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ALPINE LAKE SEDIMENT ARCHIVES AND CATCHMENT GEOMORPHOLOGY CAUSAL RELATIONSHIPS AND IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS

Lena Rubensdotter

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Alpine lake sediment archives and catchment geomorphology causal relationships and implications for paleoenvironmental reconstructions

Lena Rubensdotter Department of Physical Geography and Quaternary Geology, Stockholm University, S-10691 Stockholm, Sweden

Abstract Lake sediments are frequently used as archives of climate and environmental change. Minerogenic sediment variability in alpine lakes is often used to reconstruct past glacier and slope process activity. Alpine lake sediments can however have many different origins, which may induce errors in paleoenvironmental reconstructions. The aim of this project was to enhance the understanding of minerogenic lake sedimentation in alpine lakes and improve their use as environmental archives. Catchment geomorphology and Holocene sediment sequences were analysed for five alpine lakes. Several minerogenic sediment sources were detected in catchments and sediment sequences. Slope-, fluvial-, periglacial-, nival- and aeolian sediment transportation processes contribute to create complex lake sediment patterns. Large variations in sedimentation rates were discovered within and between lakes, which has implications for sampling strategies and age-model constructions. Similar fine-grained minerogenic laminations were found in four of the investigated lakes, despite large differences in setting. The demonstrated similarity between glacial and nonglacial lakes may complicate interpretations of glaciolacustrine sediment signals. The main conclusion is that lake sedimentation in alpine environments is highly dependent on several geomorphological factors. All lakes should therefore be viewed as unique and the geomorphology should be thoroughly investigated before environmental reconstructions are based on lake sediment proxies. This study has confirmed the multi-source origin of alpine lake sediment, which also opens possibilities of more multi-faceted paleoenvironmental studies. Different process-proxies could potentially be used to separate different climate signals, e.g. precipitation, temperature and wind, in lake sediments. Analysis of grain-size distribution, detailed mineralogy and magnetic mineralogy in combination with X-ray radiography are suggested methods for such reconstructions.

Keywords: alpine lakes, geomorphology; lacustrine sedimentation; slope processes; glaciofluvial; Holocene

© 2006 Lena Rubensdotter ISSN: 1650-4992 ISBN: 91-7155-238-3

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Lena Rubensdotter

INTRODUCTION Today there is a strong focus worldwide on climate research, both on recent climate change and climate variability back in time. Lacustrine sediments have, because of the annual deposition of particles on lake bottoms, been increasingly investigated and used in paleoclimate research as terrestrial palaeoenvironment archives, used to gain information on both within-lake hydrology and biology, and biological and physical changes in the surroundings (Hicks et al. 1990; Björck et al. 1991; Bradley et al. 1996; Bertrand et al. 2005). Small lakes in arctic and alpine environments are known to be sensitive to even modest environmental changes (e.g.Rosqvist et al. 1999; Korhola and Weckström 2000; Battarbee et al. 2001; Spooner et al. 2002) and are also often relatively undisturbed by human activities, which makes them especially suited for studies of past climate variability. Lacustrine sediments typically encompass some type of lithostratigraphical signal derived from denudation and transportation processes in the catchment (Karlén 1981; Harbor 1985; Doran, 1993; Nesje et al. 2000) and thereby also record landscape development on different time scales (Bradley et al., 1996). Lake sediments are, compared to geomorphological features, relatively easy to date and can therefore provide age control for past geomorphological activity in the catchment, including slope process activity (Jonasson 1991; Blikra and Nemec 1993; Blikra et al. 1997; Matthews et al., 2000a; Sletten et al. 2003). Glaciers erode the underlying bedrock and produce minerogenic sediments that are flushed to lakes situated downstream (c.f. Karlén, 1976; Leonard, 1986a,b; Desloges, 1999). The presence of glaciofluvial sediment in alpine lakes may thus reflect climatically forced glacier variations and proglacial lake sediment sequences containing glaciofluvial minerogenic sediment have frequently been used to detect and date variability in Holocene glacier activity (Karlén 1981; Matthews and Karlén 1992; Leemann and Niessen 1994; Matthews et al. 2000b; Nesje et al. 1991, 2000; Snyder et al. 2000; Rosqvist and Schuber 2003; Bakke et al. 2005a-c). The relation between autochthonous (produced within the lake) and allochthonous (from outside the lake) sediment components can vary significantly between lakes and over time, which may affect paleoenvironmental interpretations based on specific sediment parameters (Battarbee et al. 2001). Episodic allochthonous sediment input also makes age-depth relations more complicated and may dilute (Wagner and Melles 2002) the signals of more continuously deposited sediment components such as diatoms, chironomids and glacially derived minerogenic sediments (Souch 1994, Spooner et al. 2002). The effect of episodic sediment input is especially important in alpine environments, where much of the allochthonous sediment input is

forced by fluvial and geomorphologic activity, such as floods (Gilli et al. 2003), avalanches (Luckman 1975, Seierstad et al. 2002), debris flows (Jonasson 1991; Sletten et al. 2003) and other gravitational processes. Weathering, erosion and transportation processes are also to varying degrees controlled by climatic factors such as precipitation and temperature (Hardy 1996) and can either cancel out or enhance climate signals preserved in lake sediments. Lacustrine records from lakes affected by rapid slope processes have been used to provide local climate records (Jonasson 1991; Nesje et al. 1994; Blikra et al. 1997). Since minerogenic sediment in alpine lake sediment sequences can pose both a problem in paleoclimate studies and act as a potential paleoenvironmental proxy, it is important to gain more knowledge about its occurrence and the mechanisms affecting it. The main aim of this thesis is to increase the understanding of minerogenic alpine lake sedimentation, detect the factors contributing to variable sedimentation patterns and assess the impact of this variability on environmental reconstructions. To achieve this, I have focused on detecting and describing Holocene minerogenic sediment variability in different lakes and compare this with analysis of the geomorphological and topographical setting of the lakes. The result from the sedimentological multi-proxy analysis is therefore compared with information derived from the identification of sediment transportation processes in the catchments. The two most important geomorphological factors contributing to lake sedimentation, which are used in paleoenvironmental reconstructions from the Scandinavian mountain chain, are rapid gravitational processes and glacial denudation with subsequent glaciofluvial deposition (e.g. Jansson et al. 2005; Jonasson 1991; Blikra et al. 1997; Karlén 1976, 1981; Matthews et al. 2000; Nesje et al. 1994, 2000, 2001; Bakke et al. 2005a,c). Studies of sediments from non-glacial lakes with no contribution from glaciers nor rapid gravitational processes enabled a tentative assessment of the “background signal” that potentially may be present in all alpine lakes. Results in this thesis are presented from studies of five alpine lakes and catchments with different geomorphological settings in northern Swedish Lapland. Results from multi-proxy analysis of Holocene sediment sequences are compared with analysis of catchment geomorphology based on detailed maps compiled from aerial photographs and field verifications.

