Search

Vast amounts of methane (CH4) stored in submarine sediments are susceptible to release in a warming Arctic, further exacerbating climate change in a positive feedback. It is therefore critical to monitor CH4 over pan-regional scales to detect early signs of CH4 release. However, our ability to monitor CH4 is hampered in remote northern regions by sampling and logistical constraints, and few good baseline data exist in many areas. From high-resolution atmospheric CH4 measurements and discrete surface water samples, we estimated instantaneous sea–air CH4 fluxes at various locations. We also created a baseline study of current background levels of CH4 in North Atlantic waters based on the atmospheric CH4 data over 22 d in summer 2021 on a roughly 5100 km voyage in the northern Labrador Sea and Baffin Bay between 55 and 72∘ N. In addition, we measured CH4 concentrations across the water column at various stations. Measured atmospheric mixing ratios of CH4 ranged from 1944 to 2012 ppbv, with a mean of 1966 ± 8 ppbv and a baseline of 1954–1981 ppbv. Dissolved CH4 concentrations in the near-surface water peaked at 5.3 nmol L−1 within 1 km down-current of a known cold seep at Scott Inlet and were consistently oversaturated throughout the water column in Southwind Fjord, which is an area that has been recently affected by submarine landslides. Local sea–air CH4 fluxes ranged from 0.003–0.119 µmol m−2 d−1, indicating that the ocean released only small amounts of CH4 to the atmosphere at all stations. Atmospheric CH4 levels were also driven by meteorological, spatial, and temporal variations, and both onshore and ocean-based contributions to atmospheric CH4 mixing ratios are likely. Coupled high-resolution measurements of marine and atmospheric CH4 data have the potential to provide ongoing monitoring in a region susceptible to CH4 releases, as well as critical validation data for global-scale measurements and modelling.

Fluids leaked from oil and gas wells often originate from their surface casing─a steel pipe installed beneath the deepest underlying source of potable groundwater that serves as the final barrier around the well system. In this study, we analyze a regulatory dataset of surface casing geochemical samples collected from 2573 wells in northeastern Colorado─the only known publicly available dataset of its kind. Thermogenic gas was present in the surface casings of 96.2% of wells with gas samples. Regulatory records indicate that 73.3% of these wells were constructed to isolate the formation from which the gas originated with cement. This suggests that gas migration into the surface casing annulus predominantly occurs through compromised barriers (e.g., steel casings or cement seals), indicative of extensive integrity issues in the region. Water was collected from 22.6% of sampled surface casings. Benzene, toluene, ethylbenzene, and xylenes were detected in 99.7% of surface casing water samples tested for these compounds, which may be due to the presence of leaked oil, natural gas condensate, or oil-based drilling mud. Our findings demonstrate the value of incorporating surface casing geochemical analysis in well integrity monitoring programs to identify integrity issues and focus leak mitigation efforts.

The flow of Pacific water to the North Atlantic exerts a globally significant control on nutrient balances between the two ocean basins and strongly influences biological productivity in the northwest Atlantic. Nutrient ratios of nitrate (NO3-) versus phosphate (PO43-) have previously been used to complement salinity characteristics in tracing the distribution of Pacific water in the North Atlantic. We expand on this premise and demonstrate that the fraction of Pacific water as determined by NO3- : PO43- ratios can be quantitatively predicted from the isotopic composition of sub-euphotic nitrate in the northwest Atlantic. Our linear model thus provides a critically important framework for interpreting δ15N signatures incorporated into both modern marine biomass and organic material in historical and paleoceanographic archives along the northwest Atlantic margin.

Vast amounts of methane (CH4) stored in permafrost and submarine sediments are susceptible to release in a warming Arctic, further exacerbating climate change in a positive feedback. It is therefore critical to monitor CH4 over pan-regional scales to detect early signs of CH4 release. However, our ability to monitor CH4 is hampered in remote northern regions by sampling and logistical constraints and few good baseline data exist in many areas. To create a baseline study of current background levels of CH4 in North Atlantic waters, we collected continuous real-time atmospheric CH4 data, along with ambient air temperature and wind parameters over 22 days in summer 2021 on a roughly 5100 km voyage in the northern Labrador Sea and Baffin Bay up to 71° N. In addition, we measured CH4 concentrations in the water column using discrete water samples at selected stations. Measured atmospheric mixing ratios of CH4 ranged from 1944.7 ppb to 2012.0 ppb, with a mean of 1966.0±7.4 ppb and a baseline of 1954.2−1980.6 ppb. Dissolved CH4 concentrations in the near-surface water peaked at 56.58±0.05 nM within 1 km down-current of a known cold seep at Scott Inlet but were consistently super-saturated throughout the water column in Southwind Fjord, which is an area recently affected by submarine landslides. Local sea-air CH4 fluxes ranged from 0.1−14.1 µmol m-2 d-1 indicating that the ocean acted as a CH4 source to the atmosphere. Atmospheric CH4 levels were also driven by meteorological, spatial, and temporal variations. Highest atmospheric CH4 mixing ratios were detected in the Cumberland Sound in Nunavut, suggesting onshore sources from nearby waterbodies and wetlands, whereas ocean-based contributions at this location could not be ruled out. Coupled real-time measurements of marine and atmospheric CH4 data have the potential to provide ongoing monitoring in a region susceptible to CH4 releases, as well as critical validation data for global-scale measurements and modelling.

The accelerated increase in global methane (CH4) in the atmosphere, accompanied by a decrease in its 13C/12C isotopic ratio (δ13CCH4) from −47.1‰ to −47.3‰ observed since 2008, has been attributed to increased emissions from wetlands and cattle, as well as from shale gas and shale oil developments. To date both explanations have relied on poorly constrained δ13CCH4 source signatures. We use a dataset of δ13CCH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ13CCH4 of −39.6‰. The average δ13CCH4 weighted by US basin-level measured emissions in 2015 was −41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ13CCH4 signal in the observed decrease since 2008 (while noting that the global isotopic trend is the net of all dynamic source and sink processes). This observation strongly suggests that changing emissions of other (isotopically-lighter) CH4 source terms is dominating the increase in global CH4 emissions. Although production of shale gas has increased rapidly since 2008, and CH4 emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH4 is not dominated by emissions from shale gas and shale oil developments.

