Holocene Climate and Atmospheric Circulation Changes in Northern Fennoscandia

Holocene climate and atmospheric circulation changes in northern Fennoscandia

Interpretations from lacustrine oxygen isotope records

Christina E. Jonsson

Department of Physical Geography and Quaternary Geology Stockholm University Stockholm 2009

1 Till Moa och Ella

Layout: Christina E. Jonsson (except Paper I, III and IV) Cover photo: Lake Kalanjaure (Photo: Ove Jonsson) Printed by PrintCenter US-AB

2 Doctoral Dissertation 2009 Christina E. Jonsson Department of Physical Geography and Quaternary Geology Stockholm University

Abstract

This thesis investigates how variations in the oxygen isotopic composition of lake waters in northern Fennoscandia are recorded in lake sediment archives, especially diatoms, and how these variations can be used to infer past changes in climate and atmospheric circulation. Lake water samples collected between 2001 and 2006 indicate that the oxygen isotopic compo- 18 sition (d Olakew) of northern Fennoscandian lakes is mainly controlled by the isotopic composition 18 18 of the precipitation (d Op). Changes in local d Op depend on variations in ambient air temperature and changes in atmospheric circulation that lead to changes in moisture source, vapour transport efficiency, or winter to summer precipitation distribution. This study demonstrates that the amount 18 18 of isotopic variation in lake water δ O is determined by a combination of the original δ Olakew, the amount and timing of the snowmelt, the amount of seasonally specific precipitation and groundwater, any evaporation effects, and lake water residence time. 18 The fact that the same isotope shifts have been detected in various δ Olakew proxies, derived from hydrologically different lakes, suggests that these records reflect regional atmospheric circu- 18 lation changes. The results indicate that diatom biogenic silica isotope (δ Odiatom) records provide important information on changes in atmospheric circulation that can help explain temperature and precipitation dynamics during the Holocene. 18 18 The long-term Holocene decreasing δ Op trend recorded in lake sediment δ O in northern Sweden has likely been forced by a shift from the dominance of strong zonal westerly airflow 18 (relatively high δ Op) in the early Holocene to a more meridional flow pattern (relatively low 18 18 18 δ Op). The δ Olakew depletion recorded in the δ O records from ca. 600 cal yr BP (AD 1350) may be due to a shift to more intense meridional airflow and weaker westerlies over northern Fennos- candia together with increasing winter precipitation from the north or southeast. This climate shift probably marks the onset of the so-called Little Ice Age in this region.

Keywords: oxygen isotope, lake sediment, diatom silica, atmospheric circulation, Sweden, The Holocene, northern Fennoscandia, North Atlantic Oscillation, Little Ice Age

3 “The farther backward you can look, the farther forward you are likely to see” Winston Churchill

4 Holocene climate and atmospheric circulation changes in northern Fennoscandia Interpretations from lacustrine oxygen isotope records

Christina E. Jonsson

Thesis contents This doctoral thesis comprises a summary and six appended papers, referred to by their Roman numerals. Preliminary results are also presented and discussed in the summary.

List of papers I. Jonsson, C.E., Leng, M.J., Rosqvist, G.C., Seibert, J., and Arrowsmith, C. 2009: Stable oxygen and hydrogen isotopes in sub-Arctic lake waters from northern Sweden. Journal of Hydrology. In press.

II. Jonsson, C.E., Rosqvist, G.C., Leng, M.J, Bigler, C, Bergman, J., Kaislahti Tillman, P., and Sloane, H.: High-resolution diatom δ18O records from two sub-Arctic high-altitude lakes in the Swedish Scandes. Journal of Quaternary Science. In review.

III. Rosqvist, G., Jonsson, C., Yam, R., Karlén, W., and Shemesh, A., 2004: Diatom oxygen isotopes in pro-glacial lake sediments from northern Sweden: a 5000-year record of atmospheric circulation. Qua- ternary Science Reviews, 23, 851–859.

IV. Rosqvist, G.C., Leng, M.J., and Jonsson, C. 2007: North Atlantic region atmospheric circulation dynamics inferred from a late-Holocene lacustrine carbonate isotope record, northern Swedish Lapland. The Holocene, 17, 867–873.

V. Jonsson, C.E., Rosqvist, G.C., Leng, M.J., and Sloane, H.J. Two diatom isotope records from Naimak- ka in northern Fennoscandia: implications for Holocene palaeohydrology and atmospheric circulation dynamics. In manuscript.

VI. Jonsson, C.E., Andersson, S., Rosqvist, G.C., and Leng, M.J. 2009: Reconstructing past atmospheric circulation changes using oxygen isotopes in lake sediments from Sweden. Published in Climate of the Past (CP) Discussions, special issue, 5, 1609–1644. Co-authorship I led the writing of papers I, II, V and VI and performed most of the analyses. The co-authors contributed to all four papers by textual revision, data interpretation, and scientific discussion. Bigler performed and inter- preted the diatom species analyses presented in paper II. Kaislahti Tillman performed the diatom cleaning for the isotope analyses and Bergman produced the age-depth model for Lake Spåime presented in paper II. For the work presented in papers III and IV, I took part in the fieldwork, performed the diatom cleaning (paper III), and contributed to the interpretation and scientific discussion. This thesis project was initiated 18 by Gunhild Rosqvist who conducted the first δ Odiatom study in Fennoscandia.

5 Christina E. Jonsson

6 Holocene climate and atmospheric circulation changes in northern Fennoscandia

Introduction equator (latitude effect) or from coastal regions (continental effect) and with increasing elevation (altitude ef- Understanding the significance of anthropogenic climate fect) (Rozanski et al., 1993). The correlation between 18 2 change calls for an understanding of natural climate vari- δ O and δ H in precipitation on a global scale is well ability (e.g., Mayewski et al., 2004; IPCC, 2007; Jones understood (Craig, 1961) and is known as the global me- et al., 2009). Studying various proxy records of past cli- teoric water line (GMWL); it has been defined by Rozan- mate variability improves our understanding of how the ski et al. (1993) as: climate system varied in the past. Globally distributed climate proxy records reconstructing different climate δ2H = 8 δ18O + 10‰ (1) variables indicate that Holocene climate variations have been large enough to have significant effects on ecosys- However, the stable isotopic composition of rainfall in a tems and humans (e.g., Mayewski et al., 2004; Wanner region may represent different meteoric conditions and et al., 2008). will better be described by a local meteoric water line The Fennoscandian climate is strongly influenced by (LMWL), which may have a slightly different slope and atmospheric and oceanic circulation changes over the intercept from those of the GMWL (Ingraham, 1998). At North Atlantic (e.g., Marshall et al., 2001; Luterbacher mid and high latitudes, there is seasonal variation along et al., 2002; Hurrell et al., 2003; Moros et al., 2004). the LMWL driven by temperature, with the isotopic com- Numerous palaeoclimate reconstructions based on position being more depleted in winter precipitation than 18 2 lake sediments (Korhola et al., 2000; Seppä and Birks, in summer precipitation. Comparing δ Olakew and δ Hlakew 2001; 2002; Bigler et al., 2003; Larocque and Hall, with the LMWL reveals the hydrological setting of the 2004; Bigler et al., 2006; Weckström et al., 2006; Bjune lake. Lake waters plotted on the LMWL are isotopically and Birks, 2008; Bjune et al., 2009), dendrochronol- the same as the precipitation for that region, whereas ogy (Grudd et al., 2002; Gunnarson et al., 2003; Grudd, lake waters plotted below the LMWL along a different 2008), and speleothems (Lauritzen and Lundberg, 1999; gradient, on a local evaporation line (LEL), have under- Sundqvist et al., 2009; Linge et al., 2009) have been per- gone evaporation that progressively enriches the isotopic formed in northern and central Fennoscandia, improv- composition of the residual lake water (Clark and Fritz, ing our knowledge of variations in Holocene summer 1997). Displacement of the individual lake water along temperature and annual precipitation. Few attempts have the LEL provides information on the water balance (i.e., been made to reconstruct seasonal changes in precipi- the evaporation/inflow ratio) of the lake (Gibson et al., tation (Nesje et al., 2005; Bakke et al., 2008), and still 2005; 2008): the farther the lake water is plotted along fewer reconstructions yield information that can be used the LEL the more evaporated is the water. The slope of to reveal what type of atmospheric forcing caused the the LEL depends primarily on relative humidity together shifts in temperature and precipitation (e.g., Antonsson with wind speed and surface temperature. Lower relative et al., 2008). Hydrological changes, which can be related humidity values will generate LELs with lower slopes to atmospheric circulation, can potentially be detected in (Clark and Fritz, 1997). The intersection of the LEL with stable oxygen isotope (δ18O) archives, such as lake sedi- the LMWL corresponds to the average isotopic composi- ments, ice cores, tree-rings, and speleothems. tion of the water entering the lake. Water in non-evapo- 18 2 Due to the intimate link between the oxygen isotopic rating lakes (δ Olakew and δ Hlakew are plotted along the 18 composition of precipitation (δ Op) and climate vari- LMWL) with residence times of more than a year natu- ables (Rozanski et al., 1993; Gat et al., 1996), it is ex- rally tend to average out and integrate seasonal signals 18 pected that changes in climate will be reflected in δ Op, (Edwards et al., 2004), and their water usually has an transferred to the isotopic composition of lake water isotopic signal similar to that of weighted average annual 18 18 (δ Olakew) and finally recorded in lake sediment δ O precipitation. In lakes with shorter residence times, the proxies (Darling et al., 2004; Leng and Marshall, 2004). isotopic composition of the water may lie on the LMWL 18 Shifts in δ Op reflect changes in regional temperatures but will be dependent on other physical properties, such and precipitation patterns that follow upon changes in as lake/catchment ratio and catchment elevation. For atmospheric and oceanic circulation dynamics. Oxygen example, seasonal variation in the isotopic composition isotopic data derived from lake sediments have recently of precipitation input is likely to be more significant in displayed increasing potential as proxies that provide short-residence-time lakes, as these lakes tend to have additional information about climate change not related isotopic values that are constantly displaced by later pre- only to summer temperatures (e.g., Shemesh et al., 2001; cipitation (Leng et al., 2005). In fact, some lakes behave 18 Hammarlund et al., 2002; Leng and Marshall, 2004; St. like upland streams, having δ Olakew values similar to Amour, 2009) – the climate parameter on which most those of individual rainfalls, reflecting very short -resi 18 palaeoclimate studies focus. Proxy-based δ Op recon- dence times (Tyler et al., 2007). structions can be used to test the results of model simula- tions of past climate change, especially past changes in the atmospheric circulation pattern (Sturm, et al., 2005; The oxygen isotopic composition of lacustrine Schmidt et al., 2007; Sturm et al., 2009). materials The stable isotopic composition of carbonates has been The relationship between the isotopic compo- used in palaeoclimatic studies for more than 50 years (Urey et al., 1948; McCrea, 1950; Epstein et al., 1953). sition of precipitation and of lake water Carbonate materials in lake sediments comprise authi- The distribution of δ18O and δ2H in precipitation is gen- genic and biogenic components (Kelts and Hsü, 1978). erally more negative with increasing distance from the Authigenic carbonates precipitate when the concentra-

7 Christina E. Jonsson

tion of CaCO3 reaches supersaturation, which commonly atoms is a more recent development (e.g., Rietti-Shati occurs as a result of the photosynthetic removal of CO2 in et al., 1998; Rosqvist et al., 1999; Barker et al., 2001; the water column by algae and subsequent pH elevation Shemesh et al., 2001; Jones et al., 2004; Leng et al., and calcite precipitation. In most mid- and high-latitude 2005; Leng and Barker, 2006; Tyler et al., 2008; Schiff et 18 regions, authigenic carbonates are precipitated mainly in al., 2009). The first lake δ Odiatom study in Fennoscandia the summer months during periods of maximum phyto- was performed by Shemesh et al. (2001) on a Holocene plankton productivity (Leng et al., 1999; Hammarlund, sediment sequence from Lake 850 located in the Abisko et al., 2003; Leng and Marshall, 2004). The presence of region (Fig. 1). This study demonstrates that diatoms can carbonate material in Swedish lakes is restricted to alka- be isolated from lake sediments from this region, that 18 line lakes, which are only found in regions with calcare- δ Odiatom analysis provides only small analytical errors, 18 ous bedrock. In lakes where carbonate is absent or poorly and that δ Odiatom data can be used to reconstruct past 18 preserved, for example, in high-altitude lakes with short changes in isotope hydrology (δ Olakew) forced by shifts residence times, diatoms display increasing potential to in atmospheric circulation changes in northern Fennos- reflect past isotope hydrological variations (Shemesh candia. et al., 2001; Jones et al., 2004; Leng and Barker, 2006; Diatoms (2–200 μm) are unicellular, eukaryotic, pho- Schiff et al., 2009). tosynthetic algae (Division Bacillariophyta) that form a 18 The δ Odiatom technique was originally developed by shell, or frustule, comprising opalline or biogenic silica oceanographers and applied to marine records (Labeyrie, (SiO2 · nH2O) (Round et al., 1990; Battarbee et al., 2001; 1974; Labeyrie and Juillet, 1982; Labeyrie et al., 1984; Leng and Barker, 2006). The presence of silica leads to Shemesh et al., 1993; 1995). The study of lacustrine di- cell wall rigidity and helps preserve diatom frustules as

(b) (a) Oikojärvi Naimakka Naimakka Abisko N Karesuando Lake Keitjoru Bredkälen Katterjåkk Abisko

Finland

Norway Annual prec. (mm) >1800 1500 Pajala 1200 Sweden 1100 1000 800 700 600 <400

Torneträsk Katterjåkk Ånnsjön Riksgränsen Abisko Lake Njulla Lake Tibetanus Vuoskkujavri

Lake 850 N Lake 865

Lake Spåime N Vuolep Allakasjaure Kalanjaure Sylarna

Norway 0 km 10 Norway Below treeline

Figure 1 a) Map of Fennoscandia showing the location of the three GNIP stations. b) Mean annual precipitation (SMHI data, 1961–1990; Jutman, 1995) in northern Sweden and distribution of annual precipitation by wind direction at the meteorological stations in Katterjåkk (500 m a.s.l.) and Pajala (168 m a.s.l.) (1981–1995; Johansson and Chen, 2003). Sites mentioned in the text are shown. c) Lake Spåime in west central Sweden, in the central Scandes Mountains, 26 km southeast of Storlien and 80 km from the Atlantic coast. d) Map of the Abisko region. Sampled lakes are indicated with stars.

