October 2005
A pollen history of vegetation and climate in the most arid sector of the Atacama, the driest desert in the world
Antonio Maldonado, Universidad de La Serena, Chile
Julio L. Betancourt, Desert Laboratory, USGS & Univ. of Arizona
Claudio Latorre, Universidad Catolica, Santiago, Chile
Carolina Villagran, Universidad de Chile, Santiago
Precipitation in northern Chile is controlled by two great wind belts-
the southern westerlies over the southern Atacama and points south
(>24°S)
and the tropical easterlies over the northern and central Atacama Desert
(16-24°S). At the intersection of these summer and winter rainfall regimes
(Fig. 1),
respectively, is a Mars-like landscape consisting of expansive
surfaces devoid of vegetation (i.e., absolute desert) except in canyons
that originate high enough to experience runoff once every few years.
Pollen assemblages from 39 fossil rodent middens in one of these canyons,
Quebrada del Chaco (25°30'S), were used to infer the history of vegetation
and precipitation at three elevations (2670-2800 m; 3100-3200 m; 3450-3500
m) over the past 50,000 years. A report of this work by Desert Lab scientists
and their Chilean colleagues recently appeared in Journal of Quaternary
Science.
The study area lies near the midpoint of the Arid Diagonal, precisely at the latitude (25.30°S) where absolute desert penetrates to its highest elevation in the southern Atacama Desert. This sector defines the hinge point for plant assemblages adapted to summer vs. winter rains, and arguably supports the lowest biodiversity, as well as lowest vegetation cover and heterogeneity, in the Americas. In this unique hyperarid environment, we compared and contrasted pollen assemblage changes across the three elevations to discriminate the seasonality and source of inferred increases in precipitation at discrete times of midden deposition.
As a baseline for our paleoecological interpretations, we surveyed plant community composition and cover Fig. 2, as well as modern pollen assemblages from surface soils Fig. 3, along an elevational transect from the Andes proper to Quebrada del Chaco. We found that the upper limit of vascular plants today lies at ~ 4500 m followed by two vegetation belts that we classify here as Subnival (4300-4500 m), dominated by minute and sparse cushion plants, and Andean steppe (4100-4300 m), characterized by the bunchgrass Stipa frigida. In these two zones, plant cover never exceeds 10%, is generally less than 5% and in some plots approximates 0% cover. Puna vegetation, consisting mostly of small shrubs, is generally restricted to the canyon bottom. Modern surface pollen assemblages closely mirror the modern vegetation patterns, with Chaethanthera Fig. 4 and grass pollen being very diagnostic of the Subnival and Andean steppe zones, respectively. High grass (Poaceae) pollen percentages denote the presence of more or less even cover of Stipa frigida across the extensive flatlands that border Quebrada del Chaco; low grass pollen percentages imply that these flatlands are completely devoid of vegetation.
When compared to modern conditions and fossil records to the north and south, the pollen evidence indicates more winter precipitation at >52, 40-33, 24-17 cal kyr BP (thousands of calendar years before 1950), more precipitation in both seasons at 17-14 cal kyr BP, and more summer precipitation from 14-11 cal kyr BP. Fig. 5. Winter and summer wet events, and by extension changes in the strengths of the westerlies and easterlies, were apparently antiphased. An exception may have been the transition from full to late glacial conditions, characterized by wetter conditions in both winters and summers.
The wettest times in the midden series at all three elevations, and presumably the greatest northward extension of the southern westerlies, was the full glacial period from 24-17 cal kyr B.P. During this time the Subnival taxon Chaethantera, which today is not registered in pollen assemblages below 3700 m and is well represented only above 4350 m, shows up in fossil middens as traces at 2670-2800 m and as high percentages between 3450-3500 m. Both midden series imply a 1000 m lowering of species distributions. The high percentages of grass pollen at the higher site suggest that a diffuse cover of Stipa frigida blanketed the now-plantless "pampas" at 3500 m; the lower grass percentages in Chaco I (except in one sample at 15 cal kyr BP) indicate that this diffuse grassland gave way to absolute desert somewhere between 3500 and 2670 m, again a displacement of the modern limits of continuous vegetation of at least 1000 m (see generalized maps of modern vs. full glacial vegetation in Figs. 6 and 7).
Maldonado, A., Betancourt, J. L., Latorre, C. and Villagran, C. 2005. Pollen analyses from a 50,000-yr rodent midden series in the southern Atacama Desert (25°30'S). Journal of Quaternary Science, Vol. 20, pp. 493-507.
For more information contact:
Julio Betancourt (jlbetanc@usgs.gov)