ARTICLE IN PRESS Quaternary International 162–163 (2007) 158–165 Soil genesis related to medieval climatic fluctuations in southern Patagonia and Tierra del Fuego (Argentina): Chronological and paleoclimatic considerations Cristian M. Favier-Dubois CONICET-INCUAPA, Departamento de Arqueologı´a, Universidad Nacional del Centro, Avenida Del Valle 5737, (B7400JWI) Olavarrı´a, Argentina Available online 20 February 2007 Abstract Coastal and continental localities analyzed in southern Patagonia and Tierra del Fuego (Argentina) demonstrate the ubiquitous presence of a late Holocene Mollisol in eolian and colluvial deposits. This soil is buried in many places by a sandy layer of variable thickness, indicating a further reactivation of morphogenetic processes. Maximum and minimum ages obtained by 14C and oxidizable carbon ratio (OCR) methods place the beginning of this pedologic event about 1000 years ago. The regional character of this soil and the evidence of climatic fluctuations supplied from paleohydrological, pollen, and multi-proxy studies in the area suggest that the soil’s origin could be related to a pulse of increasing humidity following an episode of drought during medieval times. r 2006 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction and research objectives Geoarcheological work carried out in coastal and continental areas in southern Patagonia (south of the Santa Cruz river basin) and northern Tierra del Fuego has shown the recurrent presence of a pedologic event in the upper section of extended late Holocene eolian and colluvial deposits, frequently associated with archeological material (Favier-Dubois, 2001). This soil is buried in those places where erosional processes have acted or are still active, as in coastal dune areas, but it is exposed in other positions on the landscape. On a regional scale, the analyzed sequences indicate an active morphogenesis after the Middle Holocene, characterized by abundant eolian deposits, which may be related to arid conditions (Rabassa and Clapperton, 1990; Markgraf, 1993; Aniya, 1996). At about 1000 years ago, there is a pedogenetic interval represented by a soil having an A–AC–C profile and a mollic epipedon, which may be classified as a Mollisol (see stratigraphy). The soil’s development may indicate an important change in the environmental conditions due to the stabilization of regional eolian and colluvial systems at that time. Furthermore, E-mail address: cfavier@coopenet.com.ar. changes in the distribution of the archeological record are linked to this development (Favier-Dubois, 2003). Therefore, the aim of this paper is to analyze the morphology, chronology, and distribution as well as the paleoclimatic implications of this soil in the region under analysis. 2. Methods Geoarcheological surveys are a part of the archeological research projects carried out in the region for the purpose of studying the evolution of hunter-gatherer populations (Borrero, 1998). The analysis of the environmental dynamics was based on the identification of landforms, the drawing of stratigraphic profiles, the correlation of litho- and pedostratigraphic units, and the study of sedimentary facies. Soil profiles were described and characterized according to soil taxonomy (Soil Survey Staff, 2003). Two different methods were used to obtain numerical ages: (1) radiocarbon dating, using both conventional and AMS techniques, and (2) oxidizable carbon ratio (OCR) dating, a method based on the effect of the biochemical degradation of charcoal and soil humic material, measured as a ratio of the total carbon to the readily oxidizable carbon in the sample (Frink, 1994, 1995). Textures were determined by sieving and pipetting. 1040-6182/$ - see front matter r 2006 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2006.10.044 ARTICLE IN PRESS C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 The color of dry soil samples was classified according to the Munsell soil color charts. Organic C was determined by the Walkley–Black method, and pH by the use of a pH meter in a suspension with a soil:water ratio of 1:2.5. 