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  • 1. Berntsson, A.
    et al.
    Rosqvist, G. C.
    Velle, G.
    Late-Holocene temperature and precipitation changes in Vindelfjallen, mid-western Swedish Lapland, inferred from chironomid and geochemical data2014In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 24Article in journal (Refereed)
    Abstract [en]

    In this article, we present results from a palaeolimnological study from Lake Vuoksjavratje in the mountain tundra region in the Vindelfjallen Mountains, northwest Sweden. We suggest that the influence of precipitation may be one of the factors causing discrepancies between chironomid-based late-Holocene July temperature (JulyT) reconstructions from Fennoscandia. We combine quantitative temperature reconstruction using chironomids for the last 5100 years with qualitative analysis of chironomid composition and geochemical analyses, such as x-ray fluorescence (XRF), total organic carbon (TOC) and C/N analysis. The studied sequence is dated by Pb-210, Cs-137 and 11 C-14 datings from terrestrial macrofossils. The aim of the study was to use chironomids to reconstruct late-Holocene summer temperature variation on a multi-centennial to centennial timescale and to use geochemical data to identify periods during which the changes in chironomid composition might have been forced by environmental variables other than temperature, such as within lake processes or precipitation. Based on ordination techniques, and a comparison between chironomid-inferred JulyTs and changes in minerogenic sedimentation with regional temperature and wetness records, it is concluded that the JulyT signal was modulated by precipitation. The proxies indicate that both JulyT and annual precipitation have influenced the chironomid communities in Lake Vuoksjavratje, and that catchment-related processes caused by enhanced precipitation have overridden the summer temperature signal between 3000 and 2200 cal. yr BP, and between 1050 and 100 cal. yr BP.

  • 2. Holland, Hilmar A
    et al.
    Schöne, Bernd R
    Lipowsky, Constanze
    Esper, Jan
    Decadal climate variability of the North Sea during the last millennium reconstructed from bivalve shells (Arctica islandica)2014In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 24, no 7, p. 771-786Article in journal (Refereed)
    Abstract [en]

    Uninterrupted, annually resolved paleoclimate records are crucial to contextualize the current global change. Such information is particularly relevant for the Europe realm for which weather and climate projections are still very challenging if not virtually impossible. This study presents the first precisely dated, annually resolved, multiregional Arctica islandica chronologies from the North Sea which cover the time interval ad 1040-2010 and contain important information on supra-regional climatic conditions (sea surface temperature (SST), ocean productivity, wind stress). Shell growth varied periodically on timescales of 3-8, 12-16, 28-36, 50-80, and 120-240 years, possibly indicating a close association with the North Atlantic Oscillation, ocean-internal cycles of the North Atlantic controlled by ocean-atmosphere couplings, and the Atlantic Multi-Decadal Oscillation. Increased climatic instability, that is, stronger quasi-decadal variability, seems to be linked to the predominance of atmospheric forcings and some significantly decreased insolation phases (e.g. Spörer and Maunder Minima). Increased climatic variability of shorter timescales was also observed during some particularly warm phases or regime shifts (e.g. during the 'Medieval Climate Anomaly' and since c. 1970). More stable climatic conditions, that is, extended warm or cold periods ('Medieval Climate Anomaly', 'Little Ice Age'), however, fell together with a predominance of multi-decadal oceanic cycles. Whether the sunspot number and the higher frequency climate variability are causally linked and which processes and mechanisms are required lie beyond this study.

  • 3. Isaksson, E
    et al.
    Kohler, J
    Pohjola, V
    Moore, J
    Igarashi, M
    Karlof, L
    Martma, T
    Meijer, H
    Motoyama, H
    Vaikmae, R
    van de Wal, R S W
    Two ice-core delta O-18 records from Svalbard illustrating climate and sea-ice variability over the last 400 years2005In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 15, no 4, p. 501-509Article in journal (Refereed)
    Abstract [en]

