Methods reflecting glacier activity

The physical processes in glacial sedimentary environments are often complex (e.g. Ashley et al., 1985). However, a number of methods and techniques have been used to record glacier activity based on proglacial lacustrine and terrestrial sites. The methods are based on a conceptual model of glacier-meltwater induced sedimentation in which the minerogenic (nonorganic) component of the sediments is related to the occurrence of a glacier in the catchment (e.g. Karlén, 1981; Leonard, 1985). The organic component depends on many factors, including bedrock lithology and vegetation cover, local climate (temperature, precipitation and wind), size and aspect of the catchment and the lake, water depth and temperature, coverage of superficial sediments in the catchment, colluvial activity related to slope angles around the lake, as well as anthropogenic impact. The minerogenic component in proglacial lakes depends in addition on factors such as transport distance and the number of intervening lakes acting as sediment traps (Smith, 1978). In most lakes the organic component is much smaller than the minerogenic component, but the relative importance of these components is to a large extent site- and/or area dependent. Hence, Matthews and Karlén (1992) argued for the use of representative nonglacial lakes as 'control lakes' in connection with the use of proglacial lakes in the study of glacier fluctuations. The contrast between these two lake types then clarifies the glacial signal. Due to the large colour variations (light-/bluish grey to dark brown) with and without a glacier in the catchment, visual description of the layers in a core or section using a Munsell colour chart is a useful first approach. This should be supported or supplemented by various field and laboratory methods and techniques:

* Mineral magnetic susceptibility commonly reflects the concentration of magnetic minerals (e.g. Thompson and Oldfield, 1986), and may be used as an indicator of erosion and transport of clastic sediments which can be linked to glacier activity (e.g. Snowball, 1993; Leemann and Niessen, 1994; Snowball and Sandgren, 1996).

* X-ray diffraction analyses with low-density values may indicate high sediment density and hence increasing glacier activity (e.g. Karlén and Matthews, 1992; Matthews and Karlén, 1992; Leemann and Niessen, 1994; Souch, 1994).
* Weight loss-on-ignition (LOI) estimates the organic content of lacustrine sediments (Dean, 1974; see Heiri et al., 2001, for laboratory procedures), and can together with derived parameters (minerogenic material, dry weight, water content, etc.) be linked to glacier variations (e.g. Karlén, 1976; Leonard, 1986a, b; Nesje et al., 1994, 2000, 2001; Souch, 1994; Snowball and Sandgren, 1996; Menounos, 1997; Matthews et al., 2000).

* Grain-size variations of especially clay and silt may be linked to glacier fluctuations (e.g. Leemann and Niessen, 1994; Matthews et al., 2000; Nesje et al., 2001; see Blott and Pye, 2001, for the statistical treatment of grain size distribution). * Occurrence and thickness of clastic varves reflect variations in glacier activity (e.g. Leemann and Niessen, 1994; Leonard, 1997).

* Bulk density is the ratio of the mass of dry (wet) solids to the bulk volume of the sediment, and may be used to record former glacier variations/ELAs (e.g. Leonard, 1997; Menounos, 1997).

* Lithological changes in the recorded sequence or core may reflect the occurrence of glaciers in the catchment and/or variations in glacier extent (e.g. Svendsen and Mangerud, 1997; Matthews et al., 2000).

The principal coupling to former glacier activity/ ELAs is common for all these methods and techniques. In Fig. 2 both possibilities and limitations are illustrate din the schematic coupling between weight LOI and former glacier magnitude/ELA, and the glacier events reflect what is typically recorded in a core from a Norwegian proglacial lake with a temperate glacier in the catchment. The proglacial lake is located so far downstream for the glacier that (visible) turbid meltwater only enters the site during the maximum late spring/early summer flood at present. This can also easily be recorded by the bare eye in the top sediments of the core. In periods when the glacier is much smaller or nearly melted away, no visible turbid glacier meltwater enters the proglacial lake and the relative accumulation of organic sediments in the lake is much higher. The occurrence of an active glacier in the catchment cannot be recorded in the sediments by the bare eye, and only various laboratory techniques can detect an input of glacier-induced sediments to the lake. In periods when the glacier is much bigger than at present (in this example, during the early deglaciation), the influence of glacier-induced sedimentation in the proglacial Lake may be so high that variations in glacier magnitude are difficult to detect or separate out by the available laboratory methods. The main problems concerning the interpretation and calibration of these parameters are thus related to when the glacier was nearly melted away, or when it was so large that the used proxy lacked sensitivity to record variations in GS. However, the various methods may complement each other. Both as a link to other palaeoclimatic proxies and as a tool to investigate glacier activity, age control of events are essential. In studies of glacier fluctuations prior to the Little Ice Age maximum several methods have been used to obtain age-depth control:

* By measuring the magnetic declination versus sediment depth, magnetic declination records can be compared with the well-dated master curve of Lake Windermere (Creer and Tucholka, 1983). Assuming synchronous oscillations in the two magnetic declination records this can be used as a dating method (see Snowball, 1993; Leemann and Niessen, 1994, for details).

* The thickness and grain-size distribution in annual clastic glacial varves, if continuous, may represent an annually resolved record of glacier activity. However, the rhythmites in a sequence must be confirmed as real varves in the sense of De Geer (1912) before they can be used for age-depth control (e.g. Leemann and Niessen, 1994; Leonard, 1997).

* Radiocarbon dating tends to be the most important dating method for reconstructing former glacier activity.