We study the drivers behind the global atmospheric methane (CH4) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984–2016 to produce three‐dimensional fields of CH4 and δ 13C‐CH4, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH4 emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ 13C‐CH4. Increased FF CH4 emissions are unlikely to be the dominant driver for the post‐2006 global CH4 increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increase since it does not track the observed decrease in global mean δ 13C‐CH4. Different CH4 sinks have different fractionation factors for δ 13C‐CH4, thus we can investigate the uncertainty introduced by the reaction of CH4 with tropospheric chlorine (Cl), a CH4 sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH4 sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions., Key Points Increased fossil fuel emissions are unlikely the dominant driver for post‐2006 global CH4 increaseA significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increaseCH4 emission attributions are sensitive to the uncertainties in OH fractionation, tropospheric Cl sink, and wetland areas

Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

Significance There are over 900,000 active oil and gas wells in the United States. Estimates of the percentage of these wells that leak during their lifetime have been limited by data availability. We mined publicly available records from state regulatory databases and synthesized the results of various well testing methods into a uniform dataset that describes the integrity of 105,031 wells. Our analysis of the dataset provides insight into regional well leakage frequencies in three US states (Colorado, New Mexico, and Pennsylvania), spatial and temporal leakage trends among vertical and directional wells, and the characteristics of leaked fluids. Our findings demonstrate the value of statewide well testing programs and highlight the challenges of interpreting disparate interjurisdictional well testing data., Oil and gas wells with compromised integrity are a concern because they can potentially leak hydrocarbons or other fluids into groundwater and/or the atmosphere. Most states in the United States require some form of integrity testing, but few jurisdictions mandate widespread testing and open reporting on a scale informative for leakage risk assessment. In this study, we searched 33 US state oil and gas regulatory agency databases and identified records useful for evaluating well integrity in Colorado, New Mexico, and Pennsylvania. In total, we compiled 474,621 testing records from 105,031 wells across these states into a uniform dataset. We found that 14.1% of wells tested prior to 2018 in Pennsylvania exhibited sustained casing pressure (SCP) or casing vent flow (CVF)—two indicators of compromised well integrity. Data from different hydrocarbon-producing regions within Colorado and New Mexico revealed a wider range (0.3 to 26.5%) of SCP and/or CVF occurrence than previously reported, highlighting the need to better understand regional trends in well integrity. Directional wells were more likely to exhibit SCP and/or CVF than vertical wells in Colorado and Pennsylvania, and their installation corresponded with statewide increases in SCP and/or CVF occurrence in Colorado (2005 to 2009) and Pennsylvania (2007 to 2011). Testing the ground around wells for indicators of gas leakage is not a widespread practice in the states considered. However, 3.0% of Colorado wells tested and 0.1% of New Mexico wells tested exhibited a degree of SCP sufficient to potentially induce leakage outside the well.

20 pages, 8 figures, 2 tables, Recent work using compound-specific stable isotopes of amino acids (CSI-AA) in proteinaceous deep-sea corals opens a new realm of high-fidelity reconstruction for biogeochemical and ecological changes in the ocean. However, underlying these CSI-AA paleoceanographic applications are a series of fundamental assumptions, which hold first that baseline-proxy AA isotope values fixed at the base of food webs represent integrated δ13C and δ15N values of primary production, and second they are unaltered during subsequent export and incorporation from particles into corals. We explored long-term δ13C and δ15N CSI-AA data on a sediment trap time series together with contemporaneous, geographically close deep-sea bamboo corals (Isidella sp.) in the California margin, directly testing these assumptions for the first time. Our data show that isotope values of essential (δ13CEAA) and source AAs (δ15NPhe) in sinking particles quantitatively track bulk δ13C and δ15N values of export production. These CSI-AA baseline proxies varied independently of carbon flux, trophic position (TPCSI-AA) and microbial alteration, suggesting that they were well preserved in the sinking particles consumed by corals. Paired comparisons between sinking particles and corals revealed minor elevations of δ13CEAA (by ∼2‰) and δ15NPhe (by ∼1‰) in available coral specimens. We hypothesize that the difference in δ13CEAA is due to the geographic offset in δ13C values of primary production expected between the (more offshore) sediment trap site and (more onshore) coral specimens, whereas the δ15NPhe offset is likely related to expected minor trophic fractionation. Using empirical models derived from the sediment trap time series, we demonstrate for the first time that CSI-AA in proteinaceous deep-sea corals can reconstruct known bulk δ15N values of export production, source nitrogen δ15N values, and exported TPCSI-AA values with very good fidelity. Together, these findings represent a major advance in our understanding of AA isotope behavior in modern and paleoarchives, and can be used to underpin the rapidly evolving use of CSI-AA-based tools in multiple paleoceanographic studies and archives, This study was supported by National Science Foundation (OCE 1635527 to T.G. and M.M). A portion of this work was performed under the auspices of the U.S. Department of Energy (DE-AC52-07NA27344)

The presence of methane and other hydrocarbons in domestic-use groundwater aquifers poses significant environmental and human health concerns. Isotopic measurements are often relied upon as indicators of groundwater aquifer contamination with methane. While these parameters are used to infer microbial metabolisms, there is growing evidence that isotopes present an incomplete picture of subsurface microbial processes. This study examined the relationships between microbiology and chemistry in groundwater wells located in the Denver-Julesburg Basin of Colorado, a rapidly urbanizing area with active oil and gas development. A primary goal was to determine if microbial data can reliably indicate the quantities and sources of groundwater methane. Comprehensive chemical and molecular analyses were performed on 39 groundwater well samples from five aquifers. Elevated methane concentrations were found in only one aquifer, and both isotopic and microbial data support a microbial origin. Microbial parameters had similar explanatory power as chemical parameters for predicting sample methane concentrations. Furthermore, a subset of samples with unique microbiology corresponded with unique chemical signatures that may be useful indicators of methane gas migration, potentially from nearby coal seams interacting with the aquifer. Microbial data may allow for more accurate determination of groundwater contamination and improved long-term water quality monitoring compared solely to isotopic and chemical data in areas with microbial methane.

Groundwater methane (CH4) in areas of fossil fuel development has been a recent focus of study as high CH4 concentrations pose water quality concerns and potential explosive hazards. In 2013, a provincial study in Nova Scotia identified areas with elevated groundwater CH4. However, due to limited data, the specific sources and local distribution of CH4 in those areas remain unknown. In this study, we examined the Stellarton Basin in central Nova Scotia, Canada, a region with an abundance of coal formations, numerous abandoned coal mines, and an active open pit coal mine. Methane was detected in 94% of water samples that were sampled from 45 private water wells. Six water wells exceeded the 28 mg/L hazard mitigation threshold with CH4 levels of up to 72.7 mg/L. The δ13CCH4 (−85.5 to −48.5‰) and the δ2HCH4 (−280 to −88‰) indicated that >95% of samples had CH4 of microbial origin. However, the detection of ethane (C2H6) up to 2.97 mg/L and propane (C3H8) up to 0.008 mg/L, as well as the δ13CC2H6 values (−30.1 to −15.6‰) suggested a mixture of microbial CH4 with trace thermogenic gas, likely migrated from Stellarton coals (δ13CC2H6 of −27.6 to −15.35‰). A mobile greenhouse gas analyzer survey was conducted within the perimeter of residences and off-gassing from taps had atmospheric CH4 measurements as high as 66 ppmv. This study integrates multiple sampling and monitoring methods to investigate groundwater CH4 in a coal-bearing region. The findings advance the understanding of the origin and occurrence of CH4 in complex groundwater systems. The data acquired in this study may be used as a pre-drill baseline for groundwater CH4 concentrations and origins should coal-bed methane operations in Nova Scotia proceed in the future.

