Chalmers M. Clapperton

Quaternary glaciations of the southern Andes

Abstract The southern Andes comprise the southernmost portion of the Andean Cordillera, beginning at the edge of the Puna Altiplano (lat.27°S) and ending at Isla de los Estados (lat.55°S). The late Cainozoic glacial record of these mountains spans the interval from the Late Miocene to the present and is one of the most complete to be found anywhere in the world. This has arisen for several reasons: (i) the conterminous mountain ice cap extended to the piedmont zone on both flanks of the range, where the sedimentary and morphological record has been well preserved; (ii) periodic volcanism, mainly from monogenetic fissure eruptions of basalt east of the range and from central tephra-producing cones along the mountain crest, has provided opportunity for the preservation and radiometric dating of interbedded glacial deposits; (iii) a tectonically-induced interval of stream incision in the Mid Pleistocene and simultaneous uplift has preserved glacial sediments on interfluves; (iv) in the Chilean lakes region west of the mountains, Late Quaternary glaciers terminated in a well-vegetated landscape, thus creating scope for radiocarbon dating of interbedded and incorporated organic materials; consequently, the last glaciation in the Llanquihue area of Chile is one of the best dated sequences in South America; thus the ‘Llanquihue’ Glaciation is proposed as the South American equivalent of the ‘Wisconsin’ and ‘Weichsel’ glaciations of North America and north west Europe respectively.

Nature of environmental changes in South America at the Last Glacial Maximum

Abstract A review is given of geomorphological evidence suggesting the nature of environmental changes in South America during the Last Glaciation Maximum (LGM). The data are from glacial, geocryogenic, alluvial, colluvial and aeolian systems and span 68° of latitude, making the most complete land transect of major climatic systems in the Southern Hemisphere. Glaciers in the northern and central Andes may have reached their maximal extent by 27,000 yr B.P.; reduced precipitation at the LGM, caused by lower temperatures and lower atmospheric humidity, probably led to slight glacier recession in the tropical Andes. Icefields in the Southern Andes were most expansive when global temperature and sea level were lowest, because Pacific westerlies supplied abundant moisture, and western outlets advancing across the continental shelf were constrained by sea level. Low-gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the LGM. “Draw-down” of water-tables possibly impacted the forest cover, enhancing the drying influence of reduced sea surface temperature and atmospheric humidity. As forest and grass covers diminished in extent, effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation, colluvial and aeolian processes became more active and widespread. Almost 25% of the continent is covered with palaeo-aeolian features, presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the LGM. Substantial changes in precipitation totals and incidence, and in ground-water availability, probably eliminated modern-type tropical rainforest except in areas currently receiving ca. 5000 mm of annual rainfall. Equivalent changes in vegetation cover spread across much of the Guyana and Brazilian highlands and throughout the southern temperate regions.

Holocene glacier fluctuations in South America and Antarctica

Abstract Glacial geologic evidence indicates that glaciers throughout the Andes and Antarctica fluctuated during the Holocene. Radiocarbon dating and other age determinations suggest that glaciers readvanced significantly only during the last 5 ka, reaching positions from several 100 m to a few kilometres beyond their present limits. In South America tenuous evidence from radiocarbon dates, with dendrochronological data and environmental interpretations from pollen analyses indicate four main periods of Neoglacial advance, culminating 5000-4000 BP, 3000-2000 BP, 1300-1000 BP, and 15th-late 19th centuries; smaller advances may have occurred at ca. 8400 BP, ca. 7500 BP, and ca. 6300 BP. The meagre data are consistent in indicating broad synchrony throughout the Andes during the last 5 ka, suggesting response to global climatic changes. Anomalies exist in Patagonia where some tide-water glaciers reached their maximal Holocene limits recently this century. The broad spectrum of differing Antarctic environments produces interesting contrasts. Some local glaciers in the McMurdo Sound area of the East Artarctic continent are more extensive now than during the global glacial maximum ca. 18 ka BP, when they were starved of precipitation. Consistent agreement among 14 radiocarbon dates from the South Shetland Islands indicates two main Holocene glacier advances, the most extensive (2–3 km) peaking in the 12th century, the other culminating in the 18th–19th century. Glaciers in South Georgia reached their most advanced Holocene limits before 2200 BP. Moraine Fjord glacier, which culminated as a 6 km advance between 1460–1700 radiocarbon BP, may have lagged 400–650 years behind the climatic forcing because it could only advance in its deep-water fjord by building a moraine bank. Smaller advances in South Georgia culminated in the 17th–19th centuries and during the 1920s-30s. There is no firm evidence of glacier advances before 3 ka BP in the Southern Ocean-sub-Antarctic domain, but broad synchrony in glacier advances during the last ca. 3000 years appears to have occurred throughout the Andes-Antarctic transect. Caution is required in the interpretation and correlation of moraines associated with calving glaciers and of those with poorly constrained dating.

