PALEO AND PRESENT δ18O VARIABILITY OF CENTRAL ANDEAN PRECIPITATION: CONSTRAINTS FROM ISOTOPE-TRACKING CLIMATE MODELS

Recent elevation reconstructions of the Andean Plateau (AP) using stable isotope paleoaltimetry suggest rapid surface uplift of ~2.5 km during the late Miocene (10-6 Ma). Stable isotope paleoaltimetry assumes modern δ¹⁸O and temperature/altitude relationships and modern precipitation patterns are appropriate for interpreting the paleo-record preserved in soil carbonates. In this work, we use isotope-tracking climate models (REMOiso, Genesis3.0) to investigate both modern and past variations of precipitation and δ¹⁸O over the AP.

Results from simulations of the modern climate indicate: (1) a significant interannual variability in precipitation ¹⁸O exists with austral summer δ¹⁸O values over the AP ranging from -7 to -24‰. To first order, the more depleted precipitation δ¹⁸O can be associated with stronger convective rainout along the eastern flanks of the Andes. (2) The altitude effect of the Andes results in a precipitation δ¹⁸O – elevation relationship that changes with latitude. Modeled δ¹⁸O lapse rates along the eastern side of the central Andes vary between -1.46 ‰/km (~15°S lat), -1.96 ‰/km (~20°S lat) and -0.85 ‰/km (~25°S lat). These values are in good agreement with observations. (3) In the northern and central part of the AP the correlation between δ¹⁸O and elevation is not linear, but indicates a steeper slope at higher altitudes (>3000m) due to the advanced stage of condensation, while the lapse rate in the south is linear.

In our simulations, the modern isotopic signal over the AP strongly depends on precipitation amount and is less sensitive to local temperature conditions. However, precipitation and convection patterns change dramatically with the uplift of the Andes. Paleoclimate simulations of the rise of the Andes indicate that when the mountain range was less than one-half its modern elevation, precipitation over large parts of the AP was less than 40% of the modern due to a decrease in moisture transport and suppression of convection. These climate changes strongly affect the δ¹⁸O composition of AP rainwater and lead to a substantial (>50%) reduction in the δ¹⁸O lapse rate. Using this reduced δ¹⁸O lapse rate, we estimate that the late Miocene surface upliftcould have been less than 1 km.