Choice of study sites and methods Since this project aimed at detecting variable sources of minerogenic sediment deposition in alpine lakes during the Holocene, it was important to choose representative study sites with geomorphological settings common in the mountain range. It was also of importance to select sites where the geomorphological and

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Alpinelakesedimentarchivesandcatchmentgeomorphology

environmental conditions could be assumed to have remained relatively stable since the last deglaciation, which would make it possible to interpret today’s processes as representing the postglacial setting. This meant avoiding sites with signs of larger geomorphological or fluvial events, such as faulting, large rock-slides or river capture. A reasonable size of the catchments (maximum a few km2) and availability of high quality aerial photographs was also necessary to allow a thorough geomorphological analysis. The relation between catchment and lake areas size was also an aspect in the lake selection of study sites. The main sediment transportation processes of interest for lake sedimentation are glacial denudation and glaciofluvial sediment deposition and rapid slope processes (e.g. avalanches and debris flows). The studied lakes and catchments were therefore chosen so that one or more of these processes could be eliminated as an active sediment provider. The Abisko region is suited for the study since a high number of paleoclimate and paleoenvironmental studies have been published from this area (e.g. Rapp 1986; Nyberg and Lindh 1990; Berglund et al. 1996; Snowball I. and Sandgren P. 1996; Barnekow 1999, 2000; Kullman 1999; Shemesh et al. 2001; Grudd et al. 2002; Hammarlund et al. 2002), which provides information of the Holocene climate conditions and variations, that can be used to check and validate paleoenvironmental interpretations made in this study. Five lakes situated in catchments with different geomorphological settings were chosen, based on the following criteria: (i) they should represent geomorphological settings where one or more of the main processes could be excluded. (ii) they should be situated nearby each other and thereby have been exposed to the similar Holocene climate conditions and variations. (iii) they should be situated in reasonably similar geological settings to reduce major influence of varying bedrock composition on denudation rates. (iv) they should be situated well above the present day tree-line to reduce influence of Holocene vegetation changes in sedimentation rates and to be more sensitive to surface denudation processes. (v) they should be of similar sizes to reduce major scale-dependent differences in sedimentation rates and biological primary production, and have relatively small drainage areas, to enable thorough geomorphological mapping and analysis. The five lakes were chosen based on information from tophographical maps and from earlier studies (Lake Tjåmuhas, Jonasson 1991). The position in the landscape and basic characteristics of the investigated lakes and catchments are shown in Figure 1 and Table 1, and photographs of the lakes with surroundings are shown in Figure 2. All lakes are situated in, or close

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to, the mountain massif sometimes called the Abisko Alps, south of Lake Torneträsk in northern Swedish Lapland. Lakes 865 and 850 (Figs. 1 and 2, denoted by their altitude in m a.s.l.) were chosen to represent neighbouring lakes in a non-glacial setting with little or no rapid slope processes affecting the lakes. There is however differences between the Lake 865 and 850 catchments in size, topography and extent/thickness of soil-cover. Lake Tjåmuhas (Figs. 1 and 2, denoted after neighbouring mountain) was chosen to represent a non-glacial setting with rapid slope processes in direct contact with the lake. The choice was based on the topographic setting and an earlier study by Jonasson (1991) that showed that the lake sedimentation was affected by slope processes in the catchment. Lakes Suorijaure 1 and 2 (Figs. 1 and 2) were chosen as proglacial lakes with little or no direct influence of rapid slope processes. They differ in that Suorijaure 1 has a large catchment including a glacier, while Suorijaure 2 has a relatively small individual catchment but receives almost all fluvial input from Suorijaure 1, as second lake in the fluvial hierarchy. The geomorphology of all the catchments was mapped using aerial photographs and several field visits were made to validate the interpretations. Lake sediments were sampled using both a piston-corer and a gravity corer (to retrieve undisturbed surface sediment) and the number of retrieved piston-cores from each lake is shown in Table 1. Several different proxies have been measured on the sediment cores and the following list encompasses those that have been used in the papers of this thesis. Several additional measurements have been performed, especially on magnetic mineralogy, the result of which will be used in future publications. The fact that measurements have been made on unopened cores, split-cores and on discrete 1 or 2 cm samples respectively have sometimes caused small depth offsets between measured proxies. For further description of the sedimentological methods see Paper II-IV. List of measured parameters: (1) Multi-Sensor-Core-Logger (MSCL) measurements of density (attenuation of gamma rays) and magnetic susceptibility (magn. susc.) on unopened cores and split-cores (only magn. susc.). (2) X-ray radiography used for visualisation of depositional patterns and measurements of grey-scale-density (GSD) in the x-ray images (expressed as mmAl). (3) Weight loss-on-ignition (LOI) on discrete cm samples. (4) Grain-size distribution below 2 mm measured through sieving at 63 µm and for fractions smaller than that in a SediGraph. (5) Magnetic susceptibility measured on discrete cm samples. (6) Saturation Isothermal Remnant Magnetisation (SIRM) on selected cm samples.

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Figure 1. Setting of the investigated catchments and lakes in the landscape. To the left Landsat ETM+, false colour infrared image (green colours include all vegetation and red colours represent bare rock and soil). To the right a digital elevation model with artificial lightning at azimuth 90°.

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Figure 2. Photographs of the investigated lakes. Lake 850 from south, Lake 865 (with Lake 850 and the large Lake Torneträsk in background) from southwest, Lake Tjåmuhas from northeast and the Sourijaure Lakes from southwest.

(7) Anisotropi of magnetic susceptibility on 8 cm2 cube samples. (8) Backscattered electron images (BSE-Images) and

Qualitative element analysis using a energy dispersive spectrometer (EDS) detector, on homogenised cm samples.

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Alpinelakesedimentarchivesandcatchmentgeomorphology

PRESENTATION OF PAPERS

down to the lake.

Paper I

Paper II

Rubensdotter, L. 2002: Detailed geomorphological survey of a small mountain drainage area, Abisko, northern Swedish Lapland. Geografiska Annaler, 84A (3-4), 267273.1

Rubensdotter, L. and Rosqvist, G. 2003: The effect of geomorphological setting on Holocene lake sediment variability, northern Swedish Lapland. Journal of Quaternary Science 18(8): 757-767.2

In this paper the geomorphological mapping method used at all study sites included in this thesis is described in detail for the catchment of Lake 865 (Fig. 2), which was the first to be mapped. The 2 km2 drainage basin is situated above the treeline and covers an altitudinal range between the lake at 865 m a.s.l. to the Suoruoaivi peak at 1193 m, which forms the southern border of the catchment. Field visits showed that periglacial processes (e.g. non-sorted circles and stripes) are abundant close to the lake shores. To gain a good quantitative understanding of the detailed geomorphology, all features were mapped as accurately as possible. Panchromatic aerial photographs in scale 1:30 000 were interpreted in a Zeiss Jena Interpretoscope. To accommodate accurate mapping of very small features the images were digitally enhanced to approximately scale 1:1´500 and treated to optimize sharpness and contrast before traditional stereographic interpretation. This proved to be a powerful tool to rapidly map features down to meter scale, which is the smallest units detectable in aerial photographs of this scale and also the approximate size of the abundant non-sorted circles. This type of single feature mapping of primarily patterned ground features would otherwise have had to be performed in the field, or using microscope on the aerial photos. The results of the mapping (Fig. 3, left map) reveal that several types of periglacial up-freezing features (e.g. nonsorted circles and stripes, sorted circle, and debris islands) totally dominate the area close to the lake. Block fields and nonsorted stripes are situated on top of the mountain Suoruoaivi and talus cones are deposited below the mountain scarpment, while the lower part of the slope is characterised by block streams and single large boulders. A zone with few geomorphological features and well developed vegetation cover (consisting of grasses and herbs) separates the slope that constitutes the southern part of the catchment from the low angle lake surroundings with periglacial features. This zone acts as a barrier between the two more geomorphologically active systems and prevents sediment transport from the slope to the lake, limiting the potential processes affecting lake sedimentation. Only catastrophic events, such as large debris- or slush-flows are expected to be able to transport sediment through the entire slope system,

