Holocene glacier variations of Blåisen, Hardangerjøkulen, central Southern Norway

QUATERNARY

RESEARCH

Holocene

35, 25-40 (19%)

Glacier Variations of BlAisen, Hardangerjnrkulen, Central Southern Norway ATLENESJEANDSVEINOLAFDAHL

Department

of Geology,

Section

B, University

of Bergen,

Alle’gt.

41, N-5007

Bergen,

Norway

Received October 5, 1989 A l-m-deep gully section 460 m beyond the maximum Little Ice Age marginal moraines of Blaisen, Hardangerjokulen, central southern Norway, revealed alternations of minerogenic and organic sediments. The geographical/geological setting of the dated section provides a unique on/off signal of Holocene glacier fluctuations of Bltisen. Lithostratigraphy, sediment characteristics, and radiocarbon dates from the study section indicate one period of glacier (re)advance between the late Preboreal deglaciation of the inland ice sheet and 8660 2 100 yr B.P. A grey sand layer 56-57 cm below the surface is interpreted to be of fluvial/colluvial origin and is radiocarbon dated to about 7700 yr B.P. At 48 cm below the surface, a bluish-grey sand/silt layer is radiocarbon dated to 7590 -t 120 yr B.P. (6560-6240 B.C.) and interpreted to be of glaciofluvial origin. A minor glacier oscillation postdates 1130 f 70 yr B.P. (81W90 A.D.). The Medieval/Little Ice Age glacier advance of B&en beyond its modem extent occurred after 1040 2 60 yr B.P. (960-1030 A.D.). Calculations of the modem and Little Ice Age equilibrium-line altitudes (ELAs) on Hardangerjokulen suggest an ELA depression of ca. 130 m during the Little Ice Age maximum. Q 1991 University

of Washington.

dle Holocene in the western Italian Alps, consistent with pollen evidence suggesting summer temperatures at least 4°C warmer than at present. During the late Holocene many alpine glaciers throughout the world reappeared/ readvanced, an event termed Neoglaciation (Porter and Denton, 1967). Moraine records of alpine glaciers in the Northern Hemisphere led Denton and Karlen (1973) to propose climatic variations with a periodicity of ca. 2500 yr. A similar periodicity (2550 yr) was also found in 6180 oscillations in two Greenland ice cores of the last 8500 yr (Dansgaard et al., 1984). Denton et al. (1986) thus concluded that variations in solar insolation are the most likely cause of such periodic short-term climatic fluctuations, despite inconsistency in the Scandinavian record of glacier variations. The climatic periodicity proposed by Dansgaard et al. (1984) may, however, be irregular; its average value appears to be between 1200 and 2600 yr (Paterson and Hammer, 1987). Porter (1986) reviewed the pattern and forcing of Northern Hemisphere glacier

INTRODUCTION

The record of glacier variations in high altitudes and high latitudes may provide important paleoclimatic information (e.g., Porter, 1981). Studies of Holocene glacier variations show that the early to middle Holocene was, in general, a time of glacier retreat (Porter and Denton, 1967; Denton and Karlen, 1973; Mayewski ef al., 1981; Davis and Osborn, 1988) and warmer climate (Davis et al., 1980; Ritchie et al., 1983; Bartlein et al., 1984) as a result of increased summer solar radiation at high northern latitudes (Kutzbach and StreetPerrott, 1985). Karlen (1973) and Heuberger (1974) however, suggested that advances of a magnitude similar to those of recent centuries occurred during the middle Holocene, and Beget (1983) argued for worldwide glacier expansion between about 8500 and 7500 yr B.P. Later, however, Davis and Osborn (1987) questioned Beget’s evidence. Porter and Orombelli (198.5), on the other hand, presented evidence of glacier contraction during the mid25

0033-5894/91 $3.00 Copyright 0 1991 by the University of Washington. All rijthts of repmduction in any form reserved.

26

NESJEANDDAHL

variations during the last millennium and concluded that brief periods of glacier advance are associated with high volcanic aerosol production as recorded from acidity variations in a Greenland ice core (Hammer ef al., 1980). In Scandinavia there has been considerable discussion concerning the age and magnitude of pre-Little Ice Age glacier ex-

I. 1.

.

.

FIG. 1. Map showing distribution 1988).

