PALEOCEANOGRAPHY, VOL. 16, NO. 6, PAGES 549-562, DECEMBER 2001
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 themethane release 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 Dissociation: Heat Flow Model from a changeto a warm, low-latitudedeepwater source could take Here we developa simplemodel usingheat flow parametersto as long as the mixing time of the oceans(1000-2000 years). assesstemperature anomaly propagation through sediments and the Furthermore,the phase changerequired to melt the frozen gas expectedduration and timing of the observedPETM isotopic hydratewithin the sedimentswould slow the propagationof the changes.Within the contextof publishedisotopic records, this temperatureanomaly because of the latent heat consumedwhen modelplaces narrow constraints on the boundaryconditions under methanehydrate dissociates tomethane and water (4.5 x 108J/m 3 which sufficient amounts of methane could have been released via [Sloan, 1990]). Additional factorscomplicate attempts to model thermal dissociation alone at the PETM. this phasechange, such as the variable surfacearea of hydrate The deepwatertemperature warming at the onsetof the PETM is particles,formation of an isolatingice film on the hydratesurface, ,-•4ø-6øC,based on 15•80records [Kennett and Stott, 1991; Pak hydratedensity within sediments,and sublimation [e.g., Kim et al., and Miller, 1992; Bralower et al., 1995; Thomasand Shackleton, 1987; Ershov and Yakushev,1992; Hatzikiriakos and Englezos, 1996; Katz et al., 1999]. If thermal dissociationtriggered the 1993; Taylor, 1999]. Furthermore,the presenceof gashydrate in PETM methanerelease, then heat resultingfrom this deepwater the sedimentswill lower overall thermalconductivity within the warming must have propagatedthrough the sedimentsof the sedimentcolumn [e.g., Sloan, 1998]. All of thesefactors increase GHSZ and melted the concentrationof frozen hydrate that can the timespredicted by (1) and Figures1 and 2. form a seal at the base of the GHSZ [e.g., Kvenvolden,1993], Our heatflow modelshows that by ,-•2000years after the 5øC allowingmethane to escapefrom both the meltedgas hydrates and deepwaterwarming, sediment temperatures would be •3øC higher the free gasreservoir below [Dickenset al., 1997b].Heating at the at 100 m below seafloor(mbsf) and ,-•1.5øChigher at 200 mbsf base of the GHSZ must have been sufficient to release the • 1500- (Figure1), with the geothermalgradient reequilibrating in > 10,000 2200 Gt of C necessaryto explainthe CIE [Dickens,2000]. years(Figure 2). If methanewere releasedthrough thermal disso- To addressthis proposedheat flow scenario,we model the ciation in the late Paleocene,then the minimum estimateof 2000- propagationof a temperatureincrease from the sediment-water 4000 yearsrequired for the changein deepwatersource region, interfacethrough the underlyingsediments (Figure 1). This model oceanmixing, temperature anomaly propagation, and thermal dis- assumesone-dimensional heat flow through the sediments,an sociation(disregarding the phasechange) should be detectablein initial sediment surface temperatureT,-•, an increasein water geologicaltemperature proxies. Deep-sea isotopic records should KATZ ET AL.' WHAT TRIGGERED THE PETM METHANE RELEASE? 551
geothermalgradient after 10,000 yrs. .initial geothermalg 200
600 .... ' .... ' .... ' .... ' .... • 10 15 20 25 30 35
Figure 2. An instantaneous5øC water temperatureincrease will causethe geothermalgradient to readjustover >10,000 yearsas the temperatureanomaly propagates through the sediment.See text for input parameters.
showarapid 15• 80 decrease(indicating rapid warming) that precedes phere carbon isotopic value were calculatedusing an initial 40,000 a rapid6•3C decrease (indicating CH4 release). Gt ocean-atmosphericcarbon reservoir with 613, C = 0.55/00o and a carbonresidence time of 100,000 years. 