Two neighbouring lakes and catchments are compared in this paper, both regarding sediment stratigraphies and geomorphological setting. In this study the geomorphological study of the catchment of Lake 865 was continued and a generalized geomorphological map including both Lake 865 and 850´s catchments is presented (Fig 3, left map). For interpretation and comparison of sediment depositional processes in the lakes, X-ray radiographs, grey-scale density (obtained from the x-ray radiographs), LOI and sand-silt-clay composition were used. The investigated lakes have a similar catchment/lake size relation (1:19) but the altitudinal difference is higher in Lake 865´s catchment, although not in the immediate surrounding of the lake where the ground is actually flatter than around Lake 850. The geomorphological investigation revealed that the Lake 850´s catchment has a similar geomorphological setting as the periglacial zone close to Lake 865 (described in Paper 1), with nonsorted circles, -stripes and debris islands. No permanent snowfields, fluvial channels or purely gravitational features are found, and the periglacial processes are not concentrated around the lake shores to the same extent as around Lake 865, partly due to higher abundance of scoured bedrock close to the lake (Fig 2). Two piston cores were sampled in Lake 850 and six in Lake 865. The Holocene sediments in Lake 865 were found to be sparsely laminated clay-gyttja while loosely consolidated gyttja dominated in Lake 850. The minerogenic laminations in Lake 865 were both slowly and rapidly deposited, which is seen in the sharp or gradual transitions to surrounding sediments and as internal sorting (fining-upwards) in the rapidly deposited layers. The rapidly deposited layers (the major one occurring at 4800 calibrated years before present) are interpreted to be caused by sediment laden turbidity flows released outside the lake and probably originating in debris-flow events on the mountain slope in the southern part of the drainage area. We conclude that large differences in lithostratigraphy can occur in neighbouring non-glacial lakes with only minor differences in geomorphological setting. The observed variation between the lakes are probably caused by small differences in geomorphological activity in the catchment together with variability in organic primary production, likely caused by higher

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Reproduced from Geografiska Annaler Ser. A, Copyright (2002), with permission from Blackwell Publishing.

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Reproduced from Journal of Quaternary Science, Copyright (2003), with permission from John Wiley & Sons Limited.

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Figure 3. Geomorphological maps of the investigated catchments. Note the difference in scale, with the Sourijaure map with only half the scale of the 865 and Tjåmuhas maps.

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Alpinelakesedimentarchivesandcatchmentgeomorphology

summer water temperatures in the smaller Lake 850, which also lacks meltwater input from permanent snowfields. Our main conclusions in this paper are that: (i) Minerogenic sediment deposition rates in a lake with relatively stable geomorphological setting (Lake 865, lacking rapid slope or fluvial processes in close proximity) can still vary over time, causing non-linearity in sedimentation, which may complicate age-model constructions. (ii) Non-glacial processes can cause minerogenic laminations with similar density and LOI signal as those that can be found in pro-glacial lakes. (iii) The variability in the Lake 865 sediment sequence represents changing trends in the past environment with enhanced catchment erosion occurring in the early and late Holocene, with more stable conditions in between.

Paper III Rubensdotter, L. and Rousse, S. manuscript in review: Impact of Holocene slope activity on lacustrine archives in alpine environments: catchment geomorphology and sediment stratigraphies in two non-glacial lakes, Lapland, Sweden. Submitted to Journal of Paleolimnology.

In this paper we study variations in lake sediment transportation processes in relation to slope processes, through comparison of geomorphology and sediment physical properties in the two non-glacial alpine drainage basins of Lake Tjåmuhas and Lake 865 (Fig. 2). The lakes are surrounded by morphologically active and passive slopes respectively and results of a multi-proxy study of several Holocene sediment cores from each lake are compared within and between the lakes. The sediment parameters analysed in this paper are; x-ray radiography, density (from MSCLanalysis), mineral magnetic parameters: magnetic susceptibility (MSCL-analysis and on discrete samples), saturation-isothermal-remanent-magnetisation (SIRM) and anisotrophy of magnetic susceptibility (ARM), weight-loss-on-ignition (LOI) and grain-size distribution (SediGraph). Three piston cores were retrieved from Lake Tjåmuhas and six cores from Lake 865. The sediment cores are compared with each other both within and between the lakes. The catchments and lakes are of similar sizes but the geomorphology is quite different with periglacial features in the close proximity of Lake 865 and steep slopes with traces of several different rapid gravitational processes surrounding Lake Tjåmuhas (Fig. 3, central and left maps). The slope processes interpreted to directly affect lake sedimentation most in Lake Tjåmuhas are snow-avalanches, debris-flows and slush-flows. Differences in both organic and minerogenic sedimentation between cores were found in both lakes. Both the geomorphology and results from the lake sediment analysis indicate that the allochthonous

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minerogenic particles in the two lakes have several sources and different transportation processes, which affects both sediment composition and sedimentation rates. Pronounced spatial sediment variability in Lake Tjåmuhas, with active slopes, leads us to suggest that a single rapid mass-movement event (e.g. debris flow) may result in different sedimentation patterns in different parts of the lake. Hence, core location may significantly influence paleoenvironmental interpretations based on mass movement deposits in lakes. An unexpected degree of spatial variability in sedimentation rate was found in the lake with geomorphologically passive setting (gentle slopes). The highest sedimentation rate was not found in the core retrieved at the highest water depth, and the maximum sedimentation rate was not found in the same core during the whole Holocene sediment sequence. These findings has important implications for chronological control in alpine lake sediment archives and emphasizes the need for multiple cores to be sampled to confirm the most complete sediment sequence, before dated paleoenvironmental reconstructions are made. This paper also demonstrates that fine-grained minerogenic layers of similar physical characteristics (density, LOI, magnetic susceptibility and grain-size) occur in very different geomorphological lake settings, which contradicts simplified correlations between grain-size distribution and sediment source and transportation processes.

Paper IV Rubensdotter, L. and Rosqvist, G. manuscript: The significance of geomorphological setting and fluvial redeposition on sediment composition in pro-glacial lakes.

This paper uses the approach outlined in the previous papers and applies it to two neighbouring and consecutive pro-glacial lakes situated just south of the Abisko mountains in the Mårma massif. The lakes Suorijaure 1 and 2 (S1 and S2) are situated next to each other and have the size of 0.02 and 0.03 km2 respectively. Lake S1 has a catchment of 6.2 km2 (largest in this thesis) while Lake S2 receives most of its inflow from Lake S1 and has only a small catchment of its own (0.22 km2, smallest in this thesis). The sediment parameters analysed in this paper are; x-ray radiography, density (from MSCL-analysis), magnetic susceptibility (MSCL-analysis and on discrete samples), weight-loss-on-ignition (LOI), grain-size distribution (SediGraph) and scanning electron microscope (SEM) imaging. The geomorphological mapping (Fig 3, right map) shows that the major processes potentially influencing lake sedimentation in these lakes are: (1) fluvial erosion and redeposition of alluvial fan deposits in the Lake S1 catchment (dominating the Lake S1 sedimentation). (2) turbidity-flows, probably released by episodic debris-flow events in the main inflow

Lena Rubensdotter

channel to Lake S1. (3) wash-denudation of surface sediments, aeolian processes, glacial and glaciofluvial processes (minor influence) and nival processes mainly contributing to the general fluvial erosion and transportation. The results of the geomorphological mapping are compared with a critical examination of the lake sediment archives, with the aims to (i) detect sediment units produced by different geomorphological processes, (ii) detect any possible glacier signature in the sediments of Lake S1 and S2, (iii) understand lake sediment hierarchy by comparing sediment characteristics between the lakes and (iv) detect large changes in sedimentation rates and assess the resulting influence on age-depth models. Conclusions reached in this paper are that fluvial redeposition of alluvial fan deposits may significantly affect pro-glacial lake sedimentation. Depending on the geomorphological setting a non-glacial sediment signal, such as fluvial redeposition, may actually overprint any present glaciofluvial signal. In the studied lakes this results in sediment records with minerogenic laminations of alternating (or combined) glacial and non-glacial origin, which are impossible to separate if only using the most common lithological sediment parameters. This in turn emphasizes the complexity of the sediment transport system in proglacial (paraglacial) settings where redeposition of older glacial sediment is of major importance and highlights the need for thorough understanding of the geomorphological setting in lacustrine sediment studies. Both lakes in this study contain sediment sequences with both episodic (turbidites) and more continuously deposited, although laminated, sediments. The effect of episodic sedimentation on age-depth models is demonstrated by removing turbidite sediment layers from the stratigraphy before age model construction. In this study there is a potential dating error of up to several hundred years in the sediments below turbidites. This study also show the effect of fluvial hierarchy on deposition of sediment in consecutive lakes and we find that normal sedimentation rates (excluding turbidites) are up to five times higher in the first of the two lakes. Grain-size distribution patterns are also affected by the fluvial hierarchy, with especially the silt fraction displaying a general fining from coarse to medium silt between the lakes.