8 Holocene climate and atmospheric circulation changes in northern Fennoscandia fossils in the sediment. Diatoms, found in both plank- and Labeyrie, 1987; Shemesh et al., 1992; Brandriss tonic and benthic habitats, occur in almost all aquatic et al., 1998; Moschen et al., 2005). These negative re- environments wherever the nutrient supply of P, N, and sponses to changes in lake water temperature will have 18 dissolved silica (SiO2) is sufficient to sustain productiv- an opposite effect on the lake sediment δ O record and 18 ity (Battarbee et al., 2001; Leng and Barker, 2006). The damp the effect of changes in δ Op caused by air tem- selection of diatom assemblages found in lakes depends perature changes, according to the “Dansgaard relation- on a combination of physical, chemical, and biological ship” (i.e., increasing air temperature leads to increasing 18 variables in the water column, such as pH, temperature, δ Op; Dansgaard, 1964). light, ice-cover, and turbulence (Battarbee et al., 2001). Nearly pure samples of diatoms are required for the analysis of the oxygen isotopic composition of diatom Thesis objectives silica (Juillet-Leclerc 1986), as oxygen from other material in the sediment, such as organic matter, tephra, The overall objective of this thesis was to investigate silt, clay, carbonates, and chrysophyte cysts, will also how variations in the oxygen isotopic composition of be liberated during the fluorination techniques (Clay- lake waters in northern Fennoscandia are recorded in ton and Mayeda, 1963) generally used to liberate oxy- lake sediment archives, especially diatoms, and how gen from silica (Morley et al., 2004; Leng and Barker, these variations can be used to infer past changes in cli- 2006). These contaminants can be removed via a series mate and atmospheric circulation. of cleaning steps, including organic and carbonate material removal, sieving, differential settling, and heavy liq- The specific objectives of the papers were: uid separation (Morley et al., 2004). Even small amounts of the above contaminants can significantly affect the • To identify lakes in northern Sweden that have the 18 δ Odiatom reading, because these materials often have potential to retain various aspects of water isotopic 18 18 δ O values lower than δ Odiatom (Morley et al., 2004; composition that can be used for palaeoclimate re- Brewer et al., 2008). Therefore, a cleaning method in- constructions dividually adapted to the sediment and careful examina- tion of every sample is important. • To understand modern lake hydrology and their re- The diatoms consist of inner dense and isotopically lationship with climate variables in order to interpret homogenous silica, the oxygen in which represents the the δ18O sediment signal and reliably reconstruct cli- formation water, and an outer hydrous silica layer freely mate change exchangeable with any water after formation (Labeyrie and Juillet, 1982). The hydrous silica must be removed • To investigate the processes by which climate vari- 18 18 before δ O analysis to obtain an accurate isotope value ables are recorded in the lake sediment δ Odiatom representative of the lake water at the time of diatom for- records in northern Fennoscandia mation (Leng and Barker, 2006; Leng and Sloane, 2008). This can be done by vacuum dehydration (Degens and • To reconstruct Holocene climate variability and at- Epstein, 1962; Labeyrie; 1972; 1974; Brandriss et al., mospheric circulation changes in northern Fennos- 1998; Lücke et al., 2005), controlled isotope exchange candia (Labeyrie and Juillet, 1982; Juillet-Leclerc and Labeyrie, 1987), or stepwise fluorination (Haimson and Knauth, • To improve our understanding of how the climate 1983; Thorliefson and Knauth, 1984; Matheney and system has varied in the past and the forcing mecha- Knauth, 1989). Approximately 20–30% of the diatom nisms underlying these variations oxygen is thought to be removed before stable values for the δ18O are reached (Leng and Barker, 2006; Leng and Sloane, 2008). Disassociating the strong inner tetra- Site descriptions hedrally bonded silica skeleton (Si-O-Si) to extract the oxygen for the δ18O analysis requires a powerful oxidiz- Lakes from three different regions in northern and cen- ing reagent (ClF3 or BrF5) or high temperatures (Leng tral Fennoscandia were studied in this thesis (Fig. 1): and Barker, 2006). Abisko region (papers I–IV) in northwest Swedish La- The main factors (processes) that influence the sedi- pland; Naimakka region (paper V) in northeast Swedish ment d18O (both diatom and carbonate) signal are lake Lapland, 110 km northeast of Abisko; and central Jämt- water d18O and lake water temperature at the time of land (Lake Spåime) (paper II) located approximately 650 diatom bloom or carbonate precipitation. Understanding km southwest of Abisko. these processes is important in reconstructing climate The Abisko region (Fig. 1, b and d) is situated between variability from d18O records. The oxygen isotopic com- 35 and 70 km from the North Atlantic. It is characterized position of the diatom silica and carbonate is controlled by the Scandinavian mountain range, the highest peaks by the oxygen isotopic composition of the lake water of which reach approximately 1500 m a.s.l., and the large within which they formed at a given temperature. The Lake Torneträsk. This region is well suited for lake sedi- temperature-dependent isotope fractionation between ment δ18O studies because its climate is greatly affected calcite and lake water has an established negative tem- by the atmospheric circulation over the North Atlantic d18 perature coefficient of about –0.25‰/°C (Craig, 1965), and Op data are available from the meteorological while the fractionation between diatom silica and lake station in Abisko. Several published palaeoclimate stud- water is more controversial, with published estimates ies treat the Abisko area, providing information about ranging from –0.5‰/°C to –0.2‰/°C (Juillet-Leclerc Holocene climate variability on different time scales.

9 Christina E. Jonsson

In Naimakka (Fig. 1b), which has a more continental climate than Abisko, one of the few meteorological stations is located where both d18O and d2H in precipitation have been measured. This lets us calculate an LMWL and combine d18O records from lakes with different sensitivities and seasonalities. It also provides additional information about the North Atlantic influence in a west– east direction. Jämtland is located in west central Sweden, in the central part of the Scandinavian mountain range (Fig. 1c) approximately 80 km from the Atlantic coast and approximately 650 km south of the Abisko region. We compared high-resolution d18O lake records from both these regions to determine whether a regional hydrological response to climate variability could be detected.

Modern climate The modern climate in these three regions is characterized by a pronounced oceanic–continental gradient running from the Atlantic coast to the Scandinavian mountain range, where precipitation increases with altitude (Jutman, 1995; SMHI data, 1961–1990). The variable topography means that spatial variation in precipitation Figure 2 Mean winter (Oct–May) precipitation at the Storlien, and runoff is large (Jutman, 1995). On the windward Abisko, and Naimakka (SMHI) meteorological stations, plot- west side of the Scandes, the forced lifting of approach- ted together with the winter (Dec–Mar) NAO index provided ing air masses over the mountains causes the release of by the Climate Analysis Section, NCAR, Boulder, CO, USA rainfall and increased precipitation with elevation (oro- (Hurrell, 1995). Light grey bars indicate periods with highest graphic rain). The eastern leeward side of the Scandes and lowest winter NAO index values and dark grey bars indicate the years when the local δ18O data (GNIP data, IAEA/ is often drier with a more continental climate (Smith, p 1979). Strong zonal airflow with strong westerly winds, WMO) at Bredkärlen (Storlien) (1975–1988), Abisko (1975– 1980), and Naimakka (1990–1995) were measured. Dashed i.e., a positive North Atlantic Oscillation (NAO) index, lines indicate mean winter precipitation over the measured and increased cyclonic frequency across the North At- period. lantic generally lead to high winter precipitation and higher winter air temperatures in Fennoscandia (Chen and Hellström, 1999; Chen, 2000; Jacobeit et al., 2001; Marshall et al., 2001). In contrast, cold and dry winters ceives precipitation derived both from westerly (W and occur when westerly winds in the North Atlantic region NW) airflow from the North Atlantic and from - south are weak (i.e., a negative NAO index) and meridional easterly flow from Baltic Sea air masses (Johansson and airflow brings cold polar air south over Fennoscandia. Chen, 2003). Naimakka, farther east, has a more conti- Additionally, during negative NAO winters, southeast nental climate than Abisko. The mean annual precipita- airflow could bring moisture to Sweden from the Baltic tion at the Naimakka meteorological station (68°41’N, Sea (Uvo, 2003). The relationship between winter NAO 22°21’E; 403 m a.s.l.) is 460 mm (Fig. 3) (SMHI data, index and regional winter precipitation is complex in 1944–2006). Central Jämtland is located between two Scandinavia (Uvo, 2003). This is because the influence different climate regimes. A relatively more maritime of NAO varies spatially within a region (Fig. 2) due to climate dominates farther north, where air masses from local topography and regional winter atmospheric circu- the west and southwest can more easily penetrate the lation patterns. Based on instrumental temperature data relatively low mountain chain. A more continental cli- containing information on cloud cover, meridional ge- mate characterizes southern Jämtland, because of the ostrophic wind, and air pressure, Moberg et al. (2003) rain shadow caused by the higher Norwegian mountains. demonstrated that low summer temperatures over Scan- The mean annual precipitation at the Storlien meteoro- dinavia are associated with the dominance of cyclonic logical station (642 m a.s.l.), located approximately 26 circulation (cool and wet conditions), and conversely, km northwest of Lake Spåime, is 870 mm (SMHI data, high summer temperatures with the dominance of anti- 1961–1990). cyclonic circulation (warm and dry conditions). Atlantic-derived precipitation decreases along a steep gradient running from the Norwegian coast to the Isotopic composition of precipitation Abisko area (Fig. 1). The mean annual precipitation at At mid to high latitudes, the correlation between annual the Riksgränsen meteorological station is approximately surface temperature and the weighted oxygen isotopic 1000 mm (1961–1973; 508 m a.s.l.; Alexandersson et al., composition of precipitation is good, higher tempera- 18 1991), whereas at the Abisko Scientific Research Station tures corresponding to higher δ Op values (Dansgaard, (1961–1990; 388 m a.s.l.; Alexandersson et al., 1991), 1964; Rozanski et al., 1993; Fricke and O’Neil, 1999). located in the rain shadow of the mountains, it is only The present-day relationship between mean annual air 18 approximately 300 mm (Fig. 3). This is an area that re- temperature (T in °C) and δ Op for North Atlantic coast-

10 Holocene climate and atmospheric circulation changes in northern Fennoscandia

Figure 3 Monthly mean values of δ18O in precipitation at the Bredkälen (GNIP data, 1975–1988), Abisko (GNIP data, 1975– 1980), and Naimakka (GNIP data, 1990–1995) stations (IAEA/WMO) and monthly mean values for temperature and amount of precipitation at nearby meteorological stations (SMHI). Solid horizontal lines indicate weighted annual average d18O in precipitation. Grey triangles indicate individual monthly samples and black diamonds monthly means.

al stations is described by the Dansgaard relationship position of precipitation from northern Fennoscandia are 18 2 (Dansgaard, 1964): limited, though δ Op and δ Hp data are available from the meteorological station in Naimakka (1990–1995) 18 18 δ Op = 0.69T – 13.6 (2) and δ Op data from the meteorological stations in Abisko (1975–1980) and Bredkälen (1975–1988) (Table 1; Fig. In addition to being related to regional condensation 3) (IAEA/WMO, GNIP database 2008). Seasonal varia- 18 temperature, δ Op is a function of conditions at the va- tions in temperature at these three sites are responsible pour region (in the case of Fennoscandia the North At- for the seasonal variation observed in monthly mean 18 lantic Ocean or the Baltic Sea) and of the air-mass trajec- δ Op, higher values occurring in summer than in winter 18 tory producing a vapour transport history. In addition, at months. The highest δ Op amplitude is found in conti- high latitudes, change in the positions of the boundaries nental Naimakka while the lowest amplitude is observed between air masses plays a dominant role in determining in Abisko located closer to the North Atlantic Ocean. 18 local δ Op (Edwards et al., 1996; Shemesh et al., 2001). d18 Monthly Op values from meteorological stations in Sweden are available from the Global Network of Iso- Material and methods topes in Precipitation dataset (GNIP, IAEA/WMO). The 18 mean annual δ Op values (1975–1980) from 17 GNIP stations have been used by Burgman et al. (1987) to infer Fieldwork 18 isotopic isolines over Sweden. The annual mean δ Op Lake sediments were generally sampled from the deep- over Sweden decreases with increasing distance from the est part of each lake. Long sediment cores were retrieved ocean and with increasing latitude/altitude due to conti- from the ice-covered lakes using a Livingstone corer or nental and altitude effects. Records of the isotopic com- Russian peat sampler (Fig. 4). Surface sediment cores

Table 1 Data from GNIP stations (IAEA/WMO) and meteorological data from nearby stations (SMHI). Naimakka Abisko Bredkälen/Storlien Location 68°41’N, 22°21’E 68°21’N, 18°49’E 63°51’N, 15°19’E Altitude (m a.s.l.) 403 392 400/642 Measured years 1990–1995 1975–1980 1975–1988 18 Mean annual δ Op (‰) –16.1‰ –13.5‰ –13.7‰ 18 Winter δ Op (‰) –19.0‰ –14.4‰ –14.9‰ 18 Summer δ Op (‰) –12.9‰ –11.7‰ –11.3‰ 18 δ Op amplitude (‰) 10‰ 6‰ 8‰ Mean annual prec. (mm) 460 mm 300 mm 870 mm Mean annual temp. (°C) –2.6°C –0.8°C +1.1°C Mean January temp. (°C) –12.9°C –11.4°C –7.6°C Mean July temp. (°C) +12.6°C +11.1°C +10.7°C

11 Christina E. Jonsson were retrieved using an Axelsson or an HTH gravity corer (Renberg and Hansson, 2008) and were subsampled in the field (Fig. 5). Water samples for isotope analyses were collected from the lakes using a Ruttner sampler. Most of the lake water was sampled from the deepest part of the lake, from the ice in spring or from a boat in summer; a few samples were taken from the littoral zone. Lake waters were also sampled along depth profiles to determine whether there was any vertical isotopic stratification. Inflow and out- flow streams, as well as groundwater from springs, were also sampled. Water samples were collected and stored in tightly sealed polyethylene bottles until analysed for oxygen (δ18O) and hydrogen isotope (δ2H) composition. Lake water temperature was measured using automated thermistors (Thermochron iButton).

Dating Figure 4 Long-core sediment sampling from ice-covered Lake The lake sediment chronologies presented here are based Valfojaure (835 m a.s.l.) in April 2002 using a Livingstone on radiometric dating: 210Pb and 137Cs techniques were corer. used on surface sediment cores to date more recent sediments (Appleby, 2001) (paper II), whereas radiocarbon (14C) AMS dating (Björck and Wohlfarth, 2001) was used on both aquatic mosses and terrestrial macrofos- sil samples for longer time scales (papers III and IV). The chronologies of the sediment sequences from lakes Keitjoru and Oikojärvi (paper V) (St. Amour, 2009) were based on a combination of 210Pb and 14C dating; in the case of Lake Oikojärvi, additional chronological constraints were obtained by identifying lead pollution chronological markers at Umeå University using the method of Renberg et al. (2001). Because the 14C activity in the atmosphere has varied over time (Stuiver and Braziunas, 1993), all 14C dates were converted, using a calibration dataset, from radiocarbon ages into calibrated years before present (cal yr BP), where BP = AD 1950.

