Jorge Rabassa

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.

Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

Publisher Summary This chapter overviews the Patagonian and Fuegian glaciations—starting in the Late Miocene, when the junction of global, cooler climatic conditions, and the final rise of the southern Andes enabled the formation of mountain glaciers in the area. The chapter presents the absolute chronology of the Patagonian terrestrial glacial sequences, basically dated by means of 40 Ar/ 39 Ar dating techniques on volcanic rocks associated with glacial landforms and deposits, and cosmogenic isotope dating techniques on erratic boulders and glacial erosional surfaces. In some cases, the magnetostratigraphy of glacial deposits is available, thus allowing the correlation with the Pampean continental sequences and with the global ocean record. Patagonia and Tierra del Fuego are some of the regions of the world still largely covered by ice and snow. Three major mountain ice sheets can be observed along the Patagonian and Fuegian Andes. These three ice sheets are the Northern Patagonian Ice Field, the Southern Patagonian Ice Field, and the Darwin Cordillera Ice Field. The chapter includes also the most significant information available on the glaciations of the Chilean side of the Andes, along the same latitudinal belt.

Holocene raised beaches along the northern coast of the Beagle Channel, Tierra del Fuego, Argentina

Abstract Raised beaches developed during Holocene times along the northern Beagle Channel coast. These deposits contain a rich marine fauna, especially shelled organisms such as mollusks. Four Holocene marine sites along the northern coast of the Beagle Channel have been analysed; from west to east they are: Bahia Ensenada, Playa Larga, Bahia Brown and Cutalataca airfield. They were studied in relation to relative altitude, sediment type, faunal composition and radiocarbon dates. The area is characterized by several discontinuous terraces with elevations varying from about 1.5 to 10 m above sea level. Their possible origin (1) due to tectonic uplifting, in comparison with raised beaches of the northern part of Tierra del Fuego, and (2) resulting from glacio-isostatic recovery, taking into account the ice retreat, is discussed. Seventeen species of mollusks (eight bivalves, nine gastropods) were collected from the study sites. All of them are living species from the Beagle Channel and have been interpreted as representative of Holocene paleocommunities in this channel. These species show no evidence of substantial climatic changes during the Holocene, although minor temperature fluctuations cannot be ruled out yet.

Late quaternary evolution of a subantarctic paleofjord, Tierra del Fuego

Lago Roca-Lapataia valley (54°50′S, 68°34′W) is a paleofjord that was occupied by a valley-glacier system during the glacial maximum of the late Pleistocene (estimated ca. 18–20 ka BP). Deglaciation began before 10,080 ± 270 BP. The marine fauna in several marine terraces found in the area shows that early-middle Holocene climatic conditions were basically the same as at present. Species found are characteristic of cold and shallow waters, although minor temperature fluctuations cannot be ruled out for this period. A recent radiocarbon date of 7518 ± 58 BP on Chlamys patagonica (NZ # 7730) confirms that Lago Roca was transformed into a fjord ca. 7500–8000 BP. The sea reached its maximum level of 8–10 m a.s.l. around 6000 BP and at 4000–4500 BP was at least above 6 ± 1 m a.s.l. Later, when sea level fell, Lago Roca was occupied by fresh water and was no longer tidal. The relative land-sea positions during this period are a consequence of combined eustatic and neotectonic processes.

Palaeoenvironmental conditions during the early peopling of Southernmost South America (Late Glacial-Early Holocene, 14-8 ka B.P.)

Abstract Environmental conditions in Southernmost South America (the Patagonian Andes and the extra-Andean plains) during the early peopling of the region in Late Glacial/Early Holocene times are herein described. Different glacial advances or stabilization phases in valley glaciers occurred several millennia after the Last Glacial Maximum (LGM), as a consequence of regional cold climatic episodes. According to Clapperton (1993), two advances during the Late Glacial took place, ca. 15-14 14 C ka and 12-10 14 C ka B.P. A cold event during this last period, probably correlative with the “Younger Dryas” episode in the Northern Hemisphere, has been observed in the palynological record. A tundra environment dominated until ca. 13-10 14 C ka B.P.; afterwards, a gramineous steppe with forest refuges developed. The transition steppe-forest lasted to the Middle Holocene, when the Nothofagus forest took over. The arrival of man in Southernmost South America (ca. 13 14 C ka B.P. or even before) probably took place while glaciers were still present in most Andean valleys, sea level was much lower than today, and tundra environments dominated the extra-Andean plains. Moreover, peopling in the Isla Grande de Tierra del Fuego would have occurred not later than 11.8 14 C ka B.P., when the present Magellan Straits valley was still occupied by a meltwater discharge, braided stream, fed by the receding “Magellan Glacier”.

