INCREASED RUNOFF AND GROUNDWATER DISCHARGE AS EVIDENCED BY LATE PLEISTOCENE PHREATOPHYTE ASSEMBLAGES IN THE PAMPA DEL TAMARUGAL BASIN, ATACAMA DESERT, NORTHERN CHILE (21° S)

The southern portion of Pampa de Tamarugal in the hyperarid Atacama Desert (21° S lat.) is presently fed by dry, inactive washes and alluvial fans that head at the Andean Precordillera of Sierra Moreno. Barring the occasional debris flow or sheet wash, water and plant life are mostly absent from these Quebradas at present. Abundant fossil wood, however, has been collected and exploited since the 19th century from this area. How and when did this wood and other plant remains occur in this region? Did they grow in situ owing to past increases in precipitation or were these organic remains transported from elsewhere?

Here, we report on our preliminary results of macro and microfossil (pollen) content analysis from leaf litter and sediments collected along several inactive Quebradas (Lomas de la Sal, Sipuca, Tambillo and Maní). Leaves of Escallonia angustifolia (Escalloniaceae) have been dated in a debris flow at Lomas de la Sal at 12,244 ± 96 14C yr BP. Additional remains pertaining to E. angustifolia leaves and other taxa present at Q. Tambillo have been dated between 13,310 ± 180 and 12,940 ± 150 14C yr BP. At Tambillo and other localities, the organic remains identified include well preserved leaves, flowers (Fabaceae), insect exos and micromammal feces, all of which suggest in situ vegetational growth and leaf litter accumulation. The litter analysis reveals a preponderance of Escallonia angustifolia, Asteraceae (chiefly Baccharis sp and/or Tessaria sp), Cortaderia atacamensis (Poaceae) and Prosopis sp (Fabaceae). Pollen from indurated sediments shows dominance of Asteraceae, Poaceae and Prosopis- type.

The composition of these assemblages is similar to phreatophytic and riparian plant communities found in active river valleys further north (i.e. Camiña valley – 19°S). We suggest that the riparian-phreatophytic vegetation remains found in these three Quebradas represents an increase in runoff and/or elevated groundwater tables during the latest Pleistocene, a time of increased precipitation according to numerous records of past climate change described for northern Chile.