Modeling The Global Significance Of Equatorial Glaciers Of The Permocarboniferous

150 years ago, researchers inferred signs of glaciation in late Paleozoic strata in southern France, now recognized to have been near the equator on the eastern edge of the supercontinent Pangaea. Unaweep Canyon in Colorado, USA (then on the western side of equatorial Pangaea) may preserve a fossil glacial valley of late Paleozoic age. It may be possible to reconstruct these ancient glaciers and estimate their equilibrium line altitude (ELA), where accumulation and ablation balanced. Initial estimates are that the glacial termini reached 1200-1600 m elevation, which is significantly below the lowest glacial termini (~2100 m) reported for equatorial mountains at the Last Glacial Maximum (LGM). Here we model ELA for tropical mountains under pre-industrial, LGM, and a colder glacial climate with global mean temperature ~ 9 o C below the LGM. We use a downscaling framework in which Community Earth System Model version 2 (CESM2) simulations at ~2o resolution were used to force standalone simulations by the Community Land Model version 5.0 (CLM5) at ~0.01 o resolution, allowing direct simulation of glaciers over a 30 year period.

This framework underestimates ELA by 500-600 m in pre-industrial and LGM conditions. This bias originates from a combination of excessive precipitation coming from the large-scale model in many instances as well as simplifications in the downscaling framework. However, the LGM to pre-industrial change (~1000 m) in ELA is only overestimated by ~200 m, suggesting the effect of global climate change on the relative change of ELA is better simulated than individual ELA variation. The ELA values from our very cold climate simulations average ~2700 m, suggesting, upon bias correction, that ELA would average ~ 3200-3300 m under these conditions. This modeling will provide important context to future ELA estimates from the paleotropics in deep time but may be affected by uncertainties in equator-pole heat transport under higher total atmospheric pressure.

The work was funded by NSF Sedimentary Geology and Paleobiology (EAR-1849754). We would like to acknowledge high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation.