Vanessa Winchester

Characteristics of tide-water calving at Glaciar San Rafael, Chile

Glacial calving is a poorly understood process. This study tests the influence of local environmental variables on the magnitude and frequency distributions of calving behaviour at Glaciar San Rafael, Chile. Near the terminus of the glacier, surface speeds average 17 m d −1 in summer and calving is profuse and continual. The size, location and characteristics of over 7000 calving events were recorded during 32 d in 1991 and 1992, together with meteorological, bathymetric and oceanographic data. Mean daily calving exceeds 400 events per day and the mean calving flux is more than 2 Mm 3 d −1 . Mean annual calving speed and calving flux are about 4500 m a −1 and 2.0 km 3 a −1 , respectively. This calving speed is higher than that predicted by the established empirical relationship between tide-water calving speed and water depth. This is surprising, given the low salinity of Laguna San Rafael and that fresh-water calving speeds are commonly much lower than those in tide water. Daily patterns of calving frequency and flux correlate poorly or not at all with meteorological variables, but tidal stage may have some control over the timing of large submarine calving events. Submarine calving produced the largest bergs. However, the relatively small total flux recorded from the submerged part of the ice cliff may imply unusually rapid melt rates.

Dendrochronology and lichenometry : colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile

This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each others colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mm/year for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce.

Nineteenth- and Twentieth-Century Glacier Fluctuations and Climatic Implications in the Arco and Colonia Valleys, Hielo Patagonico Norte, Chile

Dendrochronology, lichenometry, and analysis of aerial photographs taken in 1944, 1979, and 1983 were used to date the 19th- and 20th-century fluctuations of the Arco, Colonia, and Arenales glaciers on the eastern side of the Hielo Patagónico Norte in southern Chile. This work has demonstrated that the glaciers retreated from their Little Ice Age maximum positions between 1850 and 1880, with retreat rates increasing during the 1940s and with surface thinning of at least 30 m since 1980. Comparison with the fluctuation behavior of other outlet glaciers of the icefield suggests a degree of synchrony in the timing of their variations and therefore argues for a common climatic control for these movements.

Recent oscillations of the San Quintin and San Rafael Glaciers, Patagonian Chile

Earlier reports, maps, aerial photographs, and tree-ring dates for moraines are used to investigate and compare the oscillations of the land-based San Quintin Glacier and the calving, tidewater San...

Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia

Glaciar Nef, a 164 km 2 eastern outlet of Hielo Patagonico Norte (the northern Patagonia icefield), terminates in a proglacial lake that has formed in conjunction with 20th-century glacier retreat. The terminus is inferred to be transiently afloat. A hinge-calving mechanism is proposed in which buoyant forces impose a torque on the glacier tongue, resulting in the release of coherent sections of the glacier tongue as “tabular” icebergs. A simple model shows how torque and tensile stress reach a maximum at the up-glacier limit of the buoyant zone, and that glacier thinning causes this point to migrate up-glacier. Empirical evidence supporting this model includes elevated thermo-erosional notches ≤6.5 m above lake level, and the ubiquitous presence since 1975 of “tabular” icebergs with surface areas ≤0.3 km 2 . Flow speeds of 1.2–1.3 m d −1 were measured near the terminus in February 1998. Extrapolations from these short-term data yield a calving rate of 785–835 m a −1 and a calving flux of 232 × 10 6 m 3 a −1 or 0.2 km 3 a −1 . The calculated mean water depth at the terminus is 190 m. This calving rate is higher than at grounded temperate glaciers calving in fresh water, but is nevertheless almost an order of magnitude less than calving rates at both grounded and floating tidewater glaciers.

Lichenometric dating of slope movements, Nant Ffrancon, North Wales

Abstract Over the last 50 years, frequent debris flows on the oversteepened slopes crossed by the main A5 road in Nant Ffrancon, North Wales, have presented a significant traffic hazard. This study uses two approaches to lichenometry to date earlier debris flows. The first approach provides dating estimates based on size/age correlations of the lichen Rhizocarpon geographicum subspecies prospectans growing in two local churchyards. The correlations supplied a growth rate of 1.47 mm year−1 for the species over a 140-year period, with a delay before colonization of 18 years. Remeasurement of the same lichens after a 4.25-year interval confirmed the growth and colonization rates in the churchyards. The second lichenometric approach, based on population size frequency distributions with size increments dated using the churchyard growth rate, provided controls for the Nant Ffrancon study site, with peaks in Rhizocarpon population size frequency distributions correlating with debris flow dates recorded by Gwynedd Council Highways Department. Circumstantial evidence supporting adoption of the growth rate on the Nant Ffrancon slopes was also supplied by data from archival and meteorological sources. The lichenometric dating estimates suggest that these slopes have a history of periodic debris flows covering at least the last 110 years, with major flows occurring in the 1890s followed by further flows in the early decades of the twentieth century.

