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El Niño Variability in the Coastal Desert of Southern Peru during the Mid-Holocene
Quaternary Research 52, 171–179 (1999) Article ID qres.1999.2059, available online at http://www.idealibrary.com on
El Nin˜o Variability in the Coastal Desert of Southern Peru during the Mid-Holocene Michel Fontugne Laboratoire des Sciences du Climat et de l’Environnement, UMR 1572-CEA/CNRS, Domaine du CNRS, F-91198 Gif sur Yvette cedex, France E-mail: [email protected]
Pierre Usselmann UMR 5651 Espace, Maison de la Ge´ographie, 17 rue de l’Abbe´ de l’Epe´e, F-34090 Montpellier, France
Danie`le Lavalle´e and Miche`le Julien Arche´ologie des Ame´riques, CNRS, Maison de l’Arche´ologie et de l’Ethnogie, 21 Alle´e de l’Universite´, F-92023 Nanterre Cedex, France
and Christine Hatte´ Laboratoire des Sciences du Climat et de l’Environment, UMR 1572-CEA/CNRS, Domaine du CNRS, F-91198 Gif sur Yvette Cedex, France Received November 19, 1998
Fourteen organic-rich sedimentary layers in the deposits at Quebrada de los Burros, in coastal southern Peru (Tacna department), lie between two debris-flow units, interpreted to result from El Nin˜o events, at 8980 cal yr B.P. and after 3380 cal yr B.P., respectively. The accumulation of the fine-grained and low-energy sediments of this deposit during the mid-Holocene is incompatible with the occurrence of El Nin˜o events in this region, as these would produce catastrophic flood deposits. The occurrence of organicrich sediments and evidence of an enhancement of upwelling strength at this time imply the existence of a permanent water supply resulting from an increased condensation of fog at midaltitudes. These results suggest a lower intensity and, perhaps, a lower frequency of occurrence of the El Nin˜o phenomenon during the mid-Holocene. It is precisely during this period that the most important human settlements are found at this site, probably indicating the presence of reliable supply of fresh water. The chronologies for wetlands in the central south altiplano are out of phase with those indicating increased soil moisture episodes on the coast, implying a long-term difference in climate between these two regions. © 1999 University of Washington. Key Words: El Nin˜o; southern Peru; mid-Holocene; 14C dating.
INTRODUCTION
The coastal region near Tacna (southern Peru), on the northern edge of the Atacama desert, is one of the most arid regions in the world. Historical precipitation has never exceeded 25
mm/yr. Surface water is only available at a few rivers draining the Andes Mountains (Rio Sama) or by cloud condensation (garua) at 600 –1000 m elevation. Compilations of geological and historical data (Ortlieb, 1995) indicate that precipitation occurs during periods of El Nin˜o (EN) events, although no direct correlation was found between the intensity of EN events and the amount of precipitation and some EN events have occurred here without causing any precipitation at these latitudes (Ortlieb, 1995). One consequence of the extreme aridity near Tacna is that no continuous sedimentary (or pollen) records have been preserved from which the early history of the El Nin˜o phenomenon could be reconstructed (Sandweiss et al., 1996, 1997; DeVries et al., 1997; Wells and Noller, 1997; Keefer et al., 1998; Rodbell et al., 1999). Palaeoclimatic reconstructions concerning the onset of EN during the late glacial or Holocene periods rely on paleosol chronology, the presence of Man in archeological sites, fluctuations of lake levels in the altiplano in the central Andes (Grosjean et al., 1995a, 1995b, 1997a, 1997b; Servant et al., 1995), changes in the distribution of mollusk fauna (Sandweiss et al., 1996; DeVries and Wells, 1990), or teleconnections with other areas in South America (Martin et al., 1993). Studies of recent meteorological data indicate that the EN phenomenon in the Altiplano and central Andes is characterized by increased dryness such that preservation of sedimentary records of EN events is poor there also. The hypothesis
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that climatic variability is out of phase between coastal regions and the altiplano during the Holocene is difficult to test mainly due to the lack of sedimentary records for comparison. New data from a continuous 5000-yr sedimentary record in Quebrada de los Burros in coastal southern Peru are presented here. The occurrence of organic loamy sands and debris-flow layers in these deposits is discussed with respect to the occurrence of the El Nin˜o phenomenon and coastal upwelling activity at this site during the mid-Holocene. The results are compared with Holocene sequences from the altiplano.
MATERIAL AND METHODS
Site Location Quebrada de los Burros, located in the Tacna department (18°19S, 70°509W), is a narrow 2.5-km-long valley orientated roughly N–S (Fig. 1). This valley is about 150 m above mean sea level (amsl) and 1 km from the coast. At the base of this valley, springs occur, creating small permanent wetlands with a weak outflow of a few 1/s. Within these pools, algae and aquatic plants, mainly mosses, accumulate to form organic-rich sediments. As no significant rains occur in this region (except during some of the EN episodes), the water supply to the Quebrada probably originates from a groundwater source at the Pampa de Lintay (600 m amsl a few kilometers to the north) (Fig. 1). The occurrence of shrubs in the bottom of the upper part of the valley suggests the presence of near-surface groundwater. The water table may be fed either by EN rains at mid-altitude or by fog-drip at the ground level of the Pampa de Lintay. However, the water discharge has been higher in the recent past because there is evidence of recent incision of this valley, resulting in the formation of a gully 2.5 m deep and 600 m long. The sediments exposed in this gully are composed of stratified organic, alluvial, colluvial, and anthropogenic deposits (Figs. 2 and 3). Three debris-flow units are observed in the sequences exposed in this gully. Human presence is indicated by seashore pebbles (“manuports”), probably used for percussion, and marine shell layers composed of Concholepas concholepas, Fissurella crassa, and Mesodesma donacium. Prehistoric settlements are indicated by the presence of shell deposits, abundant fish remains, associated hearths, lithic tools, and a few bone implements. These archeological sites have been excavated and date from 10,000 to 3000 14C yr B.P. (Lavalle´e et al., in press). Samples Fourteen organic-rich layers have been sampled along the gully within the Quebrada. We assume that these organic-rich sediment layers result from decomposition of aquatic plants and grasses growing on the bank, as they do today. It was not possible to establish direct stratigraphical correlation between
FIG. 1. Location and topography (dotted line) of the Quebrada de los Burros.
