Bajada de rahue, province of neuquen, Argentina: an interstadial deposit in northern Patagonia

Palaeogeography, Palaeoclimatology, Palaeoecology, 56 (1986): 251--258 Elsevier Science Publishers B.V., Amsterdam- Printed in The Netherlands

251

BAJADA DE RAHUE, PROVINCE OF NEUQUEN, ARGENTINA: AN I N T E R S T A D I A L DEPOSIT IN N O R T H E R N P A T A G O N I A

VERA MARKGRAF 1, J. P. BRADBURY2 and J. FERNANDEZ 3 IINSTAAR, University of Colorado, Boulder, CO 80309 (U.S.A.) 2U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 (U.S.A.) 3In*tituto Nacional Antropologia, 1370 3 de febrero, Buenos Aires (Argentina) (Received August 5, 19851 revised and accepted February 2, 1986)

ABSTRACT Markgraf, V., Bradbury, J. P. and Fernandez, J., 1986. Bajada de Rahue, Province of Neuquen, Argentina: an interstadial deposit in northern Patagonia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 56: 251--258. Pollen and diatom analyses of a radiocarbon dated lacustrine section in the temperate Andean region of Argentina (Rahue, Province Neuquen) suggest interglacial type climatic conditions between 27,000 and 33,000 yr B.P., with environments that resemble the modern conditions at the locality. This finding correlates with a woodland record from central Chile, interpreted as reflecting conditions substantially warmer and drier prior to 27,000 yr B.P. than during the following full-glacialperiod. It also appears to relate to the

global paleoclimatic scheme derived from deep-sea records, suggesting interhemispheric synchroneity of such broad-scale palaeoclimatic phases. INTRODUCTION Bajada de Rahue is t he locality of a putative interglacial sediment section (Auer, 1951, 1956) t ha t is exposed along the western flank of the Sierra Catan Lil in th e Argentine province o f Neuquen. Sierra Catan Lil parallels t he central Andean cordillera about 50 km t o the east o f the locality bet w een latitudes 39 ° and 40°S. The range's highest peaks reach over 2000 m and perennial snow banks persist on south-facing slopes. Moraines d o c u m e n t f o r m e r glacial activity, b u t have not been studied so far (D. Cobos, pets. comm., 1985). Between 1000 m and 1500 m grows a forest belt o f Araucaria araucana, Nothofagus pumilio, N. antarctica, and occasional Austrocedrus chilensis. In m o r e accessible areas this forest is strongly impacted b y overgrazing. The Patagonian scrub-steppe, with Baccharis spp. (Compositae), Mulinurn spinosum (Umbelliferae), Chacaya trinervis (Rhamnaceae), Adesmia ssp. (Leguminosae), Ephedra, grasses and herbaceous t axa occurs below 1000 m. Th e lacustrine sediment section was originally discovered by Frenguelli (1941) in a r o a d c u t a b o u t 10 k m east o f Rahue (39°22'S, 70°56'W, 1000 m, 0031-0182/86/$03.50

© 1986 Elsevier Science Publishers B.V.

252 Fig.l). Later roadwork uncovered a much larger area of unconsolidated sediments, including gravels, conglomerates, and sands, that surrounded a rather small and moderately deformed outcrop of diatomaceous lacustrine silts and clays, overlying units of peat, volcanic ash, and diatomites. Auer (1956), who studied the pollen content and general sedimentologic context of the section, concluded that the peat layers were deposited during the previous

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interglacial period. The inorganic layers, on the other hand, he thought, were subsequently deposited in periglacial lakes. According to Auer (1956) advancing glaciers from the Andes ultimately overran the site and tilted and disrupted the section. Auer (1956), however, had no radiocarbon dates to support his interpretation. Given recent advances in regional Andean chronologies of glacial events (Mercer, 1983), and of vegetation changes (Markgraf, 1983, 1984), a reassessment of the section is warranted. In 1980 and 1981 the section was resampled for detailed radiocarbon, pollen, and diatom analyses. A total of 5 levels were dated, 10 horizons analysed for pollen and 12 for diatoms. The chronology is discussed in detail by Fernandez et al. (1983). This paper deals with the palaeoenvironmental data as reflected by pollen and diatoms, and their implication on the understanding of Late Pleistocene palaeoclimates. METHODS

The thickness of the section varies considerably. For pollen and diatoms, a 450-cm section was sampled. The samples for pollen analysis were treated by standard techniques (Faegri and Iversen, 1975), with hydrochloric acid, hydrofluoric acid, and acetolysis mixture, to eliminate carbonates, silicates, and organics, respectively. Pollen was counted under 1000 X magnification with a Leitz Dialux microscope and calculated as percent of the total pollen sum excluding Cyperaceae, fern spores, and the waterplant Myriophyllum {Fig. 2). The diatoms were studied in water mounts and are given in relative abundance values ( Fig. 2). RESULTS

