Relative sea level, deglaciation and tsunami history deduced from isolation basins

Paper number 1 of the thesis is not available in Munin due to publisher's restrictions: 1. Romundset, A., Lohne, Ø.S., Mangerud, J. & Svendsen, J.I.: «The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway», Boreas 39(2010), 87-104 (Wiley). Available at http://dx.doi.org/10.1111/j.1502-3885.2009.00108.xNORSK SAMANDRAG: Denne avhandlinga tek for seg endringane i relativt havnivå – strandforskyvinga – dei siste vel 11.500 åra på kysten av Finnmark og i midtre Hardanger. Ei stor mengd kjerneprøver frå avsettingane i innsjøar som ein gong låg under havnivået, er henta inn og analysert. Grenser mellom sediment som vart avsett i salt- eller ferskvatn er bestemt ved å analysere både kiselalgar og restar etter daude dyr og planter, og tidfest ved hjelp av radiokarbondatering. I lag med presist oppmålte høgder på innsjøtersklane, gjer desse data det mogleg å rekonstruere havnivåendringane i høg detalj. Her blir det presentert havnivåkurver som syner utviklinga i fire område; Tørvikbygd i Hardanger, Sørøya og Rolvsøya i vest-Finnmark og Nordkinnhalvøya i aust-Finnmark. Desse dokumenterer samspelet mellom landheving og havnivåstiging – stort sett har landhevinga vore sterkast, men i tida mellom ca. 9-7000 år sidan var det motsette tilfelle på finnmarkskysten – da vart fleire innsjøar atter oversvømt etter å ha lege fleire tusen år over havnivået. På Rolvsøya steig havnivået opp over ein innsjø men nådde ikkje den neste som ligg rett attmed, berre omlag 60 cm høgare. Her stabiliserte det seg i meir enn tre tusen år, fram til for ca. 5000 år sidan. Etter det har havnivået falle fleire meter fram til i dag. Ei liknande utvikling er òg funne på Sørøya og Nordkinnhalvøya, men i Tørvikbygd er historia ei anna. Her vart landet mykje hardare nedpressa mot slutten av siste istid, og landhevinga har vore tilsvarande sterk, særleg det fyrste tusenåret etter at isen forsvann – da datt strandlina gjennomsnittleg med meir enn 5 cm i året. Breen som dekte Barentshavet under siste istid, smelta vekk tidlegare enn isen over Finnmark. Dette førte til sterk heving av havbotnen som også virka inn på tidleg isfrie område av ytterkysten og medførte at spranget mellom marin grense og yngre strandliner er langt større her enn langs andre delar av norskekysten. I tillegg er bidraget til hevinga frå Barentshavet, truleg i lag med ein slakare profil på innlandsisen, årsaka til at dei heva strandlinene i Finnmark hallar mindre enn dei gjer t.d. på Vestlandet. Det er funne klåre spor etter ein tsunami i fem av innsjøane på Finnmarkskysten. Dateringar syner at dette må ha vore Storeggatsunamien, som vart utløyst av eit enormt undersjøisk skred utafor Mørekysten for om lag 8100-8200 år sidan. Bølgja slo opp i alle fall 3-4 meter i høgda, og mange hundre meter innover land. Erosjonen var sterkast nært sjøen der tsunamien grov sed ned i meir enn 3000 år eldre avleiringar. Over erosjonskontakten ligg nedst eitt sandlag, fylgd av mellom anna opprivne torvbitar og gytjeklumpar som tydeleg har rulla i sanda. Den valdsamme erosjonen og dei sorterte avsettingane tilseier at tsunamien må ha råka 17 innsjøar som ikkje var islagte, og på denne tida var klimaet vesentleg kaldare enn i dag. Dette gjer det truleg at både Storeggaskredet og tsunamien hende ein gong mellom april og oktober.ABSTRACT IN ENGLISH: The work included in this thesis is based on investigations of isolation basins in mid- Hardanger, South Norway (Paper I) and at the outer coast of Finnmark, North Norway (Papers II and III). The main focus has been