Eivind W. N. Støren

A Holocene record of snow-avalanche and flood activity reconstructed from a lacustrine sedimentary sequence in Oldevatnet, western Norway

Two lacustrine sediment cores from Oldevatnet in western Norway have been studied in order to produce a record of floods, mass-wasting events and glacier fluctuations during the last 7300 years. River floods, density currents and snow-avalanches have deposited distinct ‘event layers’ at the lake floor throughout this time interval. In this study, a novel approach has been applied to distinguish event layers from the continuous background sedimentation, using Rb/Sr-ratios from X-Ray Fluorescence data. Grain-size distribution and the sedimentological parameters ‘mean’ and ‘sorting’ were used to further infer the depositional processes behind each layer. Our data suggest a record dominated by snow-avalanches, with the largest activity occurring during the ‘Little Ice Age’ (LIA). This increase in snow-avalanche activity observed during the LIA was probably caused by a combination of generally increasing winter precipitation and the advance of local glaciers towards the steep valley sides. Several fluctuations in snow-avalanche activity are also recognized prior to the LIA. Proxies of glacial activity from the background sediments indicate a similar development as earlier palaeoclimatic reconstructions from the area. It differs from previous reconstructions, however, by suggesting a lower glacial activity in the period from 2200 to 1000 cal. yr BP.

The climatic significance of artefacts related to prehistoric reindeer hunting exposed at melting ice patches in southern Norway

The main aim of this study is to describe consequences of climate change in the mountain region of southern Norway with respect to recently exposed finds of archaeological remains associated with reindeer hunting and trapping at and around ice patches in central southern Norway. In the early years of the twenty-first century, warm summers caused negative glacier mass balance and significant glacier retreat and melting of ice patches in central southern Norway. As a result, prehistoric remains lost and/or left by past reindeer hunters appeared at ice patches in mountain areas of southern Norway. In the warm summer and autumn of 2006 the number of artefact recoveries at ice patches increased significantly because of melting of snow and ice patches and more than 100 objects were recovered in the Oppland county alone. In 2009, detailed multidisciplinary investigations were carried out at the Juvfonne ice patch in Jotunheimen at an elevation of c. 1850 metres. A well-preserved Iron Age hunting station was discovered and in total c. 600 artefacts have been documented at the Juvfonne site alone. Most of the objects were recovered and brought to the Museum of Cultural History at the University of Oslo for conservation, exhibition and storing. Thirteen so called ‘scaring sticks’ recovered from the recently exposed foreland of Juvfonne were radiocarbon dated, yielding ages that group in two separate time intervals, ad 246–534 and ad 804–898 (±1 sigma). By putting the temporal distribution of the radiocarbon-dated artefacts into the context of late-Holocene glacier-size variations in the Jotunheimen and Jostedalsbreen regions, we conclude that the most extensive reindeer hunting and trapping associated with snow/ice patches was related to periods with prevailing warm summers when the reindeer herds gathered on high-altitude, contracted glaciers and ice patches to avoid insect plagues. The ‘freshness’ of the fragile organic finds strongly indicates that at least some of the artefacts were rapidly covered by snow and ice and that they may have been more-or-less continuously covered by snow and ice since they were first buried.

Separation of late-Holocene episodic paraglacial events and glacier fluctuations in eastern Jotunheimen, central southern Norway

The lacustrine sediments of proglacial lake Russvatnet in eastern Jotunheimen, central southern Norway, comprise a combination of glacier-derived material from the glacier Blackwellbreen and several episodic paraglacial processes in the catchment. This study focuses on the characteristics of lake sediments, and the decomposition of a 4000-yr complex multiprocess record from Russvatnet, in order to separate late-Holocene river floods, mass movements and glacier fluctuations. Facies models based on grain-size distribution, minerogenic content and amount of terrestrial macrofossils are used to recognize discrete mass movement and river flood deposits. Twenty-two episodic events are identified during the late Holocene, classified as 11 mass movement events (debris flows, high density turbidity currents and low density turbidity currents) and 11 river flood events. Enhanced river flood and colluvial activity is inferred at 4000—3400, 2900—2500, 2000—1400 and 1000—500 cal. yr BP. Glaciolacustrine core segments were analysed based on magnetic susceptibility, bulk density and loss-on-ignition to reconstruct variations in late-Holocene glacier magnitude. At c. 2300 cal. yr BP a shift in sedimentation regime from a paraglacial to a glacially dominated regime is recorded, and a Neoglacial expansion period after 2300 cal. yr BP is suggested. A contrasting pattern of climate deterioration and rapid episodic events are recorded in lake Russvatnet. Large regional variations in the timing of episodic events emphasize the importance of local triggering factors compared with more large-scale effects of climate change.

