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Climatic periodicity during the late Pleistocene from a loess–paleosol sequence in northwest Argentina
Quaternary International 78 (2001) 11}16
Climatic periodicity during the late Pleistocene from a loess}paleosol sequence in northwest Argentina J.A. Zinck *, J.M. Sayago ITC, P.O. Box 6, 7500 AA, Enschede, Netherlands National University of Tucuma& n, Miguel Lillo 205, 4000 S.M. de Tucuma& n, Argentina
Abstract A 42 m thick loess}paleosol sequence was studied in the pre-Andean intramountain valley of Ta"-del-Valle, 100 km west of the city of San Miguel de TucumaH n, at an elevation of 2280 m asl, in northwest Argentina. The sequence contains 28 paleosols alternating with loess layers. The loess layers are coarse loamy and have large hexagonal polyhedra. The soil layers are restricted to Bt horizons, having "ne loamy textures, prismatic structures and organo-argillans on the structural surfaces. Surface (A) and eluvial (E) horizons are absent from the whole sequence. Soil truncation is unlikely as no remains of these horizons or erosional unconformities were identi"ed. The dated part of the sequence encompasses a time span of 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the base of the exposed sequence). The youngest strata are probably missing, either because of a depositional hiatus or because of post-depositional erosion, as dated paleosols in adjacent areas indicate that loess deposition continued during at least the early Holocene. The 20 Bt/C couplets identi"ed in the dated sequence correspond to 20 climatic oscillations in 10,080 years. Thus, every 500 years on average there was a climatic change from dry}cool conditions, with loess deposition, to moist}warm conditions favouring soil development. This might have been related to recurrent northward shifts of the polar front, accompanied by weakening of the mid-latitude South-Paci"c anticyclone. Similar short-term periodicity has been identi"ed in the ice cap of Greenland and in tropical lake level records, so such frequent climatic changes during the late Pleistocene seem to have been global in character. 2001 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Because of its large extent, the Argentine loess cover shows signi"cant south}north variations in particle size distribution and mineralogical composition, leading to a distinction between Pampean loess in the south (30}403S) and subtropical loess in the north (20}303S). The latter corresponds to the present-day subtropical dry area of the Chaco plain in the east and the dry preAndean intramountain valleys to the west (Sayago, 1995). A loess}paleosol sequence was studied in the pre-Andean valley of Ta"-del-Valle, 100 km west of the city of San Miguel de TucumaH n, at the site of La Mesada located at an elevation of 2280 m asl (Fig. 1). The section is an incised arroyo wall on the right bank of the Los Alisos River, with coordinates of 26357 15 S and 65345 30 W. The exposure is a 42.5 m deep loess deposit, which developed by intermittent aggradation allowing for soil formation during periods with little or no loess in#ux
* Corresponding author. E-mail address: [email protected] (J.A. Zinck).
(Fig. 2). This paper documents the occurrence of frequent climatic oscillations during the late Pleistocene from the alternation of loess layers and paleosols.
2. Characteristics of the sequence Characterization of the sequence is based on "eld properties recorded using the guidelines for soil pro"le description of FAO (1990) and laboratory determinations using the ISRIC procedures for soil analysis (van Reeuwijk, 1992). 2.1. The loess layers In general, the loess layers are individual strata separated by one or sometimes two consecutive soil horizons. The 26 loess layers identi"ed in this way have an average individual thickness of 107 cm, but vary from 20 to 300 cm. Morphologically, they are distinguishable from the buried soil horizons by lighter colours, a massive structure within large polyhedra, greater silt content, the presence of carbonate and the absence of organo-clay
1040-6182/01/$20.00 2001 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 1 1 1 - 7
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J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
as powdery coatings. Most of the succession contains "nely divided carbonate in the matrix, ranging from 0.25 to 2.75%. The average pH value of all loess layers is 8.7, with a range of 7.8}9.9. Fossil grass roots or root imprints are common in the loess layers, and resulted from plant growth during loess deposition. Organic matter content ranges from 0.1 to 0.7%, with an average value of 0.31% which is almost twice the average value of organic matter in the intercalated soil (Bt) horizons (Table 1). The cation exchange capacity of the clay fraction is abnormally large. The average CEC value of 6 loess layers is 314 cmol (#)/kg clay; this is substantially greater than the average CEC value in the soil horizons (about 70 cmol (#)/kg clay). 2.2. The paleosols
Fig. 1. Distribution of super"cial deposits and location of the loess}paleosol sequence in the valley of Ta"-del-Valle, TucumaH n Province, Argentina (Sayago, 1995).