    Ice cores from the relatively low-lying ice caps in Svalbard have not been widely exploited in climatic studies owing to uncertainties about the effect of meltwater percolation. However, results from two new Svalbard ice cores, at Lomonosovfonna and Austfonna, have shown that with careful site selection, high-resolution sampling and multiple chemical analyses it is possible to recover ice cores from which part of the annual signals are preserved, despite the considerable meltwater percolation. The new Svalbard ice cores are positioned in different parts of Svalbard and cover the past 800 years. In this paper we focus on the last 400 years. The delta O-18 signals from the cores are qualitatively similar over most of the twentieth century, suggesting that they record the same atmospheric signal. Prior to AD 1920, the Austfonna ice core exhibits more negative delta O-18 values than Lomonosovfonna, although there are intermittent decadal-scale periods throughout the record with similar values. We suggest that the differences reflect the effect of the inversion layer during the winter. The pattern in the delta O-18 records is similar to the Longyearbyen air-temperature record, but on an annual level the correlation is low. The Austforma record correlates well with the temperature record from the more distant and southwesterly located Jan Mayen. A comparison of the ice-core and sea-ice records from this period suggests that sea-ice extent and Austforma delta O-18 are related over the past 400 years. This may reflect the position of the storm tracks and their direct influence on the relatively low-altitude Austfonna. Lomonosovfonna may be less sensitive to such changes and primarily record free atmospheric changes instead of variations in sea-ice extent, the latter is probably a result of its higher elevation.

  • 4. Kokfelt, U
    et al.
    Muscheler, R
    Solar forcing of climate during the last millennium recorded in lake sediments from northern Sweden2012In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 23, no 3, p. 447-452Article in journal (Refereed)
    Abstract [en]

    We report on a sediment record from a small lake within the subarctic wetland complex Stordalen in northernmost Sweden covering the last 1000 years. Variations in the content of minerogenic material are found to follow reconstructed variations in the activity of the Sun between the 13th and 18th centuries. Periods of low solar activity are associated with minima in minerogenic material and vice versa. A comparison between the sunspot cycle and a long instrumental series of summer precipitation further reveals a link between the 11 yr solar cycle and summer precipitation variability since around 1960. Solar minima are in this period associated with minima in summer precipitation, whereas the amount of summer precipitation increases during periods with higher solar activity. Our results suggest that the climate responds to both the 11 yr solar cycle and to long-term changes in solar activity and in particular solar minima, causing dry conditions with resulting decreased runoff.

  • 5. Melvin, Thomas M
    et al.
    Grudd, Håkan
    Briffa, Keith R
    Potential bias in ‘updating’ tree-ring chronologies using regional curve standardisation: Re-processing 1500 years of Torneträsk density and ring-width data2012In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 23, no 3, p. 364-373Article in journal (Refereed)
    Abstract [en]

    We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. ce 900?1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of ?growth-rate? based multiple RCS curves is recommended to identify and mitigate the problem of ?modern sample bias?.

  • 6. Rosqvist, Gunhild C.
    et al.
    Leng, Melanie J.
    Jonsson, Christina
    North Atlantic region atmospheric circulation dynamics inferred from a late-Holocene lacustrine carbonate isotope record, northern Swedish Lapland2007In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 17, no 7, p. 867-873Article in journal (Refereed)
    Abstract [en]

    The first high-resolution record of climate variation based on the oxygen and carbon isotope composition of authigenic carbonate for northern Scandinavia is presented. Modern lake-water isotope data indicate that controls on its oxygen and hydrogen (delta O-18(w) and delta D-w) composition are unlikely to be evaporation or temperature, and its variations must therefore reflect changes in, or at the source of, precipitation. Substantial and persistent changes of the isotopic composition of the precipitation are required to change the mean annual isotope composition of lake surface water. For this reason we argue that the recorded changes were significant and that the recurrence of such changes would greatly affect future regional climate conditions in the North Atlantic region. Oxygen isotope (delta O-18) minima occurring at similar to 200, 500, 1300, 1600 and at 2900 cal. yr BP all coincide with major peaks in North Atlantic ice rafted debris deposition. We suggest that the depletion events in delta O-18 cycles recorded in several lakes in northern Swedish Lapland are caused by the same climatic shifts as those noted in the North Atlantic marine records. This is because changes of atmospheric circulation pattern and the lower ocean and atmospheric temperatures associated with the IRD events help to explain why O-18 depletion of precipitation occurred during these events. Our findings indicate that the recorded changes in North Atlantic ice drift and surface hydrography are coupled to changes in atmospheric circulation.

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