I \ \

pansions (Karlen, 1981a; Matthews, 1982). Karl& (1973, 1976, 1982, 1988) suggested that a number of Neoglacial advances occurred from about 7500 yr B.P. to the present. In the Alps, Rothlisberger (1987) also reported a record suggesting several glacier advances throughout the Holocene. Matthews (1982, 1989), however, argued against positive moraine stratigraphic evi-

--8

Main water

dwde

Glacier-covered

of glaciers in southern Norway (Modified

area

from @trem et al.,

NORWEGIAN

GLACIER

dence of significant early Neoglacial advances in southern Norway. The Late Atlantic Chronozone seems to have been an episode with profound ecological consequences over the entire Northern Hemisphere, with downward and southward migration of vegetation zones, increased peat growth, solifluction, and glacier/snow expansion (Frenzel, 1966; Patzelt and Bortenschlager, 1973; Grove, 1979; Magny, 1982; Turner, 1984; Khotinskiy, 1984; Caseldine and Matthews, 1985, 1987; Nesje ef al., 1989; Nesje et al., in press). In Europe, the most recent Neoglacial episode, occurring between the 13th and the early 20th century was, in many cases, characterized by the most extensive Holocene glacier expansion. This is commonly referred to as the Little Ice Age (e.g., Grove, 1988). In this paper the Holocene glacial record, as inferred from the radiocarbon-dated section with alternating minerogenic and organic sediments beyond the maximum Lit-

FIG.

2. Topographic

27

VARIATIONS

tle Ice Age marginal position of Blaisen, is described and discussed in relation to the Scandinavian Holocene glacial and climatic record. Particularly important is evidence from the site for glacier extent in the early and middle Holocene, a subject of worldwide scientific interest. The chronostratigraphic subdivision of the Holocene adopted here accords with that proposed by Mangerud et al. (1974). STUDY AREA

Hardangerjokulen (60”33’N, 7”25’E) is a plateau glacier situated at the main watershed between western and eastern Norway (Fig. l), and is strategically located in relation to the North Atlantic cyclone tracks and the Arctic atmospheric polar front. The plateau glacier (Fig. 2) is the sixth largest glacier in Norway, covering an area of 73 km2, and ranges in altitude from 1850 to 1050 m. Blaisen, the focus of this study, is a

map of the Hardangejekulen

ice cap.

28

NESJEANDDAHL

northeastern outlet valley glacier from Hardangejokulen (Fig. 3) covering an area of 4.9 km*, or about 7% of the plateau glatier area. The outlet glacier descends from the glacier plateau (1850 m) to 1370 m, an altitudinal range of 480 m. The present meltwater stream from Blaisen drains in a northerly direction (Fig. 3). However, whenever BlHisen advanced across a local water divide approximately XI-100 m beyond the present frontal position, glacier meltwater flowed along a marked drainage route in a more northerly/northeasterly direction. About 460 m beyond the maximum Little Ice Age marginal position, the re-

routed meltwater deposited glaciofluvial sediments in a minor basin (Fig. 4). Advanced positions of Blaisen without glacigenie sedimentation in the basin may have been related to subglacial meltwater drainage following the present drainage pattern. This seems unlikely, however, because the drainage area is so well confined. Thus, during periods of glacier contraction behind the local water divide, peats were formed in the depression. As there is no other potential source of glacial sediments in the study section, Bllisen must have been in an advanced position whenever sediments with a texture similar to those deposited along the

FIG. 3. Map of the northeastern part of the Hardangejekulen ice cap showing the outlet glaciers Bl5isen and Midtdalsbreen and the proglacial area. The contour intervals on the glacier and in the glacier foreland are 20 and 100 m, respectively. The map is based on air photographs from 1961.

