Loess–paleosol sequence of La Mesada in Tucuman province, northwest Argentina characterization and paleoenvironmental interpretation

Journal of South American Earth Sciences 12 (1999) 293±310

Loess±paleosol sequence of La Mesada in Tucuman province, northwest Argentina characterization and paleoenvironmental interpretation J.A. Zinck a, J.M. Sayago b,* a

International Inst. for Aerospace Survey and Earth Sciences (ITC), Enschede, The Netherlands Instituto de Geociencias y Medio-Ambiente (INGEMA), TucumaÂn University, TucumaÂn, Argentina

b

Abstract A 42 m thick loess±paleosol sequence was studied in the dry pre-Andean valley of Ta®-del-Valle, at the La Mesada site (2280 masl), TucumaÂn Province in northwest Argentina. The sequence contains 28 paleosols interbedded with 26 loess layers. The latter are coarse loamy and have large hexagonal polyhedrons. The soil layers are restricted to ®ne loamy Bt horizons, with prismatic structure and organo-argillans on the structural surfaces. Surface (A) and eluviation (E) horizons are systematically absent from the whole sequence. Soil truncation is unlikely as no remains of such horizons or erosional uncomformities were identi®ed. The dated part of the section encompasses a time span of 10,080 years, from 17,580 BP at 5.2 depth to 27,660 BP at 42.3 m depth. A total of 20 Bt/C pairs developed during this time interval, corresponding to a climatic change every 500 years on the average between dry±cool conditions, promoting loess in¯ux, and moist±warm conditions favouring soil development. The depth function of the Bt/C clay ratios re¯ects the climate improvement during the soil formation interstadials, with an optimum in the upper part of the sequence. This might be related with recurrent northward shifts of the polar front, accompanied by the weakening of the mid-latitude South Paci®c anticyclone. Similar short-term periodicity has been identi®ed in the ice cap of Greenland, highlighting the global character of such frequent climatic changes during the Pleistocene. # 1999 Elsevier Science Ltd. All rights reserved.

Resumen El trabajo se enmarca en el contexto geogra®co del Loess Subtropical situado al Norte de los 308S en territorio argentino. Se estudia una secuencia de mas 40mts. situada en un valle intermontano de los cordones preandinos constituõÁ da por capas de loess que alternan con mas de 20 paleosuelos, cuyas caracterõÁ sticas extrõÁ nsicas (secuencia de horizontes, espesor, extructura, color, consistencia, etc.) e intrinsecas (granulometria, pH, capac. de intercambio, carbonatos, etc.) son detalladamente descriptas. El espesor de las capas de loess, (que no supera los 3m en la base del per®l) dismininuye juntamente con el Carbonato de Ca hacia la parte superior del per®l mientras que los paleosuelos incrementan su espesor. Teniendo en cuenta las dataciones (AMS) efectuadas se in®ere que la secuencia se desarrollo en aproximadamente 10.080 anÄos, desde 17.580 AP a 5.2.m de profundidad, hasta 27.660 AP a 42,3m, (actual base del per®l). Asumiendo que solamente la neoformacioÂn de arcilla explica las grandes diferencias existentes entre horizontes Bt y C y que dicho proceso pudo estar condicionado por el tiempo y/o el clima, mediante un indice basado en la relacioÁn arcilla Bt/C se establecieron cuatro intervalos temporales con diferentes condiciones hõÁ dricas a lo largo del desarrollo de la secuencia. En tal sentido, es sugestivo que el intervalo mas huÁmedo coincide con el maÂximo glaciar durante el Pleistoceno Tardõ o. Respecto a la variabilidad climaÁtica, dentro del intervalo temporal datado (10.080anÄos) se habrian producido 20 oscilaciones climaÁticas (correspondientes a 20 secuencias Bt/C), lo que quivaldrõ a en promedio a la presencia de un marcado cambio climaÁtico cada 500 anÄos. Ello indicarõ a que al menos durante el Pleistoceno Tardõ o el clima fue

* Corresponding author. E-mail address: [email protected] (J.M. Sayago) 0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 9 9 ) 0 0 0 1 9 - X

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extremadamente variable en el valle de TafõÂ y probablemente tambien en la regioÁn de los valles preandinos del noroeste de Argentina. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Studies conducted over the last ten years by Sayago et al. (1987), Camino (1988), Collantes and Sayago (1990), Powell et al. (1993), and Collantes et al. (1993) have contributed to the biostratigraphic and paleoenvironmental characterization of the pre-Andean valleys in northwest Argentina. An important sequence of paleosols interbedded with loess layers containing vertebrate fossils has been recorded in the intermountain valley of Ta® (TucumaÂn province) and labeled Ta®-del-Valle formation (Collantes et al., 1993). The cyclic repetition of consecutive loess±paleosol pairs suggests that climatic conditions were extremely variable during the upper Pleistocene and lower Holocene. After sketching the continental context of the Argentine loess cover and the regional context of the Ta®-del-Valle formation, this paper analyses a typical loess±paleosol sequence at the La Mesada site, with the aim of interpreting its formation and inferring the paleoenvironmental conditions prevailing during the late Pleistocene and early Holocene.

2. Pampean and subtropical loess deposits The South American loess mantle, east of the Andes, covers an area of 2000 by 1000 km between latitudes 208S and 408S (Fig. 1). Loess deposits are particularly widespread through the northern half of Argentina, but also penetrate the west of Uruguay (Anton, 1976), south of Paraguay (Bender, 1995) and south of Brazil (Bombin, 1976). Loess originated from the glacio¯uvial sediments deposited during the last Pleistocene glacial period in front of the Patagonian ice cap and on the piedmonts of the Andean glacial valleys. Selectively de¯ated ®ne sand and silt particles were blown northwards by the prevailing winds during the late Pleistocene and early Holocene. The abundance of volcanic glass indicates that the loess has been either contaminated during transportation and/or after deposition by volcanic eruptions in the Andes (Teruggi, 1957; ZaÂrate and Blasi, 1993) or that Patagonian and Andean glaciers mainly eroded pyroclastic rocks (Gonzalez Bonorino, 1965). The richness in volcanic fragments, together with low contents of calcium carbonate and quartz, clearly distinguishes the Argentine loess from other kinds of loess such as those found in Europe and North America (Sayago, 1995). Because of its large extent, the Argentine loess cover shows signi®cant south±north spatial variations in par-

ticle size distribution and mineralogical composition, re¯ecting transport distance, depositional environment and post-depositional evolution. This has led to he recognition of two main types of loess, the Pampean loess in the south (MAT: 14±188C) and the subtropical loess in the north (MAT: 18±248C). The Pampean loess occurs between latitudes 308S and 408S, throughout the extensive Pampa plains having today a moderately humid environment. North of the former, the subtropical loess covers the latitudinal belt between 208S and 308S, corresponding to the present-day subtropical area of the Chaco plain in the east, and the pre-Andean mountains and intermontane valleys to the west. The mean annual rainfall varies gradually from 600 mm in the east to 1000 mm in the west through the ¯at Chaco and Pampa plains, and between 400 and 2000 mm in the mountains according to aspect and elevation. Being closer to the Patagonian sediment sources, the Pampean loess is coarser than the neotropical loess. Typically, textures are sandy loam. Particle size fractions vary from very ®ne sand to coarse silt, and the medians range from 0.05 to 0.062 mm. In contrast, the subtropical loess is mainly silt loam and silty clay loam, with a dominance of medium silt and medians between 0.016 and 0.025 mm. The silt fraction accounts for about 80±90% of the particles (Sayago, 1995). There are also important mineralogical di€erences between the two types of loess. The Pampean loess contains abundant plagioclase feldspars (20±60%), less quartz (20±30%), but relatively high amounts of volcanic glass shards and other pyroclastic rock fragments (15±20%). Comparatively, the subtropical loess of the pre-Andean intermontane valleys contains signi®cantly fewer feldspars (3±10%) and less volcanic glass (3± 12%), although the quartz contents remain similar (18±27%). During the Holocene, the alternating wet± dry subtropical climate of this area may have accelerated the process of devitri®cation and feldspar weathering. In the drier Chaco plain, the volcanic glass content rises to 30%, suggesting either less active mineral weathering or post-depositional addition of pyroclastic fragments (Sayago, 1995). Despite di€erences in particle size and coarse mineral content, the Pampean and subtropical loesses are similar in clay mineralogy and chemical composition. In both types of material, illite is the dominant mineral, with minor proportions of smectite and kaolinite. Silicium oxides (57±60%) and aluminum oxides (15± 17%) dominate the chemical spectrum of the whole loess mantle, with minor spatial variations in MgO, K2O and Na2O (Arens, 1969; CamilioÂn, 1993). One of