Late quaternary glacial history of George VI Sound area, West Antarctica

Abstract During the last glacial maximum in West Antarctica separate ice caps developed on Alexander Island and on Palmer Land, became confluent in George VI Sound, and discharged northward from latitude 72° S. Radiocarbon (>32,000 yr) and amino acid (approximately 120,000 yr) age determinations on shell fragments ( Hiatella solida ) found in basal till suggest a Wisconsin age for the glaciation that incorporated them. The pattern of ice flow differed from that deduced for this area in the CLIMAP reconstruction. Following the maximum stage, there was a stadial event when outlet valley glaciers flowed from smaller ice caps into George VI Sound. More widespread recession permitted the George VI ice shelf to deposit Palmer Land erratics on eastern Alexander Island before isostatic recovery raised them to final elevations of about 82 m. The ice shelf may have been absent at about 6500 yr B.P., when large barnacles ( Bathylasma corolliforme ) were living in the sound. Small glaciers readvanced to form at least two terminal moraines before the ice shelf re-formed and incorporated the barnacle shells into its moraine on Alexander Island. The shells gave a 14 C age (corrected for Antarctic conditions) of about 6500 yr B.P. and an amino acid ratio consistent with a Holocene age. Valley glaciers readvanced over the ice-shelf moraine before oscillations of both valley glaciers and the ice shelf led to the formation of the present sequence of contiguous ice-cored moraines, probably during the Little Ice Age. Such oscillations may represent a climatic control not yet observed in the dry valleys of Victoria Land, the only other part of Antarctica studied in detail for glacier fluctuations.

Quaternary Glaciations in the northern Andes (Venezuela, Colombia and Ecuador)

Abstract The nature and distribution of glacial drift in the high Andes of Venezuela, Colombia and Ecuador suggest that deposits of only the last two Quaternary glaciations (Isotope Stages 6 and 4 + 2) are present. The Penultimate Glaciation in Ecuador may be represented by deeply weathered drift that is beyond the range of radiocarbon dating; in Colombia and Venezuela subdued moraines covered with loess may be of equivalent age. In places, this drift extends 1000 m lower in altitude than the limits of the Last Glaciation, which comonly reach 3400–3600 m. Radiocarbon dated organic deposits (33->43 ka BP) lying between tills composing superposed terminal moraines suggest that Andean glaciers may have been as extensive during Isotope Stage 4 as during Isotope Stage 2. A relatively long interstadial, from >43->33 ka BP preceded the glacier advance that culminated during the interval ca. 28-15 ka BP. Two late-glacial stadials are marked by moraine complexes at 3900–4200 m altitude; they are partly constrained by radiocarbon ages of >12.9 ka BP and 12-10 ka BP. Moraines apparently deposited during Neoglacial advances of the last ca. 5 ka lie in front of most of the glaciers remaining in the northern Andes.

Quaternary glaciations in the southern hemisphere: An overview

Abstract Large glacier systems in Antarctica have existed at least since the Early Miocene. Glaciers in the Patagonian Andes advanced in the Late Miocene and fluctuated several times during the Pliocene, as did ice masses elsewhere in the southern hemisphere. Sedimentological evidence of glaciation during the Early and Middle Quarternary is best preserved in volcanic regions (Patagonia, New Guinea. Africa), but is poorly constrained by dating. There is evidence of glaciation during Oxygen Isotope Stages 10, 8 and 6 in several parts of the southern hemisphere, but the only well dated advance of the Middle Quarternary is that of Stage 6 in the Transantarctic Mountains. During the last glaciation montane glaciers in parts of the southern hemisphere were as large during Isotope Stage 4 as during Isotope Stage 2; the last glaciation maximum (Stage 2) is well dated in Tasmania, New Zealand and Patagonia, culminating as 21-18 ka BP. Following slight recession from the LGM limits, late-glacial advances peaked at 15-14 ka BP and at 12-10 ka BP. Major Holocene advances occurred on at least three occasions during the last 5 ka in areas presently glacier-covered.

Late-glacial and Holocene glacier fluctuations and environmental change on South Georgia, Southern Ocean

South Georgia provides a terrestrial record of postglacial environmental change from a largely oceanic zone of the Earth. The record is representative of the southern westerlies and provides a link between Antarctica and the temperate zones of southern South America. Evidence from glacial geomorphology, slope stratigraphy, and analyses of environmental indicators in peat and lake cores is used to interpret this record. Wastage of the full-glacial ice cap was interrupted by a late-glacial stade of the outlet and valley glaciers before ca. 10,000 yr B.P. Plant growth had begun at low altitude (<50 m) on the sheltered (lee side) northeast coast within the late-glacial moraine limits by 9700 yr B.P. Environmental conditions on slopes above 80 m probably were too rigorous for a stable vegetation cover until ca. 6400 yr B.P. This was followed by a period from 5600 to 4800 yr B.P. when conditions were warmer than at present by up to 0.6°C. Periods of climatic cooling occurred at ca. 4800-3800 yr B.P., ca. 3400-1800 yr B.P., and within the last 1400 yr. The most extensive Holocene advance of South Georgia glaciers culminated just before 2200 yr B.P. These Holocene temperature changes of between 0.5 and 1.0°C are comparable in scale and timing to those identified from recent analyses of Vostok ice cores from the Antarctic ice sheet.

