65 research outputs found
Chemical and Isotopic Characterization of Size-Fractionated Organic Matter From Cryoturbated Tundra Soils, Northern Alaska
Recent studies indicate a second layer of organic matter often accumulates in the lower active layer and upper permafrost in arctic tundra soils as a result of cryoturbation. In this study, cryoturbated organic matter was characterized using a combination of physical size fractionation and modern analytical techniques for elemental composition (C and N), stable isotopes (delta(13) C and delta(15)N), radiocarbon content (Delta(14)C), and molecular fingerprinting (pyrolysis-gas chromatography/mass spectrometry, Py-GC/MS). The results indicated that cryoturbated organic matter could be highly bioavailable. Soil organic matter (SOM) associated with fine sand particles was considered to be the organic carbon pool most sensitive to the changing climate. More organic matter is stabilized on clay minerals in arctic tundra soils compared to those in temperate and tropical soils. The bioavailable soluble organic matter extracted from cryoturbated soil was found to have significant long-term effects on carbon cycling. The similar molecular composition between cryoturbated and surface soil organic matter suggests that the vegetation cover has not significantly changed since the early Holocene. Furthermore, the SOM quality in moist acidic tundra was found to be higher than that of wet nonacidic tundra. With thawing permafrost and a deepening of the active layer, cryotrubated organic matter could reenter the biogeochemical cycles in the Arctic, resulting in a positive feedback to climate change
Dissolved methane in the Beaufort Sea and the Arctic Ocean, 1992-2009; sources and atmospheric flux
Methane concentration and isotopic composition was measured in ice-covered and ice-free waters of the Arctic Ocean during 11 surveys spanning the years of 1992â1995 and 2009. During ice-free periods, methane flux from the Beaufort shelf varies from 0.14 mg CH4 mâ2 dâ1 to 0.43 mg CH4 mâ2 dâ1. Maximum fluxes from localized areas of high methane concentration are up to 1.52 mg CH4 mâ2 dâ1. Seasonal buildup of methane under ice can produce short-term fluxes of methane from the Beaufort shelf that varies from 0.28 mg CH4 mâ2 dâ1 to 1.01 mg CH4 mâ2 dâ1. Scaled-up estimates of minimum methane flux from the Beaufort Sea and pan-Arctic shelf for both ice-free and ice-covered periods range from 0.02 Tg CH4 yrâ1 and 0.30 Tg CH4 yrâ1, respectively to maximum fluxes of 0.18 Tg CH4 yrâ1 and 2.2 Tg CH4 yrâ1, respectively. A methane flux of 0.36 Tg CH4 yrâ1 from the deep Arctic Ocean was estimated using data from 1993 to 1994. The flux can be as much as 2.35 Tg CH4 yrâ1 estimated from maximum methane concentrations and wind speeds of 12 m/s, representing only 0.42% of the annual atmospheric methane budget of ⌠560 Tg CH4 yrâ1. There were no significant changes in methane fluxes during the time period of this study. Microbial methane sources predominate with minor influxes from thermogenic methane offshore Prudhoe Bay and the Mackenzie River delta and may include methane from gas hydrate. Methane oxidation is locally important on the shelf and is a methane sink in the deep Arctic Ocean
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Geochemical evidence for gas hydrate in sediment near the Chile Triple Juction
Coring at ODP Sites 859, 860, and 861 near the Chile Triple Junction failed to recover anticipated gas hydrate that was inferred
to be present from two lines of geophysical evidence: pre-cruise observation of a weak to strong bottom simulating reflector (BSR)
marking the predicted base of the gas-hydrate stability zone, and post-cruise interpretation of the velocity and resistivity logs at
Site 859 that suggests the presence of gas hydrate. In contrast to other gas-hydrate occurrences observed during previous DSDP
and ODP drilling, this Chile Margin sediment is very low in contents of total organic carbon (TOC < 0.5%) and residual methane
(Câ), that are inconsistent with an in-situ source of methane for gas-hydrate formation. However, methane/ethane (Câ/Câ) ratios
(>200) and ÎŽÂčÂł C, values (<-60â°) show little evidence that the methane came from deeper thermogenic sources. Pore-fluid
freshening does, however, suggest that gas hydrate is present, disseminated thinly and heterogeneously throughout the stability