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295

Fig. 1. Approximate distribution of loess deposits in Argentina (adapted from Teruggi, 1957).

the most striking similarities is the ubiquitous presence of rhythmic loess±paleosol sequences, which re¯ect repeated climatic oscillations, during the late Pleistocene and early Holocene, between dry±cool periods corresponding to loess deposition and moist± warm periods favoring soil formation (Imbellone and Teruggi, 1993; Sayago, 1995). Similar loess±paleosol sequences have been used as recorders of paleoclimatic variations during the last glacial-interglacial cycle in the Loess Plateau of North China (Derbyshire et al., 1995). 3. The Ta®-del-Valle formation In the TucumaÂn Province, northwest Argentina, the topographic contact between the low-lying Chaco plain to the east and the Andean Cordillera to the west is very abrupt, delineated by a series of parallel, SSW±NNE oriented fault lines. In a short distance, el-

evation rises from about 500 masl in San Miguel de TucumaÂn, the provincial capital city, to more than 4000 masl in the pre-Andean chain of Cumbres de CalchaquõÂ es. Mountain ridges are separated by tectonic valleys and basins. The valley of the Ta® river is one of these intermountain basins, squeezed between the elongated highlands of Cumbres de CalchaquõÂ es y Mala-Mala to the east and the Sierra de Aconquija to the west (Fig. 2). During the upper Pleistocene and lower Holocene, closed basins like the Ta® valley trapped considerable amounts of northward windblown loess in subtropical environments. The town of Ta®-del-Valle is located about 100 km west of the city of San Miguel de TucumaÂn, at an elevation of 2000 masl. The bottom of the valley and the lower ¯anks of the surrounding mountains are covered by primary and reworked loess sediments. The deep incision of the Ta® river and its tributary arroyos caused the exposure of loess±paleosol sequences, where

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J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

Fig. 2. Ta®-del-Valle formation and location of radiocarbon dated paleosol sites (adapted from Collantes et al., 1993).

several stratotypes have been recorded to characterize and formally establish the Ta®-del-Valle formation. The Ta®-del-Valle formation is composed of subhorizontal loess layers interbedded with paleosols. Locally, cineritic (volcanic ash) layers are included. The post-depositional incision of the drainage network transformed the original loess mantle into small dissected mesas, with 20±50 m high risers and ¯at to undulating, slightly inclined treads. Near the front of the mountain slopes, the loess surface is sometimes buried by cenoglomeratic cover materials. In general, the loess lies on a phyllite substratum belonging to the Puncoviscana formation (Late Precambrian to Cambrian). Locally, an erosional unconformity separ-

ates the loess from underlying Tertiary sedimentary rocks. Regionally, the formation is restricted to the Ta® valley, with the deepest sections in the center of the valley. The formation also extends into the lateral vales incising the surrounding mountains, but with fewer intercalated paleosols and more psephitic local material interbedded with loess layers. The holostratotype has been described from a natural wall exposure on the right bank of the Ta® river, in a place called Zanja-del-Chivo of the Angostura area, at 1870 m elevation and coordinates of 26856 '400S-65840 '400W (Collantes et al., 1993). The section is 18 m thick and includes yellowish loess layers interbedded with ®ve brownish to reddish

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

brown paleosols. The loess material contains more than 50% silt. The minerals of the sand fraction are mainly quartz, K-feldspars, plagioclase feldspars and volcanic glass, with smaller amounts of micas, tourmaline, zircon, apatite and epidote. The mineral composition of the silt fraction is similar to the former, but the proportions of plagioclases and volcanic glass signi®cantly increase. Clay minerals are mainly illite and kaolinite, with some minor components of smectite, quartz and feldspars. Intercalated paleosols are 0.7±2 m thick and exhibit morphological features of B horizons, including coarse prismatic structure and organoclay cutans. Calcium carbonate nodules and fossil roots are frequent. Paleosols are redder in the upper part of the section than in the lower. Transitional subhorizons between the paleosols and the overlying loess layers show sometimes eluviation features, causing greyish colors and high porosity. Fossil skeletons of large herbivore vertebrates have been identi®ed in an upper cineritic layer and three deeper loess layers, separated by paleosols (Powell et al., 1992). The fact that all fossils were found buried within loess material indicates that death and fossiliza-

297

tion of the animals occurred during accelerated loess deposition, with dust storms making breathing dicult and reducing the grass cover. Fossils include Scelidotheriinae at 4±5 m, Gomphotheriidae at 8±9 m, Milodontinae at 11±12 m, and Equidae at 15.5±16.5 m depth. Small intradermic bones of a Milodontinae skeleton at 11±12 m depth were dated at 8660 BP via 14 C determination. This indicates that the upper half of the 18 m section is Holocene, although the above fossil assemblage is usually considered older because it is associated, in the South American context, with other mammals of large biochronological span, such as Megatherium, Glyptodontinae and Mastodon, belonging to the Lujan Mammalian age of the Late Pleistocene. 4. Characteristics of the La Mesada sequence Since the ®rst description of the Ta®-del-Valle formation, additional stratotypes were identi®ed and documented in neighboring localities. In general, the new sections are thicker and contain a larger number

Fig. 3. Local setting of the El RincoÂn area.

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Fig. 4. Paleosol-loess sequence at the La Mesada site (adapted from Sayago, 1995).

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299

Table 1 Selected morphological properties Consistencec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 a

Horizon or layer

Thickness

Texturea

Structureb

Dry

Moist

Carbonatesd CaCO3

Cutanse Clay and humus

Material

Bt C Bt1 Bt2 C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt C Bw C Bt C Bw C Bt C Bt C1 C2 C3 C4 Bt C Bt C Bt

0.9 0.4 0.8 1.0 0.3 0.55 0.45 0.5 0.3 0.4 0.8 0.7 0.4 0.2 0.65 1.8 0.7 0.7 1.8 0.35 1.3 1.0 1.2 0.4 0.5 0.5 1.2 0.6 0.45 1.8 0.5 2.0 0.65 1.8 0.45 0.7 0.3 3.0 0.4 2.5 0.35 0.4 0.45 2.5 0.3 0.75 0.3 0.55 0.33 0.2 0.55 0.2 0.55 0.3

cl l cl l sil l sil sicl sil sicl-cl l-cl sil l sil l sil cl l-cl sil l sil sil-l sil sil sil sicl sil cl sil sil cl sil sil-sicl sil sil sil sil sil sil sil l sil sil sil sil sil sil sil sil sil sil sil sil l

pris/cr massive pris/cr pris/cr blok/md pris/md pris/wk pris/md pris/wk pris/md pris/cr massive pris/md massive pris/cr massive vh pris/md pris/md massive pris/md massive pris/cr massive pris/md massive pris/cr massive pris/md pris/cr massive pris/cr massive pris/cr massive pris/cr massive coln/md massive pris/cr massive pris/md massive pris/md massive pris/md massive blok/wk massive blok/wk pris/md massive pris/md coln/md pris/md

vh eh eh eh h vh vh eh vh eh eh vh eh h eh f eh eh vh eh h eh h eh h eh h eh eh h vh h eh sh eh h vh h eh vh eh h eh vh eh eh h h eh eh vh vh vh eh

ef f vf ef f f f ef f ef vf f vf f ef coat vf vf f vf f vf f vf f vf f vf vf f f f vf fr vf fr f fr vf f vf f vf vf vf vf f f vf vf vf f f vf