Recent comparisons between dated bulk sediment and macrofossil samples from various lakes often show marked discrepancies and radiocarbon chronologies from lake sediments are often based on AMS-radiocarbon dated terrestrial plant macrofossils (e.g. Barnekow et al., 1998). However, on certain sites and under certain conditions, AMS dates on terrestrial plant macrofossils are not superior to bulk sediment samples (e.g. Gulliksen et al., 1998), and the most reliable chronologies may be obtained not from terrestrial plant macrofossils, but from that part of the sediment fraction (the 'humic' NaOH-soluble component), where there is no contamination by
older carbon residues (Lowe and Walker, 2000). Age estimates should be given as calibrated years before present (BP) in accordance with INTCAL98 for radiocarbon dates (Stuiver et al., 1998). As sedimentation rates vary with and without a glacier in the catchment of proglacial lakes, both the initiation and the termination of glacier episodes should be dated. Hence, age-depth control in proglacial lakes relies on linear interpolation within periods with and without a glacier in the catchment.

 

Summary and conclusions

Except for historical records and observed massbalance records, knowledge of former variations in glacier activity/ELAs rely, directly or indirectly, on the maximum altitude of lateral moraines and on information from proglacial lacustrine and terrestrial sites. As lateral moraines only reflect shorter periods when the glaciers obtained steady state in advanced positions beyond later glacier advances, continuous Holocene variations in glacier activity/ELAs can only be obtained from proglacial sites beyond the Little Ice Age maximum. In this paper, various approaches and techniques for reconstructing variations in former glacier activity/ ELAs based on proglacial sites are evaluated, and criteria for site selection are discussed. The following conclusions and implications of systematic importance are proposed:
(1) Records of glacier activity/ELAs obtained from proglacial sites are based on a conceptual model of glacier-meltwater-induced sedimentation in which the minerogenic (nonorganic) component of the sediments is related to the occurrence of a glacier in the catchment (Fig. 1) (e.g. Karlén, 1981; Leonard, 1985).
(2) The principal coupling between various approaches and former glacier activity/ELAs is the same, and both possibilities and limitations are exemplified in Fig. 2. Problems in the interpretation and calibration of these parameters are primarily related to when the glacier was very small/melted away, or when it was so large that the used proxy lacked sensitivity to record variations in GS. However, the various approaches may complement each other.
(3) Within the studied time span (e.g. the Holocene), the glaciated area in the catchment must be appropriate for recording the amplitude of variations in GS/ELA. The largest glacier in the catchment must also be classified (cirque glacier, plateau glacier, etc.) to understand better which climatic factors influence the local glacier activity/ ELA (Fig. 3) (Dahl and Nesje, 1992; Dahl et al., 1997). Reconstructed ELAs must normally be adjusted for glacio-isostatic land uplift.
(4) Catchments with a high number of proglacial lakes and other sites/features (local watersheds, lateral moraines, etc.) suitable to record variations in GS/ ELAs are to be preferred (Fig. 4 and Fig. 5). Proglacial lakes should be dammed by a rock sill, and the shape of the lake basins should minimize postdepositional disturbance of the sediments. With no glaciers in the catchment, 'proglacial' lakes should have high organic production to increase the contrast, and the residence time of water in the proglacial lakes must allow both settling and further downstream transport of suspended sediments. Representative nonglacial control lakes should exist in the catchment (Matthews and Karlén, 1992), and geomorphological processes (colluvial activity, floods, etc.) which may influence on lake sedimentation must be taken into account.
(5) Ideal proglacial sites turn out to record simple systems where the topographic conditions isolate the glacier meltwater signal through natural filtering in a way that directly reflects GS/ELA. The occurrence of representative ice-marginal moraines of known age in the catchment is crucial for the calibration of glacier activity/ELAs based on proglacial sites.
(6) Combined with proglacial sites, rerouting of glacier meltwater across local watersheds may give detailed information concerning former glacier activity/ ELAs. Due to the sharp on-off signal, reconstructed variations in GS/ELA related to local watersheds can be used to calibrate ELAs based on a chain of proglacial sites (Fig. 4) (e.g. Dahl and Nesje, 1994; Dahl et al., 2002).
(7) A close relationship between GS and downstream transport distance of glacier-induced sedimentation from true suspension is suggested based on Roland and Haakensen (1985). As a consequence, smallglaciers can only be recorded at the most sensitive sites, while larger glaciers in addition can be recorded further downstream. If this catchment specific GS/SSDratio is known (Fig. 6), records of glacier-induced sediments from a chain of proglacial lakes may give continuous sensitive variations in glacier magnitude backwards in time. GS is converted to an ELA estimate using the AARmethod. Independent observations (lateral moraines, local watersheds, etc.) are used to calibrate the amplitude of ELA fluctuations for catchment dependent factors. If the investigated chain of proglacial lakes has a GS/SSDratio which allows all variations in GS within a given time span to be recorded, error bars for the estimated ELAs may be less than 750 m.
(8) With a one-site approach, variations in glacier activity/ELAs depend on the interpretation and sensitivity of the used methods. Hence, to minimize within lake variance and maximize between lake variance, basins distal to the inlet and/or the deepest part of the lake appear to give the best reproducable results when more than one core is taken into account. For one-site studies, only features that are consistently reproducible based on several independent proxies in two or more cores should be interpreted in terms of glacier variations and climate change (e.g. Snowball and Sandgren, 1996; Brauer et al., 2001).
(9) A critical factor for the use of both one-site approaches and a chain of proglacial lakes is the link between glacier advances and sediment production. Whether a longer transport length of sediments in suspension can be related to glacier maximum positions, or to periods preceding or post-dating periods of maximum ice extent (e.g. Leonard, 1997), is important for the interpretation of all proglacial sites, and must be further tested.
(10) Comparison of reconstructions using approaches based on both a single proglacial lake and a chain of proglacial lakes for the same glacier is important for the development of reliable methods/techniques to reconstruct former glacier activity/ELAs. Hence, testing and improvement of relevant field and laboratory approaches must continue, and especially how various methods/techniques complement each other must be better understood.