The Relationship Between Climate and Stable Isotopes: Faking or Breaking Andean Paleoelevation Estimates?

Tuesday, 16 December 2014
Nadja Insel, Univ of Chicago, Chicago, IL, United States and David B Rowley, Univ Chicago, Chicago, IL, United States
The elevation history of the Andes is highly debated. Rapid, substantial pulses of diachronous surface uplift in the middle to late Miocene have been claimed in the northern and southern Bolivian Altiplano. The interpretation is based on clumped isotope (Δ47) and oxygen isotope (δ18O) data and usually either (a) disregards climate change, or (b) applies arbitrary “climate corrections” that contrasts paleoclimate observations and climate model results. Here, we present new paleoelevation estimates from previously published Δ47-temperatures and δ18O data and demonstrate how paleoclimate modeling results from GCMs can be applied to Δ47-temperature estimates and how it affects paleoelevation assessments in the Andes.

In the southern Bolivian Andes, we find: (1) Δ47-derived temperatures reflect peak summer air temperatures (PSAT). Modern PSAT – elevation relationships imply Andean paleoelevation estimates of <2000m at 16 Ma, ~3000m at 13 Ma, and ~4000m at 8Ma. (2) Regional GCMs indicate a non-adiabatic PSAT change of 2.1°C/1000m. Taking regional cooling into account, estimated paleoelevations reach ~2300m at 16 Ma and ~3300m at 13Ma. (3) Assuming the middle Miocene Climatic Optimum caused a global mean temperature increase of 3°C, the newly estimated maximum paleoelevation for 16Ma would be ~2850m. In the northern Bolivian Andes, no modern Δ47-temperature estimates exist to ensure a correct correlation between modern elevation and clumped isotope temperatures. In addition, δ18O data exist only for elevations <1700m and >4000m. To establish the relationship between δ18O and elevation we use observations together with simulated δ18O at 4000m, 3000m, and 2000m from our GCM. Based on mean carbonate δ18O compositions, late Miocene paleoelevations are estimated to be ~3100m at 11Ma and ~4100m at 7Ma.

We anticipate to highlight the importance of a thoughtful interpretation of climate and climate change when inferring paleoelevations from isotopes. Our results demonstrate that middle to late Miocene carbonate clumped isotopes and δ18O data from the northern and southern Bolivian Altiplano may be interpreted to show synchronous uniform surface uplift.