Martín Iriondo

Climatic variations in the Argentine plains during the last 18,000 years

The last deglacial hemicycle was characterized by a general increase in temperature and precipitation in the region, with a few significant departures from this general trend. The present NE-SW climatic gradient was maintained throughout the entire period, except in the Upper Holocene. The following sequence of events is apparent if the present climate is taken as a reference base: 1. (a) 18,000–8500 yr B.P.: Arid and cool, with aeolian sand ad loess deposition. Patagonian fauna. Climatic isolines (temperature, precipitation, etc.) were located some 750 km northeast of their present positions. 2. (b) 8500-3500 yr B.P.: Humid subtropical, with Brazilian fauna. Pedogenesis and fluvial dynamics. Climatic limits migrated about 800/900 km southwest of their former positions. 3. (c) 3500-1000 yr B.P.: Dry subtropical; aeolian dynamics. The normal latitudinal climatic gradient was interrupted by the occurrence of an anticyclonic centre, which stabilized the climate over an area of some 1,600,000 km2. 4. (d) 1000 yr B.P.-Little Ice Age: Climate was similar to the present one over much of the plains, but the northeastern extremity was warmer. 5. (e) Little Ice Age: Climatic deterioration in the southern belt was characterized by generalized aeolian activity and migration of isolines more than 150 km to the northeast in that area. 6. (f) Present climate: 19th and 20th centuries. Subtropical, humid in the east and dry in the west.

Climatic changes in the South American plains: Records of a continent-scale oscillation

Abstract The last glacial/interglacial cycle produced a variety of climates and landscape responses in South America. The present contribution is an attempt to establish broad trends within this complex period. Two points are clear: (a) In the Cordillera de los Andes temperature variations produced advances and retreats of glaciers, which were coherent with the global climatic changes recorded in other continents. (b) The lowlands experienced a series of dry and humid periods. Humid intervals in the north of the continent (extending to latitude 10°S) are coeval with dry conditions in the south and vice versa. The broad climatic sequence is as follows: 1. Isotope stage 4 – A massive glaciation affected the whole Cordillera. Humid climate in the north. A large desert developed in the south, associated with wind-blown silts which reached a latitude of 25°S in SE Brazil. 2. Isotope Stage 3 – Mountain glaciers were of modest extent in the Andes. A humid climate with soil development and dune degradation prevailed in the south (Pampa and Chaco) while dry conditions dominated the north (Colombia and Amazonas). 3. Isotope Stage 2 – General advance of glaciers in the Andes, although smaller than in IS4. Dry and cold in the south, with extension of the Patagonian climate to the NE. Humid in Amazonas. The waning of the glaciation was accompanied by a humid environment (pedogenesis) in the Pampa and by a dry phase in the north (forest retreat in Carajas and in the Colombian Andes). Within the period 14,000 BP to 8500 BP (sensu lato Younger Dryas) a glacial advance occurred, with humid conditions in the north (pedogenesis in the Orinoco and forest readvance in Amazonas) and a dry climate characterized by strong westerly winds in the Pampa. 4. The Hypsithermal period was humid in the south and dry in the north of the continent.

Models of deposition of loess and loessoids in the upper quaternary of South America

Climatic changes occurred in South America during the Quaternary provoked deflation and deposition of large masses of silt, which formed loess and loessoid units of regional extension in several areas of the continent. An analysis of the most important Late Pleistocene and Holocene loess and loess-like deposits resulted in the identification of five types of transport and deposition of wind-blown silt. They are: 1. 1) Pampa type — Winds derived from the Patagonian ice field during the LGM transported to the NE silt and fine sand formed by frost action in the Cordillera, forming a large sand sea and a loess belt behind it. 2. 2) Chaco type — During the LGM, minor temporary streams transported frost-originated silt from the Cordillera to the low-lands in south Bolivia and NW Argentina. Dry tropical north winds deflated the sediment to the south, forming a valley-and-plain loess deposit. 3. 3) Originated in subtropical anticyclones — During the Upper Holocene, a stational anticyclonic center produced semiarid climatic conditions and dry winds in the Argentine plains and surrounding regions, eroding the Pleistocene loess and sedimenting a thin loess carpet and several sand fields over an area of 1,600,000 km2. 4. 4) Originated in trade-winds — In the Llanos del Orinoco (Colombia and Venezuela) the NE trade winds deposited a sand and loess mantle during the dry late Pleistocene. Sediments came from the Guayana Shield and from the coastal plain. 5. 5) Volcanic loessoids — Several loess-like sediment sheets, composed of partially altered volcanic ash, cover the Interandean Valley in north Ecuador.

Quaternary lakes of Argentina

Abstract The Quaternary lakes of Argentina are grouped into eight main environments: swamps and playas of the Hernandaries Formation (Middle Quaternary of northeastern Argentina) Pampean swamps; Chaco swamps; swamps and shallow lakes of the Parana system; salt lakes of the Puna; northwestern salt lakes and playas; glacial lakes of the southern Cordillera; and Patagonian closed depressions.

