G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Forum Volume 2 January 9, 2001 Paper number 2000GC ISSN: On the fate of past gas: What happens to methane released from a bacterially mediated gas hydrate capacitor? Gerald Dickens Department of Geology and, Rice University, Houston, Texas Also at School of Earth Sciences, James Cook University, Townsville, QLD 4811, Australia (jerry.dickens@jcu.edu.au) [1] Abstract: Gas hydrates and associated free gas in marine sediment may constitute a large capacitor of CH 4 in the global carbon cycle with outputs controlled by water temperature at intermediate depths of the ocean. The best support for this concept previously has come from stable isotope records of benthic foraminifera that show pronounced negative d 13 C excursions during certain brief intervals of deep to intermediate water warming. New work by Hinrichs [2001] demonstrates that such isotope excursions in the Santa Barbara Basin coincide with high accumulations of diplopterol, a biomarker for aerobic CH 4 oxidation by bacteria. This is the first direct evidence for enhanced CH 4 concentrations in the deep ocean during or immediately after bottom water warming and injection of 12 C-rich carbon. However, the formation of biomarkers indicates that a fraction of CH 4 released during warming is oxidized to biomass and CO 2 in the water column. In contrast to most literature, the primary consequences of CH 4 release from dissociated gas hydrate may be dissolved O 2 depletion and carbonate dissolution rather than atmospheric warming. Keywords: Diplopterol; methane hydrate; carbon isotopes; biomarker; deep biosphere; Paleocene. Index terms: Organic geochemistry; biogeochemical processes; paleoceanography; carbon cycling. Received December 6, 2000; Accepted December 7, 2000; Published January 9, Dickens, G., On the fate of past gas: What happens to methane released from a bacterially mediated gas hydrate capacitor?, Geochem. Geophys. Geosyst., vol. 2, Paper number 2000GC [ 1802 words, 1 figure]. Published January 9, Gas Hydrate Capacitor [2] One of the most captivating of Earth science notions is that our oceans are floored with an immense quantity of solid-phase CH 4 that is sensitive to small perturbations in temperature. Gas hydrates are crystalline substances composed of low molecular weight gas and water that are stable under certain pressures, temperatures, gas concentrations, and water activity. Marine sediments can host gas hydrates under appropriate conditions of high pressure, low temperature, and abundant gas, criteria that are met where high gas fluxes enter shallow sediment in relatively deep water. Substantial amounts of CH 4 are pro- Copyright 2001 by the American Geophysical Union

2 Volcanoes Organic Oxidation Atmosphere Biomass Cold S.W. Warm Surface Water Thermocline Carbonate & Silicate Weathering Organic Carbon Burial Carbonate Accumulation Anaerobic CH4 Oxidation Aerobic CH4 Oxidation T Deep Ocean Methane Hydrate Free CH4 Gas Diss. CH4 Methanogenesis Figure 1. The global exogenic carbon cycle with a bacterially mediated gas hydrate capacitor. Carbon enters the capacitor as a fraction of sedimentary organic matter is converted to CH 4 through bacterial methanogenesis. This CH 4 saturates pore waters to form CH 4 hydrates, which are buried past gas hydrate stability conditions to form free CH 4 gas. Although much of the free gas is returned to gas hydrates, some CH 4 leaves the system through anaerobic CH 4 oxidation by bacteria in shallow sediment. The capacitor concept arises when a significant fraction of gas hydrates are converted to free CH 4 gas during an increase in temperature (T). This CH 4 is added directly into deep water where it is oxidized by aerobic methanotrophs. Gray arrows represent external fluxes of carbon common to most conventional models of the global carbon cycle. duced in deep-sea sediment when organic 2 matter escapes oxic respiration and SO 4 reduction. Consequently, the upper few hundred meters of sediment along numerous continental margins contain laterally extensive zones of 12 C-rich CH 4 hydrate, which are often underlain by free CH 4 gas bubbles [Kvenvolden, 1993; Dickens et al., 1997]. The total mass of carbon in present-day marine gas hydrate deposits including associated free and dissolved gas probably lies between 7500 and 15,000 Gt (Gt = g) [Kvenvolden, 1993; Gornitz and Fung, 1994], or 4 to 8 times more carbon than the entire terrestrial biomass including soil and humus. [3] There have been a growing number of suggestions that the vast submarine gas hydrate reservoir serves as a bacterially mediated capacitor in the global carbon cycle (Figure 1). Carbon slowly enters gas hydrates when sedimentary organic matter is converted