Isotope analyses 18 The sediment samples were prepared for δ Odiatom analysis at Stockholm University. The biogenic silica purifica- tion followed the stepwise procedure described in Mor- ley et al. (2004) with some modification. First, hydro- chloric acid (HCl) (32%) was added to remove carbonates and then the samples were repeatedly treated with

35% hydrogen peroxide (H2O2) at 90°C to remove all organic matter. Subsequently, the samples were sieved in different size fractions. Material smaller than 20 µm was found to contain the highest proportion of diatoms from most of the lakes, except lakes Oikojärvi and Keit- joru where the 20–63-µm fraction was instead used. To separate diatoms from minerogenic material, samples Figure 5 Surface sediment sampling at Lake Kalanjaure (996 were centrifuged over a heavy liquid, i.e., sodium poly- m a.s.l) in August 2006 using an HTH gravity corer. tungstate (SPT), and checked using optical microscopy. The SPT was diluted progressively to smaller specific densities (2.3–1.8 g cm–3) until a pure sample of diatoms was retrieved. The samples from the lake Vuolep Alla- cles of <5 µm (including silt and broken fragments of kasjaure (paper III) were also treated with a mixture of diatoms). Samples were then washed again with distilled concentrated HNO3 and HClO4 at 60°C to remove or- water and finally checked again for contaminants using ganic matter (Shemesh et al., 1995). All diatom samples optical microscopy. A few samples were further checked were carefully rinsed in distilled water and wet sieved using scanning electron microscopy to ensure that no at 5 µm to remove remaining traces of SPT and parti- visible minerogenic particles were included (e.g., Fig.

12 Holocene climate and atmospheric circulation changes in northern Fennoscandia

6). Finally, the purified diatom samples were dried at 40°C for 24 h. The δ18O analysis of the diatom samples a) discussed in paper III was performed at the Weizmann Institute of Science, Israel, using a controlled isotope exchange technique (Juillet-Leclerc and Labeyrie, 1987) to remove the hydrous layer from the biogenic silica, which was then subsequently fluorinated to extract oxygen. In 18 papers II and V, the δ Odiatom analyses were performed at the NERC Isotope Geosciences Laboratory, UK, using the stepped fluorination technique to remove the hydrous layer and extract oxygen isotopes described by Leng and Barker (2006) and Leng and Sloane (2008). The oxygen liberated was converted to CO2 (Clayton and Mayeda 1963) and the 18O/16O ratios were measured on a dual- inlet mass spectrometer. The analytical reproducibility 18 of the δ Odiatom technique is 0.15–0.3‰ (1σ), based on a laboratory standard. 18 The carbonate sediment samples analysed for δ O were treated at Stockholm University with a 5% sodium- hyperchlorite solution for 16 h to oxidize the organic ma- b) terial and disaggregate the sediment. The samples were then sieved through sieve cloth onto Whatman quartz microfibre filter paper and washed in deionized water. The filter paper was dried and the carbonate removed and ground to a fine powder. The samples were reacted overnight with anhydrous phosphoric acid in a vacuum, at 25°C, using the method of McCrea (1950). CO2 measurements were made on a VG Optima mass spectrometer at NERC Isotope Geosciences Laboratory. Analytical reproducibility was better than 0.1‰ based on laboratory standards. d18 d2 Isotope analyses of Olakew and Hlakew were performed either at the NERC Isotope Geosciences Labora- tory or the Weizmann Institute of Science using standard 18 isotope techniques, mostly by CO2 equilibration for δ O and Zn or Cr reduction for δ2H. Analytical uncertainties d18 d2 are < ±0.2‰ for Olakew and ±2‰ for Hlakew. Figure 6 Scanning electron microscope images of purified dia- All the stable isotope ratios discussed here are tom samples from a) Lake Keitjoru and b) Lake Oikojärvi. expressed as “d” values, representing deviations in per mil (‰) from a standard, such that

d sample = 1000[(Rsample/RVSMOW) –1] (3) in the area. Paper I is the first systematic study of lake isotope hydrology in northwest Swedish Lapland. Lake where R is the 18O/16O or 2H/1H ratio in sample and stand- water δ18O and δ2H data were collected from the Abisko ard. The data for lake water oxygen and deuterium and region (Fig. 1d) between 2001 and 2006 from 16 lakes for diatom oxygen are presented using the Vienna Stand- and streams sharing common input water, controlled by ard Mean Ocean Water (VSMOW) scale, while carbon- the isotopic composition of regional precipitation, but ate data are presented relative to Vienna Pee Dee Belem- having different hydrological sensitivities. The lakes nite (VPDB) (Leng et al., 2005). and catchments were chosen to represent different sizes, altitudes, and residence times to achieve a better understanding of the effect of summer and winter precipitation and evaporation on lake isotope hydrology in this region. Results The results in paper I reveal lake waters with a range The major results of this thesis are presented as summa- of d18O values. These signatures are forced by local hy- ries of six papers (papers I–VI), followed by a descrip- drology and different climate variables. The isotopic tion of the preliminary results of ongoing projects. composition of almost all waters lies on or near the GMWL, indicating that these lakes mostly preserve the Paper I: Jonsson, C.E., Leng, M.J., Rosqvist, G.C., isotopic signal of seasonally changing meteoric water. Seibert, J., and Arrowsmith, C. 2009: Stable oxygen Seasonal variation is strongly determined by snow ac- and hydrogen isotopes in sub-Arctic lake waters from cumulation in winter, snowmelt in spring and early sum- northern Sweden. Journal of Hydrology. In press. mer (forced by air temperatures), and minor enrichment Making appropriate inferences from climate reconstruc- in summer due to the influence of isotopically enriched tions using lake sediment δ18O records calls for a thor- precipitation and the effect of evaporation. Due to catch- ough understanding of the local hydrological settings ment altitude, catchment and lake size, and the different

13 Christina E. Jonsson

18 18 Table 2 Characteristics of a lake that determine whether the δ Olake signal reflects season-specific or annual δ Op, or changes in water budget (evaporation/inflow ratio).

Residence time <6 months >6 months or >1 year groundwater fed

Evaporation No No Yes

18 d18 δ Olake signal Seasonality, winter or Mean annual Op Evaporation/ inflow ratio d18 summer Op (E/I)

residence times of these lakes, their waters contain dif- altitude lakes are mainly recharged by meteoric water ferent proportions of summer and winter precipitation. and that the snowmelt input plays a significant role in d18 Relatively short residence times and long periods of ice determining Olakew. The relatively short residence cover serve to minimize the effect of evaporation in sum- times and long periods of ice cover serve to minimize mer months in most of the lakes. The seasonal variation the effect of evaporation during summer months in these 18 in the isotopic composition of the lake water is larger in lakes. The results of the δ Odiatom analyses from Vuolep smaller lakes with short residence times, as they respond Allakasjaure and Lake Spåime sediments are remarkably faster to seasonal changes in precipitation than do larger similar over the last 150 years. Both records display a de- lakes with longer residence times. The small lakes in this creasing trend (~3‰) from AD 1850 to AD 1990, when region with relatively short residence times (<6 months) the lowest values of the records are reached, with a peak are non-steady-state lakes that are highly responsive to around AD 1980. After AD 1990, both records indicate changes in precipitation and meltwater inputs from snow an increase of approximately 2.5‰. Despite the short pack. duration of these records, their similarity indicates that The lakes in this region could well retain many as- the lakes have likely responded to the same main forcing pects of water isotopic composition that can be used for over the last 150 years. 18 palaeoclimate reconstructions. These reconstructions can Comparison between meteorological and δ Odiatom d18 be divided into three types depending on lake character- data indicates that changing Olakew over this time may d18 istics: (i) changes in seasonally specific Op, (ii) chang- be explained by a change in the seasonal distribution of d18 18 es in annual Op in lakes with no evaporation, and (iii) precipitation. The lowest δ Odiatom values occurred when changes in evaporation/inflow ratio (E/I) in lakes where the amount of winter precipitation was highest, ca. AD lake water is plotted on an LEL, reflecting the lake water 1990, when the NAO winter index was above +2 and balance (Table 2). The proxy signals recorded in authi- strong westerlies brought large amounts of winter pre- genic calcites, ostracod shells, diatom silica, and aquatic cipitation farther east than during less positive NAO d18 cellulose may represent different Olakew signals, de- years. We suggest that the depletion trend in Vuolep 18 pending on lake-specific hydrology and the timing of Allakasjaure and Lake Spåime δ Odiatom data between production, blooming, and precipitation. Early-bloom- AD 1900 and 1990 was mainly forced by the gradual ing diatoms in small lakes, for example, may actually be increase in winter precipitation in these regions. During 18 capturing winter δ Op through the early-summer thaw, this time, however, a change occurred from a positive while summer carbonates may capture mean weighted NAO winter index (zonal airflow) to a negative NAO 18 annual δ Op if forming in bicarbonate-rich groundwater- winter index (meridional airflow). This led us to con- fed lakes. clude that the amount of winter precipitation in fact increased even when a more pronounced southeast airflow Paper II: Jonsson, C.E., Rosqvist, G.C., Leng, M.J, dominated, bringing moisture derived from the Baltic Bigler, C, Bergman, J., Kaislahti Tillman, P., and Sea. Thus the mountain hydrology responds strongly to Sloane, H.: High-resolution diatom δ18O records winter precipitation changes caused by shifts between from two sub-Arctic high-altitude lakes in the Swed- zonal and meridional airflow. ish Scandes. Journal of Quaternary Science. In review. d18 Deriving climate information from Odiatom records call Paper III: Rosqvist, G., Jonsson, C., Yam, R., Karlén, for a detailed knowledge of the climate processes that W., and Shemesh, A., 2004: Diatom oxygen isotopes control and modify the signal. Therefore, calibrating in pro-glacial lake sediments from northern Sweden: lake sediment d18O against instrumental data is impor- a 5000-year record of atmospheric circulation. Qua- tant to help interpret long-term d18O records. In paper ternary Science Reviews, 23, 851–859. d18 II, two high-resolution Odiatom records from Vuolep Paper III describes the combined analysis of Holocene 18 Allakasjaure and Lake Spåime, located 650 km apart in glacier activity and δ Odiatom variations applied to the the Swedish Scandes (Fig. 1), were compared with me- pro-glacial lake Vuolep Allakasjaure (Fig. 1d). This was teorological records from the last century. This was done to determine whether both these proxies reflect signifi- to investigate the climate factors that influence the d18O- cant centennial–millennial-scale climate change over the d18 diatom in the sediment and whether Odiatom records can last 5000 years in northern Sweden. The sediments from be used to reconstruct seasonal precipitation variations this lake had previously been used to infer Holocene gla- and atmospheric circulation changes in these regions. cier fluctuations. 18 Modern lake water isotopic data indicate that these high- Based on results of δ Olakew analysis, we conclude

14 Holocene climate and atmospheric circulation changes in northern Fennoscandia that the modern lake water isotopic composition of addition, ice rafting records indicate that oceanographic Vuolep Allakasjaure predominantly reflects the isotopic conditions changed significantly during the Holocene. 18 composition of precipitation. The δ Odiatom record indi- This high-resolution record from Lake Tibetanus indi- 18 cates a general depletion trend of 3.5‰ over the past cates that the δ Ocarbonate minima coincide with peaks in 5000 years. Superimposed on this trend, the isotopic IRD occurring ca. 200, 500, 1300, 1600, and 2900 cal yr composition decreased (>1% excursions) on four occa- BP. An atmospheric pressure pattern promoting norther- sions ca. 4400, 3000, and 2000 and after 1200 cal yr BP. ly or northeasterly surface winds, inferred to have aided The generally high minerogenic content in the sequence drift ice transport (Bond et al., 2001), would also reduce 18 deposited over the last 5000 years indicates the presence δ Op in northern Scandinavia. Moreover, air masses of a glacier in the catchment during that period. Longer originating over the North Atlantic must have had low (>4 cm) laminated sequences with a proportionally high- δ18O because of lower atmospheric temperatures caused er minerogenic sediment content were assumed to corre- by the increased ice cover during the IRD events. The spond to periods (>200 years) when climate sustained a gradual decrease in δ18O between 2000 and 500 cal yr BP relatively large silt-producing glacier. If this assumption is likely associated with an increasing influence of low is correct, glacier advances occurred ca. 4400, 3000, and δ18O air masses. Such atmospheric circulation changes 2000 and after 1200 cal yr BP; all these events corre- could explain the late-Holocene cooling trend recorded 18 spond to δ Odiatom minima. From the similarity between in many proxy archives. We conclude in paper IV that the δ18O record derived from diatoms and the proxy lake hydrology has responded to circulation shifts on the record for glacier fluctuations, we conclude that atmos- centennial time scale over the past 3000 years, and that pheric circulation changes occurred on a centennial–mil- there was coupling between atmospheric and ocean cir- lennial time scale over the last 5000 years. The mass culation changes on the centennial timescale in the North balance of the present-day glaciers in this area primarily Atlantic area. responds to summer air temperatures. The fact that the isotope depletion minima coincide with glacier advances Paper V: Jonsson, C.E., Rosqvist, G.C., Leng, M.J., indicates that changes in atmospheric circulation affected and Sloane, H.J. Two diatom isotope records from 18 both δ Olakew and the mass balance of the glacier. Weaker Naimakka in northern Fennoscandia: implications westerlies and a more southerly polar front would allow for Holocene palaeohydrology and atmospheric cir- colder and isotopically more depleted air to influence the culation dynamics. In manuscript. d18 18 region in the summer. Interestingly, the O minima de- Paper V discusses the results of analyses of δ Odiatom sed- tected in this lake occur at times of relative ice-rafted de- iment sequences (9500 and 2800 years long) from two bris (IRD) maxima in the North Atlantic. From the IRD open-basin lakes (lakes Keitjoru and Oikojärvi) with dif- maxima it has been inferred that temperatures decreased ferent hydrological systems. The lakes are located near and winter sea ice extent increased in the vicinity of the a meteorological station in Naimakka, one of few mete- North Atlantic Current. The fact that δ18O changed at the orological stations in Fennoscandia where both d18O and same time as IRD was present reveals a significant hy- d2H in precipitation have been measured. The LMWL drological response in northern Fennoscandia to changes for Naimakka was determined from a linear regression 18 2 in the North Atlantic. of the monthly mean δ Op and δ Hp data (GNIP 1991– 1995, IAEA/WMO). The monthly mean average annual d18 Paper IV: Rosqvist, G.C., Leng, M.J., and Jonsson, C. amplitude of Op at Naimakka is 10‰. The seasonal d18 2007: North Atlantic region atmospheric circulation amplitude of Olakew in Lake Keitjoru (4‰) and Lake dynamics inferred from a late-Holocene lacustrine Oikojärvi (2.5‰) is mainly produced by the input of carbonate isotope record, northern Swedish Lap- isotopically depleted snowmelt (winter precipitation) in land. The Holocene, 17, 867–873. May/June and the influence of enriched summer precipi- The 3000-year-long stable oxygen isotope record derived tation. Our results indicate that changes in the isotopic 18 18 from authigenic carbonate (δ Ocarbonate) from Lake Tibet- composition of precipitation (δ Op) are the primary forc- 18 anus, presented in paper IV, is the first high-resolution ing mechanism decreasing or increasing the δ Olakew val- (15 yr/sample) δ18O lake record from northern Sweden. ues in both lakes. All modern lake and groundwater isotope data lie on the Changes in the isotope signals over time are inter- GMWL, indicating that evaporation is insignificant in preted as reflecting regional variations in atmospheric 18 this lake. The isotopic composition of the lake water in- circulation and climatic seasonality. The δ Odiatom record dicates that the lake, which is small, is likely constantly from Lake Keitjoru covering the last 9500 years indi- replenished by groundwater input. cates a 1.3‰ depletion trend extending from ca. 8000 18 Assuming a constant δ Olakew (i.e., no effect of chang- until shortly before 1500 cal yr BP. We believe that this 18 18 es in δ Op or of evaporation), the decreasing δ Ocarbonate change was a response to the long-term diminished in- trend of 0.8‰ detected between 2000 and 500 cal yr BP fluence of zonal airflow (strong westerlies) over Fenno- would require a summer lake water temperature increase scandia in favour of an increasing proportion of colder of 3.3°C. This is because an increase in lake water tem- meridional airflow (weak westerlies) from the north with 18 perature causes a −0.24‰/°C decrease in isotope frac- a more negative δ Op signature. A synchronous shift to tionation between the water and the calcite. The most lower δ18O values was recorded in lakes Keitjoru and 18 likely factors influencing δ Op in northern Scandinavia Oikojärvi ca. 700–600 cal yr BP (~AD 1250–1350). A during the Holocene are changes in condensation tem- similar shift is recorded in an oxygen isotopic compo- 18 perature and air mass sources. Marine diatom records in- sition record derived from aquatic cellulose (δ Ocellulose) dicate that sea surface conditions in the sub-polar North from the Lake Keitjoru sediment core, indicating a com- 18 Atlantic were unstable throughout the late Holocene. In mon response to changing δ Olakew. We argue that a cir-

