Thea Vogt
email: thea.vogt@gmail.com

Norbert Clauer

Isabelle Techer

The glaciogenic origin of the Pleistocene calcareous dust in Argentina on the basis of field, mineralogical, textural, and geochemical analyses

Keywords: Calcareous dust, Cryogenic calcite, Pleistocene, Palaeoenvironments, Argentina

Calcareous dust occurs in Argentina as layers and pockets closely associated with Pleistocene deposits and periglacial features from southernmost Patagonia to at least the Mendoza Precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials. Detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper Pleistocene deposits between 51°S and 33°S and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment. All observations and analyses lead us to conclude that: (1) the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic, (2) the dust was carried by southeasterly Antarctic winds, and (3) it was deposited over most of southern and central Argentina. Field observations, geomorphic evidence, and radiocarbon dates suggest that the dust was deposited during several Pleistocene glacial episodes.

INTRODUCTION

Calcareous dust in close association with Pleistocene deposits occurs in Argentina from southern Patagonia to the Mendoza Precordillera, either mixed with the matrix or as fine layers, pockets, and infillings of thermal contraction wedges. [START_REF] Corte | Informe preliminar del progreso efectuado en el estudio de las estructuras de crioturbación pleistocenas fósiles en la provincia de Santa Cruz[END_REF], [START_REF] Corte | Edad de la estructuras geocryogénicas de Puerto Madryn (Chubut). 2da Reuniòn del Grupo Periglacial Argentino[END_REF], [START_REF] Galloway | Fossil ice-wedges in Patagonia and their palaeoclimatic significance[END_REF], and [START_REF] Grosso | Pleistocene ice wedge casts at 34°S eastern Andes piedmont, South-West of South America[END_REF] described periglacial features and the calcium carbonate (CaCO 3 ) associated with them but did not question its origin. Abraham de Vasquez and Garleff (1985), del [START_REF] Del Valle | Morfología de la accumulaciones calcáreas en algunos paleosuelos de Patagónia Oriental (Chubut)[END_REF], [START_REF] Buschiazzo | Condiciones paleoclimáticas deducidas de indicatores pedológicos y geomorfológicos en la región pampeana central (Argentina)[END_REF], [START_REF] Bouza | Fibrous-clay mineral formation and soil evolution in Aridisols of northeastern Patagonia, Argentina[END_REF], [START_REF] Bockheim | Relict sand wedges in southern Patagonia and their stratigraphic and paleo-environmental significance[END_REF], and [START_REF] Ribolini | Late-Pleistocene wedge structures along the Patagonian coast (Argentina): chronological constraints and palaeoenvironmental implications[END_REF] considered these calcareous accumulations as soil horizons linked to warm interglacial periods. The relationship between the periglacial features in the host material and the occurrence of CaCO 3 was never addressed.

New information about the origin of these calcareous materials results from detailed observation of many outcrops ascribed to the lower to upper Pleistocene in Patagonia, the western Pampa, and the Paramillo de Uspallata in the Mendoza Pre-Cordillera at elevations from near sea level up to 2800 m asl, and from the samples of both the host material and the calcareous silt collected there. Microscopic and sedimentologic observations were carried out on more than 100 samples, and geochemical analyses were made for 24 of them [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF]. The strontium isotopic data and the distribution patterns of the rare-earth elements (REEs) clearly indicate a similar glaciogenic origin for the dust collected all over Argentina. Some of the most demonstrative exposures studied and the results of the laboratory analyses of samples collected there are detailed here.

As a preliminary statement, we note that leaching does not result in a loose micrite, and a loose calcareous material mixed with the fine matrix or in thin layers and pockets is not typical of pedogenic horizons. For instance, the CaCO 3 at all sites examined is independent from the surface soil and appears at various depths between carbonate-free layers, which excludes deposition from local percolating solutions. Also, the dust is intimately associated with the cryogenic deformations as cryoturbations or thermal contraction wedges, which makes postsedimentary penetration and crystallisation difficult to justify in a material strongly compressed and having undergone cryosuction. A supplementary fact is that, whatever the depth of the calcareous layers, colonies of calcified saprophyte fungal hyphae and sporae forming felts occur within. This means that calcitisation took place while the fungus was alive or immediately after its death before decay, as shown by [START_REF] Klappa | Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment[END_REF]. While fungi are found in soils at various depths, they only sporulate in the uppermost millimetres, indicating a subaerial environment. Therefore, the calcareous dust in which they lived must have fallen at the ground surface. This evidence supports the hypothesis that this calcareous silt is not pedogenic, but related to a dust fall over the region. Two questions arise from these observations: Where did the dust come from? How and when was it deposited? Recollection, comparison, and discussion of all available arguments about the alternative pedogenic or glaciogenic nature of the calcareous dust spread over most Argentina therefore appear justified and timely.

FIELD OBSERVATIONS

Four sites from those already described in [START_REF] Vogt | Western Anti-Atlas (Morocco) and Central Patagonia (Argentina) calcretes: the calcium carbonate origin[END_REF], [START_REF] Vogt | Calcretes and cryogenic structures in the area of Puerto Madryn[END_REF], [START_REF] Vogt | Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)[END_REF], [START_REF] Vogt | Transformations and neoformations of clay in the cryogenic environment: examples from Transbaikalia (Siberia) and Patagonia (Argentina)[END_REF], [START_REF] Vogt | Influence of the post-Miocene tectonic activity on the geomorphology between Andes and Pampa Deprimida in the area of Provincia de La Pampa, Argentina[END_REF][START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF] were chosen: Pampa del Castillo and Puerto Madryn in eastern Patagonia, Carapacha Chica in the western Pampa, and Paramillo de Uspallata in the Mendoza Precordillera (Fig. 1). They show that, despite the differences in latitude, elevation, age, and geologic and geomorphic context, the calcareous dust and its deposition mode appear to be the same everywhere.

Eastern Patagonia

This nearly 300-km-wide coastal belt forms a landscape of terraced Pleistocene gravel that incises a basement of Tertiary continental and marine sediments. At least five terraces descend gently in steps oriented southwest to northeast from 750 to 590 m asl at the Pampa del Castillo to 125 to 90 m at Puerto Madryn (42°45′S) 400 km away. All are covered by a 10-to 105-m-thick alluvial mantle called "Rodados patagónicos," "Tehuelche," or "Patagonian shingle" [START_REF] Windhausen | Líneas generales de la constitución geológica de la región situada al oeste del Golfo San Jorge[END_REF][START_REF] Sylwan | Geology of the Golfo San Jorge Basin, Argentina[END_REF][START_REF] Martínez | The "Rodados Patagónicos" (Patagonian shingle formation) of eastern Patagonia: environmental conditions of gravel sedimentation. In: Palaeogeography and Palaeoclimatology of Patagonia: Implications for Biodiversity. Special issue[END_REF] consisting of well-worn gravel of acid volcanic pebbles. Decimetre-sized cobbles with typical glacial parallel-piped shapes are visible in the well-layered and channeled, seldom cross-bedded, deposits with no fine-grained lenses and settling channels, the fine matrix being entrapped in the gravel. The sedimentary features suggest strong and regular flows, like those of a glacial discharge, carrying a voluminous load of coarse material over long distances [START_REF] Urien | Glaciomarine sediments from Southern Argentina continental shelf. Preliminary note[END_REF][START_REF] Ponce | Palaeogeographical evolution of the Atlantic coast of Pampa and Patagonia from the last glacial maximum to the Middle Holocene. In: Palaeogeography and Palaeoclimatology of Patagonia: Implications for Biodiversity. Special issue[END_REF]. The alluvial gravel is capped until about 400 km inland by calcretes 20 to 60 cm thick. They consist of calcite with the same mineralogical and geochemical characteristics as those of the underlying calcareous dust. They are soft, except for their hard upper portion (20-25 cm), and appear centimetrically bedded, each layer showing its own network of desiccation cracks, thus intraformational. Decimetric cryoturbation folds involving several layers at once are visible, which evidently formed when the material was still plastic. The general features suggest an active layer above permafrost [START_REF] Vogt | Calcretes and cryogenic structures in the area of Puerto Madryn[END_REF].