NORWEGIAN

GLACIER

29

VARIATIONS

FIG. 4. View looking northwest at the study site.

present glacier meltwater stream accumulated in the basin. The stratigraphic sequence therefore represents a unique on/off signal of Holocene glacier fluctuations of Blaisen in relation to the local water divide lying 5&100 m beyond the present glacier margin. LITHOSTRATIGRAPHY

A gully section in the proglacial depression revealed alterations between minerogenie layers and peat horizons (Figs. 5 and 6). Unit K at the base of the section consists of a grey diamicton overlain by a lightbrown sand (Unit J) with low organic content. Unit I consists of a bluish-grey silt ca. 25 cm thick. Between 68 and 18 cm is a compact, brown peat rich in macroscopic plant remains (Units H, F, D, and B), inter-layered by grey sand (Unit G) and bluish-grey sand/silt (Units E and C). Unit A consists of a weakly laminated bluish-grey sand/silt with a podzol developed on top. The minerogenic layers of units A, C, E, and I consist of bluish-grey sand/silt, analogous to glacially eroded rock flour depos-

ited along the present stream. RADIOCARBON

glacier meltwater

DATES

Six samples from the l-m-deep section (Fig. 6, Table 1) were submitted for radiocarbon dating. Due to too low organic content, however, the sample from Unit H could not be dated by conventional radiocarbon dating. The dated horizons were mainly chosen in order to obtain age constraints on periods of glacier meltwater discharge from Blaisen into the studied depression, and hence to date periods of glacier advance beyond the local water divide (Fig. 3). The utmost care was taken to avoid contamination of the peat samples, which in the field were immediately sealed in plastic bags. The samples were stored in a coldstorage chamber before submission for dating. In the Trondheim Dating Laboratory the samples were treated with dilute NaOH (5 g 100 ml-‘) to remove possible humic acids. The samples were further treated with dilute HCI (5 ml 100 ml-‘) to remove

30

NESEANDDAHL

FIG. 5. The study section. The ruler is 40 Fig. 6.

possible carbonates. fraction was dated.

The NaOH-soluble

STRATIGRAPHIC

INTERPRETATION

cm

A-

According to the geographical/geological setting of the investigated site and the sediment texture in the study section, the layers of bluish-grey sand/silt and peat are interpreted as representing periods of glaciofluvial and organic deposition, respectively. A nonglacial origin for the bluishgrey sand/silt sediments is excluded. The grey sand of unit G, on the other hand, is interpreted to be a result of fluvial/colluvial activity. Consequently, sand/silt units A, C, E, and I are inferred to represent periods when Blaisen was in an advanced position in relation to the local water divide in front of the present margin. The peats, by contrast, represent times when BlHisen was in a contracted state. As a consequence, peat Units B, D, F, and H portray periods when the glacier front was behind the local water divide. The basal diamicton (Unit K) represents the till deposited by the late Weichse-

The letters refer to the lithostratigraphic

units in

lian inland ice sheet. The overlying lightbrown sand (Unit J) probably represents a short late Preboreal ice-free interval before the glaciofluvial silt of Unit I was deposited by meltwater from Blaisen in an advanced position. The compact peat of Unit H, with a basal date of 8660 + 100 yr B.P. (T-8206) (Fig. 6, Table l), demonstrates glacier contraction during the Boreal Chronozone. As argued above, the 7700-yr-old sand layer of Unit G is interpreted as fluvial/colluvial in origin. The bluish-grey sand/silt layer (Unit E) is interpreted as representing a short-lived glacier advance across the local water divide in front of the present glacier terminus dated at 7590 + 120 yr B.P. (6560-6240 B.C., Fig. 6, Table 1). The ca. 25cm-thick peat of Unit D represents about 6500 years of glacier contraction during the Atlantic, Subboreal, and early to middle Subatlantic Chronozones. The date of 1130 + 70 yr B.P. (T-8203) (81fL990 A.D.) from the upper part of peat Unit D represents a maximum date of an

NORWEGIAN

GLACIER

VARIATIONS

31

MODERN AND LITTLE ICE AGE EQUILIBRIUM-LINE ALTITUDES ON HARDANGERJ0KULEN

Depth below S”,fX< (cm)

D

60

70

80

90

100

FIG. 6. Lithostratigraphy and radiocarbon dates from the Bliisen study section (7”3 1‘E, 60”34’N, UTM 190164, 1250 m altitude).

initial glacier advance during the Medieval period, while peat Unit B represents a short period of glacier retreat during the Medieval period. The date of 1040 ? 60 yr B.P. (T-8204) (810-990 A.D.) gives a maximum age of the Medieval/Little Ice Age glacier advance at Blaisen beyond the local water divide. Unit A represents the Little Ice Age glaciofluvial sand/silt transported by the glacier meltwater stream from Bllisen and deposited in the studied proglacial basin. The rate of sediment/peat accumulation in the dated section (Fig. 7) shows that the rate of deposition of the minerogenic sediments was, in general, about an order of magnitude greater than the accumulation rate of the peat.