2.2. Constraints on Thermal Dissociation: Paleobathymetry The model resultsbased on the isotopicresponse at Site 1051 The water depth range over which thermal dissociationcould and temperature-pressuregas hydratestability curves [e.g., Stoan, have occurredin the latestPaleocene is a functionof the magnitude 1998] showthat all gashydrate would have meltedabove 1570 m of the water temperatureincrease. We modelthe changein temper- after 2350 years.This is a minimumtime estimatebecause no time atureand gas hydrate stability field in the latestPaleocene using the was allowed for methanediffusion through the sediments,short- benthicforaminiferal 6•80 recordsfrom the Blake Nose Site 1051 ening the predictedtime for methanerelease. Our resultsprovide [Katz et at., 1999] andthe gashydrate distribution predicted by the narrower time constraintsthan Dickens e! at. , who found temperature-hydratestability relationship[Dickens and Quinby- that a changein the pressure-temperaturehydrate stability field Hunt, 1994] (Figure 3). On the basisof this modeledresponse of causedby a 4øC watertemperature increase over 10,000years in theGHSZ, we modelthe 6•3C response to the predicted methane the latestPaleocene would have meltedall gas hydratein marine release(Figure 3). sediments where the seafloor was between ,-•920 and 1460 m. At In thismodel, bottom water temperature is calculatedfrom the Site greaterwater depthsthe baseof gas hydratestability would have 1051 isotopicrecord using the paleotemperatureequation of Erez shoaled,but the base of the frozen gas hydrate-sedimentlayer andLuz: T = 16.998- 4.52(618Ocalcit e- 618Owater) q-0.028 would not have been breached and the underlying free and (618Ocalcite__618Owater)2 ' using 6• 8Owate r: -- 1.2%0[Shackteton and dissolvedgas reservoirswould have remainedtrapped (Figure 3) Kennett, 1975]. Oridorsatis spp. apparentlydo not biologically [Dickense! at., 1995; this study].This underlyinggas reservoiris fractionateoxygen isotopes [Shackteton et at., 1984], so no correc- an importantcomponent in any scenariothat involvesgas hydrate tionfactor was applied for 618Ocalcite. We assume that the water dissociationand past climatechange [Dickens e! at., 1997b]. Any temperatureincrease at Site 1051 was instantaneousin order hydratemelted at the baseof the GHSZ will eitherbe addedto the to provide an end-memberscenario that favors the viability of trappedgas reservoiror refrozenin overlying sedimentsas gases thermal dissociation.The stratigraphiclevel of the instantaneous diffuse upward. Hence melting all frozen gas hydrate wthin the water temperatureincrease was placedat the baseof the dissolution GHSZ is essentialto releasingthe substantialamounts of methane layer (512.80 mbsf; Figure 3). Sedimenttemperature is calculated necessaryto accountfor the CIE. from (1) in time stepsof 50 years and depthincrements of 10 m. A constantsedimentation rate of 20.625 m/Myr. is basedon the 3. PETM Methane Release Sites cycle countsfrom Katz et at. . Initial geothermalgradient is 0.04ø/m;• = 9.46m2/yr (--3 x 10-7 m2/s[Hyndman etat., 1979]). A critical sequenceof eventssupporting the hydratedissociation The modeled GHSZ (Figure 3) does not take into accountthe hypothesishas been documented at OceanDrilling Program(ODP) potentialeffects of dissolvedion concentrations[Katz et at., 1959], Site 1051 (presentwater depth 1980 m) on the Blake Nose [Katz et tracegases [Hovtand et al., 1995], capillaryaction [e.g., Ctennettet at., 1999], a reef-basedsalient protruding from the easternrim of at., 1999; Henry et at., 1999], or pore fluid salinity [Dickensand the Blake Plateauoff the southeasternUnited States(Figure 4). The Quinby-Hunt,1994], none of which are known for the Paleogene. onset of the CIE immediately overlies the benthic foraminiferal Themodeled 6•3C response to thepredicted methane release is extinctionevent (BFEE) at Site 1051, where ,-•55% of the benthic calculatedin time stepsof 50 years,water depthincrements of 10 foraminiferaltaxa disappear[Katz et at., 1999]. This is correlatable m, and sediment depth incrementsof 10 m for 50% sediment with similarisotopic changes and benthicforaminiferal extinctions porositywith 7% of the pore spacefilled with frozengas hydrate. in otherdeep-sea PETM sectionsfrom aroundthe world [e.g.,Pak Methanegas is assumedto remaintrapped beneath the GHSZ until and Mirlet, 1992; Kaiho et at., 1996; Thomas and Shackteton, the baseof the GHSZ shoalsto the seafloor(i.e., frozengas hydrate 1996]. The majority (60%) of the disappearancesat Site 1051 is no longer stable).Predicted changes in the mean ocean-atmos- occur within an intraformationalmud clast layer, where faunal 552 KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE?
