates of shell growth.

Abstract. The termination of Cryogenian glaciations would have undoubtedly impacted the chemistry of Neoproterozoic oceans, with possible consequences for life; but the extent and duration of this impact are poorly constrained. In this study, we use the lithium (Li) isotope composition of Ediacaran cap dolostones from South Australia (Nuccaleena Formation) and China (Doushantuo Fm) to investigate changes in ocean chemistry that followed the Marinoan deglaciation. The effect of diagenesis was evaluated and while the Nuccaleena Fm is likely to have preserved the primary composition of cap dolostone deposition, the offset in Li isotope ratios observed for the Doushantuo Fm could possibly reflect partial overprinting by diagenetic fluids. The Li isotope composition of Ediacaran seawater was estimated and we suggest it was similar to that of late Cenozoic oceans for most of the cap dolostone deposition. Using a box model for the oceanic Li cycle, we show that at the onset of deglaciation, the supply of riverine Li to the oceans was up to 50 times the modern flux. The modelled riverine Li isotope composition suggests that continents resembled modern high-latitude regions during this time. This episode was short-lived (up to 1 Myr) and the subsequent supply of riverine Li was similar to modern conditions, both in flux and isotope composition, for the whole duration of cap dolostone deposition. These results suggest that Ediacaran oceans and continents rapidly recovered from the Marinoan glaciation to reach environmental conditions similar to the late Cenozoic. From the standpoint of the Li oceanic budget, the Ediacaran oceans in which complex lifeforms emerged may have not been that different from our modern oceans.

No abstract

The Montney Formation is a highly productive hydrocarbon reservoir with significant reserves of hydrocarbon gases and liquids making it of great economic importance to Canada. However, high concentrations of hydrogen sulfide (H2S) have been encountered during exploration and development that have detrimental effects on environmental, health, and economics of production. H2S is a highly toxic and corrosive gas and therefore it is essential to understand the distribution of H2S within the basin in order to enhance identification of areas with a high risk of encountering elevated H2S concentrations in order to mitigate against potential negative impacts. Gas composition data from Montney wells is routinely collected by operators for submission to provincial regulators and is publicly available. We have combined data from Alberta (AB) and British Columbia (BC) to create a basin-wide database of Montney H2S concentrations. We then used an iterative quality control and quality assurance process to produce a dataset that best represents gas composition in reservoir fluids. This included: 1) designating gas source formation based on directional surveys using a newly developed basin-wide 3D model incorporating AGS's Montney model of Alberta with a model in BC, which removes errors associated with reported formations; 2) removed injection and disposal wells; 3) assessed wells with the 50 highest H2S concentrations to determine if gas composition data is accurate and reflective of reservoir fluid chemistry; and 4) evaluated spatially isolated extreme values to ensure data accuracy and prevent isolated highs from negatively impacting data interpolation. The resulting dataset was then used to calculate statistics for each x, y location to input into the interpolation process. Three interpolations were constructed based on the associated phase classification: H2S in gas, H2S in liquid (C7+), and aqueous H2S. We used Empirical Bayesian Kriging interpolation to generate H2S distribution maps along with a series of model uncertainty maps. These interpolations illustrate that H2S is heterogeneously distributed across the Montney basin. In general, higher concentrations are found in AB compared with BC with the highest concentrations in the Grande Prairie region along with several other isolated region in the southeastern portion of the basin. The interpolations of H2S associated with different phases show broad similarities. Future mapping research will focus on subdividing intra-Montney sub-members plus under- and overlying strata to further our understanding of the role migration plays in H2S distribution within the Montney basin.