3. Study areas and geological background The localities analyzed in Fuego–Patagonia (Fig. 1) comprise three areas: (A). Lago Argentino (locality 1), (B). Magellan Strait (locality 2), and (C). San Sebastian Bay (localities 3–5). Localities 1 and 2 are in Santa Cruz Province and localities 3–5 are in Tierra del Fuego Province. Lago Argentino is located in southwestern Santa Cruz province, very close to the Andean range, where an extensive ice field is present. This lake has a glacial origin and is surrounded by dunes and sand sheets covering Pleistocene drift deposits. Cabo Vı́rgenes is a cuspate foreland in the eastern end of the Magellan Strait, characterized by successive beach ridges and marshes over which longitudinal dunes and sand sheets were formed (see Fig. 4). The progradation of this marine landform began about 4200 years ago (Uribe and Zamora, 1981); a steep paleocliff developed on glacial drift borders in the area to the north. San Sebastian Bay is a large, semicircular coastal embayment situated on the Atlantic coast of Tierra del Fuego. A 17 km long gravel spit protects the northern part of the Bay, giving rise to a gradient in sedimentary processes from the south (barrier beach) to the north (tidal flats) (Vilas et al., 1999). Coastal sedimentation began at least 5700 years before present. A paleocliff developed on glacial drift and tertiary sandstones (footslope of Sierra de Carmen Sylva), is located in the southern area of this Bay. 159 The soil under study has also been identified, although not yet dated, in other coastal areas of southern Patagonia, including Bahı́a Grande (Santa Cruz) and the coastline between Cape San Sebastian (south San Sebastian Bay) and Rı́o Grande City (see Fig. 1). In all cases, the morphological properties and geomorphological contexts are the same as in the studied areas. The weather in the region is currently defined as cold (+6 1C on average) and semiarid, with an annual mean rainfall of 300 mm. Strong westerly winds buffet the entire area. Soil temperature regimes are isomesic (Lago Argentino and Cabo Vı́rgenes) or cryic to isomesic (San Sebastian Bay); soil moisture regimes are xeric bordering on aridic (Wambecke and Scoppa, 1980). Regional soils were mapped by the Instituto Nacional de Tecnologı́a Agropecuaria or INTA (1990) at scales 1:500.000 in Tierra del Fuego and 1:1.000.000 in Santa Cruz. Soils recognized by this survey are: Haplargids, Torriorthents, and Argixerolls in southern Lago Argentino; Paleargids, Xerorthents, and Typic Quartzipsamments (coastal dunes) in the southeastern portion of Santa Cruz which includes Cabo Vı́rgenes; and Natrixeralfs, Argicryolls, and Cumulic Haplocryolls (foothills) in San Sebastian Bay. From a vegetation point of view, this region is included in the Patagonian Steppe, characterized by a predominance of grassland and scrub communities. A grass steppe dominated by Festuca pallescens and cushion plants is present at Lago Argentino, while at Cabo Vı́rgenes and San Sebastian Bay it is dominated by Festuca gracillima with Empetrum rubrum (Mancini, 2002). On the coast, shrublands of Lepidophyllum cupresiforme and patches of Salicornia ambigua are also found. Due to the morphoclimatic characteristics of the region, the studied sequences consist primarily of eolian and colluvial deposits that overlie marine or continental units summarized above (Table 1). Examples of the buried Mollisol are shown in Fig. 2. 