Paleoclimatology of the Central Andes Workshop
Jan. 11-16, 2001, Tucson, Arizona
Convened by Julio Betancourt, Jay Quade, and Geoffrey Seltzer
Sponsored by NSF-Earth System History and InterAmerican Institute
One of the frontiers in paleoclimatology is the role and fate of the tropics in global climate change at millennial to orbital time scales. In particular, little is known about the relative influences and interactions of insolation at high vs. low latitudes, changes in oceanic heat transport, and remote teleconnections with large-scale and interrelated features of the climate system, such as the Asian Monsoon and the Walker Circulation (ENSO). Did climate change in the tropics lead or lag ice volume changes at higher latitudes? Is tropical climate variability caused by changes in seasonal insolation at low latitudes, or do insolation changes at high latitudes affect the tropics through large-scale teleconnections such as El Niño-Southern Oscillation (ENSO)? What are the effects of seasonal insolation variations over land vs. sea in the tropics? The answer to these questions in part requires high quality records of temperature and precipitation from tropical landmasses. Fossil records from the wet tropical lowlands are of limited variety and quality, so most of the effort has focused on tropical highlands or along the edges of the tropical rainfall belts.
It is therefore not surprising that the Central Andes (10-30°S) have fast become one of the most active stages for paleoclimatological research. This research has involved ice core measurements from tropical ice sheets, geomorphological evidence for glacial advances and retreats, limnological evidence from large (50,000 km2) and small (<1 km2) lakes, fossil rodent middens and various other kinds of climate proxies. The recent flurry in research, however, has yet to produce consensus about the magnitude and timing of temperature and precipitation fluctuations, much less the forcing and large-scale mechanisms involved in climate change. With the acceleration of paleoclimatological research in the last decade have come heated controversies about the history of the South American Summer Monsoon. These controversies have become evident in special sessions on South America paleoclimatology at recent annual meetings of the Geological Society of America and American Geophysical Union. Hence, we convened an international workshop on Central Andes paleoclimatology to get updates on new research and to take stock of agreements and disagreements among the various proxies.
The workshop was part of the PAGES (Past Global Changes), PEPI (Pole-Equator-Pole) initiative for coordinating paleoclimatic and paleoenvironmental research on a transect through the Americas. The necessary funds for the workshop were obtained from two programs at the National Science Foundation, Earth System History and the Inter-American Institute. On board were 60 scientists from Canada, Chile, England, France, Germany, Peru, Switzerland and the United States.
The workshop focused on points of agreement and disagreement among the various proxies and interpretations. Many of the disagreements stem partly from the sheer size of the area in question, and the complexity of atmospheric circulation over two oceans, two hemispheres and two major physiographies, the Amazon/Gran Chaco lowlands and the Andean highlands. The discrepancies could be due to poor dating, different sensitivities and response times of the various types of records, or simply different interpretations of each fossil record. Discrepancies could also arise from inadequate understanding of the geographic complexity of climate in the Central Andes, where variability in the moisture source (the Atlantic and tropical lowlands) may be decoupled from variability in the circulation mechanisms that transport moisture across the highlands.
The workshop made one thing perfectly clear. There are few fossil records or interpretations in full agreement. Over the last 1 million years, cyclicity in the sedimentology of the Amazon fan suggests that maximum lowland precipitation coincided with austral summer insolation minima. New lake level records, however, indicate that the biggest lakes on the Bolivian Altiplano were in phase with insolation maxima. The discrepancy between the high and low ground is not necessarily a problem. Drier lowlands and cooler, wetter highlands are reproduced in at least one Atmospheric General Circulation Model simulation for the Last Glacial Maximum (~21 kyr B.P.). At critical times, such as the Younger Dryas (13-11.6 kyr B.P.), there is evidence for aridity in the lowlands, but there remains disagreement about whether it was wet or dry in the highlands.
There is no universally-accepted master chronology for regional temperature fluctuations, a casualty of the "tropical paleothermometry conumdrum" in oxygen isotope measurements from Andean ice cores. In the tropics, oxygen isotopes seem to track, not only the temperature of the air mass, but also the amount of precipitation it produces. The role of cold temperatures vs. increased precipitation in maintaining glaciers and large lakes in the Andes is undetermined, and further complicated by the staggering in time of deglaciation and lake desiccation. During the last glacial, a 6.5°C depression would have brought the O°C mean annual isotherm near the highstands (~3800 m) of the paleolakes, thus eliminating terrestrial vegetation (and evapotranspiration) from their watersheds. The ages of the highstands themselves are debated, full glacial (22-16 kyr B.P.) from lake sediment cores and late glacial (16-14 kyr B.P.) from shoreline tufas in the Uyuni-Coipasa Basin.