depth (mbsf) A 512.0 512.2 512.4 512.6 512.8 513.0 513.2 18• o -1.0 - --o-- •5•0 (measured) 16ø-_, Temperature input -0.5 - 14 : •.• •,,,,,a---'- 12 '- 10 E 0.5
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1400 E,• 1400 seafloor ,i*'*•'•••%...... • 7"-•$•-:-.-.•.•.-- , ...... C
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time from CIE (k.y.) KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE? 553
diversityplummets from •25-30 to •5-10 taxa/sampleabove New seismic evidence tied to borehole data indicates that addi- the BFEE. Deposition of this clast layer has been attributedto tionalmethane may havebeen released in the latestPaleocene from slumpingthat was causedby methanerelease upslope from Site the New Jerseycontinental slope. We use down-holegeophysical 1051.A survivingbenthic fauna dominated by buliminidsindicates logs at Deep Sea Drilling Project(DSDP) Site 605 (presentdepth a decreasein dissolved02, consistentwith CH4 oxidation. At the 2194 m [van Hinte et al., 1987]) (Figure 4) to correlatea seismic sametime, partly dissolvedforaminiferal tests at Site 1051 indicate reflector(Figure 5) to the level within the core where an uncon- increaseddissolved CO2 and decreasedCaCO3 levels, as predicted formityseparates uppermost Paleocene dark green bioturbated clays for the by-productsof CH4 oxidation[Katz et al., 1999]. from lowermostEocene banded marls (563.85 mbsf [van Hinte et Seismicevidence updip from Site 1051 also is consistentwith a al., 1987; Lang and Wise,1987]. This unconformityencompasses massivemethane release:a seismicprofile shows an extensive the time intervalof the PETM, which is consistentwith correlating interval of chaotic, hummocky reflections that is marked by this unconformityto the timing of the methanerelease. The under- bidirectionaldownlap (Figure 5). Hummock-like structurescan lying Cretaceous-Paleogene(K-P) reflector is truncatedby the form wheregas-rich sediments immediately below the baseof gas Paleocene-Eocene(P-E) reflector•9 km upslopefrom Site 605 hydrate stability have convertedinto an easily mobilized, gassy (Figure 5). Tracedfarther upslope, this disconformablesurface is mud, causingdensity-driven inversions with overlying sediments expressedas a strongreflection overlying an • 160 m thick unit of [e.g., Mclver, 1982; Prior et al., 1989; Vogt, 1997; Dillon et al., irregular,chaotic reflections with bidirectionaldownlap. As on the 1998;Davies et al., 1999]. A hummockyseismic pattern can occur Blake Nose, this chaoticunit doesnot pinch out laterallyor drape where hummockshave collapsedand CH4 has escapedfrom over surroundingreflections, nor do surroundingreflections drape marine sediments[Davies et al., 1999]. The hummocky,chaotic overthe chaoticunit, indicatingthat deformationwas most likely in seismicunit on the Blake Nose is • 125-150 m thick with fairly situ. On the basisof the similar acousticexpressions of the Blake abruptlateral terminations against well-defined seismic reflections Nose chaotic seismic interval and the correlation of the reflector to (Figure 5). Deformationwas most likely in situ; the chaoticunit the hiatusin the core recordat Site 605, we proposethat the New doesnot pinch out laterallyor drapeover surroundingreflections, Jerseyslope was an additionalPETM methanerelease site. nor do surrounding reflections drape over the chaotic unit Similar chaotic seismic intervals with bidirectional downlap (Figure 5), as shouldoccur in a redepositedunit suchas a debris overlie Mesozoic reefs in several other eastern U.S. continental flow. Rather,it is a gas releasesite [Katz et al., 1999]. slopeseismic profiles: (1) USGS Line 9, shotpoints• 1740-1800, A prominent seismicreflection can be traced from the chaotic •2.3-2.6 two-way travel time [Mountain and Tucholke, 1985, seismicinterval down slope to Site 1051, where this reflection Figure 8-13] and (2) USGS Line 5, shotpoints•1800-1850, correlates with the level of the onset of the CIE and the mud clast • 50 mbsf [Grow et al., 1979, Figure 9]. We proposethat these layer, linking methanerelease to sedimentdisplacement [Katz et chaoticzones are PETM methanerelease sites. They are bracketed al., 1999]. Proximal to Site 1051, this seismicreflection is caused by seismicreflector A u (latest Eocene-earliestOligocene) and by the impedancecontrast between the displacedsediments and the reflectorA* (mid-Maestrichtian),with no nearby boreholedata overlying dissolutioninterval resulting from methane oxidation availableto firmly correlateto the PETM event.Nonetheless, the [Katz et al., 1999]. This seismicreflection correlates to a discon- stratalrelationships and two-way travel times provided by these formity at Site 1052 that spansmuch of the late Paleoceneto early reflectors are consistent with similar methane release features on middle Eocene [Norris et al., 1998], indicating that it is an the Blake Nose and the New Jerseyslope. erosionalsurface proxhmal to the releasesite. Seismicevidence [Grow et al., 1979; Jansa, 1981] showsthat Evidence from the Blake Nose links methane release features to the PETM methanerelease sites are proximal to a Mesozoicreef the onset of the PETM and CIE; however, the mass of CH4 front along the eastemU.S. continentalmargin (Figure 5). Gases releasedfrom this region alone is insufficientto accountfor the can accumulateand becometrapped below a top seal (such as a magnitudeof the global perturbationsat the PETM [Katz et al., sedimentand gas hydrate layer) that is bowedupward or wherethe 1999]. Present-daymethane volume in marinesediments provides updipportion of gas-bearingstrata terminate against a gashydrate the only available guidelineto estimatethe areal extent of the seal, creatinga gas overpressurezone that may facilitatemethane PETM methane release. Direct measurements on cores from the release[e.g., Dillon et al., 1980, 1998; Prior et at., 1989; Sloanet nearbyBlake Ridge showthat today, the GHSZ andthe underlying al., 1999]. The geometryof the Mesozoicreef front and adjacent free and dissolvedgas reservoircontain •35 Gt of C (47 Gt of sedimentsmay have been an important factor in the PETM CH4)and cover 26,000 km 2 [Dickensetat., 1997b].Mass balance methanerelease: interstitial methane may have migratedup strata equationsrequire the additionof •1500-2200 Gt of C from CH4 that dipped seawardfrom the Mesozoic reef front and became to explain the CIE [Dickens, 2000, 2001]. Assuming similar trappedbelow the overlyinggas hydrate top seals.The thicknessof volume and distribution, this indicatesthat methane from an area the chaoticlayers at the releasesites is • 125-160 m (Figure 3), at least 54 times the size of the Blake Ridge was releasedduring consistentwith the depthto the baseof the GHSZ predictedfor late thePETM (•1,400,000 km 2, or •5% of thesize of theUnited Paleocene-earlyEocene deepwater temperatures that were > 10øC States).This roughestimate shows that methane must have escaped warmer than today (based on the temperature-pressurestability from an area much larger than the Blake Nose alone. field of gashydrates [e.g., Sloan, 1998]).