4. Results 4.1. Stratigraphy Fig. 1. Map of southernmost Patagonia indicating the areas and study localities. The studied sequences present a similar stratigraphy in all localities (Fig. 3). The stratigraphic profile consists of a lower sandy unit (maximum depth 4 m) in which a Mollisol has developed under grassland vegetation. In many areas that are close to active eolian deposits, the Mollisol is buried by a more recent sandy unit (Fig. 2). This second younger sandy layer has a poorly developed soil (Cryopsamment) in it. The Mollisol is conspicuous in each area, occurring in late Holocene eolian and colluvial deposits. In all cases, this soil shows a similar morphology. The organic matter content, base saturation, thickness and Munsell color of the buried A horizon (Table 2) define it as a mollic epipedon, although there is some variability within each location according to the slope angle and erosive processes acting before burial. Structure is not well ARTICLE IN PRESS 160 C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 Table 1 Localities analyzed in Fuego–Patagonia and geomorphic context Area Locality Mollisol’s geomorphic context A. Lago Argentino 1. Estancia Alice [501200 S/721310 W] B. Magellan Strait 2. Cabo Vı́rgenes/Dungeness [521200 S/ 681210 W] 3. Cerro de los Gatos [531150 S/681340 W] Dune fields and sand sheets in the southern margin of the lake (Photo 1). Dune fields and sand sheets developed on marine terrace and glaciogenic drift. Colluvial talus of the Holocene paleocliff. Sand sheets developed on the tertiary sandstones of the hill (cerro) (Photo 2). Colluvial talus of the hill. Dune fields and sand sheets developed on the hill. Colluvial talus of the hill. Dune fields and sand sheets developed on a broad barrier beach. Colluvial talus of the Holocene paleocliff. C. San Sebastian Bay 4. Cerro Cabeza de León [531180 S/681330 W] 5. Los Chorrillos [531190 S/681170 W] Fig. 2. Photographs of the Mollisol at Estancia Alice and Cerro de los Gatos localities. Note at the first locality the presence of abundant guanaco bones (Lama guanicoe) below the soil. are sensitive to localized environmental changes, and are not well represented at regional scales (see Fig. 4). The scales of the INTA surveys in Fuego–Patagonia were inadequate for mapping this Mollisol. 4.2. Chronology Fig. 3. Representative profiles showing the general sequence at each area: A–C (for description see Table 2, localities 1–3). developed because of the sandy matrix. The mollic epipedon and the general characteristics of the profiles (Table 2, Fig. 3) enable classification of this soil as a Mollisol. According to soil moisture and temperature regimes in each area (see study areas and geological background), it can be considered a Haplocryoll in northern Tierra del Fuego, and a Haploxeroll in southern Santa Cruz (Wambeke and Scoppa, 1976). This pedologic event is evident only in eolian and colluvial deposits that Two radiocarbon ages of 3700+70 yr BP (LP-607) and 3690+70 yr BP (LP-776) from bone collagen at Cerro Cabeza de León (San Sebastian Bay) and a maximum age of 5700+180 yr BP (AC1599) from a cultural shell midden at Cerro Las Bandurrias (located near Cerro Cabeza de León), were obtained from the base of the lowest sandy unit at each location. These samples were placed approximately 2.5, and 1 m from overlaying buried soil, respectively. The late Holocene Mollisol was dated by radiocarbon and OCR methods (Table 3). Maximum ages were obtained from materials generally of archeological origin located in sedimentary units beneath the developed soil. The dated materials included bone (Lama guanicoe or, if indicated, pinnipeds [pp]), charcoal, and seashells (Mytilus edulis or, if indicated, gastropods [gp]) (see Table 3). Minimum ages were obtained by OCR in the AC horizon (lower boundary) of the Mollisol. These represent the apparent mean residence time (AMRT) (Scharpenseel, 1971; Matthews, 1985) of the organic matter in this ARTICLE IN PRESS C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 161 Table 2 Soil description at each locality Area Loc. Horizon Depth (cm) Color (dry) Text. Struct. Cons. (dry) pH A 1 (A)–C Ab1 ACb1 Cb1 0–100 100–120 120–135 135–170+ 2.5Y 2.5Y 2.5Y 2.5Y 4/3 4/2 4/2 4/3 S LS S S SG M M SG Lo So So So 7.2 6.8 7.1 7.1 (A)–C Ab1 ACb1 Cb1 