Subsurface nitrate in the Labrador Sea (NW Atlantic) and Baffin Bay is provided by North Pacific water flowing through Bering Strait and the Canadian Arctic as well as by advection from the North Atlantic. Both these nitrate sources are distinct in their isotopic signature (δ15N), owing to benthic denitrification on the Bering, Chukchi and east Siberian shelves and nitrogen fixation in the North Atlantic, respectively. Accordingly, water column profiles of δ15N(nitrate) collected off Greenland in the eastern Labrador Sea show low δ15N(nitrate), which mixes with more 15N-enriched nitrate flowing through Baffin Bay into the northern Labrador Sea. The Labrador Current carries this mixture southward along the western Labrador Sea, toward Newfoundland. The δ15N of surface sediments in the Labrador Sea closely mirrors these water column signals, suggesting that sediments can be used to trace changes in both the source signature of Atlantic versus Pacific-derived nitrate as well as in the admixture of the two source waters.Two downcore sedimentary δ15N records from the NE and NW Labrador Sea coast both show high δ15N values of ca. 7‰ during the early Holocene (9-7 kyrs BP). In the NE Labrador Sea, this is followed by a long-term decrease toward δ15N of ca. 4.5‰ at the core top, in contrast to a much more subtle decrease in the NW Labrador Sea (surface sediment δ15N of ca. 6.5‰). The decreasing δ15N values along the eastern Labrador Sea are consistent with a Holocene increase in nitrogen fixation in the North Atlantic or an increasing advection of isotopically light nitrate. In turn, an increasing admixture of North-Pacific-derived nitrate, or intensified denitrification on the Bering Shelf would be required to explain the much subdued Holocene δ15N decrease in the NW Labrador Sea.

The risk of environmental contamination by oil and gas wells depends strongly on the frequency with which they lose integrity. Wells with compromised integrity typically exhibit pressure in their outermost annulus (surface casing pressure, SfCP) due to gas accumulation. SfCP is an easily measured but poorly documented gauge of well integrity. Here, we analyze SfCP data from the Colorado Oil and Gas Conservation Commission database to evaluate the frequency of well integrity loss in the Wattenberg Test Zone (WTZ), within the Wattenberg Field, Colorado. Deviated and horizontal wells were found to exhibit SfCP more frequently than vertical wells. We propose a physically meaningful well-specific critical SfCP criterion, which indicates the potential for a well to induce stray gas migration. We show that 270 of 3923 wells tested for SfCP in the WTZ exceeded critical SfCP. Critical SfCP is strongly controlled by the depth of the surface casing. Newer horizontal wells, drilled during the unconventional drilling boom, exhibited critical SfCP less frequently than other wells because they were predominantly constructed with deeper surface casings. Thus, they pose a lower risk for inducing stray gas migration than legacy vertical or deviated wells with surface casings shorter than modern standards.

Revisions in isotopic source signatures reveal that global total fossil fuel methane emissions from industry plus natural geological seepage are much larger than thought. Stefan Schwietzke et al. re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions on the basis of long-term global methane and methane carbon isotope records. They find that total fossil fuel methane emissions (fossil fuel industry plus natural geological methane seepage) are not increasing over time, but are 60–110 per cent greater than was previously thought. They also conclude that methane emissions from natural gas, oil and coal production and their usage are 20–60 per cent greater than inventories and that methane emissions from natural gas as a fraction of production have declined from about 8 per cent to 2 per cent over the past three decades. Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions1,2,3,4,5,6,7,8,9,10 to the total atmospheric methane budget11. However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage12,13, which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates1,2,3,4,5,6,7,8,9,10 owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage12,13, we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories1,2. Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

The concentration of atmospheric methane (CH4) has more than doubled over the industrial era. To help constrain global and regional CH4 budgets, inverse (top-down) models incorporate data on the concentration and stable carbon (δ13C) and hydrogen (δ2H) isotopic ratios of atmospheric CH4. These models depend on accurate δ13C and δ2H end-member source signatures for each of the main emissions categories. Compared with meticulous measurement and calibration of isotopic CH4 in the atmosphere, there has been relatively less effort to characterize globally representative isotopic source signatures, particularly for fossil fuel sources. Most global CH4 budget models have so far relied on outdated source signature values derived from globally nonrepresentative data. To correct this deficiency, we present a comprehensive, globally representative end-member database of the δ13C and δ2H of CH4 from fossil fuel (conventional natural gas, shale gas, and coal), modern microbial (wetlands, rice paddies, ruminants, termites, and landfills and/or waste) and biomass burning sources. Gas molecular compositional data for fossil fuel categories are also included with the database. The database comprises 10 706 samples (8734 fossil fuel, 1972 non-fossil) from 190 published references. Mean (unweighted) δ13C signatures for fossil fuel CH4 are significantly lighter than values commonly used in CH4 budget models, thus highlighting potential underestimation of fossil fuel CH4 emissions in previous CH4 budget models. This living database will be updated every 2–3 years to provide the atmospheric modeling community with the most complete CH4 source signature data possible. Database digital object identifier (DOI): https://doi.org/10.15138/G3201T.

