Devensian late-glacial gas escape in the central North Sea

Continental Shelf Research Vol 12, No 10, pp 1097-1110, 1992

0278-4343/92 $5 00 + 0 00 Pergamon Press Ltd

Pnmed m Great Brttaln

Devensian Late-glacial gas escape in the central North Sea D. LONG* (Recetved 17 October 1990, m revtsed form 20 September 1991, accepted 15 January 1992) examination of deep-tow boomer records of the acoustically well-layered Witch Ground Formation in the central North Sea has revealed buned pockmarks The dlstnbutlon of these features is not even, there is a concentration at an lntraformational honzon, with a density of nearly a third of that of pockmarks at the present day seafloor This honzon is the mtermember boundary between the Witch and Fladen Members of the Witch Ground Formation It is dated to approximately 13,000 years BP and coincides with significant environmental changes Interpreted from mlcropalaeontologlcal analyses of the very soft clays Causes for such vanatlons in the density of buried pockmarks are considered, including changes in the sedimentation rate, the supply of gas to the sediments and possible chmatlc control on the migration rate of gas to the seabed Abstract--An

INTRODUCTION THE Witch G r o u n d Basin occupies a depression approximately 100 k m in d i a m e t e r in the central N o r t h Sea within the U . K . sector (Fig. 1). W a t e r depths increase f r o m 100 m a r o u n d its s o u t h e r n and western edges to a m a x i m u m of 150 m in the centre. T h e basin is floored with muds and sandy muds and is scarred by p o c k m a r k s with densities of up to 40 k m -2 (LONG, 1986). T h e basin Is lnfilled by an acoustically well-layered unit k n o w n as the Witch G r o u n d F o r m a t i o n . T h e sediments comprise very soft greenish grey to greyish b r o w n muds with subordinate sandy horizons, very rare small clasts and a b u n d a n t m o n o s u l p h i d e banding, particularly in the lower sections (STOKER et al., 1985). T h e f o r m a t i o n is up to 25 m thick; its top is the present day seabed. T h e base o f the f o r m a t i o n is m a r k e d by an Irregular erosion surface believed to have b e e n f o r m e d by sea-ice scouring ( S T O K E R and LONG, 1984). T h e Witch G r o u n d F o r m a t i o n has b e e n subdivided into three m e m b e r s on the basis of acoustic response and r e c o v e r e d lithologles ( S T O K E R e t al., 1985) T h e basal m e m b e r , called the F l a d e n M e m b e r , is up to 20 m thick. It exhibits a laterally extensive multilayered reflection pattern, particularly on the records of high frequency seismic profiling equipm e n t such as the b o o m e r or pmger. T h e s e reflectors drape over the irregular micro-rehef at the base of the f o r m a t i o n and with decreasing depth the Irregular base is " s m o o t h e d out" T h e top of the m e m b e r is generally c o n f o r m a b l e with the overlying Witch M e m b e r . R e c o v e r e d samples comprise very soft muds l n t e r b e d d e d with silty to sandy muds that show gradational contacts and have a b u n d a n t m o n o s u l p h i d e layers. T h e r e are rare granule clasts ( < 5 ram).

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On b o o m e r and pinger records the Witch M e m b e r is up to 10 m thick and varies from structureless to well-layered in appearance but is usually only faintly layered The sediments have a lower sand content than the underlying Fladen M e m b e r The small clasts and monosulphlde banding are virtually absent The uppermost m e m b e r is the silt rich Glenn M e m b e r which is usually less than 0.25 m thick but increases to 2.5 m thick in the centre of some p o c k m a r k s It IS believed that the Glenn M e m b e r represents Holocene reworking of the Witch M e m b e r by p o c k m a r k activity. BURIED

