Time scale bias in erosion rates of glaciated landscapes

Deciphering erosion rates over geologic time is fundamental for understanding the interplay between climate, tectonic, and erosional processes. Existing techniques integrate erosion over different time scales, and direct comparison of such rates is routinely done in earth science. On the basis of a global compilation, we show that erosion rate estimates in glaciated landscapes may be affected by a systematic averaging bias that produces higher estimated erosion rates toward the present, which do not reflect straightforward changes in erosion rates through time. This trend can result from a heavy-tailed distribution of erosional hiatuses (that is, time periods where no or relatively slow erosion occurs). We argue that such a distribution can result from the intermittency of erosional processes in glaciated landscapes that are tightly coupled to climate variability from decadal to millennial time scales. In contrast, we find no evidence for a time scale bias in spatially averaged erosion rates of landscapes dominated by river incision. We discuss the implications of our findings in the context of the proposed coupling between climate and tectonics, and interpreting erosion rate estimates with different averaging time scales through geologic time.

Incision rate of the Yellow River in Northeastern Tibet constrained by 10Be and 26Al cosmogenic isotope dating of fluvial terraces: implications for catchment evolution and plateau building

Unlike other large rivers flowing out of Tibet, the Yellow River escapes from the plateau towards the NE crossing no less than five NW–SE striking, actively growing ranges and intervening basins. Thick Plio-Quaternary deposits and fluvial terraces testify to a phase of aggradation and sediment infill up to the average surface elevation (3200–3250 m a.s.l.) of the Gonghe, Guide and Qinghai Lake basins. A set of seven main terraces across the Gonghe Basin suggests progressive down-cutting of the Yellow River carving the 500 m deep Longyang gorge at the basin exit. 10Be and 26Al concentrations in quartz of surface and sub-surface samples of four terraces constrain the timing of incision by determining the burial age of the deposit and the exposure age of its surface. Modelling the depth dependence of the 10Be concentration and the 26Al/10Be ratio allows us to constrain the onset of the ongoing phase of incision to 120–250 ka. These ages suggest long-term incision rates between 2–6 mm a−1. Together with the present morphology of the Yellow River terraces across the Gonghe basin and the Longyang gorge, our results imply rapid river catchment evolution and interaction between river dynamics, tectonic and climate in northeastern Tibet.

Eroding Australia: rates and processes from Bega Valley to Arnhem Land

We report erosion rates determined from in situ produced cosmogenic 10Be across a spectrum of Australian climatic zones, from the soil-mantled SE Australian escarpment through semi-arid bedrock ranges of southern and central Australia, to soil-mantled ridges at a monsoonal tropical site near the Arnhem escarpment. Climate has a major effect on the balance between erosion and transport and also on erosion rate: the highest rates, averaging 35 m Ma−1, were from soil-mantled, transport-limited spurs in the humid temperate region around the base of the SE escarpment; the lowest, averaging about 1.5 m Ma−1, were from the steep, weathering-limited, rocky slopes of Kings Canyon and Mt Sonder in semi-arid central Australia. Between these extremes, other factors come into play including rock-type, slope, and recruitment of vegetation. We measured intermediate average erosion rates from rocky slopes in the semi-arid Flinders and MacDonnell ranges, and from soil-mantled sites at both semi-arid Tyler Pass in central Australia and the tropical monsoonal site. At soil-mantled sites in both the SE and tropical north, soil production generally declines exponentially with increasing soil thickness, although at the tropical site this relationship does not persist under thin soil thicknesses and the relationship here is ‘humped’. Results from Tyler Pass show uniform soil thicknesses and soil production rates of about 6.5 m Ma−1, supporting a longstanding hypothesis that equilibrium, soil-mantled hillslopes erode in concert with stream incision and form convex-up spurs of constant curvature. Moreover, weathering-limited slopes and spurs also occur in the same region: the average erosion rate for rocky sandstone spurs at Glen Helen is 7 m Ma−1, similar to the Tyler Pass soil-mantled slopes, whereas the average rate for high, quartzite spurs at Mount Sonder is 1.8 m Ma−1. The extremely low rates measured across bedrock-dominated landscapes suggest that the ridge–valley topography observed today is likely to have been shaped as long ago as the Late Miocene. These rates and processes quantified across different, undisturbed landscapes provide critical data for landscape evolution models.

Surface dating of dynamic landforms: young boulders on aging moraines

The dating of landforms is crucial to understanding the evolution, history, and stability of landscapes. Cosmogenic isotope analysis has recently been used to determine quantitative exposure ages for previously undatable landform surfaces. A pioneering application of this technique to date moraines illustrated its considerable potential but suggested a chronology partially inconsistent with existing geological data. Consideration of the dynamic nature of landforms and of the ever-present processes of erosion, deposition, and weathering leads to a resolution of this inconsistency and, more generally, offers guidance for realistic interpretation of exposure ages.