all these layers because: (1) these anthropogenic deposits are lenticular, varying in thickness from a few centimeters to more than a meter; (2) flood events have heavily eroded these bank
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corresponds to the deeper layers of the archaeological excavations. Ten units are observed in the 2-m-thick section (Fig. 3). The Chapel cross section is located on the right bank of the gully at about 150 m amsl. Twelve units are observed in the 2-m-thick section (Fig. 3). The occurrence of a calcareous crust corresponds to a slope deposit during a period humid enough to allow formation of carbonate-saturated solutions. More detailed description of all these cross sections can be found in Table 1. Radiocarbon Analysis
FIG. 2. Photograph of Corral cross section.
deposits, making it difficult to establish spatial correlations; and (3) the slope of the stream bed is steep in the lower part of the valley. Three cross sections in the gully were selected as the most representative and the best preserved for this study. The Corral cross section is located in the upper part of the Quebrada at about 160 m amsl on the left bank of the gully. It is about 2.50 m high and overlies a debris-flow deposit (huayco) of unknown thickness. The top of the section is covered by another debris-flow deposit 1 m thick. Between these two debris-flow deposits, fourteen horizontal units are observed, consisting of organic-rich layers interbedded with fine-grained aeolian deposits (Fig. 2). In several places along the Quebrada, more recent sheet flows have cut this sequence. However, the sediments located between the two debris flows of this section were deposited under calm, low-energy conditions. The Exacavation cross section (test pit 2b-1995) is located on the left bank of the erosional gully at about 150 m amsl and
Radiocarbon analysis has been performed by b-counting (using the conventional method employing CO 2 gas proportional counters) at the L.S.C.E. at Gif-s/Yvette and at the Modane Underground Laboratory (French Alps). Results are presented as conventional 14C ages and also as calibrated dates (Table 2) calculated using the Calib 3.0 program (Stuiver and Reimer, 1993). Samples were prepared following standard procedure (De´librias, 1985). Shells were mechanically cleaned of adhering contaminants. Leaching with dilute HCl was used to remove the portions of the shell matrix suspected to have been affected by exchange reactions (Vita-Finzi and Roberts, 1984). Shells are not well preserved in the basal deposits, especially in the Corral cross section, and were therefore not dated. Organic-rich layers were sampled with special care in order to avoid possible contamination by roots. The upper part of each layer was selected for sampling where it was more than 3 cm thick. The organic carbon content of these layers ranges from 0.6 to 8.8% on a carbonate-free basis, giving a mean value of 3.1%. The carbonate fraction of these organic-rich sediment layers was comprised of small shell fragments. A hard-water effect, which could have influenced the organic deposit dates, is thought unlikely because of the surrounding volcanic rock terrain (Formacion Chocolate) and the shallow water depth of the springs (few centimeters), allowing rapid equilibration with the atmosphere. Comparison between dates of organic sediment layers and those of adjacent anthropogenic shell layers indicates that the shells yield radiocarbon ages 500 to 700 14C years older than the organic sediment layers. This is in good agreement with the present-day sea-water reservoir age measured for this region (Stuiver and Brazuinas, 1993). RESULTS AND DISCUSSION
Radiocarbon age estimates for the organic-rich layers and shells are reported in Table 2 and Figure 3. Alternating eolian sand layers and organic deposits record alternating eolian and wetland phases from 8730 6 70 to 3220 6 50 14C yr B.P. (9640 to 3380 cal yr B.P.). Except for QLB6-7, these organicrich layers are associated with marine shells or lithic artifacts that indicate a human presence in the Quebrada contemporaneous with the wetland deposition. In fact, dates of the QLB4
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FIG. 3. Description of the three cross sections of the Quebrada de los Burros deposit.
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TABLE 1 Descriptions of the Cross Sections Unit
Thickness The Corral cross section
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16
.1 m 6 cm 8 cm 10 cm 15 cm 2 cm 5 cm 6 cm 6 cm 13 cm 11 cm 5 cm 6 cm 4 cm 13 cm Unknown
Coarse debris flow (local name Huayco) with large and irregular blocks within a gray consolidated loamy-sand matrix Blackish organic loamy sands, finely stratified (sample QLB10) Coarse to fine sands horizontally stratified with oxidized stains Blackish organic loamy sands containing anthropogenic pebbles (projectiles), sample QLB9 Coarse to fine oxidized sands, like level C3 Blackish organic loamy sands (sample QLB8) Gray sands with cm-sized gravels Horizontally stratified fine gray sands Blackish organic loamy sands (sample QLB7) Coarse to fine oxidized sands horizontally stratified and consolidated in soft sandstone Blackish organic loamy sands (sample QLB6) Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB5) Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB4) Typical debris flow, very coarse The Excavation cross section (test pit 2b-1995)
E1 E2 E3 E4 E5 E6 E7 E8 E9 E10
.1 m 10 cm 10 cm 60 cm 80 cm 40 cm 50 cm 5 cm 40 cm Unknown
Coarse debris flow Blackish organic loamy sands with marine shell fragments (sample QLB17) Anthropogenic deposit of marine shells Debris flow with large blocks and sands Anthropogenic deposit of marine shells containing sands and pebbles Blackish organic loamy sands containing fragments of anthropogenic marine shells (sample QLB3) Typical debris flow within an oxidized loamy sand matrix Anthropogenic deposit of marine shells Blackish organic loamy sand (sample QLB2 (top) sample QLB1 (base)) Stratified oxidized fine sands The Chapel cross section
Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8 Ch9 Ch10 Ch11 Ch12
Variable 3 cm 2 cm 15 cm 10 cm 10 cm 25 cm 8 cm 3 cm 25 cm 10 cm 60 cm
