The catastrophic final flooding o...
1 UDK 550.344.4(261.26)"633" Documenta Praehistorica XXXV (2008) The catastrophic final flooding of Doggerland by the Storegga Slide tsunami Bernhard Weninger1, Rick Schulting2, Marcel Bradtm��ller3, Lee Clare1, Mark Collard4, Kevan Edinborough4, Johanna Hilpert1, Olaf J��ris5, Marcel Niekus6, Eelco J. Rohling7, Bernd Wagner8 1 Universit��t zu K��ln, Institut f��r Ur- und Fr��hgeschichte, Radiocarbon Laboratory, K��ln, D, firstname.lastname@example.org 2 School of Archaeology, University of Oxford, Oxford, UK 3 Neanderthal Museum, Mettmann, D 4 Laboratory of Human Evolutionary Studies, Dpt. of Archaeology, Simon Fraser University, Burnaby, CDN 5 R��misch Germanisches Zentralmuseum Mainz, D 6 Groningen Institute of Archaeology, Groningen, NL 7 School of Ocean and Earth Science, National Oceanography Centre, Southampton, UK 8 Universit��t zu K��ln, Institut f��r Geologie und Mineralogie, K��ln, D ABSTRACT ��� Around 8200 calBP, large parts of the now submerged North Sea continental shelf (���Dog- gerland���) were catastrophically flooded by the Storegga Slide tsunami, one of the largest tsunamis known for the Holocene, which was generated on the Norwegian coastal margin by a submarine landslide. In the present paper, we derive a precise calendric date for the Storegga Slide tsunami, use this date for reconstruction of contemporary coastlines in the North Sea in relation to rapidly rising sea-levels, and discuss the potential effects of the tsunami on the contemporaneous Mesolithic popula- tion. One main result of this study is an unexpectedly high tsunami impact assigned to the western regions of Jutland. IZVLE���EK ��� Okoli 8200 calBP je velik del danes potopljenega severnomorskega kontinentalnega pasu (Doggerland) v katastrofalni poplavi prekril cunami. To je eden najve���jih holocenskih cunamijev, ki ga je povzro���il podmorski plaz na norve���ki obali (Storegga Slide). V ���lanku predstavljamo natan���ne datume za cunami Storegga Slide in jih uporabimo pri rekonstrukciji takratnih obal Severnega mor- ja, v ���asu naglega dviganja morske gladine. Dotaknemo se tudi mo���nih posledic cunamija za mezo- litske populacije. Glavni rezultat ���tudije je nepri���akovano mo���an vpliv cunamija na zahodni del Jut- landa. KEY WORDS ��� Mesolithic Doggerland Storegga Slide tsunami Introduction The hypothesis that a major tsunami was generated by an underwater slide off the west coast of Norway was first proposed by Svendsen (1985) and further elaborated in a large number of studies (e.g. Bonde- vik 2003 Bondevik et al. 1997 2003 2005 2006 Dawson et al. 1988 1990 1993 Grauert et al. 2001 Haflidason et al. 2005 Long et al. 1989 Smith et al. 1985 2004). As a result of detailed fieldwork (e.g. Bondevik et al. 1997 2003 2005 Smith et al. 2004), followed by comprehensive modelling studies (Har- bitz 1992 Bondevik et al. 2005), a comparatively large number of deposits on the coasts of Norway and eastern Scotland can now be safely attributed to the Second Storegga Slide tsunami. The generation of the tsunami apparently involved some 2400��� 3200km3 of material that spread across the North Atlantic sea floor, altogether covering an area of around 95 000km2 (Haflidason et al. 2005) ��� that is about the size of Scotland. Bryn et al. (2005) sug- gest the cause of the Storegga slide was a strong
B. Weninger, R. Schulting, M. Bradtm��ller, L. Clare, M. Collard, K. Edinborough, J. Hilpert, O. J��ris, M. Niekus, E. J. Rohling, B. Wagner 2 earthquake in the North Atlantic, but further inves- tigations are necessary to substantiate this hypothe- sis. Due to the large slide/slump volume and exten- sive reworking, the direct dating of the slide sedi- ments is no easy matter. Comprehensive analysis of a long (more than 50 14C-ages) series of AMS-radio- carbon ages for stratified basal post-slide sediments, processed on purposely chosen monospecific planc- tonic foraminifera (Neogloboquadrina pachyderma and Globigerina bulloides) to reduce the risk of re- working, give an (averaged) direct date for the main slide of 7250 �� 250 14C yrs BP (Haflidason et al. 2005). Traces of the corresponding Second Storegga Slide tsunami have been identified in many regions of the North Atlantic, with the best-studied locations on the coast of Norway and eastern Scotland. On the Nor- wegian coast, at locations directly opposite to the sub-marine landslide region, the tsunami had a ma- ximum runup of 10���12m. Further north, a runup of 6���7m is reconstructed. On the eastern coast of Scot- land typical runup heights exceed 3���5m (Smith et al. 2004). Storegga