Baker et al. (2001) suggested that variations in solar insolation as a consequence of precession might play an essential role in the intensity and displacements of the monsoon systems. In order to verify this hypothesis, it is crucial to accurately define tropical paleoenvironments in the context of glacial-interglacial cycles. Whether or not the South American climate of the Last Glacial Maximum (LGM) was dry or wet has been a topic of debate for many years. The aim of our paper (Mourguiart and Ledru, 2003) was to provide new evidence for a dry LGM in Bolivian highlands. Our interpretation is supported by the analysis of the specific diversity represented by the number of identified taxa in each sample and the Shannon-Wiener index (Fig. 1). Samples associated with the LGM interval exhibit low indices compared to other biozones. We believe that colder conditions alone could not explain such a spectacular response, and that it is thus necessary to invoke drier conditions as well (see Arroyo et al., 1988). In their comment, Baker et al. disagree with this conclusion, suggesting we misinterpreted our palynological records. Their statements are based on several points that merit consideration.
1. Contrary to what Baker et al. state about vegetation changes at the Siberia location during the LGM, we consider absence of taxa such as Botryococcus or Isoetes indicative of locally dry conditions. Furthermore, the absence in pollen spectra of taxa such as Polylepis, a tree that presently grows on Nevado Sajama up to 5100 m (Liberman Cruz et al., 1997), and a decline in plant diversity both suggest regionally drier conditions. Moreover, a shift in pollen diversity and abundance is also observed at that time in records at Lake Titicaca (Paduano et al., 2003).
2. Baker et al. correctly note that factors such as temperature, water clarity, or nutrient availability could explain changes in taxa distribution during the LGM. However, in Lake Huiñaimarca, high sedimentation rates during the LGM (Mourguiart and Ledru, 2003) are not indicative of ultra- to oligotrophic lakes (Pourchet et al., 1995) or lacustrine profundal zones (Pourchet et al., 1994).
3. The validity of the diatom study by Tapia et al. (2003) is questionable for not taking the published modern database into account. Moreover, the study suffers from a lack of rigorous statistical analysis. One way or another, the well-known mid-Holocene dry phase was unquestionably characterized by Cyclotella meneghiniana, a taxum found presently in lacustrine macrophyte and in relatively high percentages during the LGM. So, we do not agree with Baker et al.'s assumption that Lake Titicaca was overflowing throughout the LGM. Their own data seem to demonstrate the reverse!
4. Furthermore, Baker et al. refer to studies by Graf (e.g., 1992) in the Valles of Cochabamba. Strahl (1998), in reference to a new pollen diagram, concluded that the LGM environments in this part of Bolivia were much drier than the Graf interpretations, according to previous conclusions drawn by Purper and Pinto (1980) on ostracode ecology. In the Bolivian lowlands, at Laguna Bella Vista and Laguna Chaplin, the same picture was observed (Mayle et al., 2000).
In conclusion, there is growing evidence that the signal of drier-than-present (but also drier than before and afterward) conditions at the LGM is not wholly an artifact of temperature depletion, and, therefore, insolation (precessional cycles) cannot be invoked to explain this situation.
The manuscript was improved by the suggestions of Mike Burn, University College London.