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Late-quaternary paleoclimates of the southern tropical Andes and adjacent regions
Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 1^3 www.elsevier.com/locate/palaeo
Foreword
Late-quaternary paleoclimates of the southern tropical Andes and adjacent regions Paleoclimate archives from the Andes of South America contain records of tropical climate variability ranging from glacial^interglacial variations in mean climate to interannual changes such as El Nin‹o Southern Oscillation (ENSO) events. This rich mosaic of information can be used to look at long-term trends in climatic change in the tropics as well as higher-resolution events that have had impacts on prehistoric and modern cultures. This special issue is a compendium of papers that are an outgrowth of the workshop, ‘Paleoclimates of the Central Andes’, held 11^16 January 2000 at the University of Arizona, convened by J. Betancourt, J. Quade, and G. Seltzer and funded by the US NSF Earth System History program and the InterAmerican Institute for Global Change Research. The majority of records presented in this issue come from the Altiplano and adjacent regions of southern Peru, Bolivia, and Chile (Fig. 1). The Altiplano is a high, closed basin located between the eastern and western cordillera of the Andes between V14 and 22‡S latitude. Moisture over the Altiplano is highly dependent on advection of water vapor in the active boundary layer from the eastern lowlands (Garreaud et al., this issue). On a seasonal basis the upper tropospheric easterlies expand to the south during the austral summer, enhancing this transport of water vapor and hence precipitation over the Altiplano and surrounding cordillera. On longer time scales changes in insolation caused by precession of Earth’s orbit may also have had a large impact on moisture availability on the Atliplano by either enhancing or decreasing the easterly tropospheric £ow. On interannual time scales, such as during
the warm phase of ENSO, conditions tend to be drier because of stronger westerly tropospheric £ow during the austral summer, which acts to block moisture transport onto the Altiplano. Thus the Altiplano and the surrounding region have experienced signi¢cant changes in precipitation on all time scales, including signi¢cant glacial^interglacial temperature variation (e.g. Paduano et al., this issue). During the last glacial maximum ( s 21 cal kyr BP) conditions were wet on the Altiplano and in some of the surrounding regions (Grove et al., this issue; Godfrey et al., this issue; Tapia et al., this issue). Lake Titicaca over£owed, and a large lake occupied the southern Altiplano, where the largest salt pan in the world, the Salar de Uyuni, exists today. Evidence from the Salar de Atacama also supports the interpretation of wet conditions in northern Chile at this time, but in neighboring Argentina the wettest time interval apparently existed s 38 kyr BP. That conditions on the Altiplano were wet at this time is signi¢cant, because it implies that easterly tropospheric transport of water vapor was active at this time and that the lowlands to the east were also wet enough to maintain high e¡ective moisture (precipitation minus evaporation) and atmospheric water vapor in southern tropical South America. The Holocene record of e¡ective moisture presents some interesting contrasts across the region. In the Altiplano and surrounding cordillera it is clear that conditions were generally drier from ca. 8 to 3.5 cal kyr BP. Glacial lakes were lower, some peaks were completely deglaciated (Abbott et al., this issue), the level of Lake Titicaca dropped V85 m and became signi¢cantly
0031-0182 / 03 / $ ^ see front matter G 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00268-2
PALAEO 3053 11-4-03
2
Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 1^3
Fig. 1.