cutans (Table 1). Dry colours on ped surfaces and in the matrix range from brown to light brown; the most frequent colour is brown (7.5YR 5/4). The loess layers are essentially massive, without apparent strati"cation. However, a distinctive structural feature, particularly in the upper layers, is the presence of large hexagonal polyhedra, 20}40 cm in diameter and as high as the thickness of each layer. The polyhedra are separated by small cracks, 2}3 mm wide and "lled with calcite sheets. Their formation probably resulted from the desiccation of originally wet loess either deposited by occasional rainstorms or trapped on swampy land surfaces. Coatings of clay}organic matter complexes (organoargillans), abundant in the soil horizons, are usually absent in the loess material, with the exception of the upper 5 m of the section where some thin cutans occur. The average particle size distribution of samples from 24 loess layers is 33% sand, 58% silt and 9% clay. All layers but one are silt loam and have more than 50% silt, and 10 of the 24 (42%) have 60% or more silt. In the upper and lower parts of the sequence, calcium carbonate is present as frequent to abundant nodules and
Compared to the loess layers, the paleosols are thinner and have slightly darker colours, prismatic structures, organo-clay cutans on the surface of the structural faces and no carbonate. They are represented only by B horizons, which are mainly Bt horizons. Neither A nor E horizons were identi"ed. The thickness of the paleosols ranges from 20 to 100 cm, with a mean of 53 cm, and is much less variable than that of the loess layers. The general relationship is 0.5 m of Bt horizon to 1 m of C horizon. The structure of the Bt horizons is medium to coarse prismatic. The prisms are covered by organo-clay coatings, which are darker than the matrix. Values and/or chromas are at least one unit darker in the Bt horizons than in the C layers beneath. The average particle size composition of 28 paleosol horizons is 27% sand, 48% silt and 23% clay. Thus, the Bt horizons have more clay than the loess layers (average clay content of 9%). Clay cutans probably contribute only a small part of the clay increase, and it is likely that neoformation of clay has occurred within the Bt horizons, a process already identi"ed in Mollisols formed from loess rich in volcanic glass in the Pampean area (Scoppa, 1976). The paleosol horizons have no "nely divided carbonate in the matrix, but a few CaCO nodules are usually present. The average pH of 28 soil horizons is 7.9, but values range from 6.7 to 9.1. The paleosol pH is usually 0.5}1 unit less than that of the loess layer beneath. Mean organic matter content is 0.17%, half that of the C horizons. Fossil roots do not occur in the Bt horizons.
3. Formation of the sequence In general, the properties of the Bt horizons (most of the paleosols) and those of the C layers (loess), including thickness, colour, texture, structure, consistence, presence/absence of CaCO , presence/absence of
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
13
Fig. 2. Selected properties of the loess}paleosol sequence of La Mesada.
organo-argillans, pH, organic matter and CEC, remain almost uniform throughout the whole sequence. This suggests that similar conditions prevailed during loess deposition on the one hand, and during subsequent episodes of soil formation on the other, for at least 24 cycles. This cyclicity is probably the most remarkable feature of the sequence.
3.1. Vertical variations of properties and features In order to detect vertical variation within the sequence, it was divided into four segments, each approximately 10 m thick and containing similar numbers of loess layers and soil horizons (Table 2). The thickness of the Bt horizons consistently increases from 31 cm in the
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J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
Table 1 Mean values of selected properties of Bt and C horizons in the loess} paleosol sequence of La Mesada Bt
C
Mean thickness (cm) Dry colour Structure Coatings
53 7.5YR 4/4}3/4 Prismatic Organo-argillans
107 7.5YR 5/4 Massive No
Sand (%) Silt (%) Clay (%)
27 48 23
33 58 9
pH (H O 1 : 2.5) CaCO (%) OM (%) CEC (cmol (#)/kg soil)
7.9 * 0.17 21
8.7 1.3 0.31 20
Bt: n"28. C: n"24 for laboratory determinations, n"26 for morphological properties.
Table 2 Mean particle size distribution of Bt and C horizons in four arbitrary segments of the loess}paleosol sequence of La Mesada Depth (m)
Horizon
0}10
Bt C
10}20
Thickness (cm)
Sand (%)
Silt (%)
Clay (%)
67 59
27 35
45 59
28 6
Bt C
61 120
28 34
49 59
23 7
20}32
Bt C
49 186
23 32
54 59
23 9
32}42
Bt C
31 94
32 31
53 58
16 11
bottom segment to 67 cm in the top segment, and that of the C horizons decreases irregularly in the same direction. The thickness of the original loess layers before soil formation started, estimated from the Bt/C couplets, ranges from 75 to 330 cm, and tends to decrease upwards. There is thus evidence that the loess deposition cycles became shorter or less intensive, while the soil development periods became longer or more intensive from the base of the sequence upwards. The loess remains remarkably homogeneous in particle size distribution throughout the sequence. From bottom to top, the sand fraction of the C horizons increases slightly from 31 to 35% and the clay fraction decreases slightly from 11 to 6%, but the silt fraction remains stable at 58}59%. Thus, even if the deposition periods have shortened from the beginning to the end of the sequence, the environmental conditions controlling loess deposition did not change over a notably
Fig. 3. Values of the ratio of clay ((2 m) in Bt horizons to that in C layers through the loess}paleosol sequence of La Mesada.