Pampa del Castillo

The entire surface of the Pampa del Castillo has been extensively disrupted in the last decades by oil exploration and extraction, so that the exposure here described has been destroyed. Therefore, we rely in this presentation on observations we made before the oil companies moved in. A gravel pit revealed at least 3 m of "Rodados" undisturbed at the bottom, but affected in the upper 2 m by thermal contraction wedges filled with gravel and fine matrix mixed with 30% to 60% loose calcite and silica concretions. The gravel appeared strongly compressed between the wedges, and some cobbles were coated by opal. The deposit was capped by a 30-to 50-cm-thick calcrete consisting of 40% to 80% CaCO 3 . Del Valle and Beltramone (1987) described three profiles in this area but did not mention the wedges. (Figs 1, site 1, and 2)

Puerto Madryn

The lowest alluvial level of Puerto Madryn slopes by 0.16 %, heading to a base level of -150 m that corresponds to the lowest sea stand during the last glacial maximum (LGM). The bedrock consists of Miocene sandstone. A loose sandy layer is spread across the whole terrace. An outcrop by the roadside exposes a 2-m-deep and 0.5-to 0.6-m-wide thermal contraction wedge [START_REF] Corte | Edad de la estructuras geocryogénicas de Puerto Madryn (Chubut). 2da Reuniòn del Grupo Periglacial Argentino[END_REF]. Filled with vertically arranged pebbles, sand and calcareous dust, it disrupts the bedrock (Figs. 1, site 2, and 3a).

Nearby, a gravel pit studied by del Valle and Beltramone (1987), [START_REF] Vogt | Western Anti-Atlas (Morocco) and Central Patagonia (Argentina) calcretes: the calcium carbonate origin[END_REF], Vogt and[START_REF] Vogt | Calcretes and cryogenic structures in the area of Puerto Madryn[END_REF]Vogt and[START_REF] Vogt | Transformations and neoformations of clay in the cryogenic environment: examples from Transbaikalia (Siberia) and Patagonia (Argentina)[END_REF] shows "Rodados" more than 3 m thick. The deposit is clearly stratified and contains thin white lenses of clay and silica concretions. Cryogenic folds and compression figures disturb the layering. Several generations of thermal contraction wedges, 2 -3 m apart and each buried by gravel beds, occur, all filled with gravel, sand, silica concretions, and calcareous dust that is strictly associated with the cryogenic deformations, whereas it is absent in the undisturbed layers at the bottom of the section. The CaCO 3 content increases downwards from 28% to 48% in the wedges, associated with the <2 µm clay-size fraction that increases from 18% to 40%, which is typical of permafrost conditions [START_REF] Corte | Vertical migration of particles in front of moving freezing plane[END_REF][START_REF] Corte | Relationship between four ground patterns, structure of the active layer and type and distribution of ice in permafrost[END_REF]. A continuous calcareous duricrust with 57% to 73% CaCO 3 tops the sequence (Figs. 1, site 2, and 3b).

Carapacha Chica exposure in the western Pampa

The western part of the Pampa extends between 40°S and 35°S and from 63°W to 65°W in the eastern foreland of the Andes chain. It represents the southernmost extension of the Brazilian craton and is composed of metamorphic and intrusive rocks (granites, diorites) together with Paleozoic and Permo-Triassic sediments. Following the uplift of the Andes chain, this region was shaped in a pediment that is gently inclined eastwards. During the late Miocene, from 11 to 5.3 Ma BP, the pediment was blanketed by siltites, a sandy silt slightly cemented by silica, free of carbonate, with a loess-like structure and containing globular or elongated siliceous concretions. Pleistocene basalts flowed across the upstream portion of the pediment; hence it is called the Basaltic Meseta. Terraced sandy alluvium capped by siliceous duricrusts covers the whole plateau [START_REF] Vogt | Influence of the post-Miocene tectonic activity on the geomorphology between Andes and Pampa Deprimida in the area of Provincia de La Pampa, Argentina[END_REF]. Calcareous dust is present in the fluvial deposits and over the basalt flows.

The Sierra Carapacha Chica is a basement outcrop of hard Permian arkosic sandstones located at the southwestern margin of the plateau (37°31.683′S, 66°24.550′W). At the bottom of the north-facing slope, a 3-to 3.5-m-high outcrop exposes a gelifluction accumulation of shattered rock. The basal portion is free of CaCO 3 and is overlain by layers slightly indurated by a fine matrix containing 20% to 25% CaCO 3 between carbonatefree layers. A siliceous duricrust tops the sequence (Figs. 1, site 3, and4).

Paramillo de Uspallata

The Paramillo de Uspallata (32°00′S, 69°00′W) is a high plateau between 3200 and 2700 m asl in the Mendoza Precordillera that possibly represents the eastern portion of the early Miocene planation surface uplifted to 6000 m in the Frontal Cordillera [START_REF] Ramos | Time constraints of the Andean deformation along the Central Andes of Argentina and Chile (32°-33°S Latitude)[END_REF]. The bedrock consists of upper Keuper sediments, mostly schists interbedded with rhyolites and sandstones, and upstanding dacite dykes. No evidence of glaciers older than the middle Pleistocene has been reported on the eastern side of the Andes further north of Patagonia. Between 33°S and 36°S, where the chain is the highest, the oldest glaciation ("Uspallata glaciation") is ascribed to the early middle Pleistocene [START_REF] Espizua | Fission-track dating of the Punta de Vacas Glaciation in the Rio Mendoza valley, Argentina[END_REF][START_REF] Espizua | Pleistocene glaciations in the Mendoza Andes, Argentina[END_REF], which means [START_REF] Corte | Edad de la estructuras geocryogénicas de Puerto Madryn (Chubut). 2da Reuniòn del Grupo Periglacial Argentino[END_REF]). It is filled with alluvial gravel from above and calcareous dust. (b) View of a >3-m-high gravel pit that exposes a series of thermal contraction wedges that disturb the alluvial deposit. Near the bottom, a gypsum crust ("g") is related to a postglacial water table; "c" is the calcrete.

that no glacier existed at the Paramillo de Uspallata during the early Pleistocene.

Periglacial processes reshaped the surface during the Pleistocene glaciations to a cryopediment 3 by 12 km wide covered by a thick gelifluction deposit spread on a smooth surface and incised by sporadic gullies. The upper portion of the deposit is visible over several hundred metres in banks about 2 m high on the road from Villavicencio to Uspallata ("ruta de los Caracoles"). Beside some poorly sorted boulders up to 50 cm in length, dacite clasts 1-20 cm across are mixed with a matrix of finely disaggregated dacitic brown-ochre sand. Devoid of CaCO 3 , the bottom is overlain by the same materials, made whitish by the presence of calcareous dust. A layer of small cryoclasts in an ochre-coloured, carbonatefree sandy matrix mixed with reddish clay forms an intermediate layer. Coarse gravel and boulders with a carbonate-rich matrix in an upper layer suggest some runoff. Fine gravel in a greyish-brown matrix and a sandy, silty, and humic surface soil cap the sequence. The whole profile displays cryoturbation features such as folds and extrusions and particle sorting with the finest fraction increasing downwards. The CaCO 3 content can be as much as 55%, and calcitic pendants at the lower face of gravels showing congelation features are frequent [START_REF] Vogt | Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)[END_REF]. The cryopediment contains the largest amounts of calcareous dust, but calcareous silt and calcitic pendants also occur in a nearby rock glacier by the road at 1800-1100 m asl, and the same kind of gravels are sparse all along the slope following the deep incision that leads to the valley bottom several hundred metres further down (Figs. 1, site 4, and 5).

CALCAREOUS DUST

Wherever collected, the calcareous dust consists of a loose material that crumbles easily even when consolidated. Its proportion in the host sediment is variable but never less than 10% by volume and can amount to 70% in some pockets. It consists of fine-grained calcite (2 to 5 µm, micrite).

Microscopic characteristics

Owing to the size of its particles, this fine material is best observed by scanning electron microscopy (SEM). The calcite appears as very well preserved, loose rhombohedral and scalenohedral idiomorphic crystals. Traces of dissolution only appear in the vicinity of fungal colonies. Recrystallisations with typical congelation features [START_REF] Vogt | Cryogenic physicochemical precipitations: iron, silica, calcium carbonate[END_REF][START_REF] Vogt | Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)[END_REF] occur in the gelifluction deposits. Round frosted and nail-scratched quartz grains typical of wind activity are mixed with the dust. Grains of sodium chloride and gypsum, clearly of allochtonous origin relative to the local bedrock, are also present. Littoral glossy ovoid quartz grains and ooids occur in the samples from eastern Patagonia. The quantity and homogeneity of the material all over Argentina is remarkable, whatever the nature of the bedrock, host sediment, and surrounding slopes (Figs. 6789).

