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Holocene ecosystem ontogeny and element cycling in the subarctic catchment of lake Torneträsk, NW Sweden: Large lake sensitivity to climate change
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
NERC Isotope Geosciences Facilities, British Geological Survey, Nottingham & Centre for Environmental Geochemistry, School of Geography, University of Nottingham, Nottingham, United Kingdom.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Understanding the response of sensitive Arctic and subarctic landscapes to climate changes is essential to determine the risks of ongoing and projected climate warming. Responses to climate change are not likely to be uniform because of site-specific differences in ecosystem susceptibility to climate forcing. Here we present a multi-proxy study of the development of the 330 km2 lake Torneträsk in northern Fennoscandia over the past 10000 years. By comparing responses of this large-lake system to past climatic and environmental change with those in small lakes in its catchment, we assessed when the magnitude of change was sufficient to affect an entire region rather than only specific areas of a watershed that may be more sensitive to environmental changes. Our results show three major periods of regional landscape alteration during the Holocene with distinct changes in the sediment composition: i) early landscape development (from c. 10000 until c. 5300 cal yr BP) following deglaciation, ii) increased soil erosion from c. 1300 until c. 100 cal yr BP, and iii) a rapid change during the past century to conditions similar to those before 1300 cal yr BP related to ongoing climate change. However, the timing and type of response to the different forcings differed fundamentally. The gradual early landscape development led to a successive change of the sediment composition, whereas the gradual climate cooling during the late Holocene, which altered terrestrial inputs to the lake, caused a rather abrupt shift in the sediment composition. However, this composition change occurred ~2000 years later than the first indications for climate cooling recorded in small lakes in the Torneträsk catchment; this suggests that a critical threshold was crossed around 1300 cal yr BP. In contrast to the gradual changes in the early/mid-Holocene and abrupt but delayed response at 1300 cal yr BP, the ongoing response to recent climate changes was immediate and of equal magnitude, emphasizing the unprecedented scale of the ongoing climate changes in subarctic Fennoscandia.

Keyword [en]
Geochemistry, soil erosion, climate change, oxygen isotopes, silicon isotopes, paleoecology, Holocene
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:umu:diva-111628OAI: oai:DiVA.org:umu-111628DiVA: diva2:872248
Available from: 2015-11-18 Created: 2015-11-18 Last updated: 2015-11-19
In thesis
1. Infrared spectroscopy as a tool to reconstruct past lake-ecosystem changes: Method development and application in lake-sediment studies
Open this publication in new window or tab >>Infrared spectroscopy as a tool to reconstruct past lake-ecosystem changes: Method development and application in lake-sediment studies
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Natural archives such as lake sediments allow us to assess contemporary ecosystem responses to climate and environmental changes in a long-term context beyond the few decades to at most few centuries covered by monitoring or historical data. To achieve a comprehensive view of the changes preserved in sediment records, multi-proxy studies – ideally in high resolution – are necessary. However, this combination of including a range of analyses and high resolution constrains the amount of material available for analyses and increases the analytical costs. Infrared spectroscopic methods are a cost-efficient alternative to conventional methods because they offer a) a simple sample pre-treatment, b) a rapid measurement time, c) the non- or minimal consumption of sample material, and d) the potential to extract quantitative and qualitative information about organic and inorganic sediment components from a single measurement.

The main objective of this doctoral thesis was twofold. The first part was to further explore the potential of Fourier transform infrared (FTIR) and visible-near infrared (VNIR) spectroscopy in paleolimnological studies as a) an alternative tool to conventional methods for quantifying biogenic silica (bSi) – a common proxy of paleoproductivity in lakes – in sediments and b) as a tool to infer past lake-water total organic carbon (TOC) levels from sediments. In a methodological study, I developed an independent application of FTIR spectroscopy and PLS modeling for determining bSi in sediments by using synthetic sediment mixtures with known bSi content. In contrast to previous models, this model is independent from conventional wet-chemical techniques, which had thus far been used as the calibration reference, and their inherent measurement uncertainties. The second part of the research was to apply these techniques as part of three multi-proxy studies aiming to a) improve our understanding of long-term element cycling in boreal and arctic landscapes in response to climatic and environmental changes, and b) to assess ongoing changes, particularly in lake-water TOC, on a centennial to millennial time scale.

In the first applied study, high-resolution FTIR measurements of the 318-m long sediment record of Lake El’gygytgyn provided a detailed insight into long-term climate variability in the Siberian Arctic over the past 3.6 million years. Highest bSi accumulation occurred during the warm middle Pliocene (3.6-3.3 Ma), followed by a gradual but variable decline, which reflects the first onset of glacial periods and then the finally full establishment of glacial–interglacial cycles during the Quaternary. The second applied study investigated the sediment record of Torneträsk in subarctic northern Sweden also in relation to climate change, but only over the recent post-glacial period (~10 ka). By comparing responses to past climatic and environmental forcings that were recorded in this large-lake system with those recorded in small lakes from its catchment, I determined the significance and magnitude of larger-scale changes across the study region. Three different types of response were identified over the Holocene: i) a gradual response to the early landscape development following deglaciation (~10000-5300 cal yr BP); ii) an abrupt but delayed response following climate cooling during the late Holocene, which occurred c. 1300 cal yr BP – about 1000-2000 years later than in smaller lakes from the area; and iii) an immediate response to the ongoing climate change during the past century. The rapid, recent response in a previously rather insensitive lake-ecosystem emphasizes the unprecedented scale of ongoing climate change in northern Fennoscandia. In the third applied study, VNIR-inferred lake-water TOC concentrations from lakes across central Sweden showed that the ongoing, observed increase in surface water TOC in this region was in fact preceded by a long-term decline beginning already AD 1450-1600. These dynamics coincided with early human land use activities in the form of widespread summer forest grazing and farming that ceased over the past century. The results of this study show the strong impact of past human activities on past as well as ongoing TOC levels in surface waters, which has thus far been underestimated. The research in this thesis demonstrates that infrared spectroscopic methods can be an essential component in high-resolution, multi-proxy studies of past environmental and climate changes.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2015. 25 p.
Keyword
Fourier transform infrared spectroscopy, visible-near infrared spectroscopy, PLS regression, biogenic silica, climate change, carbon cycling, lake-water quality, geochemistry, paleolimnology, Holocene, Lake El’gygytgyn, Torneträsk
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:umu:diva-111629 (URN)978-91-7601-372-4 (ISBN)
Public defence
2015-12-11, KBC-huset, Lilla Hörsalen, KB3A9, Umeå universitet, Umeå, 09:00 (English)
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Available from: 2015-11-20 Created: 2015-11-18 Last updated: 2015-12-02Bibliographically approved

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