Figure 3. (opposite)Response of the gashydrate stability zone (GHSZ) (shadedareas in Figures3b-3d) to a watertemperature increase at the BlakeNose (Site 1051).An instantaneouswater temperature increase provides an end-memberscenario that favors the feasibilityof thermaldissociation. Sediment temperatures are calculatedfrom (1) in time stepsof 50 yearsand depthincrements of 10 m. Age on the horizontalaxis represents the time elapsedfrom an instantaneoustemperature change. (a) The benthicforaminiferal isotopic record from Site 1051,which was used to calculatebottom water temperatures (bold line) (seetext). (b) The 1000 m waterdepth. (c) The 1400 m water depth.(d) The 1800 m waterdepth. In Figures3b-3d the isothermscurve backward during the initial stagesof the temperatureanomaly propagationbecause a temperatureinversion in the sedimentsis createdas the surfacesediments warm morerapidly than the underlying sediments.(e)Modeled response (thick line) and Site 1051 record (circles) of the 15 •3C signal to the change in GHSZand methane release (calculatedin time stepsof 50 years,water depth increments of 10 m, andsediment depth increments of 10 m andbased on 50% sediment porosity[Gornitz and Fung, 1994] with 7% of the pore spacefilled with frozengas hydrate; shaded area indicates an enveloperanging from 5 to 10% hydrate-filledpore space[e.g., Dickenset al., 1997b]). 554 KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE?
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Figure 4. Location map showing seismicprofiles and DSDP/ODP sites (the portion of each seismic profile discussedin text is indicatedrather than the entire length of each profile). Large arrow indicateslate Paleocene deepwaterflow path in the westernNorth Atlantic basin [afterMountain and Miller, 1992]. The Blake Outer Ridge is a post-Eocenedepositional feature and thereforedid not impedePaleocene deepwater flow.
Paleodepthestimates place the Blake Nose andNew Jerseyslope unpublisheddata, 1998] (Appendix A). There are no benthic releasesites at 1000-1500 m water depthbased on reassessments foraminiferaldata to constrainthe paleobathymetryof the possible ofbenthic foraminiferaldata from Sites605 [Hulsbos,1987; Saint- releasesites on USGS Lines 5 (New Englandmargin) and 9 (Long Marc, 1987; van Hinte et al., 1987], 1051 [Norris et al., 1998; Katz Islandmargin) because no boreholesare availablefrom theseareas. et al., 1999; Katz, 2001], and 1052 [Norris et al., 1998; M. E. Katz, Nonetheless,their two-way travel times and proximity to the KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE? 555
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B. SW Line TD-5 NE
.5 .,.,...... , t •,• .y•&,.• ...... ,. '• *,, . -. -. .-,'-, -m-.. -- •,x
Figure 5. (a) New Jerseycontinental margin (Shell Line 127) and(b) BlakeNose (Line TD-5; adaptedfrom Norris et al. [ 1998]) seismicprofiles showing the zoneof chaoticseismic reflections that was causedby the methaneescape that triggeredthe PETM. SeeFigure 4 for locations.See Katz et al.  or Norris et al.  for correlationof Line TD-5 to ODP Site 1051. The strongseismic reflection that overliesthe chaoticinterval on the New Jersey marginmarks the sharpacoustic and lithologic contrast between lower Eocene chert-bearing sediments [Ewing et al., 1969; Tucholke,1979] and the underlyinguppermost Cretaceous sediments that were disturbedby the PETM methanerelease. Lowermost Eocene sediments are missingfrom the regionof the Blake Nose PETM methanerelease site(ODP Site 1052 [Norris et al., 1998]);hence there is no correspondingbright seismicreflection at this location.
Mesozoic reef front indicates that they were likely at similar Publishedisotopic records do not show any time lag between paleodepths(Appendix A). deep-seawarming and methanerelease [Kennett and $tott, 1991; Bralower et al., 1995; Thomas and Shackleton, 1996; Pak and 4. Model Constraints Versus the Geologic Record Miller, 1992; Katz et al., 1999], althoughlow sedimentationrates (,-.,2 cm/kyr), dissolution,and possiblebioturbation may make it If thermaldissociation were the primary mechanism for thePETM difficult to extract a high-resolutiondeep-sea record that could methanerelease, then (1) deep-seaisotopic records should show a showthe time lag necessaryto verify thermaldissociation. None- rapid15:80 decrease (indicating rapid warming) that precedes arapid theless,there is little direct evidencefor the changefrom high- 15:3Cdecrease (indicating CH4 release) by at least 2000-4000 years, latitudeto low-latitudedeepwater source(s) that is necessaryto (2) thereshould be evidencefor a changein deepwatersource region causethe rapidwater temperature increase predicted by the thermal that caused the rapid water temperatureincrease, and (3) the dissociationhypothesis. Pak and Miller  suggestedthat a paleobathymetryof the methanerelease sites must be consistent warm salinedeepwater (WSDW) componentmight have existedin with the depthspredicted for thermaldissociation. the late Paleocene to early Eocene on the basis of benthic 556 KATZ ET AL.' WHAT TRIGGERED THE PETM METHANE RELEASE?