Origin of the calcite

Limestone outcrops occur all along the Andean chain, and the waters of rivers and lakes contain some CaCO 3 but massive precipitation of calcite is currently not observed, and if it occurs it is never powdery. The solubility of CaCO 3 increases when the temperature decreases, and therefore the colder the water, the higher the concentration of CO 2 ; close to 0°C the content of CaCO 3 can reach 299 mg/L [START_REF] Ek | Dépôt de carbonate de calcium par congélation et teneur en bicarbonate des eaux résiduelles[END_REF]. Calcium concentration in glacial water is also high relative to other cations [START_REF] Anderson | Chemical weathering in glacial environments[END_REF]. Calcite represents up to 20% of the glacial rock flour, even if the parent rocks contain very little CaCO 3 [START_REF] Bukowska-Jania | The role of glacier system in migration of calcium carbonate on Svalbard[END_REF]. When glacial rivers once discharged from subglacial environments meet subzero air temperatures, calcite precipitates by freezing as loose micrite with crystalline rhombohedral or scalenohedral shapes [START_REF] Fairchild | Calcite precipitates formed by freezing processes: kinetic controls on morphology and geochemistry[END_REF][START_REF] Clark | Kinetic enrichment of stable isotopes in cryogenic calcites[END_REF][START_REF] Zák | Cryogenic cave calcite from several Central European caves: age, carbon and oxygen isotopes and a genetic model[END_REF]. A glacial origin in a cold environment may therefore be assumed for this powdery calcite.

To verify this hypothesis, strontium, carbon, and oxygen isotopic analyses, as well as REE distribution patterns were determined on 24 samples collected from Rio Gallegos (51°30′ S) to Paramillo de Uspallata (≈32°S). The ∂ 13 C values obtained range from -1.9 to -7.2‰ (VPDB) and match with previously published values of Argentinean calcareous dust [START_REF] Mercer | Cenozoic glaciation in the Southern Hemisphere[END_REF][START_REF] Del Valle | Morfología de la accumulaciones calcáreas en algunos paleosuelos de Patagónia Oriental (Chubut)[END_REF]Zech et al., 2011), which confirms a continental, but not necessarily a pedogenic origin. Del [START_REF] Del Valle | Morfología de la accumulaciones calcáreas en algunos paleosuelos de Patagónia Oriental (Chubut)[END_REF], for instance, reported ∂ 13 C between -3.7‰ and -8.7‰, clearly outside those of pedogenic carbonates, which usually range between -8‰ and -12‰ (e.g., [START_REF] Salomons | Isotope geochemistry of carbonate dissolution and reprecipitation in soils[END_REF]. Margaritz et al. (1981) also published ∂ 13 C values of pedogenic calcite of paleosols from about 22 to 1.7 ka BP in Israel, showing that except for the ∂ 13 C of the oldest calcite at -2.6‰ (±1.9), the other 11 published values range from -7.7‰ (±0.4) to -11.7‰ (±1.3), which is, again, clearly outside the range reported by [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF]. Later, [START_REF] Cerling | Stable carbon and oxygen isotopes in soil carbonates[END_REF] showed that the ∂ 13 C of pedogenic calcite correlates with the contents of C 3 and C 4 in each soil ecosystem. In fact, the ∂ 13 C values from a compilation of C 3 and C 4 plants ranged from about -20‰ to -33‰ for the former and from about -9‰ to -17‰ for the latter, significantly off the values from -1.9‰ to -7.2‰ obtained for the calcareous dust studied here. In turn, the carbon isotopic composition of pedogenic carbonates does not simply result from that of the minerals from which they precipitated [START_REF] Salomons | Isotope geochemistry of carbonate dissolution and reprecipitation in soils[END_REF], as crystallisation temperature and isotopic composition of the interacting solution also impact the carbonate ∂ 13 C. This latter relationship between pore water moving through a soil and crystallising calcite was confirmed by the identical 87 Sr/ 86 Sr ratios for both [START_REF] Thellier | Strontium isotope evidence for soilsolution interactions during evaporation experiments[END_REF]. In fact, each pedogenic calcite yields the specific isotopic signature of soil solutions that integrate all soluble elements before precipitating the authigenic calcite. This can in no way be the case for the calcareous dust studied here, as most Argentine deposits hosting carbonate dust do not reflect homogeneous conditions. Also, there is a clear gap between the ∂ 13 C data obtained for the calcareous dust and the values that characterize pedogenic calcite.

An alternative interpretation has to be provided, which has never been done in all earlier publications referred to previously. For instance, C 3 and C 4 plants almost contemporaneous with the LGM, such as those found in Australia, did not necessarily release ∂ 13 C that was taken up by soil carbonate resulting in a pedogenic signature [START_REF] Quade | Strontium and carbon isotope tracers and the origins of soil carbonate in South Australia and Victoria[END_REF]. In fact, it has not yet been proved that such C 3 and C 4 plants were contemporaneous with the calcite dust of Argentina. Furthermore, no solid argument has been given until now about the transfer of invariable ∂ 13 C from plants to authigenic calcite. In summary, there is no reason to doubt that the ∂ 13 C values from -1.9‰ to -7.2‰ (VPDB) refer to a glacial origin for the carbonates analyzed [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF], unless sustainable contradictory arguments that are still missing are provided.

The narrow range of the 87 Sr/ 86 Sr ratio from 0.706463 ± 0.000008 to 0.707477 ± 0.000006 (2σ) reported for the carbonate dust [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF] represents another essential argument against a pedogenic origin. Indeed, it has been known for a long time that there is no way to correlate a constant 87 Sr/ 86 Sr ratio with a pedogenic process. The deposits described earlier are derived from varied bedrock sources, from basic volcanic rocks and acidic plutons to marine sediments of ages from the Paleozoic to the Triassic, that have a wide spectrum of mineral composition over the area of interest. The constitutive minerals of such

6

T. Vogt et al.

sediments are variably affected by weathering, and they release strontium with variable 87 Sr/ 86 Sr ratios that definitely cannot fit within the homogeneous overall 87 Sr/ 86 Sr ratio obtained for the observed calcite. Of all the arguments against a glaciogenic origin of the calcareous dust, this is probably the most difficult one to deny, ignore, or discard. Ultimately, no valid argument could explain homogeneous 87 Sr/ 86 Sr generated from sediments derived from such varied bedrock over such a wide area as is considered here. Last but not least, the REEs of the carbonate dust yield constant patterns with, importantly, a specific negative cerium anomaly [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF]. Again, the homogeneity of a geochemical signal for the carbonate dust all across Argentina, together with its negative cerium anomaly, is a supplementary determining argument. Indeed, continental calcite from any kind of soil does not yield the negative cerium anomaly that is rather representative of minerals crystallising in contact with seawater. In turn, both the 87 Sr/ 86 Sr ratio and the negative cerium anomaly require crystallisation in a large, homogeneous reservoir with seawater supply during periodically similar episodes and not in continental soils affected by pedogenic weathering that are necessarily heterogeneous.

There should always be more data than those available to consolidate a demonstration, but here the combination of the 14 C ages that will be discussed later, the combined ∂ 13 C values, the 87 Sr/ 86 Sr ratios, and the REE distribution patterns point consistently to the same origin for the calcareous dust.

Age of the calcareous dust

The calcareous dust was observed in all deposits ascribed to the Pleistocene by geologic and geomorphological evidences. Paramillo de Uspallata is the oldest Pleistocene formation in the Mendoza Precordillera, according to [START_REF] Harrington | Investigaciones geologicas en las Sierras de Villavicencio y Mal Pais. Provincia de Mendoza[END_REF]. As it is at a higher elevation than the middle and late Pleistocene features at about 2000-1500 m asl, and as the bottom of the nearest valley is at about 2000 m asl, this formation could be of lower Pleistocene age. [START_REF] Sylwan | Geology of the Golfo San Jorge Basin, Argentina[END_REF] ascribed the alluvial cover of the Pampa del Castillo, the oldest and highest terrace in littoral Patagonia, to the Great Patagonian glaciation (GPG; 1.17 to 1.02 Ma). The alluvium covering the plateau and its surroundings in the western Pampa contains basalt debris and is therefore of Pleistocene age. Geomorphological considerations place it between the lower and the middle Pleistocene [START_REF] Vogt | Influence of the post-Miocene tectonic activity on the geomorphology between Andes and Pampa Deprimida in the area of Provincia de La Pampa, Argentina[END_REF].