According to Qstrem et al. (1988) the present glaciation threshold (e.g., Porter 1977) at Hardangerjokulen is at ca. 1750 m, while the modern steady-state equilibriumline altitude (ELA) on the entire plateau glacier according to an accumulation-area ratio (AAR) of 0.6 +- 0.05 (e.g., Meier and Post, 1962; Andrews, 1975; Porter, 1975; Gross et al., 1977; Meierding, 1982; Sutherland, 1984; Hawkins, 1985) is calculated to lie at 1650 ‘-1-20 m. Meier and Post (1962), Pierce (1979), and Kuhle (1988), however, have reported significant departures from the AAR value of 0.6 -t 0.05 for glaciers which deviate from a “normal” area/ altitude distribution, and regard it as only a rough approximation of limited climatic significance. However, the mean ELA measured from mass balance studies during the period 1963-1982 on Rembesdalskaki, a western outlet valley glacier from Hardangerjokulen (Fig. 2), is 1695 m (Roland and Haakensen, 1985). This value is in close agreement with the figure calculated for the entire Hardangerjokulen ice cap. As the difference in the ELA between the western and eastern parts of Hardangerjokulen lies within the error limits of the ELA calculations of Bldisen and Midtdalsbreen (see below), this is not taken into account. During the maximum Little Ice Age readVance in the mid 18th century, Blaisen covered an area of 6.3 km2, which is ca. 30% larger than its present extent (Fig. 3). Due to the absence of lateral moraines from the maximum Little Ice Age glacier advance, the depression of the steady-state ELA was calculated by means of the AAR method, giving a depression from the present ELA of 45 ? 36 m (Fig. 8). Because this value is considerably less than calculated for other glaciers (e.g., Matthews, 1977) the lowering of the ELA on the adjacent Midtdalsbreen (Figs. 2 and 3) was calculated using an AAR

32

NESJEANDDAHL TABLEl.

Depth (cm)

Laboratory number

18-23 25-27 4749 55-58 64-68

T-8204 T-8203 T-8678 T-8205 T-8206

RADIOCARBON

DATESFROMTHEBLAISENSECTION

Dates (14C yr B.P.)

Calibrated age”

1040 1130 7590 7730 8660

u The Trondheim Dating Laboratory (1986).

2 2 2 * 2

60 70 120 loo 100

960-1030 810-990 6560-6240 6680-6450

Sample weight 69 19.4 56.2 5.0 33.3 12.4

A.D. A.D. B.C. B.C.

uses the program for radiocarbon age calibration

of 0.6 + 0.05. The modern and Little Ice Age ELAs of Midtdalsbreen were calculated to 1690 2 20 m and 1560 +40/-45 m, respectively, giving an altitudinal difference of 130 ? 49 m (Fig. 9). This AAR difference in modern and Little Ice Age ELAs between the two outlet glaciers is probably mainly due to glacier configuration; during the Little Ice Age Midtdalsbreen followed a marked valley depression, while Bl%isen terminated on a minor plateau at an altitude similar to that of the present glacier terminus. This gave Midtdalsbreen a “normal” hypsometric distribution during the Little

6’T - 15.2 -25.4 - 24.4 -23.1 -21.7

of Stuiver and Reimer

Ice Age, while B&en had a greater relative part of its ablation zone close to the Little Ice Age ELA (Fig. 10). An ELA depression of ca. 130 m during the Little Ice Age is thus considered to be more representative of Hardangejokulen than the calculated value of about 45 m from Bllisen. If the ELA depression of ca. 130 m was due exclusively to temperature lowering, this would correspond to a general temperature depression of approximately 0.W during the Little Ice Age glacier maximum. CUMULATIVE 0

10

20

30

40

AREA(%) 50

60

70

60

90

100

1aoo-

,+T-a20 003mm

yr ‘././

/-

/’

1700E

./

x2 1600b

0,4mm y, ,+’

2

,/c T-8205 1 Immyr- , / fT-8206

Modern ELA(AAFk0

1500-

6kO.05) = 1675?10 1675e-10

Little Ice Age ELA(AAR=0.6+0.05)= ELA(AAR=0.6k0.05)= 1630+20/-35

i 1400-

Altliudinal difference = 45+36 -h MODERN -BLA’sEN 1 LITTLE ICE AGE -

30 *

ado0

do0

RADIOCARBON

4doo

I

0

2600

AGE (YR B.P.)