more saline (Tapia et al., this issue), £uvial terraces around the lake formed as base level changed (Rigsby et al., this issue), and summer convective precipitation decreased (Servant and Servant-Vildray, this issue). In contrast, the Atacama desert of northern Chile may have become wetter at this time. Groundwater levels were apparently higher (Rech et al., this issue), and paleobotanical evidence accumulated in pack-rat middens indicates an expansion of vegetation into regions too dry to support these assemblages today (Lattorre et al., this issue). One possible explanation for this ap-
parent heterogeneity in response across the region may be that in the Atacama region relatively short wet periods occurred during a drier interval in the middle Holocene (Grosjean, this issue; Paduano et al., this issue). The resolution of this apparent paradox between the moisture records from the Altiplano and Atacama will be important as it bears on our ability to understand millennial-scale climate variability in the southern tropical Andes. Recent hydrologic variability in the Chilean Andes is explored in this issue in a study of lake sediments that have been deposited
PALAEO 3053 11-4-03
Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 1^3
in the last ca. 500 years (Valero-Garces et al., this issue). Several contributions are focused on regions north of the Altiplano: coastal Peru and the southern Ecuadorian Andes. The former has produced important records of ENSO variability from cultural deposits (Sandweiss, this issue) and £ood and debris £ow deposits (Keefer et al., this issue). The accumulation of molluscs and the isotopic analysis of ¢sh otoliths indicate reduced or no ENSO variability along the north coast of Peru prior to 5 cal kyr BP in the Holocene, with greater ENSO variability in the latter half of the Holocene. This interpretation is similar to studies from the circum-Paci¢c basin that indicate reduced ENSO variability in the early Holocene. Likewise, £ood and debris-£ow deposits provide a 38 kyr record of severe warm ENSO (El Nin‹o) events along the south coast of Peru. This record shows a gap in such severe ENSOs in the early to middle Holocene, but the recurrence of these events in the latter half of the Holocene indicates that both prehistoric and modern cultures could be subjected to events that have catastrophic impacts. However, little evidence exists that this change in ENSO frequency and magnitude corresponds to a change in mean climate in the high Ecuadorian Andes (Hansen et al., this issue). This study examines the relationship between modern vegetation and pollen rain and documents from lake sediments the late-glacial
3
to Holocene changes in vegetation and climate in the humid Andes of southern Ecuador. Finally, the occurrence of landslides in northwest Argentina in the late Pleistocene may also be linked to ENSO variability (Trauth et al., this issue). This special issue of manuscripts, focusing on paleoclimatic archives from the tropical Andes and surrounding region, contains much information on past climatic variability in the tropics. Several working hypotheses are presented to explain climatic changes on di¡erent time scales in the region. We hope that readers will ¢nd these papers useful for understanding climatic change in the Andes and the tropics in general, and we believe that this e¡ort represents only a ¢rst attempt to synthesize the broad range of information that is available.
Acknowledgements We thank the US NSF Earth System History Program for funding of activities associated with Pole^Equator^Pole 1 (the Americas transect) of the PAGES program. We dedicate this issue to the memory of Maria Bustamante de Ando, La Paz, Bolivia, who was a friend to all whom she met.
PALAEO 3053 11-4-03
Geo¡rey O. Seltzer, Donald T. Rodbell, and Herbert E. Wright
Foreword
Late-quaternary paleoclimates of the southern tropical Andes and adjacent regions Paleoclimate archives from the Andes of South America contain records of tropical climate variability ranging from glacial^interglacial variations in mean climate to interannual changes such as El Nin‹o Southern Oscillation (ENSO) events. This rich mosaic of information can be used to look at long-term trends in climatic change in the tropics as well as higher-resolution events that have had impacts on prehistoric and modern cultures. This special issue is a compendium of papers that are an outgrowth of the workshop, ‘Paleoclimates of the Central Andes’, held 11^16 January 2000 at the University of Arizona, convened by J. Betancourt, J. Quade, and G. Seltzer and funded by the US NSF Earth System History program and the InterAmerican Institute for Global Change Research. The majority of records presented in this issue come from the Altiplano and adjacent regions of southern Peru, Bolivia, and Chile (Fig. 1). The Altiplano is a high, closed basin located between the eastern and western cordillera of the Andes between V14 and 22‡S latitude. Moisture over the Altiplano is highly dependent on advection of water vapor in the active boundary layer from the eastern lowlands (Garreaud et al., this issue). On a seasonal basis the upper tropospheric easterlies expand to the south during the austral summer, enhancing this transport of water vapor and hence precipitation over the Altiplano and surrounding cordillera. On longer time scales changes in insolation caused by precession of Earth’s orbit may also have had a large impact on moisture availability on the Atliplano by either enhancing or decreasing the easterly tropospheric £ow. On interannual time scales, such as during
the warm phase of ENSO, conditions tend to be drier because of stronger westerly tropospheric £ow during the austral summer, which acts to block moisture transport onto the Altiplano. Thus the Altiplano and the surrounding region have experienced signi¢cant changes in precipitation on all time scales, including signi¢cant glacial^interglacial temperature variation (e.g. Paduano et al., this issue). During the last glacial maximum ( s 21 cal kyr BP) conditions were wet on the Altiplano and in some of the surrounding regions (Grove et al., this issue; Godfrey et al., this issue; Tapia et al., this issue). Lake Titicaca over£owed, and a large lake occupied the southern Altiplano, where the largest salt pan in the world, the Salar de Uyuni, exists today. Evidence from the Salar de Atacama also supports the interpretation of wet conditions in northern Chile at this time, but in neighboring Argentina the wettest time interval apparently existed s 38 kyr BP. That conditions on the Altiplano were wet at this time is signi¢cant, because it implies that easterly tropospheric transport of water vapor was active at this time and that the lowlands to the east were also wet enough to maintain high e¡ective moisture (precipitation minus evaporation) and atmospheric water vapor in southern tropical South America. The Holocene record of e¡ective moisture presents some interesting contrasts across the region. In the Altiplano and surrounding cordillera it is clear that conditions were generally drier from ca. 8 to 3.5 cal kyr BP. Glacial lakes were lower, some peaks were completely deglaciated (Abbott et al., this issue), the level of Lake Titicaca dropped V85 m and became signi¢cantly
0031-0182 / 03 / $ ^ see front matter G 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00268-2
PALAEO 3053 11-4-03
2
Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 1^3
Fig. 1.