How Does Climate Impact Floods? Closing the Knowledge Gap

Destructive floods impose severe consequences on societies, leaving havoc and death in their wake. Annually, an average of 9000 people are killed, and 115 million require immediate assistance or are displaced by floods worldwide. Because of population increase in flood-prone areas alone, the number of people exposed to floods in North America is expected to almost double in 2030 compared to 1970 [National Research Council, 2013]. It is no wonder that floods are considered serious threats by government agencies and municipal planners, but the impact of climate change on these numbers is still somewhat uncertain.

Magnetic and geochemical signatures of flood layers in a lake system

River floods holds the capasity to erode and transport sediments that are deposited whenever the discharge is redused. In catchments that are subjected to repeated flooding, downstream lakes can therefore contain a record of past events across multiple timescales. High-resolution core scanning analyses such as X-ray fluorescence (XRF) scanning and magnetic susceptibility (MS) provide data that are frequently used to detect flood layers in soft sediment archives such as lakes, fjords and ocean basins. Deposits of past floods also can potentially reveal information about the evolution of flood events as well as source area. Here we explore ways in which subtle variability in high-resolution data can be utilized and subsequently vetted by high-precision measurements in order to delineate the copious information that can be extracted from soft sediment records. By combining magnetic hysteresis measurements and first-order reversal curves (FORCs) with inductively coupled plasma optical emission spectrometer (ICP-OES) measurements of chemical elements on 36 samples, questions about flood dynamics and variability are raised, and also sources of noise in high-resolution scanning techniques are discussed. Specifically, we show that a lake flood record from Southern Norway containing 92 floods distributed over 10,000 years can be sub-divided into two groups of floods that were generated either by spring snow melting, intense summer rainstorms, or a combination of both. The temporal evolution of this pattern shows a marked shift towards spring floods around 2000 years ago compared to the earlier part of the record. This article is protected by copyright. All rights reserved.

DNA from lake sediments reveals long-term ecosystem changes after a biological invasion

Rabbits have had a stronger impact on the landscape and plant communities of a remote island than one century of climate change. What are the long-term consequences of invasive species? After invasion, how long do ecosystems require to reach a new equilibrium? Answering these questions requires long-term, high-resolution data that are vanishingly rare. We combined the analysis of environmental DNA extracted from a lake sediment core, coprophilous fungi, and sedimentological analyses to reconstruct 600 years of ecosystem dynamics on a sub-Antarctic island and to identify the impact of invasive rabbits. Plant communities remained stable from AD 1400 until the 1940s, when the DNA of invasive rabbits was detected in sediments. Rabbit detection corresponded to abrupt changes of plant communities, with a continuous decline of a dominant plant species. Furthermore, erosion rate abruptly increased with rabbit abundance. Rabbit impacts were very fast and were stronger than the effects of climate change during the 20th century. Lake sediments can allow an integrated temporal analysis of ecosystems, revealing the impact of invasive species over time and improving our understanding of underlying mechanisms.

Reconstructing Holocene Glacier and Climate Fluctuations From Lake Sediments in Vårfluesjøen, Northern Spitsbergen

A process-based understanding of lacustrine-deposited sediments in Arctic lakes is essential to set the present warming and hydroclimatic shift into perspective. From such a perspective, we can enhance our understanding of the natural climate variability in the Arctic. Here, we present work from the northern coast of Spitsbergen in which we unravel the sediment sequence from a distal glacier-fed lake, Varfluesjoen. Utilizing recent methodological and technological developments, we base our interpretation on new tools that better visualize and characterize the sediments cores. High-resolution X-ray Computed Tomography (X-ray CT) is used to visualize the lake sediments and quantify the sand-sized particles found in the 210Pb- and radiocarbon-dated sediments, together with a multi-proxy approach including measurement of their physical, geochemical, and magnetic properties. Our findings suggest that Varfluesjoen (6 m a.s.l.) was isolated from Woodfjorden at c. 10,200 ± 260 cal. yr. BP. During the early Holocene, the glaciers in the Varfluesjoen catchment were considerably smaller than today or had even melted completely. At the start of the Neoglacial period (c. 3500 cal. yr. BP), we find increased glacier activity in the catchment of the lake. X-ray CT reveals an increased frequency of sand-sized particles from 3500 to 1750 cal. yr. BP, suggesting greater wintertime aeolian activity. Starting c. 2250 years ago, we find a progressive increase in snowmelt runoff in the Varfluesjoen catchment, with peak runoff from 1000 to 750 cal. yr. BP. This coincides with a drop in sand-sized particles, hence less favorable environment for aeolian activity, and implying wetter conditions. During the last 2000 years, there is evidence for high glacier activity between c. 2000 to 900 and 750–350 cal. yr. BP. In between these time spans, less activity is recorded in the periods 1900–1800, 1000–800, and 350–150 cal. yr. BP.