Public concern about the potential effects of unconventional oil and gas development on water quality has grown in recent years. In this study, we considered accidental spills and releases that occurred from 2007 to 2014 in the Greater Wattenberg Area (GWA), an area of intensive oil and gas extraction located within the Denver-Julesburg Basin in northeastern Colorado. Our objective was to quantify the occurrence rates of (1) all spills and (2) spills determined by the operator to have caused groundwater impacts. Additionally, for spills affecting groundwater, we analyzed characteristics including facilities and equipment involved, causes, and resolution times in order to identify recommendations for improved regulatory or operational practices. Spills and releases were identified from publicly-available reports collected by the Colorado Oil and Gas Conservation Commission (COGCC). Spills were identified as having potentially impacted groundwater based on an operator-reported field present on the reports. Groundwater impacts were verified for this subset of spills by reviewing the narrative description of the incident and the supporting documents (e.g., consultant reports, laboratory data) available in the COGCC database. Additional information collected included the causes of the spills, the facility types, and the time required for resolution of spills. To determine spill occurrence rates, the number of annual spills was compared to several annual indicators of oil and gas development, including the volumes of oil and water produced and the number of active producing wells in the GWA. The annual spill count remained relatively stable from 2007 to 2011, but increased each year from 2011 to 2014. From 2007 to 2014, spill occurrence rates decreased or remained steady when compared to oil and water production volumes, respectively; however, the occurrence rate increased compared to the number of active producing wells. These trends suggest that fewer spills occurred for a greater volume of fluids produced and handled in the GWA, but that more spills occurred per active well. For all three normalization metrics, we observed that the occurrence rate of groundwater-impacting spills remained steady or decreased. The percentage of spills impacting groundwater compared to total spills decreased by a factor of two from 2010 (54%) to 2014 (27%). Among groundwater-impacting spills, the most common facility types were tank batteries and lines (including flow lines, gathering lines, load lines, and pipelines), and the most common cause was equipment failure. Based on our observations of spills in the GWA from 2007 to 2014, we have suggested several strategies for regulators and operators to improve spill reporting practices and to reduce the likelihood of surface spills that result in persistent impacts to shallow groundwater.

The concentration of atmospheric methane (CH4) has more than doubled over the industrial era. To help constrain global and regional CH4 budgets, inverse (top-down) models incorporate data on the concentration and stable carbon (δ13C) and hydrogen (δ2H) isotopic ratios of atmospheric CH4. These models depend on accurate δ13C and δ2H end-member source signatures for each the main emissions categories. Compared with meticulous measurement and calibration of isotopic CH4 in the atmosphere, there has been relatively little effort to characterize globally representative isotopic source signatures, particularly for fossil fuel sources, since the 1980s. Most global CH4 budget models have so far relied on outdated source signature values derived from globally non-representative data. To correct this deficiency, we present a comprehensive, globally representative end-member database of the δ13C and δ2H of CH4 from fossil fuel (conventional natural gas, shale gas and coal), modern microbial (wetlands, rice paddies, ruminants, termites, and landfills/waste) and biomass burning sources. Alkane and permanent gas molecular chemistry for fossil fuel categories are also included with the database. The database comprises 10,706 samples (8,734 fossil fuel, 1972 non-fossil) from 190 published references. Mean (unweighted) δ13C signatures for fossil fuel CH4 are significantly lighter than values commonly used in CH4 budget models, thus highlighting potential under-estimation of fossil fuel CH4 emissions in previous CH4 budget models. This living database will be updated every 2–3 years to provide the atmospheric modeling community with the most complete CH4 source signature data possible. Database Digital Object Identifier (DOI): https://doi.org/10.15138/G3201T.

Despite a reduction in nutrient supply to the North Pacific subtropical gyre, it has undergone a recent increase in nitrogen fixation, and here records of nitrogen isotopes preserved in Hawaiian corals show that this is a trend that could be linked to climate change since the end of the Little Ice Age. The deep-sea Hawaiian gold coral Kulamanamana haumeaae is a remarkably long-lived species, sometimes attaining ages of thousands of years. Over the millennia these corals provide a unique geochemical time-series as they convert sinking phytoplankton and other tiny particles into a proteinaceous coral skeleton. Primary productivity in the North Pacific subtropical gyre has increased in recent decades despite a decline in nutrient supply. An ecosystem shift towards nitrogen-fixing plankton communities has been put forward as a possible explanation, but the cause for this shift remains unclear. Owen Sherwood and colleagues use nitrogen isotopic (δ15N) records from K. haumeaae corals to establish that the increase in nitrogen fixation had already began around 150 years ago, and that it may have been linked to Northern Hemisphere climate change since the end of the Little Ice Age. The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean1. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters2,3. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes2,3,4. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend5. Here we analyse records of bulk and amino-acid-specific 15N/14N isotopic ratios (δ15N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ15N decreases between 1850 and the present. The total shift in δ15N of −2 per mil over this period is comparable to the total change in global mean sedimentary δ15N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster6. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records7,8 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

Unconventional oil and gas development has generated intense public concerns about potential impacts to groundwater quality. Specific pathways of contamination have been identified; however, overall rates of contamination remain ambiguous. We used an archive of geochemical data collected from 1988 to 2014 to determine the sources and occurrence of groundwater methane in the Denver-Julesburg Basin of northeastern Colorado. This 60,000-km(2) region has a 60-y-long history of hydraulic fracturing, with horizontal drilling and high-volume hydraulic fracturing beginning in 2010. Of 924 sampled water wells in the basin, dissolved methane was detected in 593 wells at depths of 20-190 m. Based on carbon and hydrogen stable isotopes and gas molecular ratios, most of this methane was microbially generated, likely within shallow coal seams. A total of 42 water wells contained thermogenic stray gas originating from underlying oil and gas producing formations. Inadequate surface casing and leaks in production casing and wellhead seals in older, vertical oil and gas wells were identified as stray gas migration pathways. The rate of oil and gas wellbore failure was estimated as 0.06% of the 54,000 oil and gas wells in the basin (lower estimate) to 0.15% of the 20,700 wells in the area where stray gas contamination occurred (upper estimate) and has remained steady at about two cases per year since 2001. These results show that wellbore barrier failure, not high-volume hydraulic fracturing in horizontal wells, is the main cause of thermogenic stray gas migration in this oil- and gas-producing basin.

Taphonomic degradation of cold-water gorgonian and antipatharian corals may affect the record of dead or broken corals remaining on the sea floor after damage by deep-sea fisheries. A taphonomic experiment on skeletal degradation of four species of gorgonians and one antipatharian was run in aquaria for 14 months. Paragorgia arborea skeletal fragments, composed of spongy proteins and calcite spicules, were completely degraded within 14 months. Protein-skeletons of Paramuricea and antipatharian fragments gained 10-20% wet weight through hydration of their proteinaceous skeletons. Calcitic gorgonian skeletons of Primnoa resedaeformis and Keratoisis grayi experienced no significant weight change during the experiment. Field observations of dead dislodged and dead skeletons corroborated experimental results. Dislodged and dying P. arborea skeletons were rigid in their live parts, but friable and fragile in their dead parts. Subfossil calcareous gorgonian skeletons up to 2000 years in age have been recovered from the sea floor by trawl bycatch or ROV. Documenting fisheries damage to gorgonian coral forests on the sea floor requires observation within a few months after the fisheries impacts.