long time span. In contrast, conditions during periods of soil formation seem to have changed towards warmer and moister, as the clay content of the Bt horizons increases from 16% at the base of the sequence to 28% at the top, and there is a parallel trend towards redder colours between 3 and 16 m depth. The intensity of soil formation was estimated by calculating the ratios of clay in the Bt horizons to that in the underlying C layers (their respective parent materials). A Bt/C clay ratio of 2, indicating a doubling of clay content from the C layer to the Bt horizon, was taken as an arbitrary threshold between more and less favourable conditions for soil formation, indicating a change from cooler}drier to warmer}moister climates. The Bt/C clay ratios generally increase from the base upwards (Fig. 3). Four segments can be recognized: 1. Segment I (27}42 m): ratio values mainly below 2, indicating (climatic) restrictions on soil formation. 2. Segment II (7}27 m): ratio values mainly between 3 and 4, re#ecting moderately favourable (climatic) conditions for soil formation.
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
3. Segment III (3}7 m): ratio values of 7}10 together with redder colours in the Bt horizons, corresponding to a soil formation optimum. 4. Segment IV (0}3 m): ratio slightly above 2, indicating a return to more restricting conditions for soil formation. 3.2. Inter-horizon relationships Surface A and subsurface E horizons are absent from the whole sequence, but no evidence was found for erosion before new loess layers buried the Bt horizons, such as unconformities, angular truncation of strata, buried rill and pond microtopography or local pavements of coarse fragments. Paleosol horizons and loess layers are strictly parallel and horizontal to subhorizontal. Also, 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. It is more likely that A horizons did not exist as such or are masked within the top of the Bt horizons. For a similar situation in Washington State, USA, McDonald and Busacca (1990) suggested that upward growth of an `elevating B horizona digested any incoming material until the loess in#ux became too abundant and caused fossilization. We therefore suggest that the Bt horizons resulted from two processes: (1) at the base there was weathering of the underlying C layer, and (2) at the top eluviation of organo-mineral complexes from the overlying loess cover resulted in the organoargillans coating the prism faces. B horizons started as Bw horizons, re#ecting the weathering of primary minerals from the loess substratum. Later, with further in#ux of loess the Bw horizons became Bt horizons by illuviation of humus and clay.
4. Evolution and time frame of the sequence Four loess layers at regular depth intervals spanning the full thickness of the sequence were sampled for radiocarbon dating 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. 4.1. Post-depositional erosion The dated part of the sequence encompassed a time span of 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the base of the exposed sequence). If loess in#ux during the Holocene was negligible, then the top 5 m of the sequence was deposited in the last 7000}8000 years of the late Pleistocene. This would mean that the present land surface coincides with the top of the original depositional sequence. However, even though deposition certainly decreased in the very
15
late Pleistocene and Holocene, it is unlikely that only 5 m loess were deposited in the last 17,580 years, when 37 m were deposited in the previous 10,080 years. Also some loess deposition continued in adjacent areas during at least the early Holocene. Vertebrate fossils dated to 8660 BP have been found beneath 11}12 m of loess at Zanja del Chivo, 8}10 km from the La Mesada site (Collantes et al., 1993), and at 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 (Zinck and Sayago, unpublished data). Also, outside the pre-Andean valleys, in the Chaco plain 50 km northeast of San Miguel de TucumaH n, paleosols dated from 2840 to 6290 BP occur at 100}150 cm depth, suggesting some continuing in#ux of loess even during the late Holocene (Zinck and Sayago, unpublished data). This suggests that the youngest strata at La Mesada are missing, probably because of post-depositional erosion. How much has been eroded is matter of speculation. As only 5 m of sediments are left to account for the period from 17,580 BP to the present, a considerable thickness could have been eroded, especially if loess deposition continued during the early Holocene. 4.2. Rate of loess deposition and speed of soil formation The accumulated thickness of the Bt horizons formed in each of the three main dated intervals suggests relatively fast soil formation, generating about 1 m of soil every 600}2000 years. From the total thickness of loess layers and soil horizons, the average rate of land surface accretion was about 0.5 cm/yr during the two earlier intervals (5660 years together), then decreased to 0.22 cm/yr during the later interval (4420 years) (Table 3). However, loess deposition during dust storm events must have occurred at greater rates than these, as there was little or no loess in#ux during the soil-forming periods. 