FIG. 7. Mean rates of sediment accumulation tween the dated levels in the study section.

10

20

AREA

be-

30

40

5

DISTRIBUTION(%)

FIG. 8. Modern and Little Ice Age area/altitude distribution of Bllisen.

NORWEGIAN CUMULATIVE IO

0

20

30

40

GLACIER

AREA(%) 50

60

70

80

90

1

1900

1600

1700

E x3 +

1600

5 4

Modern ELA(AAR=O 6+0 05 1690~-20

1500

L,tt,e Ice Age ELA(AAR=06?0.05 1560+401-4 A,,l,“d!“al difference = 130’49

1400

I MIDTDALSBREEN) ;F;;;;E-A;E--

1300

p 10

20

AREA

30

40

DISTRIBUTION(%)

FIG. 9. Modern and Little Ice Age area/altitude distribution of Midtdalsbreen.

CHRONOLOGY SUBSEQUENT TO THE LITTLE ICE AGE MAXIMUM

There is no historic evidence of significant ice-marginal fluctuations of BlHisen between 1750 and 1900 A.D. However, well-defined terminal moraines beyond the northwest margin of Bldisen (Fig. 3) were lichenometrically dated by Andersen and Sollid (1971). A time/distance diagram and mean annual rates of retreat subsequent to the maximum Little Ice Age position ca. 1750 A.D. are illustrated in Figure 11. The mean rate of annual retreat was slowest between 1750 and 1860 A.D.; the fastest rate of retreat occurred after the 1930s. DISCUSSION OF THE BLAISEN GLACIER CHRONOLOGY

The till (Unit K) at the base of the study section at Hardangerjokulen was probably deposited by the retreating and/or downwasting late Weichselian ice sheet. The

VARIATIONS

33

sand with low organic content (Unit J) between the till (Unit K) and the overlying bluish-grey sand/silt (Unit I) probably indicates a short ice-free period after deglaciation of the continental ice sheet. The exact time of deglaciation of the inland ice sheet from the Finse area is not dated; however, it probably occurred around the Preboreal/ Boreal transition (9000 yr B.P.). The glaciofluvial sand/silt of Unit I suggests that a glacier reactivation on the Hardangerjokulen Plateau took place subsequent to the regional deglaciation of the inland ice sheet (Fig. 12). The radiocarbondated peat at the base of Unit H shows that this readvance occurred prior to 8660 + 100 (T-8206). Sparsely distributed, wellvegetated marginal moraines with large lichen thalli (Rhizocarp-pon geographicurn) located a few meters beyond the Little Ice Age marginal moraines of BlHisen (Fig. 3), also recognized by Andersen and Sollid (197 l), may be correlated to this readvance. Apparently, the glacier front at that time reached a more advanced position than during the Little Ice Age. Similarly, marginal moraines beyond Little Ice Age moraines in front of outlet valley glaciers from Jostedalsbreen and Jotunheimen (Fig. 1) have been widely recognized (e.g., Nesje et al., in press). Radiocarbon dates proximal and distal to these moraines suggest that this readvance culminated between ca. 9200 and 8900 yr B.P., with a maximum position around 9100 yr B.P. Inferred from the altitude of lateral moraines formed during this readvance, the average ELA depression was 325 +75/ - 115 m below the modern level. By assuming a precipitation pattern similar to that at present, a mean temperature decline of about 2°C is suggested. On the basis of lithostratigraphic studies, pollen analysis and radiocarbon dating, Blystad and Selsing (1988) found similar evidence for a Preboreal/Boreal readvance of mountain glaciers in the southwestern part of southern Norway. There, maximum and minimum

34

NESJE

AND

DAHL

FIG. 10. Schematic glacier configuration explaining the difference in the AAR-calculated lowering between Midtdalsbreen (A) and BlGsen (B) during the Little Ice Age.