Using 14 C dating, [START_REF] Corte | Edad de la estructuras geocryogénicas de Puerto Madryn (Chubut). 2da Reuniòn del Grupo Periglacial Argentino[END_REF] estimated the age of the CaCO 3 collected at about 1 m below the surface in the Puerto Madryn level to be 27.2 ± 0.3 ka BP in the outer parts of the wedge and 22.7 ± 0.5 ka BP in the central part. These ages correspond to the LGM. Additional 14 C ages reported by del [START_REF] Del Valle | Morfología de la accumulaciones calcáreas en algunos paleosuelos de Patagónia Oriental (Chubut)[END_REF] in the nearby exposure are within the same time range: 36.5 ± 2.5 ka BP at about 2 m below the surface and 24.3 ± 0.95 ka BP at about 1 m below the surface. These ages agree with the estimates of [START_REF] Mercer | Cenozoic glaciation in the Southern Hemisphere[END_REF] and Zech et al. (2011). Farther southwest in Patagonia, [START_REF] Bockheim | Origin and significance of calcium carbonate in soils of southwestern Patagonia[END_REF] obtained 239 U-230 Th ages of 224, 209, and 24 ka for carbonate rinds that they did not describe, which correspond to glacial periods. [START_REF] Corte | Informe preliminar del progreso efectuado en el estudio de las estructuras de crioturbación pleistocenas fósiles en la provincia de Santa Cruz[END_REF] described succinctly an exposure about 12 km to the south of Rio Gallegos (≈51°30′S), along the road to Punta Arenas at 20 m asl. Gravel appears well layered at the bottom, whereas it is strongly disturbed between 120 and 60 cm below the surface by thermal contraction wedges filled with sand, 50 to 60 cm deep and about 1.5 m apart. The gravel between the wedges is strongly compressed, forming involutions. A line of ventifacts, perhaps a lag pavement, occurs between the contraction wedges and the sandy surface layer. Whereas the gravel at the bottom and the sand in the wedges are devoid of CaCO 3 , two layers containing calcareous dust are visible in the cryoturbated gravel between the wedges. Corte provided us with two samples from those two layers for chemical analyses. CaCO 3 content amounts to 20%, the calcite is the same loose micrite as described elsewhere, and many round aeolian-shaped grains are present in the coarse (>600 μm) sand fraction. The wedges and sand infilling are evidently younger than the underlying gravel and calcareous dust, but no 14 C ages are available. Despite a closer proximity to Antarctica, the periglacial features are far less conspicuous than those in Puerto Madryn. A cold episode (Antarctic cold reversal) occurred between 14.8 and 12.6 ka BP at 50°S [START_REF] Moreno | Renewed glacial activity during the Antarctic cold reversal and persistence of cold conditions until 11.5 ka in southwestern Patagonia[END_REF][START_REF] Murray | Northern Hemisphere forcing of the last deglaciation in southern Patagonia[END_REF], with freezing temperatures all year round to the south of this latitude [START_REF] Marsh | Observation of atmospheric and climate dynamics from a high resolution ice core record of a passive tracer over the last glaciation[END_REF]. [START_REF] Heusser | Late Quaternary vegetation and climate of southern Tierra del Fuego[END_REF] and [START_REF] Rabassa | Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina)[END_REF] also dated a last glacial occurrence at about 11 ka BP in the Beagle Channel near Ushuaia. The sand wedges and fillings at Rio Gallegos could then belong to this late cold episode.

Once more, it appears that the dust originated under glacial conditions. The next question is: Does it correspond to the climatic and edaphic environment of the glacial times in Argentina?

CENTRAL AND SOUTHERN ARGENTINA DURING THE GLACIAL EPISODES

The Andes Mountain chain and Antarctica are the two major features controlling the climate of southern and central Argentina. When the upper Miocene (≈11 to 5.3 Ma) glaciations of Antarctica caused an expansion of the ice sheet with a resulting global fall of the sea level [START_REF] Flower | The middle Miocene climatic transition: East Antarctc ice sheet development, deep ocean circulation and global carbon cycling[END_REF], the climate of southern Argentina became colder and drier. A major glaciation affected southern South America at some time between 7 and 4.6 Ma [START_REF] Mercer | Late Miocene-Earliest Pliocene glaciation in Southern Argentina. Implications for global ice-sheet history[END_REF]. Clapperton (1993b) and [START_REF] Rabassa | Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina)[END_REF] reported at least eight glacial episodes during the middle and upper Pliocene, and the GPG between 1.17 and 1.02 Ma was followed by 14 to 16 cold events until the LGM, which occurred from about 26.5 to 19 ka BP. The LGM began earlier in the southern Andes, about 39 ka according to Zech et al. (2011), which is consistent with the earlier estimation of [START_REF] Mercer | Glacier in Chile ended a major readvance about 36,000 years ago: some global comparisons[END_REF].

The Antarctic ice growth covered the southern portion of the Atlantic Ocean and resulted in a 100-50 m drop in sea level below the present-day level. During the LGM, the Antarctic grounded ice reached the shelf edge [START_REF] Anderson | The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review[END_REF][START_REF] Graham | Flow and retreat of the Late Quaternary Pine Island-Thwaites palaeo-ice stream, West Antarctica[END_REF][START_REF] Ingólfsson | Quaternary glacial and climate history of Antarctica[END_REF][START_REF] Cofaigh | Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum[END_REF], while the Antarctic sea ice expanded to 60-48°S (present position 70-60°S), doubling its surface area and reducing the surface area of the southern Atlantic Ocean [START_REF] Crosta | Application of modern analog technique to marine Antarctic diatoms: reconstruction of maximum sea-ice extent at the Last Glacial Maximum[END_REF][START_REF] Crosta | Antarctic sea ice history, Late Quaternary[END_REF][START_REF] Guilderson | Late Pleistocene sea level variations derived from the Argentine Shelf[END_REF][START_REF] Violante | The Argentine continental shelf: morphology, sediments, processes and evolution since the Last Glacial Maximum[END_REF]. Currently at 50-53°S, the Austral polar front shifted about 5-6°northwards [START_REF] Paskoff | Los cambios climaticos Plio-Cuaternarios en la franja costera de Chile semiarido[END_REF][START_REF] Caviedes | Quaternary glaciations in the Andes of north-central Chile[END_REF][START_REF] Clapperton | The quaternary glaciation of Chile: a review[END_REF], towards 51°S [START_REF] Garcia | Glacier expansion in southern Patagonia throughout the Antarctic Cold Reversal[END_REF], even to 45°S (MARGO, 2009), with a correlated 4-6°C cooling of the ocean surface [START_REF] Gersonde | Seasurface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records[END_REF][START_REF] Kaiser | A 70-kyr sea surface temperature record off southern Chile (Ocean Drilling Program Site 1233)[END_REF] and a 5-8°C drop near Argentina [START_REF] Hulton | The Last Glacial Maximum and deglaciation in southern South America[END_REF][START_REF] Kull | Evidence of an LGM cooling in NW-Argentina (22°S) derived from a glacier climate model[END_REF]. In fact, little is known about the temperatures on the South American continent. For instance, Pérez-Alberti et al. ( 2005) estimated the thermal depression at 11°C and even 13°C during the LGM in Tierra del Fuego. Alternatively, [START_REF] Kohfeld | DIRTMAP: the geological record of dust[END_REF] considered that atmospheric models have underestimated the magnitude of cooling and drying of much of the land surface during the LGM.