± vsn1 coat vsn1 vsn3 vsn2 vsn3 coat coat vsn3 ± ± vsn1 ± ± ± ± vsn1 ± vsn1 ± ± vsn1 vsn1 ± ± ± vsn1 ± coat vsn1 ± vsn1 sn2 sn2 ± ± ± vsn1 sn2 ± vsn1 vsn1 sn2 vsn1 coat vsn1 sn3 sn2 coat vsn2 vsn2 crn2 sn2

tk2 tn1 tk2 md2 tn1 tn2 tn1 md2 tn1 tk2 tk3 ± tn3 ± tk3 Loess tn2 tk2 ± tn2 ± tk2 ± tn2 ± md2 ± tn2 tn2 ± tn2 ± tk3 ± tk3 ± ± ± tn2 ± ± ± tk2 ± tn2 ± ± ± ± tn2 ± tn2 ± tn2

Paleosol Loess Paleosol Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Paleosol Loess Paleosol Loess Paleosol Paleosol Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Paleosol Loess Loess Loess Loess Paleosol Loess Paleosol Loess Paleosol

Texture: l=loam; cl=clay loam; sil=silt loam; sicl=silty clay loam. Structure: blok=blocky; pris=prismatic; coln=columner; wk=weak; md=medium size; cr=coarse size. c Consistency (dry): sh=slightly hard; h=hard; vh=very hard; eh=extremely hard. Consistence (moist): fr=friable; f=®rm; vf=very ®rm; ef=extremely ®rm. d Carbonates: vsn=very small nodule; sn=small nodule; crn=coarse nodule; coat=(powdery) coating. e Cutans: tn=thin; md=moderately thick; tk=thick. Degree of abundance: 1=few; 2=frequent; 3=abundant. b

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Table 2 Selected physical and chemical properties

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Horizon or layer

Depth (m)

Clay (%)

Silt (%)

Sand (%)

pH H2O 1:2.5

Bt C Bt1 Bt2 C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt C Bt C Bt1 Bt2 C Bt C Bt C Bt C Bw C Bt C Bw C Bt C Bt C1 C2 C3 C4 Bt C Bt C Bt

0.90 1.30 2.10 3.10 3.40 3.95 4.40 4.90 5.20 5.60 6.40 7.10 7.50 7.70 8.35 10.15 10.85 11.55 13.35 13.70 15.00 16.00 17.20 17.60 18.10 18.60 19.80 20.40 20.85 22.65 23.15 24.95 25.60 27.40 27.95 28.65 28.95 31.95 32.35 34.85 35.20 35.60 36.05 38.55 38.85 39.60 39.90 40.45 40.78 40.98 41.53 41.73 42.28 42.58

38 15 29 25 4 23

38 49 48 49 56 49

24 36 23 26 40 28

7.5 7.8 7.9 7.8 8.3 8.7

36 4 34 27 3 21

48 70 46 46 63 45

16 26 20 27 34 34

8.6 8.5 8.6 7.6 8.5 7.5

17 5 28 26 8 17 6 15 5 20 8 31 10 30 15 11 28 9 27 6 19 10 17 9 21 15 15 13 17 9 12 16 13 8 9 12 9 24 9 9

38 57 46 48 63 47 53 51 58 53 57 51 64 48 56 57 44 56 53 60 57 60 66 61 57 54 47 57 55 60 52 65 57 58 63 58 53 57 51 42

45 38 26 26 29 36 41 34 37 27 35 18 26 22 29 32 28 35 20 34 24 30 17 30 22 31 38 30 28 31 36 19 30 34 28 30 38 19 40 49

8.2 8.1 7.1 7.2 8.4 7.5 8.1 6.8 9.1 7.1 7.9 6.8 8.2 6.7 7.2 8.6 7.6 9.1 7.3 8.3 8.1 9.9 8.6 8.9 8.2 9.0 8.9 9.1 8.6 9.3 9.1 9.3 8.7 9.1 9.3 8.9 8.9 8.6 8.6 8.6

Carbonate (%)

1.0

1.5 1.0 0.5 0.75 0.75 0.25 1.5 0.5 0.5 0.5 1.75 0.5 0.75 1.25 1.9 2.5 2.75 2.75 2.5 2.5 1.5 0.25

± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Organic matter (%)

Cation exchange capacity cmol(+)/kg soil

0.28 0.10 0.17 0.14 0.30 0.10

22 20

0.14 0.30 0.10 0.28 0.19 0.21 0.14 0.15 0.31 0.34 0.10 0.21 0.35 0.24 0.35 0.21 0.23 0.31 0.39 0.17 0.31 0.41 0.07 0.43 0.10 0.72 0.14 0.50 0.07 0.21 0.17 0.38 0.14 0.30 0.14 0.26 0.07 0.17 0.10 0.50 0.10 0.07 0.50 0.07 0.50 0.10

23

20 18 16

22

21

21

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of paleosols. One of these sequences is located in La Mesada, about 8 km west of the holostratotype, along the road from El Mollar to Estancia Las Carreras (Fig. 2). The present climate of the area is a dry midmontane type, with annual rainfalls of 400±500 mm, mainly concentrated in the summer months from October to March. Average annual temperature is 13.58C, with average winter and summer values of 9 and 188C, respectively. The present soil moisture regime is ustic bordering aridic and the soil temperature regime is mesic. Vegetation is a steppe-like grassland, used for extensive grazing. The section is an arroyo-incised wall on the right bank of the Los Alisos river, at 2280 m elevation at the top of the pro®le, with coordinates of 26857 '150S and 65845 '300W (Fig. 3). The exposed pro®le is 42.5 m deep and contains 28 paleosols interbedded with loess layers (Fig. 4). The morphological, physical and chemical properties of the consecutive strata are summarized in Tables 1 and 2. The di€erential resistance of the paleosols and loess layers to erosion has created a stepwise wall topography that improves access. The pro®le corresponds to the riser of a dissected mesa, resulting from the incision of arroyos into the original loess mantle. The mesa topography is ¯at to undulating, slightly inclined towards the north, parallel to the dip slope of the paleosols and loess layers. 4.1. The loess layers In general, loess layers are individual strata separated by one or sometimes, two consecutive soil horizons. Only at the bottom of the sequence, between 38.9 and 40.8 m depth, a bed of four superposed, slightly di€erent loess layers has been recognized. The total set of 26 loess layers identi®ed in the sequence has an average thickness of 107 cm per layer, but the thickness of individual layers varies largely, from 20 to 300 cm (Table 3).