dates of 9280 -t 80 (T-3489A) and 8940 -+ 200 (T-3486) yr B.P., respectively, give an age constraint for this readvance similar to that in the Jostedalsbreen region, suggesting a regional glacier readvance in western Norway at that time. In general, little is known about the extent of glaciers during the middle Holocene (ca. 8000-2500 yr B.P.) because late Holocene and especially the Little Ice Age glacier advance(s) either destroyed or buried the evidence of earlier and less-extensive glacier fluctuations. The early and middle Holocene were, in general, times of glacier retreat and warmer climate (Davis and Os-

17w

1750 0

100

200

Dstance

300

from glacer

400

500

600

front A.D. 1961 tm)

FIG. 11. Mean rates of marginal retreat of the northwestern part of BlPisen inferred from lichenometritally dated marginal moraines (Andersen and Sollid, 1971).

ELA-

born, 1988). However, there has been a considerable discussion about the extent of alpine glaciers during the early to middle Holocene (e.g., Karlen, 1973, 1988; Heuberger, 1974; Beget, 1983; Porter and Orombelli, 1985; Riithlisberger, 1987; Dugmore, 1989). Unit G, dated to about 7730 + 100 (T-8205), is most likely fluvial/colluvial sand, indicating a humid episode close to that time. The lack of fine-grained, glacially eroded rock flour which is typical for Units A, C, E, and I shows, according to our interpretation, that Unit G does not represent a glacial readvance of Blaisen beyond the local water divide ca. 5&100 m beyond the present marginal position. The bluish-grey sand/silt of Unit E in the dated section shows that a short-lived glacier readvance occurred about 7590 2 120 yr B.P. (65606240 B.C., T-8678). Karlen (1976, 1981b) found evidence in lake sediments in northern Scandinavia that led him to infer a glacier advance at approximately 7500 yr B.P. The dates from the lake Vuolep Allakasjaure place the advance between 7765 + 100 (St-5290) and 7210 -+ 100 yr B.P. (St-5291) (Karlen, 1976). In the Jostedalsbreen region (Fig. 1) the Holocene thermal optimum is inferred to have oc-

NORWEGIAN

B.P -0

I( Zhrono zones .)-

1000

Dated section

GLACIER

35

VARIATIONS

Little Ice Age glacier maximum

Local water divide

Present glacier margin

-T-6204 -T-6203

2000

/7

6000

7000

?

, --T-6676

---n==:

6000 ? ---

-T-6206

9000

lO,OO(

-

------of the inland

ice sheet

I-

12. A schematic time/distance Hardangejdkulen. FIG.

--_-_Regional deglamtion

diagram of the Holocene

cm-red between ca. 8000 and ca. 6000 yr B.P. (Kvamme, 1984, 1989; Nesje er al., in press). The most important climatic factors determining the ELA on glaciers are summer temperature and winter precipitation (e.g., Sutherland, 1984). If the general assumption of warm summers during the Atlantic Chronozone is correct, early Atlantic glacier readvance may have been caused by increased winter humidity and precipitation. Lake-level studies in southern Sweden (Digerfeldt, 1988) show a significant lake-level rise between ca. 8000 and 7000 yr B.P. We therefore infer that more humid conditions in Scandinavia were the main cause for positive mass balance and (re)adVance of some glaciers in Scandinavia around 7500 yr B.P.

glacier fluctuations

of Bl%sen,

A modern parallel to this climatic situation was experienced during the 1988/89 winter season, when the mildest winter in southern Norway, since instrumental measurements began in the early 186Os, was accompanied by extreme winter accumulation (10-15 m snow) on some glaciers in western Norway; on Hardangerjgkulen accumulation was 8 m (Haakensen, 1989). This was due to a high frequency of North Atlantic cyclones, releasing huge amounts of latent heat by the orographic effect (Det Norske Meteorologiske Institutt, 1989). The stratigraphic record at Hardangerjgkulen demonstrates that BlGsen did not advance beyond the local water divide close to the present marginal position between ca. 7600 and 1100 yr B.P. There is