0–15 15–33 33–41 41–80+ 5Y 5Y 5Y 5Y 4/2 2.5/1 3/1 3/2 S LS S S SG M M SG Lo So So So 7.1 7.0 7.4 7.2 (A)–C Ab1 ACb1 Cb1 R 0–20 20–50 50–60 60–120 120+ 2.5Y 5/3 2.5Y 3/2 2.5Y 5/2 2.5Y 5/3 N.A. S LS S S N.A. SG M M M N.A. Lo LH So So N.A. 7.6 6.8 7.0 7.4 N.A (A)–C Ab1 ACb1 Cb1 0–35 35–70 70–80 80–120+ 2.5Y 2.5Y 2.5Y 2.5Y Sg LSg Sg Sg SG M M M Lo LH So So 7.6 7.0 6.6 7.2 (A)–C Ab1 ACb1 Cb1 0–200 200–235 235–275 275–310+ 10YR 10YR 10YR 10YR S LS S S SG M M SG Lo LH So So 7.2 6.7 7.5 7.9 B C 2 3 4 5 4/3 3/2 4/2 4/3 5/1 3/2 5/2 5/1 OM–B. sat. (%) Bound 3.3–65 A&S G&S D&W 7–92 A&S G&S D&W 3.6–70 A&S G&W D&W A&S N.A. 4.1–71 A&S G&W D&W 7–93 A&S G&W G&W Texture (Text.): S (sand)—LS (loamy sand)—g (with S or LS indicates subordinated small gravel). Structure (Struct.): SG (single grain)—M (massive). Consistence (Cons.): Lo (loose)—So (soft)—LH (slightly hard). OM: organic matter (%)/B. sat.: base saturation (%) [values obtained for Ab1 horizon only]. Boundary (Bound.): A (abrupt)—C (clear)—G (gradual)—D (diffuse)/S (smooth)—W (wavy). Fig. 4. Geographic distribution of eolian and colluvial units at Cabo Vı́rgenes (Magellan Strait) illustrating the discontinuity of the Mollisol’s expression across the landscape. 1. Glaciogenic deposits/ 2. Eolian deposits: dunes - sand sheets, and colluvial deposits: paleocliff’s talus in the southern limit of the glaciogenic unit/ 3. Beach ridges/ 4. Marshes/ T1 ¼ geologic transect’s number/ CVI ¼ archaeological site’s number at Cabo Vı́rgenes/ Faro ¼ Cabo Vı́rgenes Beacon/ RO ¼ Road Observation. horizon, producing average values which provide an age that should represent the beginning of the soil development (Ellis and Mellor, 1995). The archeological material found beneath the Mollisol at the localities studied (i.e. charcoal; lithic and bone artifacts; marine shells; bone remains of mammals, fishes, and birds) ARTICLE IN PRESS 162 C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 Table 3 Minimum and maximum ages for the beginning of the mollisol’s development in southern Patagonia and northern Tierra del Fuego Areas and localities sampled Minimum ages (OCR) horizon AC Maximum ages (14C) and material dated at horizons AC/C calibrateda A-1. Estancia Alice B-2. Cabo Vı́rgenes C-3. Cerro de los Gatos C-4. Co. Cabeza de León C-5. Los Chorrillos 632718 BP (ACT#3644) 860725 BP (ACT#3642) 828724 BP (ACT#4077) 617 BP (ACT#3227) 846725 BP (ACT#3857) 953728 BP (ACT#3856) 445713 BP (ACT#3858)b 1032730 BP (ACT#4794) 958728 BP (ACT#4793) 887726 BP (ACT#4792) 979729 BP (ACT#4796) 772 BP (ACT#2802) 620 BP (ACT#2801) 661719 BP (ACT#3643) 574 BP (ACT#2800) 980729 BP (ACT#4079) 1370770 BP (bone) (Beta11112231) 1480770 BP (bone) (Beta11112232) 1316–1259 1409–1299 13807180 BP (bone) (AC1523) 1190760 BP (charcoal) (GX-25772) 1050770 BP (bone - pp) (GX-25276-G) 1160770 BP (bone – pp) (Beta144999) 1170750 BP (charcoal) (Beta144998) 1099–720 1171–994 658–546c 750–644c 1161–988 9007115 BP (shells - gp) (AC 1483) 1600760 BP (bone) (LP-413) 578–432c 1539–1405 1070780 1479795 1483780 1420790 1060–924 1123–920 1105–932 1046–887 BP BP BP BP (charcoal) (Beta-51997) (shells) (AC 1403) (shells) (AC 1404) (shells) (AC 1484) a 1s using CALIB 3.0.3. method A (Stuiver and Reimer, 1993). Profile sampled in wet conditions. c If the reservoir effect is taken as 400 years (introduced by the calibration program) then this value is lower than the minimum age. Taphonomic analysis on these marine samples suggests sinsedimentary incorporation. Therefore, a lower reservoir effect value may be considered for this material. b does not exhibit surficial alteration such as that produced by weathering and wind abrasion. This condition indicates the predominance of high rates of sediment accumulation, and as a consequence, these materials predate the soil forming interval. It is difficult to determine an accurate age because AMRT values vary according to the depth of the sample, and the lower boundary of the AC horizon