more saline (Tapia et al., this issue), £uvial terraces around the lake formed as base level changed (Rigsby et al., this issue), and summer convective precipitation decreased (Servant and Servant-Vildray, this issue). In contrast, the Atacama desert of northern Chile may have become wetter at this time. Groundwater levels were apparently higher (Rech et al., this issue), and paleobotanical evidence accumulated in pack-rat middens indicates an expansion of vegetation into regions too dry to support these assemblages today (Lattorre et al., this issue). One possible explanation for this ap-
parent heterogeneity in response across the region may be that in the Atacama region relatively short wet periods occurred during a drier interval in the middle Holocene (Grosjean, this issue; Paduano et al., this issue). The resolution of this apparent paradox between the moisture records from the Altiplano and Atacama will be important as it bears on our ability to understand millennial-scale climate variability in the southern tropical Andes. Recent hydrologic variability in the Chilean Andes is explored in this issue in a study of lake sediments that have been deposited
PALAEO 3053 11-4-03
Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 1^3
in the last ca. 500 years (Valero-Garces et al., this issue). Several contributions are focused on regions north of the Altiplano: coastal Peru and the southern Ecuadorian Andes. The former has produced important records of ENSO variability from cultural deposits (Sandweiss, this issue) and £ood and debris £ow deposits (Keefer et al., this issue). The accumulation of molluscs and the isotopic analysis of ¢sh otoliths indicate reduced or no ENSO variability along the north coast of Peru prior to 5 cal kyr BP in the Holocene, with greater ENSO variability in the latter half of the Holocene. This interpretation is similar to studies from the circum-Paci¢c basin that indicate reduced ENSO variability in the early Holocene. Likewise, £ood and debris-£ow deposits provide a 38 kyr record of severe warm ENSO (El Nin‹o) events along the south coast of Peru. This record shows a gap in such severe ENSOs in the early to middle Holocene, but the recurrence of these events in the latter half of the Holocene indicates that both prehistoric and modern cultures could be subjected to events that have catastrophic impacts. However, little evidence exists that this change in ENSO frequency and magnitude corresponds to a change in mean climate in the high Ecuadorian Andes (Hansen et al., this issue). This study examines the relationship between modern vegetation and pollen rain and documents from lake sediments the late-glacial
3
to Holocene changes in vegetation and climate in the humid Andes of southern Ecuador. Finally, the occurrence of landslides in northwest Argentina in the late Pleistocene may also be linked to ENSO variability (Trauth et al., this issue). This special issue of manuscripts, focusing on paleoclimatic archives from the tropical Andes and surrounding region, contains much information on past climatic variability in the tropics. Several working hypotheses are presented to explain climatic changes on di¡erent time scales in the region. We hope that readers will ¢nd these papers useful for understanding climatic change in the Andes and the tropics in general, and we believe that this e¡ort represents only a ¢rst attempt to synthesize the broad range of information that is available.
Acknowledgements We thank the US NSF Earth System History Program for funding of activities associated with Pole^Equator^Pole 1 (the Americas transect) of the PAGES program. We dedicate this issue to the memory of Maria Bustamante de Ando, La Paz, Bolivia, who was a friend to all whom she met.
PALAEO 3053 11-4-03
Geo¡rey O. Seltzer, Donald T. Rodbell, and Herbert E. Wright