PALEOSOL EVIDENCE FOR UPLIFT AND FOR CLIMATE AND ECOSYSTEM CHANGES IN THE CORDILLERA TALAMANCA, COSTA RICA

Noncalcareous volcanic paleosol successions comprising 16 soil horizons from the Cordillera Talamanca, Costa Rica (Lat. 9^o41′03.3″N; Long. 83^o53′45.l9″W; 2,507 m elev.) provide evidence for changing Quaternary climate and elevation. Basal deposits (5 Cr horizon) are saprolitized, porphyritic andesite/basalt 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 kaolinite and Fe-oxides (or oxyhydroxides) on a mature paleolandscape, probably in tropical lowlands. Paleosaprolite deposits are sharply overlain by interbedded lapilli tuffs and a paleosol with a dark-colored, organic C-rich (2-3 wt %) horizon containing abundant grass pollen, interpreted as the mollic epipedon of grassland soil (paleo-Mollisol; 3Ab horizon) with δ¹³C values (-24.5 o/oo PDB) that record a former C3 grassland ecosystem. A pedogenic carbonate (3Btkb) horizon with all carbonate removed by overprinting by younger pedogenic processes, but retaining diagnostic micromorphologic features, provides evidence of former drier and drained conditions. Uppermost buried soils (1Btb and 2Btb horizons) are clay-rich, andic paleo-Ultisols that record pedogenesis under climates similar to current conditions. AMS dating of charcoal from the upper (1Btb) 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 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 is isotopically similar to δ¹³C values measured in a nearby transect of modern páramo (-24.4 to -26.3 o/oo PDB), followed by 3) continued late Pleistocene volcanism and pedogenesis in environments similar to current oak-dominated montane forest conditions. The paleosol successions provide a valuable record of dynamic climate-ecosystem change and uplift history in the Cordillera Talamanca that can complement and extend existing bog and lake sediment records.