Geo-Marine Letters (1999) 19 : 29}37 ( Springer-Verlag 1999

O. Eldholm ' E. Sundvor ' P. R. Vogt ' B. O. HjelstuenK. Crane ' A. K. Nilsen ' T. P. Gladczenko

SW Barents Sea continental margin heat flowand Has kon Mosby Mud Volcano

Abstract The high thermal gradient and heat #ow'1000 mW m~2 on Has kon Mosby Mud Volcano areascribed to rapid transport of pore water, mud, and gas ina narrow, deep conduit within a 3.1-km-thick glacial sedi-ment unit. The instability is caused by rapid loading ofdense glacial sediments on less dense oozes. Changes inpressure}temperature conditions by sudden, large-scaledownslope mass movement may induce structural defor-mation, opening transient pathways from the base of theglacial sediments to the sea #oor. This model may alsoexplain slope maxima elsewhere on the margin.

Introduction

The regional Norwegian}Greenland Sea thermal "eld was"rst described by Langseth and Zielinski (1974) based ona limited number of measurements. Subsequently, theacquisition in 1983 of a Norwegian heat #ow probe led toa series of surveys in which temperature gradients andthermal conductivities were measured along selectedocean basin and continental margin transects. All avail-able data are part of a freely available database, and theregional thermal "eld have recently been described bySundvor et al. (in press). A most intriguing phenomenon isthe observation of local continental slope maxima on theNorway}Svalbard margin, i.e., greatly enhanced local

O. Eldholm (|) ) A. K. Nilsen ) T. P. GladczenkoDepartment of Geology, University of Oslo, POB 1047 Blindern,N-0316 Oslo, Norway

E. SundvorInstitute of Solid Earth Physics, University of Bergen, Allegt. 41,N-5007 Bergen, Norway

P. R. Vogt ) K. CraneNaval Research Laboratory, Washington, DC 20375, USA

B. O. HjelstuenA. S. Geoconsult, Nedre As stveit 12, N-5083 "vre Ervik, Norway

areas of high heat #ow with respect to the surroundingregional "eld (Sundvor and Eldholm 1992; Vogt and Sun-dvor 1996).

The discovery of two 1-km-diameter circular high back-scatter objects at &1250 m (M1) and &1700 m (M2) onthe SW Barents Sea margin (Figs. 1 and 2) during NavalResearch Laboratory SeaMARC II side-scan sonar andswath-bathymetry cruises in 1989 and 1990 (Crane et al.1995) was followed by sampling and heat #ow measure-ments of M1 during the R/<Has kon Mosby cruise in 1995.A core on its #ank revealed gas and gas hydrate as well ashigh heat #ow, suggesting present-day gas venting at thesea #oor (Vogt and Sundvor 1996). Subsequent site invest-igations during the R/< Professor ¸ogachev cruise in 1996established that M1, named Has kon Mosby Mud Volcano(HMMV), oozed mud and seeped gas (Vogt et al. 1997)and that it is associated with very high '1000 mW m~2,heat #ow. In fact, the heat #ow is among the greatestreported in the ocean away from plate boundaries andhotspots. Here, we discuss the SW Barents Sea continentalmargin thermal "eld with emphasis on the HMMV slopemaximum and its relations to the observed mud volcanism.

Regional thermal field

The Norwegian}Greenland Sea heat #ow documentsa "rst-order relationship with age of the oceanic crust(Sundvor et al. in press). The mid-ocean ridge provinceexhibits heat #ow values generally '100 mW m~2, whiletypical values on crust '30 Ma are 50}75 mW m~2.The thermal conductivity in near-sea-#oor sedimentscommonly ranges from 0.85 to 1.15 Wm~1 K~1. In gen-eral, the conductivity is higher on the margins than in theocean basins, although there are local areas of enhancedthermal conductivity, for example, on the BarentsSea}Svalbard shelf and upper slope (Fig. 2). Sundvor et al.(1999) relate these di!erences to the higher content ofpelagic sediments in the ocean basins, whereas local areas

Fig. 1 SW Barents Sea marginstructural setting (Faleide et al.1993), heat #ow mesurementdistribution and regional heat#ow contours (Sundvor et al.in press). SFZ: Senja FractureZone; HFZ: Hornsund FaultZone; HB: Harstad Basin; TB:Troms+ Basin; SB: S+rvestnagetBasin; HMMV: Has kon MosbyMud Volcano

of enhanced conductivity may re#ect active erosion ornondeposition on the continental margins.

