1.Tropical Palaeoenvironments Research Group, Department of Geography, University of Wales Swansea, Swansea SA2 8PP, UK.
2.
School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.3.
Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, PA 15260, USA.4.
Department of Geography, University of Bern, 12 Hallerstrasse, 3012 Bern, Switzerland.5.
Department of Geology, Union College, Schenectady, NY 12308, USA.6.
Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA.
Glacial / interglacial variations in pCO2 and temperature were essentially global, whereas variations in tropical precipitation were regional in scale. By analysing a set of Andean lake cores located on the modern rainfall gradient from humid Venezuela (8°N, annual rainfall ~750-1500mm) to the subtropical desert region of northern Chile (23°S, <200mm rainfall), the relative impact of atmospheric pCO2 and aridity on carbon cycling in tropical lake catchment ecosystems can be assessed. d13CTOC and compound-specific isotope values show that during the last glacial, when it was colder and pCO2 was lower than today, the climate in the southern subtropical Andes was wet enough to support some C3 plants. However, the equatorial Andes experienced very dry conditions and C4 plants were abundant. The reverse situation occurred during the early and mid-Holocene, as the moist summer-monsoon inflow across Amazonia penetrated much less far south than today, due to decreased summer insolation in the southern tropics. We infer that climate was the primary influence on the changes in terrestrial vegetation rather than the direct physiological effects of atmospheric pCO2.