Geological features supporting cold-water coral habitat in Atlantic Canada are reviewed and exemplified using qualitative field observations from the Scotian margin and Southwest Grand Banks, Newfoundland, in the context of regional geology of the Atlantic Canadian continental margin. Coral habitats are concentrated in areas of shelf-crossing troughs and trough-mouth fans associated with glacial ice streams. Most habitat types are supported by glacial or glaciomarine depositional features, although some are supported by erosional features, probably related to subglacial meltwater erosion. Shelf-break and upper continental slope moraine deposits, subject to strong currents, form current-swept cobble-boulder pavements, forming the principal habitats for the large gorgonian corals Primnoa resedaeformis and Paragorgia arborea . At greater depths, upper slope till tongues are upper continental slope gravelly mud deposits, exposed at the surface as isolated cobbles and boulders in a muddy sand matrix. Common skeletal corals in these environments include the large long-lived gorgonian Keratoisis ornata , the smaller gorgonian Acanthogorgia armata , and, in muddy sand between rocks, the small gorgonian Acanella arbuscula . Erosional environments including friable Tertiary mudstones and semi-consolidated Quaternary sediments, colonized by large gorgonians and other corals, were observed in The Gully, the Stone Fence, and the Southwest Grand Banks. Weak bedrock strength may limit the size of large gorgonian coral colonies. Authigenic carbonate crusts, possibly related to cold seeps, may be locally important in supporting coral growth. Predictive models of coral distribution should consider Quaternary and surficial geology.

Despite the importance of the nitrogen (N) cycle on marine productivity, little is known about variability in N sources and cycling in the ocean in relation to natural and anthropogenic climate change. Beyond the last few decades of scientific observation, knowledge depends largely on proxy records derived from nitrogen stable isotopes ( δ 15 N) preserved in sediments and other bioarchives. Traditional bulk δ 15 N measurements, however, represent the combined influence of N source and subsequent trophic transfers, often confounding environmental interpretation. Recently, compound-specific analysis of individual amino acids ( δ 15 N-AA) has been shown as a means to deconvolve trophic level versus N source effects on the δ 15 N variability of bulk organic matter. Here, we demonstrate the first use of δ 15 N-AA in a paleoceanographic study, through analysis of annually secreted growth rings preserved in the organic endoskeletons of deep-sea gorgonian corals. In the Northwest Atlantic off Nova Scotia, coral δ 15 N is correlated with increasing presence of subtropical versus subpolar slope waters over the twentieth century. By using the new δ 15 N-AA approach to control for variable trophic processing, we are able to interpret coral bulk δ 15 N values as a proxy for nitrate source and, hence, slope water source partitioning. We conclude that the persistence of the warm, nutrient-rich regime since the early 1970s is largely unique in the context of the last approximately 1,800 yr. This evidence suggests that nutrient variability in this region is coordinated with recent changes in global climate and underscores the broad potential of δ 15 N-AA for paleoceanographic studies of the marine N cycle.

Modification of event beds by the burrowing nereidid polychaete Alitta virens (Sars) was examined using laboratory microcosms, to assess its importance as an ecosystem engineer in pristine sediments. In all microcosms, the nereidids modified their environment to permit long-term occupation, but different behavioral strategies and burrow morphologies were observed based on sediment characteristics and nutrient availability. Alitta virens utilized scavenging, surface deposit feeding, suspension feeding, microbial gardening, deposit feeding at depth, and cannibalism. Suspension feeding using mucus nets is used by many nereidids but has not been documented previously in A. virens; extended use of the technique may indicate low availability of biotic sediments for deposit feeding. Alitta virens typically produced burrows similar to Arenicolites and Skolithos, but morphologies resembling Polykladichnus, Planolites, Palaeophycus, and Thalassinoides were formed under differing sedimentary conditions and ...

In 2007, a study was undertaken of the long-term records of the ratios of stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) in the skeletons of antipatharians (black corals) growing on reefs near the city of Jeddah, Saudi Arabia. Antipatharians were collected from more than 100 sites, and detailed records obtained from 31 individuals from 16 sites. Annual growth bands were identified and counted. Precision of age estimates was verified by bomb radiocarbon analyses. Some of the specimens were >70 yr old. Samples from a comparison reef, 15 km offshore in the Red Sea, showed δ 15 N values of 4‰ and δ 13 C values of about -18‰. The early years of growth of antipatharians from affected areas also showed δ 15 N values of 4‰ and δ 13 C values of about -18‰, but then in subsequent years these values changed, presumably as water quality deteriorated. Records from specimens in the harbour showed a steady increase in δ 15 N that paralleled population growth in the city — values from 2007 frequently exceeded 8.0‰. The carbon record also showed progressive change: since 1950, values of δ 13 C in affected areas gradually moved from about -18 to -17‰. We postulate that δ 15 N values track the input of sewage-dominated wastewater. By using correlations with species of algae that co-occur with antipatharians on outer- and mid-shelf reefs, we were able to build up a spa- tial and temporal picture of 60 yr of sewage input off this coastline. The advantage of the use of bioindicators is the ability to provide baseline water quality data in places where no such data exist.

Bamboo corals (Family Isididae) are an important component of seamount benthos south of Tasmania. Besides having lifespans of up to 400 yr, little is known about their basic ecology, nor how to decode potential climate signals encoded in their skeletons. We explored the stable nitrogen isotope and radiocarbon compositions of the skeletal organic fraction of the genera Isidella, Keratoisis and Lepidisis collected from 3 Tasmanian seamounts. Analyses were performed on tissues and organic node growth rings sampled at a temporal resolution of 1 to 4 yr. Radiocarbon chronologies exhibited nuclear bomb signals characteristic of surface waters and constrained radial growth rates to ~35 ± 10 µm yr–1 for 3 specimens of the genus Lepidisis and 113 ± 17 µm yr–1 for 1 specimen of Isidella. δ15N values of the living tissue and underlying gorgonin were similar and averaged 9 to 12‰. Records of δ15N from 8 different specimens showed subtle, quasi-decadal patterns over the last ~100 yr, although the amplitude of these features (~1‰) was similar to the average intra- and intercolony reproducibility. These results demonstrate the utility of deep-sea corals to track seamount biogeochemical processes over long time scales, and suggest that the extent of nutrient depletion of surface waters and associated trophic dynamics have remained relatively constant in this region over centuries. This provides an important baseline for the evaluation of the impacts of anthropogenic climate change.

