A Mio-Pliocene palynoflora from the Eastern Cordillera, Bolivia: implications for the uplift history of the Central Andes

Graham, Alan, Gregory-Wodzicki, Kathryn M., and Wright, Kari L.

Abstract - An assemblage of 33 pollen grains and spores, recovered from the 3600 m high Pislepampa locality of E.W. Berry, Eastern Cordillera, Bolivia, adds considerably to our knowledge of the late Neogene paleoclimate and paleoelevation of the Central Andes. The plant microfossils recognized are Isoetes, Lycopodium (three types), Cnemidaria, Cyathea (three types), Grammitis, Hymenophyllum, Pteris, trilete fern spores (two types), Danaea, monolete fern spores (four types), Podocarpus, Gramineae, Palmae, Ilex, cf. Oreopanax, Cavanillesia, cf. Pereskia, Compositae (three types), Ericaceae, Tetrorchidium, and unknowns (three types). The diversity of the Compositae suggest that this flora has a maximum age around the Miocene-Pliocene transition, that is, 6-7 Ma. All members of the paleocommunity presently grow in the bosque montano húmedo (cloud forest) along the eastern slope of the Central Andes of Bolivia, which grows between mean annual temperatures of around 10 - 20 °C. The Pislepampa flora probably represents the lower limits of this forest, because the fossil leaves collected by Berry from the same locality all have entire margins, suggesting that the flora was tropical. Presently, the lower limit of this forest has mean annual temperatures of around 20 °C, a mean annual precipitation between 1000-1500 mm, and is found at elevations around 1200-1400 m. These conditions are thus inferred for the Pislepampa flora, however, because of the uncertainty in the magnitude of global climate change and in possible changes in the ecological range of the plant genera, we estimate an error of at least ± 1000 m (2 sigma) for the paleoelevation estimate. When corrected for probable amounts of erosionally-driven isostatic rebound, the paleoelevation estimate suggests that a significant amount of uplift occurred since the deposition of the Pislepampa flora, namely between 2400-3400 ±1000m, or about 40-65% of the total uplift. This estimate is consistent with other paleoelevation estimates from the Central Andes.

to be submitted to American Journal of Botany