Project: Paleoclimates of the Central Andes

Period: 1997-2000

Funding: National Swiss Foundation

Principal Investigators: Heinz Veit & Martin Grosjean (University of Bern, Department of Physical Geography).

Corresponding address:Bettina Jenny, Institute of Geography, Hallerstrasse 12, 3012 Bern, Switzerland, jenny@giub.unibe.ch

Preliminary Results:

Jenny, Bettina; Valero-Garcés, Blas; Villa, Rodrigo; Rondanelli, Mauricio; Geyh, Mebus; Kelts, Kerry; Veit, Heinz, 2000. Holocene humidity changes in mediterranean Chile. INQUA Meeting. Seville, March 2000.

ABSTRACT. How did humidity change due to the westerlies in the mid- to- late Holocene in central Chile? As archives, two shallow lakes have been selected in the coastal Cordillera of Mediterranean Chile (33°-37°S). This region is situated at the northern border of the strong westerly influence with intense winter rainfall and summer dryness. The archives in this region should have been very sensitive to past changes in the intensity or position of the westerlies. Further south (lake district), humid conditions persist throughout the year, while toward the North (Norte Chico) the climate becomes arid. The laguna Aculeo (33°50‘S, 70°55‘W, 360 m a.s.l., 6 m deep) covers an area of 12 km². A 5 m long core was recovered in 1998. The analyses include mineralogy, sedimentology, geochemistry and pollen studies. Today, the lake represents a freshwater system. The uppermost part of the core has very high biogenic silica and Pediastrum content and includes the diatom species Stephanodiscus indicating very eutrophic conditions due to tourism and agriculture in good agreement with modern limnological investigations. During about the last 3000 years, the productivity was high. In this period, also more than 20 fine-grained clastic layers, visible in the sedimentology, high density, magnetic susceptibility, low water content, and high clastic mineral content document heavy flood events. They most probably reflect stormy winters with high precipitation. Similar clastic layers are also found in nearby reservoir lakes (Adams et al. 1999). The lower part of the core indicates drier climatic conditions: The lake was a saline system with no flood-events (around 4000-5000 14C yr). The bottom of the core represents a hypersaline lake with gypsum precipitation, and hardly any organic matter. This part is not yet dated but other Radiocarbon data indicate a mid- or even early-Holocene age. This dry phase could therefore correspond to the mid-Holocene dry period in this region described by e.g. Villa et al. (1997).The other lake, Laguna de Tagua Tagua (34°30‘S, 71°10‘W, 200 m a.s.l.), has been drained in the 19th century. Geological studies (Varela 1976) and pollen studies (Heusser 1983) show a dry Mid-Holocene. Sedimentology on the new core confirms that the Mid-Holocene is represented by a shallow lake unit (banded sediments with intraclasts) indicating a dry phase. Detailed pollen and geochemical analyses are in progress. The record of Tagua Tagua reaches back to >45'000 C-14 yr (Heusser 1983). As these two lakes in mediterranean Chile are shallow, they are sensitive to lake level changes. Both show also periods of almost dry conditions (marsh or peat sequences). So changes in the moisture regime and therefore in the Westerlies were necessary as higher temperatures alone would not have been suficient.

The sediment record of Late Holocene winter floods in the Aculeo lake.

 Bettina Jenny, Julieta Massaferro, Blas Valero-Garcés, Mebus Geyh and Andrea Rizzo. 2000. Evidence of salinity changes due to marine influence during the last 3000 years in Laguna Matanzas (Central Chile) from sedimentological, geochemical and chironomid analyses. Poster to be presented by Julieta Massaferro at the Paleolimnology Congress, Kingston, Canada, August 2000.