on reconstructing the changes in relative sea level with high precision through the Holocene. The site Tørvikbygd in Hardanger was chosen because it is one of few places, well inside a fjord in western Norway, where isolation basins are found. Accurate data on relative sea-level change are important to palaeo ice-sheet modeling, since it puts constraints on the rate of glacio-isostatic adjustment after the last glacial, and in the case of Tørvikbygd, from an area where such data have previously been missing. The new reconstruction shows that the emergence rate was extremely rapid in the first millennium of the Holocene (> 5 cmyr-1), before it slowed down and reached near standstill in the mid-Holocene. The shoreline has dropped about 16 m after the standstill until today. The results are further compared to data from a site outside the fjord mouth, and used to construct a shoreline diagram for Hardangerfjorden that deviates somewhat from previous undated morphology-based reconstructions. Displacement of the shoreline has resulted in series of raised beach ridges in nearly every bay of Finnmark, and has been studied by Quaternary geologists for more than a century. Still, almost no chronological data have been obtained, due both to scarcity of datable material onshore and to the fact that no systematic isolation basin study has been undertaken. In Paper II, radiocarbon-dated isolation basin sequences used for reconstructing sea-level curves from three areas at the outer coast are presented. These curves document the timing and magnitude of the mid-Holocene Tapes sea-level fluctuation at the localities. At the easternmost locality, evidence is also found that the strong early Holocene uplift, that took place along most of the Norwegian coastline, was here delayed for about 1000 years. The data show that the gradient of Holocene raised shorelines in Finnmark is significantly lower than in comparable coastal regions of western Norway. This is attributed to the fact that coastal Finnmark lies in the periphery of the former Barents Sea Ice Sheet, and that rebound of the seafloor affected the coast in addition to the rebound from the Fennoscandian Ice Sheet. The (earlier) demise of the Barents Sea Ice Sheet and the following rebound is also the reason for anomalously high marine limits along the outer coast of Finnmark. A number of mollusk shells and macro-algae samples from basal lake sediments both in Hardanger and Finnmark have been dated and used to reconstruct the timing of regional ice- 15 sheet retreat. These show that the fjord glacier in Hardangerfjorden retreated inland of the study site around 11,300 yr BP, whereas the outermost coast of Finnmark likely became ice free following the huge temperature rise at the onset of Bølling, ca. 14,600 yr BP. Traces of strong erosion and deposition were also found in five lakes in Finnmark (Paper III). The characteristics of the erosion and deposits leave little doubt that a tsunami inundated the lakes, and ages obtained on material picked from within or just above the deposits correlate to the ca. 8100-8200 cal yr old Storegga tsunami. It is thus shown for the first time that the Storegga tsunami propagated into the Barents Sea, and reached at least 1300 km distance from the slide. Based on the pattern of erosion, the inclusion of both rip-up peat clasts and sand-coated gyttja clasts and the sorting of sediments deposited by the tsunami, it is concluded that this took place when the lakes were not ice-covered and the ground not frozen, thus the Storegga slide and tsunami event occurred in the summer season

Two thousand years of Landscape—Human interactions at a coastal peninsula in Norway revealed through pollen analysis, shoreline reconstruction, and radiocarbon dates from archaeological sites

Human impact on long-term vegetation and biodiversity changes is often discussed on a general level, connecting palynological data to archaeological time periods. In the present paper we present environmental change during the last 2,400 years on a coastal peninsula in Norway using pollen data from three sites: one lake and two bogs, in addition to 621 radiocarbon dates and in-context pollen samples from archaeological sites. Locally, a close relationship between palynological richness reflecting high landscape, habitat and floristic diversity, and the summed probability distribution of radiocarbon dates was found. During the settlement period 400 BCE–550 CE, concordant with maximum number of dates from archaeological contexts, a mosaic landscape containing infields and outfields developed. Cereals were cultivated and animals were grazing in heathlands that could provide both summer grazing and winter fodder. Additionally, seashores and wetlands were used for grazing. Settlement recession from 350 CE and abandonment following the 536 CE climate event, resulted in vegetation successions toward reforestation, abandonment of arable fields, and marshes turning into ombrotrophic peat. At the same time the distance to the sea, and to species rich shoreline meadows, continued to increase due to continuous postglacial land uplift in a flat landscape. A new increase in the summed probability distribution 900–1250 CE, is reflected in expansion of outfield pastures, heathlands in particular, a management that continued up into modern time. The local development is supported by the results on a regional scale, indicating overall climatic and social causes for observed vegetation changes. Both palynological richness and pollen-based landcover reconstructions indicate reforestation and less habitat diversity in the sixth century. On a regional scale, reforestation in the fifteenth century following the late medieval crises, is more pronounced than on the local scale, although both reflect exploitation of outfield resources

Strandforskyving og isavsmelting i midtre Hardanger

A new Holocene relative sea-level (RSL) curve is drawn, based on three firmly dated isolation basins in mid-Hardangerfjorden, southwest Norway. Several AMS-datings of Betula leaves from each isolation contact yielded very consistent ages. Following deglaciation, RSL at the site fell very rapidly a total of about 70 m during the Preboreal. The regression rate then slowed down considerably during a time span of ca. 1000 years, and a slow and even regression has taken place from then until present. No evidence of the the Tapes transgression is seen in the investigated stratigraphies, although a possible transgression or stillstand of the RSL cannot be ruled out on basis of merely these three isolation basins. The RSL curve is placed together with two other curves from the outer coast, and on this foundation a Holocene shoreline diagram representative for Hardangerfjorden is constructed. A deduced shoreline gradient curve displays a rapid decrease in the tilting of the shorelines (1,4 – 0,2 m/km) during the course of the first ca. 3000 years. From then on, the tilt has been steadily decreasing until present. Marine shells were obtained from the lowermost glaciomarine sediments in the cores. Eight datings were performed, all yielding early Preboreal ages. This indicates a final deglaciation of the area at the transition from the Weichselian to the Holocene, and supports the hypothesis of a glacier occupying Hardangerfjorden some time during the Younger Dryas. The Storegga tsunami is believed to have reached Hardangerfjorden, but no clear evidence has been found for this event

Propagation of the Storegga tsunami into ice-free lakes along the southern shores of the Barents Sea

There is clear evidence that the Storegga tsunami, triggered by the giant Storegga slide offshore western Norway 8100-8200 years ago, propagated into the Barents Sea. Cores from five coastal lakes along the coast of Finnmark in northern Norway reveal major erosion and deposition from the inundation of the tsunami. The deposits rest on a distinct erosional unconformity and consist of graded sand layers and re-deposited organic remains. Some of the organic remains are rip-up clasts of lake mud, peat and soil and suggest strong erosion of the lake floor and neighbouring land. In this part of the Arctic coastal lakes are usually covered by > 1 m of solid lake ice in the winter season. The significant erosion and deposition of rip-up clasts indicate that the lakes were ice free and that the ground was probably not frozen. We suggest that the Storegga slide and tsunami event happened sometime in the summer season; between April and October. Minimum run-up has been reconstructed to 3-4 m

Holocene relative sea-level changes and deglaciation chronology in Finnmark, northern Norway

The outer coast of Finnmark in northern Norway is where the former Fennoscandian and Barents Sea ice sheets coalesced. This key area for isostatic modelling and deglaciation history of the ice sheets has abundant raised shorelines, but only a few existing radiocarbon dates relate to them. Here we present three Holocene sea-level curves based on radiocarbon ages from deposits in isolation basins at the outermost coast of Finnmark; located at the islands Sørøya and Rolvsøya and at the Nordkinn peninsula. We analysed animal and plant remains in the basin deposits to identify the transitions between marine and lacustrine sediments. Terrestrial plant fragments from these transitions were then radiocarbon dated. Radiocarbon-dated mollusk shells and marine macro-algae from the lowermost deposits in several basins suggest that the first land at the outer coast became ice free around 14.6 cal kyr BP. We find that the gradients of the shorelines are much lower than elsewhere along the Norwegian coast because of substantial uplift of the Barents Sea. After the Younger Dryas the coast emerged 1.6-1.0 cm per year until about 9500-9000 cal yr BP. Between 9000 and 7000 yr BP relative sea-level rose 2-4 m and several of the studied lakes became submerged. At the outermost locality Rolvsøya, relative sea level was stable at the transgression highstand for more than 3000 years, between ca. 8000 and 5000 cal yr BP. Deposits in five of the studied lakes were disturbed by the Storegga tsunami ca. 8100-8200 cal yr BP

Early Holocene thinning and final demise of the Scandinavian Ice Sheet across the main drainage divide of southern Norway

The thinning and final decay of the Scandinavian Ice Sheet in the Gudbrandsdalen area in central southern Norway is described, based on (1) cosmogenic 10Be surface exposure dating of 25 glacially transported boulders, (2) radiocarbon dating of plant remains in the basal strata in four lakes and (3) mapping of large ice-dammed lakes that formed at different elevations and at different times during the last deglacation. We complement the new chronology with previously published 10Be-ages from the same region. The dated samples are spread from mountain summits 1800 m a.s.l. to the valley floor at 250 m a.s.l. Our results suggest that the ice sheet surface remained well above 1800 m a.s.l. in northern Gudbrandsdalen throughout the Younger Dryas. During the Early Holocene the ice sheet thinned rapidly, at rates estimated to 1.7–5.8 m yr−1. The final phase of deglaciation involved formation of large ice-dammed lakes, most notably the Store Dølasjø which was formed after 10.4 ka BP and finally drained around 10.0 ka BP. The ice-marginal landforms that characterize the mountain region of northern Gudbrandsdalen, i.e., moraine ridges, lateral meltwater channels, as well as deposits and shorelines from ice-dammed lakes, thus collectively originate from a period of rapid ice sheet downwasting over ca. 1600 years.ISSN:0277-379

Cappelen Damm. Ny E18 Tvedestrand-Arendal. Kystens steinalder i Aust-Agder.

E18 Tvedestrand–Arendal-prosjektet er et forvaltningsinitiert utgravningsprosjekt og organisert som en del av virksomheten til Arkeologisk seksjon ved Kulturhistorisk museum (KHM), Universitetet i Oslo (UiO). Prosjektet ble etablert ved KHM 7. juli 2014 og avsluttet 31. mai 2017. Bakgrunnen for prosjektet er bygging av ny firefelts motorvei mellom Tvedestrand og Arendal i Aust-Agder, opprinnelig i regi av Statens vegvesen Region sør. Nye Veier AS overtok som tiltakshaver 1. januar 2016. Tiltaket berører til sammen 50 automatisk fredede kulturminner. Av disse ble 35 undersøkt (se tabell i Reitan, kap. 2.1, denne bok). Utgravningene ble gjennomført i løpet av tre feltsesonger i perioden 2014–2016. Utgravningene er gjennomført i medhold av LOV 1978-06-09 Nr. 50 (kulturminneloven) med en overordnet målsetting om å ivareta og dokumentere gjenstandsfunn og dermed sikre det vitenskapelige informasjonspotensialet til de berørte kulturminnene. De faglige hovedmålsettingene var å fremskaffe kunnskap om den teknologiske, typologiske og kronologiske utviklingen gjennom steinalderen i et tidligere lite undersøkt område. Videre skulle undersøkelsene legge til rette for fremtidige analyser av blant annet landskapsbruk og regionalitet i Sør- og Øst-Norge