15 Christina E. Jonsson culation pattern dominated by meridional airflow with from more southerly westerlies. Both precipitation re- increasing winter precipitation from the southeast or constructions from northern Sweden and water balance north replaced a zonal atmospheric circulation pattern at reconstructions from south and central Sweden indicate this time. that the atmospheric circulation shifted from zonal to a We demonstrate here that this shift had a significant more meridional airflow during the Holocene. 18 influence on lake hydrology in northern Fennoscandia. Superimposed on this Holocene trend are δ Op mini- There is widespread evidence of a major change in the ma that resemble intervals of the negative phase of the natural environment and human society at this time, North Atlantic Oscillation (NAO). We conclude that the which is the beginning of the so-called Little Ice Age oxygen isotopic composition of lacustrine sedimentary (LIA). The δ18O minima detected ca. 400 and 50 cal yr materials from Sweden yields both long- and short-term BP (~AD 1550 and AD 1900) likely also mark shorter- palaeoclimate information. term shifts in circulation. Lower temperatures at that 18 time have been recorded in tree-rings and pollen data. Preliminary results of analyses of δ Odiatom in surface The fact that the same isotope shifts have been detected sediments from the Abisko region: an investigation of 18 in δ O records from hydrologically different lakes con- the relationship between modern lake water temper- firms our conclusion that these records potentially reflect ature, lake water isotopic composition, and diatom a regional response to changes in atmospheric circula- isotopic composition in the Abisko area. tion. Data revealing the modern relationship between lake 18 18 sediment δ Odiatom, δ Olakew, and lake water tempera- Paper VI: Jonsson, C.E., Andersson, S., Rosqvist, ture are few. To investigate whether such a relationship G.C., and Leng, M.J. 2009: Reconstructing past at- could be established for the Abisko region, I sampled the mospheric circulation changes using oxygen isotopes uppermost 0–0.5 cm of sediment from the deepest part in lake sediments from Sweden. Climate of the Past of six lakes in summer 2006 (Fig. 1, 7) using an HTH (CP) Discussions, special issue 5, 1609–1644. gravity corer. The lakes and catchments were chosen to Paper VI summarizes and discusses the results of lacus- represent different sizes, elevations (Fig. 8), and resi- trine stable oxygen isotope studies from Sweden. We dence times (Table 3). Lake waters were sampled twice demonstrate how Holocene-aged δ18O sediment records, in summer 2006, in early July and late August. Water derived from lakes located in different hydrological set- temperature was measured in four lakes using automated tings in Sweden, can provide information about various thermistors (Thermochron iButton) every fourth hour aspects of climate change, such as changes in precipita- from early July to late August at depths of 0.5, 6, and 12 tion pattern, atmospheric circulation, and water balance. m in the deepest parts of the lakes, near the coring sites Past records of oxygen isotopes can be obtained from (Table 3). Lake water δ18O and δ2H analyses as well as 18 a large range of materials, such as authigenic calcites, δ Odiatom analyses were performed at the NERC Isotope ostracod shells, diatom silica, and aquatic cellulose. Geosciences Laboratory, UK. Results of the lake water The signatures captured by these sedimentary materials δ18O values derived from the lakes sampled in 2006 to- 18 may represent different δ Olakew signals, depending on gether with the results of previous years’ measurements the lake-specific hydrology and the timing of produc- (Fig. 9) indicate no significant vertical isotopic stratifi- tion, blooming, and precipitation. A combination of δ18O cation at the time of sampling. This indicates that these records derived from different materials potentially re- lakes have a well-mixed water column, both shortly after 18 flect seasonal aspects of δ Op and might yield additional the early-summer snowmelt and later in summer. As ex- information about lake water temperatures. pected, temperature variation amplitude is larger in small Results derived from oxygen isotopes in lacustrine lakes, such as Lake Njulla, than in larger lakes, such as sediments from Sweden can be divided into two types Lake Vuoskkujávri. There is a lag in the temperature re- 18 of reconstruction: (i) changes in annual or seasonal δ Op sponse of the large lakes (Livingstone and Lotter, 1998), in lakes with no evaporation, and (ii) changes in evapo- as water temperature at depths of 6 and 12 m responds ration/inflow (E/I) water balance in lakes where evapo- more slowly to changes in air temperature than does the 18 18 ration is forcing the δ Olakew. The proxy δ Op records surface water temperature, measured at a depth of 0.5 m indicate that the air masses bringing precipitation to this (Fig. 10). region shifted between zonal and meridional airflow on The age models for these lakes indicate that the sedi- different time scales over the past 10,000 years. We dem- mentation accumulation rates are relatively low (paper 18 onstrate that in non-evaporative lakes, where δ Olakew is II, Bigler et al., 2002; 2003; Rubensdotter and Rosqvist, 18 near the annual δ Op, diatom and carbonate sediment 2003). It was therefore impossible to match the time range 18 18 records indicate similar changes in the Holocene δ Op. of the δ Olakew and lake water temperature measured in 18 18 The Holocene decreasing δ Op trend in northern Sweden summer 2006 with the sediment δ Odiatom value, because was likely forced by a shift from the dominance of strong the uppermost 0.5 cm represents between one and five zonal westerly airflow in the early Holocene to a more years. Sampling modern diatoms would have required 18 meridional flow pattern. Coupling independent pollen sediment traps (Lotter and Bigler, 2000). The δ Odiatom 18 and δ Op records reveals a deviation from the Dansgaard values from the uppermost 0.5 cm of the sediments from 18 δ Op–temperature relationship in northern Sweden dur- the six lakes vary between 24.2 and 26.6‰. The meas- 18 ing the early Holocene. The reconstructions based on ured δ Odiatom values are plotted against different lake sediments from the evaporative sites in south and central water and catchment parameters in Figure 11. A negative 18 Sweden reflect warm and dry conditions during the mid correlation (r = –0.98) is found between δ Odiatom and the Holocene and a shift towards wetter conditions associat- amplitude of the seasonal variation measured between 18 ed with increased zonal airflow, which is to be expected early- and late-summer δ Olakew (though the number of

16 Holocene climate and atmospheric circulation changes in northern Fennoscandia

Lake 850 (850 m a.s.l) Lake Njulla (999 m a.s.l)

Vuolep Allakasjaure (995 m a.s.l) Kalanjaure (996 m a.s.l)

Vuoskkujavri (348 m a.s.l) Lake 865 (865 m a.s.l)

Figure 7 Lakes sampled in summer 2006.

18 samples is small). Thus, a high seasonal δ Olakew ampli- longer residence times, such as Lake Vuoskkujávri, will tude indicates that the lake has a short residence time be more affected by the inherited isotopic composition and is quickly replenished with new input water. For ex- of the previous year’s summer lake water, potentially 18 ample, few days after the snowmelt, the lake water will reflecting the annual mean δ Op signal. This might ex- 18 be replaced by meltwater in a small lake, such as Lake plain why no correlation exists between the δ Odiatom and 18 18 Njulla. If we assume that the diatoms start to bloom and δ Olakew signals. The lowest δ Olakew values, caused by reproduce immediately after ice break-up, when light the influence of snowmelt on lakes with short residence and nutrition conditions are ideal, these diatoms will re- times, were probably reached before the sampling period 18 flect an early-summer δ Olakew signal. In Lake Njulla this started in early July. 18 18 signal will be a snowmelt δ O signal, representing win- Potential δ Odiatom values were calculated using Juil- ter precipitation, whereas diatoms in larger lakes with let-Leclerc and Labeyrie’s (1987) equation (–0.5‰/°C),

17 Christina E. Jonsson

18 Figure 9 Lake water depth profiles of δ Olakew for Lake 850, Vuolep Allakasjaure, and Lake Vuoskkujávri. There was no isotopic stratification in any of the lakes at time of sampling, indicating that these lakes have a well-mixed water column. Circles indicate August 1999, stars September 2000, triangles July 2006, and diamonds August 2006. Figure 8 Hypsometric curves for the lakes presented in Table 3 showing catchment elevation. The hypsometric curves are based on the 50 x 50 m2 digital elevation model (DEM) from the National Land Survey of Sweden (Lantmäteriverket). 18 for those lakes where both δ Olakew and temperature were 18 measured (Table 3). The calculated δ Odiatom values using Equation 4 (Fig. 11) are higher in early (July) than as recommended by Shemesh et al. (1992): in late (August) summer, which is caused by the effect of the larger increase in lake water temperature (2–4°C) 18 18 T°C = 11.03 – 2.03 (δ Odiatom – δ Olakew – 40) (4) (negative temperature fractionation), compared with the 18 increase in δ Olakew (0.5–1.4‰). The difference between 18 and Brandriss et al. (1998) and Moschen et al.’s (2005) measured and calculated δ Odiatom values could indicate equation (–0.2‰/°C): that the effect of lake water temperature on the oxygen isotope fractionation between lake water and silica is 18 18 T°C = 190.07 – 5.05 (δ Odiatom – δ Olakew) (5) closer to –0.2‰/°C than –0.5‰/°C.

18 18 Table 3 Characteristics of lakes sampled in summer 2006 and measured δ Olakew, δ Odiatom, and lake water temperature values. 18 Calculated δ Odiatom values using Equation 4 (–0.5‰/°C; Juillet-Leclerc and Labeyrie, 1987) and Equation 5 (–0.2‰/°C; 18 Brandriss et al., 1998; Moschen et al., 2005) for the measured δ Olakew and lake water temperatures. Vuoskkujavri Lake 865 Vuolep Kalanjaure Lake 850 Lake Njulla Allakasjaure Max water depth (m) 18 11 11 -- 8 4.2 Surface area (km2) 0.68 0.1 0.4 0.12 0.02 0.01 Catchment area (km2) 11.3 2.0 22.7 10.3 0.3 0.78 Lake:catchment ratio 1:17 1:20 1:57 1:86 1:18 1:80 Lake elevation (m a.s.l.) 348 865 995 996 850 999 Mean catchm. elev. (m a.s.l.) 680 910 1220 1270 865 1050 18 Sediment δ Odiatom (‰) 26.6 26.3 25.7 25.3 25.1 24.4 18 July δ Olakew (‰) –13.1 –13.7 –14.4 –14.8 –13.3 –14.0 18 Aug. δ Olakew (‰) –12.56 –13.1 –13.6 –13.9 –12.3 –12.6 18 Mean δ Olakew (‰) –12.8 –13.4 –14.0 –14.3 –12.8 –13.3 18 δ Olakew variation July-Aug. (‰) 0.5 0.6 0.8 0.9 1.0 1.4 July lake water temp (°C) 12.5 -- 7.8 -- 9.8 9.3 Aug. lake water temp (°C) 14.8 -- 12.1 -- 12.9 12.9 Mean lake water temp (°C) 13.5 -- 9.7 -- 11.4 11.2 18 Calc. July δ Odiatom (‰) Eq. 4 26.6 -- 26.8 -- 27.3 26.8 18 Calc. Aug. δ Odiatom (‰) Eq. 4 25.8 -- 25.6 -- 26.8 26.5 18 Calc. mean δ Odiatom (‰) Eq. 4 26.0 -- 26.4 -- 27.0 26.6 18 Calc. July δ Odiatom (‰) Eq. 5 22.1 -- 21.7 -- 22.4 21.8 18 Calc. Aug.δ Odiatom (‰) Eq. 5 22.4 -- 21.6 -- 21.8 22.5 18 Calc. mean δ Odiatom (‰) Eq. 5 22.2 -- 21.7 -- 22.6 22.1