Historical fluctuations of the Gualas and Reicher Glaciers, North Patagonian Icefield, Chile

Dendrochronology was used to date historical fluctuations of the little-visited Gualas and Reicher Glaciers on the North Patagonian Icefield in southern Chile. Vegetation trimlines dating to ad 1876, 1909 and 1954 show that glacier downwasting and retreat mirrored the patterns found at the neighbouring San Rafael and San Quintin Glaciers. Intermediate stages of recession of the Gualas and Reicher Glaciers dating to the early 1920s, mid-1930s and 1960s are also reflected by similar stages in the San Rafael and San Quintin Glaciers. We propose that the common responses shown by these four glaciers argue for climatic control of their movements and that glacier dynamics in this region are principally controlled by changes in precipitation rather than temperature since annual mean temperatures at Cabo Raper, the nearest meteorological station, have shown no trend since the early 1940s whereas precipitation, peaking in the winter months, has varied widely since the 1920s.

Recent Retreat Glaciar Nef, Chilean Patagonia, Dated by Lichenometry and Dendrochronology

This paper presents the results of a lichenometric and dendrochronological study of the recent retreat history of Glaciar Nef, an eastern outlet glacier of the Hielo Patagónico Norte. A 600-yr tree regeneration time, based on maximum tree age in the ancient forest, suggests that the forest-clad lateral moraines in the valley, southeast of the 19th century terminal moraine system, were formed some time before A.D. 1370. Dating estimates suggest that retreat from a 19th century maximum began around 1863, a decade or two earlier than the date established for other glaciers in the region, with glacier thinning near the ice front averaging 1.11 m yr−1 between 1863–1881. After 1884, retreat seems to have slowed, with glacier thinning averaging 0.09 m yr−1. Lichen and tree dating suggests that the glacier had retreated approximately 500 m by 1938; this estimate is supported by an aerial photograph showing a proglacial lake just beginning to form in 1944. Recent glacier movements around the Hielo Patagónico Norte are discussed and it is concluded that the general trend of glacier retreat around the icefield, beginning in the 1860s to 1870s, is consistent with Northern Hemisphere trends.

Glaciar Leon, Chilean Patagonia: late-Holocene chronology and geomorphology

Glaciar León is a temperate, grounded outlet of the eastern North Patagonian Icefield (NPI). It terminates at an active calving margin in Lago Leones, a 10 km long proglacial lake. We take a multidisciplinary approach to its description and use ASTER imagery and clast sedimentology to describe the geomorphology of the glacier and its associated moraines. We date periods of glacier retreat over the last 2500 years using a combination of lichenometric, dendrochronological, cosmogenic and optically stimulated luminescence techniques and show that the glacier receded from a large terminal moraine complex some 2500 years ago and underwent further significant recession from nineteenth-century moraine limits. The moraine dates indicate varying retreat rates, in conjunction with significant downwasting. The bathymetry of Lago Leones is characterized by distinct ridges interpreted as moraine ridges that dissect the lake into several basins, with water depths reaching 360 m. The fluctuations of Glaciar León appear to have been controlled by the interplay between climatic forcing and calving dynamics.

Onset of rapid calving and retreat of Glaciar San Quintin, hielo patagónico norte, Southern Chile

Abstract This note records the recent rapid retreat of the terminus of Glaciar San Quintin, which drains the western edge of the Hielo Patagónico Norte (HPN) in southern Chile. In 1993, the glacier terminus was advancing strongly into vegetated ground, while from 1996 to May 2000 the glacier underwent a transition between advance and retreat. A satellite image taken in 2000 showed that the ice front was undergoing substantial retreat and calving into proglacial lakes. Our research suggests that the glacier had lost an average of 1.89 km2 a−1 in ice surface between 1996 and 2000. The total surface loss is 7.55 km2, by far the largest retreat documented for the glaciers of the HPN since 1945.