Defining rates of landscape evolution in a south Tibetan graben with in situ‐produced cosmogenic ¹⁰Be

Abstract

The Tangra Yum Co graben is one of the active structures that accommodate the east-west extension of the southern Tibetan Plateau and hosts one of the largest Tibetan lakes, which experienced lake-level changes of~200 m during the Holocene. In this study, cosmogenic 10 Be is employed to: (1) quantify catchment-wide denudation rates in fault-bounded mountain ranges adjacent to the Tangra Yum Co graben; (2) date palaeo-shorelines related to the Holocene lake-level decline; and (3) determine the age of glacial advances in this region. The fault-bounded, non-glaciated mountain range north of Tangra Yum Co -and presumably most other areas around the lake -erode at low rates of 10-70 mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high 10 Be concentrations. As a consequence, accurate exposure dating of sediment-covered terraces and beach ridges is difficult, because the pre-depositional 10 Be concentration may exceed the post-depositional 10 Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate 10 Be exposure age of 2.3 ± 1.4 ka (i.e. an error of 60%) for a terrace that is located 67 m above the lake. Nevertheless, the age is consistent with luminescence ages for a series of beach ridges and provides further evidence for the decline of the lake level in the late Holocene. At Tangra Yum Co exposure dating of beach ridges via 10 Be depth profiles is not feasible, because the pre-depositional 10 Be component in these landforms varies with depth, which violates a basic assumption of this approach.10 Be ages for boulders from two moraines are much older than the early Holocene lake-level highstand, indicating that melting of glaciers in the mountain ranges adjacent to Tangra Yum Co has not contributed significantly to the lake-level highstand in the early Holocene.

<sup>10</sup>Be systematics in the Tsangpo-Brahmaputra catchment: the cosmogenic nuclide legacy of the eastern Himalayan syntaxis

“…0.04 to 0.2 mm yr −1 ). These low denudation rates are of the same order of magnitude as previously published bedrock and detrital denudation rates in other areas of the plateau (Lal et al, 2004;Kong et al, 2007;Hetzel et al, 2011;Strobl et al, 2012;Li et al, 2014;Rades et al, 2015). The slightly higher denudation rates measured for the two northern tributaries, the Lhasa-He (Kyi) (pt.…”

Grain size bias in cosmogenic nuclide studies of stream sediment in steep terrain

“…Equation (1) assumes that radioactive decay of the cosmogenic nuclide is negligible [e.g., Lal, 1991]. This assumption introduces errors that are significantly smaller than the grain size bias because the averaging timescale of erosion (i.e., Λ/E) is small compared to the radioactive mean lives of 10 Be and 26 Al for the range of erosion rates considered here.…”

“…The bias shown in Figure 8 spans a relative scale, from high to low, because the magnitude and sense of grain size bias for a landscape will depend on site-specific factors, such as the spatial variation in sediment size and Figure 7. Area versus relief (note log axes) for catchments where 10 Be has been used to quantify catchment-average erosion rates (based on data compiled by Portenga and Bierman [2011]). Red box shows range spanned in inset (note linear axes; area axis starts at 0.6 km 2 ), which highlights the range of values considered in our analysis of the effects of area and relief on the magnitude of the grain size bias (gray box).…”

“…Researchers have applied this approach in diverse settings, from slowly eroding Australia and Africa [ Bierman and Caffee , ; Heimsath et al ., , ; Chadwick et al ., ; Bierman et al ., ] to the rapidly eroding Himalaya [ Vance et al ., ; Wobus et al ., ; Ouimet et al ., ; Scherler et al ., ; Olen et al ., ], enabling new insights into feedbacks between erosion, climate, and tectonics in long‐term landscape evolution [ Binnie et al ., ; Wittmann et al ., ; Cyr and Granger , ; Norton et al , ; Abbühl et al ., ; Moon et al ., ; DiBiase et al ., ; Olivetti et al ., ; Olen et al ., ; Marshall et al ., ; Rades et al ., ; Ferrier et al ., 2016]. Erosion rates from cosmogenic nuclides in stream sediment can also shed light on catchment‐wide weathering rates [ Riebe et al ., , , ; von Blanckenburg et al ., ; Norton and von Blanckenburg , ] and constrain rate constants in geomorphic transport laws, such as nonlinear diffusive transport of sediment across hillslopes [ Hurst et al ., ].…”

“…Observations of cosmogenic nuclides in sediment from Inyo Creek, California, suggest that the bias can be significant: Using only sand in a 10 Be estimate of catchment-average erosion rate leads to a factor of~3 bias due to spatial variations in the size distribution of sediment produced on slopes [Riebe et al, 2015]. In comparison, propagated uncertainties in erosion rates inferred from 10 Be are typically less than 9% and replicate analyses from the same grain sizes typically agree to within 30% [e.g., Bierman et al, 1999;Matmon et al, 2003;Clapp et al, 2002;Binnie et al, 2006;Ouimet et al, 2009;DiBiase et al, 2010]. The potential for grain size bias in cosmogenic nuclide studies raises a series of questions about measurements that have been based on a narrow range of sediment sizes: Under what circumstances (i.e., in which landscapes) will erosion rates be overestimated or underestimated?…”