4.3. The rhythm of climatic oscillations In the 10,080 years delimited by the two extreme radiocarbon dates, 27,660 and 17,580 BP, 22 loess layers were deposited and 23 soil horizons were formed. Combining the double Bt horizons (three examples) and the multiple C layers (one example) into single composite units, at least 20 Bt/C couplets formed in this time interval. This indicates a climatic cycle every 500 years on average, showing that, during at least the upper-third of the last glacial period, climate was very variable in the valley of TafmH and probably also in the other pre-Andean intermontane basins. The interval from 27,660 to 24,610 BP corresponds well with segment I of the Bt/C ratio curve (Fig. 3), when conditions were below the arbitrary threshold value of 2, and thus, less favourable for soil formation. Segment II of the Bt/C ratio curve starts at 24,610 BP, but goes beyond
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J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
Table 3 Calculated mean rates of loess deposition in segments of the loess}paleosol sequence of La Mesada Depth (m)
Thickness per pro"le segment (m)
0
Age (years BP)
Time interval (years)
Average rate of loess deposition per pro"le segment (cm/yr)
7,580
0.07
4,420
0.22
2,610
0.48
3,050
0.49
Average rate of loess deposition at selected depths (cm/yr)
(10,000) 5.2
5.2
17,580 9.8
15
0.03
22,000 12.4
27.4
0.07
24,610 14.9
42.3
0.11
27,660
the next radiocarbon date of 22,000 BP, probably ending around 20,600 BP if the same thickness * time relationship is assumed. During segment II, the Bt/C clay ratios are mainly between 2 and 4, suggesting more favourable soil forming conditions. Segment III corresponds to three very high Bt/C ratios, centred around 17,580 BP, but lasting probably from 20,600 to 14,600 BP, the last date also estimated from the same thickness * time relationship. This interval must have been the most favourable part of the late Pleistocene for soil formation. In segment IV, conditions similar to those of segment II seem to have resumed.
5. Conclusions Environmental conditions during the later part of the last glacial period were far from constant in TafmH -delValle. Recurrent climatic changes favoured alternating loess deposition and soil formation. Soil development occurred repeatedly within what is elsewhere a full glacial period. Moreover, the optimum of soil formation indicated by the high Bt/C clay ratios of segment III occurred when the coldest pleniglacial conditions prevailed in other areas. The rhythmic repetition of 26 loess layers, alternating with 28 Bt horizons formed by clay illuviation and neoformation, indicates oscillating climatic conditions di!erent from those of the present. During the late Pleistocene and probably part of the Holocene, the climate oscillated between dry}cool conditions, promoting loess in#ux, and moist}warm conditions favouring soil development. This, in turn, suggests cyclic changes in the South-American air circulation pattern. Evidence for such changes has been provided by Hastenrath (1971), who postulated recurrent northward shifts of the polar front accompanied by weakening of the mid-latitude South-Paci"c anticyclone. Such short-term climatic periodicity has also been identi"ed in the ice cap of Greenland (Dansgaard et al., 1993). Here the last glacial period is divided into 24 short
0.15
warm interstadials, separated by cold stadials of longer duration (Bond et al., 1993). Correlation between ice records from both polar areas (Dansgaard et al., 1993) and lake-level records from tropical regions (Roberts et al., 1993) indicates the global character of these frequent climatic changes during the late Pleistocene, which we consider to be the cause of the loess}paleosol sequence of La Mesada. References 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, 143}147. Collantes, M., Powell, J., Sayago, J.M., 1993. FormacioH n Ta" del Valle (Cuaternario Superior), Provincia de TucumaH n (Argentina): litologmH a, paleontologmH a y paleoambientes. XII Congreso GeoloH gico Argentino y II Congreso de ExploracioH n de Hidrocarburos. Actas Tomo II, 200}206. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Ste!ensen, J.P., SveinbjoK rnsdottir, 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. FAO, 1990. Guidelines for soil description. Food and Agriculture Organization of the United Nations, Rome. Hastenrath, S.L., 1971. On the Pleistocene snow-line depression in the arid regions of the South American Andes. Journal of Glaciology 10 (59), 255}267. 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. Roberts, N., Taieb, 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, 146}147. Sayago, J.M., 1995. The Argentine neotropical loess: an overview. Quaternary Science Reviews 14, 755}766. Scoppa, C., 1976. La mineralogmH a de los suelos de la llanura pampeana en la interpretacioH n de su geH nesis y distribucioH n. Actas VII ReunioH n de la AsociacioH n Argentina de la Ciencia del Suelo. IDIA 33 (Suppl.), S659}S673. van Reeuwijk, L.P. (Ed.), 1992. Procedures for soil analysis. Technical Paper No. 9, International Soil Reference and Information Centre (ISRIC), Wageningen, The Netherlands.
