Holocene glacier and climate variations

5.6.1 Canada

The Little Ice Age was the most extensive Neo-glacial glacier advance in the Canadian Rocky Mountains (Luckman et al, 1993). Evidence of earlier, less extensive Neoglacial advances is based on wood recovered from several glacier forefields. Three radiocarbon dates, ranging between 8230 and 7550 yr bp, obtained from wood flushed out of Athabasca Glacier, and two dates from Dome Glacier ranging between 6380 and 6120yrbp, indicate that forests occurred upvalley of present glacier fronts during the Hypsithermal. Radiocarbon dates from detrital and in situ logs indicate that forests were overridden by glaciers between 3100 and 2500yrbp. This advance, termed the Peyto Advance, did not extend beyond the Little Ice Age maximum position. The earliest Little Ice Age advance is dated to ca. 900-600 yr bp.

Prior to the deposition of the Mazama tephra 6800yrbp, a minor glacier readvance (the Crowfoot Advance) left deposits in the Rockies and the interior of British Columbia

(Osborn and Luckman, 1988). The first Neo-glacial advances took place 6000-5000 yr bp. Other advances occurred between 40003000 yr bp and at about 2500 and 1800 yr bp. The Little Ice Age expansion, which started shortly after 900 yr bp, culminated in the eighteenth and nineteenth centuries.

In the American Cordillera, a Late-glacial or early Holocene glacier readvance or stillstand deposited moraines about 1-3 km beyond present glacier fronts (Thompson Davis, 1988). The earliest dated Neoglacial advances occurred at about 5000 yr bp. In most mountain ranges of the western USA, unweathered, sharp-crested moraines adjacent to modern ice margins, or near cirque headwalls without glaciers at present, date to the Little Ice Age of the last several centuries.

5.6.3 The Arctic

A review of the Holocene glaciation record in Alaska (Calkin, 1988) suggested that glacier fluctuations between Arctic, central interior and southern maritime Alaska were mostly synchronous. There is evidence of glacier expansion between 7600 and 5800 yr bp, and between 5800 and 5700 yr bp. A significant increase in glacier activity began at 4400 yr bp. During the Holocene glacial maxima, glaciers covered Glacier and Lituya Bays and the fjord of Prince William Sound. A minor glacier recession has been recorded at about 2000 yr bp. The widespread Little Ice Age glacier advances were initiated at approximately 700 yr bp. In the Arctic, glacier advances of ad 1600 dominated those of the late 1800s.

In a study by Evans and England (1992), they reported that indicators of sea-ice conditions on northern Ellesmere Island suggest that the early Holocene was a period of considerable open water. Geomorphological evidence shows that the ice shelves are presently breaking up and melting in response to recent warming. Several large glaciers in that region are still advancing in response to the mid-Holocene climatic deterioration. Some glaciers display evidence of dual advances which may reflect mid-Holocene and Little Ice Age accumulation. Other evidence of Little Ice Age cooling and recent warming includes perennial snowbank retreat and fluvi-ally eroded ice-wedge polygons near sea-level.

Bradley (1990) reviewed the Holocene record of climatic change on the Queen Elizabeth Islands. He found that temperatures were highest during the early to mid-Holocene. Temperatures declined from approximately 3000 yr bp, culminating in low temperatures from 100-400yrBP. This coldest period during the Holocene resulted in glacial advances to post-glacial maximum positions. Since 1925 ad there has been a pronounced temperature increase which has led to negative mass balance on glaciers and ice sheets.

Porter (1989) presented a 2000-year chronology of fluctuations of the tidewater glacier in Icy Bay. The outermost moraine complex at the mouth of the bay dates from ad 400 to 850. Subsequent glacier retreat led to déglaciation of the fjord by ad 1000. At the onset of the Little Ice Age (thirteenth century) the glacier expanded and reached its maximum in the early nineteenth century. A study by Wiles and Calkin (1994) from Kenai Mountains, Alaska, revealed three major intervals of Holocene glacier expansions: 3600 yr bp, ad 600, and during the Little Ice Age.

