The Little Ice Age on James Ross Island | Methods | Moraine morphology and sedimentology | Glacier extent, surface and volume changesy | Interpretation | Summary and conclusions | References | Comments |
The Late Holocene on James Ross Island
Although the Antarctic Peninsula is a region of rapid warming, the spatial and temporal pattern in this warming is complex. The wide range of glacial types across the Antarctic Peninsula has resulted in a range of responses.
The response of land-terminating glaciers across the Antarctic Peninsula is particularly interesting, because land-terminating glaciers respond in a linear fashion to changes in temperature and precipitation.
Land-terminating glaciers on James Ross Island and nearby land have been observed to be shrinking[2-4], and this has resulted in several campaigns to monitor long-term glacier mass balance in the region[5, 6]. Studies of glaciers are limited to either a short temporal scale (era of satellite observations) or are limited to small numbers of glaciers (field-based measurements).
Aims and Objectives
It is important to characterise the centennial-scale behaviour of small land-terminating glaciers in this region, in order to understand these short-term variations. This study of glaciers on James Ross Island provides a longer-term view and broader perspective of glacier character and behaviour on the northern Antarctic Peninsula during the Late Holocene, a time possibly synchronous with the northern Hemisphere’s Little Ice Age.
Over a 7-week field season in January-March 2011, Jonathan Carrivick, Bethan Davies and Neil Glasser investigated prominent moraines in front of small land-terminating glaciers. Our objectives were,
- Holistic geological descriptions of the topography, sedimentology and geomorphology of the prominent ice-cored moraines;
- Interpretation of the character and behaviour of those glaciers while they were at this relatively advanced position, and;
- Quantification of the geometric changes to these glaciers during the Late Holocene.
James Ross Island is on the NE tip of the Antarctic Peninsula, and the Ulu Peninsula on James Ross Island is one of the largest ice-free areas in Antarctica. The Ulu Peninsula comprises large areas of Cretaceous sandstone and mudstone, overlain by multiple layers of basalt and hyaloclastite.
Radiocarbon dates on organic remains on James Ross Island suggest that the Ulu Peninsula became ice-free following the Last Glacial Maximum by around 7500 years ago, with a glacial readvance that finished around 4700 years ago. Ulu Peninsula has several small cirque glaciers with pronounced ice-cored moraines, which relate to a more recent glacier readvance.
We investigated six glaciers on Ulu Peninsula: Unnamed Glacier, Triangular Glacier, IJR-45, Alpha Glacier and San José Glacier. A differential GPS (dGPS) Leica GPS500 was used in realtime kinematic mode for topographical surveys and to precisely determine the location and elevation of glacier margins, glacier snout positions and moraine crests. Glaciological structures were mapped from aerial photographs (taken in 2006); structures mapped include stratification, crevasses, streams and supraglacial debris. The landscape position of the moraines, their surface character, planform and longitudinal profile was mapped in the field using dGPS. Sedimentological analysis of sections of sediment and ice provided information on depositional history, style and environment.
Late Holocene glacier extent was interpolated from the mapped innermost moraine crests in a Geographical Information System (GIS). This gave a minimum height for the moraines and a palaeo ice-surface elevation.
Moraine morphology and sedimentology
The moraines in front of the glaciers were typically 25-40m higher in elevation than the modern glacier surface, and all the moraines are ice-cored.
Most of the moraines have multiple crests, particularly near the glacier snout, and all have a hummocky topography and slides sloping at 30° to 40°. The moraines have a complex topography, with a hummocky ridge crest, shallow surface depressions (some with ponded lakes) and numerous surficial mass movements. Sediment thickness over the ice core varies from 1-2 m.
Glacier ice exposed in the ice-cored moraines is well bedded with layers of clean white bubbly ice, ice with debris content (dispersed, laminated and stratified), and clear, massive, bubble-free blue ice with large crystals with dispersed (occasional) cobbles, pebbles or small boulders. The layers dip at 40° down glacier.
At the terminal moraines, rounded and striated boulders can be found. In contrast, the lateral moraines are formed of angular material that is very similar in composition to the scree slopes surrounding the glacier.
The form of the moraines mirrors the glaciological structures mapped (especially the stratification).
Glacier extent, surface and volume changes
All six of the glaciers investigated have undergone significant decreases in glacier extent and elevation since their maximum (when they deposited these moraines). The decrease is a function of snout recession and ice-margin surface lowering to within the lateral moraine margins. Glacier snout retreat varies between 75 m at Triangular Glacier to 130 m at San José Glacier.
Surface lowering decreases in magnitude with increasing altitude up to the maximum elevation of the moraine crests. There is a distinct east-west gradient in surface lowering across the glaciers, with greater surface lowering on the western parts of the glaciers relative to the eastern parts. This difference can be up to 20 m, as on Unnamed and Triangular glaciers. The pattern is least pronounced on IJR-45.
Triangular, Unnamed and San José glaciers have lost 20-30% of their surface area since their most recent maximum, while IJR-45 and Alpha Glacier have receded by 12-15%. The land-terminating glaciers have had a volume reduction of 0.01-0.03 km3, with a combined total of 0.1 km3. Westward-facing glaciers (Unnamed, Triangular and Lachman) have lost less volume for their relative area than San José, Alpha and IJR-45.