The southern segment of the Senja Fracture Zone (SFZ)is the main structural feature on the sheared SW BarentsSea margin (Fig. 1) (e.g., Hjelstuen et al. 1999). It demar-cates a sharp continent}ocean boundary separatingdown-faulted continental blocks from 55}33 Ma oceaniccrust. The continent-ocean transition is only 10}20 kmwide, and the oceanic crust adjacent to SFZ has a highaverage oceanic layer 2 velocity, 5.7}6.4 km s~1, and isslightly thinner than standard models (Jackson et al.1990).

Relative to the scatter in individual heat #ow measure-ments observed elsewhere in the Norwegian}GreenlandSea, the values on oceanic crust west of SFZ are surpris-ingly consistent (Sundvor et al. in press). For example, thenorth-trending transect 1 on the lower slope (Fig. 1) re-veals a very gentle increase in heat #ow with crustal age,from 53 to 75 mW m~2 (Fig. 3). The heat #ow is alsoquite stable, averaging 63.5$5.3 mW m~2, in transect2 across SFZ along the magnetic anomaly 13 lineationtrend (Fig. 1). Only few values exist at the SFZ (Figs. 1 and3), but several values are (50 mW m~2. Farther east inthe Barents Sea, shallow drill hole measurements yieldvalues from 54 to 74 mW m~2. The SFZ proper does notexhibit an appreciable heat #ow signature, but reliable

values of 121}122 mW m~2 are measured in transect2 (Fig. 3). Slope maxima elsewhere on the Norway}Svalbard margin indicate that such local anomalies mayexist without being of structural origin.

Thermal measurements

The thermal gradient measurements during the 1995 and1996 cruises were made by the Norwegian heat #ow probemanufactured by Lamont-Doherty Geological Observ-atory. The instrument employs thermistors mounted onoutriggers several centimeters away from a solid lancethat is attached to a corehead weight. It also recordsbottom-water temperature and lance tilt. The thermalgradient measurements were complemented by gravityand box cores at or near the probe locations. Thermalconductivities were measured in the gravity core sedi-ments at 10-cm intervals by the needle-probe method(Herzen and Maxwell 1959). The same procedure wasapplied to vertical &&subcores'' extracted from the boxcores. At thermal gradient stations without sedimentcores, conductivities from neighboring stations were usedto calculate the heat #ow. Tables 1 and 2 list the data fromthe two cruises, including some measurements outsidethe SW Barents Sea margin.

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Fig. 2 Distribution of thermalconductivity measurements andaverage values for the SWBarents Sea margin and LofotenBasin physiographic provinces asde"ned by Sundvor et al. in press.Annotations in Figure 1

The "ve 1995 thermal gradient measurements nearHMMV showed normal values, 54}71 mKm~1, exceptfor heat #ow station 20 on the northern #ank whichmeasured 314$10 mKm~1 (Table 1, Figs. 4 and 5). Ther-mal conductvities are also normal for the SW Barents Seamargin, and 12 measurements averaged 1.10 Wm~1K~1.However, the two cores nearest the mud volcano, stations72 and 86, yielded values of 0.95 and 1.20 Wm~1K~1corresponding to heat #ow values of 298 and 377 mWm~2,respectively. However, the lower conductivity value isconsidered most representative. The improved targetingof the central mud volcano in 1996 led to even greaterthermal gradients. In particular, heat #ow stations 5a}bmeasured 817 and 637 mKm~1 on the western moat(Table 2), and the gradient exceeded the thermistor calib-ration range at stations 6a}d in the central part ofHMMV. However, temperature measurements in thesediment cores just after retrieval showed a maximum of163C, and gradients reaching 10000 mKm~1 have beenestimated by Vogt et al. (1997) in the thermal &&eye'' of themud volcano.

