Paleo-Glacier Equilibrium Line Altitudes around Nevado de Chañi, Arid Subtropical Andes

Glaciation along the high Andes exhibits remarkable geographic variation. The arid subtropical region is particularly unique because it hosts peaks exceeding 5000 m asl that are not presently glaciated. Several peaks intersect the annual 0˚C isotherm, but the region is too dry to support glaciers. Nevertheless, glacial valleys and moraines throughout the region document extensive glaciations in the past, which were driven by greater snowfall accumulation, associated reductions in net shortwave radiation, and reduced temperatures. However, the variability and magnitude of glacial expansion across the region is still largely unresolved. Here, we use a new model to calculate the steady-state equilibrium line altitude (ELA) of paleo-glaciers in the arid subtropical Andes. Our model is based on mass continuity equations and thus bypasses the subjective process of selecting or tuning empirical coefficients, which is an important advantage in regions without modern glaciers for reference. Another novel advantage is that it leverages Monte Carlo sampling to inform the ELA estimates with uncertainties in modeled paleo-glacier geometry. The result is a first-order distribution of plausible ELA values for each paleo-glacier. The model requires only a DEM of the valley topography and a shapefile of the valley outline as inputs, and from there automatically generates the central flowline, glacier thickness and width at discrete points along the flowline, and robust uncertainty bounds for the glacier geometry. We focus our study on the Nevado de Chañi massif in Cordillera Oriental, northwest Argentina. Glacial valleys are far more extensive on the east side of the massif than the west side, making Nevado de Chañi an ideal natural laboratory for testing how interactions between climate and topography locally control the extent of glaciation. Beryllium-10 ages from prior work around the massif indicate moraine formation during Marine Isotope Stage 3, the global Last Glacial Maximum, Heinrich Stadial 1, and the Younger Dryas. With chronology in hand, we apply the ELA model to glacial valleys across the massif to quantify the time-evolution of glaciation through the late Pleistocene. Finally, we use statistical relationships to quantify the influence of topographic parameters on ELA variation between valleys.