Andrea Coronato

The Physical Geography of Patagonia and Tierra del Fuego

Publisher Summary This chapter focuses on the physical geography of Late Cenozoic of Patagonia and Tierra del Fuego. This chapter discusses a junction of natural elements in the southern end of South America. Bedrock substratum and structure, climate, distribution of the superficial runoff, soils, and vegetation, all act together in a systematic, cause–effect relationship and form a group of homogeneous geographical spaces that are joined under first-order geological and atmospheric elements, such as tectonic plates and the general atmospheric circulation. The interaction of the wetter Pacific winds and the Andean Cordillera is the basic natural condition that determines the geographical aspects of the natural, physical environment of the southernmost end of South America. The pluvial and snow regime of the streams, the allochtonous character of the tableland streams, the development of contrasting biomes, such as jungle-like forests and rainshadow deserts, with a modest thermal regime are a consequence of the orographic barrier interaction with the air masses of the Southern Pacific anticyclonic centers and the polar front.

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.

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.

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.

The Late Cenozoic Fluvial Deposits of Argentine Patagonia

Publisher Summary This chapter focuses on the Late Cenozoic fluvial deposits of Argentine Patagonia. The study of the Patagonian fluvial deposits comprises the analysis of a very extensive group of units that have historically been known as “Rodados Patagonicos” or “Patagonian gravels.” The chapter discusses the present stage of knowledge and proposes a classification based upon genetic, spatial, and chronological criteria. The gravel mantles that extend almost parallel to the present drainage lines may be genetically related to fluvial valley processes, whereas the rest of the units must be associated with climatic fluctuations, the glacial and interglacial periods, and to modifications in base levels because of tectonic, epirogenic, eustatic, and glacioisostatic movements during the Late Cenozoic. The approach adopted in this chapter is basically genetic, allowing the differentiation of (1) fluvial valley units, (2) glaciofluvial units, and (3) piedmont units. Additionally, the use of other criteria of spatial, geomorphological, and temporal nature has permitted a better characterization of the identified morphostructural units. The marked climatic, tectonic, and geological differences existing throughout Patagonia do not encourage the use of a single genetic scheme for all these units. Evidence seems to indicate that it is necessary to develop specific models that would explain the origin and occurrence of these gravel sheets.

Geomorphological evidence of different glacial stages in the Martial cirque, Fuegian Andes, southernmost South America

Abstract The geomorphological features of a cirque partially occupied by receding ice bodies are presented as evidences of different glacial stages in the Fuegian Andes summits since the Last Glaciation (IOS 2). A glacierization scheme involving: (i) regional glacier accumulation basin; (ii) glacier individualization from a main regional; (ii) a stillstand during the general retreat and (iii) two advancing stages are suggested. Geomorphic and sedimentary units, named Martial I, II, III and IV, have been distinguished as a consequence of the glacial scheme. Unfortunately, the lack of datable materials has impeded establishment of an absolute chronology. Recognized glacier advances in the region are discussed. The palaeoclimatic framework for the surroundings, based on previous studies, is presented.

Paleoecology and paleobiogeographic patterns of mid-Holocene mollusks from the Beagle Channel (southern Tierra del Fuego, Argentina)

As the Beagle Channel lies between the Atlantic and the Pacific oceans, this is a critical region for the interpretation of faunal distributions in the Magellan Region. This work proposes a paleoenvironmental interpretation of the Holocene, inferred from malacological data. The development of different local benthic paleocommunities during the mid-Holocene is associated with the diversity of habitats inside this channel. The analysis of records of Venus antigua and Ensis macha through time and their geographic links to ecological factors showed that these taxa are typical elements of the cold-temperate seas throughout the Magellan Region, showing a similar range of distribution and paleodistribution. The presence of these taxa in life position (AMS 14C of 6,276±41 years BP) suggests that during the mid-Holocene the Beagle Channel was a path for dispersion of these taxa between the two oceans. Whether Venus antigua andEnsis macha are Pleistocene survivors or Holocene migrants is herein discussed. In the future, integrated cross-disciplinary studies will be necessary for a better understanding of the biogeographic relationships among magellanic mollusks.

Late-glacial elevated dust deposition linked to westerly wind shifts in southern South America

Atmospheric dust loadings play a crucial role in the global climate system. Southern South America is a key dust source, however, dust deposition rates remain poorly quantified since the last glacial termination (~17 kyr ago), an important timeframe to anticipate future climate changes. Here we use isotope and element geochemistry in a peat archive from Tierra del Fuego, to reconstruct atmospheric dust fluxes and associated environmental and westerly wind changes for the past 16.2 kyr. Dust depositions were elevated during the Antarctic Cold Reversal (ACR) and second half of the Younger Dryas (YD) stadial, originating from the glacial Beagle Channel valley. This increase was most probably associated with a strengthening of the westerlies during both periods as dust source areas were already available before the onset of the dust peaks and remained present throughout. Congruent with glacier advances across Patagonia, this dust record indicates an overall strengthening of the wind belt during the ACR. On the other hand, we argue that the YD dust peak is linked to strong and poleward shifted westerlies. The close interplay between dust fluxes and climatic changes demonstrates that atmospheric circulation was essential in generating and sustaining present-day interglacial conditions.