According to 5 radiocarbon dates from different levels (Fig.2), the sediment section was deposited between 27,900 and 33,500 yr B.P. (27,900 + 1200, AC-282; 28,800 + 1100, AC-175; 29,000 -+ 1400, AC-176bis; 32,600 + 1500, AC-177; 33,500 + 1500, A-3553). Thus, contrary to Auer's {1956) interpretation, the whole lacustrine section represents a continuous but relatively short interval of time. From the 450 cm long section, 10 samples were analysed for pollen (Fig.2). The well-preserved pollen types represented are Gramineae, other herbaceous taxa, Compositae, Nothofagus, Cyperaceae, and Myriophyllum. The proportions of these types remain comparatively similar throughout the section, with Gramineae dominant (50--70%), Nothofagus (10-20%), Compositae, primarily of the tribe of Tubuliflorae {7--12%), followed by herbs (10-20%) that included Umbelliferae, Caryophyllaceae, Acaena (Rosaceae), Ranunculaceae, Cruciferae, Plantago, and Chenopodiaceae. Most of the identifiable Nothofagus pollen is N. dombeyi, with traces of N. antarctica and N. obliqua type. In addition there are traces from other canopy trees, such as Podocarpus, Austrocedrus, Maytenus, Araucaria, and understory trees such as Myrtaceae, Schinus, Weinmannia, Drimys, and Buddleja.

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255 This pollen assemblage reflects a local sedge-marsh/shallow pond environment surrounded by steppe-scrub, with Nothofagus forests nearby. Of the numerous modern pollen rain samples (Markgraf et al., 1981), the sample from Estancia Condor in the steppe/forest zone east of Bariloche (Fig.l) most closely resembles the Rahue pollen assemblage. This site is a seasonally inundated marsh, 10--20 km from the nearest Austrocedrus and Nothofagus dombeyi forests. Among all the modern steppe sites, this sample appeared the least altered by overgrazing and by subsequent invasion of European weeds (Rumex and Plantago lanceolata account for less than 5% of the total sum) and thus seems best suited for comparison. The similarity of both the modern and the fossil pollen assemblages lies in the proportions of arboreal vs. non-arboreal pollen, as well as in the non-tree pollen composition (Fig.2). Thus, the occurrence of 10--20% of Nothofagus dombeyi pollen in the interstadial section, compared to 18% in the modern steppe sample, suggests that the Nothofagus forest 30,000 years ago was at least 10 km distant from the site (Markgraf et al., 1981). At present N. dombeyi reaches its eastern limit about 20 km west of the Rahue site. The low percentages of other arboreal pollen in the Rahue section compare equally well to the modern pollen assemblage of Estancia Condor. Both Araucaria and Austrocedrus pollen disperse only in the immediate vicinity of the trees, and thus are represented only by low values in steppe sites (Markgraf et al., 1981). The similarity of the occurrences of non-arboreal taxa, such as Compositae, Umbelliferae, and Caryophyllaceae in both the modern and the fossil sample suggests that the floristic setting of the steppe environment 30,000 years ago was also similar to that of today. The limnological environment of the Rahue site can be deduced by study of fossil diatoms and the pollen of aquatic plants. The diatoms from the Rahue section are all species that live in shallow, freshwater environments. Diatom concentration varies considerably throughout the section; the upper and lower units are entirely composed of diatoms (diatomites) and the intermediate units range from barren to highly diatomaceous (Fig.2). Volcanic ash, sand, clay, and peat are the principal lithological components that occur with the diatoms and reduce diatom concentration. The influx of nondiatomaceous materials probably reflects stream activity and water depth. The most highly diatomaceous units presumably formed in somewhat deeper water in areas of a marsh or pond that were surrounded by extensive emergent and submergent vegetation that traps incoming clastic sediment. Highly organic or peaty units apparently represent shallow water conditions within the surrounding vegetation. Typically samples from the organic units contain significant concentrations of sand and silt. Semi-quantitative evaluation of diatom concentration (Fig.2) shows that Fragilaria pinnata (= F. construens v. elliptica, according to Cappannini, as cited in Auer, 1956) is the predominant species in these deposits. This is particularly so in the highly diatomaceous units at the top and base of the section. Cymbella cyrnbiformis and Epithemia adnata, which are frequent