is not clear but typically gradual to diffuse. Older AMRT values are closest to the beginning of the pedologic event, due to the fact that the regional development of the Mollisol should have constituted a relatively synchronous process in response to climatic factors (see next part); however, minor chronological variations at a local scale are to be expected. Maximum ages were obtained from bones, shells, and charcoal (hearths) recovered from the AC and C horizons. Taphonomic variables, such as depth, position, size, shape, and completeness, were used to determine whether the bones were intrusive and, therefore, whether they are reliable indicators of ages before the development of the soil. The same criteria were used for molluscs. The earlier ages obtained are considered to be closest to the regionwide origin of the Mollisol. This suggests that the Mollisol began developing close to, or soon after, AD 1000. Soil samples for OCR dates were obtained from the top of the A horizon at several locations and are considered maximum ages for the burial of the soil in those places (Table 4). Ages do not vary in relation to the thickness of the deposit above the Mollisol. This suggests that the beginning of the burial is synchronous in most localities. In the case of the Cabeza de León dunes, however, the soil is older than average, which may indicate an earlier burial at this location. 4.3. Paleoclimatic considerations Chronological data discussed above suggest a correspondence between the beginning of the Mollisol development and the presence of climatic fluctuations in southern Patagonia that would coincide with the so-called medieval warm period (MWP) from the 10th to 12th centuries AD (Lamb, 1977) or medieval climatic anomaly (MCA), a less precise characterization of conditions (Stine, 1994). These climatic fluctuations are present in both southern and northern Patagonia, according to different proxy data. 4.3.1. Dendroclimatic and relict stump records Stine (1994) has proposed that a moist period followed severe drought conditions at Lago Argentino (LA) and Lago Cardiel (LC), based on studies of relict tree stumps of Nothofagus sp. from Santa Cruz. The wetter conditions are related to a tree-killing transgression of these lakes. Radiocarbon ages place the time of tree death at the two lakes as AD 1051–1226 and AD 1021–1228, respectively (Stine, 1994). The previous severe drought is related to hydrologic disturbances referred as to the MCA (Stine, 1994, 2000). On the other hand, dendroclimatic studies in northern Patagonia (Villalba, 1990, 1994) based on larch (Fitzroya cupressoides) from Rı́o Alerce in Rı́o Negro Province, ARTICLE IN PRESS C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 163 Table 4 OCR dates from the top of the a horizon and thickness of the overlying deposit Localities sampled OCR dates (Horizon A) Thickness of overlying deposit (cm) 2. Cabo Vı́rgenes 12073 BP (ACT#3854) 14474 BP (ACT#3855) 170 BP (ACT#2034) 17775 BP (ACT#4080) 558716 BP (ACT#4076) 170 BP (ACT#2035) 202 BP (ACT#3226) 70 160 20 40 103 5 20 3. 4. 4. 5. Cerro de los Gatos Cabeza de León—colluvial talus Cabeza de León—dunes Los Chorrillos Table 5 Comparison of dendroclimatic and multi-proxy results during medieval time in patagonia Northern Patagonia Fuego–Patagonia Villalba, 1990, 1994 Stine, 1994 Mauquoy et al., 2004 Cold and wet AD 900–1070 Warm and dry AD 1080–1250 Severe drought Wet AD 1021–1228 (LA) Wet AD 1051–1226 (LC) Low water tables AD 960–1020 Cooler/wetter AD 1030–1100 established the following intervals: (1) AD 900–1070, cold and moist; (2) AD 1080–1250, warm and dry (correlating with the MWP of Villalba); and (3) AD 1270–1660, cold and moist. Thus, Villalba’s studies in northern Patagonia during medieval times indicate a recurrence of warm and dry conditions, while Stine’s research in southern Santa Cruz demonstrates an increase in moisture in that area for the same period (see Table 5). These results should not be considered contradictory but, rather, illustrative of the likely atmospheric circulation pattern at those times. 