Slope deposit (clast, sand, silt) Blackish organic loamy sands containing marine shell fragments (sample QLB 14) Anthropogenic deposit of marine shells Loamy sand calcareous crust Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB 13) Anthropogenic deposit of marine shells Fine sands containing shell fragments Blackish organic loamy sands containing marine shells (sample QLB12) Anthropogenic deposit of marine shells Fine sands and oxidized gravels Blackish organic loamy sands layer containing marine shells and lying on slope deposits with blocks and shells (sample QLB11)
and QLB7 layers correspond to the level N2 of human settlements in the excavation (Lavalle´e et al., in press). Since 3380 cal yr B.P. until very recently, no organic layer has been deposited at this site, and there is no evidence of human occupation in the Quebrada or in the neighboring Canyon site since 3000 14C yr B.P. (Lavalle´e et al., in press). Based on chronostratigraphical constraints, the following correlations between the sections are proposed: QLB1 and QLB11, QLB4
and QLB12, and QLB5 and QLB13. QLB17 in the Excavation section (test pit 2b-1995) appears to have been partly reworked due to human activity, or it may have been disrupted by the overlying huayco deposit. QLB17 may be correlated with either QLB9 or QLB10, or it could be a mixture of both. These results indicate ten periods of increased soil moisture between 9640 and 3380 cal yr B.P. The duration of each of these episodes is short: radiocarbon age estimates constrain the
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TABLE 2 Radiocarbon Dates from Quebrada de los Burros
Laboratory no. Excavation cross section GIF-10632 GIF-10633 GIF-10400 GIF-10634 GIF-10401 GIF-10648 GIF-10404 Corral cross section GIF-10635 GIF-10636 GIF-10637 GIF-10638 GIF-10639 GIF-10640 GIF-10641 Chapel cross section GIF-10642 GIF-10643 GIF-10646 GIF-10644 GIF-10645 GIF-10647 Canyon site GIF-10629 GIF-10722
Sample no.
5085 5086 5035 5087 5031–2 5088 5011
Unit no.
QLB1 QLB2 QLB3 QLB17
Nature
Organic Organic Shell Organic Shell Organic Shell
layer layer layer layer
Radiocarbon ages ( 14C yr)
d 13C (%)
Calibrated age (cal yr B.P.)
Calibrated age range (cal yr B.P.)
8730 6 70 8160 6 70 8860 6 130 8040 6 105 8780 6 70 3700 6 40 4445 6 40
215,1 215,0 21,0 219,7 0,3 215,0 0,3
9643 8991
9857–9496 9358–8678
8950
9201–8511
3975
4085–3864
5066 5067 5068 5069 5070 5071 5072
QLB4 QLB5 QLB6 QLB7 QLB8 QLB9 QLB10
Organic Organic Organic Organic Organic Organic Organic
layer layer layer layer layer layer layer
7320 6 80 6940 6 60 6180 6 60 5390 6 100 4555 6 50 4010 6 55 3220 6 50
214,9 216,3 215,1 212,5 212,7 212,3 211,9
8064 7666 7008 6175 5255 4415 3378
8286–7904 7876–7564 7172–6878 6307–5918 5307–4987 4559–4239 3476–3262
5079 5080 5083 5081 5082 5084
QLB11 QLB12 QLB15 QLB13 QLB14 QLB16
Organic Organic Shell Organic Organic Shell
layer layer
8650 6 7390 6 8125 6 7105 6 6595 6 7160 6
70 50 30 55 75 80
213,4 215,3 20,5 215,0 213,6 20,7
9524 8124
9840–9443 8299–7968
7894 7393
7937–7718 7531–7277
3120 6 80 3595 6 90
225,3 0,0
3330
3434–3033
5074 5074
layer layer
Charcoals Shell
thickest layer (QLB1-2) to a duration of about 570 14C yr (650 cal yr), giving an accumulation rate around 0.6 mm/yr for this layer. Using this sedimentation rate, the accumulation time for the other wetland deposits ranges between 30 and 210 yr. Between 8060 and 3380 cal yr B.P., the deposition of fine-grained sediment implies that low-energy sedimentation conditions prevailed at this site. Only one fine gravel layer (,1 cm), between organic horizons QLB7 and QLB8 in the Corral cross section, is indicative of a higher energy deposition between 5390 and 4555 14C yr B.P. This episode may correlate to the EN event observed at 4550 6 60 14C yr B.P. at Quebrada Tacahuay (17°509S, 71°W) (Keefer et al., 1998), or beach ridge “J” at Chira (northern Peru), speculated to be strongly related to El Nin˜o events (Ortlieb et al., 1993). Huaycos apparently did not occur here between 8060 and 3380 cal yr B.P. The oldest huayco was deposited at around 8980 cal yr B.P. between organic horizons QLB2 (8160 6 70 14C yr B.P.) and QLB3 (8040 6 105 14C yr B.P.). Keefer et al. (1998) observe a huyaco deposit of similar age at Quebrada Tacahuay. Although the youngest huayco in our sequence cannot be dated precisely, it is constrained to have occurred after 3380 cal yr B.P. Such huayco deposits are thought to result from EN events (Wells, 1990; Keefer et al., 1998). If this is the case, then the
huayco deposit in our sequence constitutes one of the older records of EN during the Holocene. The youngest huyaco deposit at our site ca. 3380 cal yr B.P. could be synchronous with paleo EN indicators observed at the Cerro Sechin site (Wells, 1987), or within beach-ridge series in north Peru (Ortlieb et al., 1993), or at the Puripica site (22°409 S, 4000 m) in northern Atacama (Grosjean et al., 1997a). The organic-rich horizons of the Quebrada de los Burros deposit are not thought to be directly associated with enhanced precipitation associated with EN events because there is no evidence of debris-flow deposits at these times. During the last 40 yr, two extensive debris-flow deposits have been observed to be produced by EN events in the Quebrada area. During this 5000-yr mid-Holocene period represented by the organic-rich sediments of the Quebrada, a variety of data indicates climates very different from that of the present: (1) all sedimentary units are fine-grained deposits derived mainly from aeolian origin, (2) the presence of fisherfolk and maritime gatherer settlements indicate availability of permanent water, (3) faunal remains in the middens indicate the presence of camelids, cervids, and birds. Furthermore, in this region, the occurrence of an EN event does not always produce rain (Ortlieb, 1995). Although each EN (4-yr pseudo-periodicity) yielded rainfall here, the
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FIG. 4. Comparison of the chronologies of organic deposits of Quebrada de los Burros and of wetlands at the Puripica site (in gray) in the Andes mountains (from Grosjean et al., 1997a). Calibrated age ranges are expressed at 1-sigma confidence level. Dashed areas represent periods of wetlands in Puripica.
