Uncorking the Bottle: What Triggered the Paleocene/Eocene Thermal Maximum Methane Release? Miriame
Total Page：16
File Type：pdf, Size：1020Kb
PALEOCEANOGRAPHY, VOL. 16, NO. 6, PAGES 549-562, DECEMBER 2001 Uncorking the bottle: What triggered the Paleocene/Eocene thermal maximum methane release? MiriamE. Katz,• BenjaminS. Cramer,Gregory S. Mountain,2 Samuel Katz, 3 and KennethG. Miller,1,2 Abstract. The Paleocene/Eocenethermal maximum (PETM) was a time of rapid global warming in both marine and continentalrealms that has been attributed to a massivemethane (CH4) releasefrom marine gas hydrate reservoirs. Previously proposedmechanisms for thismethane release rely on a changein deepwatersource region(s) to increasewater temperatures rapidly enoughto trigger the massivethermal dissociationof gas hydratereservoirs beneath the seafloor.To establish constraintson thermaldissociation, we modelheat flow throughthe sedimentcolumn and showthe effectof the temperature changeon the gashydrate stability zone throughtime. In addition,we provideseismic evidence tied to boreholedata for methanerelease along portions of the U.S. continentalslope; the releasesites are proximalto a buriedMesozoic reef front. Our modelresults, release site locations, published isotopic records, and oceancirculation models neither confirm nor refute thermaldissociation as the triggerfor the PETM methanerelease. In the absenceof definitiveevidence to confirmthermal dissociation,we investigatean altemativehypothesis in which continentalslope failure resulted in a catastrophicmethane release.Seismic and isotopic evidence indicates that Antarctic source deepwater circulation and seafloor erosion caused slope retreatalong the westemmargins of the North Atlanticin the latePaleocene. Continued erosion or seismicactivity along the oversteepenedcontinental margin may have allowed methaneto escapefrom gas reservoirstrapped between the frozen hydrate-bearingsediments and the underlyingburied Mesozoicreef front, precipitatingthe Paleocene/Eoceneboundary methanerelease. An importantimplication of this scenariois thatthe methanerelease caused (rather than resulted from) the transienttemperature increase of the PETM. Neither thermaldissociation nor mechanicaldisruption of sedimentscan be identifiedunequivocally as the triggering mechanism for methanerelease with existingdata. Further documentation with high- resolutionbenthic foraminiferal isotopic records and with seismicprofiles tied to boreholedata is neededto clarify whether erosion,thermal dissociation, or a combinationof thesetwo was the triggeringmechanism for the PETM methanerelease. 1. Introduction al., 2000]; it was followedby an exponentialreturn to preexcursion 1513Cvalues as excess •2C was eventually transferred out of the A catastrophic,transient event at the Paleocene/Eocene(P/E) ocean-atmospherecarbon reservoir, which is consistentwith esti- boundary(,,•55 Ma) led to rapidglobal warming, severe perturba- mates of the modem residencetime of carbon [Dickens, 2000]. tion of the Earth'sexogenic carbon cycle, and dramaticchanges in Methane derivedfrom frozen gas hydrateand underlyingfree and marine and terrestrialecosystems. During the Paleocene/Eocene dissolvedgas reservoirs (mean 15•3C -- ,,•-60%o [see Kvenvolden, thermalmaximum (PETM; formerlyknown as the latestPaleocene 1995]) is a plausiblesource for the large, rapid carbonisotopic thermalmaximum, or LPTM), deep-oceanand high-latitudesur- changerecorded at the P/E boundary[Dickens et al., 1995; Kaiho facewater temperatures rose by ,,•4ø-6øC[e.g., Kennett and Stott, et al., 1996]. The rapid escapeof 1500-2200 gigatons(Gt) of 1991; Zachos et al., 1993; Thomasand Shackleton, 1996; Katz et isotopicallylight CH4 from multiplemarine gas hydrate reservoirs al., 1999] in <5000 years [Katz et al., 1999; Norris and Rdhl, stored within continentalslope sedimentsand the subsequent 1999; R6hl et al., 2000], archaicmammals died out while modem transferof CH4 to the ocean-atmospheresystem [Dickens et al., mammalianancestors appeared in the geologicrecord [Hooker, 1995, 1997a;Kaiho et al., 1996; Dickens,2000, 2001] hasbecome 1996; Clyde and Gingerich, 1998], and many deep-seaspecies known as the "methane hydrate dissociationhypothesis." An became extinct or disappearedtemporarily [e.g., Tjasma and alternativesource that may accountfor some or all of the CIE is Lohmann, 1983; Steineckand Thomas,1996]. a 12C-enrichedbolide [I45'lde and Quinby-Hunt, 1997]. The PETM coincided with a dramatic decrease of ,,•2-4%o in In this paper,we establishconstraints on the proposedPETM 613Cvalues worldwide, called the "carbonisotope excursion" thermal dissociation by modelingheat flow throughthe sediment (CIE) [e.g.,Kennett and Stott, 1991;Koch et al., 1992;Bralower et columnand determiningthe effect of the temperaturechange on al., 1995;Katz et al., 