.... Site1051 •l}1
.•l•'•l • j"l.• '• . I j ...... eq. ,,
I I I I I I 0 km 5
Figure6. A sectionof BlakeNose seismic Line TD-5 (adapted from Norris et at. )showing reflector Ab equivalentand overlying current-controlledsedimentation. See Katz et at.  or Norris et al.  for correlation to Site 1051.
foraminiferal15:80 values that show warmer deepwater temper- (via erosion) precededthe rapid temperatureincrease. In this atures in the Pacific and North Atlantic than in the Antarctic near scenario,erosion along the oversteepenedU.S. continentalmargin the Paleocene/Eoceneboundary. However, these comparisons may have allowedmethane to escapefrom gas reservoirstrapped suffer from substantialcorrelation uncertainties [Aubry, 1998]. It betweenthe frozenhydrate-beating sediments and the underlying has been proposedthat Tethyanwaters may have becomedense buffed Mesozoic reef front. An important implication of this enoughto form WSDW [Kennettand Stott, 1990, 1991; Katz and scenariois that the methanerelease was the cause (rather than Miller, 1991; Pak and Miller, 1992; Zachos et at., 1993, 1994; the result)of the rapid temperatureincrease. Methane that escapes Thomasand Shackleton,1996; Bratower et at., 1997], but avail- the seafloor will be oxidized in the oceans;excess methane will able carbon isotopic data do not supportsuch a hypothesized escapeto the atmosphere[e.g., Ehhatt, 1974; Ward et at., 1987; WSDW [e.g., Pak and Miller, 1992; Schmitzet at., 1996], and Ciceroneand Oremtand,1988]. Increasedconcentrations of atmos- climatemodels show that prolonged WSDW productionis unlikely pheric CH4 [Peters and Sloan, 2000] and/or its oxidant, CO2 [e.g., Sloan and Thomas,1998]. Thereforeneither isotopiccom- [Kennettand Stott, 1991], could have been the cause(rather than parisonsnor modelingefforts substantiate the changein watermass the result) of warming during the PETM. sourceregion that is requiredto triggerthermal dissociation. Seismicprofiles show that widespread deepwater erosion occurred The paleobathymetricdistribution of the releasesites must be in the westernNorth Atlantic in the late Paleocene,providing a consistent with the causal mechanism for methane release. All four potentialmechanical trigger for methanerelease. On the westem potential PETM release sites lie within the water depth range BermudaRise, an angularunconformity marked by seismicreflector predictedfor completegas hydratethermal dissociationgiven the Ab resultedfrom swiftly flowing bottom currents [Mountain and late Paleoecenedeepwater temperatures [Dickens et at., 1995; this Miller, 1992].Seismic ties to pistoncores and boreholes (e.g., DSDP study]. However, the releasesites are limited to a narrow region Site 386) showthat peak erosionoccurred on the westemBermuda proximal to the Mesozoicreef front ratherthan extendingacross Riseprior to circa57.3 Ma (Mountainand Miller [ 1992], revisedto the broaderdepth range predicted for thermaldissociation. There- the timescaleof Berggrenet al. [ 1995]) (Figure7). Late Paleocene b fore thermaldissociation as the trigger for PETM methanerelease erosionassociated with reflector A canbe distinguishedclearly from cannotbe unequivocallyconfirmed using the distributionof the a more widespreadand severe erosional event associatedwith PETM release sites. reflectorA u(latest Eocene-earliest Oligocene) [Tucholke andMoun- Methane can also escapevia the mechanicaldisruption of sedi- tain, 1979;Mountain and Tuchotke,1985] that was caused by a pulse ments [e.g., Gornitz and Fung, 1994]. Regardlessof the initial of northern componentwater flowing from the northernNorth triggeringmechanism (thermal or mechanical),methane can only Atlantic [Miller and Tuchotke,1983]. escapefrom hydratedeposits through sediment failure [Pautt et at., Hiatuses documentedin drill cores from the U.S. margin 1996; Dillon et at., 1998], complicatingthe searchfor the causal indicatethat A b erosionwas most likely basin-wide; seismic data mechanism for methane release. In thermal dissociation, frozen presentedbelow strengthenthis argument.A coeval hiatus has hydrate dissociatesto water and free gas, which destabilizesthe been identifiedat DSDP Site 390, 700 m downslopefrom Site sedimentcolumn, leading to slopefailure and gas releaseinto the 1051 on the Blake Nose [Gradsteinet at., 1978; Schmidt,1978]. A overlyingwaters. In contrast,mechanical disruption of sediments prominentseismic reflection associated with an unconformityat can breachthe baseof a frozengas hydrate and sedimentlayer to Site 1051 on the Blake Nose (,-0544mbsf [Norris et at., 1998]) open a conduit to a free gas reservoirtrapped below, with the (Figure 6) is datedat 56.8-57.5 Ma (Figure 7). This is referedto potentialto releaselarger quantities of methanethan predicted from as "reflectorAb equivalent"because although it is potentially thermaldissociation alone [Gornitz and Fung, 1994]. coevalwith reflector Ab, it cannotbe tracedseismically to the Bermuda Rise with existing data. A similar seismicreflection on 5. An Alternative Hypothesis the New Jerseycontinental rise is associatedwith an unconformity at Site 605, wherebiostratigraphic correlations reveal a hiatusfrom In the absenceof firm evidenceto verify thermal dissociation, 56.3 to 56.8 Ma at ,-0605.0mbsf (Figure 7). We alsorefer to this as we investigatean alternativehypothesis in which methanerelease "reflectorAb equivalent." KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE? 557