301

4.1.1. Physical characteristics Morphologically, loess layers are distinguishable from the buried soil horizons because of light colors, massive structure in large polyhedrons, high silt content, e€ervescence in HCl and absence of organo-clay cutans. Dry colors, on ped surfaces and in the matrix, vary from brown (7.5 YR 4/3, 4/4, 5/3 and 5/4) to light brown (7.5 YR 6/3 and 6/4). The most frequent color is brown (7.5 YR 5/4), followed by light brown (7.5 YR 6/3 and 6/4). There are no signi®cant changes in color along the section. The material is essentially massive, without apparent strati®cations. However, a distinctive structural feature, particularly in the upper layers, is the presence of large hexagonal polyhedrons, 20±40 cm in diameter and as high as the thickness of each layer. Polyhedrons are separated by small cracks, 2±3 mm wide and ®lled with calcite sheets. When a polyhedral loess layer is exposed to rainsplash at the terrain surface, water erosion causes the formation of a polygonal microtopography and convex rounding of the polyhedron tops, which contribute to simulating columnar-like structure. Polyhedron formation might result from the dessication and retraction of wet loess deposition, loess being either precipitated by occasional rainstorms or trapped by moist to slightly swampy land surfaces. When dry, loess material is hard to very hard, but friable when moist. Being more porous and thus more resistant to water erosion than the over- and underlying soil horizons, the loess layers form usually overhanging vertical walls. Water easily percolates through the loess material and seeps along the Bt horizon below, causing its backward wasting under the roof of the loess layer above. This results in giving a stepwise pro®le to the outcropping vertical section. Coatings of clay and organic matter complexes (organo-argillans), abundant in the soil horizons, are usually absent in the loess material, with the exception

Table 3 Statistical variations of paleosol and loess properties Properties

Horizon or layer

Mean

Standard deviation

Range

Number of samples

Horizon thickness (cm)

Bt C Bt C Bt C Bt C Bt C Bt C Bt C

53 107 27 33 48 58 23 9 7.9 8.7 (0.5) 1.33 0.17 0.31

22 80 9.4 5.2 9.8 4.8 7.6 3.6 0.7 0.5

20±100 20±300 16±49 19±41 38±66 49±70 9±38 3±16 6.7±9.1 7.8±9.9 ± 0.25±2.75 0.07±0.34 0.10±0.72

28 26 28 24 28 24 28 24 28 24 (1) 22 28 24

Sand (%) Silt (%) Clay (%) ph (H2O 1:2.5) CaCO3 (%) Organic matter (%)

0.8 0.08 0.16

±

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Fig. 5. Textural grouping of the paleosols and loess layers.

of the upper 5 m of the section where thin zonal cutans may occur. The average particle size composition, from 24 loess strata, includes 33% sand, 58% silt and 9% clay. All layers but one are silt loam, belonging to the coarse loamy family, and have more than 50% silt (Fig. 5). Ten out of 24 layers (42%) have 60% or more silt. The clay content might be underestimated because of the diculty of dispersing the loess material, rich in volcanic components, for particle size distribution analysis. 4.1.2. Chemical characteristics Calcium carbonate is present as nodules, from frequent to abundant, and as powdery coatings only in the upper (0±5 m) and lower (35±42 m) parts of the sequence. But the largest part of the section (22 out of 26 layers) contains ®nely divided lime in the mass of the material, ranging from 0.25% to 2.75%. The average pH value of all loess layers is 8.7, with a range from 7.8 to 9.9. Thus, the material is fully base-saturated and free CaCO3 is present in the system. Exchangeable cation population is largely Ca+Mg, but some layers with very high pH might contain appreciable exchangeable Na, coming from the weathering of the plagioclase feldspars. Fossil grass roots or root imprints are frequent in

loess layers, re¯ecting the cumulization process taking place during loess deposition. Organic matter content varies from 0.1% to 0.7%, around an average value of 0.31% which doubles the average value of organic matter in the intercalated soil horizons. Cation exchange capacity is controlled by the amorphous colloidal material contained in the loess, in the presence of high pH values (volcanic components identi®ed in the holostratotype at Zanja-del-Chivo). The average CEC value from 6 layers is 300 cmol (+) per kg clay, a clear overestimation probably caused by insucient clay dispersion during particle size analysis. CEC values are substantially higher in the loess than in the soil horizons (about 70 cmol (+) per kg clay), indicating that the original amorphous volcanic material has been largely preserved in the loess layers. The latter fact con®rms that loess deposition occurred in dry conditions, preventing rapid weathering of volcanic glass and transformation of amorphous colloids into other types of clay minerals. 4.2. The paleosols Compared to the loess layers, the paleosols are thinner, darker, have a prismatic structure, and organoclay cutans on the surface of the structural faces, and

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

303

Table 4 Variations of paleosol and loess properties with depth Horizon thickness (cm)

Sand (%)

Silt (%)

Clay (%)

Depth (m)

Horizon or layer

na

M

SD

R

n

M

SD

R

M

SD

R

M

SD

R

0±10.15

Bt C Bt C Bt C Bt C Cb

9 7 6 5 6 5 7 9 6

57 59 61 120 49 186 31 94 140

22 56 24 46 12 82 9 90 102

40±100 20±180 35±100 50±180 30±65 70±300 20±45 30±250 40±250

9 5 6 5 6 5 7 9

27 35 28 34 23 32 32 31

8.4 5.4 6.5 6.1 4.6 2.3 10.2 6.1

16±45 26±40 18±36 26±41 17±29 30±35 19±49 19±40

45 59 49 59 54 59 53 58

4.3 7.9 2.7 4.5 7.7 22 60 4.6

38±49 49±70 46±53 53±64 44±66 56±61 42±58 51±65

28 6 23 7 23 9 16 11

7.1 5.0 6.4 6.0 6.4 1.9 5.3 3.0

17±38 3±15 15±31 5±10 15±30 6±11 9±24 8±16

10.15±19.80 19.80±31.95 31.95±42.58

a b

n= number of samples; M=mean; SD=standard deviation; R=range of values. Includes one loess layer composed of 4 consecutive strata.

show no e€ervescence to HCl. A striking feature is that paleosols are restricted to B horizons, mostly Bt, and few Bw. Neither A nor E horizons were identi®ed. In general, the soil bodies interbedded with the loess layers are individual horizons. In some cases, two consecutive Bt horizons were distinguished. The thickness of the paleosols is much less variable than that of the loess layers: 20±100 cm for the former, instead of 20± 300 cm for the latter. The average thickness of a set of 28 paleosols is 53 cm, half the thickness of the loess layers (107 cm). Thus, the general relationship is 0.5 m Bt horizon for 1 m loess (Table 3). 4.2.1. Physical characteristics Paleosols are darker than loess layers. The most frequent dry color on ped surfaces is brown (7.5 YR 4/4), but colours vary mostly in the 7.5 YR hue, including value/chroma combinations of dark brown (3/3 and 3/ 4), brown (4/3, 4/4 and 5/4) and strong brown (4/5 and 4/6). In the upper part of the sequence (3.4±16 m depth), soils are systematically redder, mostly dark reddish brown (5 YR 3/4). The ped surfaces are darker coloured than the matrix. Similarly, values and/or chromas are at least one unit darker in the Bt horizons than in the subsequent C layers. Soil structure is medium to coarse prismatic. Prisms are covered by organo-clay coatings, darker colored than the matrix. Cutans vary from thin to thick and from discontinuous to continuous. As there are no overlying A or E horizons, the presence of dark colored organo-argillans in the Bt horizons might be related to a process of dispersion and mobilization of colloids from the overlying C layers, but this is still a debatable issue. The consistency of the soil material varies from hard to extremely hard when dry and from ®rm to extremely ®rm when moist. The combination of extremely hard and very ®rm is clearly dominant. The average particle size composition, from 28 soil horizons, includes 27% sand, 48% silt and 23% clay.

