Paleosol evidence for Quaternary uplift and for climate and ecosystem changes in the Cordillera de Talamanca, Costa Rica

摘要

A noncalcareous volcanic paleosol succession from the upper montane rain forest of the Cordillera de Talamanca, Costa Rica (Lat. 9°41′03.3″ N; Long. 83°53′45.9″ W; 2507 m elev.) provides evidence for changing Quaternary elevation, climate, and ecosystems. Basal deposits (Crb horizons) are saprolitized, porphyritic andesite enriched in Si, Al, and Fe, and depleted in base cations, which record an earliest period of very intense weathering forming a leached residuum of smectite–kaolinite and Fe-oxides (or oxyhydroxides) on a mature paleolandscape, probably in tropical lowlands. Paleo-saprolite deposits are sharply overlain by interbedded lapilli tuffs and two paleosols with dark-colored, organic C-rich (2–3 wt.%) Ab horizons (one containing abundant grass pollen), interpreted as mollic epipedons of grassland soils (andic Mollisol-like paleosols) with δ13C values (- 24.5‰ PDB) that record a former C3 grassland ecosystem. Possible pedogenic carbonate (Btkb) horizons (with all carbonate removed by overprinting by younger pedogenic processes) provide evidence of former drier and drained conditions. Uppermost buried soils (mainly Btb horizons) are non-calcareous, clay-rich, andic Ultisol-like paleosols that record pedogenesis under climates similar to current conditions, with highly weathered clay mineral suites. AMS dating of charcoal from the upper (Btb) paleosol constrains a latest glacial age (36,640 ± 650 14C yr BP; ages for two additional deeper samples were > 43–48 ka) for the 2Btb paleosol. Uplift rates of the Cordillera de Talamanca are estimated as 1 km/1 Ma. Consequently, the described paleosols provide evidence for: 1) initial lowland pedogenesis, perhaps at 2–3 Ma, 2) regional uplift and major unconformity, volcanism and development of a grassland ecosystem that has δ13C values similar to those measured in modern, high-elevation páramo sites in the cordillera (- 24.4 to - 26.3‰ PDB) but formed mollic epipedons and pedogenic carbonates, which do not characterize modern páramo soils, followed by 3) continued late Pleistocene volcanism and pedogenesis in environments similar to current oak-dominated montane rain forest conditions. The paleosol succession provides a valuable record of dynamic climate-ecosystem change and uplift history in the Cordillera de Talamanca that can complement and extend existing bog and lake sediment records.