5.6.4 Greenland

Investigations along the western Greenland ice sheet margin in the region of the Jacobshavn Isbrae indicate that the glacier was at least 15 km behind the present position between 4700-2700 cal. yr bp (Weidick et al., 1990). Gordon (1980) demonstrated that cirque and valley glaciers in western Greenland have advanced since 1968 simultaneously with the retreat of larger valley and icefield outlet glaciers.

5.6.5 Iceland

Historical records indicate that the maximum Holocene extent of Fjallsjôkull and Breidamer-kurjôkull was in the latter half of the nineteenth century (Kugelmann, 1991). The sequence of Holocene development in Iceland can be summarized as: (1) Late Weichselian and early Holocene glaciation; (2) Holocene non-glacial conditions and prehuman colonization; (3) Holocene non-glacial conditions with human occupation; (4) Little Ice Age glaciation (Norddahl, 1990; Ingolfsson, 1991). There are, however, uncertainties concerning the timing of the wastage of the Weichselian glaciers, and the timing and extent of glacial readvances between the Late Weichselian deglaciation and the Little Ice Age glaciation (Gudmundsson, 1997). However, stratigraphic evidence from Skalafellsjokull, SE Iceland, and Eyjabakkajokull, eastern Iceland, suggests no glacier advance during this interval (Sharp and Dugmore, 1985).

A study of the Holocene record of glacier fluctuations from northern Iceland (Stotter, 1991) indicated two glacial advances during the time interval 6000-4800 yr bp. Dugmore (1989) used tephra layers interbedded with soils to date Holocene glacier fluctuations in southern Iceland. This study demonstrated that a large ice mass existed in the mid-Holo-cene in Iceland, because Solheimajokull extended up to 5 km beyond its present limits between 7000 and 4500 yr bp. Major advances also culminated before 3100 yr bp and between 1400-1200 yr bp. In the tenth century the glacier was also longer than during the Little Ice Age (ad 1600-1900). In contrast, some other glaciers reached their maximum Holocene extent during the Little Ice Age. The anomalous behaviour may be the result of changes in the catchment areas over the last 5000 years. Recently, Stotter et al (1999) suggested that Holocene glacier advances in northern Iceland took place at around 4700, 4200, 3200-3000, 2000, 1500 and 1000 yr bp.

A study at Fjallsjokull, SE Iceland, indicated a mid-Holocene glaciation ending at about 4500 yr bp (Rose et al, 1997). The Little Ice Age glaciation occurred ca. ad 1850-1965. At Fjallsjokull this glaciation caused glaciotectonic deformation and subsequent erosion of the mid-Holocene land surface.

5.6.6 New Zealand

In the central Southern Alps of New Zealand, glacier retreat exposed sections in lateral moraines which made possible studies of moraine stratigraphy and genesis (Gellatly et al, 1988). Radiocarbon dating of buried wood and soils indicates that glaciers in New Zealand expanded at about 5000, 4500-4200, 3700, 3500-3000, 2700-2200, 1800-1700, 1500, 1100, 900, 700-600 and 400-100 yr bp.

5.6.7 South America and Antarctica

Different dating approaches indicate that glaciers readvanced significantly only during the last 5000 yr bp (Clapperton and Sugden, 1988). In South America there is evidence for four main periods of Neoglacial advance: 5000-4000, 3000-2000, 1300-1000 yr bp, and fifteenth to late nineteenth centuries. Minor glacier advances may have occurred at ca. 8400, 7500 and 6300 yr bp.

Different Antarctic environments produce interesting contrasts. Some glaciers in McMurdo Sound (East Antarctica) are more extensive now than during the global glacial maximum at 18,000 yr bp. Radiocarbon evidence from the South Shetland Islands indicates two main Holocene glacier advances, the most extensive peaking in the twelfth century.