3 to CH 4 via bacterial methanogenesis and subsequently concentrated [Paull et al., 1994; Buffett and Zatsepina, 2000]. At steady state conditions, carbon also slowly leaves gas hydrates when CH 4 is advected to overlying SO 2 4 -rich pore waters and converted to HCO 3 via bacterial anaerobic CH 4 oxidation [Borowski et al., 1996]. The capacitor concept arises because carbon might also rapidly escape gas hydrates with external forcing, specifically a rise in temperature [Dickens et al., 1995; Hesselbo et al., 2000; Kennett et al., 2000]. In theory, bottom water warming should increase temperatures below the seafloor, dissociating a portion of gas hydrate to free CH 4 gas. This gas would then increase pore pressures at depth, perhaps resulting in a substantial release of CH 4 to the ocean or atmosphere through slumping or faulting [Kayen and Lee, 1991; Dillon et al., 1998; Katz et al., 1999]. [4] Until this year, the strongest geochemical support for a gas hydrate capacitor came from detailed stable isotope records of deep-sea carbonate across an unusual time interval circa 55 million years ago (Ma) coined the Late Paleocene thermal maximum (LPTM). Benthic foraminiferal isotope records across the LPTM at many locations display rapid, pronounced, and near-coincident 2 to 3% excursions in d 13 C and d 18 O, suggesting that bottom waters suddenly became enriched in 12 C when they were warmed [Kennett and Stott, 1991; Bralower et al., 1995; Katz et al., 1999]. The only satisfactory explanation for these isotope anomalies appears to be thermal release of 12 C-rich CH 4 to the ocean or atmosphere followed by oxidation [Dickens et al., 1995]. Isotopic signatures that are consistent with thermal dissociation of gas hydrate and release of CH 4 have now been documented across other brief intervals of the geological record, notably events in the Aptian and Toarcian circa 120 and 183 Ma [Hesselbo et al., 2000]. Nonetheless, there has been no geochemical evidence to directly link the deep-sea or other (e.g., terrestrial) carbon isotope anomalies to seafloor CH 4 release, and the capacitor concept has remained highly speculative. 2. Support and Constraints From Bacterial Biomarkers [5] In a current brief, Hinrichs [2001] presents a simple data set that at once escalates and focuses ideas on the deep marine CH 4 cycle. Ocean Drilling Program (ODP) Site 893 recovered a greatly expanded late Pleistocene to Holocene sediment sequence from the central Santa Barbara Basin, a gas-rich graben off the southern California coast. At 580-m water depth the location sits at pressure and temperature conditions where CH 4 hydrate is stable but highly sensitive to fluctuations in bottom water temperature [Kennett et al., 2000]. A recent and fascinating find has been that benthic and some planktonic foraminifera from Site 893 display pronounced negative d 13 C excursions during brief intervals of bottom water warming [Kennett et al., 2000]. As for isotope records across the LPTM, Kennett and colleagues have attributed the d 13 C excursions to thermal dissociation of gas hydrates and release and oxidation of CH 4. What Hinrichs has done is to test this hypothesis by examining the organic geochemistry of three sediment samples from Site 893 where only two samples were deposited during supposed CH 4 release events. He then shows that these two samples contain significant amounts of diplopterol, a hopanoid specifically linked to aerobic methanotrophic bacteria [Summons et al., 1994]. The conclusion is straightforward: CH 4 was present in the water column during or immediately after bottom waters were warmed in the Santa Barbara Basin. [6] The data set by Hinrichs begs two immediate questions before its significance can be