Cold temperatures, extension of glaciated areas, and a decreased ocean surface all restricted evaporation with a concomitant estimated decrease in precipitation of 50% in Antarctica [START_REF] Jouzel | Global change over the last climatic cycle from the Vostok ice core record (Antarctica)[END_REF]. The fall in sea level exposed the continental shelf and widened the continental surface area. The Argentinean shoreline shifted several hundred kilometres eastwards (640 km to the south of Santa Cruz according to [START_REF] Ponce | Palaeogeographical evolution of the Atlantic coast of Pampa and Patagonia from the last glacial maximum to the Middle Holocene. In: Palaeogeography and Palaeoclimatology of Patagonia: Implications for Biodiversity. Special issue[END_REF]), increasing in turn the continentality of the climate. In any case, the fall in sea level caused the water tables to drop, which intensified the ambient aridity. Central and southern Argentina were cold deserts [START_REF] Iriondo | El sistema eólico pampeano. Comunicaciones Museo Provincial Ciencias Naturales "Florentino Ameghino[END_REF], much drier than the presentday cold deserts, as evaporation was lower and the air was drier.

The presently submerged eastern continental shelf was a vast plain on which rivers much larger than those of today were fed by more extensive glaciers [START_REF] Cavallotto | Geological aspects and evolution of the Patagonian continental margin. In: Palaeogeography and Palaeoclimatology of Patagonia: Implications for Biodiversity. Special issue[END_REF] and deposited gravel extensively. It is known from submarine exploration that the continental shelf contains fluvial valleys, braided river networks, blowouts, lagoons, and deltas, in addition to loess and glaciofluvial materials interlayered with and capped by calcretes (Clapperton, 1993a;[START_REF] Urien | Glaciomarine sediments from Southern Argentina continental shelf. Preliminary note[END_REF][START_REF] Violante | Evidences of subaerial exposure of the Argentine continental shelf during the Last Glacial Maximum[END_REF][START_REF] Violante | The Argentine continental shelf: morphology, sediments, processes and evolution since the Last Glacial Maximum[END_REF]. The emerged marine platforms were described as marginal deserts [START_REF] Ochsenius | Pleniglacial desertization, large-animal mass extinction and Pleistocene-Holocene boundary in South America[END_REF]. The ice-free regions suffered a harsh cold climate, and permafrost formed near sea level in southernmost South America [START_REF] Benn | Glacial sediment-landform associations and paleoclimate during the Last Glaciation, Strait of Magellan, Chile[END_REF]. Therefore, it is almost certain that rivers fed by glacial outwash at the outlet from the Andes deposited calcite-rich sediment on the alluvial plains and wide barren surfaces that emerged as sea level fell. The submarine landscape of the Argentinean shelf was similar to current periglacial deltaic plains such those of the Yukon or Lena Rivers, where river and marine waters mix.

PEDOGENIC INTERPRETATION OF THE CALCAREOUS DUST: A DISCUSSION

The authors who suggested that the calcareous deposits resulted from leaching and concentration of the CaCO 3 in soil horizons that they called B Ca , K, or petrocalcic also assumed that this process took place during the warm and wet interglacial periods. This interpretation calls for several remarks.

First, the homogeneous characteristics, especially the constant 87 Sr/ 86 Sr of the calcite studied, could not be generated by pedogenesis, as pedogenic evolution depends strongly on the bedrock. The studied samples were collected on a wide area extending from 51°S (Rio Gallegos) to 32°S (Paramillo de Uspallata) and from near sea level up to 2800 m asl, with a variety of bedrocks comprising Palaeozoic granite and marine sediments, Tertiary volcanics and littoral sandstone, and Pleistocene deposits. From rocks consisting of alkali-rich minerals, fluids will be enriched in 87 Sr, whereas from rocks consisting of alkali-poor minerals, fluids will be depleted in 87 Sr. No strontium isotopic homogeneity can therefore be expected as a result of pedogenesis on these highly varied bedrocks.

Furthermore, idiomorphic calcite crystals cannot result from pedogenic leaching, which by definition is favoured by organic acids, as organic matter affects the crystallisation of calcite [START_REF] Berner | Calcium carbonate concretions formed by decomposition of organic matter[END_REF][START_REF] Inskeep | Kinetics of calcite precipitation in the presence of water soluble organic ligands[END_REF][START_REF] Lebron | Calcite nucleation and precipitation kinetics as affected by dissolved organic matter at 25°C and pH > 7.5[END_REF]. Microscopic observations also reveal that pedogenic calcite is intimately mixed with the host materials and is often impure, containing iron oxides, for instance. Pedogenic leaching and associated recrystallisation do not produce loose crystals, but often produce heterogeneous concretions or nodules of various sizes. And finally, it is difficult to consider that solutions could percolate easily through sediments strongly compressed by thermal contraction.

Second, the authors seem to have disregarded not only the fact that the calcareous dust is closely linked with cryogenic features, but also that the features they consider as indicative of pedogenesis, such as coatings, pendants, and concretions, can be produced by freezing as well.

For instance, [START_REF] Cailleux | Quaternary secondary chemical deposition in France[END_REF][START_REF] Cailleux | Actions du vent et du froid entre le Yukon et Anchorage, Alaska[END_REF][START_REF] Cailleux | Periglacial of McMurdo Strait (Antarctica)[END_REF]) described cryogenic deposits of calcium carbonate, silica, and iron oxides within Würmian formations from Europe and the permafrost of Alaska, Yakutia, and Antarctica. [START_REF] Adolphe | Etude de quelques cristallisations provoquées par gel expérimental[END_REF][START_REF] Adolphe | Obtention d'encroûtements carbonatés par gel expérimental[END_REF] obtained calcitic pendants on a limestone gravel by experimental congelation; interestingly, the calcite crystals were rooted in the cortex, which was decarbonated. In natural conditions, [START_REF] Vogt | Croûtes calcaires quaternaires de période froide en France méditerranéenne[END_REF] reported identical features on the gravels of a Würmian terrace in southern France. Further SEM examination revealed the peculiar micromorphology of the cryogenic calcite crystals, far different from pedogenic (vadose) pendants [START_REF] Vogt | Croûtes calcaires d'origine cryogénique[END_REF]. Later, [START_REF] Vogt | Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)[END_REF] described calcitic pendants and fringes in the permafrost of Antarctica, the Mendoza Pre-Cordillera, and southern Siberia.

As for silica, in addition to the cryogenic siliceous crusts reported by [START_REF] Cailleux | Quaternary secondary chemical deposition in France[END_REF] in Antarctica and Siberia, [START_REF] Hallet | Subglacial silica deposits[END_REF] described subglacial calcium carbonate and silica deposits coating the bedrock. In the terrace near Puerto Madryn studied here, opal coatings occur on volcanic gravels. SEM and energy-dispersive spectrometer analyses showed that silica derived from the inner gravel, which was hollowed; the same was observed for magnesium, iron oxide, and aluminium [START_REF] Vogt | Cryogenic physicochemical precipitations: iron, silica, calcium carbonate[END_REF]). The explanation for both cryogenic calcitic pendants and opal coatings and concomitant impoverishment of the gravel are explained by the strength of the cryogenic suction: 1.2 MPa (≈10 atm)/ -1°C at -10°C = 10.2 MPa (≈100 atm).

Concerning the silica concretions, which are ubiquitous in glacial age deposits in Patagonia, they result from a different process. Smectite, the dominant clay mineral in these deposits, loses its aluminium, the fate of which in permafrost conditions remains to be investigated. The impoverished smectite transforms into opal, which still contains up to 27% magnesium. This transformation was showed by SEM and transmission electron microscopy observation [START_REF] Vogt | Transformations and neoformations of clay in the cryogenic environment: examples from Transbaikalia (Siberia) and Patagonia (Argentina)[END_REF].

