Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Large boulders, ca. 10 m in diameter or more, commonly linger in Himalayan river channels. In many cases, their lithology is consistent with source areas located more than 10 km upstream, suggesting long transport distances. The mechanisms and timing of “exotic” boulder emplacement are poorly constrained, but their presence hints at processes that are relevant for landscape evolution and geohazard assessments in mountainous regions. We surveyed river reaches of the Trishuli and Sunkoshi, two trans-Himalayan rivers in central Nepal, to improve our understanding of the processes responsible for exotic boulder transport and the timing of emplacement. Boulder size and channel hydraulic geometry were used to constrain paleo-flood discharge assuming turbulent, Newtonian fluid flow conditions, and boulder exposure ages were determined using cosmogenic nuclide exposure dating. Modeled discharges required for boulder transport of ca. 103 to 105 m3 s−1 exceed typical monsoonal floods in these river reaches. Exposure ages range between ca. 1.5 and 13.5 ka with a clustering of ages around 4.5 and 5.5 ka in both studied valleys. This later period is coeval with a broader weakening of the Indian summer monsoon and glacial retreat after the Early Holocene Climatic Optimum (EHCO), suggesting glacial lake outburst floods (GLOFs) as a possible cause for boulder transport. We, therefore, propose that exceptional outburst events in the central Himalayan range could be modulated by climate and occur in the wake of transitions to drier climates leading to glacier retreat rather than during wetter periods. Furthermore, the old ages and prolonged preservation of these large boulders in or near the active channels shows that these infrequent events have long-lasting consequences on valley bottoms and channel morphology. Overall, this study sheds light on the possible coupling between large and infrequent events and bedrock incision patterns in Himalayan rivers with broader implications for landscape evolution.


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Augengneiss, likely Ulleri-gneiss of Lesser Himalayan sequence, outcrops only just below the MCT in the study region, no intrusions mapped or known to the authors which are located south of these areas (Shrestha et al., 1986;Dhital, 2015), with garnets.  Betrawati fill-terrace at river-cut seen from different angle than Figure 2B. Deposit has sorting, some grading and clast-supported texture.

Supplement 3 (S3): Boulder exposure ages
Surface exposure dating with cosmogenic nuclides developed substantially in the last two decades and has become a powerful tool in analysing landscape evolution in Quaternary Geology and Geomorphology (e.g. Ivy-Ochs and Kober, 2008). Taking into account local cosmogenic nuclide production and topographic shielding, which lowers production, a surface exposure age is calculated from the cosmogenic nuclide concentrations by solving for t in Equation S3-1 below, where nuclide concentration N [atoms g -1 ] is given as a function of time t [a] with production rate P [atoms g -1 a -1 ] and decay constant λ [a -1 ]. Equation S3-1 simplifies the evolution of cosmogenic nuclide concentrations by neglecting inheritance and erosion. Following standard chemical separation procedures (details provided below), concentrations of cosmogenic nuclides are measured with accelerated mass spectrometry (AMS). The radionuclide 10 Be ( 16 O(n,4p3n) 10 Be) is used in this study for cosmogenic nuclide dating because the target mineral quartz (SiO2) is abundant in the sampled lithologies. Exposure dating with 10 Be is a well-established method, comparably easy to apply and delivers reliable results for the targeted time-frame (Dunai, 2010).