POCKMARKS

Pockmarks have been widely reported from the seafloor of the Witch G r o u n d Basin since they were first discovered in the area in 1970 (HOVLAND and JUDO, 1988). Both seismic profiles and sonar records indicate their presence on the seabed. They are typically 50-100 m across and 2-3 m deep but active pockmarks may be up to 400 m wide and 17 m deep. Seismic profiles of the Witch Ground Formation have also revealed the presence of buried p o c k m a r k s These can be defined as pockmarks that have ceased venting and have subsequently been covered by sediments_ A seismic survey [British Geological Survey (BGS) cruise 81/04] was shot over much of the Witch Ground Basin as part of a regional survey of the U K. continental shelf. The profiles were run at approximately 12 km spacing and most included high frequency surveying using Huntec deep-tow b o o m e r (BRErr, 1981) The records are of high quality and exhibit resolution of less than 1 ms A study of these records was undertaken to examine the distribution of buried pockmarks within the Witch G r o u n d Formation. As buried p o c k m a r k s are seen only on two-dimensional seismic sections and not on sidescan sonar, it is possible to confuse them with linear features such as Ice scour marks (Fig 2) However, surveys containing closely spaced lines such as rig site-surveys have revealed that these features are singular and could be circular, supporting the interpretation that they are buried pockmarks. Buried pockmarks show up well within zones of acoustic

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multilayering, particularly if the base of the p o c k m a r k is defined by a strong reflector (Fig. 3). However, they may be less apparent within a zone of acoustically structureless sediment. Sometimes the p o c k m a r k lnfill is opaque but the shape of the p o c k m a r k is revealed by the acoustically well-layered sediments adjacent to It (Fig 4) Occasionally there are hyperbolae within the structureless sections which may equate with the hyperbolae often seen beneath present day seabed pockmarks (Fig 5). Buried boulders occasionally produce similar acoustic features but are thought unlikely to be large enough in this area to cause the features seen in the Witch Ground Basin Examination of the b o o m e r records reveals that the distribution of buried pockmarks is not r a n d o m throughout the Witch Ground Formation, but is concentrated at particular levels (e.g Fig. 4) Buried pockmarks are recorded within both the Fladen and Witch M e m b e r s of the formation but the greatest concentration appears close to the Fladen/ Witch boundary (Fig 6) There are more buried pockmarks at this boundary level than in the entire Witch M e m b e r and many times those found in the Fladen m e m b e r The Fladen M e m b e r contains only a few buried features (Fig. 6a). These are usually towards the top of the unit There are also some features near the base which appear wider than the other buried pockmarks. These may more correctly be attributed to ice scouring similar to that at the base of the Fladen M e m b e r , where a sea-ice scoured surface separates the Witch G r o u n d Formation from the underlying Swatchway Formation (STOKERand LONG, 1984) There IS evidence of buried pockmarks within the Witch M e m b e r (Fig. 6b) but they do not occur in such densities as at the Fladen/Wltch boundary (Fig. 6c) However, due to the more structureless appearance of the Witch M e m b e r some features may have been overlooked. This method of assessing p o c k m a r k density seems reasonable as the distribution of surface pockmarks determined solely from the 81/04 b o o m e r records (Fig 6d) compares well with the density maps produced from side scan sonar records of many surveys, both BGS and commercial (LONG, 1986). ENVIRONMENTAL CONDITIONS AND DATING The change in acoustic appearance between the Fladen and Witch Members has been attributed to changes in the hydrographic regime, climatic conditions and sediment inputs (STOKERet a l , 1985) This was supported by a detailed analysis of a vibrocore sample (BGS sample n u m b e r 58 + 0 0 / 1 l I V E ) taken through a condensed sequence of the Witch G r o u n d Formation (LONG et a l , 1986) On the basis of sedlmentological and micropalaeontological studies, the Fladen M e m b e r was shown to be a distal glaclomarlne sediment deposited in shallow arctic water affected by sea-ice and with little or no contact with more t e m p e r a t e North Atlantic waters The Witch M e m b e r in sample 58 + 00/111VE contains more t e m p e r a t e microfossils and suggests a decrease or absence of sea-ice cover concomitant with a greater influx of warmer North Atlantic waters The upper part of the Witch M e m b e r shows a return to colder conditions with some sea-ice The identification of volcanic glass shards in the upper part of the Witch M e m b e r and their geochemical similarity with the 10,600 year old Vedde Ash Bed in western Norway confirms the upper cool period as of Loch L o m o n d Stadial (Younger Dryas) age (LONG e t a l . , 1986). Prior to the study on core 58 + 00/111VE almost all radiocarbon dates obtained from Witch Ground Formation sediments were based on whole sample analyses in order to obtain sufficent carbon for a standard radiocarbon analysis These results are susceptible to contamination by reworked carbon (HOLMES,1977) and may therefore be suspect The