Third, the same authors quoted previously assumed that the interglacial episodes were warm and wet in central and southern Argentina. Since the LGM, central and southern Argentina belong to the arid diagonal caused by the rain shadow effect of the Andean mountain chain (Mancini et al., 2008). All studies dealing with the late glaciation and Holocene climates in southern Argentina record two major cold reversals (between 13.1 and 11.1 ka BP) and a series of neoglaciations until the present (e.g., [START_REF] Mercer | Variations of some Patagonian glaciers since the Late-Glacial[END_REF][START_REF] Iriondo | Climatic variation in the Argentine plains during the last 18,000 years[END_REF][START_REF] Andres | Southern Ocean deglacial record supports global Younger Dryas[END_REF][START_REF] Glasser | Late Pleistocene and Holocene palaeoclimate and glacier fluctuations in Patagonia[END_REF][START_REF] Borromei | Late Pleistocene-Holocene environments in Valle Carbajal, Tierra del Fuego, Argentina[END_REF][START_REF] Haberzettl | Lateglacial and Holocene wet-dry cycles in southern Patagonia: chronology, sedimentology and geochemistry of a lacustrine record from Laguna Potrok Aike, Argentina[END_REF][START_REF] Aniya | Holocene glaciations of Hielo Patagónico (Patagonia Icefield), South America: a brief review[END_REF]. In summary, the climate was dominantly dry with polar winds reaching Amazonia [START_REF] Kronberg | Geochemistry and geochronology of surficial Acre basin sediments (western Amazonia): key information for climate reconstruction[END_REF][START_REF] Servant | Tropical forest changes during the late quaternary in African and South American lowlands[END_REF][START_REF] Carneiro Filho | Amazonian paleodunes provide evidence for drier climate phases during the Late Pleistocence-Holocene[END_REF]. A mantle of sand was spread over Argentina during the late glaciation and early Holocene (Carignano and Cioccale, 2005), which is confirmed by luminescence dating of dune fields in San Luis Province (≈33°S) (Tripaldi andForman, 2007, 2016). The lower portion of this aeolian formation has been dated at 11-8 ka BP in Buenos Aires Province [START_REF] Tonni | Predominance of arid climates indicated by mammals in the pampas of Argentina during the Late Pleistocene and Holocene[END_REF]. This sand cover is 50-100 cm thick and occasionally thicker, forming sheets and dunes in Patagonia and the western Pampa, but no radiometric or luminescence dating is available for those areas. According to [START_REF] Tonni | Predominance of arid climates indicated by mammals in the pampas of Argentina during the Late Pleistocene and Holocene[END_REF], the climate was dry and cool during the early Holocene (11-8 ka BP). [START_REF] Paez | Fossil pollen from Los Toldos locality: a record of the Late-glacial transition in the Extra-Andean Patagonia[END_REF] considered the environmental conditions to be extremely arid, with precipitation lower than 200 mm before ca. 11 ka BP and effective moisture increasing between ca. 11 ka BP and ca. 10 ka BP, probably related to an increase in precipitation to about 200 mm under cold conditions. The edaphic moisture was lower than today during the late Pleistocene and the early and middle Holocene [START_REF] Pendall | Multiproxy record of Late Pleistocene-Holocene climate changes from a peat bog in Patagonia[END_REF], and most of the Holocene (12 to 0.8 ka) was characterised by drier than present conditions, according to [START_REF] Tripaldi | Eolian depositional phases during the past 50 ka and inferred climate variability for the Pampean Sand Sea, western Pampas, Argentina[END_REF].

Dry climate means that there is little movement of water through the soil and little leaching of solutes. [START_REF] Bouza | Micromorphological, physical, and chemical characteristics of soil crust types of the central Patagonia region, Argentina[END_REF], for instance, described a centimetre-thick surficial crust formed by raindrop impact with redeposition by rainwash on top of an upper zone of laminated sand containing silt and clay with large empty vesicles, and a dense lower zone with fewer and smaller air vesicles. Vesicular soil horizons reduce infiltration [START_REF] Turk | Distribution and properties of vesicular horizons in the western United States[END_REF] from 3-to 100-fold decline [START_REF] Young | Hydraulic properties of a desert soil chronosequence in the Mojave Desert, USA[END_REF], so this structure indicates lateral rather than vertical transport. The 14 C age of 5440 ± 160 yr BP for the calcareous dust in the early Holocene sand cover [START_REF] Del Valle | Morfología de la accumulaciones calcáreas en algunos paleosuelos de Patagónia Oriental (Chubut)[END_REF] confirms that no recrystallisation occurred during the late millennia (from 5440 ± 160 yr BP to the present). Wetter and milder phases are only recorded in eastern and northern Pampa and in northern Argentina, where the climate is driven by a northeastern atmospheric circulation. The geographic situation was identical during the previous interglaciations, and it Pleistocene calcareous dust in Argentina can be assumed that the climatic conditions were not so different.

Finally, the great proportion of CaCO 3 in the deposits presumes a massive mobilisation and crystallisation of calcite, and its good preservation, which is difficult to consider in warm and wet environments. This good preservation of calcite is better explained during glacial periods, when atmospheric CO 2 decreased drastically [START_REF] Jouzel | Global change over the last climatic cycle from the Vostok ice core record (Antarctica)[END_REF][START_REF] Kotlyakov | Global changes over the last climatic cycle from Antarctic ice core records[END_REF][START_REF] Caillon | Timing of atmospheric CO 2 and Antarctic temperature changes across Termination III[END_REF] with a lower effect on carbonate recrystallisation. And finally, such a loose material resisted all climatic changes during the whole of the Pleistocene, confirming the persistence of an arid environment. The lack of dust deposits in the eastern and northern Pampas and in northern Argentina, where the climate is warmer and wetter, then becomes understandable. If calcareous dust was also deposited in those regions, which is likely, it could have been weathered and leached away under wetter and warmer local conditions, as was the case during the interglacial episodes elsewhere, and still is the case according to the literature [START_REF] Sayago | The Argentine neotropical loess: an overview[END_REF][START_REF] Sayago | Genesis and distribution of the Late Plelstocene and Holocene loess of Argentina: a regional approximation[END_REF]Zinck andSayago, 1999, 2001).

GLACIOGENIC AND AEOLIAN EXPLANATION

In summary, the calcareous dust is intimately associated with the periglacial deformations of the host material. Locally, calcite may be recrystallised as cryogenic fringes. Radiocarbon dating of the younger deposits places them within the LGM. Microscopic and chemical analyses show that the calcite characteristics are homogeneous in all occurrences and that it was deposited at the ground surface, as proved by fungal colonisation.

All evidence points to the following succession: (1) increased solubilisation of CaCO 3 under cold conditions leading to a carbonated glacial outwash; (2) CaCO 3 crystallisation under cold temperatures as idiomorphic micrite crystals; and (3) remobilisation of calcite as dust across the region.

How did the calcareous dust form?

The huge volume of calcareous dust could form only on a wide continental area allowing crystallisation of large amounts of homogeneous carbonates, an area necessarily located near the ocean, as interactions with seawater are assumed from REE distribution spectra. The Argentinean shelf is one of the largest submarine platforms in the world, stretching from 35°S to 55°S, over 170 to 850 km, and covering about 1 million square kilometres. Its depth is 110 to 165 m with a smooth eastward slope [START_REF] Violante | Evolución de las llanuras costeras del este bonaerense entre la Bahía Samborombón y la laguna Mar Chiquita[END_REF], so the shelf emerged as an immense littoral plain when the sea level dropped by 100 to 150 m during glacial maxima [START_REF] Rabassa | Chronology of the Late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina)[END_REF][START_REF] Ponce | Palaeogeographical evolution of the Atlantic coast of Pampa and Patagonia from the last glacial maximum to the Middle Holocene. In: Palaeogeography and Palaeoclimatology of Patagonia: Implications for Biodiversity. Special issue[END_REF]. [START_REF] Walter | Provenance and transport of terrigenous sediment in the South Atlantic Ocean and their relations to glacial and interglacial cycles: Nd and Sr isotopic evidence[END_REF] showed that the Argentinean shelf was an important source for wind-transported silt during the glacial periods, while [START_REF] Basile | Patagonian origin of glacial dust deposited in East Antarctica (Vostok and Dome C) during glacial stages 2, 4 and 6[END_REF] reported 87 Sr/ 86 Sr ratios for sediments of this Argentinean continental shelf to be within the values of the glaciogenic calcareous dust of the same area [START_REF] Techer | Origin of calcareous dust in Argentinean Pleistocene periglacial deposits traced by Sr, C and O isotopic compositions, and REE distribution[END_REF]. The climate cooled slowly, more effectively on the continent than on the wide ocean, allowing mountain glaciers to extend more rapidly than sea level fell. The glaciofluvial discharge containing calcium (and strontium) was deposited over the alluvial plains on a flat, frozen, and muddy surface and under shallow water in lagoons. During the glacial episodes, the CaCO 3 transported to these flat estuarine areas by the rivers was dissolved due to water temperature and pH changes. Calcium concentrated again afterwards and crystallised into new authigenic, chemically homogeneous calcite in an environment that had slightly varied strontium isotopic ratios depending on the ratio of fresh/seawater during crystallisation.

