Results

89.4.1 Timing of deglaciation of the Long Range Mountains type localities based on TCN in erratics

The exposure ages of boulders in the two type localities of weathering zones show that the summits were deglaciated in the late Pleistocene. On the highest weathering zone of the Highlands of St John (Fig. 89.1), four boulders (>1.5m height, resting on bedrock or perched on cobbles) have 10Be ages of 13.8 ± 0.4, 13.1 ± 0.6, 13.6 ± 0.5 and 13.9 ± 0.6ka (±1 o AMS precision), for an average age of deglaciation of the summit of 13.6 ± 0.7ka (±2 o coefficient of variation, i.e. 5%) (Table 89.1). To the south, on Big Level between the fjords of Bakers Pond and Western Brook Pond,

104 105

Figure 89.3 26Al/10Be versus 10Be isotope plot for bedrock samples from western Newfoundland. Production rates according to Stone (2000) and interpretation according to Gosse & Phillips (2001). Uncertainties are 1 o AMS precision. Samples above the shaded field are theoretically impossible and may represent a measurement error. Samples in the field have continuous exposure although they may be gradually eroding. Samples below the field have had their exposures interrupted at least once by shielding cosmic rays, or have experienced recent plucking (episodic erosion >1m). The latter is unlikely in this case because the samples are from very weathered surfaces (based on quartz vein protrusion and other weathering characteristics). Downward triangles, zone A; diamonds, zone B; upward triangles, zone C.

104 105

Figure 89.3 26Al/10Be versus 10Be isotope plot for bedrock samples from western Newfoundland. Production rates according to Stone (2000) and interpretation according to Gosse & Phillips (2001). Uncertainties are 1 o AMS precision. Samples above the shaded field are theoretically impossible and may represent a measurement error. Samples in the field have continuous exposure although they may be gradually eroding. Samples below the field have had their exposures interrupted at least once by shielding cosmic rays, or have experienced recent plucking (episodic erosion >1m). The latter is unlikely in this case because the samples are from very weathered surfaces (based on quartz vein protrusion and other weathering characteristics). Downward triangles, zone A; diamonds, zone B; upward triangles, zone C.

only one large erratic (80cm high, but perched on a tor-like ridge) was found and dated with 10Be at 20.9 ± 2.9ka (±1 o AMS precision). Significantly, the exposure ages of these erratics place ice cover on the highland summits of western Newfoundland until after the last global glacial maximum. The perched boulder on Big Level was closer to the edge of the fjord and therefore should have become uncovered prior to the deglaciation of the centre of the summits and the adjacent fjords. The ages also agree with the timing of deglaciation in weathering zone A based on bedrock ages.

89.4.2 Concentration of TCN in bedrock surfaces in the Long Range Mountains

The TCN concentration in eight bedrock surfaces in different weathering zones in the Big Level area indicates that even under ice cover the landscapes of the highest weathering zones preserved their mature appearance. Concentrations (normalized to account for differences in production rate due to atmospheric and geomagnetic field effects) from the highest weathering zone ('C' of Grant, 1977) range from 60.8 to 104 x 103atomsg-1. Concentrations in the intermediate weathering zone ('B') range from 21.3 to 125 x 103atomsg-1. In the lowest, least weathered, zone ('A') the bedrock surface has the lowest concentration of 18.7 x 103 atoms g-1. The concentration of the erratic in weathering zone 'C' is 21 x 103 atoms g-1, which is much lower than the TCN concentrations in the adjacent bedrock. These results show that the bedrock in the lowest and intermediate weathering zones typically have lost more than 2 m of bedrock (and essentially have had their TCN clocks reset about 18ka). In contrast, the highest weathering zone has retained a memory of exposure prior to glaciation, despite ice cover as recently as 18 ka.

Table 89.1 Sample data (word format)

Sample

Elevation

Zone

Description

Normal

Age

Uncertainty

26Al/10Be

Uncertainty

ID

(m)

(Grant, 1998)

concentration (atoms g-1 qtz)

(ka)

(ka)

(atom atom-1)

(atom atom-1)

NF-92-015

680

A

Quartz vein, 7 cm W, 0cm H

18730

18.8

1.0

5.94

0.53

NF-92-009

708

B

Quartz vein 4 cm W, 2cm H

26291

26.5

1.1

6.40

0.36

NF-92-010

762

B

Pegmatite dyke, 15cm W, 2cm H

21296

21.4

1.4

5.75

0.72

NF-92-011

696

B

Granitic gneiss

125031

129

5

NF-92-008

734

C

Quartz vein, 5 cm W, 3cm H

98847

101

4

5.08

0.17

NF-92-006

760

C

Quartz vein, 2 cm H

60795

61.7

2.1

4.39

0.36

NF-92-007

760

C

Granitic gneiss erratic

20804

20.9

3.0

NF-92-003

726

C

Quartz vein, 4 cm W, 3cm H

99941

102

4

NF-92-016

641

C

Quartz vein, 3cm W, 3cm H

104233

107

4

NF-02-SJH-301

606

C

Granitic gneiss erratic

13730

13.8

0.4

NF-02-SJH-302

608

C

Granitic gneiss erratic

13019

13.1

0.6

NF-02-SJH-303

618

C

Granitic gneiss erratic

13563

13.6

0.5

NF-02-SJH-304

614

C

granitic gneiss erratic

13867

13.9

0.6

89.4.3 26Al/10Be to substantiate the previous results for western Newfoundland

The 26Al/10Be for weathering zone 'A' is 5.94 ± 0.53 (±1 o), which is close to the ratio expected for a continuously exposed surface in the late Pleistocene (ca. 6.0 ± 0.1; Gosse & Phillips, 2001) (Fig. 89.3). The intermediate weathering zone 'B' ratio is 5.75 ± 0.72, which, although lower than weathering zone 'A' as expected, is within 1 o error of the ratio for a continuously exposed surface. The ratio of a second bedrock surface in zone 'B' is 6.40 ± 0.36, which is not a possible ratio based on the production systematics of these two isotopes, and therefore must have an analytical error (Fig. 89.3). The ratios of weathering zone 'C' are substantially lower, at 4.39 ± 0.36 and 5.08 ± 0.17. Collectively, these ratios show that at least some of the bedrock surfaces within the highest weathering zone ('C') have an exposure history that was substantially interrupted by prolonged shielding (200 and 100kyr minimum total burial durations), that weathering zone 'B' was interrupted by a shorter burial duration or not at all, and weathering zone 'A' surfaces record only continuous exposure since the last glaciation.

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