Using bomb-14C dating and growth ring counting methods, we calculate life spans and growth rates of six species of deep-sea corals collected at depths of between 400 and 900 m from the continental slope of Newfoundland and Labrador. Bamboo corals ( Acanella arbuscula , Keratoisis ornata ) and antipatharians ( Stauropathes arctica ) secrete concentric growth rings in their axial skeletons, which were proven to form annually for K. ornata and S. arctica. These species had the lowest radial growth rates of 33 ± 11 to 75 ± 11 μm·year–1. Primnoa resedaeformis and Paramuricea spp. had higher radial growth rates of 83 ± 6 to 215 ± 37 μm·year–1. Paragorgia arborea had the highest radial growth rate of 830 ± 120 μm·year–1. Axial growth rates ranged from 0.56 ± 0.05 cm·year–1 for a specimen of Paramuricea spp. to 2.61 ± 0.45 cm·year–1 for a specimen of Primnoa resedaeformis . Life spans ranged from 18 ± 4 years for a live-collected P. resedaeformis to 200 ± 30 years for a subfossil specimen of K. ornata. Because all of the corals were from heavily fished areas, it is likely that age distributions are biased towards smaller and younger colonies. Recovery of deep-sea corals from fishing-induced damage will take decades to centuries.

Deep-sea gorgonian corals secrete a 2-part skeleton of calcite, derived from dissolved inorganic carbon at depth, and gorgonin, derived from recently fixed and exported particulate organic matter. Radiocarbon contents of the calcite and gorgonin provide direct measures of seawater radiocarbon at depth and in the overlying surface waters, respectively. Using specimens collected from Northwest Atlantic slope waters, we generated radiocarbon records for surface and upper intermediate water layers spanning the pre- and post-bomb- 14 C eras. In Labrador Slope Water (LSW), convective mixing homogenizes the pre-bomb Δ 14 C signature (− 67 ± 4‰) to at least 1000 m depth. Surface water bomb- 14 C signals were lagged and damped (peaking at ∼ + 45‰ in the early 1980s) relative to other regions of the Northwest Atlantic, and intermediate water signals were damped further. Off southwest Nova Scotia, the vertical gradient in Δ 14 C is much stronger. In surface water, pre-bomb Δ 14 C averaged − 75 ± 5‰. At 250–475 m depth, pre-bomb Δ 14 C oscillated quasi-decadally between − 80 and − 100‰, likely reflecting interannual variability in the presence of Labrador Slope Water vs. Warm Slope Water (WSW). Finally, subfossil corals reveal no systematic changes in vertical Δ 14 C gradients over the last 1200 yr.

With the aim of understanding of the trophic ecology of cold-water corals, this paper explores the tissue d 13 C and d 15 N values of 11 ‘coral’ species (8 alcyonacean, 1 antipatharian, 1 pennatulacean, 1 scleractinian) collected along the Newfoundland and Labrador continental slope. Isotopic results delimit species along continua of trophic level and food lability. With an isotopic signature similar to macrozooplankton, Paragorgia arborea occupies the lowest trophic level and most likely feeds on fresh phytodetritus. Primnoa resedaeformis occupies a slightly higher trophic level, likely supplementing its diet with microzooplankton. Bathypathes arctica, Pennatulacea and other alcyonaceans (Acanella arbuscula, Acanthogorgia armata, Anthomastus grandiflorus

Community changes centuries in the making How might climate change affect the base of the marine food chain? Phytoplankton, the foundation of the marine ecosystem, depend on ambient oceanographic conditions such as temperature, salinity, and nutrient availability, which affect ocean chemistry and isotopic distributions. McMahon et al. report carbon isotopic composition changes in the North Pacific Ocean over the past 1000 years, which reflect changes in the community composition of phytoplankton in the region (see the Perspective by Vogt). An ongoing trend toward greater prevalence of nitrogen-fixing cyanobacteria that began 100 years ago might lead to a more efficient carbon pump and remove increasing amounts of CO 2 from the atmosphere. Science , this issue p. 1530 ; see also p. 1466

Primnoa resedaeformis is a deep-sea gorgonian coral with a two-part skeleton of calcite and gorgonin (a fibrillar protein), potentially containing long-term records of valuable paleo-environmental information. For various reasons, both radiocarbon and U/Th dating of these corals is problematic over the last few centuries. This paper explores aspartic acid racemization dating of the gorgonin fraction in modern and fossil specimens collected from the NW Atlantic Ocean. Radiocarbon dating of the fossil specimen indicates a lifespan of 700 ± 100 years, the longest yet documented for any octocoral. Gorgonin amino acid compositions were identical in the fossil and modern specimens, indicating resistance to organic diagenesis. Similar to bone collagen, the fibrillar protein of gorgonin may impose conformational constraints on the racemization of Asp at low temperatures. The rate of racemization of aspartic acid ( d / l -Asp) was similar to previously published results from an 1800 year old anemone (Gerardia). The age equation was: age (years BP 2000 AD) = [( d / l − 0.020 (±.002))/.0011 (±.0001)]2 (r2 = 0.97, p d / l -Asp was marginally better than that for 14C dating over the most recent 50–200 years, although the dating error may be improved by inclusion of more samples over a broader time range. These results suggest that d / l -Asp dating may be useful in augmenting 14C dating in cases where 14C calibrations yield two or more intercept ages, or in screening samples for further 14C or U/Th dating.

The deep-sea gorgonian octocoral Primnoa resedaeformis is distributed throughout the Atlantic and Pacific Oceans at depths of 65-3200 m. It has a two-part skeleton of calcite and gorgonin. Towards the inside of the axial skeleton gorgonin and calcite are deposited in concentric growth rings, similar to tree rings. Colonies were collected from the Northeast Channel (northwest Atlantic Ocean, southwest of Nova Scotia, Canada) from depths of 250-475 m. Radiocarbon was measured in individual rings isolated from sections of each colony, after dissolution of calcite. Each {Delta}{sup 14}C measurement was paired with a ring age determined by three amateur ring counters. The precision of ring counts averaged better than {+-} 2 years. Accurate reconstruction of 20th century bomb-radiocarbon shows that (1) the growth rings are formed annually, (2) the gorgonin is derived from surface particulate organic matter (POM) and (3) useful environmental data are recorded in the organic endoskeletons of deep-sea octocorals. These results support the use of Primnoa resedaeformis as a long-term, high resolution monitor of surface ocean conditions, particularly in temperate and boreal environments where proxy data are lacking.

