Rekonstruksjon av sen-holosene brevariasjoner og skråningsprosesser - En studie av innsjøsedimenter fra Russvatn i Øst-Jotunheimen
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This study considers an area located around the western part of lake Russvatn in eastern Jotunheimen, central southern Norway. I have mapped processes in the catchment of this lake, and tried to connect these processes to the sedimentation in the lake. To do this, a 293 cm long sediment core (BL1-05) was taken from the westernmost part of lake Russvatn. The coring site is situated at a place where it is likely to get a signal from both glacial and non-glacial processes that have been active throughout the Holocene. A multiproxy approach with signals from magnetic susceptibility, bulk density, loss on ignition and grain-size distribution in the core has been used to analyse the BL1-05 core. I have done a brief evaluation of all the methods used in the core analysis. There are nine episodic sedimentation events related to debris flows in the core. These events are recognized mostly by their grain-size distribution, sorting and grading, and coupled with facies-models such as the Bouma sequence. On the basis of mean grain-size, degree of sorting, and the amount of macrofossils, seven flooding events are recognized in BL1-05. From these episodic events I have suggested a debris flow chronology, and thus an interpretation of extreme rainfall episodes in the area. The debris flow episodes was most frequent at 3850-4000 cal. yr BP, 2500-2600 cal. yr BP and around 1000 cal. yr BP. There were also smaller episodes at 685 and 520 cal. yr BP. The debris flow episodes seem to have a tendency to occur in periods with high ELA on the glacier Blackwellbreen. This could indicate the importance permafrost degradation in relation to these events. The properties of the sediments deposited at a more or less continuous rate throughout the Holocene are believed to reflect variations in the extent of the glacier Blackwellbreen. With the use of the analytic parameters magnetic susceptibility, loss on ignition, bulk density, and coarse silt in these sediments, a late-Holocene glacierfluctuation- curve has been constructed. From this curve it is suggested that there was no glaciers in the catchment in three periods at 3935-4037 cal. yr BP, 3630-3782 cal. yr BP, and 1878-2854 cal. yr BP. There was higher glacial activity than at present in the periods at 0-130 cal. yr BP, 450-575 cal. yr BP, 600-605 cal. yr BP, 900-1050 cal. yr BP, 1080-1410 cal. yr BP, 1610-2200 cal. yr BP, 2530-2650 cal. yr BP, 2950-3070 cal. yr BP, 3250-3670 cal. yr BP, and 3800-3930 cal. yr BP. In an attempt to determine the dominant thermal regime for each glacial period I have sketched a method that with by use of grain size distribution in a proglacial lake can indicate the thermal regime of glaciers in the catchment. This method is at this point tentative, and has elements of uncertainty. However, to investigate the potential of this method, it was used in a reconstruction of former glacier properties at Blackwellbreen. On the basis of this method it seems that periods at 3800-4000 cal. yr BP, 2850-3000 cal. yr BP, and also short periods around 630 cal yr BP and 100 cal. yr BP, have been dominated by a cold based thermal regime. This study demonstrates the importance of having a good understanding of the catchment, before doing any climatic interpretation based on lake sediments.