Glaciations in Argentine Patagonia, southern South America

Publisher Summary This chapter reviews that the reconstruction of the southern hemisphere glaciations in Argentine Patagonia is favoured by their excellent geomorphological and stratigraphical records that are being progressively studied. Along the Andean Cordillera, 13–14 glacial advances have been identified from the latest Late Miocene to the Late Glacial. The chapter discusses seven or eight tills of the Cerro del Fraile taken as individual advances. The chapter demonstrates the occurrence of several Pliocene glaciations. These glaciations may have been relatively coeval at different latitudes. The most extensive glaciation, known as the “Great Patagonian Glaciation” (GPG), has been chronologically constrained by 40 Ar/ 39 Ar and K/Ar dates in the Lago Buenos Aires valley and the Rio Gallegos basin. The later glaciations have been developed at least during three main episodes recognised throughout the entire southern Patagonian region and at least two in all the glacierization regions. One of the major problems to be solved is whether the morainic arcs, younger than the Last Glacial Maximum, correspond to the minor glacier advances or to the stabilisation phases of general ice recession during the late glacial. The radiometric dating confirms that Late Miocene and Pliocene glaciations, identified at several sites along the Patagonian Andean Cordillera, suggests the full glacierization of the Antarctic Peninsula occurred in the Miocene. It also indicates that the establishment of the circumpolar Antarctic convergence had an almost immediate climatic response in the Patagonian environments. Further studies are required to complete this model and to refine the absolute chronology of this extended land based glacial chronology.

40Ar/39Ar chronology of late pliocene and early pleistocene geomagnetic and glacial events in southern Argentina

K-Ar dating and paleomagnetic directions from the lava sequence atop Cerro del Fraile, Argentina, contributed to the nascent Geomagnetic Polarity Time Scale (GPTS), recording the Reunion event, and the Olduvai and Jaramillo subchrons [Fleck et al., 1972]. New stratigraphy, paleomagnetic analyses, 40 Ar/ 39 Ar incremental heating ages, and unspiked K-Ar dating of 10 lava flows on Cerro del Fraile place these eruptions between 2.181±0.097 and 1.073±0.036 Ma and enhance this unique record, which includes seven tills interbedded with the lavas. The Reunion event is recorded by three lavas with transitional, normal, and reversed polarity that yielded identical 40 Ar/ 39 Ar isochron ages and a weighted mean age of 2.136±0.019 Ma. When combined with 40 Ar/ 39 Ar ages from lavas on Reunion Island and a normal tuff in the Massif Central, the age of the Reunion event is 2.137±0.016 Ma and is older by ∼50 kyr than the 2.086±0.016 Ma Huckleberry Ridge event. The onset and termination of the Olduvai are similarly constrained to 1.922±0.066 Ma and 1.775±0.015 Ma, whereas the onset of the Jaramillo occurred 1.069±0.011 Ma. A discordant age spectrum from another transitional lava gave a total fusion age of 1.61 Ma and an unspiked K-Ar age of 1.43 Ma. It is uncertain whether this corresponds to the Gilsa, Gardar, Stage 54, or Sangiran events, or represents an unrecognized period of geomagnetic instability. Deposition of till on the piedmont surface prior to 2.186 Ma and six subsequent tills between 2.186 Ma and ∼1.073 Ma mark frequent glaciations of southern South America during marine oxygen isotope stages 82 to 48.

Glaciations in the Magellan Straits and Tierra del Fuego, southernmost South America

Publisher Summary This chapter reviews that the landscape morphology of the southernmost end of South America shows many erosional and depositional features of glacial origin. Many of these features are related to ancient glaciations, although most of them represent the more recent Quaternary glacial events. In all cases, the glaciers that formed this landscape were derived from the mountain ice sheet of the Darwin Cordillera, from which ice tongues emerged in all directions. The genesis of the various ice lobes depended upon the ice thickness in each glacial episode and the underlying superficial relief. According to the regional morphology and stratigraphy, the formation of the different ice lobes would have been physically defined from the older to the younger glaciations in a progressive succession. As indicated in the map, the Sierra de los Frailes glaciation would have expanded over a vast extension, between the Rio Gallegos valley and Central Tierra del Fuego. The existence of an ancient glaciation, named the Rio Grande Glaciation, has been interpreted from the observation of scattered erratic boulders along the Rio Grande valley and neighbouring areas and more, a few tens of kilometres further north in the Rio Chico valley. This area belongs to a large basin that drains the western and central portions of Tierra del Fuego toward the Atlantic ocean. Moreover, this area received the outwash discharge of the glaciers occupying the Carmen Sylva and Beauvoir ranges during the Pleistocene. The region adjacent to the Atlantic coast in southeastern Tierra del Fuego is shown in the map as covered by “undifferentiated drifts” because the till deposits observed in various positions along the mountain valleys have not been studied in detail.

Fuegian floating tree-ring chronology from subfossil Nothofagus wood

The discovery of subfossil Nothofagus wood buried in several peat bogs of Argentine Tierra del Fuego is reported. A provisional, fragmented-floating tree-ring width chronology covering the last 1400 years has been obtained. Statistical and spectral analysis revealed no significant differences between the present and floating Tierra del Fuego tree-ring chronologies. Some cyclical growth peculiarities found in both modern and floating chronologies are discussed.

Chapter 51 - Pleistocene Glaciations in Southern Patagonia and Tierra del Fuego

Abstract Geomorphological evidence of Quaternary glaciations that developed along the larger glacial valleys of the Eastern side of the Andes in Southern Patagonia and the Fuegian Archipelago, between 47°08′S and 54°55′S, are herein presented. The boundaries of the glacial advances that took place during the Early, Middle and Late Pleistocene, including the Last Glacial Maximum and Late Glacial moraines, have been indicated.