The glacier ice exposed in the moraine is interpreted as basal glacier ice formed by adfreezing near the glacial margin[7-9]. The basal ice facies becomes interbedded with the clear blue basal ice by shearing and thrusting. This produces stacked basal, englacial, supraglacial and proglacial ice. Debris laminations were formed through the attenuation of debris and interstitial ice (the ice frozen around the debris) at the boundary with clean glacier ice. Folding occurs due to ice creep, particularly around larger obstacles.
Subglacial and englacial thrusting has therefore entrained sediment, which melts out at the surface to bury the ice core and produce the present-day surface sediment veneer. Shearing and thrusting of basal ice is commonly associated with polythermal glaciers[10-12]. These processes produce the arcuate belts of aligned ridges and superimposed minor moraine ridges. The orientation of the thrusting closely mirrors the angle of dip and dip direction observed in the stratified glacier ice in the moraines; this closely controls moraine morphology and surface slopes, and also moraine height, width and character.
The sediment drape, particularly at the terminal moraines, has melted out from the debris within the ice-cored moraines. Once the ice core has melted, there will be little geomorphological expression of the moraine. They therefore have a limited preservation potential. This may explain the lack of other prominent moraines on Ulu Peninsula. Sediment redistribution by mass flows and mass movements during melting further limits preservation potential. These processes have been observed in other high-latitude environments[14, 15].
These are ‘controlled moraines’; i.e., the moraine morphology is controlled by englacial structures. The moraine form is specifically controlled by stratification, which is produced by marginal shear and by the melt out of debris-rich basal ice.
We therefore conclude that these moraines record the advance of polythermal glaciers, because the englacial and proglacial thrusting and stacking and thick deformed basal ice is indicative of a compressive regime near the snout of the glacier[11, 12, 15].
Past glacier extent and volume changes
All glaciers have retreated, and retreat is most evident at the glacier snouts, although retreat is also observed at the lateral margins. The magnitude of surface lowering decreases with altitude, and it is therefore likely to have been driven by air temperature.
The evolution of surface lowering and the observed east-west gradients reflects the importance of wind-blown snow. Enhanced accumulation on the eastern parts is a product of orographically enhanced snow drifting by the prevailing westerly/southwesterly winds, which are typical of this region. A change in the dominance of controlling factors could be a cause and a symptom of a changing mass balance.
The glaciers previously were thicker, more extensive and had a convex long-profile. They now have an asymmetric surface morphology and a linear slope long profile, and an absence of modern, active moraines or crevasses. These factors would imply a modern negative mass balance. Finally, the present lack of dynamism or movement is due to glacier stagnation and a transition to a cold-based thermal regime, as reported for other glaciers on James Ross Island [cf. 12]. We therefore surmise that there has been a transition from a polythermal to a cold-based glacier thermal regime.
Rate of glaciological change
There are no absolute dates available for the age of these moraines. Dating them is difficult due to the lack of organic matter for radiocarbon dating, and because of the difficulties in cosmogenic nuclide dating of basalt boulders of such a young age. However, the moraines are much younger and fresher than the moraine in Brandy Bay, dated to 4500 years old by radiocarbon dating[18, 19]. These moraines are most likely to date from a Neoglacial readvance 700-1000 years ago, broadly synchronous with the early stages of a Little Ice Age, which has been postulated but undated for James Ross Island and from around the Antarctic Peninsula[21, 22]. This would produce mean snout recession rates of 0.17-0.1 meters per annum (m a-1), mean surface lowering of 0.03-0.02 m a0.02 m a-1 and mean areal decline of 0.03% m a-1. These rates are much lower than those calculated for the glaciers of James Ross Island from 1995 onwards[2, 23]. Recent work has shown a mean annual surface lowering of 0.79 m a-1 of IJR-45m a-1 between 1979-2006.
Summary and conclusions
In summary, the combined topographical, sedimentological and geomorphological measurements and observations of glaciers on Ulu Peninsula, James Ross Island describe the first meso-scale changes in the character and behaviour of land-terminating glaciers in the Antarctic Peninsula region. They have retreated 75-120 m each, and glacier surfaces have lowered 9-23 m on average since a Late Holocene readvance. These changes are relatively uniform across the glaciers, but there is a west-east gradient in these changes that may be due to wind-blown snow.
These Late-Holocene moraines are ice-cored, and reflect the importance of thrusting and shearing when the moraines were formed. The composition of the moraines and the ice surface reconstructed from them indicates that the glaciers were polythermal during the Late Holocene, and that the glaciers were more dynamic than at present. The glaciers are now cold based, and down wasting in situ. Comparison of reconstructed past glacier dynamics with the present glaciers permits speculation that glacier shrinkage, caused by warming temperatures, resulted in a transition from a polythermal to a cold-based thermal regime. Land-terminating glaciers on James Ross Island are now cooler despite a warmer climate.
Was the Little Ice Age a global phenomenon? Increasing evidence of a LIA in Antarctica seems to suggest it was so.
Please refer to the original article and cite as:Jonathan L. CARRIVICK, Bethan J. DAVIES, Neil F. GLASSER, Daniel NÝVLT, Michael J. HAMBREY, 2012. Late Holocene changes in character and behaviour of land-terminating glaciers on James Ross Island, Antarctica. Journal of Glaciology 58 (212).
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