Jórunn Hardardóttir

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.

A high-resolution Holocene sediment record from Húnaflóaáll, N Iceland margin: centuryto millennial-scale varability since the Vedde tephra

MD99-2269 is a 25 m long core from Hunaflóadll, a deep trough which runs northward towards the shelf break from the narrow neck of land which joins the Northwest Peninsula of Iceland with the ‘mainland’. The core was recovered from the thickest part of an elongate shelf sediment body, underlain by glacial diamictons with ages 26 ka BP. 3.5 kHz profiles indicate the presence of a reflector, RI, which can be traced over long distances. Twelve AMS radiocarbon dates and the presence of the Saksunarvatn tephra (which is the origin for the RI reflector at 21 m core depth) define a linear depth/age relation with an accumulation rate of 200 cm/ky. The date on the core top is ‘modem’. X-radiographs indicate that the sediments are principally massive. bioturbated muds. The data were measured at resolutions of between 5 and 40 yr/sample. Our primary data represent aspects of (a) multisensor track measurements, (b) colour, (c) grain size, (d) sediment properties and (e) sediment magnetic properties. Principal component analysis (PCA) was used to simplify the analysis of these five data sets, and resulted in nine significant axes compared to over 70 original variables. The explained variance on the first axes varied between 47 and 96%. These time series were reduced to a common 50-yr sample interval which were then also subject to PCA. Using the resulting PC scores we present three primary axes of environmental variability in Hunaflóaall that have century-scale quasi-periodicities, and demonstrate that these are highly correlated with specific ‘raw’ variables. We focus on variables linked to changes in grain size and productivity and show regional coherence (cores B997-321 and 330) in the coercivity/grain size measure ARMJ(20)/ARMJ(0), which have a close similarity to the detrended A 14C series. This provides a potential link to the vigour of the thermohaline circulation to the north of the Iceland shelf in the Iceland and Greenland Seas. Our data indicate increased variability in our proxies during the Neoglacial period.

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.

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.

Holocene Sediment Magnetic Properties along a Transect From Ísafjardardjúp to Djúpáll, Northwest Iceland

ABSTRACT Holocene changes in terrestrial provenance and processes of sediment transport and deposition are tracked along a fjord-to-shelf transect adjacent to Vestfirdir, Iceland, using the magnetic properties of marine sediments. Magnetic susceptibility (MS) profiles of 10 cores (gravity and piston) were obtained onboard using a Bartington MS loop. Remanent magnetizations were measured at 1-cm intervals from u-channel samples taken from six cores on a cryogenic magnetometer. Between six and nine alternating field demagnetization steps were used to isolate the characteristic magnetization directions. The chronologies of the cores used in this study were determined from AMS 14C dates on mollusks and foraminifera and constrained by the regional occurrence of the 10,200 ± 60 cal yr. BP Saksunarvatn tephra. Correlative fluctuations in magnetic concentration are noted between the fjord and shelf sites, though these fluctuations are partially masked by regional variations in carbonate content. The onset of Neoglaciation is interpreted by changes in magnetic properties including an increase in mass magnetic susceptibility that began approximately 3000 cal yr. BP. The maximum angular deviation and the median destructive field (generally <20 mT) suggest that the natural remanent magnetization is carried by a coarse ferrimagnetite mineralogy, likely magnetite or titano-magnetite. Reproducible paleomagnetic inclination values are observed in several records, including a nearly vertical inclination around 8000 cal yr. BP, suggesting that the magnetic pole may have been proximal to Iceland, followed by an interval of much shallower inclination (6000–7000 cal yr. BP).