zone, and occupies less than 25% of the available pore space. The environment of the gas hydrate in sediment near the Chile Triple
Junction has unique characteristics relative to known gas-hydrate occurrences elsewhere
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Bacterial dominance in subseafloor sediments characterized by methane hydrates
The degradation of organic carbon in subseafloor sediments on continental margins contributes to the largest reservoir of methane on Earth. Sediments in the Andaman Sea are composed of ~ 1% marine-derived organic carbon and biogenic methane is present. Our objective was to determine microbial abundance and diversity in sediments that transition the gas hydrate occurrence zone (GHOZ) in the Andaman Sea. Microscopic cell enumeration revealed that most sediment layers harbored relatively low microbial abundance (10Âłâ10â” cells cmâ»Âł). Archaea were never detected despite the use of both DNA- and lipid-based methods. Statistical analysis of terminal restriction fragment length polymorphisms revealed distinct microbial communities from above, within, and below the GHOZ, and GHOZ samples were correlated with a decrease in organic carbon. Primer-tagged pyrosequences of bacterial 16S rRNA genes showed that members of the phylum Firmicutes are predominant in all zones. Compared with other seafloor settings that contain biogenic methane, this deep subseafloor habitat has a unique microbial community and the low cell abundance detected can help to refine global subseafloor microbial abundance.Keywords: Andaman Sea, molecular sequence data, geologic sediments/chemistry/microbiology, pyrosequencing, lipidsKeywords: Andaman Sea, molecular sequence data, geologic sediments/chemistry/microbiology, pyrosequencing, lipid
Macondo-1 well oil-derived polycyclic aromatic hydrocarbons
Mesozooplankton (>200 mm) collected in August and September of 2010 from the northern Gulf of Mexico show evidence of exposure to polycyclic aromatic hydrocarbons (PAHs). Multivariate statistical analysis revealed that distributions of PAHs extracted from mesozooplankton were related to the oil released from the ruptured British Petroleum Macondo-1 (M-1) well associated with the R/V Deepwater Horizon blowout. Mesozooplankton contained 0.03ĂąâŹâ97.9 ng g 1 of total PAHs and ratios of fluoranthene to fluoranthene + pyrene less than 0.44, indicating a liquid fossil fuel source. The distribution of PAHs isolated from mesozooplankton extracted in this study shows that the 2010 Deepwater Horizon spill may have contributed to contamination in the northern Gulf of Mexico ecosystem
Methane-derived authigenic carbonates from the northern Gulf of Mexico and their relation to gas hydrates
Author Posting. © The Author(s), 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Journal of Geochemical Exploration 95 (2007): 1-15, doi:10.1016/j.gexplo.2007.05.011.Authigenic carbonates were sampled in piston cores collected from both the Tunica Mound and the Mississippi Canyon area on the continental slope of the northern Gulf of Mexico during a Marion Dufresne cruise in July 2002. The carbonates are present as hardgrounds, porous crusts, concretions or nodules and shell fragments with or without carbonate cements. Carbonates occurred at gas venting sites which are likely to overlie gas hydrates bearing sediments. Electron microprobe, X-ray diffraction (XRD) and thinsection investigations show that these carbonates are high-Mg calcite (6 - 21 mol % MgCO3), with significant presence of framboidal pyrite. All carbonates are depleted in 13C (Ύ13C = -61.9 to -31.5 ⰠPDB) indicating that the carbon is derived mainly from anaerobic methane oxidation (AMO). Age estimates based on 14C dating of shell fragments and on regional sedimentation rates indicate that these authigenic carbonates formed within the last 1,000 yr in the Mississippi Canyon and within 5,500 yr at the Tunica Mound. The oxygen isotopic composition of carbonates ranges from +3.4 to +5.9 ⰠPDB. Oxygen isotopic compositions and Mg2+ contents of carbonates, and present in-situ temperatures of bottom seawater/sediments, show that some of these carbonates, especially from a core associated with underlying massive gas hydrates precipitated in or near equilibrium with bottom-water. On the other hand, those carbonates more enriched in 18O are interpreted to have precipitated from 18O-rich fluids which are thought to have been derived from the dissociation of gas hydrates. The dissociation of gas hydrates in the northern Gulf of Mexico within the last 5,500 yr may be caused by nearby salt movement and related brines.Financial support for this work was provided by the Grant-in-Aid from the Ministry of Education and Science and the Research Grant from JAPEX