The second high backscatter object, &50 km south-west of HMMV, was also surveyed during the 1996 cruisewithout revealing any active ventlike features. Three heat#ow stations (4a}c, Table 2) were deployed, of which

station 4b shows a local maximum of 108 mW m~2(Figs. 3 and 4).

Tables 1 and 2 show that few thermal conductivityvalues correspond directly to gradient measurements, i.e.,most values are not &&in-situ.'' Similarly, exsolution andhydrate decomposition in HMMV cores prior tomeasurement also cause deviations from in-situ condi-tions. Nonetheless, the magnitude and character of thethermal gradient distribution indicate that the corre-sponding error in heat #ow is, in a relative sense, small.Venting from HMMV is also con"rmed by measurabletemperature anomalies and anomalously high dissolvedmethane in the water column implying an at least 100 mhigh hydrothermal plume (Vogt et al. 1997), a heightcomparable to plumes over some hot vents on themid}oceanic ridge (Humphris et al. 1995).

In summary, these observations suggest a pronouncedthermal maximum over the central mud volcano charac-terized by very steep lateral gradients in the sea #oorsediments for a distance of 6}700 m towards the periph-ery, reaching background level values just beyond thecircumferential moat. In terms of thermal character,the HMMV resembles mud volcanos on the Barbadosaccretionary wedge (Foucher et al. 1990). An anomaly ofthis kind will also induce and maintain a thinspot in the

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Table 1 Has kon Mosby cruise 1995

HF Area Core N lat E long WD (m) TM TG TC HF(mKm~1) (Wm~1K~1) (mW/m~2)

1 SM 7 79 45.0 07 00.0 820 3 73$4 1.09 802 SM 10 79 06.9 06 00.2 1238 4 116$8 0.90 1043a SM 15 79 08.4 05 11.8 1342 4 145$9 0.85 1233b SM 15 79 08.5 05 12.0 1342 4 140$10 0.85 1194 SM 18 77 00.4 09 04.8 2054 4 122$9 0.91 1045 SM 24 75 27.5 12 56.6 1556 4 76$3 0.80 616 SM 30 74 50.5 14 40.4 1537 4 69$5 0.89 617 SM 33 74 39.0 11 26.4 2402 5 68$4 1.11 758 SWB 35 74 24.4 12 14.2 2264 4 75$4 1.06 809 SWB 39 74 23.7 12 43.7 2202 4 59$4 1.20 71

10 KR 42 73 50.7 09 16.0 2455 5 193$6 1.16 22411 KR 45 73 39.4 10 11.5 2267 5 80$3 1.18 9412 KR 47 73 30.8 08 52.6 2459 5 126$7 1.20 15113 LB 53 73 00.4 10 04.9 2201 5 76$5 1.10 8414 MR 55 73 00.8 08 46.8 2360 5 128$5 1.13 14515 MR 57 73 00.7 08 35.6 2360 5 109$5 1.05 11416 SWB 61 73 12.5 13 45.9 1192 5 62$3 1.08 6717 SWB 62 73 22.3 13 45.2 1253 5 60$3 1.14 6818 SWB 66 72 29.4 14 45.2 709 *19 SWB 67 72 17.4 14 45.5 1004 4 54$3 1.02 5520 HMMV 70 72 00.5 14 43.7 1260 4 314$10 0.95 29821 SWB 79 71 56.4 14 04.0 1506 4 71$3 0.99 7022a SWB 82 71 58.8 14 54.4 1147 5 68$3 1.20 8222b SWB 82 71 59.0 14 54.4 1155 5 54$3 1.20 6522c SWB 82 71 59.4 14 50.7 1215 4 56$3 1.20 6723 SWB 88 70 28.7 15 08.7 2310 4 63$3 1.09 69

Thermal gradient (TG) and thermal conductivity (TC) measurements, and calculated heat #ow (HF) values. All cores are gravity cores of&4.7 m penetration. Positions in degrees and minutes. WD: water depth; TM: number of thermistors in sediment; SM: Svalbard margin;SWB: SW Barents Sea margin; KR: Knipovich Ridge; MR: Mohns Ridge; LB: Lofoten Basin*Temperature out of thermistor range.