4.3.2. Pollen and lake sediment records Pollen evidence from archaeological sequences at Lago Argentino (Charles Fuhr 2 and El Sosiego 4 sites) and the Rı́o Chico basin (Don Ariel Cave and Markatch Aike 1 sites) indicates an increase in moisture about 1500–1100 yr BP and about 1200 yr BP, respectively (Mancini, 1998a, b; Borromei and Nami, 2000). At Lago Argentino there is an increase in Poaceae, Cyperaceae, and Caryophyllaceae and a decrease in shrubs (Mancini, 1998a). In the Rı́o Chico basin, the xeric shrub steppe is replaced during this period by a herbaceous shrub steppe of Graminae and Compositae Tubuliflorae, which indicates greater water availability (Borromei and Nami, 2000). At Potrok Aike Lake, south of the Rı́o Gallegos basin, there is recorded evidence of wet and dry periods of differing length beginning in AD 400. Between AD 1120 and 1240 a lake level high stand was recorded indicating humid conditions (Haberzettl et al., 2005). 4.3.3. Multiproxy analyses Changes in temperature and/or precipitation were inferred from plant macrofossils, pollen, fungal spores, testate amoebae, and peat humification of a raised bog deposit in Tierra del Fuego (Mauquoy et al., 2004). These studies suggest that a period of low local water tables occurred in the bog between AD 960 and 1020, which may correspond to the Medieval Warm Period from AD 950–1045 according to tree ring data recorded in the Northern Hemisphere. A period of cooler or wetter conditions was detected from about AD 1030–1100, and a later period of cooler or wetter conditions from about AD 1800–1930 (Mauquoy et al., 2004). In summary, the period of wetter conditions detected about 1000 AD by the former proxy data in Fuego–Patagonia would have favored the stabilization of eolian and colluvial sediments by plant colonization, and marked the beginning of the regional-scale soil-forming interval. According to Stine (1994), the aberrant atmospheric circulation of medieval time seems to have brought to some regions of the world a far greater departure in precipitation than in temperature. The variables involved in the burial of this late Holocene soil are already being explored at several sites. According to Table 4, the burial process appears to be relatively recent. This process may have been the result of climatic factors, such as the occurrence of dry periods after the improvement in environmental conditions detected by dendroclimatic and glacial studies (see Röthlisberger, 1986; Rabassa and Clapperton, 1990; Aniya, 1996; Luckman and Villalba, 2001, for the latter), or it may have been due to anthropic reasons, such as the human impact on the process of erosion and desertification of Patagonia since the 19th century. These factors may also have acted together or may have been superimposed. 4.4. Simulation models and climatic patterns Global warming climatic simulation models for future conditions in Patagonia (Labraga, 1997, 1998; Labraga ARTICLE IN PRESS 164 C.M. Favier-Dubois / Quaternary International 162–163 (2007) 158–165 and López, 1997) show a rainfall pattern similar to extant paleoclimatic data for the period considered (Table 5). The models forecast warmer and drier conditions for northern Patagonia but greater humidity in southern Patagonia and Tierra del Fuego. This coincidence requires discussion. If the MWP or MCA was due to a global thermal maximum (of probable solar origin, according to authors such as Jirikowic and Damon, 1994), the correspondence between the data from the simulations and the available climatic–environmental evidence for the period could be tested. Such an analogy is postulated assuming similar responses within the climatic system. Wetherald and Manabe (1975) had already demonstrated that a 2% increase in the solar constant would produce a heating of the troposphere of