²⁶Al and ¹⁰Be concentrations from alluvial drill cores across the Indo-Gangetic plain reveal multimillion-year sediment-transport lag times

Sediment burial can lead to significant lag times during sediment transport and is an important component of the sediment cycle. The Indo-Gangetic Plain (IGP) is a large sediment conveyer belt and understanding sediment lag times on various time scales is important for short- and long-term sediment transport and denudation-rate estimations. In this study, we have quantified the transport lag time in terms of burial duration with 14 different alluvial drill cores from 4 major Himalayan river basins, including the Ghaggar, Paleo-Yamuna, Ganges, and Kosi rivers. We rely on 56 samples using the paired cosmogenic radionuclides, 26Al and 10Be. The coring locations of paleo-Yamuna and Kosi, along with one of the Ghaggar cores are within ∼100 km from the mountain front, while the Ganges cores are ∼350 km away. The coring locations cover an E-W distance of ∼1200 km on the IGP and the depositional ages range from ∼120 kyr BP to the present day based on previous age estimations. Out of 56 samples, 39 samples show a burial signal ranging from 0.61±0.33 Myr to 3.75±1.9 Myr, with outlier values as large as ∼6 Myr. We show that the Kosi samples due to their proximity to the mountain front show no burial or a lower mean burial duration of ∼1.01 Myr, compared to the samples from the coring locations well within the IGP, such as the Ghaggar (∼2.48 Myr) and the Ganges (∼2.36 Myr). However, despite its proximity to the mountain front, the paleo-Yamuna samples have a mean burial duration of ∼2.20 Myr. Along with the floodplains and megafan surface of the IGP, we identify two potential major sediment-lag sources in the Himalayan catchment: The high and low elevation broad valleys and the Siwaliks. Increased storage spaces in the valleys north of the main Himalayan crest, including drainage-captured catchments are likely to provide long burial durations such as in the case of the Kosi and Ghaggar basins. The upper and middle Siwaliks with burial duration in the range of up to ∼6 Myr constitute a second source. We attribute the no-burial signals of 7 samples of the Kosi basin to rapid sediment generation and transportation, driven by dynamic hillslope erosion and fluvial dynamics. Importantly, our data suggest that even samples close to the mountain front exhibit million years of burial duration. These findings suggest that denudation rates derived from modern river sands or in paleo settings will need to account for burial duration and associated processes.