1999]. The CIE occurredin <5 kyr and lasted the gas hydrate stability zone (GHSZ) through time. We also ,,•150-220 kyr [Katz et al., 1999; Norris and R6hl, 1999; R6hl et provideseismic evidence tied to boreholedata for methanerelease alongportions of the U.S. continentalslope. Because our modeling 1Departmentof Geological Sciences, Rutgers University, Piscataway, results and the paleobathymetric distribution of the methanerelease New Jersey,USA. sitesdo not conclusivelyconfirm thermal dissociation as the causal 2Lamont-DohertyEarth Observatory, Columbia University, Palisades, mechanism for the PETM methanerelease, we considererosion of New York, USA. continentalslope sediments as an alternativetrigger for methane 3Departmentof Earthand EnvironmentScience, Rensselaer Poly- release. technicInstitute, Troy, New York, USA. Copyright2001 by the AmericanGeophysical Union. 2. Thermal Dissociation Hypothesis Papernumber 2000PA000615. The PETM and CIE were short-termclimatic perturbations 0883-8305/01/2000PA000615512.00 superimposedon a long-termglobal warmingtrend that beganin 549 550 KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE? the late Paleocene at •56.5 Ma and continued for ,-•5 Myr, seafloor culminatingin the early Eoceneat ,-•51.5 Ma [Shackletonet al., 1984;Miller et al., 1987; Zachoset al., 2001]. A gradualwarming of the deepsea duringthis long-termwarming trend would have causeda slow changein the temperature-pressurestability field of 200 frozengas hydrate, resulting in a gradualtransfer of methanefrom gashydrate reservoirs to the oceans.This gradualtransfer could not have producedthe rapid isotopicchanges observed at the CIE, contraryto previousstudies that have proposedincorrectly that 400 long-termwarming passeda critical temperaturethreshold for methanehydrate stability [e.g., Norris, 2001]. Instead,a rapidwater temperatureincrease is requiredto trigger widespreadthermal 6OO dissociation,presumably through a changein deepwatersource region [Dickenset al., 1995]. As proposed[Dickens et al., 1995, 1997a;Kaiho et al., 1996], 8OO the methanehydrate dissociation hypothesis maintains that long- 0 2000 4000 6000 8000 term globalwarming during the late Paleocene[Shackleton et al., time (years) 1984; Miller et al., 1987; Zachoset al., 2001] pushedthe ocean- atmospheresystem past a criticalthreshold [Zachos et al., 1993; Thomas,1998], causingwarm surfacewaters to sink and inter- Figure 1. An instantaneous5øC water temperature increase will mediateto deepocean temperatures to rise. This warmingwould propagatethrough the underlyingsediments as a functionof time. have propagatedinto the sediments,melting once solid CH4 See text for input parameters. hydratesand releasing free gas bubbles into the sediments[Dickens temperatureat the sedimentsurface AT, d, a geothermalgradient et al., 1995, 1997a; Kaiho et al., 1996]. Thermal dissociation •, and a thermaldiffusivity • (adaptedfrom Carslawand Jaeger would have createdan increasein sedimentpore pressureand [1959, equation(13), p. 61] and Turcotteand Schubert[1982, destabilizedthe sedimentcolumn, resulting in slopefailure and the problem4-34]). An analysisof this model givesthe changein release of massive quantitiesof CH4 into the ocean. Methane temperaturedTz as a functionof time t and depthz: releasewould have occurredon continentalslopes between 900 and 2000 m water depthbecause all gas hydrateat thesedepths drz//X ri• : erfc[z/ (2(•t) •/2)] , (1) wouldhave been prone to dissociationin thePaleocene with a 4ø- 6øC rise in bottomwater temperature[Dickens et al., 1995]. The where erfc is the complementaryerror function(erfc = 1 - error gaseousCH4 would have reactedwith dissolved02 (likely via function;values are from standardtables, e.g., Carslawand Jaeger bacterialactivity [Paull et al., 1995; Buffettand Zatsepina,2000]) [1959,p. 485] andTurcotte and Schubert[1982, p. 161]),Tz = T,•+ •2 to produce C-enrichedCO2, adding carbon to all globalexogenic •3z+dTz, • = 3 x 10-7 m2/s(=9.46 m2/yr [Hyndman etal., 1979; carbonreservoirs and significantlyshoaling the depthof carbonate Dickenset al., 1995]), and AT,• = 5øC (approximateobserved dissolution in the ocean [e.g., Dickens et al., 1995, 1997a; deepwaterwarming at the PETM). Thomas,1998; Katz et al., 1999; Dickens,2000]. Higher bottom We simplifyour heat flow modelwith severalassumptions that water temperature,lower dissolved02, changesin surfacewater result in a maximum rate of temperatureanomaly propagation productivity,and/or more corrosivedeep watersprecipitated the (therebyfavoring thermal dissociation); a more realisticanalysis extinctionof many deep-seaspecies [e.g., Thomas,1998]. would lengthenthe time requiredfor the temperatureincreases predictedby this model. We use an instantaneous5øC water temperatureincrease; in reality,the temperatureincrease resulting 2.1. Constraints on Thermal