36

NESJE

AND

now a general recognition that the Little Ice Age glacier advances started in the 13th century (Porter, 1986). The radiocarbon dates from the B&en foreland, however, suggest that the initial glacier advances there started even earlier, in the 9th to 1lth century (Table I), a period termed the “Medieval optimum” (Lamb, 1984; Porter, 1986), possibly as a delayed response to early Medieval climatic deterioration (Williams and Wigley, 1983; Porter, 1986). The glacier advance of Bldisen after ca. 1100 yr B.P. is more or less contemporaneous with several glacier advances in northern Scandinavia postdating 1500 to 1100 yr B.P. (550 to 970 A.D.) (Karlen, 1988). In southern Norway stratigraphic studies of palaeosols buried by glacier advances indicate restricted glacier extent during most of the Holocene prior to the Little Ice Age (Griffey and Matthews, 1978; Matthews and Dresser, 1983; Caseldine, 1983; Matthews, 1980, 1981, 1982, 1984, 1985; Ellis and Matthews, 1984; Caseldine and Matthews, 1985, 1987; Matthews et al., 1986; Harris et al., 1987). Matthews (1989) has recently presented conclusive evidence that the Little Ice Age glacier limits were not exceeded at any time since regional deglaciation ca. 9000 yr B.P. from soils buried by glacier advance during the last 500 radiocarbon yr B.P. at seven of nine investigated glaciers. Karlen (1988), however, suggested glacier expansions in Scandinavia about 5 IOO4500,3200-2800,2200-1900, and 1500-I 100 radiocarbon yr B.P. Less-extensive glacier advances were inferred at 6300,5600,2500, 940, 600-560, and 380 yr B.P. Minimal data also suggest advances close to 1050, 600, and 430 yr B.P. (Karlen, 1982). However, Mottershead et al. (1974) obtained two radiocarbon dates from the base and top of a peat exposed by recent retreat of a southeasterly valley glacier from the Jostedalsbreen ice cap (Fig. 1) showing that the glacier front terminated continuously upvalley from the site between 8083 2 100 (SRR-50) and 3855 * 55 (SRR-87) yr B.P.

DAHL

The stratigraphic records from the Jostedalsbreen region suggest that the glaciers might have contracted between ca. 3000 yr B.P. and the Little Ice Age (Nesje et al., in press). However, initiation of gelifluction processes around Jostedalsbreen dating to 3200-2800 yr B.P. (Nesje et al., 1989) and increased frequency of turbidity currents entering lake sediments at ca. 2000 yr B.P. suggest that climatic reversals occurred at those times. Stratigraphic records from the Jostedalsbreen region (Nesje et al., in press) also indicate that glacier activity was reduced during the Medieval Period and that initial glacier expansion occurred about 900 yr B.P. (ca. 1140 A.D.). This glacier expansion is also recorded in tree rings and lake sediments in northern Scandinavia (Karlen, 1984). Dates of sheared-off trees in front of Engabreen, northern Norway, of 1230 2 80 (HAR-386) and 1060 ? 80 yr B.P. (HAR385) (Worsley and Alexander, 1975) suggest an advance at the end of early Medieval time. During wastage of the central part of Omnsbreen, a small glacier lying north of Hardangerjokulen (Fig. l), an extensive area of fresh till with humus and plant remains, was exposed. Elven (1978) obtained radiocarbon dates of the plant remains ranging from 550 * 110 (T-1485) to 430 + 100 yr B.P. (T-1479). The dates relate to the development of a glacier from perennial snow fields (Elven, 1978) and suggest formation of glaciers shortly after 1400 A.D. The glaciers were much reduced in size at that time, compared to their maximum extent during the Little Ice Age. Organic material mainly consisting of Graminaea, Carex, and mosses (unidentified) were discovered by the authors on the Midtdalsbreen glacier surface about 300 m from the present glacier margin (Figs. 2 and 3). The plant remains are interpreted as part of a soil transported from the base of the glacier through flow lines/shear planes to the glacier surface. The plant macrofossils were dated and gave an age of 1630 rir 60 yr