Patagonian dust in Antarctica

Abstract Recent advances in the knowledge of Antarctica have demonstrated that the atmospheric dust trapped in the West Antarctica glaciers during IS2, 4 and 6 has a Patagonian origin. A climatic scenario is developed in this article in order to explain such a process. Patagonia is unique, as it is the only sizable land mass located in the belt of Westerlies in the Southern Hemisphere. The Westerlies are formed by cyclonic structures, which normally elevate the air masses and aerosols to the upper troposphere. There, the phenomena of mass compensation occur with the Antarctic anticyclone, which (as any anticyclone) sinks air and aerosols to the surface. Large quantities of loose surficial sediments and atmospheric dust are typical of Patagonia and can easily be picked up by the wind. According to this scenario, which is based on the general atmospheric circulation, Australia and Southern Africa are hindered in providing aerosols to Antarctica, because those territories are located in the subtropical high-pressure belt.

A probable scenario for a dry climate in central Amazonia during the late Quaternary

Abstract A semi-quantitative and dynamic scenario is proposed for a dry climatic phase during the late Quaternary of central Amazonia. This scenario has been developed on the basis of previous studies by different authors, augmented by results of several months of field work, by using the present climate as a comparison system. The main features of that probable climate are: a general savanna vegetation nature; a strong seasonality in precipitation; annual precipitation of the order of 850–1000 mm; general circulation systems similar to the present ones; regional atmospheric systems were very weak; wet season from November to April; a dry season from May to October; a 10-fold variation in river discharge and seasonal oscillations of about 12 m (i.e. double the present value) at Obidos; minor tributaries with intermittent regime and arroyo/alluvial dynamics; trade winds stronger and drier than the present ones; little recycling of local waters in precipitation. Two unexpected conclusions became apparent. (1) The excess of humidity in the present climate of central Amazonia is not as high as it appears at first glance. (2) No more than second-order changes in the regional dynamics are needed for the occurrence of a dry climatic phase.

Last Glacial Maximum and Hypsithermal in the Southern Hemisphere

Abstract A synthesis of the two contrasting climates of the upper Quaternary in the Southern Hemisphere, based on continental geological indicators, suggests that the Antarctic Anticyclone is the most important climatic system in the hemisphere. According to glaciological and oceanographic studies, the Antarctic Anticyclone covered an area in the LGM that was double of that during the Hypsithermal. This phenomenon produced a consequent shifting of the climatic belts by approximately 10° of latitude to the north in the LGM and a migration to the south during the Hypsithermal. Oceanic and continental anticyclones were active systems in shaping climates in South America, Australasia, and Southern Africa. These structures were enhanced throughout the LGM (provoking generalized droughts), and weakened at the Hypsithermal (permitting more rains in tropical latitudes). The results presented here were obtained by integrating our own results with regional and continental syntheses of other authors. The paleoclimatic proposals are amply validated by more than 1500 dates in the four continents. Geological and geomorphological features, such as dunefields, paleosols, fluvial terraces and lake levels were the main tools employed in the work.

The Quaternary of Ecuador

Abstract Ecuador is composed of three major regions: the coast, the Cordillera and the Oriente or Amazon. In the eastern amazonic lowlands, the Quaternary begins with the deposition of the Mera/Mesa Formation, composed of coarse- and medium-grained sediments; that unit was sedimented during a dry climate, forming two large alluvial fans. An erosive episode followed, with formation of pediments at a later stage. The humid climate appeared in the Late Pleistocene, with formation of a landscape of round convex hills and narrow valleys. The last important event in the region was the deposition of a thick ash mantle in the Early Holocene. The Quaternary of the Cordillera is characterized by important volcanic activity, glacial processes and the occurrence of ‘cangahua’, a loess-like aeolian silt composed of piroclastic materials. Lahars, debris avalanches and related phenomena are common. The coastal region during the Quaternary underwent the influence of mud flows, debris avalanches and other Cordilleran phenomena, which aggradated the area (San Tadeo Formation and other units). Sea level oscillations deposited tidal sediments in several areas during the Middle Holocene, and the occurrence of El Nino produced alterations in the oceanic and coastal regime during the Late Holocene.

The origin of silt particles in the loess question

Abstract Many researchers consider loess as solely the product of glacial activity. Investigations throughout South America, however, have demonstrated that loess and loess-like sediments can be formed during geological periods when glaciation was absent. Loess deposits may be derived from volcanic action, from weathering processes, and as a result of pedogenic activity. Production of silt, regardless of the geologic process, can result in loess formation under suitable climatic conditions.

Planation Surfaces on the Paraná Basaltic Plateau, South America

SRTM data constituted a good resource for morphometrical analyses of the extensive Parana Basaltic Plateau (southern Brazil and northeastern Argentina, South America). The plateau is a stepped system of high-level surfaces separated by escarpments and with incised fluvial valleys, mainly belonging to the Upper Parana and the Upper Uruguay River basins. Palaeosurface remnants of such basins preserve attributes that have been identified in digital elevation models. Generation of hypsometric curves in six representative tributary basins of the Uruguay River basin and also in one tributary basin of the Parana River in the region permitted to identify, classify and map the main Cenozoic planation surfaces of the basaltic plateau. Other morphometric parameters such as longitudinal profiles and isobase lines were produced also to delimit such surfaces. Field geomorphological analyses were performed, also including the description of toposequences. Three groups of hypothetical hypsometric curves are deduced from proximal value sets for predicted base levels. Each mapped surface was considered between the minimum height for correlative surfaces in all of the subbasins and the minimum height of the next higher surface. Using that approach, which is based on the clustering from the modelled base level in the river mouth of the subbasins, three main palaeosurfaces were defined in northeastern Argentina. Complementarily, three intermediate or secondary surfaces also were identified based on morphometric analyses, taking into account that small flat remnants at the same level suggest that they could be remnants of a formerly extensive plain.