events would not have sustained a permanent water supply necessary for the formation of an organic-rich sediment. Even if EN events contribute to increased humidity of the coastal region (Ferreyra, 1993), our observations suggest a lower strength and frequency of EN events throughout the midHolocene compared to today. This is in good agreement with Keefer et al. (1998), who proposed a decreasing number of EN during the mid-Holocene (see also Sandweiss et al., 1999). Conditions more suitable for the maintenance of a permanent water supply at the Quebrada de los Burros would be obtained by an increasing frequency of intense coastal fogs, which would consequently lower the frequency of EN events. Precipitation originating from cloud condensation during the winter of 1949 A.D. (the year without an EN event), at Pampa de Lachay north of Lima, was observed to reach 1240 mm (Ellenberg, 1959). Such winter fogs could be abundant enough to feed the aquifer of Pampa de Lintay a few kilometers to the north of our deposits. This interpretation agrees with other evidence for the enhanced coastal upwelling during the early and mid-Holocene, which would result in an increased fog production (Schader and Sorknes, 1991). Such an increase in
the strength of the upwelling at this time may be reflected in the reservoir ages of sea waters (R) we calculate from paired 14C ages of shell and organic-rich layers (Table 2), which are slightly greater (about 100 yr) that those observed today (Stuiver and Brazuinas, 1993). The three sections at Quebrada de los Burros provide evidence for an extremely dry mid-Holocene climate which was interrupted by short high soil-moisture events. Wetland episodes similar to those observed by Grosjean et al. (1997a) at Puripica or by Veit et al. (personal communication, London, PAGES Meeting 1998) in the northern Chilean Andes plateau are found at Quebrada de los Burros. However, with the exception of QLB6, the chronology of wetlands deposits in Puripica is very different from that of Quebrada de los Burros (Fig. 4). These results indicate contrasting climates between the Andes and the coast, with increased aridity in the mountains corresponding to increased levels of soil moisture on the coast and vice versa. Such a pattern could be produced by an increase in the frequency of EN events recharging the aquifers, since modern meteorological records during the last century also show this opposition (Francou and Pizarro, 1986). From
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3380 cal yr B.P. to the present, humid periods have not been found in coastal records and less arid conditions prevail in the Andes (Veit et al., personal communication, London, PAGES Meeting 1998), confirming a long-term difference in climate evolution between these two regions. If this hypothesis is true, it would indicate that dry periods in the altiplano of the northern Atacama would not necessarily be linked to EN events, and as suggested by Grosjean et al. (1997a) other mechanisms, like convective summer storms and outbursts of frontal winter precipitation from westerlies, might play a significant role.
DeVries, T. J., Ortlieb, L., Diaz, A., Wells, L., and Hillaire-Marcel, C. (1997). Determining the early history of El Nin˜o. Science 276, 965–966. ¨ ber den Wasserhaushalt tropischer Nebelaosen in Ellenberg, H. (1959). U
CONCLUSIONS
Grosjean, M., Geyh, M. A., Messerli, B., and Schotterer, U. (1995a). Late glacial and early Holocene lake sediments, groundwater formation and climate in the Atacama altiplano 22–24°S. Journal of Paleolimnology 14, 2412–2452.
The sedimentary records of Quebrada de los Burros are comprised of ten organic deposits indicating that the aridity during the mid-Holocene was interrupted by short (less than 200 yr) more humid spells. These increases in soil moisture recorded between 8060 and 3380 cal yr B.P. have been related to an enhancement of the strength of upwelling resulting in an increase in condensation of winter fogs. The fine-grained deposits of Quebrada de los Burros indicate that the southern Peru coast has not been affected by EN events during this period. These observations concerning upwelling strength and sedimentary records indicate a lower intensity of the EN phenomenon and perhaps a lower periodicity in the occurrence of EN at these times as suggested by Rodbell et al. (1999). These results also suggest that the opposing climates between the south central Andes and the coastal region which prevail today were also in existence during the mid-Holocene. It is particularly interesting to point out that past humid but calm periods are strictly correlated with intense human occupation in the Quebrada. Finally, these results provide new evidence for two debris-flow deposits probably resulting from large EN events at around 8980 cal yr B.P. and younger than 3380 cal yr B.P. ACKNOWLEDGMENTS We thank L.E. Wells and D. Rea, who reviewed the manuscript and proposed noticeable improvements. Thanks are also due to L. Ayliffe, M.A. Juillet-Leclerc, and M. Elliot for their comments, C. Noury and S. Martin for preparation of samples for 14C dating, and A. Bolan˜os and C. Dolorier for their help in the field. The Peru-Sur Archaeological Project was supported by Ministe`re des Affaires Etrange`res, Centre National de la Recherche Scientifique (CNRS), and Institut Francais d’Etudes Andines (IFEA, Lima). M.F. and C.H. are supported by Commissariat a` l’Energie Atomique (CEA). LSCE contribution 0250.
