Geochronology and Stratigraphy of Late Pleistocene Lake Cycles on the Southern Bolivian Altiplano: Implications for Causes of Tropical Climate Change
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Geochronology and stratigraphy of late Pleistocene lake cycles on the southern Bolivian Altiplano: Implications for causes of tropical climate change Christa Placzek† Jay Quade P. Jonathan Patchett Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT Zone (ITCZ). Pole-to-equator sea-surface cooling in the tropics was as much as 5–8 °C temperature (SST) and atmospheric gradi- (e.g., Stute et al., 1995; Thompson et al., 1995; Large paleolakes (~33,000–60,000 km2) that ents may have infl uenced the position of the Colinvaux et al., 2000), but during global gla- once occupied the high-altitude Poopo, Coi- ITCZ, affecting moisture balance over the ciations, the magnitude of hydrologic changes pasa, and Uyuni Basins in southern Bolivia Altiplano and at other locations in the Ama- in the Amazon Basin, an ecosystem with large (18–22°S) provide evidence of major changes zon Basin. Links between the position of the potential impacts on global biogeochemical in low-latitude moisture. In these now-dry or ITCZ and the ca. 23 ka precessional solar cycles, remains disputed. A number of biologic oligosaline basins, extensive natural exposure cycle have been postulated. March insola- proxies suggest little or no change in the tropi- reveals evidence for two deep-lake and several tion over the Altiplano is a relatively good cal forest during the late Pleistocene (Haberle minor-lake cycles over the past 120 k.y. Fifty- fi t to our lake record, but no single season or and Maslin, 1999; Colinvaux et al., 2000; Kast- three new U-Th and 87 new 14C dates provide latitude of solar cycling has yet to emerge as ner and Goñi, 2003). By contrast, pollen records a chronologic framework for changes in lake the primary driver of climate over the entire (Behling, 2002), clay mineralogy of Amazonian level. Deposits from the “Ouki” deep-lake Amazon Basin. Temperature may infl uence sediment (Harris and Mix, 1999), and some cycle are extensively exposed in the Poopo Altiplano lake levels indirectly, as potentially models (e.g., Garreaud et al., 2003) suggest Basin, but no deep lakes are apparent in the dry glacial periods in the Amazon Basin are relative aridity over this same interval. record between 98 and 18.1 ka. The Ouki lake linked to dry conditions on the Altiplano. A long-range lake record from the southern cycle was ~80 m deep, and nineteen U-Th Intensifi cation of the trade winds associated Bolivian Altiplano can serve as a critical test of dates place this deep-lake cycle between 120 with La Niña–like conditions currently brings the relationship between tropical moisture and and 98 ka. Shallow lakes were present in the increased precipitation on the Altiplano, and global climate. The Bolivian Altiplano (14– terminal Uyuni Basin between 95 and 80 ka deep-lake development during the Tauca 22°S) is a large, high-elevation (~3800 m) pla- (Salinas lake cycle), at ca. 46 (Inca Huasi lake lake cycle coincided with apparently intense teau situated at the western end of the Amazon cycle), and between 24 and 20.5 ka (Sajsi lake and persistent La Niña–like conditions in the rainfall belt. The South American summer mon- cycle). The Tauca deep-lake cycle occurred central Pacifi c. This suggests that SST gradi- soon, which brings copious rains to the Amazon between 18.1 and 14.1 ka, resulting in the ents in the Pacifi c are also a major infl uence Basin, is responsible for more than 80% of the deepest (~140 m) and largest lake in the basin on deep-lake development on the Altiplano. annual rainfall on the Altiplano (Vuille, 1999). over the past 120 ka. Multiple 14C and U-Th Over long time scales, the position of the Alti- dates constrain the highest stand of Lake Keywords: lakes, U-Th, climate, Bolivia, in- plano at the distal end of the Amazon rainfall Tauca along a topographically conspicuous solation, ENSO. belt makes this location sensitive to relatively shoreline between 16.4 and 14.1 ka. A prob- modest changes in climate. Modern interan- able post-Tauca lake cycle (the Coipasa) pro- INTRODUCTION nual variability on the northern Altiplano is also duced a ≤55-m-deep lake that is tentatively strongly tied to ENSO, which modulates the dated between 13 and 11 ka. The tropics are implicated in forcing global strength of the trade winds, generally causing El We suggest that paleolakes on the Bolivian climate shifts at interannual to orbital time scales Niño years to be dry (Vuille, 1999). Altiplano expanded in response to increased (e.g., Cane, 1998), and the global impact of El Most existing paleoclimate studies (e.g., moisture in the Amazon and