Thus the Bt horizons have signi®cantly more clay than the loess layers (average clay content of 9%). Probably, clay cutans contribute only to a small extent to the clay increase, so that neoformation of clay must take place within the Bt horizons, a process already identi®ed in Mollisols of the Pampean area (Scoppa, 1976). Unlike the loess material which is exclusively silt loam, the soil material spreads over several textural classes, including loam, clay loam, silt loam and, to a lesser extent, silty clay loam, thus covering the ®ne and coarse loamy families (Fig. 5). 4.2.2. Chemical characteristics Soil horizons have no ®nely divided lime in the soil mass, but usually a few CaCO3 nodules are present. The average pH of 28 soil horizons is 7.9, but values vary largely from 6.7 to 9.1. The soil pH is usually 0.5 to 1 unit less than the pH of the associated loess layer. Organic matter content is very low, with a mean value of 0.17%, half the organic matter content of the C layers. Fossil roots do not occur in Bt horizons. The average value of cation exchange capacity in the Bt horizons is 72 cmol (+) per kg of measured clay, thus signi®cantly less than in the loessial material (average of 314 cmol (+) per kg clay). Although Bt horizons contain more clay than the C layers, the contribution of amorphous material to CEC is much less in the former than in the latter. In the Bt horizons, the amorphous colloids of the parent material are transformed into clay minerals, mainly illite, suggesting a dry-wet alternating (ustic) type of climate. 5. Formation of the La Mesada sequence In general, the properties of the Bt horizons (paleosols) and those of the C layers (loess), including thickness, color, texture, structure, consistency, presence/ absence of CaCO3, presence/absence of organo-argil-

304

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

lans, pH, organic matter and CEC, remain strikingly homogeneous throughout the whole sequence of 42 m. Thus, the same conditions prevailing during the loess deposition, on the one hand, and during the subsequent soil formation, on the other, have systematically recurred, with only minor modi®cations, during at least 24 cycles covering the upper Pleistocene and lower Holocene. This cyclicity is the most remarkable feature of the sequence. Similar concentration of paleosols exists elsewhere but is spread over larger vertical sections, as for instance, in the proximity of Luochuan in Shaanxi Province, PR, China, where exposures around 150 m thick contain up to 30 paleosols (Derbyshire et al., 1997). 5.1. Vertical variations of properties and features With the purpose of detecting and analyzing vertical variations along the pro®le, the sequence was divided into four segments approximately 10 m thick each and containing similar numbers of loess layers and soil horizons (Table 4). Thickness of the Bt horizons consistently decreases from 67 cm in the top segment to 31 cm in the bottom segment. In contrast, the thickness of the C layers increases in the same direction from 59 to 140 cm. There is thus evidence that the loess depositional cycles became shorter or less intensive from upper Pleistocene to lower Holocene. Soil development periods became longer or more intensive with evolving time. The loess material remains remarkably homogeneous along the whole section. From top to bottom, the sand fraction of the C horizons slightly decreases from 35% to 31% and the clay fraction slightly increases from 6% to 11%. However, the silt fraction remains stable around 58±59%. Thus, even if the depositional periods shortened from the beginning to the end of sequence, the environmental conditions controlling the loess deposition did not change over a notably long time span. In contrast, the soil formation conditions might have changed over time towards a warmer and moister regime. Indeed, the clay content of the Bt horizons increases from 16% at the bottom of the sequence to 28% at the top, paralleling a trend towards redder colors between 3 m and 16 m depth. Assuming that the enrichment in clay of the Bt horizons through clay illuviation alone cannot satisfactorily explain the large di€erences in clay content between the Bt horizons and the underlying C layers (their respective parent materials), it can be hypothesized that part of the clay content in the Bt horizons results from neoformation in situ. The magnitude of the clay neoformation is then essentially a function of either time span (the longer, the more clay) and/or climate (the more favourable, the more clay). Therefore, the ratio of clay content in the Bt horizon to the clay content in the C layer could be an indicator of clay neoformation. Bt/C clay ratios

Fig. 6. Values of the clay ratio between Bt horizons and C layers.

were established for each couple of Bt horizon±C layer couplet. Adopting a criterion similar to the concept of abrupt textural change used in Soil Taxonomy (USDA, 1975), a ratio of 2, indicating a doubling of clay content from C layer to Bt horizon, could be a threshold value between more favourable and less favourable conditions for clay formation, hence between cooler±drier and warmer±moister climates. The depth function of the Bt/C clay ratios shows a general tendency to higher ratio values from bottom to the upper part of the sequence (Fig. 6). Four consecutive segments can be recognized: . Segment I (27±42 m): ratio values mainly below 2, indicating (climatic) restrictions for soil formation. . Segment II (7±27 m): ratio values moderately high, mainly between 3 and 4, re¯ecting moderately favourable (climatic) conditions for soil formation.

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

305

. Segment III (1.3±7 m): very high ratio values (6.75± 10.16) together with redder colors in the Bt horizons, corresponding to a soil formation optimum. . Segment IV (0±1.3 m): ratio slightly above 2, indicating a return to more restrictive soil formation conditions. To complement the former, a depth function plotting the Bt/C thickness ratios was also constructed, assuming that thicker Bt horizons would re¯ect more favourable soil formation conditions (Fig. 7). The curve highlights a clear break point, at about 7 m depth, between an upper segment where Bt horizons are thicker than the respectively underlying C layers, and the rest of the sequence with Bt horizons being narrower than C layers. Thus, in the upper part of the sequence, incoming loess has been more intensively pedogenized, an outcome which supports the conclusion derived from the Bt/C clay ratio curve. 5.2. Inter-horizon relationships An essential feature of the sequence is the cyclic repetition of B horizons, mostly Bt, assumed to have formed from weathering of underlying loess parent material and from eluviation of overlying A or E horizons. The systematic absence of A and E horizons in the whole sequence calls for an explanation, as well as for a di€erent hypothesis about the formation of the Bt horizons. An obvious assumption is that any existing A horizon was removed by wind or water erosion, before a new loess layer buried the remaining Bt horizon. No erosional evidence, such as incision unconformities, angular truncation of strata, buried rill and pond microtopography, local pavements of coarse fragments and the like, was found. Paleosol horizons and loess layers are strictly parallel and have concordant dip and strike values. Moreover, it is quite unlikely that all A horizons, if they originally existed, would have been fully removed in all 24 Bt/C cycles of the sequence. Thus, it may be that A horizons never existed as such. Explanation of a similar situation was proposed by McDonald and Busacca (1990), when analyzing the absence of A horizons in a paleosol developed from the Palouse loess of eastern Washington state. The authors suggest that, under loess or dust in¯ux, an A horizon will transform to a B horizon upon ®rst burial, and that this new B horizon will start accumulating illuvial material eluviated from the added sediments. Further, as more layers are deposited on the rising land surface, the B horizon will eventually be removed from the active zone of soil formation and preserved in the stratigraphic record. The ``elevating B horizon'' mechanism would digest any incoming material, until the loess in¯ux becomes too abundant and causes fossilization. Thus, Bt horizons result from a dual formation pro-

Fig. 7. Values of the thickness ratio between Bt horizons and C layers.

cess: (1) at the bottom, through weathering of the underlying C layer, leading to neoformation of clay from the amorphous volcanic components of the loess (present in the holostratotype); and (2) at the top, through the eluviation of organo-mineral complexes from the overlying loess cover, causing the coating of the prism faces by organo-argillans. B horizons start being Bw horizons, re¯ecting the weathering of primary minerals from the loess substratum. In a second evolution step, after new in¯ux of loess has taken place on top, they become Bt horizons through humus±clay illuviation. 6. Evolution and time frame of the La Mesada sequence Four loess layers were selected at approximately

0.15 654

0.11 898

0.49 205 1034 a

b

27,660 42.3

Composite strata including more than one C layer or Bt horizon. Assumed date in the hypothesis that loess in¯ux would have stopped at the beginning of the Holocene.