Fig. 3 SW Barents Sea margin heat #ow and bathymetry transects.Heat #ow values, "lled circles, are projected onto the transects inFigure 1. Maximum projection distances are 5 and 2 km for tran-sects 1 and 2, respectively (Sundvor et al. in press). Approximatelocation of magnetic sea-#oor spreading anomalies shown at thebase. SFZ: Senja Fracture Zone

hydrate stability zone, and the absence of hydrate inthe thermal eye of the HMMV suggests that the hydratestability zone may rise to the sea #oor at this point.

Origin of the Has kon Mosby thermal anomaly

The question of why local heat #ow anomalies exist on thecontinental slope was discussed by Vogt and Sundvor(1996), who evaluated several models to explain slopemaxima at 600}800 m water depths on the Norway}Sval-bard margin. These models consider deep crustal fractures,igneous activity, gas hydrates, changes in oceanographiccurrent regime, and sediment dewatering. To evaluatepossible heat generation processes responsible for the

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Table 2 Professor ¸ogachev cruise 1996

HF Area Core, N lat E long WD (m) TM CP TG TC HFD (m) (m) (mKm~1) (Wm~1K~1) mW/m~2

4a SWB 17 G-117 71 43.4 13 31.9 1754 3 2.90 79.3$23 0.95 754b SWB 17 G-372 71 43.2 13 32.1 1785 3 2.90 113.7$45.9 0.95 1084c SWB 17 G-746 71 43.0 13 32.0 1765 3 2.90 34.9$3.2 0.95 335a HMMV 36 B-296 72 00.4 14 44.0 1228 2 0.40 816.7** 1.28 10455b HMMV 36 B-507 72 00.3 14 44.2 1228 4 0.40 637.3$94.3 1.28 8155c HMMV 36 B-784 72 00.3 14 44.8 1249 3 0.40 67.6$6.4 1.28 865e HMMV 36 B-1320 72 00.3 11 45.8 1218 3 0.40 51.4$5.1 1.28 666a HMMV 27 G-186 72 00.4 14 42.2 1223 2.80 * 1.186b HMMV 27 G-460 72 00.3 14 43.0 1247 2.80 * 1.186c HMMV 36 B-219 72 00.4 14 43.4 1220 0.40 * 1.286d HMMV 36 B-376 72 00.3 14 43.7 1233 0.40 * 1.287a HMMV 27 G-230 72 00.3 14 42.6 1224 3 2.80 140.9$21.8 1.18 1667b HMMV 27 G-342 72 00.4 14 42.7 1224 3 2.80 167.0$13.4 1.18 1977c HMMV 36 B-496 72 00.6 14 42.8 1233 3 0.40 108.1$14.9 1.28 138

HMMV 47 G 72 00.1 14 40.8 &1230 2.70 1.131a MM 3 B-564 64 20.7 05 36.9 660 4 0.50 112.1$17.2 1.31 1471b MM 3 B-564 64 20.7 05 36.9 687 3 0.50 91.4$1.8 1.31 1202a MM 9 G-0 64 49.4 04 18.4 967 5 3.00 59.7$1.9 0.86 512b MM 8 G-0 64 49.1 04 18.5 968 5 3.10 56.2$1.8 0.86 483a VP 12 G-228 65 52.6 03 52.5 1340 4 2.90 45.4$1.2 0.99 453b VP 12 G-228 65 52.6 03 51.9 1340 4 2.90 65.9$0.8 0.99 65

MM 4 B 64 18.6 05 06.7 1249 0.40 0.77MM 5 B 64 18.4 05 06.7 1262 0.35 0.78KR 49 B 76 55.0 07 12.4 &3000 1.30 0.80SM 70 B 80 17.7 12 02.7 198 0.40 1.39SM 81 B 78 10.1 13 54.2 395 0.40 0.98