18 Holocene climate and atmospheric circulation changes in northern Fennoscandia

studies in northern Sweden (Shemesh et al., 2001; Ham- marlund et al., 2002) indicate that the lake water temperature effect is not the main factor responsible for variations in δ18O records. Based on the results presented here, I conclude that the effect of lake water temperature 18 on the δ Odiatom signal is small. This conclusion is sup- 18 ported by recent analyses of δ Odiatom values in surface sediments across Europe (Tyler et al., 2008) and Alaska (Schiff, 2007; Schiff et al., 2009) demonstrating that the effect of temperature may be less than previously esti- mated, and suggesting an almost constant fractionation 18 value, in the range of 35.5–42.8‰ between δ Olakew and 18 δ Odiatom. iii) The seasonal timing of the diatom productivity in the lakes must be considered when making climate infer- 18 ences based on δ Odiatom records, especially records from 18 lakes with short residence times. In these lakes, δ Olakew and temperature might change between early and late summer, reflecting different seasonal conditions. The seasonal cycle of diatom productivity in high-altitude lakes depends largely on the timing of the ice break-up Figure 10 Lake water temperature in Vuolep Allakasjaure, (Lotter and Bigler, 2000). The diatom blooms occur both Lake Njulla, Lake 850, and Lake Vuoskkujávri measured using immediately after ice break-up, when light conditions in automated thermistors every fourth hour from early July to late the water columns improve and nutrients become availa- August at the deepest part of each lake, near the coring sites ble, and later in summer when environmental conditions (Table 3). Black lines indicate temperatures at 0.5 m, dark grey are less stressful for the algae (Lotter and Bigler, 2000; 6 m, and light grey 12 m water depths. Daily mean temperature Catalan et al., 2002; Forsström et al., 2005, Forsström, at the Abisko Scientific Research Station (385 m a.s.l.). 2006). Changes in nutrient supply to a lake over time, for example, due to soil and vegetation development in the catchment or to human influence, might change the timing of the maximum diatom bloom (Lotter and Bigler 2000, Sorvari et al., 2002; Bigler et al., 2006), thus af- Discussion 18 fecting the δ Odiatom signal. Several factors/processes might be influencing the The preliminary results presented here indicate that 18 18 δ Odiatom (δ Ocarbonate) record in the studied lake sedi- the main part of modern diatom productivity in the stud- ments, including: i) the oxygen isotopic composition of ied lakes occurs early in summer, reflecting an early-sum- 18 the input water to the lakes, ii) the temperature-depend- mer lake water signal. It is therefore likely that δ Odiatom ent oxygen isotope fractionation between diatom silica records from small lakes with short residence times will (carbonate) and lake water, and iii) the timing of diatom have an isotope signal weighted towards winter precipi- production in the lakes. tation. In fact, comparisons between meteorological and 18 2 18 i) The δ O and δ H composition of the lake waters δ Odiatom data presented in paper II indicate that the most d18 presented here (Fig. 12) lie on or near the GMWL, in- likely explanation of changing Olakew values in high- dicating that the isotopic composition of these lakes is altitude lakes with relatively short residence times is a mainly controlled by the isotopic composition of the change in the seasonal distribution of precipitation. The d18 18 precipitation. Changes in local Op depend on varia- main factor affecting δ Odiatom values in these lakes over tions in ambient air temperature and atmospheric circu- the last 150 years has been changes in the amount of win- lation that lead to changes in moisture source, vapour ter precipitation. transport efficiency, and winter to summer precipitation distribution. Due to lower temperatures and rain-out ef- 18 2 Regional shifts in atmospheric circulation fects, relatively lower δ Olakew and δ Hlakew values are found in lakes located at high altitude and/or latitude. during the Holocene This study demonstrates that the amount of isotopic vari- This thesis presents lake sediment δ18O studies, with dif- ation in lake water δ18O is determined by a combination ferent resolutions and timescales, of lakes with different 18 18 of the original δ Olakew, the amount and timing of the hydrological settings. All the analysed δ O records indi- 18 snowmelt, the amount of seasonally specific precipita- cate that changes in δ Olakew are the main factor respon- tion and groundwater, any evaporation effects, and lake sible for changes in the δ18O record, and that the effect of water residence time. lake water temperature on isotope fractionation is of mi- ii) The temperature-dependent fractionation ranges nor importance. Furthermore, the similarities in the re- 18 18 between –0.5‰/°C and –0.2‰/°C (Juillet-Leclerc and sponse of δ Odiatom to changes in δ Olakew in sites located Labeyrie, 1987; Shemesh et al., 1992; Brandriss et al., 650 km apart (paper II) indicates that the signal is region- 18 1998; Moschen et al., 2005) for diatoms and approxi- al, reflecting changes in δ Op following upon changes mately –0.25‰/°C (Craig, 1965) for carbonates. Results in atmospheric circulation. From the relatively coherent 18 d18 18 derived from δ Odiatom and Ocarbonate records from lake pattern of isotopic changes evident in the Holocene δ O sediments presented here (papers II–V) and from other records, I infer that the isotope hydrology of the studied

19 Christina E. Jonsson

18 Figure 11 Analyzed sediment (0–0.5 cm) δ Odiatom values from six lakes (Table 3) in the Abisko region sampled in 2006, plotted 18 against lake water temperature measured in summer 2006, catchment mean elevation, measured δ Olakew in early July and late 18 18 August, δ Olakew variation between early July and late August 2006, and calculated δ Odiatom values using Equation 4 (–0.5‰/°C; Juillet-Leclerc and Labeyrie, 1987) and Equation 5 (–0.2‰/°C; Brandriss et al., 1998). Circles indicate early July 2006, squares late August 2006, and diamonds calculated mean value for July and August. lakes has responded to the same main forcing (Fig. 13, (Shemesh et al., 2001; Hammarlund et al., 2002; St 18 14). The results further indicate that δ Op changes due Amour 2009) (Fig. 13) has likely been forced by a long- to shifting moisture sources and changes in the seasonal term shift from a dominant strong zonal westerly airflow distribution of precipitation were generally greater than in the early Holocene to a more variable circulation with those that result from changes in condensation tempera- a dominant meridional airflow pattern. The long-term ture over the same period. Therefore, the δ18O data imply Holocene trend follows the pattern of orbital forcing of that the relative influences of precipitation-bearing air summer insolation at 65°N, with considerably higher masses has changed, caused by shifts between zonal and insolation values during the early Holocene than today 18 meridional airflow over northern Fennoscandia on dif- (Berger and Loutre, 1991). Therefore, the δ Olakew re- ferent time scales over the past 10,000 years. sponse could perhaps be reflecting a general insolation- forced decreasing air (condensation) temperature and its Long-term Holocene trend 18 effect on the δ Op. A relatively cool and moist climate 18 The long-term Holocene decreasing δ Op trend evi- has, however, been reconstructed using vegetation prox- dent in the records presented here and in other studies ies (Seppä and Birks, 2001; Seppä et al., 2009) for the

20 Holocene climate and atmospheric circulation changes in northern Fennoscandia

18 2 Figure 12 The isotopic composition of modern lake waters (δ Olakew and δ Hlakew), 2002–2006, from lakes in the Abisko, Naimakka (lakes Oikojärvi and Keitjoru), and Jämtland (Lake Spåime) regions. The global meteoric water line (GMWL) is based on the global distribution of precipitation defined as δ2H = 8δ18O + 10 (Craig, 1961). Regression through lake water samples from Lake Oikojärvi defines an average local evaporation line (LEL) (δ2H = 5.3δ18O – 28.4). early Holocene. Therefore, the relatively high δ18O val- eral lakes indicates that the age chronologies are good ues recorded in the early Holocene suggest a deviation enough for comparison on the centennial time scale. 18 18 from the present-day relationship between δ Op and air From the similarity between the δ O record derived 18 temperature, where δ Op = 0.69T – 13.6 (T in °C) (the from diatoms and the proxy record for Holocene glacier Dansgaard relationship; Dansgaard, 1964). A strong zon- activity in Vuolep Allakasjaure we conclude in paper al circulation with enhanced Atlantic airflow across the III that the coinciding of all isotope depletion minima Scandes Mountains possibly caused a decreased rain-out with glacier advances indicates that changes in atmos- 18 effect and reduced distillation process, generating large pheric circulation affected both δ Olakew and the mass 18 amounts of precipitation with relatively high δ Op val- balance of the glacier. The mass balance of present-day ues in the early Holocene. The successively diminishing glaciers in this area primarily responds to summer air influence of maritime westerly (or zonal) airflow over temperatures, but winter precipitation amounts are also Fennoscandia in favour of an increasing proportion of important (Holmlund et al., 1996). Weaker westerlies more continental meridional airflow from the north or and a more southerly polar front would allow colder and 18 east with a more negative δ Op signature would lead to isotopically more depleted air from the north to influ- 18 lower temperatures and δ Olakew values in the mid and ence the region in summer. However, because increased 18 late Holocene (Shemesh et al., 2001; Hammarlund et al., winter precipitation would result in decreasing δ Olakew 2002). This is supported by a pollen-based temperature values and an increasing glacier mass balance, the record reconstruction indicating that the Fennoscandian climate might alternately reveal changes in atmospheric circula- has become increasingly continental over the last 7000 tion in winter, as seen in paper II. years (Gisecke et al., 2008). Results from Vuolep Allakasjaure and Lake Tibeta- nus indicate a tentative coupling with North Atlantic ice Late-Holocene palaeoclimate implications d18 d18 rafting events. This is because Odiatom and Ocarbonate Age–depth models have several limitations, so using minima occurred ca. 200, 500, 1300, 1600, and 2900 cal them in identifying shorter-term changes and in detailed yr BP, simultaneously with ice-rafted debris (IRD) maxi- comparisons with other proxy records should be done ma (Bond et al., 1997; 2001). The IRD maxima are used with caution. The similarity of the δ18O records from sev- to identify changes in Holocene drift ice in the vicinity of

21 Christina E. Jonsson

shift to weaker winter NAO conditions and stronger meridional circulation occurred around AD 1450 (500 cal yr BP). A cold summer climate ca. 400 and 50 cal yr BP has been detected in several terrestrial proxy archives from northern Fennoscandia (Karlén, 1988; Karlén and Kuylenstierna, 1996; Kirchhefer, 2001; Grudd et al., 2002; Seppä and Birks, 2002; Moberg et al., 2005; Nesje et al., 2005; 2008; Grudd et al., 2008; Bjune et al., 2009). 18 Thus, the reconstructed δ Olakew signatures probably also 18 include the effect of lower δ Op values caused by summer cooling. SST cooling in the North Atlantic occurred after 600 cal yr BP, the lowest temperatures being recorded ca. 350 and 50 cal yr BP (Andersson et al., 2003; Risebrobakken et al., 2003). The latter two brief minima are also characterized by an increase in IRD (Bond et 18 al., 2001). We argue that the significant δ Olakew depletions recorded ca. 400 and 50 cal yr BP mark a shift to more intense meridional airflow, with weaker westerlies over northern Fennoscandia producing colder summers and considerable winter precipitation from the south- 18 east or north-northwest lowering the δ Olakew. Bütikofer (2007) suggested that the IRD maxima might resemble Figure 13 July insolation at 65°N based on Berger and Loutre prolonged intervals of the negative phase of the contem- (1991) plotted together with pollen-inferred annual precipitation and July lake water temperature in Lake Tsuolbmajavri porary NAO, which is marked by cooler conditions in d18 northern Europe. Changes in atmospheric circulation (Seppä and Birks, 2001). Lake sediment Odiatom records from lake 850 (Shemesh et al., 2001) and Keitjoru (paper V). Note during different NAO phases might then result in shifts the different d18O scales. in the location of precipitation moisture sources in Fen- noscandia as well as changes in the atmospheric fractionation temperature. This effect of NAO variability on 18 δ Op has been identified for precipitation in Greenland in a study using a new moisture source diagnostic (Sode- the North Atlantic Current (Bond et al., 2001; Moros et mann et al., 2008). 18 al., 2006). The fact that the δ Odiatom level seems to have changed over Fennoscandia in the same periods as IRD deposition changed might indicate a coupled atmospher- Study uncertainties and limitations ic and oceanic forcing, and that these events occurred on a regional scale. The δ18O signal derived from lacustrine or marine dia- A significant depletion shift is seen in several δ18O toms may be complicated by changes in diatom taxa/ records starting ca. 700–600 cal yr BP (AD 1250–1350) assemblages. The diatom samples analyzed for δ18O are (Fig. 14). A minima is reached ca. 400 cal yr BP (AD bulk samples usually comprising multiple species that 1550), after which the δ18O values increase (visible only may vary throughout the stratigraphic sequence (e.g., in relatively high-resolution records) over the next ap- Bigler et al., 2003). Species-related differences in oxy- proximately 300 years. Another minimum is reached ca. gen isotope fractionation (vital effects) have been found 18 50 cal yr BP (~AD 1900). The most likely explanation in the δ Odiatom signal from marine cores (Swann et al., 18 for the significant δ Olakew depletions recorded by the 2007; 2008). Still, little is known about any specific frac- diatoms and carbonates starting ca. 700–600 cal yr BP tionation factor between lake water and different diatom is that a large contribution of winter precipitation low- species in lake sediments. Although lake diatom species 18 ered the δ Olakew. We therefore argue that the lake waters might not be affected by any species-specific differ- responded to a significant shift in atmospheric circula- ence in fractionation (e.g., Shemesh et al., 1995; Schiff 18 tion at this time, which changed the annual δ Op. Ma- et al., 2009), species that bloom in different habitats jor changes in the atmospheric circulation systems that might be affected by different lake water temperatures 18 would confirm this scenario have indeed been reported and δ Olakew values. Therefore, changes, for example, in from elsewhere around this time. Based on the variabil- the planktonic/benthic ratio over time, may drive part of 18 ity of the concentration of sea salt-derived ions in the the δ Odiatom signal. To bloom and reproduce, planktonic GISP2 ice core, Meeker and Mayewski (2002) conclud- diatoms are dependent on turbulent mixing in the water ed that a major shift in winter atmospheric circulation column, so they benefit from a long period without ice in the northern North Atlantic region took place ca. 550 cover (Bigler et al., 2002). Most of the studied lakes have cal yr BP (AD 1400–1420). This is the most dramatic a well-mixed water column, so I assume that this effect is change registered by this circulation proxy in the GISP2 negligible. In any case, a 5-mg diatom sample analysed ice core over the last 4000 years (Kreutz et al., 1997) and for δ18O is a bulk sample consisting of >100,000 diatoms is considered to mark the onset of the LIA ( Meeker and dominated by the best-preserved specimens. Each meas- Mayewski, 2002; Dawson et al., 2003; 2007). Accord- ured δ18O value thus represents a mean δ18O value from ing to an NAO mode reconstruction based on tree-ring >100,000 diatoms. and speleothem data, (Trouet et al., 2009) a significant Non-climatic effects, such as long-term lake develop-

22 Holocene climate and atmospheric circulation changes in northern Fennoscandia

Figure 14 Pollen-inferred a) annual precipitation and b) July lake water temperature from Lake Tsuolbmajavri (Seppä and Birks, d18 d18 2001). Lake sediment records from: Lake Keitjoru, c) Odiatom (paper V) and d) Ocellulose (St.Amour, 2009); Lake Oikojärvi, e) d18 d18 d18 Odiatom (paper V); Lake 850, f) Odiatom (Shemesh et al., 2001); and Lake Tibetanus, g) Ocarbonate (paper IV). Grey bar indicates a period with low values in all d18O records, low July air temperatures, and high amounts of precipitation. Note the different d18O scales.

ment during the Holocene, can influence the δ18O signal. δ18O record shifting from an annual to a seasonal sig- For example, vegetation succession, soil development, nal. Another non-climatic factor that might affect the 18 and hydrological changes will affect the water input, δ Odiatom signal is the silica maturation process in the nutrient concentration, and pH of a lake (Engstrom et diatoms during sedimentation. There are indications that 18 al., 2000), which in turn might affect lake productivity maturation leads to δ Odiatom enrichment after deposition (timing and community) and mineral precipitation. The (Schmidt et al., 1997; 2001; Brandriss et al., 1998; Mo- lake volume might, for example, gradually decrease as schen et al., 2006; Tyler, 2007). This secondary isotope the basin is filled with sediments and the residence time exchange is not fully understood, but has been related 18 will, especially in shallow lakes, shift towards shorter to 2.5‰ enrichment in surface sediment δ Odiatom levels, times, assuming the same runoff. A 50% loss in volume versus�� levels in diatoms from sediment traps in the wa- would reduce the residence time by half. This might ter column in a lake in Germany (Moschen et al., 2006). change the lake water δ18O and lead to the sedimentary After a rapid initial increase, the maturation process