DISCUSSION Geomorphology of the catchments

The investigated catchments contain a number of fluvial, periglacial, glacial and gravitational features indicative of geomorphological processes normally acting in alpine landscapes. The geomorphological maps are presented in Figure 3 and photographs (not shown in the papers) of some of the geomorphological features, are presented in Figure 4.

Active fluvial channels are few, except at the inflow to Lake Suorijaure 1, indicating that much of the fluvial inflow reach the lakes through surface overland flow and small seasonal rills spread evenly over the landscape. Features indicative of fluvial deposition (alluvial fans and delta-like features) are today both active (small features at Lake Tjåmuhas), semi-active (only erosion of fans at Lake Suorijaure 1), and passive (fans and delta surface indicative of past lake levels and drainage directions around Lake 865). Many of the small channels present are today only active during the most intensive snow-melt period and may have been initiated during the last deglaciation. Some traces of intense fluvial erosion are found in the upper parts (mountain cols) of the Lake 865 and Tjåmuhas catchments (Fig. 5), consisting of erosional scarps in sediment and channelised stripping of sediment cover (up to boulder size). These traces were verified in the field and indicates northward glaciofluvial drainage over these cols (direct from glacier or through ice damming of lakes) during ice retreat towards the south, which is consistent with the known deglacial pattern in the Torneträsk region (Melander 1977; Kleman 1992, 1997; Boulton et al. 2001). Different types of periglacial up-freezing features (e.g. nonsorted circles and stripes and debris islands, e.g. Fig 4D, Washburn 1973) are abundant in the catchments, and occur either in depressions (associated with water-logged conditions) and near the lakes or in the upper parts of the catchments and on low angle slopes. Other periglacial features are boulder streams (at Lake 865, Fig. 4B) and solifluction lobes (Fig. 4C), which are associated with slopes and whose occurrence is dependent on a combination of factors including sediment composition, slope angle and vegetation (Strömqvist 1983). Nival features are present in Lake 865, Tjåmuhas and Suorijaure 1 catchments and is most developed around the larger snowfield situated in a niche-shaped depression southwest of Lake 865, where an almost flat surface has formed under and proximal to the snowfield (Fig. 4E and F). Visits in August 1999 and 2002 revealed that this snowfield was almost melted, exposing a waterlogged even stone-paved surface, from which all fine (< sand) sediment was removed. Features indicative of glacial activity are in the investigated catchments restricted to the large ice-cored frontal moraine at the Skadnjalákhu glacier in Lake Suorijaure 1 catchment (Fig. 4A). No other signs of Holocene glacial activity (e.g. front or lateral moraines or lateral channels not correlated to the last big ice sheet) were found in the catchments, confirming the original choice of Lake 865, 850 and Tjåmuhas as non-glacial lakes. Purely gravitational features are to different degrees found in all except the Lake 850 catchments. In the Lake 865 and Suorijaure 2 catchments, rapid slope processes are only represented by talus deposits

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Alpinelakesedimentarchivesandcatchmentgeomorphology

A. Ice cored moraine, Sourijaure 1, with 2 m scale .

E. Nivation hollow, Lake 865, person for scale.

B. Boulder streams, Lake 865.

F. In front of snowpatch in E.

C. Solifluction lobes, person for scale, Sourijaure 1.

G. Avalanche chute, Lake Tjåmuhas.

D. Debris islands, Lake 850, 40 cm bag for scale.

H. Passive talus with lichens and mosses, Sourijaure 2.

Figure 4. Photographs of geomorphological features in the mapped catchments.

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below the steeper slopes. In the case of Lake 865, the talus occurs far from the shoreline and at Lake Surijaure 2 the talus seems mostly passive, with extensive cover of lichen and mosses (Fig 4H). Several gravitational processes are represented in the Lake Tjåmuhas catchment, where rock fall and avalanche generated talus (Fig 4G), together with slush- and debris flow tracks are the most important features, which in many places reach the lake shoreline. In Lake Sourijaure 1 catchment the gravitational features consists of a few well developed avalanche boulder tongues and a little talus, however none of these features reach the lake shore.

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Figure 5. Photographs showing features of intense fluvial erosion in Lake 865 and Tjåmuhas catchments. White broken lines show interpreted flow direction. A, From the ridge towards northeast and Lake 865. B, Southward from the same position as in A. C, The scoured and stripped mountain col south of Lake Tjåmuhas.

Sediment transportation processes detected in the catchments The geomorphological mapping of the catchments (Fig. 3) revealed several features associated with different sediment transportation processes, which can be assumed to bring varying amounts of minerogenic particles into the lakes. The geomorphological features occur to varying extent in the investigated catchments and the list below is therefore not in order of importance. (i) Aeolian transportation and deposition likely contributes to the sedimentation in most alpine lakes. Based on topography (also outside the catchments), which affects the wind-catch area, and general wind directions (often from the west) I postulate that aeolian deposition might be proportionally more important in Lake 865 and 850 than in the rest of the lakes. (ii) Glaciofluvial transportation and deposition of glacially eroded sediment. This process can, because of the topographic settings in combination with the Holocene climate conditions, only occur in the joint catchment of Lake Sourijaure 1 and 2. The absence of glacial activity in the other catchments is confirmed by the geomorphological investigation. (iii) Fluvial erosion and transportation is a general description of several different processes. The most obvious fluvial processes occurring in the studied catchments are; Wash-denudation by saturation overland flow, which occurs to different degrees in all catchments. The transportation efficiency may vary depending on e.g. angle of slope, vegetation cover and plant species composition; Channel erosion and transportation, which is relatively most important in the Sourijaure 1 catchment and least important around Lake 850, due to the occurrence and lengths of fluvial channels; Fluvial redeposition of alluvial material in old fans and deltas is dominating in the Lake Sourijaure 1 catchment and is considered as a separate fluvial process here, since it is judged to be of large importance for the lake sedimentation in that lake. (iv) Rapid slope processes are to varying degree present in all but Lake 850´s catchment, but is most important for the sedimentation in Lake Tjåmuhas. The different rapid slope processes identified in this study are rockfall, snow-avalanches, debris-flows and slush-flows (also called slush-avalanches or slush-torrents depending on the velocity and relative water and/or snow content). (v) Wave and ice erosion of shore-line deposits and within lake transportation of sediments by currents are processes that probably occurs in all lakes. However, most of the lake shore-lines consists of stones and boulders with little exposed fine sediment, and the small lake sizes limit wind-catch and hence the depth of wave action. I therefore believe these processes to be of minor importance for the general sedimentation patterns