deposits are also known from the Faroes (Grauert et al. 2001) and the Shetland Islands, where runup exceeds 20m (Bondevik et al. 2005). Recent studies show that the tsunami proba- bly even reached the east coast of Greenland (Wag- ner et al. 2007). This would agree with modelling studies (Bondevik et al. 2005), according to which the wave front would have crossed the North Atlan- tic within 3 hours, with maximal elevation on the open ocean of 3m. The size of these waves, and their spread over such a large area, indicate that most of the volume of the slide was involved in the genera- tion of the tsunami (Bondevik et al. 2005). On the Norwegian coast, the arrival of the first wave would have been associated with a major water withdra- wal, corresponding to a predicted initial sea-level drop of 20m. The model also predicts that multiple waves should occur. This is confirmed for deposits probably laid down by the Storegga slide tsunami on the east coast of Greenland, where the grain-size composition, biogeochemical and macrofossil data indicate that the Loon Lake basin was inundated by at least four waves (Wagner et al. 2007). The effects of the tsunami on other North Sea coasts ��� and no- tably on Mesolithic Doggerland (Coles 1998) ��� have not yet been modelled. As a starting point for our studies towards the potential effects of the Storegga Slide tsunami in the southern North Sea, we assume that runup in this region is likely to have been around 3m (pers. comm. Bondevik 2007). Tsunami deposits The accurate dating of the Storegga Slide Tsunami represents a major challenge to established radiocar- bon methodology. As already recognised by Bonde- vik et al. (2006), the accurate radiocarbon dating of palaeotsunamis is problematic for three reasons: (1) erosion of the underlying strata, (2) redeposition of organic material within the tsunami deposit, and (3) redeposition of organic matter following the tsunami event. Due to the importance of these issues for ra- diocarbon dating, we begin with a brief description of the tsunami deposits under study on the coasts of Norway and Great Britain. Norway In Norway, the Storegga Slide tsunami deposits are typically recognised as a distinct layer of sand in peat outcrops, with an underlying and often sharply erod- ed surface (Bondevik et al. 1997 2003). Similar ob- servations have been made all along the eastern coast of Scotland, where the inferred tsunami depo- sits are readily recognised by a recurring sand layer within raised estuarine sediments that pass into peat in a landward direction (Dawson et al. 1993). This sand layer, both in Norway and Scotland (see be- low), contains a variety of chaotically redeposited organic materials, including twigs and bark. These are the samples, typically described as deriving from ���within the tsunami layer���, that were carefully selec- ted during field-work. When short-lived (annual growth) dating material (e.g. twigs, bark) is avail- able, this is the preferred material submitted for ra- diocarbon dating, in contrast to peat samples, which are expected to have an in-built ���older��� age due to peat growth processes. Along the Norwegian coast, as observed at higher levels, the tsunami inundated a number of fresh-wa- ter bodies, again leaving behind a characteristic sand layer. These deposits contain redeposited lake mud, rip-up clasts, and churned up marine fossils. This sand layer has many of the characteristic properties known from modern tsunami deposits. In particular, the observations made for the Storegga Slide tsuna- mi are consistent with the modern observation that tsunamis are commonly associated with at least two waves, with the second wave arriving within minu- tes, but even up to a few hours after the first, depen- ding on distance to the source (Bondevik et al. 2005). Regarding the geological situation in Norway, the first wave typically appears to have eroded the peat surface, producing huge amounts of rip-up peat clasts, which were then chaotically redeposited along with
