Springer 2006
Journal of Paleolimnology (2006) 35: 99–109 DOI 10.1007/s10933-005-7871-2
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Lake sediment core records of sulphur accumulation and sulphur isotopic composition in central Ontario, Canada lakes M. Catherine Eimers1,*, Andrew M. Paterson2, Peter J. Dillon1, Sherry L. Schiff3, Brian F. Cumming4 and Roland I. Hall5 1
Environmental and Resource Science Department, Trent University, Peterborough, ON K9J 7B8, Canada; Ontario Ministry of Environment, Dorset Environmental Science Center, ON P0A 1E0, Canada; 3Earth Sciences Department, University of Waterloo, Waterloo, ON N2L 3G1, Canada; 4Biology Department, Queen’s University, Kingston, ON K7L 3N6, Canada; 5Biology Department, University of Waterloo, Waterloo, ON N2L 3G1, Canada; *Author for correspondence (e-mail:
[email protected])
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Received 6 January 2005; accepted in revised form 15 May 2005
Key words: Acid deposition, Redox reactions, S-isotopes, Sediment cores
Abstract Stable isotopic compositions and concentrations of total sedimentary sulphur (S) were determined in cores from 6 lakes in the acid-sensitive Muskoka-Haliburton region of south-central Ontario. The isotopic composition of S in deep sediment (>20 cm) was approximately constant in all lakes, and indicated a preindustrial d34S value between +4.0 and +5.3&, which is similar to current bulk deposition. Similarly, total S concentrations in deep sediment were relatively low (1.9–5 mg S g 1 dwt) and approximately constant with depth within cores. All lakes exhibited up-core increases in total S and decreases in d34S at a depth corresponding to the beginning of industrialization in the Great Lakes region (1900), resulting in a generally reciprocal depth pattern between total S concentration and d34S ratios. While initial shifts in total S and d34S were likely due to enhanced SO4 reduction of newly available anthropogenic SO4, both the magnitude and pattern of up-core S enrichment and shifts in d34S varied greatly among lakes, and did not match changes in S deposition post 1900. Differences between lakes in total S and d34S were not related to any single hydrologic (e.g., residence time) or physical (e.g., catchment-area-to-lake area ratio) lake characteristic. This work indicates that sediment cores do not provide consistent records of changes in postindustrial S deposition in this region, likely due to redox-related mobility of S in upper sediment.
Introduction Lake sediment profiles have been used to infer historical changes in perturbations such as acid deposition, eutrophication, forest fires, harvesting and extreme climatic events (e.g., Murray and Gottgens 1997). Specifically, S concentration and isotope profiles in lake sediment cores have been used to reconstruct historical changes in the magnitude of S inputs to lakes and to evaluate the
effects of pollutant S deposition on S cycling and alkalinity generation (Nriagu and Coker 1983; Fry 1990; Giblin et al. 1990). The analysis of S isotopes provides additional insight into the processes that control S retention in lake sediment. When sulphate is not limiting, microbes preferentially reduce the lighter 32S, producing sulphides that are depleted in 34S relative to the initial sulphate. Dissimilatory sulphate reduction (DSR) is often limited by low SO4
100 availability in soft-water lakes; however, rates of DSR are stimulated by increased sulphate availability, such that changes in atmospheric deposition can alter both S-diagenesis and the isotope signature of sedimentary sulphur (Nriagu and Soon 1985; Fry 1990). The isotopic composition of S in sediment is also influenced by the availability of labile carbon (C), and temperature, both of which affect the rate of SO4 reduction and consequently the extent of isotope fractionation. Finally, SO4 reduction can only occur under conditions of anoxia, and lakes with well-oxygenated hypolimnia often exhibit different depth patterns of d34S in sediment compared to lakes that are regularly anoxic at depth (Fry et al. 1995). Oxygen availability also determines the net storage of reduced S compounds in sediment. However, there is debate over whether S concentration profiles in sediment accurately reflect historical changes in atmospheric deposition, because sediment S profiles can vary even within a single lake (e.g., Baker et al. 1992). In addition, the zone of SO4 reduction (up to 10 cm; Hormer and Storkholm 2001) can correspond to a relatively broad time interval of sediment deposition (i.e., decades), particularly in lakes with slow rates of sediment accumulation. Therefore, diffusion of SO4 into sediment and migration of reduced sulphides may obscure the sedimentary record of S inputs to lakes (Holdren et al. 1984; Carignan and Tessier 1985; Novak et al. 1994). The goal of this study was to determine whether the S record contained in sediment cores was reflective of past changes in S deposition in 6 lakes located within a 50-km radius, in the acid-sensitive region of southcentral Ontario, Canada. The 6 lakes have identical histories of acid deposition and similar geology, and therefore any differences in S accumulation among lakes must be due to local factors (e.g., hypolimnetic oxygen availability). In addition, sediment d34S was analyzed in order to gain insight into the processes governing S retention in sediment.
Methods Study sites The 6 study lakes are located in the District of Muskoka (Harp, Chub, Blue Chalk, Red Chalk
and Dickie Lakes) or Haliburton County (Plastic Lake) in south-central Ontario, Canada on a southern extension of the Precambrian Shield (Figure 1). The surficial geology of this region is characterized by thin, discontinuous till deposits, overlying granite and gneiss bedrock. Soils are mainly acidic, weakly developed sandy (90% sand) podzols, and range in depth from >1 m at Harp and Blue Chalk Lakes to