0.33

27.4

14.9

24,610

3050

6

15

12.4

22,000

2610

5 4420 9.8

17,580 5.2

0

5.2

(10,000)b

7580

4

11(8)a

9

2.95

0.07 1467

0.48 210 637 4.1 7(6)a

0.59

0.03

0.22 451 1605 4.0 7(5)a

0.57

3.75 5(4)a

0.75

2021

1458

0.07

3380

(years/m) (cm/year) Mean thickness of Bt (m) Accumulated thickness of Bt (m) Number of soil horizons (Bt) Number of Loess layers (C) Time interval (years) Age (years BP) Thickness per pro®le segment (m)

Table 5 Age and estimated rates of formation of the paleosol±loess sequence

The dated part of the sequence encompasses a time span of about 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the actual bottom of the exposed sequence). If loess in¯ux during the Holocene is assumed to be negligible or merely sporadic, then the top 5 m of the sequence would have deposited in the last 7000±8000 years of the upper Pleistocene. This would mean that the present terrain surface coincides with the summit of the original depositional sequence. However, dated paleosols in adjacent areas indicate that loess deposition substantially continued during, at least, the lower Holocene. This may indicate that the terminal strata of the sequence are missing, either because of a depositional hiatus or because of post-depositional erosion. It is unlikely that only 5 m loess would have deposited in the last 17,580 years, while 37 m deposited in the anterior 10,080 years, although depositional intensity certainly decreased in the very late Pleistocene and Holocene. There is clear evidence of past and present erosion in the immediate surroundings of the pro®le. The original loess mantle has been deeply incised by a dense network of arroyos, subdividing the table-shaped landscape into smaller undulating mesas and convex promontories. The sequence of La Mesada is located at the end of such an eroded promontory. In the vicinity, remains of the original loess are found hanging on lower hill slopes, several meters above the present terrain surface of the described pro®le, and forming small micro-escarpments of a few meters high and wide. It is thus likely that the upper part of the sequence is missing. After completion of the sequence, Holocene erosion partially dismantled the loess cover through drainage incision, sur®cial sheet erosion and, locally, soli¯uxion. As mentioned earlier, such phenomena did not occur during the development of the sequence. How much has been eroded is matter of speculation. Considering that a thickness of only 5 m of sediments represent the period from 17,580 BP to the present, a considerable part of the section may well have been eroded, especially if loess deposition continued during the lower Holocene. Evidence of the latter is provided by dated paleosols from the surrounding areas (Table 6). For instance, vertebrate fossils dated 8660 BP were found in a loess layer at 11±12 m depth in the locality of

Time per accumulated thickness of Bt (years/m)

6.1. Post-depositional erosion of the sequence

(years/m)

Average rate of loess deposition per pro®le segment

Average rate of loess deposition at selected depths

similar depth intervals and sampled for radiocarbon dating to obtain a ®rst time frame of the sequence. Determination of 14C was carried out at the Center for Isotope Research of the University of Groningen, the Netherlands, using the alkali-extract fraction of the organic material <180 m, after removing fossil roots. Primary and derived data are presented in Tables 5 and 6.

(cm/year)

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

Depth (m)

306

Ta® del Valle

Ta® del Valle

Ta® del Valle

Ta® del Valle

Ta® del Valle

Ta® del Valle

BurruyacuÂ

BurruyacuÂ

BurruyacuÂ

3

4

5

6

7

8

9

10

11

b

26833'500S 64834'200W

26827'320S 64844'590W

26827'320S 64844'590W

26857'150S 65845'300W

26857'150S 65845'300W

26857'150S 65845'300W

26857'150S 65845'300W

26857'100S 65845'200W

26856'400S 65840'400W

26854'300S 65847'200W

26850'300S 66844'050W

380

575

575

2280

2280

2280

2280

2230

1870

2540

2180

Elevation of terrain surface (masl)

Fluvio-eolian plain

Dissected piedmont glacis

Alluvial fan with eolian cover Interbedded lacustrine and glacio-¯uvial deposits Dissected glacis with loess± paleosol sequence Dissected mesa with loess± paleosol sequence Dissected mesa with loess± paleosol sequence Dissected mesa with loess± paleosol sequence Dissected mesa with loess± paleosol sequence Dissected mesa with loess± paleosol sequence Dissected piedmont glacis

Geomorphic context

Loess

Loess

Loess

Garmendia-4

BurruyacuÂ-2B

BurruyacuÂ-2A

La Mesada-53

La Mesada-34

Loess

Loess

La Mesada-21

La Mesada-9

Estancia Las Carreras

Zanja del Chivo (La Angostura)

Rio MunÄoz

Rio Blanco I

Site identi®cation

Loess

Loess

Vertebrate bone in loess layer

Vertebrate bone in loess layer

Varves

Alluvium

Type of material (dated)

Center for Isotope Research, University of Groningen, The Netherlands. Gakushuin University, Natural Radiocarbon Measurements Laboratory, Tokyo, Japan.

Ta® del Valle

2

a

Ta® del Valle

1

Coordinates

C determinations in TucumaÂn Province, Northwest Argentina

14

Locality

Table 6 Record of

4.9±5.2

13.7±15.0

25.6±27.4

41.73±42.28

0.91±0.97 1.4±1.6 1.17±1.5

GrN-22632a

GrN-22633a

GrN-22634a

GrN-22635a GrN-22636a GrN-22637a

9±10

GrA-912a

GrN-22630a

10.7±11.9

2.33±2.86

0.91±1.09

Depth (m)

GAK13899bb

GAK13898b

GrN-21783a

Laboratory number

3Bw2B

3Ab

2Ab

C

C

C

C

C

C

Oe

2Bw2

Type of soil horizon

ITC-INGEMA (Zinck and Sayago, 1997) ITC-INGEMA (Zinck and Sayago, 1997) ITC-INGEMA (Zinck and Sayago, 1997)

ITC-INGEMA (Zinck and Sayago, 1997)

ITC-INGEMA (Zinck and Sayago, 1997)

ITC-INGEMA (Zinck and Sayago, 1997)

ITC-INGEMA (Zinck and Sayago, 1997)

ITC-INGEMA (Zinck and Sayago, 1997) INGEMA (Sayago and Collantes, 1991) INEGMA (Sayago and Collantes, 1991) ITC-INGEMA (Zinck and Sayago, 1997)

Reference

6.2902120

3.780240

2.840260

27.6602 1830/1490

24.6102 1230/1070

22.0002 420/400

17.5802 500/470

10.020260

8.6602150

5950290

28402110

Age (years BP)

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310 307

308

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

Zanja del Chivo, 8±10 km distant from La Mesada site (Powell et al., 1992). Similarly, at the locality of Estancia Las Carreras, a short distance northeast of La Mesada, vertebrate fossils dated 10,020 BP were found in a loess layer at 9±10 m depth. It can thus be inferred that loess in¯ux continued during the lower Holocene. In some favorably exposed trapping sites, eolian accretion occurred even more recently, as suggested by a paleosol dated 2480 BP at 54 cm depth in an eolian deposit covering an alluvial fan at the Rio Blanco site, about 15 km north of La Mesada. Also outside the preAndean valleys, in the Chaco plain 50 km northeast of San Miguel de TucumaÂn, paleosols dated from 2840 BP to 6290 BP, at 100 to 150 cm depth, support the hypothesis of continuous in¯ux of loess during the Holocene. At the bottom of the section, the deepest stratum at 42 m depth is dated 27,660 BP, indicating that probably only the upper part of the whole sequence is exposed. Assuming that the last glacial period lasted approximately 100,000 years, as conventionally accepted, the full stratigraphic sequence at La Mesada site might be considerably thicker than the exposed pro®le. 6.2. Rate of loess deposition and speed of soil formation The stratigraphic intervals between dated loess layers are still too large to allow detailed estimates of loess deposition and soil formation. Soil development was not continuous as it was interrupted by periodic loess deposition and, conversely, loess in¯ux repeatedly halted or diminished suciently to allow for soil formation. Moreover, soil formation and loess deposition did not proceed at comparable rates. At maximum intensity, loess deposition was certainly faster than the most rapid soil formation. According to Arnold and Riecken (1964), the formation of one meter solum of a Hapludalf from a weathered loess in Iowa required 4000 years, i.e., a rate of 40 years per cm of soil. The accumulated thickness of the Bt horizons formed in each dated interval suggests accelerated soil formation, from about 600 to 2000 years per m of soil (Table 5), probably related to the ease with which the volcanic components contained in the loess of Ta®-del-Valle are weathered. To more accurately estimate the rates of loess deposition and soil formation, shorter time frames, as for example between two consecutive loess layers separated by a Bt horizon, are needed. With the four radiocarbon dates currently available, only coarse thickness/time ratios can be established, considering that all material of the sequence was originally loess. This does not account for the di€erential speed of loess deposition (faster) and soil formation (slower), nor does it account for the post-depositional thickness modi®cations through compaction of the loess layers and increased bulk density of the soil horizons.