Broad synchrony of a Late-glacial glacier advance and the highstand of palaeolake Tauca in the Bolivian Altiplano

The morphology and sedimentology of glacially influenced fan-deltas on massifs at the margin of the southern Altiplano, Bolivia, suggest a broadly synchronous expansion of glaciers and palaeolake Tauca during the late-glacial interval. This is shown by sedimentary successions of glacigenic, glacifluvial and glacideltaic facies linking palaeoglaciers with palaeolake Tauca on the flanks of Cerro Azanaques and Cerro Tunupa at altitudes of 3770–3720 m. Radiocarbon dates from peat overlain by glacial diamict and glacifluvial outwash indicate that glaciers in this area reached their last glacial maximum extent after ca. 13 300 14 C yr BP. Glacifluvial fan-deltas contiguous with the moraines confirm that the advance coincided with a highstand of palaeolake Tauca radiocarbon dated to the interval ca. 13 500–11 500 yr BP. Modeling of climatic controls on this glacier advance suggests the primary forcing was increased summer (wet season) moisture, possibly amounting to 600 mm above the modern values of 200–400 mm. Greater cloud cover probably depressed local temperatures and reduced the evaporation rate. The consequent rise in effective annual moisture ( P  −  E ) comfortably accommodates a palaeolake 48–50 × 10 3  km 2 in area and up to 100 m deep in the southern Altiplano. Because the palaeoglacier equilibrium-line altitudes rose toward the south and west, like the gradient of modern precipitation totals, we conclude that the increased late-glacial moisture was brought by weather systems similar to those of the present, but that atmospheric conditions were cloudier and cooler.

Quaternary glaciations in the Southern Ocean and Antarctic peninsula area

Abstract There are three main difficulties in constructing detailed time series for Late Quaternary glacier fluctuations in the Southern Ocean-sub-Antarctic region: sea level control on ice extent, differential tectonics and lack of material for radiometric dating. South of 60°S, the glacial Equilibrium Line Altitude is low enough for glaciers to expand without a decrease in temperature, if sea level falls. Thus most tidewater glaciers in this region are not reliable indicators of small scale fluctuations in climate. Tectonic uplift during the Quaternary may explain why the Falkland Islands did not develop most of their glacial and nivoglacial features until the last glaciation. The South Shetland Islands have a unique assemblage of raised marine features in the sub-Antarctic, possibly because the crustal block overlies a zone of magmatism and may respond sensitively to isostatic changes imposed by fluctuating ice masses. Despite the lack of vegetation, some radiometric dates have been obtained from peat, seaweed and fossil remains of marine shells and bones associated with glacial and raised beach deposits. Together with relative weathering criteria and drift distribution, these suggestthat glaciers in the Southern Ocean and sub-Antarctic region have fluctuated synchronously with glaciers elsewhere in the southern hemisphere during the last 100 ka. The last glaciation maximum culminated after 26 ka BP and glacier advances are inferred for the late-glacial intervals (15-14 ka and 12-10 ka BP) and the Neoglacial interval (last 5 ka).

Modeling the growth and decay of the Antarctic Peninsula Ice Sheet

A model of the growth and decay of the Antarctic Peninsula Ice Sheet during the last glacial/ interglacial cycle is used to identify the main controls on ice sheet behavior. Using as input glaciological assumptions derived by W. F. Budd and I. N. Smith (1982, Annals of Glaciology 3, 4249), bedrock topography, isostatic compensation, and mass balance relationships, the model is driven by sea-level change over the last 40,000 yr in association with assumed changes in the rate of melting beneath ice shelves. An ice sheet dome over 3.5 km thick grows on the offshore shelf and straits west of the Antarctic Peninsula and reaches a maximum at 18,000 yr B.P. Collapse begins at 14,000 yr B.P. but becomes rapid and continuous after 10,000 yr B.P. The present stable ice cover is achieved at 6500 yr B.P. Ice growth and decay are characterized by thresholds which separate periods of steady state from periods of rapid transition; the thresholds usually relate to topography. Tests show that ice sheet behavior is most sensitive to sea-level change, basal marine melting, and accumulation and is less sensitive to isostasy, spatial variation in accumulation, calving rates, and ice flow parameterization. Tests of the model against field evidence show good agreement in places, as well as discrepancies which require further work. o