Disagreements extend into the Holocene and onto the Pacific slope of the Central Andes. Saline lake deposits from Salar de Atacama indicate highest lake levels during the full glacial. Small lake records, paleowetlands, and vegetation invasions into Absolute Desert indicate maximum summer wetness in the late glacial-early Holocene (16-10 kyr B.P.. For the middle Holocene (7-3 kyr B.P.), when Lake Titicaca dropped 100m below its current level, reconstructions differ among records from the Pacific slope of the Andes. Even on the Bolivian Altiplano, there are notable discrepancies in the timing of mid- and late Holocene lake level fluctuations. Archeologists interested in climatic impacts on Andean prehistory are on standby, waiting for the smoke to clear. To follow the latest developments on central Andes paleoclimatology, consult some of the latest references provided below.
Abbott, M. B. et al., 2000: Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordilera Real, Bolivia. Quaternary Science Review 19, 1801-1820.
Baker, P. A. et al., 2001: Tropical climate changes at millennial and orbital timescales on the Bolivian Altiplano. Nature 409, 698-701.
Baker, P.A. et al. 2001: The history of South American tropical climate for the past 25,000 years from the sedimentary record of Lake Titicaca (Bolivia/Peru). Science 291, 640-643.
Betancourt, J. L. et al.: 2000. A 22,000-yr record of monsoonal precipitation from northern Chile's Atacama Desert. Science 289, 1542-1546.
Clement, A.C., et al., 2000: Suppression of El Niño during the mid-Holocene by changes in the earth's orbit. Paleoceanography 15, 731-737.
Cross, S. L., 2000: A new estimate of the Holocene lowstand level of Lake Titicaca, central Andes, and implications for tropical palaeohydrology. The Holocene 10, 21-32.
Garreaud, R. D., 2000: Intraseasonal variability of moisture and rainfall over the South American Altiplano. Monthly Weather Review 128, 3337-3346.
Grosjean, M. et al., 2000: A 22,000 14C year BP sediment and pollen record of climate change from Laguna Miscanti (23°S), northern Chile. Global and Planetary Change 553, 1-17.
Harris, S. E. and Mix, A. C., 1999: Pleistocene precipitation balance in the Amazon Basin recorded in deep sea sediments. Quaternary Research. 51, 14-26.
Hostettler, S. W., and Mix, A. C., 1999: Reassessment of ice-age cooling of the tropical ocean and atmosphere. Nature 17, 673-678.
Hostettler, S.W., and P. U. Clark, 2000: Tropical climate at the Last Glacial Maximum inferred from glacier mass balance modeling. Science 290, 1747-1750.
Klein, A. G. et al., 1999: Modern and last glacial maximum snowlines in the Central Andes of Peru, Bolivia, and northern Chile, Quaternary Science Reviews 18, 65-86.
Liu, Z., et al., 2000: Modeling climate shift of El Niño variability in the Holocene. Geophysical Research Letters 27, 2265-2268.
Maslin, M. A., and Burns, S. J., 2000: Reconstruction of the Amazon Basin effective moisture availability over the past 14,000 years. Science 290, 2285-2287.
Rodbell, D.T. et al., 1999. An ~15,000-year record of El Niño-driven alluviation in southwestern Equador. Science 283, 516-520.
Rodbell, D.T. and Seltzer, G., 2000, Rapid Ice Margin Fluctuations during the Younger Dryas in the Tropical Andes. Quaternary Research 54, 328-338.
Seltzer, G., et al., 2000: Isotopic evidence for late Quaternary climatic change in tropical South America. Geology 28, 35-38.
Sylvestre, F. et al., 1999: Lake-level chronology on the southern Bolivian Altiplano (18°S-23°S) during late-glacial time and the early Holocene. Quaternary Research 51, 54-66.
Thompson, L. G. et al., 2000: Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum. Journal of Quaternary Science 15, 377-394.
Vuillle, M. et al., 2000: Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing. Journal of Geophysical Research 105, 12447-12460.