Climatic periodicity during the late Pleistocene from a loess}paleosol sequence in northwest Argentina J.A. Zinck *, J.M. Sayago ITC, P.O. Box 6, 7500 AA, Enschede, Netherlands National University of Tucuma& n, Miguel Lillo 205, 4000 S.M. de Tucuma& n, Argentina
Abstract A 42 m thick loess}paleosol sequence was studied in the pre-Andean intramountain valley of Ta"-del-Valle, 100 km west of the city of San Miguel de TucumaH n, at an elevation of 2280 m asl, in northwest Argentina. The sequence contains 28 paleosols alternating with loess layers. The loess layers are coarse loamy and have large hexagonal polyhedra. The soil layers are restricted to Bt horizons, having "ne loamy textures, prismatic structures and organo-argillans on the structural surfaces. Surface (A) and eluvial (E) horizons are absent from the whole sequence. Soil truncation is unlikely as no remains of these horizons or erosional unconformities were identi"ed. The dated part of the sequence encompasses a time span of 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the base of the exposed sequence). The youngest strata are probably missing, either because of a depositional hiatus or because of post-depositional erosion, as dated paleosols in adjacent areas indicate that loess deposition continued during at least the early Holocene. The 20 Bt/C couplets identi"ed in the dated sequence correspond to 20 climatic oscillations in 10,080 years. Thus, every 500 years on average there was a climatic change from dry}cool conditions, with loess deposition, to moist}warm conditions favouring soil development. This might have been related to recurrent northward shifts of the polar front, accompanied by weakening of the mid-latitude South-Paci"c anticyclone. Similar short-term periodicity has been identi"ed in the ice cap of Greenland and in tropical lake level records, so such frequent climatic changes during the late Pleistocene seem to have been global in character. 2001 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Because of its large extent, the Argentine loess cover shows signi"cant south}north variations in particle size distribution and mineralogical composition, leading to a distinction between Pampean loess in the south (30}403S) and subtropical loess in the north (20}303S). The latter corresponds to the present-day subtropical dry area of the Chaco plain in the east and the dry preAndean intramountain valleys to the west (Sayago, 1995). A loess}paleosol sequence was studied in the pre-Andean valley of Ta"-del-Valle, 100 km west of the city of San Miguel de TucumaH n, at the site of La Mesada located at an elevation of 2280 m asl (Fig. 1). The section is an incised arroyo wall on the right bank of the Los Alisos River, with coordinates of 26357 15 S and 65345 30 W. The exposure is a 42.5 m deep loess deposit, which developed by intermittent aggradation allowing for soil formation during periods with little or no loess in#ux
* Corresponding author. E-mail address: [email protected] (J.A. Zinck).
(Fig. 2). This paper documents the occurrence of frequent climatic oscillations during the late Pleistocene from the alternation of loess layers and paleosols.
2. Characteristics of the sequence Characterization of the sequence is based on "eld properties recorded using the guidelines for soil pro"le description of FAO (1990) and laboratory determinations using the ISRIC procedures for soil analysis (van Reeuwijk, 1992). 2.1. The loess layers In general, the loess layers are individual strata separated by one or sometimes two consecutive soil horizons. The 26 loess layers identi"ed in this way have an average individual thickness of 107 cm, but vary from 20 to 300 cm. Morphologically, they are distinguishable from the buried soil horizons by lighter colours, a massive structure within large polyhedra, greater silt content, the presence of carbonate and the absence of organo-clay
1040-6182/01/$20.00 2001 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 1 1 1 - 7
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as powdery coatings. Most of the succession contains "nely divided carbonate in the matrix, ranging from 0.25 to 2.75%. The average pH value of all loess layers is 8.7, with a range of 7.8}9.9. Fossil grass roots or root imprints are common in the loess layers, and resulted from plant growth during loess deposition. Organic matter content ranges from 0.1 to 0.7%, with an average value of 0.31% which is almost twice the average value of organic matter in the intercalated soil (Bt) horizons (Table 1). The cation exchange capacity of the clay fraction is abnormally large. The average CEC value of 6 loess layers is 314 cmol (#)/kg clay; this is substantially greater than the average CEC value in the soil horizons (about 70 cmol (#)/kg clay). 2.2. The paleosols
Fig. 1. Distribution of super"cial deposits and location of the loess}paleosol sequence in the valley of Ta"-del-Valle, TucumaH n Province, Argentina (Sayago, 1995).
