Glacier history

Glaciers are a striking feature of many alpine landscapes globally, and glacial moraines provided early evidence for recent glacial fluctuations in response to climate change. Denton and Karlen (1973) dated glacial termini in the St Elias Mountains on the Yukon/Alaska border and in the mountains of Swedish Lapland and showed repeated intervals of glacial expansion and retreat in both areas during the Holocene. They suggested that major glacial expansions had occurred in the "Little Ice Age" of the past several centuries, and at 1050-1250, 2300-3000, 4900-5800, and 8000 years ago. They proposed a recurrence of major glacial activity about every 2500 years and hypothesized that Holocene glacial and climatic fluctuations were caused by varying solar activity because of close correlations with short-term atmospheric 14C variations (see also Karlen and Kuylenstierna 1996). The glacial advances and retreats identified by Denton and Karlen (1973) are now termed "rapid climate changes" by Mayewski et al. (2004) in their survey of some globally distributed paleoclimate records and are now dated to 8000 -9000, 5000-6000, 3800-4200, 2500-3500, 1000-1200, and 150-600 years ago by tuning to the high-resolution GISP2 ice-core record.

In some areas it is difficult to reconstruct reliably the Holocene history of alpine glaciers because moraines that pre-date the "Little Ice Age" of the 18 th-19th centuries have often been overrun by "Little Ice Age" or Neoglacial ice advances.

Karlen (1976), working in the mountains of northern Sweden, showed the potential of sediments in lakes downstream of glaciers to provide a continuous record of Holocene glacial activity. By measuring the organic content (and hence the inorganic content) and quantifying changes in sediment color by X-radiography, Karlen (1976) reconstructed glacial activity for much of the Holocene in the drainage basin of the study lake. This approach of using proglacial lakes to reconstruct glacial activity and equilibrium-line altitudes (ELA) has been greatly developed by Nesje and Dahl (2000, 2003) (see, for example, Dahl et al. 2003; Bakke et al. 2005a,b).

Glacial ELAs are a curvilinear relationship between mean annual winter precipitation and mean ablation-season (summer) temperature (Nesje and Dahl 2000, 2003). If glacial ELA can be reconstructed using proglacial lake sediments and terrestrial geologic evidence (Dahl et al. 2003) and summer temperatures are reconstructed using biologic proxies (e.g. pollen) and modern organism-climate transfer functions (e.g. Bjune et al. 2005), changes in winter precipitation during the Holocene can be inferred (e.g. Bakke et al. 2005b; Bjune et al. 2005). Nesje et al. (2005) provide a recent synthesis of Holocene glacial history and reconstructed variations in winter temperature in southern Norway.

Although different glaciers in southern Norway show different Holocene histories, some general patterns emerge (Nesje et al. 2005). Glaciers in the early Holocene mainly retreated but with some significant glacier readvances at about 10 000, 9700, and 8000-8500 years ago. Most, if not all, glaciers may have melted completely at least once during the mid-Holocene. Most glaciers appear to have reformed between 4000 and 6000 years ago and advanced between 3000 and 1500 years ago ("Neoglaciation"). Most glaciers reached their maximum position relative to their position at 10 000 years ago during the "Little Ice Age", ranging in time from the early 18th century to the 1940s. Reconstructed changes in winter precipitation through the Holocene show striking millennial-scale fluctuations between periods with predominantly mild and wet winter conditions (similar to a positive North Atlantic Oscillation (NAO) index weather-mode) and periods with mainly cold and dry winters (negative NAO index weather-mode) (Nesje et al. 2005). The causes of these striking changes in winter precipitation are unclear but they suggest that there may have been major broad-scale variability in winter atmospheric circulation over north-west Europe during the Holocene.

Studies on Holocene glacial history using a combination of geologic approaches to reconstruct ELA and biologic approaches to reconstruct summer temperatures have provided a new paradigm in Holocene climate research, namely the detection of millennial-scale fluctuations in winter precipitation. Alpine glaciers thus have the potential to record not only changes in summer temperature but also changes in winter precipitation.

0 0

Post a comment