In W. C. Mahaney (ed.), Correlation of. Quaternary Chronologies. .... Toolin, L. J. & C. J. Eastoe, 1993. Late Pleistocene-recent atmospher- ic 13C record in C4 ...
Journal of Paleolimnology 18: 219–233, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.
219
Multi-component carbon isotope evidence of early Holocene environmental change and carbon-flow pathways from a hard-water lake in northern Sweden Dan Hammarlund1;2, Ramon Aravena1 , Lena Barnekow2 , Bjørn Buchardt3 & G¨oran Possnert4 1
Department of Earth Sciences, Quaternary Sciences Institute and Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 2 Department of Quaternary Geology, Lund University, Tornav¨agen 13, S-223 63 Lund, Sweden 3 Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark 4 Tandem Laboratory, Uppsala University, Box 533, S-751 21 Uppsala, Sweden Received 29 August 1996; accepted 13 January 1997
Key words: northern Sweden, stable carbon isotopes, carbon isotope fractionation, limnic sediments, Holocene, lake Abstract A 9000-year carbonate-rich sediment sequence from a small hard-water lake in northernmost Sweden was studied by means of multi-component stable carbon isotope analysis. Radiocarbon dating of different sediment fractions provides chronologic control and reveals a rather constant hard-water effect through time, suggesting that the lake has remained hydrologically open throughout the Holocene. Successive depletion of 13 C in fine-grained calcite and carbonate shells during the early Holocene correlate with a change in catchment vegetation from pioneer herb communities to boreal forest. The vegetational change and associated soil development likely gave rise to an increased supply of 13 C-depleted carbon dioxide in groundwater recharging the lake. This process is therefore believed to be the main cause of decreasing values of 13 C in dissolved inorganic carbon of the lake and thereby in limnic carbonates. Strongly 13 C-depleted sedimentary organic matter may be related to enhanced kinetic fractionation during photosynthetic assimilation by means of proton pumping in Characean algae. This interpretation is supported by a substantial offset between 13 C of DIC as recorded by mollusc shells and 13 C of fine-grained calcite. Introduction Lake sediments may contain a variety of organic and inorganic components from which valuable palaeoclimatic and palaeoenvironmental information can be obtained. Stable isotope analysis of biogenic and inorganic lacustrine carbonates is a frequently used method for reconstructions of climate-related changes in limnic environments (Buchardt & Fritz, 1980; Talbot & Kelts, 1990). However, whereas the processes responsible for oxygen isotope fractionation within the hydrologic cycle and during precipitation of carbonates are reasonably well known, the relation between carbon isotope composition of dissolved inorganic carbon (DIC), limnic carbonates and organic material are still not suf-
*133577
ficiently understood. Although lacustrine carbon isotope records may represent the combined effect of several processes and respond to widely different environmental conditions, such as bedrock geology, vegetational composition in the lake and its catchment, productivity, soil conditions and hydrology, the measured values are often directly related to 13 C of DIC (Kelts & Hs¨u, 1978). The most important processes influencing 13 C of DIC can be discerned from examination of carbon isotope data obtained on different components of lake sediments. Such composite records may thus serve as useful proxies for palaeolimnology and vegetation history. Without being directly related to temperature, carbon isotope records often provide information on biological processes and hydrologic conditions that
Article: jopl 415 Pips nr 133577 BIO2KAP jopl415.tex; 5/09/1997; 7:58; v.7; p.1
220 may facilitate the interpretation of oxygen isotope data and results of other geochemical as well as palaeoecological analyses (e.g. Schwalb et al., 1995; Wolfe et al., 1996). Here we present carbon isotope records obtained on organic material and various limnic carbonates, supplemented by elemental carbon data, in a complete postglacial lake sediment sequence from a small hard-water lake close to Abisko in northernmost Sweden. Corresponding oxygen isotope records will be published elsewhere. The study is part of a multi-disciplinary project aimed at reconstructions of Holocene environmental changes and treeline fluctuations in the area (Berglund et al., 1996). Radiocarbon dating was applied to different components of the sediments in order to establish a chronology, to estimate the hardwater effect, and to assist with the interpretation of stable isotope data. Stable carbon isotopes can sometimes be used for validation of radiocarbon dates as demonstrated by Aravena et al. (1992a). Similarly, radiocarbon dating may provide information that facilitates the interpretation of 13 C records. The study contributes to mounting evidence for marked shifts in the carbon isotope composition of DIC in response to vegetational development and soil maturation subsequent to deglaciation (Hammarlund, 1993; Hammarlund & Keen, 1994; Hammarlund & Lemdahl, 1994; NoeNygaard, 1995). The results also bear implications for the understanding of carbon isotope fractionation during aquatic photosynthesis in hard-water environments dominated by submersed aquatic plants.