4 fully assessed. Do all negative d 13 C excursions in foraminifera from the Santa Barbara Basin relate to high quantities of diplopterol in sediment and vice versa? Can diplopterol or other organic biomarkers for aerobic CH 4 oxidation be used to investigate more ancient times of inferred CH 4 release such as the LPTM? Nonetheless, the discovery of diplopterol in sediment deposited during an interval of bottom water warming and benthic foraminiferal 12 C enrichment is very exciting. [7] There remains, however, a fundamental issue regarding the role of gas hydrates in the global carbon cycle. Many authors [e.g., Kvenvolden, 1993; Kennett et al., 2000] have discussed the idea of CH 4 release from a gas hydrate capacitor in terms of direct carbon transfer between marine sediments and the atmosphere. Accordingly, bottom water warming leads to an increase in atmospheric CH 4, perhaps leading to a significant rise in Earth surface temperatures and a positive feedback for additional CH 4 release [e.g., Kvenvolden, 1993]. Recent studies of sediment records across the LPTM and earlier time intervals support an alternative process (Figure 1) whereby most of the CH 4 reacts with dissolved O 2 in the water column to produce CO 2 [Katz et al., 1999; Dickens, 2000; Hesselbo et al., 2000]. In this case, the primary consequences of CH 4 release are decreased dissolved O 2 concentrations and deep-sea carbonate dissolution rather than atmospheric warming [Dickens, 2000]. Diplopterol and a series of other compounds are formed when methanotrophs use CH 4 and O 2 to form biomass and CO 2 in the water column [Summons et al., 1994]. Although organic biomarkers for aerobic CH 4 oxidation may provide a fantastic new means to assess past CH 4 release from the gas hydrate capacitor, their application will force us to consider a fate for CH 4 that is much different than presented in almost all previous literature. References Bralower, T. J., J. C. Zachos, E. Thomas, M. Parrow, C. K. Paull, D. C. Kelly, I. Premoli Silva, W. V. Sliter, and K. C. Lohmann, Late Paleocene to Eocene paleoceanography of the equatorial Pacific Ocean: Stable isotopes recorded at Ocean Drilling Program Site 865, Allison Guyot, Paleoceanography, 10, 841 ±865, Borowski, W. S., C. K. Paull, and W. Ussler III, Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate, Geology, 24, 655±658, Buffett, B. A., and O. Y. Zatsepina, Formation of gas hydrate from dissolved gas in natural porous media, Mar. Geol., 69± 77, Dickens, G. R., Methane oxidation during the Late Palaeocene Thermal Maximum, Bull. Soc. Geol. France, 171, 37±49, Dickens, G. R., J. R. O'Neil, D. K. Rea, and R. M. Owen, Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene, Paleoceanography, 10, 965± 971, Dickens, G. R., C. K. Paull, and P. Wallace, and ODP Leg 164 Scientific Party, Direct measurement of in situ gas volumes in a large gas hydrate reservoir, Nature, 385, 426±428, Dillon, W. P., W. W. Danforth, D. R. Hutchinson, R. M. Drury, M. H. Taylor, and J. S. Booth, Evidence for faulting related to dissociation of gas hydrate and release of methane off the southeastern United States, in Gas Hydrates: Relevance to World Margin Stability and Climatic Change, edited by J.-P. Henriet and J. Mienert, Geol. Soc. London Spec. Publ., 137, 293± 302, Gornitz, V., and I. Fung, Potential distribution of methane hydrates in the world's oceans, Global Biogeochem. Cycles, 8, 335± 347, Hesselbo, S. P., D. R. Grocke, H. C. Jenkyns, C. J. Bjerrum, P. Farrimond, H. S. M. Bell, and O. R. Green, Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event, Nature, 406, 392± 395, Hinrichs, K.-U., A molecular recorder of methane hydrate destabilization,geochem. Geophys. Geosyst., vol. 2 (Research Letter), 2000GC [1717 words], (Available at Katz, M. E., D. K. Pak, G. R. Dickens, and K. G. Miller, The source and fate of massive carbon input during the latest Paleocene thermal maximum, Science, 286, 1531±1533, Kayen, R. E., and H. Lee, Pleistocene slope instability of gas hydrate-laden sediment on the Beaufort Sea Margin, Mar. Geotechnol., 10, 125 ± 141, Kennett, J. P., and L. D. Stott, Abrupt deep sea warming,

5 paleoceanographic changes and benthic extinctions at the end of the Palaeocene, Nature, 353, 319 ±322, Kennett, J. P., K. G. Cannariato, I. L. Hendy, and R. J. Behl, Carbon isotopic evidence for methane hydrate instability during Quaternary interstadials, Science, 288, 128± 133, Kvenvolden, K. A., Gas hydrates: Geological perspective and global change, Rev. Geophys., 31, 173±187, Paull, C. K., W. Ussler III, and W. S. Borowski, Sources of biogenic methane to form marine gas hydrates, Ann. N.Y. Acad. Sci., 715, 392 ±409, Summons, R. E., L. L. Jahnke, and Z. Roksandic, Carbon isotopic fractionation in lipids from methanotrophic bacteria: Relevance for interpretation of the geochemical record of biomarkers, Geochim. Cosmochim. Acta, 58, 2853±2863, 1994.