EROSIONAL PULSE AND GLACIER-LANDSCAPE FEEDBACKS DURING PLIOCENE GLACIATION ONSET IN THE WESTERN ALASKA RANGE

Pliocene-Pleistocene glaciation has modified the erosion and topography of most middle and high latitude mountain belts. Examination of how erosion rates change during repeated glaciation can help test hypothesized glacier erosion-landscape feedbacks across a range of settings. The topographic evolution of a catchment controls long-term glacier mass balance and erosion. To better understand how glaciations and landscapes coevolve on geologic timescales, here I quantify erosion rates with low-temperature thermochronometric data and inverse modeling from the glaciated western Alaska Range. Zircon (U-Th)/He and apatite fission track data suggest Cenozoic mountain building was underway by early Miocene time. In contrast, lower-temperature apatite (U-Th)/He age-elevation and grain age-kinetic data indicate that erosion accelerated coincident with regional Pliocene glaciation ca. 4 Ma. Furthermore, the data and inverse modeling require an initial Pliocene pulse of rapid 0.8 mm/yr erosion rates followed by more moderate 0.3 mm/yr erosion rates. The initial erosion pulse suggests a significant transient landscape adjustment to the introduction of efficient glacial erosion. The subsequent decrease in Pliocene erosion rates is consistent with a negative feedback between continuing glaciation and glacier size and erosivity. If glacial erosion outpaces rock uplift, glacier erosion decreases over time as topography, mass balance, valley gradients, and ice flux are reduced. In simple terms, as glacial erosion propagated toward the heads of valleys, pre-existing topography diminished and valleys flattened with the effect that subsequent glaciations became smaller and less erosive. These findings may provide a useful analog for other glaciated middle and high latitude alpine regions with moderate rock uplift rates. Although Plio-Pleistocene glaciation represents a change of state, it may have been punctuated by an initial pulse of rapid landscape change. Once this pulse propagated through the landscape, change was more gradual. As a result, much of the topographic form observed in these glaciated settings with moderate rock uplift may be a relict of the first several glaciations with only minor modification since that time.