The authors address additional comments on their hypothesis for the origin of Neoproterozoic postglacial cap carbonates and their isotopic excursions

Christie-Blick, Nicholas

Kennedy, Martin J.

Sohl, Linda E.

English

Columbia University Academic Commons

GEOLOGY, August 2002 763REPLYMartin J. Kennedy*Department of Earth Science, University of California, Riverside,California 92521, USANicholas Christie-BlickLinda E. SohlLamont-Doherty Earth Observatory of Columbia University,Palisades, New York 10964, USAWe welcome the opportunity to address additional comments onour hypothesis for the origin of Neoproterozoic postglacial cap car-bonates and their isotopic excursions.Response to R.S. Shapiro. Shapiro (2002) summarizes field, pet-rographic, and geochemical criteria for the recognition of ancient hy-drocarbon seep deposits, and suggests that we have insufficient evi-dence to link the deposition of cap carbonates with the destabilizationof permafrost gas hydrate (Kennedy et al., 2001).According to our hypothesis, destabilization of gas hydrate wasnot ‘‘catastrophic,’’ and it did not involve the ‘‘entire seabed.’’ Indeed,the seabed itself was disrupted only in the vicinity of inferred seeps.Methane was released over thousands of years from gas hydrate as aresult of late glacial transgression and warming of the sediment surface.Permafrost gas hydrate is stable only at depths in excess of 150 m,and it would have responded slowly to changes in surface temperature.The source of the excess alkalinity required for carbonate precipitationneeds to be distinguished from the location of precipitation and themanner in which the carbonate was deposited. Field data suggest thatseeps were local features, in some cases associated with buried faultsin the manner described by Shapiro.We infer that alkalinity was produced near the seafloor at seepsas a result of the oxidation of methane by a consortium of methano-trophs using sulfate as an electron acceptor. Cap carbonates are nev-ertheless laterally persistent sheets because the bulk of the carbonatewas precipitated in a marine environment, most likely at the chemo-cline. The common presence of mechanical laminae, cross-laminae,subtle scoured surfaces and textural grading, along with paleocurrentmeasurements, shows that much of the sediment was deposited fromlow-concentration turbidity currents.Order of magnitude estimates for the amount of methane requiredto account for the 25‰ carbon isotope excursions observed in capcarbonates compare favorably with estimates of the mass of cap car-*E-mail: martink@mail.ucr.edu.bonate deposited at any particular stratigraphic level. We suspect thatthe apparent absence of expected isotopic heterogeneity in the vicinityof inferred seeps may be due in part to the lack of a concerted effortto acquire such data, and in part to the mineralogical and geochemicalhomogenization of these ancient rocks at most locations. Ongoing stud-ies of isotopic variation in mm-thick fringing cements within cap car-bonates in eastern California reveal d13C variations as great as 9‰.Our estimate for the amount of methane that might have beenstored in permafrost gas hydrate ($100 times the present inventory)was obtained from a consideration of the area of subaerially exposedshelves and intracratonic basins on an Earth sufficiently cold for con-tinental ice sheets to develop at low latitude (,108). The ultimatesource of methane would have been buried organic matter. To ourknowledge, there are no conflicts introduced by our hypothesis withrespect to existing tectonic models or the Precambrian biosphere.The anomalous cap facies, inferred in our paper to indicate po-tential pathways for hydrate-derived methane, do include many of thefeatures described by Shapiro (Kennedy, 1996; Kennedy et al., 2001):peloidal and microbial textures, breccia-filled cavities lined by iso-pachous and botryoidal cements, and meter-scale seafloor relief. Ad-ditional evidence for gas release includes the presence of tubes, sheetcracks, and ‘‘gas blisters.’’ Fossil remains (annelids and molluscs) arenot expected in Neoproterozoic examples, and the tubes in cap car-bonates are not related to such organisms. The larger scale of someformerly aragonite and barite seafloor fans in cap carbonates comparedwith modern interstitial cement fans is not of concern to us.In contrast to seeps linked to hydrocarbon systems, such as thoseof the Gulf of Mexico, methane seeps related to the destabilization ofpermafrost gas hydrate are not expected to be associated with bitumenbecause there is no reason for bitumen to have been produced. More-over, most cap carbonates passed through the oil window long ago,and contain only indirect evidence for the former presence ofhydrocarbons.Response to M.D. Max and W.P. Dillon. Max and Dillon (2002)suggest that methane may have been released also during times of ice-sheet growth, eustatic lowering, and destabilization of oceanic gas hy-drate through pressure release. We do not disagree with much of whatthey write, but we doubt that the scenario they describe applies to capcarbonates. Cap carbonates developed during the late stages of degla-ciation. They are associated almost exclusively with the termination ofmajor ice ages, and not with glacial-interglacial cycles. Available ev-idence suggests that they represent time scales that are inconsistentwith catastrophic methane release (Kennedy and Christie-Blick, 1998).REFERENCES CITEDKennedy, M.J., 1996, Stratigraphy, sedimentology, and isotopic geochemistryof Australian Neoproterozoic postglacial cap dolostones: Deglaciation,d13C excursions, and carbonate precipitation: Journal of Sedimentary Re-search, v. 66, p. 1050–1064.Kennedy, M.J., and Christie-Blick, N., 1998, Sequence stratigraphy of late Neo-proterozoic glacial to post-glacial transition: Implications for the timingof d13C excursions: Society for Sedimentary Geology–International As-sociation of Sedimentologists Research Conference (Strata and Sequenceson Shelves and Slopes): Catania, Sicily, Italy.Kennedy, M.J., Christie-Blick, N., and Sohl, L.E., 2001, Are Proterozoic capcarbonates and isotopic excursions a record of gas hydrate destabilizationfollowing Earth’s coldest intervals?: Geology, v. 29, p. 443–446.Max, M.D., and Dillon, W.P., 2002, Are Proterozoic cap carbonates and isotopicexcursions a record of gas hydrate destabilization following Earth’s coldestintervals?: Comment: Geology, v. 30, p. 762.Shapiro, R.S., 2002, Are Proterozoic cap carbonates and isotopic excursions arecord of gas hydrate destabilization following Earth’s coldest intervals?:Comment: Geology, v. 30, p. 761.

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