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Late Quaternary Paleolakes, Rivers, and Wetlands on the Bolivian JOURNAL OF QUATERNARY SCIENCE (2005) 20(7–8) 671–691 Copyright ß 2005 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.986 Late Quaternary palaeolakes, rivers, and wetlands on the Bolivian Altiplano and their palaeoclimatic implications CATHERINE A. RIGSBY,1* J. PLATT BRADBURY,2 PAUL A. BAKER,3 STEPHANIE M. ROLLINS1 and MICHELLE R. WARREN1 1 Department of Geology, East Carolina University, Greenville, NC, USA 2 Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA 3 Division of Earth and Ocean Science, Duke University, Durham, NC, USA RIGSBY, C. A., BRADBURY, J. P., BAKER, P. A., ROLLINS, S. M. and WARREN, M. R. 2005. Late Quaternary palaeolakes, rivers, and wetlands on the Bolivian Altiplano and their palaeoclimatic implications. J. Quaternary Sci., Vol. 20 pp. 671–691. ISSN 0267–8179. Received 1 June 2005; Revised 21 September 2005; Accepted 27 September 2005 ABSTRACT: Drill cores of sediments from the Rio Desaguadero valley, Bolivia, provide new infor- mation about the climate of tropical South America over the past 50 000 years. The modern Rio Desaguadero is fed by Lake Titicaca overflow (and by local tributaries) in the wetter northern Altiplano and discharges into Lake Poopo in the more arid central Altiplano. During the late Quatern- ary the Rio Desaguadero valley was the site of several generations of palaeolakes and wetlands that formed during periods of increased precipitation and local runoff, augmented by increased overflow from Lake Titicaca. Sediments recovered by drilling in eight localities along the 390-km long valley of the Rio Desaguadero yield a regional history of lacustrine sedimentation and effective precipitation. Lacustrine strata in the drill cores record 12 distinct wet periods in the past 50 000 years. Four of these wet periods resulted in the formation of major palaeolakes in the Rio Desaguadero valley: during the last glacial maximum from before 20 000 to 16 000 cal. yr BP, during the late glacial from about 14 000 to 12 000 cal. yr BP, in the early Holocene from about 10 000 to 7900 cal. yr BP, and in the late Holocene from 4500 cal. yr BP to present. The period that appears to have been most arid was between 7900 and 4500 cal. yr BP. The Altiplano wet periods were generally synchronous with North Atlantic cold events (respectively, the last glacial maximum, the Younger Dryas, the 8200 cal. yr BP event, and the Neoglacial) implying a relationship between past precipitation variability in tropical South America and North Atlantic sea-surface temperature. Copyright ß 2005 John Wiley & Sons, Ltd. KEYWORDS: Bolivia; Altiplano; Quaternary; palaeoclimate; sedimentology; lacustrine; fluvial. Introduction for the past 50 000 years was poorly known. In the past few years there has been a spate of relevant publications, yet no clear consensus has emerged on several important problems: The tropics of South America play a fundamental role in Earth’s whether it was arid (e.g. Heine, 2000; Mourguiart and Ledru, climate, host the world’s largest river, and have higher total ter- 2003) or wet (e.g. Colinvaux and DeOliveria, 2001; Baker restrial primary productivity and biological diversity than any et al., 2001a, 2001b) during the Last Glacial Maximum in the other continental region of similar area. How this region’s cli- South American tropics; whether it was arid (e.g. Argollo and mate, freshwater runoff, and biological communities will Mourguiart, 2000; Cross et al., 2000) or wet (Betancourt et al., respond to future large-scale deforestation and anthropogenic 2000; Rech et al., 2002) during the early and middle Holocene; atmospheric change is a matter of research and concern (e.g. whether apparent differences between coeval climatic records Myers, 1991; Zeng et al., 1996), and helps motivate our study. in different locations are the result of misinterpretation of these One common approach to assessing future climate changes is records or real regional variation of climate (e.g. Vuille and to deduce the nature and causes of past atmospheric variability, Kemig, 2004); whether there was a significant climatic event especially during times of different climatic mean states. Until in South America synchronous with the Younger Dryas chron- recently, there were few such palaeoclimatic studies in tropical ozone (e.g. Baker et al., 2001a) or not, and, if yes, whether it South America and even the basic character of climate change was characterised predominantly by cold (e.g. Clapperton, 1997; Clayton and Clapperton, 1997), dry (e.g. Maslin and Burns, 2000), or wet conditions (e.g. Baker et al., 2001b) or something more complex (Rodbell and Seltzer, 2000); whether * Correspondence to: Catherine A. Rigsby, Department of Geology, East Carolina there were other millennial climatic events in tropical South University, Greenville, NC 27858, USA. E-mail: [email protected] America (e.g. Abbott et al., 1997) and, if so, whether these also 672 JOURNAL