A 6 Ma record of palaeodenudation in the central Himalayas from in situ cosmogenic ¹⁰Be in the Surai section

Abstract

To better constrain late Neogene denudation of the Himalayas, we analysed in situ 10 Be concentrations in 17 Neogene sediment samples of the Surai section (central Nepal) and two modern sediment samples of the Rapti River. We first refined the depositional ages of the Surai section from 36 new paleomagnetic analyses, five 26 Al/ 10 Be burial ages, and, based on the Dynamic Time Warping algorithm, 10 4 automatically calculated likely magnetostratigraphic correlations. We also traced changing sediment sources using major element and Sr-Nd isotopic data, finding at 4-3 Ma a switch from a large, trans-Himalayan river to a river draining only the Lesser Himalaya and Siwalik piedmont, increasing the contribution of recycled sediments at that time. 10 Be concentrations in Neogene sediments range from (1.00 ± 0.36) to (5.22 ± 0.98) × 10 3 at g-1 and decrease with stratigraphic age. Based on a flood plain transport model, our refined age model, and assuming a drainage change at 4-3 Ma, we reconstructed 10 Be concentrations at the time of deposition. Assuming cosmogenic production rates similar to those of the modern basins, we calculated palaeodenudation rates of 0.9 ± 0.5 to 3.9 ± 2.7 mm a-1 from ca. 6 to 3 Ma in the palaeo-Karnali basin and 0.6 ± 0.2 to 1.6 ± 0.8 mm a-1 since ca. 3 Ma in the palaeo-Rapti basin. Given the uncertainties and similar modern values of ~2 mm a-1 , the palaeo-Karnali denudation rates may have been steady at ~1.7 ± 0.3 mm a-1 for the last ca. 6 Ma. A transient acceleration of the denudation in the palaeo-Rapti basin of ~1.5 mm a-1 since ca. 1.5 Ma was likely due to the reworking of older, 10 Be-depleted Siwalik sediments in the foreland. If true, this steadiness of the denudation rates may suggest that Quaternary glaciations did not largely affect Himalayan denudation. K E Y W O R D S cosmogenic 10 Be, Himalaya, late Neogene, palaeodenudation rates | 1219 EAGE CHARREAU Et Al.

Impact of Quaternary glacial cycles on denudation in Western Mediterranean mountains

“…Our new record brings two main results: i) At the million years timescale, the appearance of the Quaternary glaciations (2.6 Ma) had a negligible impact on the denudation rates of these Alpine reliefs. This observation is in good agreement with other 10 Be-denudation rates records from Asia (Tianshan and Himalaya) that report a limited impact of the Pleistocene climatic transition [1][2][3], but at odds with other regions of the American Cordilleras, where tectonic may have played a role [4][5].…”

Milankovitch-paced erosion in the southern Central Andes

“…Because P 0 varies with latitude, altitude, and topographic shielding, modern mean effective production rates (topographic shielding factor x production rate) across the Río Iruya watershed were estimated using the 90-meter Shuttle Radar Topography Mission digital elevation model and the CRONUScalc Matlab codes 75 assuming a sea-level high-latitude (SLHL) production rate of 4.01 atoms g −1 yr −1 and based on the time-independent Lal-Stone scaling (St) 31,76,77 . Assuming rapid transport of sediments from the hillslopes into the fluvial network and proper amalgamation in the fluvial system of an integrated catchment signal, the concentration of 10 Be in a river sand sample can be used to calculate the erosion rate, R E , of the upstream catchment 24 where…”

“…excursions in climate [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Unfortunately, these studies are rarely straightforward, and commonly rely on less-than-ideal field exposures, temporal controls, and source-area and cosmogenic radionuclide systematic constraints.…”

Cosmogenic Nuclide Tracking of Sediment Recycling From a Frontal Siwalik Range in the Northwestern Himalaya

“…This shortcoming is due in part to challenges in estimating catchment-wide erosion rates using the concentration of 10 Be measured in detrital quartz grains recycled from the older foreland successions. The sediment pathway from hinterland to foreland is complex and multi-step as follows: (1) sediment creation via hinterland erosion, (2) transport from hinterland to foreland, (3, 4) burial and storage within the sedimentary basin, and (5) later uplift via thrusting and folding, which makes sediment available for re-exhumation (recycling) (Figure 2) (e.g., Charreau et al, 2011Charreau et al, , 2020Mandal et al, 2021). As a consequence, the recycled quartz grains contain nuclides acquired during prior exposure (steps 1-4), resulting in inheritance and, as a result, anomalously low erosion rates.…”

“…Therefore, to estimate erosion rates using present‐day 10 Be concentrations in fluvial sediments, we must be able to separate the number of 10 Be atoms (per gram of quartz) gained during recycling (step 5) from the inheritance. Although a few studies have attempted to reconstruct hinterland paleoerosion rates using present‐day 10 Be concentrations in tectonically uplifted late Miocene‐Pleistocene foreland basin deposits (Charreau et al., 2020; Mandal et al., 2021), this challenge has yet to be explored and applied to a regional case study.…”