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Wells, L. E. (1987). An alluvial record of El Nin˜o events from northern coastal Peru. Journal of Geophysical Research 92(C13), 14,463–14,470. Wells, L. E. (1990). Holocene history of the El Nin˜o phenomenon as recorded in flood sediments of northern coastal Peru. Geology 18, 1134 –1137. Wells, L. E., and Noller, J. S. (1997). Determining the early history of El Nin˜o. Science 276, 966.
El Nin˜o Variability in the Coastal Desert of Southern Peru during the Mid-Holocene Michel Fontugne Laboratoire des Sciences du Climat et de l’Environnement, UMR 1572-CEA/CNRS, Domaine du CNRS, F-91198 Gif sur Yvette cedex, France E-mail: [email protected]
Pierre Usselmann UMR 5651 Espace, Maison de la Ge´ographie, 17 rue de l’Abbe´ de l’Epe´e, F-34090 Montpellier, France
Danie`le Lavalle´e and Miche`le Julien Arche´ologie des Ame´riques, CNRS, Maison de l’Arche´ologie et de l’Ethnogie, 21 Alle´e de l’Universite´, F-92023 Nanterre Cedex, France
and Christine Hatte´ Laboratoire des Sciences du Climat et de l’Environment, UMR 1572-CEA/CNRS, Domaine du CNRS, F-91198 Gif sur Yvette Cedex, France Received November 19, 1998
Fourteen organic-rich sedimentary layers in the deposits at Quebrada de los Burros, in coastal southern Peru (Tacna department), lie between two debris-flow units, interpreted to result from El Nin˜o events, at 8980 cal yr B.P. and after 3380 cal yr B.P., respectively. The accumulation of the fine-grained and low-energy sediments of this deposit during the mid-Holocene is incompatible with the occurrence of El Nin˜o events in this region, as these would produce catastrophic flood deposits. The occurrence of organicrich sediments and evidence of an enhancement of upwelling strength at this time imply the existence of a permanent water supply resulting from an increased condensation of fog at midaltitudes. These results suggest a lower intensity and, perhaps, a lower frequency of occurrence of the El Nin˜o phenomenon during the mid-Holocene. It is precisely during this period that the most important human settlements are found at this site, probably indicating the presence of reliable supply of fresh water. The chronologies for wetlands in the central south altiplano are out of phase with those indicating increased soil moisture episodes on the coast, implying a long-term difference in climate between these two regions. © 1999 University of Washington. Key Words: El Nin˜o; southern Peru; mid-Holocene; 14C dating.
INTRODUCTION
The coastal region near Tacna (southern Peru), on the northern edge of the Atacama desert, is one of the most arid regions in the world. Historical precipitation has never exceeded 25
mm/yr. Surface water is only available at a few rivers draining the Andes Mountains (Rio Sama) or by cloud condensation (garua) at 600 –1000 m elevation. Compilations of geological and historical data (Ortlieb, 1995) indicate that precipitation occurs during periods of El Nin˜o (EN) events, although no direct correlation was found between the intensity of EN events and the amount of precipitation and some EN events have occurred here without causing any precipitation at these latitudes (Ortlieb, 1995). One consequence of the extreme aridity near Tacna is that no continuous sedimentary (or pollen) records have been preserved from which the early history of the El Nin˜o phenomenon could be reconstructed (Sandweiss et al., 1996, 1997; DeVries et al., 1997; Wells and Noller, 1997; Keefer et al., 1998; Rodbell et al., 1999). Palaeoclimatic reconstructions concerning the onset of EN during the late glacial or Holocene periods rely on paleosol chronology, the presence of Man in archeological sites, fluctuations of lake levels in the altiplano in the central Andes (Grosjean et al., 1995a, 1995b, 1997a, 1997b; Servant et al., 1995), changes in the distribution of mollusk fauna (Sandweiss et al., 1996; DeVries and Wells, 1990), or teleconnections with other areas in South America (Martin et al., 1993). Studies of recent meteorological data indicate that the EN phenomenon in the Altiplano and central Andes is characterized by increased dryness such that preservation of sedimentary records of EN events is poor there also. The hypothesis
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that climatic variability is out of phase between coastal regions and the altiplano during the Holocene is difficult to test mainly due to the lack of sedimentary records for comparison. New data from a continuous 5000-yr sedimentary record in Quebrada de los Burros in coastal southern Peru are presented here. The occurrence of organic loamy sands and debris-flow layers in these deposits is discussed with respect to the occurrence of the El Nin˜o phenomenon and coastal upwelling activity at this site during the mid-Holocene. The results are compared with Holocene sequences from the altiplano.
MATERIAL AND METHODS
Site Location Quebrada de los Burros, located in the Tacna department (18°19S, 70°509W), is a narrow 2.5-km-long valley orientated roughly N–S (Fig. 1). This valley is about 150 m above mean sea level (amsl) and 1 km from the coast. At the base of this valley, springs occur, creating small permanent wetlands with a weak outflow of a few 1/s. Within these pools, algae and aquatic plants, mainly mosses, accumulate to form organic-rich sediments. As no significant rains occur in this region (except during some of the EN episodes), the water supply to the Quebrada probably originates from a groundwater source at the Pampa de Lintay (600 m amsl a few kilometers to the north) (Fig. 1). The occurrence of shrubs in the bottom of the upper part of the valley suggests the presence of near-surface groundwater. The water table may be fed either by EN rains at mid-altitude or by fog-drip at the ground level of the Pampa de Lintay. However, the water discharge has been higher in the recent past because there is evidence of recent incision of this valley, resulting in the formation of a gully 2.5 m deep and 600 m long. The sediments exposed in this gully are composed of stratified organic, alluvial, colluvial, and anthropogenic deposits (Figs. 2 and 3). Three debris-flow units are observed in the sequences exposed in this gully. Human presence is indicated by seashore pebbles (“manuports”), probably used for percussion, and marine shell layers composed of Concholepas concholepas, Fissurella crassa, and Mesodesma donacium. Prehistoric settlements are indicated by the presence of shell deposits, abundant fish remains, associated hearths, lithic tools, and a few bone implements. These archeological sites have been excavated and date from 10,000 to 3000 14C yr B.P. (Lavalle´e et al., in press). Samples Fourteen organic-rich layers have been sampled along the gully within the Quebrada. We assume that these organic-rich sediment layers result from decomposition of aquatic plants and grasses growing on the bank, as they do today. It was not possible to establish direct stratigraphical correlation between
FIG. 1. Location and topography (dotted line) of the Quebrada de los Burros.
