Lake Sediments

Lakes accumulate sediments from their surrounding environment and so sediment cores recovered from lakes can provide a record of environmental change. Accumulation rates in lakes are often high, so lake sediments offer the potential for highresolution records of past climate, providing they can be adequately dated. Lake sediments are made up of two basic components: allochthonous material, originating from outside the lake basin; and autochthonous material, produced within the lake itself. Allochthonous material is transported to lakes by rivers and streams, overland flow, aeolian activity, and (in some cases) subsurface drainage. It is made up of varying amounts of fluvial or aeolian clastic sediments, dissolved salts, terrestrial macrofossils, and pollen. Autochthonous material is either biogenic in origin or it may result from inorganic precipitation within the water column (often as a consequence of seasonally varying biological productivity that can significantly alter the water chemistry). Both allochthonous and autochthonous material can be useful in paleoclimatic reconstruction.

Pollen is the most widely studied component of lake sediments and is discussed at length in Chapter 9. Plant macrofossils can be especially helpful in corroborating vegetation reconstructions based on pollen (Hannon and Gaillard, 1997; Jackson et al., 1997). Insect parts, both terrestrial and aquatic, are often found in lake sediments and these may provide additional quantitative paleoclimatic data (see Section 8.3). The character of the inorganic (clastic) sediments transported to lakes can also provide useful paleoenvironmental information, based on sediment geochemistry, grain size variations, magnetic properties etc., though interpretation of such data can be complicated due to post-depositional diagenetic changes in the sediments. In some cases, the thickness of annually laminated sediments (varves) may be of cli matic significance, reflecting climatic controls on sediment flux into a lake; this is discussed in what follows.

Biological productivity in lakes is, in part, climatically dependent and so the remains of organisms that lived in the water column can be of paleoclimatic significance. For example, different species of ostracods (small crustaceans, typically ~1 mm in size) are characteristic of specific salinity conditions and so ostracods in lake sediments can be useful indicators of paleosalinity, which may reflect the overall water balance of a lake. Furthermore, geochemical changes in the carbonate shells of ostracods reflect variations in lake water chemistry (De Decker and Forester, 1988; Chivas et al., 1993). Thus, a change from freshwater to brackish (athalassic) conditions, resulting from a shift in the precipitation-evaporation balance, would be reflected in the chemistry of the ostracod shells and/or in the species composition (Holmes, 1996; Xia et al., 1997).

Changes in diatoms (unicellular algae of the class Bacillariophyceae) may also reflect water chemistry (Moser et al., 1996). Certain diatoms favor particular salinity ranges so a down-core shift in species composition could reflect a change in water-balance, particularly in arid and semiarid environments (Fritz et al., 1991, 1993; Gasse et al., 1997). In arctic and subarctic freshwater lakes of northwest Canada, modern diatom assemblages have been related to lake water temperature in summer (or to temperature-dependent variables), enabling paleotemperatures to be estimated from diatoms in lake sediments (Pienitz et al., 1995). Oxygen isotopes in the silica of diatoms (biogenic opal) has also been used to derive paleotempera-ture estimates (Shemesh and Peteet, 1998).

Inorganic precipitation within carbonate-rich lakes can provide an isotopic record that may reflect the varying composition of meteoric waters entering the drainage basin over time (Eicher and Siegenthaler, 1976; McKenzie and Hollander, 1993). For example, in Lake Gerzensee, Switzerland, changes in S180 of lake carbonates appear to vary in parallel with isotopic changes seen in the GRIP ice core during late glacial time, suggesting that there were widespread changes in the isotopic composition of precipitation at that time (Eicher, 1980; Oeschger et al., 1984).

In many parts of the world, the annual climatic cycle is the strongest part of the overall spectrum of climate variability. This is commonly reflected in the seasonal deposition of sediments in lakes. However, this cyclicity is only rarely identifiable in lacustrine sediments because a variety of processes within lakes act to mix or disturb the seasonally varying flux of material to the lake floor. In particular, benthic organisms mix the sediments and this may prevent the identification of annually deposited sequences. Where the input of sediment is large enough to overwhelm any benthic disturbance and/or where anoxic conditions exist at depth in a lake (thereby eliminating benthic organisms), the seasonal cycle of sediment deposition may be preserved; the resulting annual layers are called varves26 (O'Sullivan, 1983; Saarnisto, 1986). Varves

26 In the marine environment, varved sediments are much rarer, but outstanding examples have been found in areas of high biological productivity (upwelling regions) and where deep anoxic basins allow sediment preservation (e.g., the Cariaco Basin off the coast of Venezuela [Hughen et al., 1996b] and the Santa Barbara Basin off southern California [Sancetta, 1995; Pike and Kemp, 1996]).

are most common in cold-temperate environments, particularly in deep anoxic lakes that are frozen over for part of the year and where sediment input is strongly seasonal (Zolitschka, 1996a). In some lakes, varved sediments have accumulated throughout the Holocene and provide a chronological yardstick that can be used in calibrating the radiocarbon timescale (Anderson et al., 1993; Zolitschka, 1991) (see Section Varve thickness variations can provide useful paleoclimatic information, if carefully calibrated against instrumentally recorded climatic data (Zolitschka, 1996b; Overpeck, 1996). For example, detailed studies of Lake C2 in the Canadian High Arctic showed that summer temperature above the local surface inversion controlled sediment flux to the lake; this was confirmed by using long-term climatic data to "predict" how varve thickness would have changed over the previous 40 yr, producing a good fit with the recorded varve thickness variations (Hardy, 1996; Hardy et al., 1996). Studies of varved sediments in the Swiss Alps have also demonstrated the importance of summer temperatures in controlling sedimentation in that region, enabling a late Holocene paleotemperature record to be reconstructed (Leeman and Niessen, 1994a, 1994b). However, in glacierized basins, changes in the extent of glaciers can alter sediment flux downstream, making the interpretation of long-term varve thickness variations more complex (Leonard, 1997).

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