enhanced trans- Niño–Southern Oscillation (ENSO) variability Baker et al., 2001a, 2001b; Seltzer et al., 2000; port of that moisture onto the Altiplano by is one example of how the tropical moisture can Rowe et al., 2002; Fritz et al., 2004) point to strengthened trade winds or southward dis- propagate rapid changes worldwide. The scar- local insolation changes as the cause of rainfall placement of the Intertropical Convergence city of well-dated paleoclimate records from variability over the Altiplano. Altiplano lake- low latitudes, however, leaves open many ques- level records, based largely on proxy evidence tions about the interplay between tropical and from cores, provide crucial support for the idea †E-mail: [email protected]. global climate. Many now recognize that glacial that the precessional solar cycle is the principal GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 515–532; doi: 10.1130/B25775.1; 13 fi gures; 3 tables; Data Repository item 2006100. For permission to copy, contact [email protected] 515 © 2006 Geological Society of America PLACZEK et al. increases the number of exposures studied, and A 70°W 68°W 66°W we replicate many dates using both 14C and U- Th methods. Most critically, a fi rm U-Th chro- Atlantic nology for these basins is required in order to Moisture Trades test for possible hard-water effects on 14C dates 15°S E N (Sylvestre et al., 1999) and to date older lakes at Amazon or beyond the limit of 14C (Rondeau, 1990). For Basin Lake Titicaca H many stages of the basins’ various lake cycles, ITC we are able to closely constrain absolute lake El Niño Z level by dating nearshore or beach deposits. Winds Bolivian High La Paz STUDY AREA Four very large lake basins (Titicaca, Poopo, 17°S Coipasa, and Uyuni) occupy a vast intermontane depression (the Altiplano) spanning 14–22°S Río Desaguad B (Fig. 1). In the north, Lake Titicaca (3808 m; 8560 km2) is a deep (>285 m) (Argollo and ero Mourguiart, 2000), freshwater lake that loses ≤10% of its annual water budget to overfl ow Sajama Río into the Rio Desaguadero (Roche et al., 1992). La The Rio Desaguadero empties into the oligosa- uca line Lake Poopo (3685 m; 2530 km2), which is separated by the Laka topographic sill (3700 m) 19°S Salar de Lake Poopo from the salars of Coipasa (3656 m) and Uyuni (3653 m). These salt pans occupy 12,100 km2 Coipasa (Argollo and Mourguiart, 2000) and in wet years are seasonally fi lled with shallow water (<4 m) Salar de originating primarily from the Rio Lauca and Salar de Uyuni Rio Grande. Paleolakes deep enough to cover Empexa the Laka sill at 3700 m integrated the Poopo, Coipasa, and Uyuni Basins, spanned more than Río Río 3.5° latitude, and occupied an area of 33,000– 60,000 km2 (Blodgett et al., 1997). The Empexa, 21°S Salar de Laguani Gran Laguani, and Ascotan subbasins extend beyond the Uyuni Basin and were integrated into the de N deepest lakes at various stages (Fig. 2). Salar de The Bolivian Altiplano lies on the western Ascotan 100 km end of the Amazon rainfall belt, and precipita- tion occurs primarily during the austral summer. Moisture traverses the Amazon Basin in the summer months when the Intertropical Conver- Figure 1. (A) Map of the Titicaca, Poopo, Coipasa, and Uyuni hydrographic basins. Mod- gence Zone (ITCZ) is displaced southward and ern lakes are in gray and salars are dotted. The dashed line represents the approximate convection is intense in the Amazon (Lenters maximum extent of paleo–Lake Tauca at ~3780 m. (B) The major climate features of South and Cook, 1997). Deep convection over the America. The Titicaca, Poopo, Coipasa, and Uyuni drainage basins are striped. ITCZ— Altiplano releases large amounts of energy into Intertropical Convergence Zone. the middle and upper troposphere, producing a monsoonal moisture regime (Zhou and Lau, 1998). A moisture source (Fig. 1B) to the north control on Altiplano moisture balance. Evidence 14 ka, in-phase with a summer insolation maxi- and east of the Altiplano produces a pronounced for two deep lakes, generally referred to as the mum (Baker et al., 2001a). Radiocarbon dates north-south and east-west precipitation gradient. Tauca and Minchin phases, has been studied for the Minchin lake phase range from 28 14C yr High spatial variability within any rain event or since before the turn of the century (Minchin, B.P. to the limit of 14C dating (Servant and Fon- rainy season is caused by the sporadic nature 1882). Several researchers date the Tauca paleo- tes, 1978; Rondeau, 1990; Baker et al., 2001a). of the convective afternoon and evening storms lake phase between 19 and 12 ka (Servant and Here we present a U-Th and 14C chronology that bring rain to the Altiplano (Aceituno and Fontes, 1978; Rondeau, 1990; Bills et al., 1994; spanning 120 k.y.