The gorgonians Plexaura spp. occur throughout the Florida Reef Tract, and lay down annual bands of a tough protein, gorgonin, in their skeletons. We analyzed stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) from individual annual bands in skeletons of Plexaura homomalla and P. flexuosa from 10 locations on the Florida Reef Tract, producing a proxy record of nutrient inputs dat- ing from the mid-1970s to 2002. Isotope data were compared with tissue from offshore Belize reefs, and inshore reefs from Xel-Ha, Mexico (Yucatan Peninsula), collected in 2003. Based on previous nutrient work, Florida sites were classified as 'clean' (relatively low nutrient levels) and 'dirty' (ele- vated nutrient levels). Samples from clean and dirty sites had significantly different nitrogen isotope values, which were correlated with the average total nitrogen of the seawater. Dirty sites (3.9 ± 0.2‰ δ 15 N) were isotopically enriched over clean sites (2.9 ± 0.1‰) throughout the entire duration of the study. Dirty sites had an enrichment of 0.8‰ δ 15 N throughout the study period, with the greatest increase in the 10 yr from 1993 to 2002 inclusive. Clean sites showed no significant change since 1974. Levels and trends in δ 13 C showed a similar picture of high and increasing eutrophication stress. Clean sites had lower δ 13 C values, consistent with predominantly autotrophic nutrition; dirty sites had higher values, suggesting a shift to a more heterotrophic mode of feeding. The methods outlined herein suggest that long-term records of organic pollution on coral reefs may be retrieved from gorgonian skeletons.

The deep-sea gorgonian coral Primnoa spp. live in the Atlantic and Pacific Oceans at depths of 65 to 3200 m. They have an arborescent growth form with a skeletal axis composed of annual rings made from calcite and gorgonin. Lifespans may exceed several hundreds of years. It has been suggested that isotope profiles from the gorgonin fraction of the skeleton could be used to reconstruct long-term, annual-scale variations in surface productivity. We tested assumptions about the trophic level, intra- and inter-colony isotopic reproducibility, and preservation of isotopic signa- tures in a suite of modern and fossil specimens. Measurements of gorgonin δ 15 N indicate that Prim- noa spp. feed mainly on zooplankton and/or sinking particulate organic matter (POMsink), and not on suspended POM (POMsusp) or dissolved organic carbon (DOC). Gorgonin δ 13 C and δ 15 N in specimens from NE Pacific shelf waters, NW Atlantic slope waters, the Sea of Japan, and a South Pacific (South- ern Ocean sector) seamount were strongly correlated with surface apparent oxygen utilization (AOU; the best available measure of surface productivity), demonstrating coupling between skeletal iso- topic ratios and biophysical processes in surface water. Time-series isotopic profiles from different sections along the same colony, and different colonies inhabiting the same area were identical for δ 13 C, while δ 15 N profiles were less reproducible. Similarity in C:N, δ 13 C and δ 15 N between modern and fossil specimens suggest that isotopic signatures are preserved over millennial timescales. These results support the use of Primnoa spp. as historical recorders of surface water processes such as bio- logical productivity and the isotopic composition of source nutrients.

From September to November 1997, most of Indonesia was covered by a dense blanket of haze, originating from fires on Sumatra and Borneo. Specimens of Porites lobata were collected from two locations, i.e. the Riau Archipelago, south of Singapore, where the haze was most dense, and the Karimunjawa Islands, north of Central Java, where the effects were less severe. All corals exhibited strong Kinetic Isotope Effects (KIE). On plots of δ 18 O vs. δ 13 C, shifts in coral metabolism associated with the haze event could be estimated from the distance individual values are positioned from the theoretical KIE line. Skeletons of corals affected by the haze showed decreased δ 13 C values, perhaps produced by a shift to a more heterotrophic mode of feeding. These results suggest that wildfires and major forest fire events on tropical coastlines may be recorded in nearby corals, as could temporal variation in frequency of major fires. Moreover, information on coral metabolism may be determined by examining shifts of coral skeletal values in C–O space.

Nature 538, 88–91 (2016); doi:10.1038/nature19797 In this Letter, Supplementary Figs 10 and 12 were not updated. The analyses, calculations, and text are correct. The Supplementary Information to this Corrigendum contains the corrected Supplementary Figs 10 and 12. In Supplementary Fig. 10a the blueline was about 30 Tg of CH4 per year too low, and represents results from a previous model version.

Summary The stable carbon (δ 13 C) and hydrogen (δ 2 H) isotopic composition of hydrocarbons has been used in petroleum exploration and development since the 1960s. With the development of tight oil and gas plays over the last decade, analysis of stable isotopes has re-emerged as a powerful interpretive and predictive technology. In particular, isotopic analysis of mud and production gases is useful in the analysis of hydrocarbon maturity, migration and reservoir compartmentalization. Here, we report on a publicly available Colorado Oil and Gas Conservation Commission (COGCC) database of discrete and co-mingled production gases from the Lower to Upper Cretaceous Muddy “J” Sand, Codell, Niobrara and Sussex formations of the Wattenberg Field of Colorado. Production gases are characterized as early-mature to post-mature, oil-associated gases, ranging from -55 to -40 ‰ in δ 13 C methane and 5-45 % C2+ concentration. Going down-section from the Sussex to J Sand formation, δ 13 C of methane, ethane and propane components all increase, reflecting increasing maturity with depth, and the presence of multiple, discrete source rock/petroleum systems. Using a previously established calibration developed for the Upper Cretaceous Uinta Basin in Utah, we convert δ 13 C ethane and propane values to a Vitrinite Reflectance Equivalent (VREiso) maturity scale. Mapped VREiso coincides with the regional thermal gradient. These results indicate that production gases from the Codell through Sussex have generated and accumulated in situ, with no apparent migration from distant source areas. Highest maturities, up to 1.5 VREiso, are centered over the geothermal hotspot, and indicate an area of gas–prone hydrocarbon production. Lower maturities (VREiso < 1.2) are located around the margins of the field, and indicate the liquids-rich “sweet-spot”. By contrast, some J Sand formation gases located along the Longmont wrench fault zone exhibit maturity values as high as 2.1 VREiso, implying vertical migration of these gases from a deeper source. These results highlight gas stable isotope signatures as a useful and reliable hydrocarbon phase prediction and migration analysis tool.

Production of oil and gas from shales and other unconventional hydrocarbon-bearing reservoirs has increased dramatically in recent years, largely as a result of technological developments in horizontal drilling and hydraulic fracturing. This increase has been accompanied by growing awareness of the environmental impacts of drilling and production activities, with a particular emphasis on groundwater quality. The presence of natural gas in groundwater is a health and explosion hazard. It also may indicate hydraulic connectivity with deeper hydrocarbon reservoirs, along which formation and drilling fluids that could migrate upward. Gas in groundwater occurs naturally through microbial activity and connection with microbial or thermogenic hydrocarbon accumulations. More rarely, hydrocarbon gases can migrate upward along the annuli of well bores and into shallow aquifers. The ability to distinguish between in situ and migrated natural gas underlies recent baseline water quality regulations in jurisdictions such as the state of Colorado. Stable isotope analysis forms a critical element of natural gas identification and fingerprinting protocols. This chapter reviews analytical and interpretive methods of compound-specific stable isotope analysis of natural gas.