all these layers because: (1) these anthropogenic deposits are lenticular, varying in thickness from a few centimeters to more than a meter; (2) flood events have heavily eroded these bank
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corresponds to the deeper layers of the archaeological excavations. Ten units are observed in the 2-m-thick section (Fig. 3). The Chapel cross section is located on the right bank of the gully at about 150 m amsl. Twelve units are observed in the 2-m-thick section (Fig. 3). The occurrence of a calcareous crust corresponds to a slope deposit during a period humid enough to allow formation of carbonate-saturated solutions. More detailed description of all these cross sections can be found in Table 1. Radiocarbon Analysis
FIG. 2. Photograph of Corral cross section.
deposits, making it difficult to establish spatial correlations; and (3) the slope of the stream bed is steep in the lower part of the valley. Three cross sections in the gully were selected as the most representative and the best preserved for this study. The Corral cross section is located in the upper part of the Quebrada at about 160 m amsl on the left bank of the gully. It is about 2.50 m high and overlies a debris-flow deposit (huayco) of unknown thickness. The top of the section is covered by another debris-flow deposit 1 m thick. Between these two debris-flow deposits, fourteen horizontal units are observed, consisting of organic-rich layers interbedded with fine-grained aeolian deposits (Fig. 2). In several places along the Quebrada, more recent sheet flows have cut this sequence. However, the sediments located between the two debris flows of this section were deposited under calm, low-energy conditions. The Exacavation cross section (test pit 2b-1995) is located on the left bank of the erosional gully at about 150 m amsl and
Radiocarbon analysis has been performed by b-counting (using the conventional method employing CO 2 gas proportional counters) at the L.S.C.E. at Gif-s/Yvette and at the Modane Underground Laboratory (French Alps). Results are presented as conventional 14C ages and also as calibrated dates (Table 2) calculated using the Calib 3.0 program (Stuiver and Reimer, 1993). Samples were prepared following standard procedure (De´librias, 1985). Shells were mechanically cleaned of adhering contaminants. Leaching with dilute HCl was used to remove the portions of the shell matrix suspected to have been affected by exchange reactions (Vita-Finzi and Roberts, 1984). Shells are not well preserved in the basal deposits, especially in the Corral cross section, and were therefore not dated. Organic-rich layers were sampled with special care in order to avoid possible contamination by roots. The upper part of each layer was selected for sampling where it was more than 3 cm thick. The organic carbon content of these layers ranges from 0.6 to 8.8% on a carbonate-free basis, giving a mean value of 3.1%. The carbonate fraction of these organic-rich sediment layers was comprised of small shell fragments. A hard-water effect, which could have influenced the organic deposit dates, is thought unlikely because of the surrounding volcanic rock terrain (Formacion Chocolate) and the shallow water depth of the springs (few centimeters), allowing rapid equilibration with the atmosphere. Comparison between dates of organic sediment layers and those of adjacent anthropogenic shell layers indicates that the shells yield radiocarbon ages 500 to 700 14C years older than the organic sediment layers. This is in good agreement with the present-day sea-water reservoir age measured for this region (Stuiver and Brazuinas, 1993). RESULTS AND DISCUSSION
Radiocarbon age estimates for the organic-rich layers and shells are reported in Table 2 and Figure 3. Alternating eolian sand layers and organic deposits record alternating eolian and wetland phases from 8730 6 70 to 3220 6 50 14C yr B.P. (9640 to 3380 cal yr B.P.). Except for QLB6-7, these organicrich layers are associated with marine shells or lithic artifacts that indicate a human presence in the Quebrada contemporaneous with the wetland deposition. In fact, dates of the QLB4
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FIG. 3. Description of the three cross sections of the Quebrada de los Burros deposit.
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TABLE 1 Descriptions of the Cross Sections Unit
Thickness The Corral cross section
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16
.1 m 6 cm 8 cm 10 cm 15 cm 2 cm 5 cm 6 cm 6 cm 13 cm 11 cm 5 cm 6 cm 4 cm 13 cm Unknown
Coarse debris flow (local name Huayco) with large and irregular blocks within a gray consolidated loamy-sand matrix Blackish organic loamy sands, finely stratified (sample QLB10) Coarse to fine sands horizontally stratified with oxidized stains Blackish organic loamy sands containing anthropogenic pebbles (projectiles), sample QLB9 Coarse to fine oxidized sands, like level C3 Blackish organic loamy sands (sample QLB8) Gray sands with cm-sized gravels Horizontally stratified fine gray sands Blackish organic loamy sands (sample QLB7) Coarse to fine oxidized sands horizontally stratified and consolidated in soft sandstone Blackish organic loamy sands (sample QLB6) Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB5) Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB4) Typical debris flow, very coarse The Excavation cross section (test pit 2b-1995)
E1 E2 E3 E4 E5 E6 E7 E8 E9 E10
.1 m 10 cm 10 cm 60 cm 80 cm 40 cm 50 cm 5 cm 40 cm Unknown
Coarse debris flow Blackish organic loamy sands with marine shell fragments (sample QLB17) Anthropogenic deposit of marine shells Debris flow with large blocks and sands Anthropogenic deposit of marine shells containing sands and pebbles Blackish organic loamy sands containing fragments of anthropogenic marine shells (sample QLB3) Typical debris flow within an oxidized loamy sand matrix Anthropogenic deposit of marine shells Blackish organic loamy sand (sample QLB2 (top) sample QLB1 (base)) Stratified oxidized fine sands The Chapel cross section
Ch1 Ch2 Ch3 Ch4 Ch5 Ch6 Ch7 Ch8 Ch9 Ch10 Ch11 Ch12
Variable 3 cm 2 cm 15 cm 10 cm 10 cm 25 cm 8 cm 3 cm 25 cm 10 cm 60 cm