256

associates of F. pinnata, are most commonly found attached to submerged plants. These three species are common in Patagonia today. For example, they are common taxa in the shallow, littoral margins of Lago Quillen and Lago Mascardi, and are reported to be frequently present and occasionally abundant in Holocene bog profiles throughout Patagonia and Tierra del Fuego (Cleve-Euler, 1948; Krasske, 1949). Because these species are not planktonic forms, but must live on the bottom or attached to rooted aquatic plants within the photic zone {where sunlight is sufficient for photosynthesis) they indicate comparatively shallow water, probably not exceeding 1--2 m depth. All other diatoms in the Rahue section (see Cappannini, in Auer, 1956) occur in comparatively low concentrations. The restriction of Melosira distans and species of Eunotia to the more organic units in the middle part of the section (Fig.2) indicates that during this period of deposition the small lake or marsh was receiving an abundant supply of terrestrial organic material, and that the water became slightly acidic and perhaps somewhat dystrophic. The organic matter may have been derived from the marsh plants growing around the margins of the water body or from nearby terrestrial vegetation. Pollen from Cyperaceae and from Myriophyllum in variable quantities throughout the section suggest that wetland and aquatic plants were responsible for at least some organic influx to the basin. Nevertheless, the continuation of the dominant species (Fragilaria pinnata and Cymbella cymbiformis) throughout this interval implies that limnologic changes were minor and related to local conditions near the small lake. Diatoms as well as pollen and age-determinations suggest that the whole lacustrine section was deposited during a relatively short time interval under one environmental regime that resembled m o d e m environments of the area. The dates of the deposit indicate a late Wisconsin interstadial age. Thus, Auer's {1951, 1956) conclusion that the Rahue section represented interglacial age sediments, overlain by periglacial deposits can no longer be supported. It also seems highly improbable that Andean glaciers reached that far to the east during the upper Pleistocene (Caldenius, 1932; Flint and Fidalgo, 1964; Mercer, 1982), or that the local glaciers of the Sierra Catan Lil descended to the elevation of the site. DISCUSSION

Even though many pre-full-glacial sections have been analysed for pollen from comparable latitudes in Chile (Heusser, 1974, 1981) few of these records encompass the interval 27,000--33,000 yr B.P. represented by the Rahue section. These records are: Taiquem0, in Valdivian rainforest on the Chiloe Island (Heusser and Flint, 1977; Heusser et al., 1981); Los Pellines, an allochthonous sample set dated 30,400 + 1500 yr B.P. from rainforest environment in the Chilean Lake District (Heusser, 1974); and Laguna Tagua Tagua in subtropical sclerophyllous woodland in central Chile (Heusser, 1983; see Fig.l).

257 According to the records from the rainforest region, palaeoenvironmental, and hence palaeoclimatic, conditions were basically full-glacial in character from before 33,000 to 14,000 yr B.P., based on the pollen data that show the same Nothofagus woodland throughout that time interval. Thus, summer temperatures were calculated to have been about 4°C cooler and mean annual precipitation about 1500 mm less than today (Heusser, 1974; Heusser et al., 1981). In contrast, the Laguna Tagua Tagua record from subtropical latitudes suggests that the 27,000--33,000 yr B.P. interval was substantially warmer and drier than the following full-glacial interval, although not quite as warm as the Holocene (Heusser, 1983). This latter palaeoclimatic scenario, which contrasts to the Valdivian rainforest one, compares well with the Rahue interpretation. This suggests that there is a difference in palaeoclimatic sensitivity between the woodland (Rahue and Tagua Tagua) and the rainforest sites, the subtropical woodland sites showing a far greater difference between pre- and full-glacial conditions than the rainforest sites. Even though this palaeoclimatic difference refers to both temperature and precipitation, it is affecting the respective environments quite differently. This difference can be explained by the difference in modern climatic amplitude of the environments. While modern rainforests grow under a wide range of precipitation (from 5000 to 2000 mm annually), suggesting a low degree of sensitivity for precipitation changes, sclerophyllous-woodlands are characterized by a steep climatic gradient, and an even minor climatic shift would result in environmental response. This suggests that the overall palaeoclimatic scenario prior to 27,000 yr B.P. in southern South America was characterized by a precipitation pattern similar to the modern one, and with substantially less precipitation and/or substantially higher temperatures than reported for the full-glacial period. In conclusion, the Rahue record supports the notion of interglacial-type climatic conditions for southern South America prior to 27,000 yr B.P. This was suggested previously on basis of high sealevels along the Argentine coast dated at 34,000 yr B.P. (Vogel and Lerman, 1969), which correlates with the global palaeoclimatic scheme of deep-sea records (e.g. Cline and Hays, 1976). At this broad-scale of palaeoclimatic global comparison, both hemispheres appear to have followed a similar pattern during the last 30,000 yr B.P. ACKNOWLEDGEMENTS This research has been supported under the International Cooperative Research Program by the National Science Foundation (ATM-82-12836 to V. Markgraf) and Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONtCET to J. Fernandez). The administration of the Argentine National Parks facilitated the field work and Dr. J. Mead assisted sampling in the field.

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