23 Christina E. Jonsson in laboratory experiments proceeds slowly and is sug- in many long cores is lost or compacted during core ex- gested to lead to successive isotopic enrichment in the traction. In addition, correlating 210Pb-dated surface sedi- sedimentary archive over longer time scales (Moschen ment cores in which the uppermost loose sediments are et al., 2006). However, the fact that the high-resolution present with 14C-dated long cores is often complicated by 18 δ Odiatom records from Vuolep Allakasjaure and Lake different sedimentation rates. Spåime (paper II) start with decreasing values, and not a 18 downcore increase, and the similarity between δ Odiatom and δ18O records suggest that the maturation proc- Carbonate Conclusions ess has a minor effect on the isotope signal in these lakes. It is also suggested that this diagenetic process might re- • This study demonstrates that lakes in this sub-Arctic duce the fractionation coefficient between lake water and region are currently mainly recharged by meteoric silica (Tyler et al., 2008). water. The isotopic variation in lake water δ18O rep- d18 resents a combination of the original Olakew, the amount and timing of the snowmelt, the amount of Contamination seasonally specific precipitation and groundwater, In small high-altitude lakes, most diatoms are between 5 any evaporation effects, and lake water residence and 20 μm in size. Sediment samples from these lakes, time. Seasonal variation in the isotopic composition especially sediments from pro-glacial lakes, often have of the lake waters is larger in smaller lakes with short a high content of minerogenic particles in the same size residence times (<6 months), as they respond faster fraction, because of the input of glacially eroded silt and to seasonal changes in precipitation, than in larger clays. Because the analysis of δ18O will liberate oxygen lakes with longer residence times (>6 months) that d18 from all oxygen-bearing minerals in the sample, these retain a signal similar to that of mean annual Op. lake samples require careful inspection using light microscopy and sometimes even scanning electron micro- • The proxy signals recorded in authigenic calcites, scopy after cleaning to ensure that a pure diatom sample ostracod shells, diatom silica, and aquatic cellulose 18 d18 is analysed. The δ O values of minerogenic materials may represent different Olakew signals depending 18 are usually significantly lower than the δ Odiatom values, on a lake’s residence time and the timing of produc- so relatively small amounts of contamination can disturb tion, blooming, and precipitation. Early-blooming 18 the δ Odiatom record (Leng and Barker, 2006; Brewer et diatoms in small lakes, for example, may capture 18 d18 al., 2008). Siliceous materials, such as chrysophytes and winter δ Op through its dominance of the Olakew sponges, are difficult to remove by cleaning, because during the early-summer thaw. Summer carbonates 18 they are usually similar to the diatoms in size and densi- may instead capture the mean weighted annual δ Op, ty. Little is known about the isotope fractionation factor if forming in bicarbonate-rich groundwater-fed lakes. between lake water and the silica in these materials. We If forming in a lake where evaporation is dominant, can therefore only assume that their fractionation factors the isotope signal will instead reflect the summer lake are similar to those of the diatoms and that their influ- water balance. The present results indicate that in 18 18 ence on the δ Odiatom signal is insignificant. non-evaporative lakes, where δ Olakew represents an- 18 18 nual δ Op, both diatom- and carbonate-derived δ O data indicate similar changes in the Holocene. Age control uncertainty • 18 Reliable climate change reconstructions from lake sedi- The δ Odiatom data derived from sub-Arctic high-alti- ment δ18O records require sufficient age–depth control to tude lake sediments with short residence times reflect 18 enable correlation with meteorological data and detailed changes in the δ Olakew and therefore also changes in comparison with other proxy records. 14C radiocarbon the seasonality of precipitation. Meteorological data dating measurements should preferably be performed indicate that the AD 1900–1990 period was charac- on small, carefully selected terrestrial plant samples, terized by increased winter precipitation and high due to the many uncertainties in identifying the carbon winter/summer precipitation ratios. This change is source in bulk samples (Barnekow, 1998; Björck and recorded in the diatom δ18O signal and appears as a Wohlfarth, 2001). Due to the lack of suitable macrofos- decreasing δ18O trend, so changes in the winter pre- sils, some lake sequences have been dated using bulk cipitation pattern are an important factor influencing 18 samples, aquatic mosses, or pieces of unidentified wood. the δ Odiatom signal in these high-altitude lakes with For example, seven bulk samples from Oikojärvi were short residence times. analysed and all were found to be too old (St. Amour, 2009). In addition, 14C dates obtained for terrestrial mac- • Lake sediment δ18O is an important climate proxy in 18 rofossils from Lake Keitjoru yielded ages that were ob- northern Fennoscandia, and δ Odiatom records can im- viously too old, likely due to the reworking of catchment prove our understanding of past regional responses soils (St. Amour, 2009). Calibration of 14C dates and the to climate forcing and provide important information selection of statistical method and age–depth model will about atmospheric circulation changes. This study add further uncertainty to the chronologies (Telford et also emphasizes the importance of properly under- al., 2004a; 2004b) standing current lake isotope hydrology and system It is often assumed that the uppermost portions of sensitivity to changes in climate variables. long cores obtained using Livingstone corers or Russian peat samplers represent modern sediments. However, • More than one lake should be studied in a region if 210Pb dating indicates that the topmost surface sediment regional changes in δ18O variation are to be identified

24 Holocene climate and atmospheric circulation changes in northern Fennoscandia

and understood. The fact that the same isotope shifts NAO modes/shift can be detected. The mean winter 18 have been detected in different δ Olakew proxies, de- (December- March) NAO index in Naimakka is highly rived from hydrologically different lakes, allows us correlated with the deuterium-excess values (d-excess = to conclude that these records reflect significant -at d2H – 8 d18O, Dansgaard, 1964) in winter precipitation (r mospheric circulation changes on a regional scale. = 0.9) (Baldini et al., 2008). Therefore, combining lake sediment δ18O records and δ2H records from n-alkanes • 18 The results presented here indicate that δ Op changes produced by phytoplankton and macrophytes (Sachse et due to shifting moisture source and/or changes in the al., 2004; Tyler et al., 2008) from this region might let seasonal distribution pattern are generally greater us reconstruct past meteoric water lines and calculate d- 18 than those that result from changes in condensation excess, to identify changes of the δ Op sources reflecting temperature over the same period. shifts in the NAO mode.

• 18 The long-term Holocene decreasing δ Op trend recorded in lake sediment δ18O in northern Sweden is Acknowledgements likely forced by a decrease in the influence of zonal 18 airflow (strong westerlies, relatively high δ Op) over This project was financially supported by the Swedish Fennoscandia in favour of an increasing proportion Society for Anthropology and Geography (SSAG), Lille- of colder meridional airflow (weak westerlies, lower mor och Hans W:son Ahlmanns Fond, Carl Mannerfelts 18 18 δ Op) from the north. The δ Olakew depletion record- Fond, Axel Lagrelius Fond, Abisko Scientific Research ed in the δ18O records ca. 600 cal yr BP (AD 1350) Station (ANS), Helge Ax:son Johnsons Stiftelse, Stif- may be due to a shift to a more intense meridional telsen Axel Hambergs Testamentsfond, and L & E Ki- circulation pattern and weaker westerlies over north- nanders Stipendiestiftelse. ern Fennoscandia, together with increasing winter First, I would like to thank my main supervisor, precipitation from the southeast or north. This cli- Gunhild Rosqvist. Ninis encouraged and supported me mate shift probably marks the onset of the Little Ice throughout the work on this thesis. She gave me the op- Age in this region. portunity to conduct a diatom oxygen isotope study in the beautiful Abisko region as an undergraduate, and then to continue this interesting and challenging project Future perspectives as a PhD student. Without her enthusiasm, knowledge, and guidance this thesis would not have been possible. This study demonstrates that lake waters in Sweden I am very grateful to Melanie Leng at the NERC Iso- could well retain many aspects of water isotopic com- tope Geosciences Laboratory, UK, for always taking the position that can be used for palaeoclimate reconstruc- time to share her knowledge and for valuable discussions tions. Ongoing monitoring of the isotopic composition of isotopes. Thanks are also deserved by my co-supervi- of precipitation, groundwater, and lake water over differ- sor Jan Seibert for his hydrology expertise and valuable ent seasons is needed if we are to keep improving our un- comments on the manuscript. derstanding of the modern relationship between climate Many thanks to Wibjörn Karlén for always taking an 18 18 change, δ Op, and δ Olakew. interest in my project and for sharing Vuolep Allakas- An interesting and important issue for future studies jaure with me. will be to continue to investigate the effect of tempera- Aldo Shemesh and Miri Rietti-Shati, at the Weiz- ture on the oxygen isotope fractionation between lake mann Institute of Science in Israel, are acknowledged water and silica and any species-specific fractionation. for introducing me to the diatom cleaning process and Improved techniques and newly developed methods kindly letting me use their laboratory during the first may allow the isolation of single diatom species for δ18O stage of this project. analysis, for example, the split-flow lateral-transport I am thankful to the staff at ANS for their logistical thin separation cells (SPLITT) (Giddings, 1985; Rings et support whenever I visited Abisko, in particular, Lin- al., 2004; Leng and Barker, 2006), a separation method nea and Bengt Wanhatalo, Nils Åke Andersson, Anders based on gravity, and laser extraction techniques allow- Eriksson, and Thomas Westin (CIRC). I would also ing the use of smaller diatom samples (1–2 mg) in analy- like to thank Christian Bigler and Jonatan Klaminder at sis (Dodd et al., 2007; Schiff et al., 2009). There is also Umeå University for diatom and lead pollution analysis. a need to relate the timing of diatom productivity to the Thanks to all my friends and colleagues at the De- 18 δ Odiatom signal. partment of Physical Geography and Quaternary Geol- Identifying decadal climate change requires highly ogy for creating an inspiring working environment. I resolved lake sediment δ18O records. This emphasizes would like to acknowledge the administrative staff at the the necessity of more accurately dated lake sediments department, expressing special thanks to Håkan Granell, and improved chronologies for interpreting decadal to Yanduy Cabrera, Berit Berggren, Carina Henriksson, centennial timescale variations and more reliable com- and Susanna Blåndman. Sven Karlsson provided lab parisons with other proxy records. support and Päivi Tillman Kaislathi and Annika Bernts- The results of this study clearly indicate the impor- son shared the secret and magical world of diatom clean- tance of carefully selecting sites with suitable lakes and ing. Special thanks to Ola Svanered, Pernilla Schuber, sediment materials recording the appropriate δ18O sig- Andreas Beskow, Mattias de Woul, Martin Spångberg, nal required for the palaeoclimate reconstruction. For Hanna Sundqvist, Karin Ebert, Jan Risberg, Krister Jans- example, it would be interesting to identify sites where son and Björn Gunnarson for stimulating lunchtime dis- 18 pronounced variation in δ Olakew according to different cussions and for creating a pleasant atmosphere. I would

25 Christina E. Jonsson also like to thank my roommates over the years, Lena 14C chronologies of Holocene lake sediments in the Rubensdotter, Hanna Sundqvist, Martina Hättestrand, Abisko area, northern Sweden – a comparison be- and Sofia Andersson. I would especially like to thank tween dated bulk sediment samples and macrofos- Hanna for all those interesting and encouraging talks sil samples. GFF 120, 59-67. about life in general and oxygen isotopes in particular – I Battarbee, R.W., Jones, V.J, Flower, R.J, Cameron, N.G, truly appreciate our discussions of warm summers and Bennion, H., Carvalho, L. and Juggins, S., 2001: cold winters! Diatoms. In: Tracking Environmental Change Us- ing Lake Sediments. Volume 3: Terrestrial, Algal, This project could not have been completed without and Siliceous Indicators. Smol, J.P, Birks, H.J.B, field assistance from Lena Rubensdotter, Henrik Törn- and Last, W.M (eds.). Kluwer Academic Publish- berg, Urban Pettersson, Ove Jonsson, Bert Jonsson, and ers, Dordrecht, The Netherlands. pp. 155-202. Magnus Linde. Berger, A. and Loutre, M.F. 1991: Insolation values for I am very grateful to Binnie Kristal-Andersson for the climate of the last 10 million years. Quaternary support and encouragement. I would also like to thank Science Reviews 10, 297-317. Karl-Erik Tronner for medical expertise. Bigler, C., Barnekow, L., Heinrichs, M.L. and Hall, R.I. Last but not least, I would like to thank my family. 2006: Holocene environmental history of Lake My parents, Ingegerd and Ove, deserve my special grati- Vuolep Njakajaure (Abisko National Park, north- tude for their love and support, baby-sitting in Abisko, ern Sweden) reconstructed using biological proxy indicators. Vegetation History and Archaeobotany and for always believing in me. I am also grateful to my 15, 309-320. parents-in-law, Christina and Ingemar, for their support Bigler, C., Grahn, E., Larocque, I., Jeziorski, A. and Hall, and for all those relaxing visits to Gotland. Katarina and R. 2003: Holocene environmental change at Lake Bert, thanks for always being there. Njulla (999 m a.s.l), northern Sweden: a compari- Magnus, you are the best. Your endless love, pa- son with four small nearby lakes along an altitudi- tience, and optimistic view of life helped me finish this nal gradient. Journal of Paleolimnology 29, 13-29. thesis. Moa and Ella, you made this journey longer but Bigler, C., Larocque, I., Peglar, S.M., Birks, H.J.B. so much more joyful. I love you! and Hall, R.I. 2002: Quantitative multiproxy assessment of long-term patterns of Holocene environmental change from a small lake near Abisko, northern Sweden. The Holocene 12, 481-496. References Björck, S. and Wohlfart, B. 2001: 14C chronostratigraphic techniques in paleolimnology. In: Tracking Envi- Alexandersson, H., Karlström, C. and Larsson-McCann, ronmental Change using Lake Sediments. Volume S. 1991: Temperature and precipitation in Sweden, 1: Basin Analysis, Coring and Chronological Tech- 1961-90. Reference normals. Swedish Meteoro- niques. W.M. Last and J.P. Smol (eds.), Kluwer logical and Hydrological Institute, Norrköping, Academic Publishers, Dordrecht, The Netherlands. Sweden, Meteorologi 81. pp. 205-245. Andersson, C., Risebrobakken, B., Jansen, E. and Dahl, Bjune, A.E. and Birks, H.J.B. 2008: Holocene vegetation S.O. 2003: Late Holocene surface conditions of the dynamics and inferred climate changes at Svanå- Norwegian Sea (Vøring Plateau). Paleoceanogra- vatnet, Mo I Rana, northern Norway. Boreas 37, phy 18, 1-13. 146-156. Antonsson, K., Chen, D., and Seppä, H. 2008: Anticy- Bjune, A.E., Seppä, H. and Birks, H.J.B. 2009: Quanti- clonic atmospheric circulation as an analogue for tative summer-temperature reconstructions for the the warm and dry mid-Holocene summer climate last 2000 years based on pollen-stratigraphical data in central Scandinavia. Climate of the Past 4, 215- from northern Fennoscandia. Journal of Paleolim- 224. nology 41, 43-56. Appleby, P.G. 2001: Chronostratigraphic techniques Bond, G., Kromer, B., Beer, J., Lotti, R., Muscheler, R., in recent sediments. In: Tracking Environmental Evans, M.N., Showers, W., Hoffmann, S., Lotti- Change using Lake Sediments. Volume 1: Basin Bond, R., Hahdas, I. and Bonani, G. 2001: Persist- Analysis, Coring and Chronological Techniques. ent solar influence on North Atlantic climate during W.M. Last and J.P. Smol (eds.), Kluwer Academic the Holocene. Science 294, 2130-2136. Publishers, Dordrecht, The Netherlands. pp. 171- Bond, G., Showers, W. J., Cheseby, M., Lotti, R., Alma- 203. si, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, Bakke, J., Lie, Ø., Dahl, S. O., Nesje, A., and Bjune, A.E. I., and Bonani, G. 1997: A pervasive millennial- 2008: Strength and spatial patterns of the Holocene scale cycle in North Atlantic Holocene and glacial wintertime westerlies in the NE Atlantic region. climates. Science 278, 1257-1266. Global Planetary Change 60, 28-41. Brandriss, M.E., O’Neil, J.R., Edlund, M.B. and Stoer- mer, E.F. 1998: Oxygen isotope fraction between Baldini, L.M., McDermott, F., Foleyl, A.M. and Bald- diatomaceous silica and water. Geochimica et Cos- ini, J.U.L. 2008: Spatial variability in the European 18 mochimica Acta 62, 1119-1125. winter precipitation δ O-NAO relationship: Im- Brewer, T.S., Leng, M.J., Mackay, A.W, Lamb, A.L, plications for reconstructing NAO.mode climate Tyler, J.J. and Marsh, N.G. 2008: Unravelling con- variability in the Holocene. Geophysical Research tamination signals in biogenic silica oxygen iso- Letters 35, L04709, doi:10.1029/2007GL032027.. tope composition: the role of major and trace ele- Barker, P.A., Street-Perrott, F.A., Leng, M.J., Green- ment. Journal of Quaternary Science 23, 321-330. wood, P.B., Swain, D.L., Perrott, R.A., Telford, R.J. Burgman, J. O., Calles, B., and Westman, F. 1987: Con- and Ficken, K.J. 2001: A 14,000-year oxygen iso- clusions from a ten year study of oxygen-18 in pre- tope record from diatom silica in two alpine lakes cipitation and runoff in Sweden. In Proceedings on on Mt. Kenya. Science 292, 2307-2310. an International Symposium on the use of Isotope Barnekow, L., Possnert, G. and Sandgren, P. 1998: AMS techniques in Water Resources Development, Iner-