Slope deposit (clast, sand, silt) Blackish organic loamy sands containing marine shell fragments (sample QLB 14) Anthropogenic deposit of marine shells Loamy sand calcareous crust Anthropogenic deposit of marine shells Blackish organic loamy sands (sample QLB 13) Anthropogenic deposit of marine shells Fine sands containing shell fragments Blackish organic loamy sands containing marine shells (sample QLB12) Anthropogenic deposit of marine shells Fine sands and oxidized gravels Blackish organic loamy sands layer containing marine shells and lying on slope deposits with blocks and shells (sample QLB11)
and QLB7 layers correspond to the level N2 of human settlements in the excavation (Lavalle´e et al., in press). Since 3380 cal yr B.P. until very recently, no organic layer has been deposited at this site, and there is no evidence of human occupation in the Quebrada or in the neighboring Canyon site since 3000 14C yr B.P. (Lavalle´e et al., in press). Based on chronostratigraphical constraints, the following correlations between the sections are proposed: QLB1 and QLB11, QLB4
and QLB12, and QLB5 and QLB13. QLB17 in the Excavation section (test pit 2b-1995) appears to have been partly reworked due to human activity, or it may have been disrupted by the overlying huayco deposit. QLB17 may be correlated with either QLB9 or QLB10, or it could be a mixture of both. These results indicate ten periods of increased soil moisture between 9640 and 3380 cal yr B.P. The duration of each of these episodes is short: radiocarbon age estimates constrain the
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TABLE 2 Radiocarbon Dates from Quebrada de los Burros
Laboratory no. Excavation cross section GIF-10632 GIF-10633 GIF-10400 GIF-10634 GIF-10401 GIF-10648 GIF-10404 Corral cross section GIF-10635 GIF-10636 GIF-10637 GIF-10638 GIF-10639 GIF-10640 GIF-10641 Chapel cross section GIF-10642 GIF-10643 GIF-10646 GIF-10644 GIF-10645 GIF-10647 Canyon site GIF-10629 GIF-10722
Sample no.
5085 5086 5035 5087 5031–2 5088 5011
Unit no.
QLB1 QLB2 QLB3 QLB17
Nature
Organic Organic Shell Organic Shell Organic Shell
layer layer layer layer
Radiocarbon ages ( 14C yr)
d 13C (%)
Calibrated age (cal yr B.P.)
Calibrated age range (cal yr B.P.)
8730 6 70 8160 6 70 8860 6 130 8040 6 105 8780 6 70 3700 6 40 4445 6 40
215,1 215,0 21,0 219,7 0,3 215,0 0,3
9643 8991
9857–9496 9358–8678
8950
9201–8511
3975
4085–3864
5066 5067 5068 5069 5070 5071 5072
QLB4 QLB5 QLB6 QLB7 QLB8 QLB9 QLB10
Organic Organic Organic Organic Organic Organic Organic
layer layer layer layer layer layer layer
7320 6 80 6940 6 60 6180 6 60 5390 6 100 4555 6 50 4010 6 55 3220 6 50
214,9 216,3 215,1 212,5 212,7 212,3 211,9
8064 7666 7008 6175 5255 4415 3378
8286–7904 7876–7564 7172–6878 6307–5918 5307–4987 4559–4239 3476–3262
5079 5080 5083 5081 5082 5084
QLB11 QLB12 QLB15 QLB13 QLB14 QLB16
Organic Organic Shell Organic Organic Shell
layer layer
8650 6 7390 6 8125 6 7105 6 6595 6 7160 6
70 50 30 55 75 80
213,4 215,3 20,5 215,0 213,6 20,7
9524 8124
9840–9443 8299–7968
7894 7393
7937–7718 7531–7277
3120 6 80 3595 6 90
225,3 0,0
3330
3434–3033
5074 5074
layer layer
Charcoals Shell
thickest layer (QLB1-2) to a duration of about 570 14C yr (650 cal yr), giving an accumulation rate around 0.6 mm/yr for this layer. Using this sedimentation rate, the accumulation time for the other wetland deposits ranges between 30 and 210 yr. Between 8060 and 3380 cal yr B.P., the deposition of fine-grained sediment implies that low-energy sedimentation conditions prevailed at this site. Only one fine gravel layer (,1 cm), between organic horizons QLB7 and QLB8 in the Corral cross section, is indicative of a higher energy deposition between 5390 and 4555 14C yr B.P. This episode may correlate to the EN event observed at 4550 6 60 14C yr B.P. at Quebrada Tacahuay (17°509S, 71°W) (Keefer et al., 1998), or beach ridge “J” at Chira (northern Peru), speculated to be strongly related to El Nin˜o events (Ortlieb et al., 1993). Huaycos apparently did not occur here between 8060 and 3380 cal yr B.P. The oldest huayco was deposited at around 8980 cal yr B.P. between organic horizons QLB2 (8160 6 70 14C yr B.P.) and QLB3 (8040 6 105 14C yr B.P.). Keefer et al. (1998) observe a huyaco deposit of similar age at Quebrada Tacahuay. Although the youngest huayco in our sequence cannot be dated precisely, it is constrained to have occurred after 3380 cal yr B.P. Such huayco deposits are thought to result from EN events (Wells, 1990; Keefer et al., 1998). If this is the case, then the
huayco deposit in our sequence constitutes one of the older records of EN during the Holocene. The youngest huyaco deposit at our site ca. 3380 cal yr B.P. could be synchronous with paleo EN indicators observed at the Cerro Sechin site (Wells, 1987), or within beach-ridge series in north Peru (Ortlieb et al., 1993), or at the Puripica site (22°409 S, 4000 m) in northern Atacama (Grosjean et al., 1997a). The organic-rich horizons of the Quebrada de los Burros deposit are not thought to be directly associated with enhanced precipitation associated with EN events because there is no evidence of debris-flow deposits at these times. During the last 40 yr, two extensive debris-flow deposits have been observed to be produced by EN events in the Quebrada area. During this 5000-yr mid-Holocene period represented by the organic-rich sediments of the Quebrada, a variety of data indicates climates very different from that of the present: (1) all sedimentary units are fine-grained deposits derived mainly from aeolian origin, (2) the presence of fisherfolk and maritime gatherer settlements indicate availability of permanent water, (3) faunal remains in the middens indicate the presence of camelids, cervids, and birds. Furthermore, in this region, the occurrence of an EN event does not always produce rain (Ortlieb, 1995). Although each EN (4-yr pseudo-periodicity) yielded rainfall here, the
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FIG. 4. Comparison of the chronologies of organic deposits of Quebrada de los Burros and of wetlands at the Puripica site (in gray) in the Andes mountains (from Grosjean et al., 1997a). Calibrated age ranges are expressed at 1-sigma confidence level. Dashed areas represent periods of wetlands in Puripica.