Considering the above reservations, the average rate of land surface accretion was about 0.5 cm/year during the two lower intervals (5660 years together), while it decreased to 0.22 cm/year during the upper interval (4420 years). Obviously, loess deposition during dust storm events must have occurred at higher rates to cause vertebrate death and fossilization. The above ®gures are much higher than those reported for other regions exposed to long-range eolian dust (Inoue and Naruse, 1991). The narrow, elongated, pre-Andean valleys, with complex air circulation, are more favorable to trapping air-borne sediments than large open areas. 6.3. The rhythm of climatic oscillations In the time frame (10,080 years) delineated by the two extreme radiocarbon dates, 27,660 BP and 17,580 BP respectively, 22 loess layers were deposited and 23 soil horizons formed. Reducing the double Bt horizons (3 cases) and the multiple C layers (one case) to one composite unit in each case, at least 20 Bt/C pairs are present in the considered time interval. They correspond to 20 climatic oscillations in 10,080 years, thus a climatic change every 500 years on average. This indicates that, during at least the upper third of the last glacial period (according to the dated interval of the exposed sequence), climate was extremely variable in the valley of Ta® and probably also in the other preAndean intermontane basins, leading to alternating loess deposition and soil development. The radiocarbon dates also allow some framing of the soil formation periods, previously inferred from the depth function of the Bt/C clay ratios. Four segments were recognized (Fig. 6), assumed to correspond to major climatic periods, either more favourable or less favourable to soil development. The interval from 27,660 BP to 24,610 BP ®ts remarkably well segment I of the Bt/C ratio curve, when soil formation conditions were below the threshold value of 2, thus less favourable. Segment II of the Bt/C ratio curve starts at 24,610 BP, but goes beyond the following date of 22,000 BP. If some thickness Ð time proportionality is applied, the segment might end around 20,600 BP. During this time interval (from 24,610 BP to 20,600 BP), the Bt/C clay ratios are above 2, mainly between 2 and 4, re¯ecting more favourable soil formation conditions. Segment III corresponds to a prominent curve peak of three consecutive very high Bt/C ratios, centered around 17,580 BP, but lasting probably from 20,600 BP to 14,600 BP (the latter two dates being estimated from thickness Ð time proportionality). This interval must have been the most favourable interstadial for soil formation in the late Pleistocene. Segment IV, which corresponds to the assumed missing top of the sequence, has conditions similar to those of segment II.

J.A. Zinck, J.M. Sayago / Journal of South American Earth Sciences 12 (1999) 293±310

In conclusion, environmental conditions corresponding to the last glacial period of the Pleistocene, at least in its upper part, were far from homogeneous in Ta®del-Valle. Recurrent climatic changes occurred, favouring alternations of loess deposition or soil formation. Thus soil development repeatedly took place within a comprehensive glacial period. Moreover, the optimum of soil formation, identi®ed from Bt/C clay ratios of segment III, took place over a period of time during which pleniglacial conditions prevailed in other areas.

7. The La Mesada sequence in the global context 7.1. Relationships with the South American air circulation Today's climate around the study site is a dry midmontane steppe climate, with concentrated rainfall in a few summer months when evapotranspiration is high. The soil moisture regime is ustic limiting with aridic. Presently forming soils in the regional context of Ta®del-Valle are mainly Inceptisols, but also Al®sols are developing in favorable topographic settings, such as slightly sloping glacis positions. Thus Bt horizons form under current climatic conditions. The rhythmic repetition of 24 loess layers, interbedded with 28 Bt horizons featuring clay illuviation and pedogenic clay formation, required the recurrence of alternating climatic conditions di€erent from those of today. Thus, during the upper Pleistocene and probably part of the Holocene, climate must have oscillated between dry±cool, promoting loess in¯ux, and moist± warm favoring soil development. This, in turn, points to cyclic changes in the South American air circulation. Evidence of such changes has been provided by Hastenrath (1971), who postulates the hypothesis of recurrent northward shift of the polar front, accompanied by the weakening of the mid-latitude South Paci®c anticyclone. In its displacement towards the north, the polar front would cause westerly humid air masses to cross the lower-elevation Andes in the south and bring, as a consequence, more moisture to the eastern ¯ank of the cordillera. This situation would favor soil development in the inter-Andean valleys. During the intermediate periods, when the polar front was con®ned to higher latitudes, south of the continent, winds blowing northwards from Patagonia would cause cool and dry conditions along the eastern ¯ank of the cordillera, with loess deposition. These changes in the South Paci®c air circulation, if repeated, could constitute the global context explaining the rhythmic alternation of paleosol formation during warmer and moister periods and their fossilization by loess in¯ux during cooler and drier periods.

309

7.2. Correlation with climatic changes in polar regions Short-term (in geologic time) periodicity, similar to that exhibited by the loess±paleosol sequence of La Mesada, has been identi®ed in the ice cap of Greenland (Dansgaard et al., 1993). The last glacial period is divided into 24 warm interstadials, separated by cold stadials of 10,000±15,000 years duration (Bond et al., 1993). Changes from warm to cold are gradual and long-lasting; changes from cold to warm are abrupt and short-lasting. For instance, the Younger Dryas period started with an increase of 78C average temperature in 50 years and a 75% decrease of dust in¯ux in only 20 years (Dansgaard et al., 1989). Correlation between ice records from both polar areas (Dansgaard et al., 1993) and stratigraphic records from tropical regions (Roberts et al., 1993) highlights the global character of such frequent climatic changes during the late Pleistocene.

8. Conclusion In the 42.5 m deep pro®le of the La Mesada sequence, 28 paleosols are cyclically interbedded with loess layers. Paleosols are restricted to Bt horizons, with morphological, physical and chemical characteristics clearly di€erent from those of the loessial C layers. The properties of the Bt horizons and those of the C layers do not change signi®cantly along the whole sequence, indicating that the controlling formation conditions have systematically repeated, with only minor modi®cations, during at least 24 cycles covering the upper Pleistocene and lower Holocene. A striking feature is the complete absence of A and E horizons over the whole sequence, a situation considered to be unlikely to have arisen due to erosion. In this context, the development of the Bt horizon is attributed to a dual formation process, including the neoformation of clay from the underlying C layer and the eluviation of organo-mineral complexes from the overlying loess cover. Bt/C clay ratios were used to divide the pro®le into four stratigraphic segments with either more or less favourable conditions for clay formation and soil development. Length and/or intensity of pedogenesis increases towards the upper part of the sequence. The dated section of the pro®le encompasses a time span of about 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 at 42.3 m depth. This raises the hypothesis that the terminal strata of the sequence might be missing, probably because of post-depositional erosion. The erosion assumption is supported by the presence, in the surroundings of the La Mesada site, of paleosols containing vertebrate fossils dated lower