Thermal gradient (TG) and thermal conductivity (TD) measurements, and calculated heat #ow (HF) values. Conductivities refer to the coreclosest to the heat #ow station. D: distance between heat #ow and core stations; G: gravity core; B: box core; CP: core penetration.SM: Svalbard margin; SWB: SW Barents sea margin; MM: M+re margin; VP: V+ring Plateau; KR: Knipovich Ridge*Temperature out of thermistor range**Minimum value, lower core temperature out of thermistor range

Fig. 4 HMMV (insert) andbackground heat #ow values(Tables 1 and 2). LOC: limit ofwell-de"ned oceanic crust inseismic pro"les (Faleide et al.1993)

HMMV maximum, we analyze the regional geologicalsetting, the regional and local heat #ow, as well as thegeological data obtained during the 1995}1996 cruises onand near the mud volcano. Key boundary conditions are

(Hjelstuen et al. 1998): (1) the HMMV location withina large slide scar on the post-Late Pliocene glacial BearIsland Fan underlain by &34- to 37-Ma-old oceaniccrust; (2) the &6-km-thick, and unfaulted post-opening

33

Fig. 5 HMMV (insert) andbackground thermalconductivity values (Tables 1 and2). LOC: limit of well-de"nedoceanic crust in seismic pro"les(Faleide et al. 1993)

sediment wedge; and (3) the &3.1-km-thick glacial se-quence. In fact, the glacial wedge represents 50}70% ofthe total sediment volume on the SW Barents Sea margin,documenting a dramatic late Pliocene}Pleistocene in-crease in sedimentation rates. Furthermore, numeroussmall, steep features, interpreted as mud diapirs, have beenobserved in the side-scan data just outside the slide valley(Crane et al. 1995; Vogt et al. 1997).

A potential heat-generating mechanism is #uid migra-tion from deep crustal heat sources, for example, mag-matic intrusions possibly related to structural reactivationalong the SFZ (Crane et al. 1982; Sundvor and Eldholm1992). The absence of enhanced seismicity on the uppercontinental slope has been considered to argue againstsuch structural reactivation (Vogt and Sundvor 1996).However, the entire region west of the shelf edge is seismi-cally active, but the seismicity is related to rapid glacialsediment loading (U. Byrkjeland personal communica-tion). Nonetheless, the location of HMMV over oceaniccrust without clear evidence of active faulting or intrusiveactivity on either side of the fracture zone does not favordeep local heat sources. Furthermore, the moderate anduniform level of heat #ow adjacent to the continent}oceanboundary (Figs. 1 and 3) rule out a deep regional heat-#ow source in the oceanic lithosphere.

Large-scale late Pliocene}Pleistocene downslope massmovements have played a key role in the construction ofthe glacial SW Barents Sea margin (Laberg and Vorren1993; Fiedler and Faleide 1996; Elverh+i et al. 1997). Inthis context we note that several heat #ow slope maximahave been found within the huge Storegga slide on theM+re margin (Sundvor et al. in press), and infer that thespatial correlation of slope maxima and huge slide scarsindicate a causal relationship. However, detailed localsurveys are required to con"rm whether the thermal

anomalies are associated with mud volcanism or moresubdued local escape of pore water. Although the removalof near-sea-#oor sediments and subsequent cooling byseawater will increase the near-sea-#oor thermal gradient,the e!ect is transient and a linear gradient in the upper fewmeters will be reestablished in 100}1000 years (Vogt andSundvor 1996). These inferences point towards intrasedi-mentary heat sources, thus we focus on the SW BarentsSea margin postopening sedimentary sequences.

The Cenozoic depositional history of the Norway}Sval-bard margin comprises three main stages, all associatedwith distinct climatic regimes. The warm Paleocene}middle Eocene interval, characterized by sedimentation inshallow and restricted basins, was followed by a period ofmargin subsidence, modest sedimentation, and climatedeterioration that lasted until Late Pliocene time. Theonset of the Northern Hemisphere glaciation at &2.6 Maled to greatly increased erosion and sedimentation ratesand to major margin progradation. Data on the V+ringmargin farther south show pronounced changes in lithol-ogy and physical properties at the base of the glacialsequence. The underlying biosiliceous oozes have higherporosity and water content and lower density than thesandy glacial muds above. These observations made Hjel-stuen et al. (1997) propose that the Late Pliocene}Pleis-tocene progradation induced di!erential loading of thepreglacial oozes, creating a density inversion and sedi-mentary instability what mobilized the sediments andcaused some structural deformation. The unstable oozes,accompanied by #uids and probably also gas, weresqueezed up along zones of weakness through the relative-ly thin glacial cover in front of the prograding glacialdepocenter. This process, in turn, is probably responsiblefor the extensive areas of mud diapirs, interspersed withmud volcanos, at 1200}1500 m water depth on the V+ring