At South Georgia, Clapperton et al (1989) used glacial geomorphology, slope stratigraphy, and analyses of environmental indicators in peat and lacustrine cores to present a Holocene record. At low altitudes plant growth had begun by 9700 yr bp. Fairly cool conditions prevailed until 6400 yr bp, followed by a period from 5600 to 4800 yr bp when conditions were slightly warmer than at present. Periods of climatic deterioration occurred at around 48003800 yr bp, 3400-1800 yr bp, and during the last 1400 years. The most extensive Holocene glacier advance on South Georgia culminated slightly before 2200 yr bp.

5.6.8 Mount Kenya and other East African mountains

Many of the East African mountains are covered with late Pleistocene and late Holocene glacial sediments which are poorly dated. Only on Mount Kenya is there a chronology emerging based on dating. During the Pleistocene, glacier fluctuations appear to have been broadly synchronous in the East African mountains. Terminal moraines on Mount Kenya extend to 3200 m in most drainage basins and yield minimum radiocarbon dates of approximately 15,000 yr bp (Mahaney, 1988). Recessional moraines at 3700-4200 m appear to date from the Late-glacial, while younger Neoglacial moraines are either Little Ice Age or older late Holocene deposits. The Neoglacial sequence on Mount Kenya is younger than 1000 yr bp, while on other East African mountains glaciers may have existed during the last 2000 yr bp.

Lacustrine sedimentary evidence indicates that glaciers occupied the southwestern cirques on Mount Kenya during much of the last 6000 years (Karlen, 1998; Karlen et al, 1999). Pro-glacial lacrustrine sediments obtained from Hausberg Tarn suggest six major periods of glacier advances at about 5700, 4500-3900, 3500-3300, 3200-2300, 13001200, and 600-400 cal. yrBP. The 5700 cal. yr bp advance reached approximately 1 km beyond the Little Ice Age moraines.

5.6.9 The Alps

Rothlisberger (1986) summarized the Holocene glacier fluctuations in the Alps. According to his data compilation, the glaciers were in advanced positions at 8600, 8200, 7500, 6400, 6100, 5200, 4800, 4500, 3600-3000, 2800, 2100, 1100, 600, 400 and 180yrBP. On the Swiss Plateau and at timberline in the Alps, Haas et al (1998) found, on the basis of palaeobiological evidence, eight synchronous pre-Roman cold phases at 9600-9200, 8600-8150, 7550-6900, 6600-6200, 5350-4900, 4600-4400, 3500-3200 and 2600-2350yrBP.

Cores recovered from the proglacial Lake Silvaplana in the Swiss Alps (Leemann and Niessen, 1994) showed that glacial varves were deposited during glacial retreat in the early Holocene until 9400 yr bp. Glacial activity was absent or negligible within the catchment between 9400 and 3300 yr bp. Maximum varve thickness, interpreted as reflecting the size of the glaciers in the catchment, was observed between ad 1790 to 1870.

Grove (1997) summarized the history of Holocene glacier variations in the Alps. The sequence of déglaciation in the Austrian Ôtztal is documented by peats dated at 10,000 bp within the Egesen moraines, correlated with the Younger Dryas in NW Europe. A glacier advance took place in the Swiss and Austrian Alps at around 9300yrbp, termed the Schlaten advance period between 9400 and 9000yrbp. The period between 9000 and 6000yrbp was characterized by episodes of recession interrupted by glacier advances between 6600 and 6000yrbp, between 7700 and 7300yrbp, and between 8400 and 8100yrbp. The interval between 6000 and 4600 yr bp was generally warm, but broken by glacier advances at around 4600 and 4200 yr bp. A warm climate prevailed between 4600 and 3600yrbp, but during the period 3600-3000 yr bp a significant glacier advance occurred (the Lôbben Advance). The period from 3000 to 1100yrbp was warm and characterized by glacier contraction. Between 900 and 800yrbp glaciers readvanced. During the Medieval period (ca. ad 900-1300) glaciers retreated and reached positions comparable to those in the late twentieth century.