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Holocene at 9±12 m depth. It can thus be inferred that loess in¯ux continued during the Holocene. On the basis of the four radiocarbon data available, the average rate of land surface accretion was estimated 0.5 cm/year in the lower part of the sequence and 0.2 cm/year in the upper part. Actual rates of loess deposition during dust storms must have been much higher. Accelerated soil formation took place during interstadials of the last glacial period, at rates of about 600±2000 years per meter of soil. In the dated part of the sequence, corresponding to a time interval of about 10,000 years, 20 Bt/C pairs are present, suggesting a climatic change every 500 years on average, from drier and cooler for loess deposition to moister and warmer for soil development. This shortterm climatic periodicity during the upper Pleistocene and lower Holocene, which is comparable to recent ®ndings in the polar regions, required cyclic changes in the general air circulation of South America. The above results correspond to the inception phase of the investigation conducted at the La Mesada site. Additional key layers are being dated with carbon 14 to reduce the span of time intervals and improve the estimation of loess deposition and soil formation rates. Paleoenvironmental indicators, such as clay minerals, magnetic susceptibility, and soil cation exchange capacity, are under determination for soils of the preAndean valleys and the Chaco plain to better approximate the climate changes of the Late Quaternary in northwest Argentina.

References Anton, D., 1976. Relaciones entre las super®cies de aplanamiento y los suelos en el Uruguay. IDIA, INTA Supl. 33, 409±413. Arens, P. 1969. Algunos paisajes geoquõ micos de la regioÂn pampeana. Actas V ReunioÂn Argentina de la Ciencia del Suelo, Santa Fe. Arnold, R.W., Riecken, F.F., 1964. Grainy gray ped coatings in Brunizem soils. Proc. Iowa Acad. Sci. 71, 350±360. Bender, H., 1995. The impact of indirect recharge in ¯at semi-arid and arid areas. Two case histories from South America. In: Natural Resources and Development, vol. 42. TuÈbingen, Germany. Bombin, M., 1976. Modelo paleoecoloÂgico evolutivo para o neoquaternario da regiaÄo da campanha-oeste de Rio Grande do Sul (Brasil), a FormacaÄo Touro Passo, seu conteuÂdo fossilõ fero e a pedogeÂnese poÂs-deposicional. Comun. Mus. PUCRGS, Porto Alegre 15, 1±90. Bond, G., Broecker, W., Johnsen, S., MacManus, J., Labeyrie, L., Jouzel, J., Bonani, G., 1993. Correlations between climate records from North Atlantic sediments and Greenland ice. Nature 365. CamilioÂn, M.C., 1993. Clay mineral composition of pampean loess (Argentina). Quaternary International 17, 27±31. Camino, M. 1988. Estratigrafõ a y evolucioÂn paleoambiental durante el perõ odo cuaternario del valle de La Sala. Unpublished thesis. Facultad de C. Naturales, TucumaÂn University, Argentina. Collantes, M.M., Sayago, J.M., 1990. CaracterizacioÂn estratigra®ca e interpretacioÂn paleoambiental del complejo loess-paleosuelos de

Ta® del Valle, TucumaÂn, Argentina. In: Proc. Intern. Symp. on loess (Extended abstracts). Mar del Plata, Argentina. Collantes, M.M., Powell, J., Sayago, J.M., 1993. FormacioÂn Ta® del Valle (Cuaternario Superior), provincia de TucumaÂn (Argentina): litologõ a, paleontologõ a, paleoambientes. In: Proc. XII Congreso GeoloÂgico Argentino, II, pp. 200±206. Dansgaard, W., White, J.W.C., Johnsen, S.J., 1989. The abrupt termination of the Younger Dryas climate event. Nature 339. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Ste€ensen, J.P., SveinbjoÈrnsdottir, A.E., Jouzel, J., Bond, G., 1993. Evidence for general instability of past climate from a 250-kyr ice-core record. Nature 364, 218±220. Derbyshire, E., Keen, D.H., Kemp, R.A., Rolph, T.A., Shaw, J., Meng, X., 1995. Loess±paleosol sequences as recorders of paleoclimatic variations during the last Glacial-Intergalcial cycle: some problems of correlation in north-Central China. Quaternary Proceedings 4, 7±18. Derbyshire, E., Kemp, R.A., Meng, X., 1997. Climate change, loess and paleosols: proxy measures and resolution in North China. Journal of the Geological Society, London 154, 793±805. Gonzalez Bonorino, F., 1965. Mineralogõ a de las fracciones arcilla y limo del Pampeano en el aÂrea de la ciudad de Buenos Aires y su signi®cado estratigra®co y sedimentoloÂgico. Revista AsociacioÂn GeoloÂgica Argentina 20. Hastenrath, S.L., 1971. On the pleistocene snow-line depression in the regions of Southamerican Andes. Journal of Glaciology 10, 59. Imbellone, P.A., Teruggi, M.E., 1993. Paleosols on loess deposits of the Argentine pampas. Quaternary International 17, 49±55. Inoue, K., Naruse, T., 1991. Accumulation of Asian long-range eolian dust in Japan and Korea from the Late Pleistocene to the Holocene. CATENA supplement 20, 25±42. McDonald, E.V., Busacca, A.J., 1990. Interaction between aggrading geomorphic surfaces and the formation of a Late Pleistocene paleosol in the Palouse loess of eastern Washington state. Geomorphology 3, 449±470. Powell, J.E., Duarte, R.G., Ru®no, S.D., Mule, P.V., 1992. Mamõ feros cuaternarios del Valle de Ta®, Provincia de TucumaÂn, Argentina. In: ResuÂmenes IX Jornadas Cientõ ®cas de la Sociedad de Biologõ a de Tucuman, p. 44. Powell, J.E., Ru®no, S.D., Mule, P.U., 1993. Hippidiformes (Pleistoceno Superior) del valle de Ta® (Provincia de TucumaÂn, Argentina). Consideraciones tafonoÂmicas y paleoambientales. In: X Jornadas Argentinas de Paleontologõ a de Vertebrados, ResuÂmenes. La Plata (Argentina). Roberts, N., Taleb, M., Barker, Ph, Damnati, B., Icole, M., Williamson, D., 1993. Timing of the Younger Dryas event in East Africa from lake-level changes. Nature 366. Sayago, J.M., Powell, J., Esteban, G., Collantes, M., 1987. Informe preliminar sobre la bioestratigrafõ a y paleogeomorfologõ a de los sedimentos loeÂsicos de La Angostura, Depto. Ta® del Valle, prov. de TucumaÂn, Rep. Argentina. In: X Congreso GeoloÂgico Argentino, 3, pp. 317±320. Sayago, J.M., 1995. The Argentine neotropical loess: an overview. Quaternary Science Reviews 14, 755±766. Scoppa, C., 1976. La mineralogõ a de los suelos de la llanura pampeana en la interpretacioÂn de su geÂnesis y distribucioÂn. In: Actas VII ReunioÂn de la AsociacioÂn Argentina de la Ciencia del Suelo, IDIA suppl 33, pp. 659±673. Teruggi, M., 1957. The nature and origin of Argentine loess. In: Smalley, I.J. (Ed.), Loess Lithology and Genesis, pp. 195±295. USDA, 1975. Soil Taxonomy: a basic system of soil classi®cation for making and interpreting soil surveys. In: Agr. Handbook 436. Superint. of Doc., US Govt. Printing Oce, Washington, DC. ZaÂrate, M., Blasi, A., 1993. Late Pleistocene-Holocene eolian deposits of the southern Buenos Aires province, Argentina: a preliminary model. Quaternary International 17, 15±20.