cutans (Table 1). Dry colours on ped surfaces and in the matrix range from brown to light brown; the most frequent colour is brown (7.5YR 5/4). The loess layers are essentially massive, without apparent strati"cation. However, a distinctive structural feature, particularly in the upper layers, is the presence of large hexagonal polyhedra, 20}40 cm in diameter and as high as the thickness of each layer. The polyhedra are separated by small cracks, 2}3 mm wide and "lled with calcite sheets. Their formation probably resulted from the desiccation of originally wet loess either deposited by occasional rainstorms or trapped on swampy land surfaces. Coatings of clay}organic matter complexes (organoargillans), abundant in the soil horizons, are usually absent in the loess material, with the exception of the upper 5 m of the section where some thin cutans occur. The average particle size distribution of samples from 24 loess layers is 33% sand, 58% silt and 9% clay. All layers but one are silt loam and have more than 50% silt, and 10 of the 24 (42%) have 60% or more silt. In the upper and lower parts of the sequence, calcium carbonate is present as frequent to abundant nodules and
Compared to the loess layers, the paleosols are thinner and have slightly darker colours, prismatic structures, organo-clay cutans on the surface of the structural faces and no carbonate. They are represented only by B horizons, which are mainly Bt horizons. Neither A nor E horizons were identi"ed. The thickness of the paleosols ranges from 20 to 100 cm, with a mean of 53 cm, and is much less variable than that of the loess layers. The general relationship is 0.5 m of Bt horizon to 1 m of C horizon. The structure of the Bt horizons is medium to coarse prismatic. The prisms are covered by organo-clay coatings, which are darker than the matrix. Values and/or chromas are at least one unit darker in the Bt horizons than in the C layers beneath. The average particle size composition of 28 paleosol horizons is 27% sand, 48% silt and 23% clay. Thus, the Bt horizons have more clay than the loess layers (average clay content of 9%). Clay cutans probably contribute only a small part of the clay increase, and it is likely that neoformation of clay has occurred within the Bt horizons, a process already identi"ed in Mollisols formed from loess rich in volcanic glass in the Pampean area (Scoppa, 1976). The paleosol horizons have no "nely divided carbonate in the matrix, but a few CaCO nodules are usually present. The average pH of 28 soil horizons is 7.9, but values range from 6.7 to 9.1. The paleosol pH is usually 0.5}1 unit less than that of the loess layer beneath. Mean organic matter content is 0.17%, half that of the C horizons. Fossil roots do not occur in the Bt horizons.
3. Formation of the sequence In general, the properties of the Bt horizons (most of the paleosols) and those of the C layers (loess), including thickness, colour, texture, structure, consistence, presence/absence of CaCO , presence/absence of
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
13
Fig. 2. Selected properties of the loess}paleosol sequence of La Mesada.
organo-argillans, pH, organic matter and CEC, remain almost uniform throughout the whole sequence. This suggests that similar conditions prevailed during loess deposition on the one hand, and during subsequent episodes of soil formation on the other, for at least 24 cycles. This cyclicity is probably the most remarkable feature of the sequence.
3.1. Vertical variations of properties and features In order to detect vertical variation within the sequence, it was divided into four segments, each approximately 10 m thick and containing similar numbers of loess layers and soil horizons (Table 2). The thickness of the Bt horizons consistently increases from 31 cm in the
14
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
Table 1 Mean values of selected properties of Bt and C horizons in the loess} paleosol sequence of La Mesada Bt
C
Mean thickness (cm) Dry colour Structure Coatings
53 7.5YR 4/4}3/4 Prismatic Organo-argillans
107 7.5YR 5/4 Massive No
Sand (%) Silt (%) Clay (%)
27 48 23
33 58 9
pH (H O 1 : 2.5) CaCO (%) OM (%) CEC (cmol (#)/kg soil)
7.9 * 0.17 21
8.7 1.3 0.31 20
Bt: n"28. C: n"24 for laboratory determinations, n"26 for morphological properties.
Table 2 Mean particle size distribution of Bt and C horizons in four arbitrary segments of the loess}paleosol sequence of La Mesada Depth (m)
Horizon
0}10
Bt C
10}20
Thickness (cm)
Sand (%)
Silt (%)
Clay (%)
67 59
27 35
45 59
28 6
Bt C
61 120
28 34
49 59
23 7
20}32
Bt C
49 186
23 32
54 59
23 9
32}42
Bt C
31 94
32 31
53 58
16 11
bottom segment to 67 cm in the top segment, and that of the C horizons decreases irregularly in the same direction. The thickness of the original loess layers before soil formation started, estimated from the Bt/C couplets, ranges from 75 to 330 cm, and tends to decrease upwards. There is thus evidence that the loess deposition cycles became shorter or less intensive, while the soil development periods became longer or more intensive from the base of the sequence upwards. The loess remains remarkably homogeneous in particle size distribution throughout the sequence. From bottom to top, the sand fraction of the C horizons increases slightly from 31 to 35% and the clay fraction decreases slightly from 11 to 6%, but the silt fraction remains stable at 58}59%. Thus, even if the deposition periods have shortened from the beginning to the end of the sequence, the environmental conditions controlling loess deposition did not change over a notably
Fig. 3. Values of the ratio of clay ((2 m) in Bt horizons to that in C layers through the loess}paleosol sequence of La Mesada.