In the Italian Alps, glaciers were reduced to their present size, or even smaller, after déglaciation at about 9000yrbp (Orombelli and Mason, 1997). In the western Italian Alps, Rutor Glacier had its terminus upvalley of the present position during most of the period between 9000 and 5000 yr bp. Subsequent to 5000 yr bp, several Neoglacial advances occurred, the most pronounced between 3000 and 2500 yr bp. The first Little Ice Age glacier advance was recorded as post-Medieval floods from the Rutor Glacier. The maximum Little Ice Age extent was reached during the seventeenth and eighteenth centuries. The majority of the glaciers reached their maximum Holocene extent around ad 1820 or 1850. Minor readvances took place at approximately ad 1890, 1920-1925 and 19701980. The glaciated area has been reduced by around 40 per cent since the last century.

Dendrochronological analyses of fossil larches in the forelands of Grosser Aletch

Glacier show that the glacier advanced during the Lobben cold period at around 32003100 yr bp, during the Goschenen cold period I at approximately 2800-2700 and 24002300 yr bp (Holzhauser, 1997). Grosser Aletch Glacier was 600-1000 m less advanced than at present between 3200 and 2400-2300 yr bp. Den-droclimatological research proved that the Gorner Glacier advanced during the fourteenth century and that the Medieval warm period lasted from the eighth to the end of the thirteenth century (Fig. 5.10).

Tree trunks and wood fragments in minero-trophic peat accumulated in the outwash plain of Unteraargletscher, Switzerland, have been radiocarbon-dated to represent Holocene retreat phases of the glacier (Flormes et al, 1998). The radiocarbon dates suggest that the glacier was at least several hundred metres less advanced at around 8100-7670, 6175-5780, 4580-4300 and 3380-3200 yr bp. The warmest and driest period occurred between 4100-3600 yr bp. Growth of peat between 3800 and 3600 yr bp, was, however, attributed to more humid conditions. Based on the chronology, the authors suggested an approximately 2000-year cyclicity of tree and/or peat growth in the study area.

5.6.10 China

In China, the last 10,000 years have been climatically divided into three parts. Humid conditions prevailed in the early Holocene (10,000-7500 yr bp), followed by the climatic optimum at 7500-3000 yr bp. At about 3000 yr bp, temperatures dropped, causing numerous glacier advances over the Tibetan Plateau.

5.6.11 Scandinavia

Karlen (1988) presented evidence of significant Neoglacial episodes in northern Scandinavia at about 7500, 5100-4500, 3200-2800, 2200-1900 and 1500-1100 yr bp, while less extensive glacier readvances, for which there is less evidence, occurred at 6300, 5600, 2500, 940, 600-560 and 380 yr bp. Pre-Little Ice Age moraines have been mapped around Jostedalsbreen in western Norway, the largest ice cap on mainland Europe. The moraines have been radiocarbon-

dated to 9100 yr bp, and termed the Erdal event by Nesje et al. (1991), Nesje and Kvamme (1991) and Nesje (1992). Similar moraines have also been reported from the Hardangerjokulen and Jotunheimen areas. At Hardangerjokulen, a short-lived glacier phase is dated at 7600 yr bp (8200 cal. yr bp) (Nesje and Dahl, 1991a; Dahl and Nesje, 1994, 1996) and termed the Finse event. Radiocarbon dates indicate that glaciers on the northern part of the Hardangerjokulen Plateau were totally melted between 7560 and 6285 yr bp. In northern Scandinavia, at Jostedalsbreen, and at the northern part of Hardan-gerj0kulen, glacier readvances occurred between 6300 and 5300 yr bp (Karlen, 1988; Nesje et al, 1991; Dahl and Nesje, 1994, 1996). Glaciers at the northern part of Hardanger-j0kulen were small or totally absent between 5280 and 4830 yr bp. At the northern sector of Hardangerjokulen, the period from 4830 to 3790 yr bp was characterized by minor, high-frequency glacier oscillations. Hardangerjokulen has existed continuously from 3790 yr bp to the present (Dahl and Nesje, 1994, 1996). This is also in close accordance with a date of 3710 yr bp for evidence of significant Neoglacial expansion in the Jostedalsbre region (Nesje et al, 1991). At the southwestern margin of the Jostedalsbre ice cap, Nesje and Dahl (1991b) showed that a cirque glacier isolated from the main ice cap was present between 2595 and 2360 yr bp, 2250 and 2150 yr bp, 1740 and 1730 yr bp, and subsequent to 1430 yr bp.

