tings. Pioneering contributions on Pampean loess composition and distribution came from one key locality (Mar del Plata; see Figs. 4a and 4b) on the coast of the southern Pampas, from which results were extrapolated to the rest of Argentina (Teruggi et al., 1957; Terug-gi, 1957). Second, inadequate chronological control, based on few ages, hinders stratigraphic correlation of eolian records between different areas of the Pampas, making even the present interpretations tentative and speculative.

12.2.1. Full Glacial and Late Glacial Times

Iriondo (1990a, 1997) proposed a regional model of eolian deposition that he named the Pampean Eolian System. The eolian source areas are thought to be the floodplains and fans of the combined fluvial system of the Bermejo, Salado, Desaguadero, and Colorado Rivers (Fig. 4a), which drained the glaciated Andes Mountains from approximately 28°-38°S latitude. A minor amount of material was supplied locally from the mountain ranges in central Argentina (Córdoba and

San Luis). Volcanic ashfalls were a secondary source of eolian particles transported in high suspension, which came from volcanoes south of 38°S (southern Mendoza and Neuquén; Fig. 4a) and from volcanoes from the Puna region (Bloom, 1990), and were deposited in the extensive Chaco-Pampean plains.

The initiation of late Quaternary eolian deposition has not been extensively dated. Traditionally, the latest episodes of major deposition of the loess and sandy eo-lian facies have been related to the LGM (Groeber, 1936; Iriondo, 1997; Zarate and Blasi, 1991), and this is supported broadly by magnetostratigraphic data from the classic Mar del Plata section (see Zarate and Fasano, 1989). Recently, Krohling (1999) reported thermoluminescence (TL) ages of —36,000 cal. B.P. near the base of loess assigned to the Tezanos Pinto formation and —9000 cal. B.P. near the top of this unit, in the Carcarana River basin. These bracketing ages, if correct, broadly support a last glacial age for major loess deposition in this area, although Krohling (1999) also shows eolian sand deposited prior to the loess deposition, with TL ages going back to at least 52,000 cal. B.P. During the

FIGURE 4 (a) Location map of provinces within central Argentina and surrounding countries. (b) Sediment sources and transport directions during the last glacial maximum (LGM) in southern Buenos Aires Province, Argentina. (Redrawn from Zarate and Blasi, 1993.)

LGM, the aggradation of fluvial valleys was mostly related to eolian deposition under dry conditions, with eolian material partially reworked and redeposited by fluvial processes.

The eolian sediments are of volcano-pyroclastic composition, characterized by an abundance of volcanic glass shards and plagioclase, with quartz as a minor component. A relatively coarse grain size is characteristic of these eolian deposits, which are classified as loessial sands, sandy loess, and loess (Zarate and Blasi, 1991, 1993; Bidart, 1996). Mobilization of the eolian grains and particles is attributed to short-term suspension and modified saltation. These mechanisms are related to the occurrence of low-level dust storms, which account for the bulk of the eolian deposits. The source areas of the loessial sands and sandy loess are thought to be the distal segments of the Colorado and Negro Rivers, located ca. 200 km south of the southern Pampas, when sea level was about 120 m below present levels at the LGM (Fig. 4b). Consequently, the eolian deposits of the southern Buenos Aires Province can be regarded as proximal facies and ascribed to a medium-distance transport of dust. Only a minor amount of sed iment (estimated to be less than 10%) was transported in high suspension during volcanic ashfalls (Zárate and Blasi, 1991).

