Comment on 'A Middle Wisconsin interstadial in the northern Andes' by Valenti Rull

摘要

Collectively, the stratigraphic evidence available suggests that the Peat Ⅶ layer of the PED5 section may represent a maximum of up to 5000 years of accumulation and date from the earliest part of the Middle Wisconsinan interstadial or the later part of the Early Wisconsinan/Merida stadial. The pollen sequence presented by Rull (2005), described in one instance as corresponding to the Middle Wisconsinan and the 'Pedregal Interstadial,' in reality corresponds to a much compressed span of Wisconsinan time and needs to be interpreted within the entire PED5 interstadial complex. The pollen sequence undoubtedly represents a cycle of warming and cooling and associated altitudinal shifts of the Paramo/Superparamo boundary in close proximity to the PED5 site. Assuming that lake level fluctuations correspond to climatically driven water balance fluctuations, most of the peats/paleosols in the PED5 sequence represent either relative decreases in precipitation or increases in evapotranspiration/temperature. According to the lithostratigraphy, Peat Ⅶ and the associated paleosol just predate the initial formation of a lake in the Pedregal basin (following deposition of an early Merida till) and/or represent the first of several drying cycles affecting the lake (Dirszowsky et al., 2005). The occurrence of Isoetes spores in the peat sequence related to 'local flooding' (Rull, 2005) appears to support the first interpretation, though the details of water level fluctuation at the site are difficult to reconstruct from the available data.