Sensitivity of the carbon cycle in the Arctic to climate change
The recent warming in the Arctic is affecting a broad spectrum of physical, ecological, and human/cultural systems that may be irreversible on century time scales and have the potential to cause rapid changes in the earth system. The response of the carbon cycle of the Arctic to changes in climate is a major issue of global concern, yet there has not been a comprehensive review of the status of the contemporary carbon cycle of the Arctic and its response to climate change. This review is designed to clarify key uncertainties an vulnerabilities in the response of the carbon cycle of the Arctic to ongoing climatic change. While it is clear that there are substantial stocks of carbon in the Arctic, there are also significant uncertainties associated with the magnitude of organic matter stocks contained in permafrost and the storage of methane hydrates beneath both subterranean and submerged permafrost of the Arctic. In the context of the global carbon cycle, this review demonstrates that the Arctic plays an important role in the global dynamics of both CO2 and CH4. Studies suggest that the Arctic has been a sink for atmospheric CO2 of between 0 and 0.8 Pg C/yr in recent decades, which is between 0% and 25% of the global net land/ocean flux during the 1990s. The Arctic is a substantial source of CH4 to the atmosphere (between 32 and 112 Tg CH4/yr), primarily because of the large area of wetlands throughout the region. Analyses to date indicate that the sensitivity of the carbon cycle of the Arctic during the remainder of the 21st century is highly uncertain. To improve the capability to assess the sensitivity of the carbon cycle of the Arctic to projected climate change, we recommend that (1) integrated regional
studies be conducted to link observations of carbon dynamics to the processes that are likely to influence those dynamics, and (2) the understanding gained from these integrated studies be incorporated into both uncoupled and fully coupled carbonâclimate modeling efforts
Macondo-1 well oil-derived polycyclic aromatic hydrocarbons in mesozooplankton from the northern Gulf of Mexico
Copyright 2012 by the American Geophysical UnionMesozooplankton (>200 ĂÂŒm) collected in August and September of 2010 from the northern Gulf of Mexico show evidence of exposure to polycyclic aromatic hydrocarbons (PAHs). Multivariate statistical analysis revealed that distributions of PAHs extracted from mesozooplankton were related to the oil released from the ruptured British Petroleum Macondo-1 (M-1) well associated with the R/VDeepwater Horizon blowout. Mesozooplankton contained 0.03ĂąâŹâ97.9 ng gĂąËâ1 of total PAHs and ratios of fluoranthene to fluoranthene + pyrene less than 0.44, indicating a liquid fossil fuel source. The distribution of PAHs isolated from mesozooplankton extracted in this study shows that the 2010 Deepwater Horizon spill may have contributed to contamination in the northern Gulf of Mexico ecosystem
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Macondo-1 well oil-derived polycyclic aromatic hydrocarbons in mesozooplankton from the northern Gulf of Mexico
Mesozooplankton (>200 ÎŒm) collected in August and September of 2010 from the northern Gulf of Mexico show evidence of exposure to polycyclic aromatic hydrocarbons (PAHs). Multivariate statistical analysis revealed that distributions of PAHs extracted from mesozooplankton were related to the oil released from the ruptured British Petroleum Macondo-1 (M-1) well associated with the R/V Deepwater Horizon blowout. Mesozooplankton contained 0.03â97.9 ng gâ»Âč of total PAHs and ratios of fluoranthene to fluoranthene + pyrene less than 0.44, indicating a liquid fossil fuel source. The distribution of PAHs isolated from mesozooplankton extracted in this study shows that the 2010 Deepwater Horizon spill may have contributed to contamination in the northern Gulf of Mexico ecosystem
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