LATE MIOCENE PATTERNS OF PRECIPITATION, UPLIFT, AND EROSION IN THE ANDEAN FORELAND

The Andean orogen including its arid and hyperarid plateau and desert regions represents one of the most extreme terrestrial environments. Especially the late Miocene Andean evolution is of global importance as this time interval is characterized by dramatic global changes in sediment deposition, erosion, and potentially surface uplift rates. Due to its high elevation the modern Andes are also the most important topographic element controlling southern hemispheric atmospheric flow. The topography of the Andes induces two major climatic effects: a) S-ward deflection of Atlantic-derived moisture that results in monsoonal climate along the E and hyperaridity on the W side of the central Andes and b) north-to-south asymmetry in SST and position of the ITCZ in the equatorial Pacific. Terrestrial stable isotope records reflect the distribution and amount of precipitation and eventually changes in mean annual temperature and hence provide important archives for the reconstruction of long-term climate change. We present multi-isotope (δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr) data from late Miocene pedogenic carbonate in the Eastern Andean foreland (Bolivia) that record the onset of such seasonality and distribution of precipitation. δ¹⁸O and δ¹³C data from pedogenic carbonate suggest a rapid change in precipitation consistent with a deflection of the low-level Andean jet to more southerly latitudes at ca. 8.5 Ma. A contemporaneous increase in ⁸⁷Sr/⁸⁶Sr of pedogenic carbonate indicates a transition to higher silicate weathering rates and/or accelerating headward erosion and higher river discharge. Such a rapid (<100 ka) transition to more cyclic climate either due to enhanced seasonality or as a response to the onset of ENSO, strongly affects isotopes in precipitation and provides previously unavailable boundary conditions for Andean climate and topography reconstruc