Site 1051 Site 605 450 55O
O 5OO 6OO Rection Abe•uiv. 55O Reflection••bequiv__. ¸ 650
56 3 • I , i . i i ,I i , i 65O( 750 . 15,6'3. 62 6O 58 56 54 60 59 58 57 56 55 Age (Ma) Age (Ma) Site 386 610 13 nannofossil lowest occurrence • 620 nannofossilhighest occurrence
12 630 Reflection A b planktonicforaminiferal lowest occurrence ¸ planktonicforaminiferal highest occurrence r,.) 640 [•66•.0.... , ß . 1.57'.3 65 60 55 Age (Ma)
Figure7. Age-depthplots used to assignages to reflector Ab (Site386) and reflector Ab equivalent(Sites 605 and 1051). Site 386 data are afterMountain and Miller ; Site 1051 data are afterNorris et al. ; Site 605 data are after van Hinte et al. , Lang and Wise, and Saint-Marc .
On thebasis of thedistribution of reflectorA b, its associatedSeaward of Cape Fear, North Carolina, the landward edge of hiatuses,and Pak and Miller's [ 1992] interbasinalcarbon isotopic predominantlyslumped sediments are bracketedby reflectorsA u b comparisons,Mountain and Miller concluded that A b and A equivalent(Figure 9; see captionfor discussionof diapir erosionwas causedby SouthernOcean bottom water that flowed tectonism). Similar reflector geometriesare observedelsewhere into the western Atlantic in the late Paleocene. Geostrophic alongthe margin,and we believethat all arethe resultof Paleocene considerationsdictate that this southerncomponent water (SCW) undercuttingand slopefailure. This resultedin the seawardadvance must have flowed north along the South American continental of Paleogeneturbidites containing slope sediments, as documented margin, swung east as it crossedthe equator,and flowed north at DSDP Sites386 and387 on the westernBermuda Rise (Figure4) alongthe westernflank of the mid-AtlanticRidge (this ridge crust [Tucholke et al., 1979; Tucholke and Mountain, 1979], where was ,-•50 Myr old in the Paleocene,and thereforeit was relatively turbiditesonlap reflector Ab [Mountainand Miller, 1992]. shallow).Mountain and Miller  proposedthat SCW circu- Pervasivelate Paleoceneerosion at widely separatedlocations lated cyclonically around the westernNorth Atlantic basin and (BermudaRise, New Jerseyslope, North Carolinaslope, and Blake sweptsouthward along the U.S. continentalrise (Figure4). On the Nose) is consistentwith a basin-wide cause: a pulse of SCW. BlakeNose, reflector Ab equivalent erosion and overlying current- Seismic evidenceindicates that this vigorous deepwatercurrent controlled sedimentationprovide evidence that late Paleocene undercutthe baseof the U.S. continentalslope, which was already deepwaterflow affectedthe U.S. continentalslope (Figure 6). steepbecause it was underlainby extensiveMesozoic carbonate This SCW erodedthe baseof the easternU.S. continentalslope platforms and reefs [see Grow and Sheridan, 1988]. Current- during the late Paleocene,resulting in slopeinstability. Extensive controlled sedimentation on the Blake Nose shows that erosion sedimentfailure [Tucholkeand Mountain, 1986; Mountain, 1987] that beganin the late Paleocenecontinued through the time of the (Figures 8 and 9) resultedin the landwardretreat of the U.S. CIE and into the early Eocene,although this featureis not adjacent continentalslope by ,-•15 km in the early Tertiary [Schlee,1981 ]. to the methanerelease site (Figure 6). Erosionat the base of the Evidence from the New Jersey and North Carolina margins slopevery likely causedmargin oversteepening,widespread slope confirms that late Paleocenedeepwater flow undercutthe con- failure,and the landwardretreat of the slopein the latestPaleocene. tinental margin and caused slope failure (Figures 8 and 9). We proposethat continentalslope failure may have breachedthe Downslopefrom Site 605 on the New Jerseycontinental rise, an baseof the GHSZ proximalto the reef front, therebyaccessing the along-strikeseismic profile shows that reflector Ab equivalentfree anddissolved methane gas reservoirs below, releasing methane truncatesunderlying sedimentsand is immediately overlain by andprecipitating the PETM. Slopefailure may haveresulted from turbiditesthat onlap the adjacenttopographic high (Figure 8). ongoingSCW erosionor from seismicactivity. 558 KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE?