In the southern Pampas, no evidence has been found to support the physical weathering suggested by Irion-do (1990a) as the main mechanism for particle formation. The dominant explosive vulcanism of the Andes seems to have played an important role in particle formation, generating extensive pyroclastic deposits along the Andean piedmont and northern Patagonia, which were easily reworked by glacial, fluvial, and eo-lian processes (Zárate and Blasi, 1993). This interpretation is supported by recent geochemical and isotopic studies, which confirm the importance of volcanic-source sediments for loess in this region (Gallet et al.,

The southern Pampas facies of fine sand sheets grade into dune fields toward La Pampa (Gardenal, 1986; Salazar Lea Plaza, 1980), southern San Luis (Ra-monell et al., 1992), northwest of Buenos Aires, and southern Córdoba (Giménez, 1990; Iriondo, 1990a). According to Iriondo (1997, 1999), this central region of Argentina is characterized by a sand sea that covers an area of 180,000 km2. Hence, the loess distribution is much more restricted than what was mapped by Terug-gi (1957) and slightly modified by Sayago (1997) (compare Figs. 3a and 3b).

The eolian facies of the southern Pampas have been deposited by westerly and southwesterly winds. Geo-morphological analysis revealed the occurrence of southwest-to-northeast- and west-to-east-oriented eo-lian landforms, partially superposed in the western Buenos Aires Province (Gardenal, 1986). The inferred south-southwesterly and westerly paleowinds have been related to high-pressure cells situated on the ice fields of the Patagonian Andes (Groeber, 1936; Tricart, 1973; Iriondo, 1997). The subhumid-semiarid climate of the late glacial inferred from pollen analysis in this region could have been due to a more northerly location of the subtropical circulation or a stronger Atlantic anticyclone. An asymmetry with the Pacific anticyclone would imply a meridional circulation predominantly from the southwest (Prieto, 1996).

In the northern Pampas (northern Buenos Aires, Santa Fe, and northern Córdoba; see Fig. 4a), silty eo-lian facies are dominant. In these areas, Iriondo (1997) located a loess belt that rims the sandy facies of central Argentina (Fig. 3b), with TL ages spanning the late glacial period (Krohling and Iriondo, 1998; Krohling,

1999), as was discussed earlier. This distribution also suggests a general trend of decreasing grain size northward and northeastward.

The rate of eolian deposition decreased remarkably between 11,000 and 10,000 B.P. and was followed by an interval of pedogenesis under more humid conditions. Instead, Iriondo (1999) and Tonni et al. (1999) suggested that cold and dry conditions extended until ca. 8500 B.P. This humid period gave rise to most of the present cultivated soils in the southern Pampas (Zárate and Flegenheimer, 1991). The change to more humid conditions could be related to a poleward shift of the Atlantic Convergence Zone, with a predominant circulation from the northeast, or alternatively to a weakening of the southern Atlantic anticyclone (Prieto, 1996).

12.2.2. Mid- and Late Holocene

The lack of chronological control does not permit a detailed reconstruction for the mid- and late Holocene, but a broad approximation of the environmental conditions can be made. The eolian activity, which was very much reduced or negligible during the early and mid-Holocene, increased sometime between 5000 and 4000 14C B.P. and continued during the late Holocene period. In the southern Pampas, after 5000 B.P., soils were partially truncated at some locations, whereas small ponds and swampy areas of floodplains dessi-cated and were somewhat modified by soil formation (Zárate et al., in press). Sandy loess accumulation started at ca. 5200 B.P. in Bahía Blanca (González and Weiler, 1987/1988) and at ca. 4600 B.P. in the Tandilia Range (Fig. 4a), where localized eolian activity mostly reworked and redeposited late Pleistocene sandy loess (Zárate and Flegenheimer, 1991); this eolian activity continued for an unknown span of time. In a fluvial setting, loessial sand up to 2.5 m thick with an A/C soil horizonation gave a TL age of —4300 cal. years B.P. in its lower section (Prieto, 1996). At another locality (also in a fluvial setting), 1-m thick sandy loess with a cu-mulic A horizon is bracketed between alluvial deposits (which have been modified by pedogenesis) dated at ca. 2500 B.P. and modern alluvium (Zárate et al., in press). A more recent and localized episode of eolian redeposition in the Tandilia Range occurred after the Spanish arrival in the sixteenth century (Zárate and Fle-genheimer, 1991).