Role of wind

However, these facts do not explain the natural dispersion of the calcareous dust across all geomorphic surfaces of Argentina. For instance, glacial outwash could not flow across the geomorphic levels of the western Pampa and Paramillo de Uspallata, because there were no glaciers upstream and obviously no glaciogenic calcite. How chemically homogeneous calcite could have been dispersed over more than 20°in latitude even up to the mountain chains at 2800 m asl has to be explained. Wind seems not only a plausible carrier of the material, but also the only reasonable explanation for such a wide horizontal and vertical spread.

Arguments to strengthen an aeolian hypothesis come from microscopic analysis, which reveals that the material contains wind-transported littoral grains and allochtonous minerals associated with the calcareous silt. Yet deflation requires wide surfaces made of fine-grained materials devoid of vegetation: presently, most aeolian dust comes from desert regions. Several authors invoked an aeolian supply for the calcareous silt in the exposures they observed in Argentina, for example, [START_REF] Buschiazzo | Condiciones paleoclimáticas deducidas de indicatores pedológicos y geomorfológicos en la región pampeana central (Argentina)[END_REF] in the western Pampa and [START_REF] Bockheim | Relict sand wedges in southern Patagonia and their stratigraphic and paleo-environmental significance[END_REF] and [START_REF] Ribolini | Late-Pleistocene wedge structures along the Patagonian coast (Argentina): chronological constraints and palaeoenvironmental implications[END_REF] in the southernmost Patagonia, without proposing a possible source area. [START_REF] Ribolini | Late-Pleistocene wedge structures along the Patagonian coast (Argentina): chronological constraints and palaeoenvironmental implications[END_REF] suggested southwesterly winds, yet the westerlies reaching Patagonia come from the southern Pacific Ocean and, according to [START_REF] Blisniuk | Climatic and ecologic changes during Miocene surface uplift in the Southern Patagonian Andes[END_REF], deposition in the eastern foreland of the southern Patagonian Andes had essentially ceased by ≈14 Ma as the result of a rain shadow caused by the Miocene surface uplift. The only periods of extensive desert areas in Argentina occurred during glaciations.

Currently, the belt of westerly winds extends over the Southern Hemisphere from about 40°S almost to Antarctica. Oriented orthogonally to these winds, the Andes create an orographic barrier, with heavy precipitation on the western Chilean side and a marked drought on the eastern side with a föhn effect. Two powerful winds blow over southern and central Argentina: the cold and dry pampero from the southwest and the surestada from the southeast, carrying cold and wet polar air. The surestada sometimes reaches the Amazonian basin, but once on the continent it is stopped by 10

T. Vogt et al.

the Andes, turns counterclockwise southwards along the mountain chain, and continues with attenuated strength toward the Antarctic [START_REF] Weischet | Regionale Klimatologie. Teil 1[END_REF]. The South Atlantic high-pressure area is centred around 30°S, bringing summer rain to central and northern Argentina. With the Austral polar front at 40°S, the Antarctic anticyclone and its belt of polar easterly winds shifted northwards during glacial times and, consequently, the southern westerlies moved farther north [START_REF] Markgraf | Paleoenvironments and paleoclimates in Tierra del Fuego and southermost Patagonia, South America. Palaeogeography[END_REF][START_REF] Sayago | The Argentine neotropical loess: an overview[END_REF][START_REF] Wenzens | The influence of tectonically derived relief and climate on the extent of the last Glaciation east of the Patagonian ice fields (Argentina, Chile)[END_REF][START_REF] Stuut | Climate variability at the southern boundaries of the Namib (southwestern Africa) and Atacama (northern Chile) coastal deserts during the last 120,000 yr[END_REF][START_REF] Toggweiler | Shifted Westerlies, atmospheric CO 2 , and climate change during the ice ages[END_REF][START_REF] Zech | Timing of the late Quaternary glaciation in the Andes from 15 to 40°S[END_REF][START_REF] Zech | Chronologies of the Last Glacial Maximum and its Termination in the Andes (~10-55°S) based on surface exposure dating. In, Past Climate Variability in South America and Surrounding Regions from the Last Glacial Maximum to the Holocene[END_REF]Zech et al., , 2011;;[START_REF] Zolitschka | Environment and climate of the last 51,000 years-new insights from the Potrok Aike maar lake Sediment Archive Drilling project (PASADO)[END_REF][START_REF] Liu | The de-correlation of westerly winds and westerly-wind stress over the Southern Ocean during the Last Glacial Maximum[END_REF] to 18°S (Stuut and Hebeln, 2007), the highest point of the mountain chain, and its already limited influence on the eastern side vanished. The distance between polar and tropical belts narrowed, thereby steepening the pole-toequator temperature gradient and, consequently, the wind strength [START_REF] Petit | Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica[END_REF][START_REF] Compagnucci | Atmospheric circulation over Patagonia from the Jurassic to present: a review through proxy data and climatic modelling scenarios[END_REF][START_REF] Crouvi | Intensive winds during glacial periods increased sand dune activity and loess deposition[END_REF]. Southeasterly winds were reinforced [START_REF] Latrubesse | A climatic model for southwestern Amazonia in Last Glacial times[END_REF][START_REF] Shi | Southeast trade wind variations during the last 135 kyr: evidence from pollen spectra in eastern South Atlantic sediments[END_REF][START_REF] Stuut | A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic[END_REF][START_REF] Kim | Reconstruction of SE trade-wind intensity based on sea-surface temperature gradients in the Southeast Atlantic over the last 25 kyr[END_REF]. The littoral quartz grains and ooïds found together with the calcareous dust in the LGM terrace of Puerto Madryn, when the shore was more than 200 km to the east, suggest strong winds (Fig. 10). The deflation of the calcareous dust and its transport over thousands of kilometres on the continent by strong and dry winds sweeping over the desertic plains covered by glaciofluvial deposits fits this context well. Under long-lasting cryogenic conditions, rock debris suffers comminution to the 50-10 µm silt grain size [START_REF] Konishchev | Characteristics of cryogenic weathering in the permafrost zone of the European USSR[END_REF]. Quartz is the most fragile because its microfissures are about 1-10 µm apart and can be reduced to the limiting size of solid particles, about 1 µm [START_REF] Schwamborn | Depositional dynamics in the El'gygytgyn Crater margin: implications for the 3.6 Ma old sediment archive[END_REF]. This grain size is well adapted for wind deflation and explains the fineness of the glacial dust observed in Argentina and also collected in the Antarctic ice. [START_REF] Petit | Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica[END_REF], [START_REF] Legrand | Vostok (Antarctica) ice core: atmospheric chemistry changes over the last climatic cycle (160,000 years)[END_REF][START_REF] Jouzel | Global change over the last climatic cycle from the Vostok ice core record (Antarctica)[END_REF], [START_REF] Kotlyakov | Global changes over the last climatic cycle from Antarctic ice core records[END_REF][START_REF] Fischer | Reconstruction of millennial changes in dust emission, transport and regional sea ice coverage using the deep EPICA ice cores from the Atlantic and Indian Ocean sector of Antarctica[END_REF][START_REF] Reader | The mineral dust aerosol cycle during the Last Glacial Maximum[END_REF] evaluated the amount of dust in the atmosphere during the cold periods to be at least 15 times more than the amount seen now, because the wind was so much stronger. [START_REF] Delmonte | Late Quaternary interglacials in East Antarctica from ice-core dust records[END_REF] estimated that dust fluxes during the last five interglacials were reduced 10-to 25-fold compared with dust fluxes occurring during glacial periods.