NORWEGIAN

GLACIER

B.P. (350-450 A.D.) (T-7041A), suggesting that the front of Midtdalsbreen had at least retreated to a position 300-400 m behind its present extent at that time. About 6500 yr of glacier contraction behind Blaisen’s modern terminal position between ca. 7600 and 1100 yr B.P. contrasts with the evidence of pre-Little Ice Age glacier advances of similar magnitude reported from elsewhere in Scandinavia (e.g., Karlen, 1988), although minor glacier oscillations of a contracted Blaisen may have occurred during periods of mid-Holocene glacier formation and expansion in the Jostedalsbreen area (Nesje et al., in press) and in northern Scandinavia (Karlen, 1988). The record from Blaisen is, however, in accordance with recently presented data from palaeosols on restricted glacier extent in southern Norway prior to the Little Ice Age (e.g., Matthews, 1989). Apparent discrepancies in the Scandinavian Holocene record of glacier variations may be due to dating problems of palaeosols (e.g., Matthews, 1985) and/or to different topographic and climatic effects on glaciers in different climatic regimes (maritime glaciers in western Norway in contrast to continental glaciers in eastern Norway and northern Scandinavia). SUMMARY

AND CONCLUSIONS

(1) A l-m-deep gully section 460 m beyond the Little Ice Age marginal moraines of Bllisen, Hardangerjokulen, shows alternating layers of minerogenic sediments and peat. Except Unit G, the bluish-grey sand/ silt layers unequivocally represent glaciofluvial deposition from Blhisen at the study section. Thus, the geographical/geological setting of the study site in the Blaisen foreland area represents a unique on/off signal of Holocene glacier fluctuations. (2) A glacier (re)advance from the Hardangerjokulen Plateau occurred after the wastage of the inland ice sheet, and prior to 8660 4 100 yr B.P. This is correlated with a glacier (re)advance of similar magnitude between ca. 9200 and 8900 yr

VARIATIONS

37

B.P. reported from other regions of western Norway. (3) A thin sand layer (Unit G) implies a fluvial/colluvial episode about 7730 + 100 yr B.P. (4) A thin, bluish-grey glaciofluvial sand/ silt layer 48 cm below the surface of the dated section indicates a short period of glacier readvance about 7600 yr B.P. This possibly occurred as a response to positive mass balance due to increased precipitation as recorded by high lake levels in southern Sweden (Digerfeldt, 1988). This readvance is correlated with a glacier advance between about 7700 and 7200 yr B.P. recorded in lake sediments in northern Sweden (Karlen, 1976, 1988). (5) About 23 cm of compact peat rich in macroscopic plant remains demonstrates nearly 6500 yr of glacier contraction behind its modern limits between ca. 7600 and 1100 yr B.P. This contrasts with the evidence reported elsewhere in Scandinavia of preLittle Ice Age glacier advances of magnitude similar to that at present (e.g., Karlen, 1988). The record from BlHisen is, however, in accordance with recently presented data from palaeosols on restricted glacier extent in southern Norway prior to the Little Ice Age (e.g., Matthews, 1989). Apparent discrepancies in the Scandinavian Holocene record of glacier variations may be due to dating problems of palaeosols (e.g., Matthews, 1985) and/or different topographic and climatic effects on glaciers in different climatic regimes in Scandinavia. (6) The initial Neoglacial advance of Blaisen beyond the local water divide about 50-100 m beyond the present margin of BlHisen is dated to 1130 2 70 yr B.P. (810990 A.D.), while the Medieval/Little Ice Age glacier advance postdates 1040 & 60 yr B.P. (960-1030 A.D.). These advances are probably more or less contemporaneous with significant Medieval/Little Ice Age glacier advances postdating 1500 to 1100 yr B.P. (55&970 A.D.) in northern Scandinavia (Karlen, 1988). (7) Calculations of the modem and Little

38

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Ice Age equilibrium-line altitudes on Hardangejgkulen suggest an ELA depression of ca. 130 m during the Little Ice Age glacier maximum. ACKNOWLEDGMENTS The radiocarbon datings were carried out at the Trondheim Dating Laboratory (The Norwegian Research Council for Science and the Humanities) under the guidance by Reidar Nydal and Steinar Gulliksen. We are grateful to Wibjdm Karlen, John A. Matthews, and Stephen C. Porter, who read a draft of the manuscript and whose perspective comments and suggestions aided in improving its clarity. We are also indebted to P. Thompson Davis and an anonymous reviewer for helpful comments on the manuscript. Jane Ellingsen, Ellen Irgens, and Else Lier drew the figures.

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