34

Fig. 6 Schematic model for HMMV pore water, gas and mud en-trapment and transport based on the seismic section of Hjelstuenet al. (1999)

Plateau. No local heat #ow anomalies have yet beenrecorded within the diapir "elds, however.

Similar slope processes are likely also on the SWBarents Sea margin. However, the thicker glacial unit mayimply that the glacial sediments form a more impenetrablelid than on the margin farther south. Thus, diapirism andassociated phenomena will be expected on the most distalparts of the Bear Island Fan in the NE Lofoten Basin, anarea that is experiencing brittle, most likely load-induced,crustal deformation (U. Byrkjeland personal communica-tion). In fact, a single channel pro"le recorded by ;SSHayes in 1977 shows this type of deformation. On theother hand, the more frequent and more voluminousglacigenic debris #ow activity (Vogt et al. 1993; Dowdes-well et al. 1996) throughout the entire glacial period mayhave destroyed or masked such features.

The seismic pro"les show deformation below HMMVextending at least to the base of the glacial section, belowwhich the seismic resolution is relatively poor (Hjelstuenet al. 1999). The base of the glacial unit may act asa barrier for the unstable and mobile underlying #uids,gases and/or oozes, thus inducing local overpressurezones. Vogt and Sundvor (1996) have estimated that the'340 mWm2 heat #ow anomaly discovered in 1995 maybe explained by pore waters convecting at rates on theorder of 10 cmyr~1 from depths corresponding to the topof the preglacial unit. Moreover, we estimate the #uidpressure at this source level below 1.25 km water(1030 kg m~3) over 3.1 km glacial mud at 2100 kgm~3(Fiedler and Faleide, 1996), to &78 MPa; and the temper-ature to&1833C, for an assumed gradient of 60 mKm~1.The abundant biogenic gas on the Norway}Svalbardmargin also suggests the likelihood of trapped biogenic,possibly also thermogenic methane and higher order hy-drocarbons, at several depth levels, and we note thatcarbon isotopes from CH

4samples (gas and hydrate) at

HMMV are at the low end of the thermogenic "eld.A major slide on the upper slope, i.e., over the main

glacial depocenter, will unload the underlying Cenozoicsediments and basement. Consequently, the pore pressuregradients in the glacial unit will be maintained or in-creased. If the unloading induces structural adjustments,new pathways for gravity-driven #ow of pore water maybe opened, in some cases allowing the #uid to reach thesurface (Fig. 6). Similarly, the glacial}preglacial gravi-tational instability could induce structural adjustmentsthat provide conditions for mud and gas transport. Thisadvecting system, or &&miniplume,'' is a very favorableagent for transporting heat. Moreover, if the #uid rises fastenough, the cooling becomes adiabatic, and the advectingsystem is maintained by the ambient temperature in thesediments without requiring &&new'' heat sources such asigneous intrusions. We suggest that the local heat #owanomaly at HMMV is caused by #uid expulsion at the sea#oor, enhanced by the coeval venting of mud and gas.Evidence for structural deformation is found as circularnormal, collapse-type faults at the outer boundary of the&200-m-wide moat around HMMV and, more impor-

tantly, as &50- to 100-m-long linear fractures at theperiphery, suggesting brittle-type deformation of hydratesthat have cemented the near-sea-#oor sediments (Vogtet al., 1997).