OF QUATERNARY SCIENCE correlate with North Atlantic variability (e.g. Baker et al., 2001a, 2001b); and whether Pacific Ocean sea-surface tem- perature (SST) variability dominates the forcing of precipitation variability in the tropical Andes (e.g. Garreaud et al., 2003; Bradley et al., 2003) or Atlantic SST variability also is a factor during some periods (e.g. Baker et al., 2001b; Hastenrath et al., 2004; Vezy and Cook, 2005). This paper presents sedimentological and stratigraphic ana- lyses of drill cores from the central Altiplano of Bolivia and dis- cusses their significance in the context of these important palaeoclimate questions. The discussion is limited to the 14C- dated portion of the cores, approximately the past 50 000 years. These cores, by virtue of their key locations between Lake Titicaca to the north and the Salar de Uyuni to the south, and because of the manner in which they record wet intervals (as lacustrine/wetland sedimentary sequences) and dry intervals (as fluvial sequences), provide new information that helps to resolve some of the palaeoclimatic controversies concerning tropical South America. Geomorphic and climatic setting The Bolivian (and Peruvian) Altiplano is situated between the fold and thrust belt of the eastern Cordillera and the active vol- canic arc of the western Cordillera. It covers 200 000 km2, extends from 14 to 22 S latitude and 66 to 71 W longi- tude, and has an average elevation of about 4000 m (Wirrman Figure 1 Map showing the location of the Altiplano and of the eight and de Oliveira Almeida, 1987). Regional geophysical data core sites used in this study. See Fig. 2 for site names and elevations (Bills et al., 1994), the flat-lying nature of Quaternary strata in Lake Titicaca (Seltzer et al., 1998), and the flat-lying nature of Desaguadero valley strata, all suggest that tectonics probably ing to their relative ages, from youngest to older: Coipasa, did not have a major influence on Holocene and late Pleisto- Tauca and Minchin (there are also many yet-older lacustrine cene sedimentation in this area (Baucom and Rigsby, 1999). periods). We have previously argued (Baker et al., 2001a; Fritz However, information from Lake Titicaca and Salar de Uyuni et al., 2004) for the following choices of the ages of each drill cores (Baker et al., 2001a, 2001b; Fritz et al., 2004) and palaeolake period. The youngest palaeolake, sometimes desig- from high lake-stands around Lake Titicaca at approximately nated palaeolake ‘Coipasa’ (Sylvestre et al., 1999), has been 40 000 cal. yr BP (Farabaugh and Rigsby, 2005) suggest that radiocarbon dated to between 13 400 and 11 500 cal. yr BP either tectonic or hydrologic thresholds may have changed (Servant et al., 1995; Baker et al., 2001a). The much larger basin configurations during the last 50 000 years. palaeolake ‘Tauca’, obtained a maximum depth of 140 m The modern Rio Desaguadero, which lies near the centre of above the modern salar surface (Bills et al., 1994) and was con- the Altiplano, is a transitional river system (Baucom and tinuously present from 26 000 to 15 000 cal. yr BP (Baker et al., Rigsby, 1999) that drains from Lake Titicaca, in the relatively 2001a; Fritz et al., 2004). Although palaeolake ‘Tauca’ per- wet northern Altiplano, to the drier central Altiplano (Fig. 1). sisted throughout the period, it is clear from diatom and Interannual changes in the level of Lake Titicaca have a dra- stable-isotopic compositions of drill core samples (Fritz et al., matic influence on the discharge of the Rio Desaguadero. 2004) that the palaeolake was relatively shallow during the The latter is also augmented by more local runoff, especially LGM and that the maximum depth of this lake, determined that of the Rio Mauri. At present, the Rio Desaguadero flows from palaeo-shoreline elevations (Bills et al., 1994), was 390 km south from Lake Titicaca (elevation 3810 m a.s.l.) to attained near the end of this period. The existence of an older Lake Poopo (a shallow, saline lake; 3685 m a.s.l.). During the palaeolake ‘Minchin’ was originally postulated on the basis of past, in times significantly wetter than present, Lake Poopo two radiocarbon dates on shells from outcropping sediments drained into the salars (salt flats) of Coipasa (3657 m a.s.l.) (Servant and Fontes, 1978). Palaeolake Minchin is now known and Uyuni (3653 m a.s.l.). to have been continuously present between about 46 000 and Geomorphic and sedimentary features, including lake ter- 36 000 cal. yr BP based on radiocarbon and uranium/thorium races, carbonate bioherms and lacustrine sediments around dates (Baker et al., 2001a; Fritz et al., 2004). Lake Poopo and the salars (e.g. Servant and Fontes, 1978; Bills Formation of these palaeolakes is attributed to increased et al., 1994; Servant et al., 1995; Sylvestre et al., 1999), show effective moisture, resulting from increased precipitation or that the Altiplano was the site of several generations of late decreased evaporation, or both.
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