Laboratory work
Sample preparation was performed in the laboratories of the Geological Institute in the Earth Science Department at ETH Zurich. The procedure employed is based on Ivy-Ochs (1996) with modifications from Norton et al. (2008), which itself is adapted after Von Blanckenburg et al. (1996Blanckenburg et al. ( , 2004. Samples were crushed with high-voltage pulse power fragmentation (SELFRAG), sieved to grain sizes between 1000 μm to 250 μm and magnetically separated to remove unwanted magnetic minerals from each sample. Repetitive acid treatment with diluted hydrochloric (HCL), hexafluorosilicic (H2SiF6) and hydrofluoric (HF) acids was used to remove minerals, mainly oxides, carbonates and feldspars from the sample material and isolate quartz (Norton et al. 2008). In order to fully remove meteoric 10 Be from the remaining crystals, the grain boundaries of the quartz were leached with HF 3 times so as to dissolve 10% of the quartz mass at each step. Approximately 200 to 250 μg 9 Be carrier solution was added to a sample weight of ~50 g to enable appropriate sample size and isotope ratio for a later measurement. Beryllium was then extracted and purified using ion exchange column chromatography. The final steps before measurement, including pressing and loading of the samples into cathodes, were performed at the Ion Beam Laboratory at ETH Zurich, Hönggerberg where the samples were measured at the LIP 0.5 MV compact accelerator mass spectrometry (AMS) facility (Tandy).
Results were normalized to secondary in-house standards S2007N and S2010N with nominal values of 10 Be/ 9 Be = 28.1 x 10 -12 and 10 Be/ 9 Be = 3.3 x 10 -12 , respectively. S2007N and S2010N have been calibrated with our new primary standard ICN 01-5-1. ICN 01-5-1 is produced by K. Nishiizumi and has a nominal 10 Be/ 9 Be value of 2.709 x 10 -11 (Nishiizumi et al., 2007, Christl et al., 2013. Blank corrections were performed using an arithmetic mean of 14 10 Be blanks with zero outliers measured at the Tandy facility in the period of 4 months before our last measurement was conducted (20 blanks with one outlier in a period of one year before measurement for sample NEQ/162 79). AMS measurements were performed in June and September 2017 (June 2018 for sample NEQ/162 79).

Calculation of ages
Subsequently cosmogenic exposure ages were computed from the 10 Be/ 9 Be ratios including analytical errors measured at the AMS facility. The "Cosmic Ray Exposure program" (CREp) code, which is accessible online via the URL http://crep.crpg.cnrs-nancy.fr (Martin et al., 2017), was used to calculate exposure ages from nuclide concentrations. This web-based computational tool was chosen because it utilizes a robust production rate calibration database set up by the Informal Cosmogenic-nuclide Exposure-age Database (ICE-D) project (http://calibration.ice-d.org). The database is continuously updated and compiles and aligns production rate calibration data published for a variety of locations globally (Martin et al., 2017). Parameters input into CREp include the 10 Be concentration in the samples (calculated from the measured ratios) with 1σ-error, sample location coordinates and altitude, topographic shielding, an assumed uniform rock sample density of 2.7 g cm-3 and the average sample thickness. We applied the Lifton-Sato-Dunai (LSD) theoretical scaling scheme (Lifton et al., 2014) for our age computation which uses analytical approximations to modelled cosmic ray particle fluxes giving specific atmospheric cross-sections for the 10 Be-nuclide and the other particles involved in the corresponding nuclear reactions (Martin et al., 2017). Another input scheme is the ERA-40 atmosphere model (Uppala et al., 2005) based on a 45 year spanning database of atmospheric pressures for any locations on earth. It gives a pressure distribution approximation necessary because atmospheric pressure has an impact on the local production rate of cosmogenic nuclides. The geomagnetic record Lifton 2016 VDM (Pavón-Carrasco et al., 2014;Laj et al., 2004;Ziegler et al., 2011) was chosen to account for variations in the earth's magnetic field in the past. We chose a global mean production rate because no production rate calibration data was available for the whole Asian continent (see full list of references on http://crep.crpg.cnrs-nancy.fr or Martin et al., 2017). We computed our ages on the 7 th of June 2018. (1) elevation of sampling point (2) 6.616 x 10¹⁹ atoms 9 Be per gram carrier (3) after blank correction: 1.36 x 10 5 ± 2.44 x 10 4 ¹⁰Be atoms n = 14 blank measurements over 4 months in same laboratory (except for NEQ/162 79 that was corrected for a blank contribution of 1.43 x 10 5 ± 2.77 x 10 4 ¹⁰Be atoms, n = 20 blank measurements over 1 year in same laboratory) (4) calculated with online version of CREp (Martin et al.2017) on 7.6.2018, see text for set parameters and production rate.