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major problem of dating these sediments is the rarity of macroscopic shell material, hence radiocarbon dating of the thin-walled mollusc Portlandla arctica (Gray) has had to await the development of accelerator mass spectrometry methods. Even using these new techniques it was necessary to group shell material from a 20 cm slice of the core so as to obtain 0.1 g of calcium carbonate. Results on shell material taken just below the Fladen/ Witch boundary in vibrocore 58 + 00/111 (2 4-2 6 m depth) gave an age of approximately 13,250 years BP (13,255 + 184 years BP: 13,660 _+ 180 less 405 _+ 40 years for the effect of the marine carbon reservoir) (HEDGES et al., 1988) which correlates with dates obtained elsewhere for the climatic improvement marking the start of the Windermere (Aller0d) Interstadial. The location of sample 58 + 00/111 (58°34.5'N, 00°24.1'E) is in an area of few surface pockmarks (LONG, 1986) SO It IS reasonable to assume that the influences of lSOtopically light "old" carbon is minimal In the carbonate of molluscs found in the core. The radiocarbon ages determined are believed to represent the true age of the water masses in which the animals lived. The presence of "old" carbon probably only affects the radiocarbon age of organisms IH the very centre of methane seeps (PAULL et al., 1989). DISCUSSION Variations in the frequency of pockmarks within a sequence may be controlled by three factors, the supply of shallow gas, the migration of that gas to the seabed and the sedimentation rate of the area. If the gas is thermogenlc then it is reasonable to assume that, within the tlmespan of the last glaciation, gas migration from deep accumulations into the shallow zone has been steady. However if the gas is blogenlc in origin then the formation of shallow gas will be dependent on sediment influx and the chemical environment of deposition. Work in the North Sea has suggested that the gas is thermogenlc in origin (HOVLANDand JUDD, 1988) and that a steady supply of gas into the shallow sediments could be expected. Hence it IS likely that the constraints on pockmark density are restricted to the sedimentation rate and the migration of gas to the seabed. The correlation of the higher frequency of buried pockmarks with the period of rapid climatic improvement may be explained by examining these two variables. Assuming that the rate of migration of shallow gas to the seabed is constant and, by inference, the rate of pockmark formation also, then to explain the variations in the frequency of buried pockmarks, the rate of sedimentation must be variable If the change in the climate created a change in the sedimentation rate, for example, a rising sea-level will cause the retreat of sediment input sources, then the change would have had to cause an almost complete cessation of sedimentation to allow such a large number of pockmarks to be formed as recorded at the Fladen/Witch Member boundary This would suggest that the Fladen / Witch boundary was a palaeoseabed for longer than any other horizons within the Witch G r o u n d Formation. This could be analagous to the present day seafloor where 6C 14dating has suggested that there has been virtually no sediment input since about 8000 years BP (ERLENKEUSER, 1979, personal communication; JOHNSON and ELK1NS, 1979) If the present day density of pockmarks represents 8000 years of pockmark formation, comparing the relative densities of present day seabed pockmarks with that of the Fladen/ Witch boundary pockmarks permits an estimate to be made of the time span that, that horizon represents. Comparison of numbers south of 58°20'N suggests that the Fladen/Witch boundary has