The geographic source of the dust contained in the Antarctic ice has been often assumed as derived from southern South America [START_REF] Basile | Patagonian origin of glacial dust deposited in East Antarctica (Vostok and Dome C) during glacial stages 2, 4 and 6[END_REF][START_REF] Delmonte | Quaternary variations and origin of continental dust in East Antarctica[END_REF][START_REF] Delmonte | Comparing the Epica and Vostok dust records during the last 220,000 years: stratigraphical correlation and provenance in glacial periods[END_REF][START_REF] Delmonte | Mineral dust in East Antarctica: Assessing the contribution from remote and local dust sources from the last glacial maximum to present-day[END_REF][START_REF] Fischer | Reconstruction of millennial changes in dust emission, transport and regional sea ice coverage using the deep EPICA ice cores from the Atlantic and Indian Ocean sector of Antarctica[END_REF][START_REF] Li | Distribution, transport, and deposition of mineral dust in the Southern Ocean and Antarctica: contribution of major sources[END_REF]. According to [START_REF] Ackert | Palaeoclimate: Patagonian dust machine[END_REF], most of the dust in the Antarctic ice cores came from the glacial outwash of Patagonia. [START_REF] Sugden | Influence of Patagonian glaciers on Antarctic dust deposition during the last glacial period[END_REF], [START_REF] Albani | Changes in mineral dust transport and deposition to Antarctica between the Last Glacial Maximum and current climates: modelling concentration, size and provenance[END_REF][START_REF] Compagnucci | Atmospheric circulation over Patagonia from the Jurassic to present: a review through proxy data and climatic modelling scenarios[END_REF][START_REF] Mcculloch | Climatic inferences from glacial and palaeoecological evidence at the last glacial termination, southern South America[END_REF] also considered South America to be the main source for the dust during the LGM, because of the extension of glaciogenic dust sources on active outwash plains, as well as a likely more efficient transport to West Antarctica. [START_REF] Delmas | Present Antarctic aerosol composition: a memory of Ice-Age atmospheric dust?[END_REF], Kaiser and[START_REF] Kaiser | Links between Patagonian ice sheet fluctuations and Antarctic dust variability during the last glacial period (MIS 4-2)[END_REF][START_REF] Weber | Dust transport from Patagonia to Antarctica-a new stratigraphic approach from the Scotia Sea and its implications for the last glacial cycle[END_REF] emphasised the role of the emerged shelf. [START_REF] Li | Toward understanding the dust deposition in Antarctica during the Last Glacial Maximum: Sensitivity studies on plausible causes[END_REF] considered that the extension of the dust source caused by the lower sea level was two to three times greater than that due to the reduction of continental vegetation. Dust transported from Patagonia and even from La Pampa can be well explained by the counterclockwise shift of the southeasterly winds once stopped by the Andes chain.

By analogy with satellite observation of current dust plumes, [START_REF] Gaiero | Dust provenance in Antarctic ice during glacial periods: from where in southern South America[END_REF], [START_REF] Gaiero | The signature of river-and wind-borne materials exported from Patagonia to the southern latitudes: a view from REEs and implications for paleoclimatic interpretations[END_REF][START_REF] Delmonte | Geographic provenance of aeolian dust in East Antarctica during Pleistocene glaciations: preliminary results from Talos Dome and comparison with East Antarctic and new Andean ice core data[END_REF] suggested that the Bolivian Altiplano could also represent a potential geographic source. This option is debatable when considering the environment during the glacial periods. The Altiplano was wetter than today during the LGM [START_REF] Baker | The history of South American tropical precipitation for the past 25,000 years[END_REF]. Consequently, glaciers developed not only on the mountain chains, but also on the northern Altiplano [START_REF] Mark | Late Quaternary glaciations of Ecuador, Peru and Bolivia[END_REF], and wide palaeolakes formed [START_REF] Clayton | The last glacial cycle and paleolake synchrony in the southern Bolivian Altiplano: Cerro Azanaques case study[END_REF][START_REF] Clapperton | Late Quaternary glacier advances and palaeolake highstands in the Bolivian Altiplano[END_REF][START_REF] Moon | III. The geology and physiography of the Altiplano of Peru and Bolivia[END_REF]. These lakes were frozen during winter and melted in summer. There were not enough barren surfaces to supply dust towards Antarctica, and it is difficult to identify the wind circulations that would have been responsible for the transfer.

CONCLUSIONS

Field observations as well as laboratory analyses prove that the calcareous dust associated with Pleistocene periglacial features of central and southern Argentina is of glacial origin, transported by and precipitated from river waters over the alluvial plains to the emerged continental shelf, where it recrystallised and was deflated and dispersed by winds during the cold episodes. The observations detailed herein provide also information about the environment during the glacial periods. The role of reinforced Antarctic southeasterly winds, for instance, was minimised in previous publications, whereas it has been asserted in the southeastern Atlantic [START_REF] Kim | Reconstruction of SE trade-wind intensity based on sea-surface temperature gradients in the Southeast Atlantic over the last 25 kyr[END_REF]. Alternatively, the calcareous concentrations in soils of central and northern Argentina could have resulted from secondary precipitation of CaCO 3 after weathering under warmer and wetter postglacial conditions.

Figure 1 .

 1 Figure 1. Map of Argentina with the location of Mendoza (M), Buenos Aires (BA), Bahia Blanca (BB), Puerto Madryn (PM), Comodoro Rivadavia (CR), and Rio Gallegos (RG). The white arrows and numbers indicate the sites studied: Pampa del Castillo (1), Puerto Madryn (2), Carapacha Chica (3), Paramillo de Uspallata (4). The dotted line delimits the emerged continental shelf during the glaciations.

Figure 2 .

 2 Figure 2. An exposure in the Pampa del Castillo terrace (45°07′S, 68°04′W), but no longer exists due to human modification. The calcareous dust is mixed with the gravels. A series of contraction wedges are visible. The spade indicates the scale.

Figure 3 .

 3 Figure 3. Two exposures in the Puerto Madryn level (42°44.750′ S, 65°15.950′W). (a) A 2-m-deep thermal contraction wedge breaks up and folds the Miocene sandstone (studied by[START_REF] Corte | Edad de la estructuras geocryogénicas de Puerto Madryn (Chubut). 2da Reuniòn del Grupo Periglacial Argentino[END_REF]). It is filled with alluvial gravel from above and calcareous dust. (b) View of a >3-m-high gravel pit that exposes a series of thermal contraction wedges that disturb the alluvial deposit. Near the bottom, a gypsum crust ("g") is related to a postglacial water table; "c" is the calcrete.

Figure 4 .

 4 Figure 4. The Carapacha Chica exposure (37°31.683′S, 66°24.550′W). The gelifluction deposit consists of shattered siliceous sandstone mixed with scarce matrix. The clasts are angular and heterometric. The white material ("c") is CaCO 3 . The deposit is capped by a duricrust ("d"). The hollows are bird nests in the soft calcareous accumulation below the duricrust. The spade at the bottom gives the scale.

Figure 5 .

 5 Figure 5. An exposure by the Ruta de los Caracoles, at the Paramillo de Uspallata: two carbonate-rich layers in the gelifluction deposit affected by folds, underlined by the black and the white dashes. The hammer in the middle gives the scale.

Figure 6 .

 6 Figure 6. Scanning electron microscopy photographs showing the calcite within the calcareous dust: Calcite crystals with scalenohedral shape from (a) the Paramillo de Uspallata and (b) the Carapacha Chica exposure, with incipient cryogenic crystallization in the centre. The shape of the calcite is the same in all studied sites.

Figure 7 .

 7 Figure 7. Cryogenic lamellar crystals: (a) dendritic assemblage of needles growing from calcitic dust; (b) the lamellar crystals assemble together to form needles; (c) detail of the lamellar crystals.

Figure 8 .

 8 Figure 8. Scanning electron microscopy photographs showing the colonization of the calcareous dust by fungi: (a) and (b) calcitized fungal hyphae; (c) a cluster of calcitized sporae.

Figure 9 .

 9 Figure 9. Scanning electron microscopy photographs showing (a) a cluster of ooids cemented by calcite; (b) neoformed opal; (c) gypsum platelets, a cryogenic structure also observed in Antarctica[START_REF] Vogt | Secondary precipitates in Pleistocene and present cryogenic environments (Mendoza Precordillera, Argentina, Transbaikalia, Siberia, and Seymour Island, Antarctica)[END_REF].

Figure 10 .

 10 Figure 10. Wind circulation over Argentina. (a) The present-day circulation in a surestada situation: the black arrows indicate the polar wind, the white arrows represent the tropical wind. (b) The proposed wind circulation during the glacial times, according to the situation of polar ice and polar front as described by several authors.

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ACKNOWLEDGMENTS

One of us, T. Vogt, did the fieldwork with A. Corte and H. Vogt, both of whom have since passed away. We remember the animated discussions in the field and the outcomes that we report here. We also acknowledge the field organisation and cooperation of the teams of CENPAT in Puerto Madryn and the University of La Pampa in Santa Rosa. And we are grateful to the senior editor and to the two reviewers whose remarks and suggestions helped to improve our text.