Aw sw NE
> • ______.... - ..... ---1 L Ionlappingturbiditeslx ...
'S'R'Li'n.... I 0I 5I 10Ikrn
Figure8. NewJersey continental margin seismic profile (BGR Line 201) showing turbidites that onlap reflector Ab equivalent.See Figure 4 for location.(a) Uninterpretedseismic profile. (b) Interpretedseismic profile.
Widespreaddeepwater erosion and marginfailure may extendto and sediment cores from the western South Atlantic is needed to othercontinental margins. A counterpartto reflector Ab hasbeen assessthis possibility.Even thoughthe deepbasins of the Atlantic tentativelyidentified in the westernSouth Atlantic [Mountain and were smaller in the Paleocenethan they are today, the volume of Miller, 1992]. Seismicand lithologic evidencefor the northward the margin (where most gas hydrate reservoirsoccur today) was flow of SCW through the South Atlantic at this time (outlined not drasticallydifferent. above) has been noted in the sub-Antarcticsector of the South Methanemay have escapedfrom marginsthroughout the Atlan- Atlantic [Ciesielski et al., 1988] and the Brazil Basin and Rio tic, yet it is difficult to explainmassive slope failure over a large Grande Gap [Gamboa et al., 1983]. We suggestthat as in the area within a brief time period. Limited evidence for multiple western North Atlantic, base-of-slopeerosion along the South methanereleases [Bains et al., 1999] may indicate that methane Americanmargin could have oversteepenedthe margin and pre- release occurred over •20,000 years. If this is the case, then cipitatedmethane release also. Detailed analysis of seismicprofiles methanemay have escapedthrough both mechanicaldisruption KATZ ET AL.' WHAT TRIGGERED THE PETM METHANE RELEASE? 559
NW SE NW SE
0 5 10 km :! .... USGS Line 32 I I I
Figure 9. North Carolinacontinental margin seismic profile (USGS Line 32) showingslump deposition associated with late Paleocenedeepwater erosion. See Figure 4 for location.(a) Uninterpretedseismic profile. (b) Interpreted seismicprofile. A saltdiapir affected stratal geometries immediately seaward of the Mesozoicreef front,complicating the interpretationof this profile.Because (1) the slumpedunit is more deformedthan the underlyingand overlying units and (2) irregularseismic reflectors in this unit are observed> 10 km seawardof the diapir,we believethat this unit resultedfrom large-scaleslumping from the adjacentslope, independent of salt tectonism. and thermal dissociation.Greenhouse warming (including deep to a narrowregion proximal to a Mesozoic reef front, in contrastto and bottom water warming) caused by an initial mechanical the broaderdepth range predictedfor methanerelease via thermal methanerelease may have triggeredsubsequent thermal dissocia- dissociation. tion, resultingin additionalmethane release. As we have shown, 4. Our model results,release site locations,published isotopic the ,-•20,000 year duration for successivemethane releases sug- records, and ocean circulation models neither confirm nor refute gestedby Bains et al.  providessufficient time for prop- thermaldissociation as the trigger for the PETM methanerelease. agationof a thermalanomaly and destabilizationof the GHSZ. 5. In light of inconclusive evidence confirming the PETM thermaldissociation hypothesis, we considermechanical disruption 6. Summary and Conclusions of sedimentsas an alternatecausal triggering mechanism for the methanerelease. (1) Seafloorerosion caused by SCW circulation The major conclusionsof this paper are the following. led to oversteepeningand retreatof the continentalslope along the 1. Our heat flow model provides an end-member,minimum westernmargins of the Atlantic in the late Paleocene.(2) Con- estimateof the time requiredfor thermaldissociation of methane tinental slope failure may have breachedthe base of the GHSZ hydratesat thePaleocene/Eocene boundary; this predicts that a rapid proximal to a reef front, therebyaccessing the free and dissolved ]8 •50 decrease(indicating rapid warming)should precede the rapid methanegas reservoirs below, releasing methane, and precipitating •513Cdecrease (indicating CH4 release) by at least 2000-4000 years. the PETM. Slope failure may have resultedfrom ongoingSCW 2. We use the Site 1051 •5180record to model the effect of the erosionor from seismicactivity. temperaturechange on the GHSZ through time, showing that 6. If mechanical disruption were the primary trigger, then >2350 years is necessaryto melt all gas hydrate at locations methanerelease may have been the cause(rather than the result) shallower than 1570 m. of the rapid temperatureincrease. The resultinggreenhouse warm- 3. A PETM releasesite was identifiedon the Blake Nose [Katz et ing