18,000 year of environmental change in the Eastern Cordillera of the Bolivian Andes

Mountainous regions are considered to be early warning sites for climatic change because narrow vertical species ranges mean even small temperature/precipitation variation can result in species movement. This is especially true in the tropical Andes where the complex topography of the Andean valleys allows biodiverse woodland to be separated from grassland and snow dominated peaks by just a few kilometers, with microclimates clearly playing an important role. To begin to predict the likely impacts of future climatic changes and to help protect Andean woodlands, an understanding of baseline ecological conditions and previous responses to longer-term climatic shifts is vital. The Cochabamba Basin and surrounding mountain peaks is situated within the Eastern Andean Cordillera on the margin between the Altiplano and Yungas cloud forest. We present here multi-proxy data from two high elevation (>3400 m) lake sediment records which reveal sub-500 year ecosystem response to climatic shifts since the last glacial period and the impact of pre-Hispanic human populations. The sediment cores recovered from Lakes Challacaba (17°33’ S, 65°34’ W, 3400 m) and Khomer Kocha (17°16’ S, 65°43’ W, 4153 m) span the last c. 4000 and c. 18,000 years respectively. The two sites are only 35 km apart but are positioned within very different climatic and vegetation zones; Challacaba is within a cold and seasonally dry valley, and Khomer Kotcha is located on the steep slopes above the Yungas cloud forests. Analysis of pollen, chironomid, charcoal, geochemical and physical proxies from within the sediment cores provided insight into the drivers of environmental change at a local and regional scale. The Challacaba and Khomer Kocha records are the first from the eastern flank of the Bolivian Andes to record the last 4000 years and help to fill a gap in our understanding of vegetation succession and subsequent climatic variability since the late glacial. Our results suggest that, prior to increased human impact c. 1000 years ago, moisture balance was likely to have been the most important driver of environmental change. Increased drought activity during a mid-Holocene dry period is evident at both sites (Challacaba dry before 4000 yr BP, Khomer Kocha lower lake level c. 8500-5500 yr BP). In addition, within the Challacaba record there is evidence of millennial scale variations probably linked to shifts in El Niño-Southern Oscillation (ENSO). In this paper we present a new record that improves our understanding of longer-term climate-human-environment interactions in the Andes. Our findings offer insight into the spatial extent, timing and intensity of environmental impacts since the last glacial period; particularly with regard to ENSO, the mid-Holocene dry event and pre-Hispanic human impact. The adjustments in moisture balance recorded, as well as impacting the local vegetation and lake level, would have also had major influence on other biotic and abiotic processes, and so the value of land based palaeolimnological records should not be overlooked or limited to just ecological reconstruction.