11

Alpinelakesedimentarchivesandcatchmentgeomorphology

(vi) I also want to mention sediment exposure processes, enabling secondary fluvial or aeolian erosion, which potentially has had effect on the lake sedimentation. These processes contribute to bring “new” sediment particles to the soil or sediment surface, where it can be subsequently affected by other erosion and transportation processes. This might be especially important in the Lake Tjåmuhas, i.e. debris-flows and slush-flows, and Lake 865 catchments, i.e. periglacial up-freezing.

differ however between these lakes, as sediment from Lake Sourijaure 1 is dominated by coarse silt and sand while the sediment from Sourijaure 2 is generally finer with higher clay-content (up to 30% in the bottom of the Holocene sequence) but show a bimodal grain-size composition with both high sand and medium-silt content. Lake 865 and 850 has similar maximum Holocene sedimentation, around 120 cm, but different sediment composition. Between Lake Sourijaure 1 and 2 the difference in Holocene sedimentation might be up to 5 m, although both laminated sediment sequences are minerogenic with similar organic content. Lake Sourijaure 1 displays a much higher sedimentation rate due to high inflow of coarse grained minerogenic sediment from alluvial and delta-like fans, together with higher organic sedimentation rate. The results reveal large differences in sedimentation rate and pattern between neighbouring lakes and emphasize that catchment size and geomorphological setting plays a significant role in determining sedimentation characteristics. A comparison of all lake sediment stratigraphies (Figs. 6 and 7) reveals that the most similar in terms of deposition pattern, density, and LOI are Lake 865 and Sourijaure 2. Their settings are however very different since Sourijaure 2 has a much smaller primary catchment compared to Lake 865, but a much larger total drainage area through the Lake Sourijaure 1 catchment (including the small glacier). The sediment signal in Lake 865 is definitively non-glacial, while some of the Holocene sediment variability (although not the turbidites) in Lake Sourijaure 2 could have a glacial origin even though it is the second sedimentation basin from the glacial source (Souch 1994). The similarity of these sediment records points again to the risk of over-interpreting the glacial sediment signals in pro-glacial lakes. Similar depositional patterns as in proglacial lakes, reflected in density, LOI, magnetic susceptibility and distribution of fine grainsizes (silt and below) is, as shown here, found also in non-glacial lakes. The non-glacial processes detected as responsible for these deposits are also present in many, if not most, glaciated catchments, and may to

Comparison of the sediment sequences Between two and six piston cores have been retrieved from each of the studied lakes (Table 1). The position of the cores in relation to the sediment-water interface has been established through comparisons of xray radiographs with surface gravity cores obtained in close proximity to the longer piston cores. X-ray radiograph composites, MSCL density and LOI are presented for one core from each lake in Figure 6 and the grain-size distributions from all lakes but Lake 850 are presented in Figure 7. Note that only the uppermost 135 cm are shown for Lake Sourijaure 1 in Figure 6, while the maximum sampled sequence (388 cm) is seen in Figure 7 for the same lake. The total Holocene lacustrine sediment sequences vary between ~78 cm, for the shortest core from Lake Tjåmuhas, up to around 6 m in Lake Suorijaure 1 (based on extrapolation of the mean sedimentation rate calculated for the longest 388 cm core). The Holocene sediment sequence in Lake 865 is composed of clay-gyttja (5-10% LOI) with thin minerogenic laminations and high relative content of silt and clay. In Lake 850 the sediment consists of homogenous gyttja with LOI values of 20-30%. The three piston cores from Lake Tjåmuhas are quite different from each other, but all display variable, laminated minerogenic sediments (LOI mostly below 3%) with high content of random clasts of varying sizes. The minerogenic layers varies from mm to dm in thickness and from well sorted silt or sand to coarse-grained and diamict deposits. Both Lake Sourijaure 1 and 2 have laminated minerogenic sediment sequences with LOI values generally between 2 and 6%. Grain-sizes

Maximum Lake size Catchment Lake:catchment depth (m) size (km2) relation (km2)

Number of piston cores

Lake

Position

850

68°18' N19°07' E

8m

0.02

0.35

1:17.5

3

865

68°18' N19°07' E

11m

0.1

2.0

1:20

6

Tjåmuhas

68°16' N18°42' E

11m

0.15

2.8

1:18.6

3

Sourijaure 1

68°09' N18°46' E

6.4m

0.02

6.2

1:310

4

Sourijaure 2

68°09' N18°46' E

23.5m

0.03

0.22

1:7.3

2

Table 1. Geographical information about the lakes and catchments in this study together with the number of retrieved cores.

12

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Figure 6. X-ray radiograph composites, MSCL density (grey-scale density for Lake 850), radiocarbon dates and LOI for one core from each lake. The radiocarbon dates are, in all but Lake 850, transferred from other cores. The contrast in the x-ray radiographs have been differentially enhances between the cores and grey-scale values should therefore not be compared between cores.

varying degree (depending on setting in relation to the lakes) contribute to the total minerogenic lake sedimentation. If their sedimentological signal resembles that of the glacial sediment it will inevitably lead to over-interpretation of the glacial activity. Based only on a comparison of the sand/silt/clay relation and grain-size distribution patterns below 63 µm (Fig. 7), it seems that Lake Tjåmuhas and Sourijaure 1 are most similar. Both lakes have a high inflow of coarse silt and sand, however in Lake Tjåmuhas particles coarser than sand (dropstones) are abundant all through the Holocene while the sediments in Lake Sourijaure 1 are more well sorted with almost no particles larger than the sand fraction. The reasons for the difference in coarse grain-size distribution between the lakes are the fact that Lake Tjåmuhas receives slope-process deposits while the sedimentation pattern in Lake Sourijaure is mostly affected by fluvial redeposition of alluvial deposits. These comparisons of sediment sequences reveals both similarities and differences in sediment pattern and composition, of which many would not be predictable from only a general knowledge of the topographical and glacial/

non-glacial setting. This study is mainly focussed on discussing the variability of minerogenic sedimentation but organic sedimentation is however also important in forming the overall deposition pattern. For example in Lake Sourijaure 1, the organic sedimentation rate also contributes to the high sedimentation rate. This is illustrated by the LOI values that are similar as in Lake Sourijaure 2, even though the total Holocene sediment sequence is (up to) six times thicker. The total Holocene organic sedimentation may thus be up to six times higher in Lake Sourijaure 1 compared to Sourijaure 2. This large difference may be due both to higher primary production, higher allochthonous organic input from the larger catchment and/or a lower degree of organic decomposition caused by the high minerogenic deposition, which buries the organic material and thereby protects it against decomposition.

Complexity in alpine lake sedimentation mechanisms Two independent variables, or sediment forcing mechanisms, are important for the understanding

13

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Figure 7. Grain-size distribution in four of the five lakes. To the right of each lake plot, volume percent of grain-size in fractions sand, silt and clay are shown. Surface plots of grain-size distributions below 63 µm are shown to the left. The surface plots are calculated for continuous 2 cm samples (in Lake 865 and Tjåmuhas) and for selected 1 cm samples from Lake Sourijaure 1 and 2 (position of samples marked with diamonds) with interpolation between the sampled levels. It is important to note that the depth scales are different and that the plummet to zero at 63 µm is not true, but an artefact of the method.

of variability in minerogenic sedimentation. Firstly, provenance signals, i.e. differences between sediments originating from different parent material. Minerogenic particles can for example originate from local catchment bedrock or from a wider geographical area i.e. material transported by a larger ice-sheet. Secondly it is of interest to detect and separate minerogenic material delivered by different transportation processes, e.g. fluvial transport or avalanches. The results from this study show that these two factors together form a quite complicated pattern. One can, and in formerly glaciated landscapes inherently will, have situations where particles originating from glacial or glaciofluvial deposits subsequently will be transported to a lake by one or several different other processes. The measurable physical parameters of such particles in a sediment sequence provide a signal derived from separate sources and processes. An example is sediment with a mineralogy indicating a bedrock source