may have triggeredsubsequent thermal dissociation. al., 1999]; here we identify three additional release sites on the Neither thermal dissociationnor erosionand margin failure are northeasternU.S. continentalmargin. All releasesites are limited fully supportedby existingrecords as the triggeringmechanism for 560 KATZ ET AL.' WHAT TRIGGERED THE PETM METHANE RELEASE? methanerelease at the PETM. Additionalhigh-resolution benthic than,-•2000 m; furthermore,species typical of deeperwater depths foraminiferalisotopic records are needed to confirmthat (1) anocean are rare or absent(e.g., Clinapertinaspp., Quadrimorphina pro- circulationchange and a rapidwater temperature increase preceded funda).The faunalpaleodepth estimates at Site605 aresupported by the methanerelease (thermal dissociation) or (2) methanerelease geophysicalbackstripping efforts that yield a paleodepthof 2000- precededthe water temperature increase (mechanical disruption). In 2300 m for the early Eocene[Hulsbos, 1987]. addition,further documentation with seismicprofiles and corrobo- Site 1052 is ,-•100 m downslopefrom the Blake Nose PETM rativeborehole data are needed to confirmthe plausibility of erosion releasesite and providesthe best constrainton paleodepthesti- as the causal mechanism for the initial methane release. mates. Site 1052 faunaslack severaldeeper water indicatorsthat are present at Sites 605 or 1051 (e.g., Abyssaminaspp., A. praeacuta, and C. havanensis)and containshallower water indi- Appendix A catorsthat are absentfrom the deepersites (e.g., B. impendens,C. mexicanus,N. jarvisi, andP costata);these faunas place Site 1052 Paleobathymetricestimates were basedon benthicforaminiferal and the nearbymethane release site in the upperhalf of the lower faunasat Sites605 [Hulsbos,1987; Saint-Marc, 1987; van Hinte et bathyalzone (1000-1500 m). This paleodepthrange is consistent al., 1987], 1051 [Norris et al., 1998; Katz, 2001], and 1052 [Norris with the vertical displacementbetween the deeperSites 605 and et al., 1998; M. E. Katz, unpublisheddata, 1998] (Table A1). 1051 and the New Jerseymargin and Blake Nose releasesites. Paleodepthcriteria are providedfor the commonoccurrences of benthicforaminifera in the late Paleocene-earlyEocene [Tjasma and Lohmann, 1983; van Morkhovenet al., 1986] (Table A1). Acknowledgments. We thank Jerry Dickens and William Dillon for their helpful reviewsof this manuscript.This materialis basedupon work Paleodepthsof ,-•2000 m (+100 m) are indicatedat Sites605 and supportedunder a RutgersUniversity Graduate Research Fellowship (M. E. 1051 by the overlappingoccurrences of speciesthat are typically Katz), a National ScienceFoundation Graduate Research Fellowship (B. S. restrictedto (or mostabundant at) depthseither shallower or deeper Cramer), and NSF grant OCE0084032.
Table A1. BenthicForaminiferal Occurrences at Sites605, 1051, and 1052a Species Paleodepth,m Site605 Site 1051 Site 1052 Abyssaminaquadrata >2000 x Abyssaminapoagi >2000 x Anomalinoides danica <2000 x Aragonia spp. >2000 x x x Anomalinoides semicribratus >600 x Anomalinoidescapitatus >600 x x Anomalinoidespraeacuta >2000 x Bolivinoides delicatulus <2000 x x x Buliminairapendens 500- 2000 x Bulimina semicostata > 1000 x x x
Bulimina trinitatensis <2500 x x x
Bulimina velascoensis > 1000 x Bulimina midwayensis <2500 x x Buliminella beaumonti <2000 x x x Buliminellagrata <2000 x x x Cibicidoides eocaenus <2000 x x Cibicidoidesgrimsdalei >1000 x x Cibicidoides havanensis >2000 x Cibicidoideshyphalus <2000 x x Cibicidoides laurisae >600 x
Cibicidoides mexicanus 600- 2000 x Cibicidoidespraemundulus > 1000 x x Coryphostomamidwayensis <2000 x Gaudryinapyramidata <2000 x x x Gyroidinoidesglobulosus >200 x Hanzawaia ammophila <2000 x x Lenticulinaspp. <2000 x x x Neofiabe llina jarvisi <2000 x Nuttallinellaflorealis > 1000 x x Nuttallidestruempyi > 1000 x x Osangulariaspp. <2000 x x Planulina costata <1500 Pullenia coryelli <2000 x Pyrarnidinarutida <2000 Quadrirnorphinapro funda >2000 Stensioinabeccariiformis <2000 x Tappaninaselmensis <2000 Uvigerina rippensis <2000 Vulvulina mexicana <2500 x Vulvulinaspinosa <2500 x Geophysicalbackstxipping 2000-2300m aSeeSites 605 Hulsbos , Saint-Marc , and van Hinte et al. for Site605; Norriset al. and Katz  for Site 1051;and Norris et al.  andM. E. Katz (unpublisheddata, 1998) for Sitel052. Paleodepthcriteria are basedon the commondistxibutions of speciesin the late Paleocene-earlyEocene [Tjasma and Lohmann,1983; van Morkhovenet al., 1986].Geophysical backstripping is for the earlyEocene [Hulsbos, 1987]. KATZ ET AL.: WHAT TRIGGERED THE PETM METHANE RELEASE? 561
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