Two radiocarbon-dated soils on the surface of Blaisen and Midtdalsbreen, at the northern margin of Hardangerjokulen, demonstrate that both outlet glaciers were behind the modern terminal position until about 15002000 yr bp (Dahl and Nesje, 1994). Two minor glacier oscillations also occurred at about 1100 and 800yrBP (Nesje and Dahl, 1991a; Dahl and Nesje, 1994, 1996), while the Little Ice Age is dated to have occurred since 575 yr bp (cal. yr ad 1300-1420).

Karlen and Matthews (1992) and Matthews and Karlen (1992) found evidence of glacier expansions in the More/Jostedalsbreen/Jotunheimen regions during the following intervals: 6400-5900, 3400-3000, 3000-2600, 2500-2300, 2200-2100,1600-1400 and after 1000 yr bp.

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Figure 5.10 Fluctuations of the Grosser Aletch Glacier (top) and Gorner Glacier (bottom) in the Swiss Alps over the last 1200 years. (Adapted from Holzhauser, 1997)

Matthews (1991) presented convincing evidence from radiocarbon-dated buried soils for a late Neoglacial maximum within the last 400-600 years in the Jostedalsbreen/Jotunhei-men region. Griffey and Worsley (1978) found no evidence for any extensive glacier episode after the deglaciation of the continental ice sheet between approximately 9000 and 3000yrbp. Two significant pre-Little Ice Age periods of glacier advance took place in the Okstindan region (Griffey and Worsley, 1978). The first was dated to between 30002500 yr bp, and the second to between the eighteenth century and 1600yrbp, with a tentative age of 1250-1000yrbp.

In front of glaciers on Lyngshalv0ya, northern Norway, up to four Neoglacial moraines occur (Ballantyne, 1990), representing five episodes of glacier expansion, one of which predated the Little Ice Age. Lichenometric, den-drochronological and historical evidence indicates that the oldest Little Ice Age moraines date to the mid-eighteenth century, and the youngest to ad 1910-30. At nine small glaciers, the ad 1910-30 moraine represents the Neoglacial maximum.

5.6.12 Tatra Mountains

In the Tatra Mountains, about ten small glaciers (glacierets) occur (Jania, 1997). The absence of typical glaciers makes reconstruction of Holo-cene glacier fluctuations difficult. Geomorpho-logical evidence suggests a total melt of glaciers in the highest cirques subsequent to 8300 yr bp. A fossil soil found beneath one of the glacierets indicates that the snow patches did not exist in the Tatras during the Atlantic period. During the Little Ice Age, however, glacierets and snow patches were considerably larger than during recent decades. During the last 20 years, the number of snow patches has decreased and those remaining are also reduced in size and thickness (Jania, 1997).

5.6.13 Jan Mayen

On the island of Jan Mayen (373 km2), there have been at least two periods of glacier advance in the Holocene (Anda et al., 1985).

The first may have occurred at approximately 2500yrbp, while some glaciers reached their maximum extent around ad 1850. Subsequently the glaciers experienced an oscillating retreat, however, with a significant expansion around ad 1960.

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