long time span. In contrast, conditions during periods of soil formation seem to have changed towards warmer and moister, as the clay content of the Bt horizons increases from 16% at the base of the sequence to 28% at the top, and there is a parallel trend towards redder colours between 3 and 16 m depth. The intensity of soil formation was estimated by calculating the ratios of clay in the Bt horizons to that in the underlying C layers (their respective parent materials). A Bt/C clay ratio of 2, indicating a doubling of clay content from the C layer to the Bt horizon, was taken as an arbitrary threshold between more and less favourable conditions for soil formation, indicating a change from cooler}drier to warmer}moister climates. The Bt/C clay ratios generally increase from the base upwards (Fig. 3). Four segments can be recognized: 1. Segment I (27}42 m): ratio values mainly below 2, indicating (climatic) restrictions on soil formation. 2. Segment II (7}27 m): ratio values mainly between 3 and 4, re#ecting moderately favourable (climatic) conditions for soil formation.
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
3. Segment III (3}7 m): ratio values of 7}10 together with redder colours in the Bt horizons, corresponding to a soil formation optimum. 4. Segment IV (0}3 m): ratio slightly above 2, indicating a return to more restricting conditions for soil formation. 3.2. Inter-horizon relationships Surface A and subsurface E horizons are absent from the whole sequence, but no evidence was found for erosion before new loess layers buried the Bt horizons, such as unconformities, angular truncation of strata, buried rill and pond microtopography or local pavements of coarse fragments. Paleosol horizons and loess layers are strictly parallel and horizontal to subhorizontal. Also, 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. It is more likely that A horizons did not exist as such or are masked within the top of the Bt horizons. For a similar situation in Washington State, USA, McDonald and Busacca (1990) suggested that upward growth of an `elevating B horizona digested any incoming material until the loess in#ux became too abundant and caused fossilization. We therefore suggest that the Bt horizons resulted from two processes: (1) at the base there was weathering of the underlying C layer, and (2) at the top eluviation of organo-mineral complexes from the overlying loess cover resulted in the organoargillans coating the prism faces. B horizons started as Bw horizons, re#ecting the weathering of primary minerals from the loess substratum. Later, with further in#ux of loess the Bw horizons became Bt horizons by illuviation of humus and clay.
4. Evolution and time frame of the sequence Four loess layers at regular depth intervals spanning the full thickness of the sequence were sampled for radiocarbon dating 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. 4.1. Post-depositional erosion The dated part of the sequence encompassed a time span of 10,080 years, from 17,580 BP at 5.2 m depth to 27,660 BP at 42.3 m depth (the base of the exposed sequence). If loess in#ux during the Holocene was negligible, then the top 5 m of the sequence was deposited in the last 7000}8000 years of the late Pleistocene. This would mean that the present land surface coincides with the top of the original depositional sequence. However, even though deposition certainly decreased in the very
15
late Pleistocene and Holocene, it is unlikely that only 5 m loess were deposited in the last 17,580 years, when 37 m were deposited in the previous 10,080 years. Also some loess deposition continued in adjacent areas during at least the early Holocene. Vertebrate fossils dated to 8660 BP have been found beneath 11}12 m of loess at Zanja del Chivo, 8}10 km from the La Mesada site (Collantes et al., 1993), and at 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 (Zinck and Sayago, unpublished data). Also, outside the pre-Andean valleys, in the Chaco plain 50 km northeast of San Miguel de TucumaH n, paleosols dated from 2840 to 6290 BP occur at 100}150 cm depth, suggesting some continuing in#ux of loess even during the late Holocene (Zinck and Sayago, unpublished data). This suggests that the youngest strata at La Mesada are missing, probably because of post-depositional erosion. How much has been eroded is matter of speculation. As only 5 m of sediments are left to account for the period from 17,580 BP to the present, a considerable thickness could have been eroded, especially if loess deposition continued during the early Holocene. 4.2. Rate of loess deposition and speed of soil formation The accumulated thickness of the Bt horizons formed in each of the three main dated intervals suggests relatively fast soil formation, generating about 1 m of soil every 600}2000 years. From the total thickness of loess layers and soil horizons, the average rate of land surface accretion was about 0.5 cm/yr during the two earlier intervals (5660 years together), then decreased to 0.22 cm/yr during the later interval (4420 years) (Table 3). However, loess deposition during dust storm events must have occurred at greater rates than these, as there was little or no loess in#ux during the soil-forming periods. 