14

from a specific rock slope talus, while the grain-size distribution in the specific sediment unit containing the particle might indicate a high degree of fluvial sorting. At the same time the magnetic susceptibility could be affected by fluvial sorting (enriching or depleting grain-sizes rich of magnetic minerals) showing high values similar to e.g. deglacial glaciofluvial deposits, while the position in the core is associated with a turbidite (well defined stratigraphic unit with fining-upward of grain-sizes). The complexity of the processes and triggering mechanisms affecting minerogenic lake sedimentation in alpine settings is illustrated by Figure 8. The diagram is by no means a complete explanation of reality but puts the research using alpine lake sediments to reconstruct environmental change in some perspective regarding the complexity of the system. The box diagram shows that one trigger mechanism may have different process responses and different

Lena Rubensdotter

processes may result in similar sediment deposits. This thesis concerns mostly what is included in the shadowed area. For this study it is the last transportation processes affecting the sediments before deposition in the lake that is of interest, as is mostly the case in research on minerogenic sedimentation aiming to reconstruct and date paleoenvironmental variations. Knowledge of the original sediment source can sometimes lead to that information, however it can as mentioned also be totally misleading due to processes of redeposition. With these factors in mind, the results from high resolution x-ray radiographs can be of decisive importance for the final interpretation, since the xray radiographs reflects the actual process of deposition (and possible post depositional processes), while other physical parameters sometimes tell different stories. Also thin-section analysis of sediments (not performed in this study) can provide detailed depositional and post depositional information (Francus and Karabanov 2000; Last and Smol 2001).

Varying influence of different sediment transport processes on the sediment sequences Different combinations of sediment characteristics indicative of different depositional mechanisms are identified in this study. Well defined high density minerogenic layers (as high as in the deglacial glaciofluvial deposits), low organic content, sharp lower transition and fining upwards of grain-sizes are indicative of sediment deposition from turbidity flows. These can

be released both within lakes, through slumping of bottom sediments on slopes, or result from episodic fluvial or gravitational events bringing sediment laden water to a lake. Turbidites are identified in Lake 865 and in Sourijaure 1 and 2. Very thin turbidites (mm-scale) may be present in both Lake 865 and the Sourijaure lakes, but are difficult to identify with the sample resolution (0,5-2 cm) mostly used in this study. Such thin turbidites could possibly be identified using x-ray radiography in combination with thin-slide analysis, which can reveal grain-size trends over sub mm distance. Thin (mm-cm) turbidite layers may also be present in Lake Tjåmuhas, but the generally high density and clast content in these sediments makes them difficult to distinguish. The density and magnetic susceptibility of the sediment sequences mostly co-varies, but not always. High density layers with low magnetic susceptibility may therefore be indicative of a different sediment source or grain-size sorting process (type of sediment transportation process and/or transportation distance) compared to similar layers with high magnetic susceptibility. Examples of this are seen at one level in the bottom of the Lake 865 sediment sequence and in parts of the deglacial deposit in Lake Sourijaure 2. Grain-sizes vary significantly in the sediment sequences, which is also evident in the x-ray radiographs. This variability is probably underestimated in the data presented in this thesis because the sampling resolution for grain-size analysis (2 cm in Lake Tjåmuhas and 865 and 1 cm in Lake Sourijaure 1 and 2) is much lower compared with much of the laminae thicknesses (sub-cm). The x-ray radiographs can

Minerogenic lacustrine sediments in alpine environments �����������������

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Figure 8. Diagram of some of the complex relationships governing minerogenic sedimentation in alpine lakes, and the potential relations to climate triggering mechanisms. It is important to note that this diagram is not perceived as a full description of reality but is assembled to illustrate the complexity of the system and potential problems in interpretation.

15

Alpinelakesedimentarchivesandcatchmentgeomorphology

however to some extent compensate for this, since they can be used to identify grain-size forced density variations on sub-mm scale (image resolution of 300 dpi in the digital version, scale 1:1).The most obvious grain-size shifts are found in association with the turbidites and between deglacial and Holocene sediment. However there are more finely laminated grainsize variations revealing higher frequency variability in the minerogenic sedimentology. Two types of thin minerogenic layers can be identified visually on the cores and in the x-ray radiographs. These are thin silt and clay laminations down to almost sub-mm scale, which occur in all lakes but Lake 850 (well illustrated in Fig. 14B and E, Paper III this thesis), and mm-cm layers of sorted sandy material occurring in Lake Tjåmuhas and Sourijaure 1 (e.g. Fig 14F, Paper III, this thesis). The sandy layers, although similar between the lakes in visible grain-size, have different mineralogy and probably also separate transportation processes origin, due to the different geomorphological setting of the catchments. The sandy layers are probably associated with debris-flow or slush-flow events in Lake Tjåmuhas and with high energy fluvial events in Lake Sourijaure 1.

Glacial and non-glacial signals in proglacial lakes Many, if not most, of the alpine sediment sources are dependent on paraglacial preconditions (Ballantyne 2002). Past glacier activity thus has a long-lasting effect on sedimentation patterns in the investigated lakes, which makes distinction of sediment transportation processes difficult, since much of the sediment may be redeposited glacial and glaciofluvial deposits. This is specifically evident when trying to identify evidence for neo-glacial activity based on multi-proxy variability in a sediment sequence. This problem probably exists in most alpine proglacial settings. Almost all sediment denudation and transportation processes discussed in this thesis are affected by sediment availability and topographical preconditions created by the last glaciation. Till and glaciofluvial deposits, which can be reworked and eroded by fluvial, aeolian, gravitational or nival processes, are together with sharpened topographical profiles, triggering slope processes, all largely induced by former glacial activity. According to Ballantyne (2002), and earlier suggested by Church and Ryder (1972), Karlén (1981) and Leonard (1986b), the maximum sediment delivery to proglacial lakes occurs during the initial stage of glacial retreat, rather than during periods of glacier advance. Ballantyne also interprets this pattern as representing the large influence of paraglacial reworking of sediments on recently exposed glacier forelands, more than the direct sediment delivery from glacier erosion to the proglacial melt-water system. He argues that that enhanced minerogenic sediment delivery to

16

lakes might actually represent warming and glacier recession rather than cooling and glacier advance. Increased fluvial process activity, e.g. caused by increased precipitation or increased snowmelt intensity caused by raised spring temperatures may coincide with maximum glacial sediment output (Jansson et al. 2005) to cause a high minerogenic lake sedimentation, which is not correlated to cold conditions and glacier growth. In this study it was not possible to reliably distinguish any Holocene glacial signal in the proglacial lakes, Sourijaure 1 and 2, since sediment input by fluvial redeposition is dominating the sedimentation and diluting and obscuring the potential glacial sedimentation signal. Silty minerogenic sediment laminations are frequently used to infer variability in glacial denudation (Matthews and Karlén 1992; Seierstad et al. 2002: Dahl et al. 2003; Bakke et al. 2005b,c). Such laminations are identified in all Holocene lake sediment sequences analysed in this study, except in Lake 850, regardless of geomorphological settings. The different processes that produces and/or deposits these silty sediments are likely active in many alpine catchments, i.e. maybe especially at proglacial sites were old glacial deposits are abundant, slopes often are steep and weathered and vegetation cover is scarce. Reconstructions of past glacial activity based on the occurrence of minerogenic fine-grained sediments in proglacial lakes thus probably often overestimate the glacial input to the lake. This could to some extent, in addition to error margins in dating and that different glaciers may have varying response time to climate changes, explain the difficulty to precisely correlate glaciolacustrine records from relatively nearby glaciers and lakes (seen in Leonard 1986; Bakke et al. 2005c).