Devenslan Late-glacialgas escape

1109

a density approximately 28% of that of the present day seabed. This would imply a time interval of approximately 2250 years (although this is probably an underestimate due to differential recognition; seabed pockmarks are more easily recognized than buried features). If the boundary had been a stable sea floor for 2250 years, deposition would have recommenced about 11,000 years BP during the onset of the Loch L o m o n d stadial However, such a time period does not fit with the micropalaeontological evidence which suggests that a sustained period of sedimentation during lnterstadial conditions occurred after the Fladen/Wltch boundary. Also, If sedimentation did not begin again until 11,000 years BP this would also imply that about 2.5 m of sediments were deposited in the 400 years prior to deposition of the Vedde Ash Bed, a surprisingly high sedimentation rate. If there was a negligible sedimentation rate over a period of 2250 years, then it could be expected to be evident in the shear strength properties of the horizon by an increase in the consolidation of the sediments. No such increase has been reported in any of the many measurements taken at this level from many cores analysed from the Witch Ground Basin It is therefore presumed that changes m sedimentation rate are not sufficient to totally explain the high density of buried pockmarks at the Fladen/Witch boundary and that changes in the rate of gas migration must also be involved An increase in the sedimentation rate could also explain the very low frequency of buried pockmarks within the Fladen member This might be supported by other radiocarbon dates from vlbrocore 58+00/111 (HEDGES et a l , 1988) However, results on a nearby, less condensed sequence m BH77/2 suggest a much more complex pattern of sedlmentaUon rates with accelerator radiocarbon dates extending back to approximately 22,000 years BP and amino-acid raclmlzation ratios suggestive of early-Devenslan ages for the Fladen Member (H.-P. Sejrup, personal communication), implying that sedimentation rates were not particularly high. The opposing control on pockmark frequency is the migration of gas to the seabed If sedimentation was at a near constant rate then there would need to have been significant changes m the flux of the gas, particularly between the time of deposition of the Fladen Member and the Fladen/Wltch boundary sediments The most significant climatic control on gas flux would have been temperature An increase in temperature would marginally decrease the viscosity but this would not have been sufficient to explain the differences in pockmark density However, if the temperatures had been low enough during the late Devensian, ice is hkely to have formed in the pore spaces below the seabed as permafrost lenses extending out from the shoreline of an adjacent tundra land area. Also, had the seafloor been exposed dunng glacial condmons sometime prior to the deposition of the Witch Ground Formation, ground ice would have formed and could have remained when the area became flooded as the Witch Ground Sea transgressed The subseabed permafrost ice lenses would have provided an impenetrable barrier to shallow gas migration, and acted as a cap rock to gas accumulations It is envisaged that as the climate improved and marine temperature rose, the subseabed permafrost melted permitting increased gas migration to the palaeoseabed and the formation of pockmarks. The release of accumulated gas would explain the decrease m buried pockmark numbers within the Witch Member with respect to the Fladen/Wltch boundary. This hypothesis suggests that towards the end of the Late Devenslan, conditions in the Witch G r o u n d Basin were similar to that offshore arctic Canada and Siberia today, where submarine rehct permafrost has been indentified in shallow water. The hypothesis is supported by measurements of bubbles trapped in the Greenland and Antarctic ice