events would not have sustained a permanent water supply necessary for the formation of an organic-rich sediment. Even if EN events contribute to increased humidity of the coastal region (Ferreyra, 1993), our observations suggest a lower strength and frequency of EN events throughout the midHolocene compared to today. This is in good agreement with Keefer et al. (1998), who proposed a decreasing number of EN during the mid-Holocene (see also Sandweiss et al., 1999). Conditions more suitable for the maintenance of a permanent water supply at the Quebrada de los Burros would be obtained by an increasing frequency of intense coastal fogs, which would consequently lower the frequency of EN events. Precipitation originating from cloud condensation during the winter of 1949 A.D. (the year without an EN event), at Pampa de Lachay north of Lima, was observed to reach 1240 mm (Ellenberg, 1959). Such winter fogs could be abundant enough to feed the aquifer of Pampa de Lintay a few kilometers to the north of our deposits. This interpretation agrees with other evidence for the enhanced coastal upwelling during the early and mid-Holocene, which would result in an increased fog production (Schader and Sorknes, 1991). Such an increase in
the strength of the upwelling at this time may be reflected in the reservoir ages of sea waters (R) we calculate from paired 14C ages of shell and organic-rich layers (Table 2), which are slightly greater (about 100 yr) that those observed today (Stuiver and Brazuinas, 1993). The three sections at Quebrada de los Burros provide evidence for an extremely dry mid-Holocene climate which was interrupted by short high soil-moisture events. Wetland episodes similar to those observed by Grosjean et al. (1997a) at Puripica or by Veit et al. (personal communication, London, PAGES Meeting 1998) in the northern Chilean Andes plateau are found at Quebrada de los Burros. However, with the exception of QLB6, the chronology of wetlands deposits in Puripica is very different from that of Quebrada de los Burros (Fig. 4). These results indicate contrasting climates between the Andes and the coast, with increased aridity in the mountains corresponding to increased levels of soil moisture on the coast and vice versa. Such a pattern could be produced by an increase in the frequency of EN events recharging the aquifers, since modern meteorological records during the last century also show this opposition (Francou and Pizarro, 1986). From
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3380 cal yr B.P. to the present, humid periods have not been found in coastal records and less arid conditions prevail in the Andes (Veit et al., personal communication, London, PAGES Meeting 1998), confirming a long-term difference in climate evolution between these two regions. If this hypothesis is true, it would indicate that dry periods in the altiplano of the northern Atacama would not necessarily be linked to EN events, and as suggested by Grosjean et al. (1997a) other mechanisms, like convective summer storms and outbursts of frontal winter precipitation from westerlies, might play a significant role.
DeVries, T. J., Ortlieb, L., Diaz, A., Wells, L., and Hillaire-Marcel, C. (1997). Determining the early history of El Nin˜o. Science 276, 965–966. ¨ ber den Wasserhaushalt tropischer Nebelaosen in Ellenberg, H. (1959). U
CONCLUSIONS
Grosjean, M., Geyh, M. A., Messerli, B., and Schotterer, U. (1995a). Late glacial and early Holocene lake sediments, groundwater formation and climate in the Atacama altiplano 22–24°S. Journal of Paleolimnology 14, 2412–2452.
The sedimentary records of Quebrada de los Burros are comprised of ten organic deposits indicating that the aridity during the mid-Holocene was interrupted by short (less than 200 yr) more humid spells. These increases in soil moisture recorded between 8060 and 3380 cal yr B.P. have been related to an enhancement of the strength of upwelling resulting in an increase in condensation of winter fogs. The fine-grained deposits of Quebrada de los Burros indicate that the southern Peru coast has not been affected by EN events during this period. These observations concerning upwelling strength and sedimentary records indicate a lower intensity of the EN phenomenon and perhaps a lower periodicity in the occurrence of EN at these times as suggested by Rodbell et al. (1999). These results also suggest that the opposing climates between the south central Andes and the coastal region which prevail today were also in existence during the mid-Holocene. It is particularly interesting to point out that past humid but calm periods are strictly correlated with intense human occupation in the Quebrada. Finally, these results provide new evidence for two debris-flow deposits probably resulting from large EN events at around 8980 cal yr B.P. and younger than 3380 cal yr B.P. ACKNOWLEDGMENTS We thank L.E. Wells and D. Rea, who reviewed the manuscript and proposed noticeable improvements. Thanks are also due to L. Ayliffe, M.A. Juillet-Leclerc, and M. Elliot for their comments, C. Noury and S. Martin for preparation of samples for 14C dating, and A. Bolan˜os and C. Dolorier for their help in the field. The Peru-Sur Archaeological Project was supported by Ministe`re des Affaires Etrange`res, Centre National de la Recherche Scientifique (CNRS), and Institut Francais d’Etudes Andines (IFEA, Lima). M.F. and C.H. are supported by Commissariat a` l’Energie Atomique (CEA). LSCE contribution 0250.
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