In the northeastern Pampas of Buenos Aires, several indicators of much drier conditions during the late Holocene also have been reported. Deflation basins were reexcavated and lunettes formed, whereas soils were truncated and then covered by a thin blanket of loess, which in turn was modified by pedogenesis (Tri-cart, 1973). According to Iriondo and García (1993), further northwest, in Santa Fe (Fig. 4a), a thin eolian layer was deposited regionally during a dry interval between 3500 and 1000 B.P. Recently, a basal TL age of 2050 ± 100 years B.P. was reported by Kröhling and Iriondo (1998). Although soil truncation certainly oc curred at some localities, the ubiquity and regional impact of soil erosion processes currently assumed by some investigators pose critical questions on the intensity and magnitude of the hypothesized dry interval, the vegetation changes associated with it, and the processes involved in the transport and deposition of the eroded soil material. These assumptions are untested by the available information.

Several dune fields are assumed to have formed during the late Holocene in southern San Luis, Córdoba, Buenos Aires (Fig. 4a), and along the distal eastern piedmont of the Andes (Ramonell et al., 1992; Iriondo, 1997, 1999). In northwestern Buenos Aires, included in the so-called sandy Pampa (Pampa arenosa) are longitudinal dunes over 100 km long, 2-5 km wide, and up to 6 m high. These dunes are arranged as extensive southwest-to-northeast archlike forms and are presently stabilized by vegetation; Iriondo (1999) considers them to be of late Pleistocene age. However, Iriondo (1999) also maps smaller dunes, both longitudinal and parabolic, that he considers to be of late Holocene age. A field of parabolic dunes, most of them also stabilized and oriented in a southwest-to-northeast direction, extends south of the longitudinal dunes. The parabolic dunes have developed from the reworking of preexisting sandy deposits. Their dimensions are variable, between 0.2 and 6 km long and from 0.2-3 km wide (Cabral et al., 1988; Giménez, 1990). Soils in both the longitudinal and the parabolic dunes show weak development (Entisols) with A/C horizon sequences, including buried A horizons that indicate previous intervals of stability (Hurtado and Giménez, 1988).

The late Holocene eolian reactivation was apparently triggered by a climatic shift to drier conditions at about 5000 B.P. that may have lasted until ca. 1000 B.P. (Tonni 1992) when subhumid-semiarid conditions returned (Prieto, 1996). This climatic change is interpreted as a northward shift of the anticyclonic centers (Prieto 1996). The shift may have been seasonal, equa-torward during the winter and poleward during the summer (Markgraf, 1993a). A seasonal anticyclonic anomaly that covered most of the region and generated a warm and semiarid climate is proposed by Irion-do (1990b, 1997). His estimated paleowind trajectories are based on an assumption that eolian deposits in the region are synchronous. In the area of southern Córdoba, geomorphological studies do not support the inferred paleowind trajectories from this anticyclonic anomaly, suggesting northeasterly winds for supposedly late Holocene eolian deposits (Cantú and Degiovanni, 1984). Little evidence is currently available to support the notion of synchronous, regionwide dune formation. It is likely, as the few available ages suggest, that several pulses of eolian reactivation may have oc curred during the mid- and late Holocene that did not necessarily affect the heterogeneous Pampean region in the same manner. Factors such as geomorphic setting, vegetation, and soils may have exerted local controls, resulting in responses of different magnitudes. For the most recent dry interval after the fifteenth century, Markgraf (1993a) proposed a transitional or intermediate stage of the circulation patterns that remained between the extreme positions.

The distribution, age, and thickness of the mid- and late Holocene eolian deposits show that the Pampean region may have presented a diverse sensitivity to climate change, with much more intense eolian activity at the southern and western margins. These areas are currently characterized by transitional climatic conditions toward the semiarid / subhumid environments of central Argentina, where sandy eolian deposits were re-mobilized and dunes formed or were reactivated.

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