A similar model was discussed in general terms byVogt and Sundvor (1996), who assumed maximum dew-atering in areas of maximum sediment thickness, i.e.,on the upper slope. Here, we relate the process to theprobable gravitational instability at the base of the glacialunit and propose that the properties of the glacial lidwill inhibit #uid migration to the sea #oor unless a!ectedby a triggering event, for example, sudden unloadingcaused by large-scale slumping (Fig. 6). A source in thepreglacial sediments, as we propose here, also solves the&&carbon problem,'' in that glacial deposits tend to be poorin TOC and therefore poor methane sources (Vogt et al.1997).

Decomposition of gas hydrates may trigger mass wast-ing on the continental slopes by liberation of gas trappedbeneath the hydrate. Gas evolution in turn weakens theoverlying sediments (Carpenter 1981). The SW BarentsSea continental slope lies well within the hydrate stability"eld, and the 500- to 3000-m water depth region is con-sidered a hydrate likelihood area by Max and Lowrie(1992). The extent of the hydrate stability zone is mainlydependent on water depth and the thermal gradient in thesediments, and Vogt and Sundvor (1996) calculateda stability zone thickness on the order of 600 m from thethermal gradient data. This zone roughly corresponds to

35

the base of glacial sequence GIII (Fig. 6); thus the occur-rence of gas hydrates is restricted to the upper glacialsediments. Gas hydrates have been inferred from bottomsimulating re#ectors in several locations on the Norway}Svalbard margin (Mienert and Bryn 1997), but no suchre#ectors have been traced near HMMV (Hjelstuen et al.,1999). However, gas hydrates have been cored in areaswithout such seismic images (Booth et al. 1995). A stablehydrate layer will be a!ected by slumping, both by re-moval of near-sea-#oor hydrate-"lled sediment and byreduced pressure potentially causing decompositionand release of free gas trapped below the base of thehydrate layer. This mechanism is conceptually similar tothe sediment dewatering mechanism described above, themain di!erence being the nature and depth of the barriersinhibiting upward migration of gas and pore water(Fig. 6). In addition, lithological changes related to theinterglacial-glacial cyclicity may locally trap #uids andgas (e.g., Shipboard Scienti"c Party 1987). Thus, leaksfrom all three potential trap types may develop dueto sediment deformation induced by an extensive andvoluminous slide.

Concluding remarks

The heat #ow anomaly and the heat #ow distribution isconsistent with many other observations decribed in thisvolume documenting the limited extent of HMMV, andthe very local nature of its feeder. It is an enigma, perhaps,that this local hotspot is allowed to exist within a verycold and frigid environment. However, most of the heatfrom a narrow, local feeder for advecting and convectinggas- and mud-"lled #uids will dissipate in the overlyingwater column. Thus, the observations of hydrate-like fea-tures in the central plain of the mud volcano representeither the pre-HMMV gas hydrate environment orrefrozen hydrate, whereas the small central area withoutobvious hydrate indicators represents the feeder and itsassociated thermal plume in the water column (Vogt et al.1997). Consequently, there is a direct relationship of thedramatic HMMV thermal slope maximum and the pro-cesses causing sea-#oor venting and mud volcanism.

The fact that two 1-km-diameter circular objects wereidenti"ed in the backscatter data, while only one objectwas found during the 1996 cruise, may suggest that theventing is a transient and intermittent process. This infer-ence is supported by the absence of evidence for gasbubbles into the water column (Vogt et al. 1997). None-theless, the geological setting of the HMMV on a gashydrate-prone margin, characterized by mass wasting, byfrequent debris #ow as well as by recent mud #ows fromHMMV, combined with a time-variable plume #ux, maylead to a complex pattern of sealing and opening of localpathways for this gravity-driven plume.

Acknowledgments We thank the o$cers and crews and the par-tcipating scientists and technicians onboard F/S Has kon Mosby and

NIS Professor ¸ogachev for their e!orts and dedication. We aregrateful to the O$ce of Naval Research, US National ScienceFoundation, Russian Ministry of Science and Technology, andNorsk Hydro Research Centre for "nancial support. The projectwas completed while the senior author was on sabbatical at Institutefor Geophysics, University of Texas at Austin. Finally, we thankthe Research Council of Norway and Statoil for support of theNorwegian shore-based work.

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