Varying sedimentation between cores Differences between cores are found in all lakes except in Lake 850 where the homogeneity of the organic Holocene sediment sequence prevents correlations and thereby comparisons of the sampled cores. The difference between cores is most evident in Lake Tjåmuhas, where the only certainly correlatable units were the glaciofluvial clays and silt encountered in the bottom of all three piston cores. Both the total thickness and composition of the Holocene sediment sequence is significantly different between the cores. In Lake 865 the differences between cores are not as striking at first but a comparison of the sediment thickness between well correlated levels shows that sedimentation varies between cores obtained from similar water depths. The variability also is inconsistent through the Holocene in that one single core does not show the highest sedimentation rate throughout the whole sequence. In contrast to what we expected, the core retrieved from the highest water depth has the lowest sedimentation rate of the compared cores.

Lena Rubensdotter

In Lake Sourijaure 1 and 2 obvious differences occur between cores sampled at similar water depths. This is especially evident in the Lake Sourijaure 2, where core S2-1 has at least 20 cm longer Holocene sediment sequence compared to core S2-2. This is a large difference considering that the total of the longest sequence (in S2-1) is only around 100 cm. In this lake the upper parts of both cores suffer from disturbances caused by the coring process. The four cores from Lake Sourijaure 1 are easy to correlate because of the turbidites and other large density variations, but the small scale sediment pattern varies both within the sediment sequence and between cores, with the highest sedimentation rates (e.g. distances between correlated layers) at different depths in different cores (as in Lake 865). The results from the study of sediments from Lake Tjåmuhas, dominated by rapid slope processes, clearly show that it is impossible to obtain a representative sediment profile from this type of lake based on only one core. Cores from lakes directly affected by gravitational processes should only be used to infer changes at that specific coring site (or possibly part of the lake), which makes them unsuitable for regional paleoenvironmental reconstructions. The discussion above shows that it is unexpectedly difficult to find the core with highest sedimentation rate all through the sequence, which has implications for other palaeoenvironmental studies using lake sediments. Often authors describe sampling only one core from the deep or deepest part of a lake (Björck et al. 1991; Nesje et al. 2004; Bigler et al. 2002), without any information about how well this coring site represent maximum sedimentation rates. To resolve this type of problem it is important to sample and e.g. x-ray several replicate cores to provide stratigraphic information that can be used to correlate and compare individual sections between cores. This is, as demonstrated, necessary even in small lakes with simple bathymetry. It is only through comparison of cores that the maximum or mean sedimentation rate can be estimated and used in age-model constructions and paleoenvironmental inferences.

Varying sedimentation rates and agedepth relations The distinct turbidites in the Sourijaure Lakes and Lake 865 were deposited during a very short time interval compared to the surrounding sediment. This is demonstrated by the depositional pattern of finingupwards grain-size distribution visible in the grainsize distribution plots for the thicker examples, and visibly noted in colour and appearance on the sediment surface of the thinner turbidites. It is also detectable in the SEM analysis and images, which show a lack of diatoms in the turbidite sediments while the

surrounding sediment units have abundant diatoms, indicating a deposition rate much larger than that of annually produced diatom frostules. The difference in age-depth relations when including and removing detected turbidites from the age-depth model is calculated in paper IV. The difference in sediment age for levels below the two thickest turbidites are up to ~350 years in Lake Sourijaure 1 and up to ~1200 years in Sourijaure 2. Also the organic sedimentation rate has been shown to vary within cores (Paper II and III, this thesis), and potential variability in the many different types of minerogenic laminations discussed above are possible, even likely, but can, due to poor age constraints in this study, not be safely determined. The results from this study indicate that the chronological precision of lake sediment archives might suffer from, potentially large, inherent errors due to variable sedimentation rates within the cores. The only way to overcome this potential problem is to sample and compare several cores and/or radiocarbon date numerous levels within one core.

The sediment sequences in relation to known Holocene climate changes The lake sediment sequences from Lake 865 and Tjåmuhas indicate an increase in episodic and/or precipitation induced sediment transport events in the later part of the Holocene (2000-3000 years in Lake 865, and not datable in Lake Tjåmuhas). A similar pattern can tentatively also be detected in Lake Sourijaure 1 during the last ~1200 years where it is recorded as a higher sedimentation rates (not counting potential turbidite in the mid Holocene) and higher frequency changes in the variability of minerogenic sediment inflow, reflected in density and magnetic susceptibility, together with a slight increase in the same values. This signal is however also suggested to be a possible sign of neoglaciation in the Sourijaure 1 catchment. Either way the changes could be a sign of the same climate signal as the changes in Lake 865, i.e. higher precipitation and /or lower temperatures.

Future studies This project has helped to identify methods which can be used to resolve the problem of attributing a specific sediment layer to a specific erosion/transportation process. In addition to the main measurements made in this project (stated above), more discriminatory information can be obtained using the following techniques, of which a few were tested in this study. X-ray radiography and detailed grain-size analysis are important complements to the methods mentioned below. Measurements of mineral magnetic parameters (in Lake 865 and Tjåmuhas) and especially magnetic anisotropy, indicates differences in magnetic mineralogy and orientation of magnetic particles between some

17

Alpinelakesedimentarchivesandcatchmentgeomorphology

of the minerogenic layers. Detailed changes in mineralogy could further be explored using e.g. X-Ray Diffraction (XRD, Last and Smol eds. 2001; Yuste et al. 2004; Flogeac et al. 2005). A more precise qualitative way to gain information on mineralogy is through Scanning Electron Microscope (SEM) analysis (Last and Smol eds. 2001; Yuste et al. 2004) together with element analysis through a energy dispersive spectrometer (SEM-EDS, Acquafredda and Paglionico 2004; Pereira et al. 2004), which was tested on some samples from the Suorijaure lakes (Fig 9). SEM analysis is a powerful tool to gain elemental composition and through this mineralogical information from single grains, at the same time as achieving visual information on grain shapes and microtextures (Mahaney et al. 2001; Van Hoesen and Orndorff 2004), which are dependent on mineralogy, denudation and weathering history of the particles. New core-scanning techniques, using X-ray fluorescence (XRF) and digital X-ray radiography, have become available which allows element analysis to be made together with digital x-ray analysis with submm spatial resolution, directly on split cores. This is a method which enables rapid qualitative determination of the geochemical composition of the sediment, indirectly linked to both depositional processes and

50

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Figure 9. Example of the information achieved through SEM (scanning-electron-microscope) analysis using a BSE detector and performing qualitative element analysis on specific grains in the images. The sample is from the thick “turbidite” layer at 320 cm depth in Lake Sourijaure 1.

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mineralogy. Another method that can yield detailed information of detailed (sub-mm) depositional pattern of different sediment deposits is thin-section analysis (Last and Smol eds. 2001). This technique is frequently used to gain micro scale information of both terrestrial and fluvial sediment deposition patterns (Van der Meer 2000; Francus and Karabanov 2000) and might be of use to detect e.g. the occurrence of two different sediment transportation processes working simultaneously but contributing with different types of particles. The results from this study clearly shows that lakes directly affected by rapid slope processes are not suited as paleo-archives. Potentially it would instead be possible to achieve a more generalised record of slope activity if a second lake downstream, ideally with only a small individual catchment, was investigated. In such a lake, studies of the fine grain sizes (i.e. silt and clay) could reflect the combined activity of several slope processes e.g. debris-flows and avalanches affecting the lake upstream. This of cause requires a good control over the whole sediment signal, in order to separate the sediment derived from the upstream lake from other potential sources. Following this study it would be interesting to try to produce qualitatively refined lacustrine records of environmental change by careful selection of study sites, based on the detailed geomorphological setting. This approach would be focused on finding different sites where only one of aeolian, fluvial, glaciofluvial and rapid slope processes would dominate minerogenic lake sedimentation. A combination of the above mentioned techniques would then be implemented on the cores to validate the interpretations of sediment depositional processes made from the geomorphological evidence and use the sediment record to achieve a dated record of variations over time. If certain physical attributes, or combinations of such, can be used to discriminate between minerogenic sediment units linked to different processes, calculation of mass accumulation rates of these sediment types would yield information of their temporal variability. Comparison of records from such sites would then enable a better qualitative understanding of environmental variations in a region, through the results from proxies sensitive to different climatological triggering factors, e.g. precipitation, wind and temperature.