The catastrophic final flooding of Doggerland by the Storegga Slide tsunami 3 other organic remains, during the backwash. The second wave then appears to have buried these ma- terials in a layer of sand (Bondevik et al. 1997). In order to accurately measure the runup heights for the Storegga tsunami, Bondevik et al. (2005) devel- oped a novel method for runup reconstruction, which is applicable to the large number of tsunami depo- sits known from the Norwegian coast. The method is to map the precise heights of the tsunami deposits in a series of increasingly higher lake basins, until the maximum height is reached. By this method, it appears that the waves inundated the coastal lakes up to 10���12m above contemporary sea-level, but failed to reach lakes at a height of 13m (Bondevik et al. 2005). Similar to the Shetland islands, as de- scribed below, the reconstructed maximal runup depends strongly on the established local contempo- rary sea-level, but in this case that level is well con- strained (to within 1m), due to previous studies of Glacial uplift for the Fennoscandian ice-shield. According to Bondevik et al. (2003), the tsunami de- posits in Norway were sampled for radiocarbon da- ting by the careful selection of short-lived plant ma- crofossils. Such samples are available both from peat outcrops, as well as lakes. From the peat deposits, the ages judged most reliable were obtained on seeds found immediately below the sand layer. Fur- ther sampling emphasis is on leaves and seeds from lake mud just above the tsunami deposit. In one case, a radiocarbon age was obtained on a stick immedi- ately above the sand layer. Following critical sam- ple selection, Bondevik et al. (1997) propose that the tsunami most likely dates to c. 7300 14C-BP. This age is supported by Bondevik et al. (2003), who give a calibrated age value of c. 8150 calBP. Scotland Geological observations probably relating to the Sto- regga tsunami are also available for the east coast of Scotland, where a conspicuous sand layer is recog- nised at numerous localities (Dawson et al. 1988 1993 Smith et al. 2004). According to Dawson et al. (1990), this sand layer was deposited by a major tsu- nami believed to have overwhelmed a Mesolithic occupation at Inverness, and it may also have flooded other Scottish archaeological sites, e.g. at Morton. Ballantyne (2004) urges interpretational caution, however, since localised storm events would have had equally catastrophic effects, particularly during a period of rapidly rising sea-levels. The sand layer is not found on the west coast of Scotland. This would be indicative of a tsunami coming from the east. Britain A useful review of all the currently known sites in the United Kingdom with evidence of the Storegga Slide tsunami is given by Smith et al. (2004). These authors demonstrate that the tsunami affected a much larger coastal area than previously described, with the total length of the inundated coastline reaching more than 600km along eastern Scotland. In addition to giving information on the altitude, distribution, stratigraphi- cal context, and microfossil characteristics of the de- posits, it is shown by detailed particle size analysis that the majority of tsunami sand deposits have a marked fining-upwards characteristic. This is impor- tant, because it gives information pertaining to the dynamics of the wave at different heights. Since sedi- mentation is only possible when the suspended sand particles are released, the implication is that the tsu- nami runup is likely to have exceeded the measured maximal height of the sand layer by several metres (Smith et al. 2004, with references). This study is of further interest, since the authors invest some effort in discussing the taphonomic properties of the dated samples, in search of a useful dating strategy. According to Smith et al. (2004), based on a total of 47 radiocarbon dates from the United Kingdom, the tsunami event took place sometime around 7100 14C-BP (7900 calBP). This estimate seems about 200 years later than that from Norway (Bondevik et al. 1997 2003), but this ���offset��� likely results from the different dating approaches in the Norwegian and British studies. In their 14C-analysis, which is of special interest to us for the purposes of comparison, Smith et al. (2004) describe and classify the UK 14C-dates accor- ding to whether the samples have a ���transgressive��� or ���regressive��� overlap with the tsunami sand layer. The idea is that it might be possible to produce a sta- tistical ���sandwich��� date for the tsunami, when large numbers of such paired dates are analysed. As men- tioned by Smith et al. (2004), this approach could be problematic, since the derived dates from the con- tact zone might turn out too young, if there is a delay in peat growth on the sand layer, following the tsu- nami. To further analyse the UK dates, and notably to compare the results of applying different descrip- tive approaches to the tsunami deposits, we have adopted the database of Smith et al. (2004) essen- tially unchanged (Appendix, Tab. 8). England (Howick case study) Further south, deposits that have been attributed to the Storegga tsunami have been identified in the