Perennial ice cap glaciation and progressive valley incision beginning by ~6 Ma in Patagonia

The extent of mountain glaciation is driven both by changing climate and by evolving mountain topography. Where the effect of topographic change is comparable to that of climate change, mountain glaciations may not have grown progressively larger during late Cenozoic global cooling as is commonly assumed. Here we estimate glacial extent and bedrock topography at ~6 Ma in the Patagonian Andes, the earliest known mid-latitude glaciation. We use a numerical model of steady-state glaciation to identify climate-topography combinations that produce glaciers consistent with provenance constraints from glacial sediments. Model results demonstrate that the ~6 Ma glaciation in Patagonia was a large-scale ice cap (≥ 77% Last Glacial Maximum ice volume), supported in the warm prevailing climate by relatively smooth bedrock topography on the east side of the range. Numerous younger glacial deposits similar to the ~6 Ma till indicate that large-scale ice caps are a recurrent feature of the region. Since the early ice caps would have had ice discharges similar to the Pleistocene ice caps, we infer that the deeply incised alpine landscape of this region was cut continuously over the entire interval from ~6 Ma to present. This valley incision suppressed glacier growth and offset contemporaneous global cooling, partially decoupling glacier extent from climate change and resulting in the perennial presence of ice caps since ~6 Ma.