Áslaug Geirsdóttir

Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland

Multi-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 14C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 14C yr B.P. and that July temperatures were around 4°C at ca. 9500 14C yr B.P. Temperatures then rose to ca. 8°C at the time of the deposition of the Saksunarvatn tephra (9100 14C yr B.P.), reaching ca. 10°C by 8500 14C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 14C yr B.P. There is some evidence for cooling immediately preceding 9100 14C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 14C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 14C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 14C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone.

A 36 Ky record of iceberg rafting and sedimentation from north-west Iceland

Evidence from north-west Icelands shelf and fjords is used to develop a scenario for environmental change during the last 36 cal Ky. The retreat history of the Iceland Ice Cap during the last deglaciation is delineated through lithofacies studies, carbon analyses and magnetic susceptibility, and studies of ice-rafted debris (IRD) in sediment cores. Sedimentological data from lake Efstadalsvatn, Vestfirdir peninsula, trace the glacier retreat on land. In two of the high resolution shelf cores we detect near continuous IRD accumulation from 36 to 11 cal Kya. However, IRD is absent in the cores from ca. 22 to 19 cal Kya, possibly indicating more extensive landfast sea ice conditions. All cores show intensified IRD during the Younger Dryas chronozone; the fjord cores show a continuous IRD record until 10 cal Kya. Magnetic susceptibility and carbon analyses from Efstadalsvatn reveal the disappearance of local ice in the basin just before 10.5 cal Kya. No IRD was detected in the sediment cores during 10 to ˜ 4 cal Kya. Some indication of cooling occurs between 4 and 3 cal Kya, with a fresh input of IRD in fjord cores after 1 cal Kya.

Late Quaternary ice extent and glacial history from the Djúpáll trough, off Vestfirdir peninsula, north-west Iceland: a stacked 36 cal. Ky environmental record

Djúpáll is a ˜90 km long by 15 km wide trough which extends from ĺsafjardardjúp to the shelf break above Blosseville Basin, north of the Denmark Strait. We present 3.5 kHz seismic profiles from this trough and data from cores collected in 1996 (JM96-1232 and ?1234) and five cores collected on cruise B997. We pay particular attention to B997-338 as this core recovered sediments ranging in age between 12 and 36 cal. Ky BP. This is the first such record from the Iceland continental shelf. Dating control is provided by AMS 14C dates and the occurrence of the Saksunarvatn tephra. X-radiographs of the cores enable us to quantify the input of iceberg-rafted detritus (IRD) and to describe the lithofacies. The sediment matrix is fine-grained and might represent either rain-out of suspended sediment plumes or distal turbidites. IRD is present from ca. 12 cal. Ky BP throughout the next 24 cal. Ky with some IRD-free intervals. Using sediment magnetic properties, sampled at 1 cm (ø100 yrs/sample) resolution, we provide a stacked environmental record which includes marine isotope stages 1, 2 and part of 3. The sediment magnetic properties kARM and IRM(60), and carbonate and TOC, show multi-millennia quasi-periodic cycles, but there are no obvious events coeval with the North Atlantic Heinrich events. Our data indicate that at the Last Glacial Maximum on the Vestfirdir peninsula (VP), north-west Iceland, ice did not reach the shelf break, but was probably grounded near the mouth of ĺsafjardardjúp. A rapid increase in the rate of sediment accumulation suggests that deglaciation of the VP occurred mainly between 11 and 15 cal. Ky BP.

Holocene tephra from Iceland and Alaska in SE Greenland Shelf Sediments

Abstract The record of Icelandic volcanic events in Holocene marine sediments off SE Greenland provides evidence for the frequency and timing of atmospheric tephra plume dispersal from Iceland towards Greenland. Geochemistry of tephra abundance peaks from two SE Greenland shelf cores: MD99-2322 and JM96-1215-2GC are compared with core MD99-2269, north Iceland shelf, to evaluate the dispersal direction of Icelandic eruptions. Glass shard counts (106–1000 µm) in MD99-2322 revealed 16 distinct cryptotephra peaks. Geochemical analyses of eight cryptotephra peaks in MD99-2322 and two in JM96-1215 indicate sources in the volcanic systems of Iceland and Alaska. A tephra layer matching in geochemistry and stratigraphy to the c. 3600 BP eruption of the Aniakchak Volcano in the Aleutian Islands was identified in JM96-1215/2GC. The Settlement Tephra (AD 871±2) and Hekla B (H-B) were identified in MD99-2322. A new marker horizon, Katla EG-6.73, was found in both SE Greenland cores. Three basaltic peaks between 9.9 and 10.4 cal kyr BP, exhibit major-element geochemistry indistinguishable from the c. 10.2 kyr Saksunarvatn tephra. These layers represent 3 out of≥seven westward and northward-dispersed Grímsvötn layers found on the SE Greenland shelf and the north Iceland shelf between 9.9 and 10.4 cal kyr BP. supplementary material: a list of all analyses performed for this study is available at http://www.geolsoc.org.uk/SUP18715

Synchronizing Holocene lacustrine and marine sediment records using paleomagnetic secular variation

High sediment accumulation rates in lacustrine and shallow-marine archives around Iceland offer the potential to compare high-resolution paleoclimatic reconstructions from terrestrial and marine archives; however, direct comparisons are hampered by difficulties in stratigraphic correlation and in deriving accurate age models for lacustrine archives. Icelandic paleomagnetic secular variation (PSV) has the potential to synchronize these records. Here we compare Holocene PSV from a well-dated marine core on the North Iceland shelf with PSV from two lacustrine archives with comparable sediment-accumulation rates, HVT03–1A, a glacier-dominated lake, and HAK03–1B, in a nonglacial catchment. Geochemically characterized tephra layers combined with unique high-amplitude structures in the PSV records provide secure tie points every ∼200 yr. Once the records are synchronized, the chronology from the marine core can be reliably transferred to the two lacustrine records. The resultant lacustrine age models reveal large changes in sediment accumulation rate at submillennial scales that escape detection in conventional age models with independent dates every ∼1 k.y. Sediment accumulation rate changes occur at similar times in both lakes, despite very different catchment properties. Low and regular accumulation rates during the Holocene thermal maximum suggest regionally stable, vegetated catchments, followed by a stepped landscape destabilization during the transition into neoglaciation, culminating with maximum sedimentation rates during the Little Ice Age. PSV allows synchronization between multiple records from nearby marine and lacustrine archives, providing improved age models and a means of assessing leads and lags between marine and terrestrial environments.

Late-Holocene terrestrial glacial history of Miki and I.C. Jacobsen Fjords, East Greenland

We present a late-Holocene glacial and environmental history of three valleys within Miki and I.C. Jacobsen Fjords, East Greenland (68°N). The fjords have no direct connection with the main ice sheet and are presently glacierized by a series of local ice caps and glaciers. Little sediment has been deposited within the study area, but bedrock is distinctly glacially striated, which suggests that extensive valley glaciers reached the present coastline in the past. Moraines are the prominent morphological features including distinct end and lateral moraines. Most of the moraines are related to the existing glaciers in the area. Apparent moraines extend as far as 2 to 2.5 km in front of present-day glaciers. Based on morphological and stratigraphical studies of the sediments in these valleys, five glacier advances during the late Holocene are implied. Stabilization of the oldest moraines is lichenometrically dated toc. 1450 to 1650 years ago. Younger moraines in both Miki and I.C. Jacobsen Fjords are lichenometrically dated toc. 1050–1100 years ago,c. 750–950 years ago and c. 600–700 years ago. The youngest moraines containing measurable lichens stabilizedc. 300–400 years ago. The timing of this moraine formation correlates to marine records from previous research on the inner part of Miki Fjord, where apparent intensification in sediment stratification is related to a glacier advance into the fjordc. 290 years ago. This glacial advance may correlate to the‘Little Ice Age’ cooling observed in many records from the North Atlantic region. Our glacial record from Miki and I.C. Jacobsen Fjords also shows good correlation with temperature proxies within the GRIP ice core in Greenland and foraminifera records from Nansen Fjord, East Greenland. Less pronounced correlation is observed with the Icelandic Sea Ice Index and records of glacier fluctuation in Iceland during the last .1000 years. These results suggest that widespread climatic cooling was the direct cause of many of the late-Holocene glacier variations documented in this study, although the glacial advance atc. 1050–1100 years ago coincides with the so-called‘Medieval Warm Period’, and could reflect the response of glaciers to increased precipitation within the area.

A record of Pliocene and Pleistocene glaciations and climatic changes in the North Atlantic based on variations in volcanic and sedimentary facies in Iceland

Abstract The location of Iceland directly below the Arctic Circle and on the North Atlantic plate boundary gives the island a characteristic record of interbedded lavas, glacial deposits and other sediments, opening a window on the geological and climatic history of the North Atlantic. Frequently extruded lava flows tend to shield underlying sediments from subsequent erosion. Explosive volcanism keeps the sedimentation rate high and produces extensive time markers. Continuing studies of diamictites interbedded within basaltic lava suites of Cainozoic age in Iceland deal specifically with the problem of differentiating between glacial and non-glacial sediments that have very similar attributes. A multifaceted approach has been taken in order to differentiate these deposits. This includes both lateral and vertical lithofacies description and clast fabric studies based on extensive field work, a comparison with modern analogues, and detailed laboratory work (textural studies on thin sections and palaeomagnetic studies). The results suggest that both the nature and extent of late Pliocene and early Pleistocene glaciations can be inferred from the preserved record. The rock sequence offers excellent opportunities for pinning down the age of each event through palaeomagnetic correlation and radiometric dating of the lava flows. The oldest tillite along the north coast of Iceland has been bracketed down to approximately 2.1 Ma, and in inland mid-western Iceland to approximately 2.6 Ma. It is anticipated that continuing work elsewhere in Iceland can further define the trends of Pliocene glaciation in the North Atlantic. The significance of the frequency of identified glacial deposits in the Icelandic sections may be tested through comparison with the latest deep-sea record of glacial-interglacial cycles.

Late quaternary spatial and temporal changes in clay- and silt-size mineral assemblages of fiord and shelf cores, western Baffin Bay, northwest North Atlantic

Clay- and silt-size mineral assemblages are described from eight piston cores from the fiords and shelf on the western margin of Baffin Bay, Arctic Canada. Radiocarbon dates indicate that all the cores extend back in time to the last local glacial/interglacial transition (i.e. 8–10 ka); four extend back to between 10 and 12 ka, and HU77-021-156, located on the Southeast Baffin Island shelf, includes the entire late Foxe glacial stage. Silt- and clay-size particles constitute ca 40 and 55%, respectively, by weight of the bulk sediment. The clay-size fraction is dominated by mica; feldspars and quartz are the main constituents of the silt fraction. The fiord sediments are mainly composed of local mineralogies, but on the shelf, and at times in the fiords, exotic mineral species occur. The most important of these are detrital carbonates, derived from erosion of the Paleozoic basins in Arctic Canada and/or northwest Greenland. Both calcite and dolomite occur; calcite is the major carbonate mineral in the “southern” cores, whereas dolomite is the most abundant in cores north of 66°N. Higher inputs of carbonate species occur during regional deglaciation, 7–10 ka, and during the last 5 ka (probably reflecting increased iceberg production from northwest Greenland). Thus variations in the precentages of the carbonate minerals indicate significant shifts in Late Quaternary glacial-sediment source areas and oceanographic regimes.

14C AMS dating of Icelandic Lake sediments

We report an age-depth profile for the sediments of the Lake Hestvatn, southern Iceland, based on (super 14) C analyses of the organic fraction of bulk sediment samples, molluscs and foraminifera. Our age-depth curve is supported by the occurrence of the well-dated Vedde ash in the lowermost part of the sediments. Comparison of foraminifera dates with the age of the Vedde ash indicates a reservoir age of ca. 400 yr. The results suggest that the sediments at Hestvatn accumulated in a marine environment until ca. 8700 BP and thereafter in freshwater. Owing to the lack of terrestrial macrofossils and the low concentration of molluscs and foraminifera, we were forced to attempt to date most of the core with the organic fraction of the bulk sediment samples. We found, however, that this fraction is not homogeneous in density or (super 14) C age. We believe that during sample pretreatment we managed to isolate a light organic fraction, which closely represents the true age of the sediment, whereas the denser fraction yields ages that are too high. This age diversity may to some extent be explained by the large drainage area of the lake, from which plant remains of different ages may have been washed into the lake.

Extent and chronology of glaciations in Iceland; a brief overview of the glacial history

This chapter discusses the extent and chronology of glaciations in Iceland. The distribution of glacial deposits and the correlation between rock sequences in Iceland show a distinct trend during the Pliocene–Pleistocene transition indicating a gradually growing ice sheet from the southeast toward the west and north. The oldest glacial deposit identified based on detailed sedimentological analyses is found imbedded within c. 3.8 to 4.0 My old lava flows in Fljotsdalur, eastern Iceland. Another glacial bed found higher up in the sequence at the same site has an estimated age of 3.4 My. As it has not been possible to trace these two glacial beds over a larger area, they are thought to represent only local glacier activity. Recent palaeomagnetic study and K–Ar radiometric dating on the stratigraphy of Skaftafell, southeast Iceland, also suggests an increased glaciation at c. 2.6 Ma. More extensive glaciation is indicated by glacial deposits in all parts of the country dating from 2.4 to 2.5 My ago. From the period between 2.9 and 1.6 My, at least 6 glaciations and possibly 8 glaciations are indicated, but the geological record implies a decrease in glacial activity in the period between 1.6 to 1.2 My.