Consistent slow exhumation in a late Cenozoic glaciated landscape: The Presidential and Carter ranges of the White Mountains in New Hampshire, USA

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To investigate the ability of the onset of Quaternary glaciations to drive an acceleration in erosion rates we compare short timescale (10 –10 yr) erosion rates using the in situ terrestrial cosmogenic nuclide (TCN) Be to longer time scale exhumation rates (10 –10 yr) using the apatite (U[sbnd]Th)/He method in the Presidential and Carter ranges of the White Mountains in New Hampshire, USA. Our results are generally consistent with but refine previous studies using similar techniques. Resulting (U[sbnd]Th)/He ages and thermal modeling from Mt. Washington in the Presidential Range (n = 4, 91.7 to 117.3 Ma) suggest that exhumation rates from the Late Cretaceous to the present were slow, <0.01–0.02 mm yr . Be-10 analyses of high elevation bedrock and talus from the Presidential Range (n = 4; 1.5–1.8 km above mean sea level [amsl]; 18–130 ka) and Carter Range (n = 4; 1.0–1.4 km amsl; 14–22 ka) yield exposure ages that dominantly predate the retreat of the last glacial maximum ice sheet (13.9 ka) and indicate inheritance or inefficient glacial erosion of peaks. A non-linear positive relationship between exposure age and elevation suggests a component of elevation dependent polythermal basal ice conditions. A comparison of Be concentrations measured in modern fluvial sand and flood deposits in the region (n = 14; 5–19 × 10 atoms g SiO ) to till (n = 5; ~2–7 × 10 atoms g SiO ), colluvium (n = 2; 2–7 × 10 atoms g SiO ), bedrock, and talus suggests that the modern day sediment budget continues to be dominantly sourced from remobilized glacial sediment rather than postglacial weathering. As such, late Cenozoic glaciation and postglacial processes modified existing topography at lower elevations but did not significantly accelerate exhumation rates in the higher elevation regions of White Mountains of New Hampshire. These results refute the idea of a global acceleration in erosion rates resulting from late Cenozoic climatic cooling and glaciation as a result of frozen-based minimally erosive ice cover in conjunction with low rates of tectonic uplift. 3 4 10 6 7 −1 10 4 −1 4 −1 4 −1 2 2 2



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