26 Holocene climate and atmospheric circulation changes in northern Fennoscandia

national Atomic Energy Agency, Vienna, 579-590. 1996: Influence of changing atmospheric circula- Bütikofer, J. 2007: Millennial scale climate variability tion on precipitation δ18O - temperature relations during the last 6000 years - tracking down the Bond in Canada during the Holocene. Quaternary Re- cycles. Diploma thesis University of Bern (http:// search. 46, 211-218. www.giub.unibe.ch/klimet/docs/diplom_jbuetikof- Engstrom, D. R., Fritz, S. C., Almendinger, J. E., and er.pdf). Juggins, S. 2000: Chemical and biological trends Catalan, J., Ventura, M., Brancelj, A., Granados, H., during lake evolution in recently deglaciated ter- Thies, H., Nickus, U., Korhola, A., Lotter, A.F., Bar- rain. Nature 408, 161-166. bieri, A., Stuchlík, E., Lien, L., Bitušík, P., Bucha- Epstein, S., Buchsbaum, R., Lowenstam, H. A., and ca, T., Camarero, L., Goudsmit, G.H., Kopáćek, J., Urey, H. C. 1953: Revised carbonate-water isotopic Lemcke, G., Livingstone, D.M., Müller, B., Rautio, temperature scale. Bulletin of the Geological Soci- M., Šiško, M., Sorvari, S., Šporka, F., Strunecký, ety of America 64, 1315-1326. O. and Toro, M. 2002: Seasonal ecosystem vari- Forsström, L. 2006: Phytoplankton ecology of subarctic ability in remote mountain lakes: implications for lakes in Finnish Lapland. PhD-thesis, Department detecting climate signals in sediment records. Jour- of Biological and Environmental Sciences, Univer- nal of Paleolimnolgy 28, 25-46. sity of Helsinki. Chen, D. 2000: A monthly circulation climatology for Forsström, L., Sorvari, S., Korhola, A. and Rautio, M. Sweden and its application to a winter temperature 2005: Seasonality of phytoplankton in subarctic case study. International Journal of Climatology Lake Saanajärvi in NW Finnish Lapland. Polar Bi- 20, 1067-1076. ology 28, 846-861. Chen, D. and Hellström, C. 1999: The influence of the Fricke, H. C., and O’Neil, J. R. 1999: The correlation North Atlantic Oscillation on the regional tempera- between 18O/16O ratios of meteoric water and sur- ture variability in Sweden: spatial and temporal face temperature: its use in investigating terrestrial variations. Tellus 51A, 505-516. climate change over geologic time. Earth and plan- Clark, I.D. and Fritz, P. 1997: Environmental Isotopes in etary science letters 170, 181-196. Hydrogeology, Lewis Publishers, Boca Raton. 328 Gat, J. R. 1996: Oxygen and hydrogen isotopes in the hy- pp. drological cycle. Annual Review of and Planetary Clayton, R.N. and Mayeda, T.K. 1963: The use of bro- Sciences 24, 225-262. mine pentaflouride in the extraction of oxygen from Gibson, J. J., Birks, S. J., and Edwards, T. W. D. 2008: 2 18 oxides and silicates for isotopic analysis. Geochim- Global precipitation of δA and δ H-δ O evaporation ica et Cosmochimica Acta 27, 43-52. slopes for lakes and soil water accounting for sea- Craig, H., 1961: Isotopic variations in meteoric waters. sonality. Global Biogeochemical Cycle 22, 55-74. Science 133, 1702-1703. Gibson, J. J., Edwards, T. W. D., Birks, S. J., St.Amour, Craig, H. 1965: The measurement of oxygen isotope N. A., Buhay, W. M., McEachern, P., Wolfe, B. B., palaeotemperatures,. In: Stable isotopes in Oceano- and Peters, D. L. 2005: Progress in isotope tracer graphic Studies and Palaeotemperatures, Tongior- hydrology in Canada. Hydrological Processes 19, gi, E., Consiglio Nazionale delle Ricerche (eds.), 303-327. Pisa. pp. 161-182. Giddings, J.C. 1985: A system based on split-flow later- Dansgaard, W. 1964: Stable isotopes in precipitation. al-transport thin (SPLITT) separation cells for rap- Tellus 16, 436-468. id and continuous particle fractionation. Separation Darling, W.G. 2004: Hydrological factors in the pre- Science and Technology 20, 749-768. cipitation of stable isotopic proxy data present and Giesecke, T., Bjune, A. E., Chiverrell, R. C., Seppä, H., past: a European perspective. Quaternary Science Ojola, A.E.K., and Birks H. J. B. 2008: Exploring Reviews 23, 743-770. Holocene continentality changes in Fennoscandia Dawson, A.G., Elliott, L., Mayewski, P., Lockett, P., using present and past tree distributions. Quater- Noone, S., Hickey, K., Holt, T., Wadhams, P. and nary Science Reviews 27, 1296-1308. Foster, I. 2003: Late-Holocene North Atlantic cli- Grudd, H. 2008: Torneträsk tree-ring width and density mate ‘seesaws’, storminess changes and Greenland AD 500-2004. 2008: A test of climatic sensitivity ice sheet (GISP2) paleoclimates. The Holocene 13, and a new 1500-year reconstruction of north Fen- 381-392. noscandian summers. Climate Dynamics 31, 843- Dawson, A.G., Hickley, K., Mayewski, P.A. and Nes- 857. je, A. 2007: Greenland (GISP2) ice core and his- Grudd, H., Briffa, K.R., Karlén, W., Bartholin, T.S., torical indicators of complex North Atlantic cli- Jones, P.D. and Kromer, B. 2002: A 7400-years mate changes during the fourteenth century. The treee-ring chronology in northern Swedish Lap- Holocene 17, 427-434. land: Natural climatic variability expressed on an- Degens, T.E. and Epstein, S. 1962: Relation between nual to millennial time scales. The Holocene 12, 18O/16O ratios in coexisting carbonates, cherts and 657-665. diatomites. American Association of Petroleum Ge- Gunnarson, B., Borgmark, A. and Wastergård, S. 2003: ologists Bulletin 46. 534-542. Holocene humidity fluctuations in Sweden inferred Dodd, J.P., Sharp, Z.D., Fawcett, P.J., Schiff, C. and from dendrochronology and peat stratigraphy. Kaufman, D.S. 2007: A laser-extraction technique Boreas 32, 347-360. for oxygen isotope analysis of diatom frustules. Haimson, M. and Knauth, L.P. 1983: Stepwise fluorina- American Geophysical Union, B13A-0893. tion – a useful approach for the isotopic analysis Edwards, T. W. D., Wolfe, B. B., Gibson, J. J., and Ham- of hydrous minerals. Geochimica et Cosmochimica marlund, D. 2004: Use of water isotope tracers in Acta 47, 1589-1595. high latitude hydrology and paleohydrology, In: Hammarlund, D., Barnekow, L., Birks, H.J.B., Buchardt, Long-Term Environemntal Change in Arctic and B. and Edwards, W.D.E. 2002: Holocene changes Antarctic Lakes, Pienitz, R., Douglas, M. S. V. and in atmospheric circulation recorded in the oxygen- Smol, J. P. (eds.), Kluwer Academic Publishers, isotope stratigraphy of lacustrine carbonates from Dordrecht, The Netherlands. pp. 187-207. northern Sweden. The Holocene 12, 339-351. Edwards, T. W. D., Wolfe, B. B., and MacDonald, G.M. Hammarlund, D., Björck, S., Buchardt, B., Israelson,

27 Christina E. Jonsson

C., and Thomsen, C. 2003: Rapid hydrological Kirchhefer, A.J. 2001: Reconstruction of summer tem- changes during the Holocene revealed by stable peratures from tree-rings of Scots pine (Pinus isotope records of lacustrine carbonates from Lake sylvestris L.) in coastal northern Norway. The Igelsjön, southern Sweden. Quaternary Science Holocene 11, 41-52. Reviews 22, 353-370. Korhola, A., Weckström, J., Holmström, L. and Erästö, Holmlund, P., Karlén, W. and Grudd, H. 1996: Fifty P. 2000: A quantitative Holocene climatic record years of mass balance and glacier fron observations from diatoms in northern Fennoscandia. Quater- in the Tarfala research station. Geografiska Annaler nary Research 54, 284-294. 78A, 105-113. Kreutz, K.J., Mayewski, P.A., Mekker, L.D, Twickler, Hurrell, J.W. 1995: Decadal trends in the North Atlantic M.S., Whitlow, S.I. and Pittalwala, I.I. 1997: Bi- Oscillation: Regional temperatures and Precipita- polar changes in atmospheric circulation during the tion. Science 269, 676-679. Little Ice Age. Science 277, 1294-1296. Hurrell, W. J., Kushnir, Y., Ottersen, G. and Visbeck, M. Labeyrie, L. 1972: Composition isotopique de l’oxygène 2003: An Overview of the North Atlantic Oscilla- de la silice biogènique. CR Academy of Science, tion. Geophysical Monograph 134, 1-35. Paris 274, 1605-1608. IAEA/WMO, Global Network of Isotopes in Precipita- Labeyrie, L.D. 1974: New approach to surface seawater tion. The GNIP Database, (2007) Accessible at: palaeotemperatures using 18O/16O ratios in silica of http://isohis.iaea.org diatom frustules. Nature 248, 40-42. Ingraham, N. 1998: Isotopic variations in precipitation. Labeyrie, L.D. and Juillet, A. 1982: Oxygen isotopic ex- In: Isotope tracers in catchment Hydrology, Ken- changeability of diatom valve silica; interpretation dall, C. and McDonnell, J. J. (eds.), Elsevier Sci- and consequences for paleoclimatic studies. Geo- ence, Amsterdam, The Netherlands.pp. 87-118. chimica et Cosmochimica Acta 46, 967-975. IPCC, Intergovernmental Panel of Climate Change. Labeyrie, L.D., Juillet, A. and Duplessy, J.C. 1984: 2007:. Climate Change 2007 Fourth Assessment Oxygen isotope stratigraphy: fossil diatoms vs fo- Report (AR4). Valencia, Spain. raminifera. In: Proceedings of the 7th International Jacobeit, J., Jönsson, P., Gundestrup, N., Bärring, L., Symposium on Recent and Fossil Diatoms, Mann, Beck, C. and Ekström, M. 2001: Zonal indices for D.G. (ed.), Koeltz, Philadelphia, pp. 477-491. Europe 1780-1995 and running correlations with Larocque, I. and Hall, R.I. 2004: Holocene temperature temperature. Climate change 48, 219-241. estimates and chironomids community composi- Johansson, B., and Chen, D. 2003: The influence of tion in the Abisko Valley, northern Sweden. Qua- wind and topography on precipitation distribution ternary Science Reviews 23, 2453-2465. in Sweden: Statistical analysis and modelling. In- Lauritzen, S.-E. and Lundberg, J. 1999: Calibration of ternational Journal of Climatology 23, 1523-1535.. the speleothem delta function: an absolute tempera- Jones, P.D., Briffa, K.R., Osborn, T.J., Lough, J.M., van ture record from the Holocene in northern Norway. Ommen, T.D., Vinther, B.M., Luterbacher, J., Wahl, The Holocene 9, 659-669. E.R., Zwiers, F.W., Mann, M.E., Schmidt, G.A., Leng, M.J. and Barker, P.A. 2006: A review of the oxy- Ammann, C.M., Buckley, B.M., Cobb, K.M., Es- gen isotope composition of lacustrine diatom silica per, J., Goosse, H., Graham, N., Jansen, E., Kiefer, for palaleoclimate reconstruction. Earth-Science T., Kull, C., Küttel, M., Mosley-Thompson, E., Reviews 75, 5-27. Overpeck, J.T., Riedwyl, N., Schulz, M., Tudhope, Leng, M. J., and Marshall, J. D. 2004: Palaeoclimate in- A.W., Villalba, R., Wanner, H., Wolff, E. and Xo- terpretation of stable isotope data from lake sedi- plaki, E. 2009: High-resolution palaeoclimatology ment archives. Quaternary Science Reviews 23, of the last millennium: a review of current status 811-831. and future prospects. The Holocene 19, 3-49. Leng, M.J., Metcalfe, S.E. and Davies, S.J. 2005: Inves- Jones, V.J., Leng, M.J., Solovieva, N., Sloane, H.J. and tigate Late Holocene climate variability in central Tarasov, P. 2004: Holocene climate of the Kola Pe- Mexico using carbon isotope ratios in organic ma- ninsula; evidence from the oxygen isotope record terial and oxygen isotope ratios from diatom silica of diatom silica. Quaternary Science Reviews 23, within lacustrine sediments. Journal of Paleolim- 833-839 nology 34, 413-431 Juillet-Leclerc, A. 1986: Cleaning process fot diatoma- Leng, M.J., Roberts, N., Reed, J.M. and Sloane, H.J. ceous samples. In. Ricard, M. (Ed.), Proceedings 1999: Late Quaternary palaeohydrology of the of the 8th International Symposium on Recent and Konya Basin, Turkey, based on isotope studies of Fossil Diatoms. Koeltz, Philadelphia, pp. 733-736. modern hydrology and lacustrine carbonates. Jour- Juillet-Leclerc, A. and Labeyrie, L. 1987: Temperature nal of Paleolimnology 22, 187-204. dependence of the oxygen isotopic fraction be- Leng, M.J. and Sloane, P.A. 2008. Combined oxygen and tween diatom silica and water. Earth and Planetary silicon isotope analysis of biogenic silica. Journal Science Letters 84, 69-74. of Quaternary Science 235, 313-319. Jutman T. 1995: Runoff. In: Climate, lakes and rivers, Linge, H., Lauritzen, S-.E., Andersson, C., Hansen, J.K, National Atlas of Sweden, Raab B, Vedin H (eds). Skoglund, R.Ø. and Sundqvist H.S. 2009: Stable Stockholm. pp.106-111. isotope records for the last 10 000 years from Ok- Karlèn, W. 1988: Scandinavian glacial and climatic fluc- shola cave (Fauske, northern Norway), and region- tuations during the Holocene. Quaternary Science al comparision. Climate of the Past Discussion 5, Reviews 7, 199-209. 1763-1802. Karlén, W. and Kuylenstierna, J. 1996: On solar forcing Lotter, A.F. and Bigler, C. 2000: Do diatoms in the Swiss of Holocene climate: evidence from Scandinavia. Alps reflect the length of ice-cover? Aquatic Sci- The Holocene 6, 359-365. ences 62, 125-141. Kelts, K., and Hsü, K. J. 1978: Freshwater carbon- Lücke, A., Moschen, R. and Schleser, G.H. 2005: High ate sedimentation. Iin: Lakes, chemistry, geology, temperature carbon reduction of silica: a novel physics, Lerman, A. (ed.), Springer-Verlag, New approach for oxygen isotope analysis of biogenic York. pp. 295-323. opal. Geochimica et Cosmochimica Acta 69, 1423-