Site description Lake Tibetanus (Ekman, 1957) is located in the northern part of the Scandes Mountains in northernmost Sweden (68 200 N, 18 420 E; Figure 1). It is situated in the upper mountain-birch region at c. 560 m a.s.l. on the south-facing slope of Mount Sl˚attatj˚akka (1191 m) in Abisko National Park. The hydrologically open basin is 85–100 m across with a maximum depth of c. 4 m. Apart from direct precipitation and limited surface run-off, water input to the lake is confined to groundwater springs at the northern edge, and the outflow consists of shallow streams through a small mire at the southern side of the lake. Based on rough estimates of volume and flow velocities, a residence time of less than two months can be assumed. Along the slope above the lake is an outcrop of calcite marble (Kulling, 1964) that constitutes a substantial supply of
dissolved inorganic carbon to groundwater and surface run-off. Thus, the lake water exhibits relatively high values of electrical conductance and pH (622 S at 0 C and 7.7 respectively as sampled under the ice in mid March 1996). The adjacent Abisko valley has a dry climate with a mean annual precipitation at Abisko of c. 300 mm. In contrast, the mountains 10–30 km to the west are influenced by oceanic air-masses and receive 600–1000 mm per year. Mean annual temperature at Abisko is 0.8 C, and the mean monthly values of temperature and precipitation respectively are 11.9 C and 25 mm for January and + 11.0 C and 54 mm for July (1961–1990; Alexandersson et al., 1991). The ratio of snow relative to total precipitation is c. 50%. Small lakes in the area such as Lake Tibetanus are generally covered by ice from mid October to late May.
Methods Fieldwork and subsampling Multiple cores were sampled from the deepest, western part of the ice-covered lake in March 1995 at a water depth of 3.9 m, using 1 m-long Russian peat samplers, 10 and 6.5 cm in diameter. The sediment sequence was described in detail in the field. After correlation in the laboratory based on clearly visible sedimentological changes (Figure 2) the cores were subsampled into 61 main sections, 20–68 mm thick, taking into account lithostratigraphic boundaries. Minor aliquots were used for carbon analysis and stable isotope analysis. For radiocarbon dating the remaining part of the main sections were further divided into two or three subsamples, 17–26 mm thick. Carbon content and carbonate mineralogy The carbon content of the sediments was determined by temperature-controlled combustion in pure oxygen with subsequent detection of carbon dioxide by infrared absorption photometry in a Leco RC 412 Multiphase Carbon Determinator. The results are expressed as elemental organic carbon and carbonate carbon content in percentages of total dry weight. Theoretically, pure calcium carbonate would yield a carbon content of 12%, whereas pure organic material generally has a carbon content of c. 40%. The mineralogic composition of carbonates from selected sediment samples was
jopl415.tex; 5/09/1997; 7:58; v.7; p.2
221
Figure 1. Map of the study area.
Figure 2. Photograph of the studied sediment sequence showing the correlation of individual cores. Note the transition from almost pure lake marl to laminated calcareous gyttja with abundant terrestrial macrofossils at c. 2.0 m (see Table 2). The scale refers to depth in m below the sediment surface.
jopl415.tex; 5/09/1997; 7:58; v.7; p.3
222 Table 1. Radiocarbon dates Sample depth (m)
Lab. and No.
Material analysed
0.25–0.28 0.58–0.60
Ua–10 117 Ua–10 118
0.88–0.90
Ua–10 119
1.07–1.10 1.14–1.16 1.44–1.46 1.78–1.80 1.99–2.01 2.36–2.42
Ua–10 399 Ua–10 120 Ua–10 121 Ua–10 122 Ua–10 123 Ua–10 124
2.63–2.67
Ua–10 400
2.78–2.80
Ua–10 125
0.25–0.28 0.58–0.60 0.88–0.90 1.14–1.16 1.78–1.80 1.99–2.01 2.90–2.95 2.12–2.16 2.38–2.42 2.12–2.16 2.38–2.42 2.67–2.71 2.90–2.95
Ua–10 111 Ua–10 112 Ua–10 113 Ua–10 114 Ua–10 115 Ua–10 116 Ua–10 937 Ua–10 935 Ua–10 936 Ua–10 907 Ua–10 908 Ua–10 909 Ua–10 910
Leaf of Salix Leaves and twigs of Betula and Salix Leaves and twigs of Betula and Salix Leaves of Betula Twigs of Betula and Salix Needles of Pinus Needles of Pinus Needles of Pinus Needle and needle scales of Pinus, fruits and catkin scales of Betula Fruits and catkin scales of Betula, leaves of Salix Leaves of Dryas, fruits and catkin scales of Betula SOL fraction of bulk organics SOL fraction of bulk organics SOL fraction of bulk organics SOL fraction of bulk organics SOL fraction of bulk organics SOL fraction of bulk organics SOL fraction of bulk organics INS fraction of bulk organics INS fraction of bulk organics