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D LONO

c o res wh i ch r e v e a l a r a p i d i n c r e a s e in m e t h a n e in t h e a t m o s p h e r e at t h e o n s e t of d e g l a c i a t i o n to a b o u t d o u b l e the typical c o m p o s i t i o n d u r i n g glaciations (STAUFFER et al., 1988). This s u d d e n i n c r e a s e in t h e r a t e o f m e t h a n e r e l e a s e into t h e a t m o s p h e r e p r o v i d e s a p o s i t i v e f e e d b a c k into t h e c l i m a t e m o d e l a n d has b e e n used to ex p l ai n t h e r ap i d rise in t e m p e r a t u r e at the e n d o f a glaciation (NISBET, 1990). CONCLUSIONS D a t i n g o f the W i t c h G r o u n d F o r m a t i o n confirms that t h e r e h a v e b e e n significant c h a n g e s in the s e d i m e n t a t i o n r a t e within the W i t c h G r o u n d Basi n during the last d e g l a c l a t i o n , in p a r t i c u l a r c o m p a r i n g pre- a n d post-10,000 years ago. T h e r e h a v e also b e e n c h a n g e s in the r a t e o f p o c k m a r k activity, m o s t n o t a b l y with a p e r i o d o f i n c r e a s e d gas r e l e a s e a b o u t 13,000 years ago. It is suggested that the d e g r a d a t i o n o f s u b s e a b e d p e r m a f r o s t ice lenses i n c r e a s e d the flux causing the h i g h e r density o f b u r i e d p o c k m a r k s n o t e d at t h e F l a d e n / W l t c h M e m b e r b o u n d a r y h o r i z o n . P r i o r to its m e l t i n g , g r o u n d ice h a d t r a p p e d h y d r o c a r b o n gases n e a r to t h e s e a b e d . F u r t h e r dating o f m sttu shell m a t e r i a l w o u l d h e l p to r e s o l v e th e c o n t r o l s on L a t e - g l a c i a l p o c k m a r k activity in the W i t c h G r o u n d Basin. Acknowledgements--The data on which this work is based were collected as part of the Offshore Regional Mapping Programme funded by the Department of Energy Helmut Erlenkeuser and Hans-Petter Selrup are thanked for the supply of their, as yet unpublished, age determinations of material from the Witch Ground Formation This paper is published with permission of the Director, British Geological Survey (NERC) REFERENCES BRE'I'r C P (1981) Operations Report on Prolect 81/04, a Regional Geophysical Survey m the northern North Sea Manne Geophysics Unit Report, 120, Institute of Geological Sciences, Contenental Shelf Division, 15 pp. ERLENKEUSERH (1979) Environmental effects on radiocarbon in coastal manne sediments In Radiocarbon dating, R BEGGERand H E SUESS,editors, Proceedings of the Ninth International Conference, Los Angeles and La Jolla, 1979, University of Cahforma Press, pp 453-469 HEDGES R E M , R A HOOSLEY,I. A_ LAW, C PERRYand E HENDY(1988) Radiocarbon dates from the Oxford AMS system Archaeometry datelist 8 Archaeometry, 30, 291-305 HOLMESR (1977) Quaternary deposits of the central North Sea, 5 The Quaternary geology of the U K sector of the North Sea between 56° and 58°N Report of the Institute of Geological Sciences, 77/14, 50 pp HOVEANDM and A. G JUDD(1988) Seabed pockmarks and seepages impact on geology, biology and the manne envzronment, Graham and Trotman, London, 293 pp JOHNSONT C, and S R ELKINS(1979) Holocene deposits of the northern North Sea evidence for dynamic control of their mineral and chemical composition Geologze en Mqnbouw, 58, 353-366 LONGD- (1986) Seabed sedtments, Fladen Sheet 58°N~90° British Geological Survey, 1 250 000 map series LONGD , A BENT,R HARLANO,D_ M GaEGORY,D K GRAHAMand A C MORTON(1986) Late Quaternary palaeontology, sedimentology and geochemistry of a vibrocore from the Witch Ground Basin, central North Sea Marme Geology, 73, 109-123 NISBETE G. (1990) The end of the ice age Canadian Journal of Earth Sctences, 27,148-157 PAULLC K , C S MARTENS,J P CHANTON,A C NEUMANN,J COSTON, A J. T JULLand L J TOOLIN(1989) Old carbon in hying organisms and young CaCO3 cements from abyssal brine seeps Nature, 342,166-168 STAUFFERB , E LOCHBRONNEB,H OESCHGERand J SCHWANDER(1988) Methane concentraUon in the glacial atmosphere was only half that of the pre-lndustnal Holocene Nature, 332, 812-814 STOKERM S and D LONG(1984) A relict ice-scoured erosion surface m the central North Sea. Marine Geology, 61, 85-93 STOKERM S , D LONGand J A FYFE(1985) A rewsed Quaternary strattgraphyfor the centralNorth Sea Report of the British Geological Survey, 17/2, 35 pp