Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are Stable

Department of Geosciences and the Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA.
Science (Impact Factor: 33.61). 02/2006; 311(5762):838-40. DOI: 10.1126/science.1121235
Source: PubMed


One explanation for the abrupt increases in atmospheric CH4, that occurred repeatedly during the last glacial cycle involves clathrate destabalization events. Because marine clathrates have a distinct deuterium/hydrogen (D/H) isotope ratio, any such destabilization event should cause the D/H ratio of atmospheric CH4 (deltaD(CH4)) to increase. Analyses of air trapped in the ice from the second Greenland ice sheet project show stable and/or decreasing deltaD(CH4) values during the end of the Younger and Older Dryas periods and one stadial period, suggesting that marine clathrates were stable during these abrupt warming episodes. Elevated glacial deltaD(CH4) values may be the result of a lower ratio of net to gross wetland CH4 emissions and an increase in petroleum-based emissions.

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Available from: Todd Anthony Sowers, Oct 07, 2015
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    • "The validity of this " clathrate gun hypothesis " is contentious. Methane emissions from wetlands may exceed those from gas hydrates hosted in marine sediments, as suggested by isotopic analysis of methane within ice core records (Sowers, 2006). Here we consider whether our two study locations provide evidence that landslides may have helped to drive climate change through methane emissions. "
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    ABSTRACT: Previous studies propose that submarine landslides and turbidity currents may become more likely due to future rapid global warming. Determining whether global warming increases likelihood assists in assessment of landslide-triggered tsunami hazards and risk to seafloor structures. Other studies propose that landslides helped to trigger past rapid climate change due to sudden release of gas hydrates. Two deep-water turbidite records show prolonged hiatuses in turbidity current activity during the Initial Eocene Thermal Maximum (IETM) at ∼55 Ma. The IETM represents a possible proxy for future anthropogenically-induced climate change. It is likely that our records mainly represent large and fast moving disintegrative submarine landslides. Statistical analysis of long term (>2.3 Myr) records shows that turbidity current frequency significantly decreased after the IETM. Our results indicate that rapid climate change does not necessarily cause increased turbidity current activity, and do not provide evidence for landslides as a primary trigger for the IETM.
    Earth and Planetary Science Letters 06/2015; 420. DOI:10.1016/j.epsl.2015.03.022 · 4.73 Impact Factor
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    • "Ferretti et al., 2005; Fischer et al., 2008; Sowers, 2010; Sapart et al., 2012; Möller et al., 2013) provide further insight into processes and sources controlling the global methane cycle. For instance, knowledge of the temporal evolution of the hydrogen isotopic composition of methane (δD(CH 4 ) or δ 2 H(CH 4 )) over the termination of the last ice age (14 000– 18 000 years before present) (Sowers, 2006) as well as during rapid warming events between 32 000–42 000 years before present (Bock et al., 2010b) made it possible to reject the " clathrate gun hypothesis " proposed by Kennett et al. (2003) as the trigger for the steep atmospheric methane increases. However, we are still far from a complete picture of the biogeochemistry of methane in the past. "
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    ABSTRACT: Firn and polar ice cores offer the only direct palaeoatmospheric archive. Analyses of past greenhouse gas concentrations and their isotopic compositions in air bubbles in the ice can help to constrain changes in global biogeochemical cycles in the past. For the analysis of the hydrogen isotopic composition of methane (δD(CH4) or δ2H(CH4)) 0.5 to 1.5 kg of ice was hitherto used. Here we present a method to improve precision and reduce the sample amount for δD(CH4) measurements in (ice core) air. Pre-concentrated methane is focused in front of a high temperature oven (pre-pyrolysis trapping), and molecular hydrogen formed by pyrolysis is trapped afterwards (post-pyrolysis trapping), both on a carbon-PLOT capillary at −196°C. Argon, oxygen, nitrogen, carbon monoxide, unpyrolysed methane and krypton are trapped together with H2 and must be separated using a second short, cooled chromatographic column to ensure accurate results. Pre-and post-pyrolysis trapping largely removes the isotopic fractionation induced during chromatographic separation and results in a narrow peak in the mass spectrometer. Air standards can be measured with a precision better than 1‰. For polar ice samples from glacial periods, we estimate a precision of 2.3‰ for 350 g of ice (or roughly 30 mL – at standard temperature and pressure (STP) – of air) with 350 ppb of methane. This corresponds to recent tropospheric air samples (about 1900 ppb CH4) of about 6 mL (STP) or about 500 pmol of pure CH4.
    Atmospheric Measurement Techniques 07/2014; 7(7). DOI:10.5194/amt-7-1999-2014 · 2.93 Impact Factor
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    • "Insights into the source of atmospheric methane from deuterium isotopes within ice cores provide the most unambiguous evidence against landslides (or indeed gas hydrates in marine sediments more generally) being a cause of climate change (Sowers, 2006). "
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    ABSTRACT: Submarine landslides on open continental slopes can be prodigious in scale. They are an important process for global sediment fluxes, and can generate very damaging tsunamis. Submarine landslides are far harder to monitor directly than terrestrial landslides, and much greater uncertainty surrounds their preconditioning factors and triggers. Submarine slope failure often occurs on remarkably low (< 2°) gradients that are almost always stable on land, indicating that particularly high excess pore pressures must be involved. Earthquakes trigger some large submarine landslides, but not all major earthquakes cause widespread slope failure. The headwalls of many large submarine landslides appear to be located in water depths that are too deep for triggering by gas hydrate dissociation. The available evidence indicates that landslide occurrence is either weakly (or not) linked to changes in sea level or atmospheric methane abundance, or the available dates for open continental slope landslides are too imprecise to tell. Similarly, available evidence does not strongly support a view that landslides play an important role in methane emissions that cause climatic change. However, the largest and best-dated open continental slope landslide (the Storegga Slide) coincides with a major cooling event 8,200 years ago. This association suggests that caution may be needed when stating that there is no link between large open slope landslides and climate change.
    Oceanography (Washington D.C.) 06/2014; 27(2):32-45. DOI:10.5670/oceanog.2014.38 · 2.94 Impact Factor
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