the same magnitude as a doubling in the CO2 concentration. Through d 18O studies of seashells (Obelic et al., 1998), an increase in mean temperature, in agreement with the simulation results obtained by Labraga (1997, 1998) and Labraga and López (1997), was recorded for the beginning of the last millennium in Beagle Channel waters. In the central region of Argentina, several climatic indicators suggest warm and humid conditions for the MWP (Iriondo and Kröhling, 1995; Iriondo, 1999; Cioccale, 1999), that are also consistent with the simulation models. This interval corresponded to a climatic improvement in this region indicated by soil development, expansion of fluvial and lacustrine systems, and the formation of swamps in depressions from about 1000 BP to shortly before the Little Ice Age in the 14th century (Carignano, 1997; Cioccale, 1999). A way to explore the correspondence between climatic patterns and soil distribution in Patagonia is by mapping Mollisol limits throughout the entire region. This task has begun, and it is interesting to note that investigations of dune fields and sand sheets along 300 km of the northern Patagonia coastline (San Matı́as Gulf, Rı́o Negro Province) have not indicated the presence of this soil. This finding agrees with expectations from the climatic patterns predicted by the simulation models. 5. Conclusions The sequences studied at three areas in Patagonia and Tierra del Fuego correspond to middle and late Holocene eolian and colluvial deposits. They exhibit active morphogenesis from approximately 5700 yr BP, related to regionalscale dry conditions. From about 1000 years ago improved environmental conditions are indicated by the development of a Mollisol in eolian and colluvial systems, which are highly sensitive to changing conditions. This soil is buried where reactivation of erosion-deposition processes occurred due to natural and anthropic causes; the case at many of the localities analyzed. On a macro regional scale, the Mollisol functions as a chronostratigraphic unit, enabling the evaluation of long-term changes in archeological distributions (Favier-Dubois, 2003). The Mollisol’s emergence in southern Patagonia and Tierra del Fuego was related to an environmental anomaly (e.g. an important increase in humidity) linked to fluctuations during the MWP as indicated by study of relict stumps and other multiproxy indicators (Stine, 1994; Mauquoy et al., 2004). The pollen and lake sediment records in localities of southern Santa Cruz also suggest an increase in humidity at that time (Mancini, 1998a, b; Borromei and Nami, 2000; Haberzettl et al., 2005). The paleoclimatic results obtained in northern and southern Patagonia are consistent with the results of the global warming climatic models for the region (Labraga, 1997, 1998; Labraga and López, 1997) and allow us to evaluate similarities in the response of the climatic system. The regional mapping of the distribution of this Mollisol in Patagonia, currently in progress, will contribute to a better understanding of the climatic patterns through time. Acknowledgments I wish to thank Dr. Marcelo A. Zárate, whose insightful comments helped strengthen this paper. I am grateful to Santiago G. Arrondo for correcting my english. I would like to thank all my colleagues for their continual collaboration. I remain very grateful to Carolyn Olson for her help in many ways and to the anonymous reviewers of this manuscript. Finally, I would like to express my gratitude to the Consejo Nacional de Investigaciones Cientı́ficas y Técnicas (CONICET), the Universidad de Buenos Aires, and the Agencia de Promoción Cientı́fica y Tecnológica for providing funds for this research. References Aniya, M., 1996. Holocene variations of Ameghino Glacier, Southern Patagonia. The Holocene 6, 247–252. Borrero, L.A., (Ed.), 1998. Arqueologı́a de la Patagonia Meridional (Proyecto Magallania). 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