4.3. The rhythm of climatic oscillations In the 10,080 years delimited by the two extreme radiocarbon dates, 27,660 and 17,580 BP, 22 loess layers were deposited and 23 soil horizons were formed. Combining the double Bt horizons (three examples) and the multiple C layers (one example) into single composite units, at least 20 Bt/C couplets formed in this time interval. This indicates a climatic cycle every 500 years on average, showing that, during at least the upper-third of the last glacial period, climate was very variable in the valley of TafmH and probably also in the other pre-Andean intermontane basins. The interval from 27,660 to 24,610 BP corresponds well with segment I of the Bt/C ratio curve (Fig. 3), when conditions were below the arbitrary threshold value of 2, and thus, less favourable for soil formation. Segment II of the Bt/C ratio curve starts at 24,610 BP, but goes beyond
16
J.A. Zinck, J.M. Sayago / Quaternary International 78 (2001) 11}16
Table 3 Calculated mean rates of loess deposition in segments of the loess}paleosol sequence of La Mesada Depth (m)
Thickness per pro"le segment (m)
0
Age (years BP)
Time interval (years)
Average rate of loess deposition per pro"le segment (cm/yr)
7,580
0.07
4,420
0.22
2,610
0.48
3,050
0.49
Average rate of loess deposition at selected depths (cm/yr)
(10,000) 5.2
5.2
17,580 9.8
15
0.03
22,000 12.4
27.4
0.07
24,610 14.9
42.3
0.11
27,660
the next radiocarbon date of 22,000 BP, probably ending around 20,600 BP if the same thickness * time relationship is assumed. During segment II, the Bt/C clay ratios are mainly between 2 and 4, suggesting more favourable soil forming conditions. Segment III corresponds to three very high Bt/C ratios, centred around 17,580 BP, but lasting probably from 20,600 to 14,600 BP, the last date also estimated from the same thickness * time relationship. This interval must have been the most favourable part of the late Pleistocene for soil formation. In segment IV, conditions similar to those of segment II seem to have resumed.
5. Conclusions Environmental conditions during the later part of the last glacial period were far from constant in TafmH -delValle. Recurrent climatic changes favoured alternating loess deposition and soil formation. Soil development occurred repeatedly within what is elsewhere a full glacial period. Moreover, the optimum of soil formation indicated by the high Bt/C clay ratios of segment III occurred when the coldest pleniglacial conditions prevailed in other areas. The rhythmic repetition of 26 loess layers, alternating with 28 Bt horizons formed by clay illuviation and neoformation, indicates oscillating climatic conditions di!erent from those of the present. During the late Pleistocene and probably part of the Holocene, the climate oscillated between dry}cool conditions, promoting loess in#ux, and moist}warm conditions favouring soil development. This, in turn, suggests cyclic changes in the South-American air circulation pattern. Evidence for such changes has been provided by Hastenrath (1971), who postulated recurrent northward shifts of the polar front accompanied by weakening of the mid-latitude South-Paci"c anticyclone. Such short-term climatic periodicity has also been identi"ed in the ice cap of Greenland (Dansgaard et al., 1993). Here the last glacial period is divided into 24 short
0.15
warm interstadials, separated by cold stadials of longer duration (Bond et al., 1993). Correlation between ice records from both polar areas (Dansgaard et al., 1993) and lake-level records from tropical regions (Roberts et al., 1993) indicates the global character of these frequent climatic changes during the late Pleistocene, which we consider to be the cause of the loess}paleosol sequence of La Mesada. References 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, 143}147. Collantes, M., Powell, J., Sayago, J.M., 1993. FormacioH n Ta" del Valle (Cuaternario Superior), Provincia de TucumaH n (Argentina): litologmH a, paleontologmH a y paleoambientes. XII Congreso GeoloH gico Argentino y II Congreso de ExploracioH n de Hidrocarburos. Actas Tomo II, 200}206. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Ste!ensen, J.P., SveinbjoK rnsdottir, 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. FAO, 1990. Guidelines for soil description. Food and Agriculture Organization of the United Nations, Rome. Hastenrath, S.L., 1971. On the Pleistocene snow-line depression in the arid regions of the South American Andes. Journal of Glaciology 10 (59), 255}267. 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. Roberts, N., Taieb, 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, 146}147. Sayago, J.M., 1995. The Argentine neotropical loess: an overview. Quaternary Science Reviews 14, 755}766. Scoppa, C., 1976. La mineralogmH a de los suelos de la llanura pampeana en la interpretacioH n de su geH nesis y distribucioH n. Actas VII ReunioH n de la AsociacioH n Argentina de la Ciencia del Suelo. IDIA 33 (Suppl.), S659}S673. van Reeuwijk, L.P. (Ed.), 1992. Procedures for soil analysis. Technical Paper No. 9, International Soil Reference and Information Centre (ISRIC), Wageningen, The Netherlands.