Lena Rubensdotter

CONCLUSIONS This project has shown the multi-source origin of minerogenic alpine lake sediment. Detailed studies of both geomorphological setting and lake sedimentology reveal several different types of minerogenic laminations with different origins in both proglacial and non-glacial alpine lakes. The fact that numerous geomorphological processes contribute with minerogenic sediments to alpine lakes means that it is crucial to understand the geomorphological setting when reconstructing environmental change using lacustrine sediment records. Another important conclusion is that non-glacial lakes can contain laminated minerogenic silty sediments, similar in character (elevated density and magnetic susceptibility and lowered LOI) to sediment found in proglacial lakes, where they may be used for reconstructions of glacier fluctuations. Because sediment erosion and transportation processes (e.g. fluvial, nival, periglacial and slope processes) present in non-glacial catchments are common also in proglacial settings, proglacial lake deposits likely contain nonglacial minerogenic sediment signals easily confused with directly derived glacigenic sediment. This cause overestimation of glacier activity and extent back in time. Therefore detailed knowledge of catchment geomorphology, and especially occurrences of alluvial deposits exposed for fluvial redeposition, is very important when studying proglacial lake sediments to be able to identify non-glacial minerogenic contribution to the sediment sequence. Physical parameters used in this study that are suited to detect variability in sediment sources and transportational and depositional processes are; Density variations detected in X-ray images, which enables detection of sub-mm scale depositional features; Mineralogy, which in sediment deposits is governed by both the mineralogy of the parent material and transportation distance and process (changing the composition through sorting and mechanical and chemical weathering); Grain-size distribution, which shows sorting patterns dependent on source material and/or type of transportation process and distance; Magnetic mineralogy, which is forced by magnetic mineralogy in the parent material, and secondarily by grain-size distribution; Magnetic anisotropy, which can indicate depositional conditions, such as flow direction and turbid contra grain-by grain deposition. This study also shows that x-ray radiographic analysis is an important tool to detect disturbances in core stratigraphy, caused by depositional processes or coring techniques, which both may affect both the numerical values of analysed sediment proxies and estimates of sedimentation rates.

The demonstrated multi-source origin of minerogenic lake sediment in alpine settings can, in combination with new techniques for sediment analysis, enable a reduction of errors in paleoenvironmental reconstruction based on variability in minerogenic lake sediment. It also opens possibilities of more multi-faceted paleoenvironmental studies in which several different minerogenic process-proxies, in combination with thorough studies of catchment processes, could be used to detect and separate past changes in different climate parameters (e.g. precipitation, temperature and wind). This study demonstrates that large differences in lake sedimentation pattern occur in neighbouring non-glacial lakes. The differences are probably depending on varying position of geomorphological features and processes in relation to the lakes together with general differences in catchment size and surface sediment availability. Variable sedimentation rates between cores occur in all investigated lakes and is not directly linked to water depth. It is therefore important to sample multiple cores in alpine lakes to obtain a representative sediment sequence and avoid misinterpretations based on coring-site specific sedimentological factors. Variable sedimentation rates within individual cores are common in alpine lakes. This complicates age-depth model constructions and emphasise the need for closely spaced age determinations to constrain changes in sedimentation rate caused by episodic or variable sedimentation processes. This is especially important if the aim is to achieve reliably dated high resolution paleoclimate information. The lake directly affected by rapid slope processes (Lake Tjåmuhas) show very large variability in sediment sequences between cores, suggesting variable and highly local sedimentation patterns within the lake. Inferences of stratigraphical changes related to environmental conditions from such cores are thus only valid for the specific core site. This type of lake is therefore not suitable for studies of regional environmental changes. An increase of slope processes (in Lake Tjåmuhas) and/or precipitation induced sediment transport events (i.e sediment composition changes and/or increases in frequency of variability) are in some lakes (Lake 865, Tjåmuhas and S1) recorded in the later part of the Holocene. This is excluding the identified turbidites in Lake 865 and both Sourijaure lakes, which are believed to be more linked to meteorological anomalies (e.g. extreme rainfall) than climate variations.

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A critic might say that this study only is valid for the small specific drainage areas and lakes studied, and that other conditions might exist elsewhere. This statement is valid and in itself points to the overall most important conclusion of this project: Lacustrine sedimentation patterns in alpine environments are highly dependent on very local factors in geomorphological setting, which together with other environmental factors create the lake sediment sequences.

ACKNOWLEDGEMENT The multi-proxy approach of this study in combination with my elongated PhD period (due to the arrival of my two wonderful children) has meant that many people have been involved along the way to which my thanks are due. My main supervisor Gunhild Rosqvist (Ninis) has followed me from the start and all the way for to finally push me over the finishing line. She enthusiastically helped initiate this project and mentored my development as a scientist. I especially want to thank her for supporting me in getting a family and “a normal life”. The rest of my supervisor group; Christer Jonasson (the first three years), Ingmar Borgström, Eve Arnold and Jens-Ove Näslund (the last years) have all contributed in many ways, both in the field (Ingmar and Eve), laboratory and in the writing process, for which I want to sincerely thank them. I thank Camilla Hansen, Christian Bigler, Mats Eriksson, Ola Fredin, Bengt Wanhatalo, Gunhild Rosqvist, Henrik Törnberg, Eva-Stina Gran and Urban Pettersson for field assistance and Christer Jonasson for logistic support at Abisko Scientific Research Station (ANS). Wibjörn Karlén and Arjen Stroeven gave valuable comments on earlier drafts of manuscripts in this thesis, and I also want to thank Peter Kuhry for constructive comments on the whole thesis. I want to thank all colleagues at the Department of Physical Geography and Quaternary Geology (INK) for shared fun over the years. The administrative and logistical staff at the department is particularly acknowledged for all their help and support. The PhD group at INK has served as a source of many laughs and talks around the coffee-table. I especially want to mention Christina, Hanna and Maria as fellow mothers in this business. -Hang in there! The staff at the Institute of Rock Magnetism (IRM) in Minneapolis, USA was most helpful during my visit there in 2000. I also would like to extend my warm thanks to the people in the Landscape and Climate and the “Geohazard”-groups at NGU, Trondheim, for their support and encouragement during the last year. Special thanks to Eiliv Larsen for research discussions (førstyrrelser) and to my roommate Valentin Burki for help and encouragement. Last, but of course not the least, I want to thank my family and parents who has supported me in

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something as weird as staying a student to the age of 33. Especially my husband Ola and darling daughters Mina and Tea are just the best. I could not have finished this thesis without Ola´s help with everything from washing up to MatLab and DEMs – thank you! This project was supported by grants from the The Royal Swedish Academy of Sciences (ANS in Abisko), Svenska Sällskapet för Antropologi och Geografi, Hans W:son Ahlmanns Fond för Geografisk Forskning, Carl Mannerfelts stipendiefond, Liljevalchs stipendiefond and the Swedish Research Council (VR) to Gunhild Rosqvist.

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