Stable nitrogen isotope (delta(15)N) analysis has proven an effective "fingerprint" of sewage contamination in coral reef environments; however, short-term variability in nitrogen cycling and isotopic fractionation may obscure long-term trends. Here, we examine delta(15)N signatures in the organic endoskeletons of long-lived (20-40 years) gorgonian corals. Specimens were collected from relatively pristine reefs off Green Turtle Cay, Bahamas, and from reefs off southeast Florida heavily impacted by multiple sources of anthropogenic nitrogen. The delta(15)N of the most recently grown skeleton (branch tips) ranged from +2 to +3 per thousand at Green Turtle Cay, and +4.5 to +10 per thousand off Florida. These values closely match the delta(15)N of macroalgae collected from the same locations, indicating that gorgonian corals are isotopically similar to primary producers, and therefore suitable for assessing sources of dissolved inorganic nitrogen. Differences in the delta(15)N between younger and older skeleton indicated an overall decline of -0.34 +/- 0.06 per thousand (1 s.e) over the last 20 - 40 years at Green Turtle Cay, reflecting a possible increase in nitrogen fixation and/or atmospheric deposition of anthropogenic nitrogen. Off southeast Florida, there was an overall increase in delta(15)N over the same time period, reflecting increasing wastewater discharges from the rapidly growing population. These results highlight the usefulness of delta(15)N recorded in gorgonians and other long-lived organisms in assessing spatiotemporal patterns of nitrogen sources to coastal marine environments.

While coral reefs decline, scientists argue, and effective strategies to manage land-based pollution lag behind the extent of the problem. There is need for objective, cost-effective, assessment methods. The measurement of stable nitrogen isotope ratios, delta(15)N, in tissues of reef organisms shows promise as an indicator of sewage stress. The choice of target organism will depend upon study purpose, availability, and other considerations such as conservation. Algae are usually plentiful and have been shown faithfully to track sewage input. The organic matrix of bivalve shells can provide time series spanning, perhaps, decades. Gorgonians have been shown to track sewage, and can provide records potentially centuries-long. In areas where baseline data are lacking, which is almost everywhere, delta(15)N in gorgonians can provide information on status and trends. In coral tissue, delta(15)N combined with insoluble residue determination can provide information on both sewage and sediment stress in areas lacking baseline data. In the developed world, delta(15)N provides objective assessment in a field complicated by conflicting opinions. Sample handling and processing are simple and analysis costs are low. This is a method deserving widespread application.

Publisher Summary This chapter focuses on the deep-sea corals. Deep-Sea corals provide a widespread archive of oceanographic processes, with annual- to decadal-resolution records spanning several centuries in length. They are ideally suited to the study of rapid changes, such as that which characterized the Pleistocene–Holocene transition. Deep-sea scleractinians exhibit a wide variety of growth forms, from simple, solitary cup corals to complex, branching colonies. The corallite is lengthened by periodic uplift of the polyp and formation of a new dissepiment. The wall of the corallite is called the theca. A series of thin vertical sheets, the septa, radiate from the theca into the center of the corallite. The key utility of corals in paleoceanography is that long records of ambient conditions are locked into the growing skeleton in successive layers. When dealing with tropical reef scleractinians, massive colonies are selected for climate reconstructions because of the simplicity of their growth banding. Deep-sea scleractinians have more complicated cup and branching morphologies, making it difficult to visualize and sample the growth layers. The chapter discusses the growth and sclerochronology in deep-sea scleractinians, growth and sclerochronology in horny corals, sources of carbon to deep-sea corals, and stable isotopic disequilibria in deep-sea corals.

Bioerosion by Cliona delitrix and Cliona lampa was assessed at 43 sites along the Florida Reef Tract, USA, in the summer of 2001. Sponge abundances were estimated using rapid visual assessment. Tissue samples of sponges were taken for analysis of d 15 N. Comparison samples were taken from Belize. Annual trends in sponge abundance were estimated from archived videos covering the period from 1996 to 2001. Sites with the greatest boring sponge size and cover were in the Backcountry and Lower Keys, where total nitrogen, ammonium, and d 15 N levels were highest. The sites with the largest relative increase of C. delitrix and C. lampa over the 5 year period were in the Upper Keys, where the greatest relative decline in stony coral cover has occurred. Florida sponge d 15 N values were 5.2(±0.1)%, suggesting the influence of human waste; in comparison, offshore Belize samples were 2.1(±0.1)%. These results suggest sewage contamination of the Florida Reef Tract, shifting the carbonate balance from construction to destruction. � 2005 Elsevier Ltd. All rights reserved.

It has been suggested that the deep-sea gorgonian coral Primnoa resedaeformis may be an important paleoceanographic archive. Seventeen colonies collected from the upper slope of the NW Atlantic margin (229 – 447 m) were analyzed to see if skeletal Mg/Ca is related to temperature. Analyses were focused on the calcite cortex region of skeletal sections to avoid interference from organic Mg in the horny layers found closer to the center of sections. Comparison of bulk skeletal Mg/Ca with hydrographic temperature yielded the relationship Mg/Ca (mmol/mol)=5 (+/− 1.4) T (°C)+64 (+/− 10). This relationship was used to calibrate profiles of Mg/Ca measured across the annual rings of one large, well-dated colony, over the period 1950–2002. Mg/Ca profiles were broadly consistent among three sections spaced 10 cm apart along the main trunk of the colony. These profiles were in general agreement with the local instrumental record of temperature at 375 – 450 m. Some discrepancies between the coral and instrumental records of temperature may be a result of chronological error, poor sampling density, or additional factors influencing Mg partitioning in the coral. Overall, these preliminary results support the hypothesis that temperature drives Mg/Ca in the skeletal calcite of this species. It appears that environmentally meaningful records from Primnoa resedaeformis will be found at decadal scales or longer.

Two samples of the calcitic deep-sea coral Primnoa resedaeformis have been analysed for Mg/Ca ratios by micro-beam methods ( laser-ablation ICP-MS and electron microprobe). Continuous profiles of Mg/Ca have been studied with the aim of establishing the reproducibility of the variations in different parts of the coral, and therefore the potential use of Mg/Ca as a paleoceanographic or paleoclimatic tracer.