28 Holocene climate and atmospheric circulation changes in northern Fennoscandia

1433. Nesje, A., Jansen, E., Birks, H.J.B., Bjune, A., Bakke, Luterbacher, J., Xoplaki, E., Dietrich, D., Rickli, R., J., Dahl, C.A, Dahl, S.O., Kiltgaard-Kristensen, Jacobeit, J., Beck, C., Gyalistras, D., Schmutz, C. D., Lauritzen, S.E., Lie, Ø., Risebrobakken, B. and and Wanner, H. 2002: Reconstruction of sea level Svendsen, J.I. 2005: Holocene climate variability pressure fields over the Eastern North Atlantic and in the northern North Atlantic region: a review of Europe back to 1500. Climate Dynamics 18, 545- terrestrial and marine evidence. In: The Nordic 561. Seas: An integrated Perspective. Geophysical Mon- Marshall, J., Kushnir, Y., Battisti, D., Chang, P., Czaja, ograph Series, Drange, H., Dokken, T., Furevik, T., A., Dickson, R., Hurrell, J., McCartney, M., Sara- Rüdiger, G., Bergen, W. (eds). pp. 289-322. vanan, R. and Visbeck, M. 2001: North Atlantic Renberg, I., Bindler, R. and Brännvall, M.L. 2001: Using climate variability: Phenomena, impacts and mech- the historical atmospheric lead-deposition record as anisms. International Journal of Climatology 21, a chronological marker in sediment deposits in Eur- 1863-1898. ope. The Holocene 11, 511-516. Matheney, R.K. and Knauth, L.P. 1989: Oxygen-isotope Renberg, I. and Hansson, H. 2008: The HTH sediment fractionation between marine biogenic silica and corer. Journal of Paleolimnology 40, 655-659. seawater. Geochimica et Cosmochimica Acta 53, Rietti-Shati, M., Shemesh, A. and Karlén, W. 1998: A 3207-3214. 3000-year climatic record from biogenic silica ox- Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlén, ygen isotopes in an Equatorial High-altitude lake. W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Science 281, 980-982. Gasse, F., van Kreveld, S., Holmgren, K., Lee- Rings, A., Lucke, A. and Schleser, G.H. 2004: A new Thorp, J., Rosqvist, G., Rack, F., Staubwasser, M., method for the quatitative separation of diatom Schneider, R.R. and Steig, E. J. 2004: Holocene frustules from lake sediments. Limnology and Oce- climate variability. Quaternary Research 62, 243- anography: Methods 2, 25-34. 255. Risebrobakken, B., Jansen, E., Andersson, C., Mjelde, McCrea, J.M. 1950: On the isotopic chemistry of car- E. and Hevrøy, K. 2003: A high-resolution study bonates and a paleotemperature scale. Journal of of Holocene paleoclimatic and paleocenographic Chemical Physics 18, 849-857. changes in the Nordic Seas. Paleoceanography 18, Meeker, L.D. and Mayewski, P.A. 2002: A 1400-year 1-14. high-resolution record of atmospheric circulation Rosqvist, G.C, Rietti-Shati, M. and Shemesh, A. 1999: over the North Atlantic and Asia. The Holocene 12, Late glacial to niddle Holocene climatic record of 257-266. lacustrine biogenic silica oxygen isotopes from a Moberg, A., Alexandersson, H., Bergström, H. and Southern Ocean island. Geology 27, 967-970. Jones, P.D. 2003: Were southern Swedish summer Round, F.E., Crawford, R.M. and Mann, D.G. 1990: The temperatures before 1860 as warm as measured? Diatoms. Biology and Morphology of the Genera. International Journal of Climatology 23, 1495- Cambridge University Press, Cambridge. 747 pp. 1521. Rozanski, K., Araguás- Araguás, L. and Gonfiantini, Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, R. 1993: Isotopic patterns in modern global pre- N.M. and Karlén, W. 2005: Highly variable North- cipitation. Climate Change in Continental isotopic ern Hemisphere temperatures reconstructed from Records. American Geophysical Union, Geophysi- low- and high-resolution proxy data. Nature 433, cal Monograph 78, 1-36. 613-617. Rubensdotter, L. and Rosqvist, G. 2003: The effect of Morley, D.W., Leng, M.J., Mackay, A.W., Sloane, H.J., geomorphological setting on Holocene lake sedi- Rioual, P. and Battarbee, R.W. 2004: Cleaning of ment variability, northern Swedish Lapland. Jour- lake sediment samples for diatom oxygen isotope nal of Quaternary Science 18, 757-767. analysis. Journal of Paleolimnology 31, 391-401. Sachse, D., Radke, J. and Gleixner, G. 2004: Hydrogen Moros, M., Andrews, J. T., Eberl, D. D., and Jansen, E. isotope ratios of recent lacustrine sedimentary n- 2006: Holocene history of drift ice in the northern alknes record modern climate variability. Geochim- North Atlantic: Evidence for different spatial and ica et Cosmochimica Acta 68, 4877-4889. temporal modes. Paleoceanography 21, PA2017, Schiff, C. J. 2007: A modern survey and Holocene record doi:10.1029/2005PA001214. of lake water and diatom isotopes from south Alas- Moros, M., Emei, K., Risebrobakken, B., Snowball, I., ka, PhD-thesis, Northern Arizona University, Flag- Kuijpers, A., McManus, J. and Jansen, E. 2004: Sea staff, Arizona. surface temperatures and ice rafting in the Holocene Schiff, C.J., Kaufman, D.S., Wolfe, A.P., Dodd, J. and North Atlantic: climate influences on northern Eu- Sharp, Z. 2009: Late Holocene storm-trajectory rope and Greenland. Quaternary Science Reviews changes inferred from the oxygen isotope composi- 23, 2113-2126. tion of lake diatoms, south Alaska. Journal of Pale- Moschen, R., Lücke, A., Parplies, J., Radtke, U. and Sch- olimnology 41, 189-208. leser, G.H. 2006: Transfer and early diagenesis of Schmidt, M., Botz, R., Rickert, D., Bohrmann, G., Hall, biogenic silica oxygen isotope signals during set- S.R. and Mann, S. 2001: Oxygen isotopes of ma- tling and sedimentation of diatoms in a temperate rine diatoms and relations to opal-A maturation. freshwater lake (Lake Holzmaar, Germany). Geo- Geochimica and Cosmochimica Acta 65, 201-211. chimica et Cosmochimica Acta 70, 4367-4379. Schmidt, M., Botz, R., Stoffers, P., Anders, T. and Bo- Moschen, R., Lücke, A. and Schleser, G.H. 2005: Sensi- hrmann, G. 1997: Oxygen isotopes in marine dia- tivity of biogenic silica oxygen isotopes to changes toms: a comparative study of analytical techniques in surface water temperature and palaeoclimatol- and new results on the isotopic composition of re- ogy. Geophysical Research Letters 32, 1935-1938. cent marine diatoms. Geochimica and Cosmochim- Nesje, A., Bakke, J., Dahl, S.O., Lie, Ø. and Matthews, ica Acta 61, 2275-2280. J.A. 2008: Norweigan mountain glaciers in the Schmidt, G. A., LeGrande, A. N., and Hoffman, G. 2007: past, present and future. Global and Planetary Water isotope expressions of intrinsic and forced Change 60, 10-27. variability in a coupled ocean-atmosphere model. Journal of Geophysical Research, 112.

29 Christina E. Jonsson

Seppä, H. and Birks, H.J.B. 2001: July mean tempera- 23, 389-400. ture and annual precipitation trends during the Telford, R.J., Heegaard, E. and Birks H.J.B. 2004a: The Holocene in the Fennoscandia tree-line area: pol- intercept is a poor estimate of a calibrated age. The len-based climate reconstruction. The Holocene 11, Holocene 14, 296-298. 527-539. Telford, R.J., Heegaard, E. and Birks H.J.B. 2004b: All Seppä, H. and Birks, H.J.B. 2002: Holocene climate re- age-depth models are wrong: but how badly? Qua- constructions from the Fennoscandia tree-line area ternary Science Reviews 23, 1-5. based on pollen data from Toskaljavri. Quaternary Thorliefson, J.T. and Knauth, L.P. 1984: An improved Research 57, 191-199. stepwise fluorination procedure for the oxygen iso- Seppä, H., Bjune, A.E., Telford R.J., Birks H.J.B. and topic analysis of hydrous silica. Geological Society Veski S. 2009: ��Last nine-thousand years of temper- of America Abstracts Progress 16, 675. ature variability in Northern Europe. Climate of the Trouet, V., Esper, J., Graham, N.E., Baker, A., Scourse, Past Discussion 5, 1521-1552. J.D. and Frank D.C. 2009: Persistent positive North Shemesh, A., Burckle, L.H. and Hays J.D. 1995: Late Atlantic Oscillation mode dominated the Medieval Pleistocene oxygen isotope records of biogenic sil- Climate Anomaly. Science 324, 78-80. ica from the Atlantic sector of the Southern Ocean. Tyler, J.J., Leng, M.J. and Arrowsmith, W. 2007: Sea- Paleoceanography 10, 179-196. sonality and the isotope hydrology of Lochnagar, a Shemesh, A., Charles, C.D. and Fairbanks, R.G. 1992: Scottish mountain lake: implications for palaeocli- Oxygen isotopes in biogenic silica: Global changes mate research. The Holocene 17, 717-727. in Ocean temperature and isotopic composition. Tyler, J.J., Leng, M.J., Sloane, H.J., Sachse, D. and Science 256, 1434-1436. Gleixner, G. 2008: Oxygen isotope ratios of sedi- Shemesh, A., Macko, S.A., Charles, C.D. and Rau G.H. mentary biogenic silica reflect the European trans- 1993: Isotopic evidence for reduced productivity in continental climate gradient. Journal of Quater- the Glacial Southern Ocean. Science 262, 407-410. nary Science 23, 341-350. Shemesh, A., Rosqvist, G., Rietti-Shati, M., Rubensdot- Urey, H. C., Epstein, S., McKinney, C. and McCrea, J. ter, L., Bigler, C., Yam, R. and Karlén W. 2001: 1948: Method for measurement of paleotempera- Holocene climatic change in Swedish Lapland in- tures (abstract), Bulletin of the Geological Society ferred from an oxygen-isotope record of lacustrine of America, 59, 1359-1360. biogenic silica. The Holocene 11, 447-454. Uvo, C.B. 2003: Analysis and regionalization of north- Smith, R.B. 1979: The influence of mountains on the ern European winter precipitation based on its re- atmosphere. Advances in Geophysics, 21, 87-229. lationship with the North Atlantic Oscillation. In- Sodemann, H., Masson-Delmotte, V., Schwierz, C., ternational Journal of Climatology 23, 1185-1194. Vinther, B. M., and Wernli, H. 2008: Interan- Wanner, H., Beer, J., Bütikofer, J., Crowley, T.J., Cu- nual variability of Greenland winter precipitation basch, U., Flückiger, J., Goosse, H., Grosjean, M., sources.: 2. Effects of North Atlantic Oscilla- Joos, F., Kaplan, J.O., Küttel, M., Müller, S.A., tion variability on stable isotopes in precipitation. Prentice, I.C., Solomina, O., Stocker, T.F., Tarasov, Journal of Geophysical Research 113, D1211, P., Wagner, M. and Widmann, M. 2008: Mid- to doi:10.1029/2007JD009416. Late Holocene climate change: an overview. Qua- Sorvari, S., Korhola, A. and Thompson, R. 2002: Lake ternary Science Reviews 27, 1791-1828. diatom response to recent Arctic warming in Finn- Weckström, J., Korhola, A., Erästö P. and Holmström, L. ish Lapland. Global Change Biology 8, 153-163. 2006: Temperature patterns over the past eight cen- St.Amour, N.A. 2009: A multi-proxy study of Holocene turies in northern Fennoscandia inferred from sedi- atmospheric circulation dynamics recorded in lake mentary diatoms. Quaternary Research 66, 78-86. sediments in Fennoscandia. PhD Thesis, Waterloo, Ontario, University of Waterloo, 253 pp. Sturm, K., Hoffman, G., Langmann, B., and Sticher, W. 2005: Simulation of δ18O in precipitation by the re-

gional circulation model REMOISO. Hydrological Processes 19, 3425-3444. Sturm, C., Zhang, Q. and Noone D. 2009: An introduction to stable water isotopes in climate models: benefits of forward proxy modelling for paleoclimatology Climate of the Past Discussion 5, 1697-1729. Stuvier, M. and Braziunas, T.F. 1993: Modeling atmospheric 14C influnces and 14C ages marine samples back to 10.000 BC. Radiocarbon 35, 137-189. Sundqvist, H.S., Holmgren, K., Moberg, A., Spötl, C. and Mangini, A. 2009: Stable isotopes in a stalag- mite from NW Sweden, document changes in the environment as well as in the proxy-environment relationship over the last 4000 years. Boreas, 10.1111/j.1502-3885. 2009.00099.x. ISSN 0300- 9483. Swann, G.E.A., Leng, M.J., Sloane H.J., Maslin M.A. and Onodera J. 2007: Diatom oxygen isotopes: evidence of a species effect in the sediment record Geochemistry, geophysics, Geosystems 8, Q06012. Swann, G.E.A., Leng, M.J., Sloane H.J. and Maslin M.A. 2008: Isotope offsets in marine diatom δ18O over the last 200 ka. Journal of Quaternary Science