This data publication contains the data sets of a study aiming to trace variations in organic carbon sourcing along the Kali Gandaki River in Central Nepal. The data are on samples from different materials in the landscape (litter, soil, bedrock) and river sediments. On these samples we measured total organic carbon content, stable carbon and nitrogen isotopes, radiocarbon content and surface area. The data was generated between 2015-05 and 2017-12. The tabular data are provided as csv and Excel verisons.

River sediment samples (n=53) and end-member samples (n=70) were taken in the KG catchment during two consecutive Monsoon seasons (July 2015 and June 2016). While the 2016 sampling campaign coincided with the arrival of the Monsoon in the catchment, sampling in 2015 was during the height of that year’s monsoon season. River sediment samples comprise surface suspended sediment load samples (n=30), suspended sediment samples collected at depth (n=2), and river bank sediment samples (n=21). Surface suspended sediment load was sampled from bridges with a metal bucket, targeting the upper meter of the fast-flowing part of the river. Depth samples were taken in the KG River at Bharatpur several decimetres above the river bed. Suspended sediment samples were filtered on the same day using a custom made high-capacity filtration system equipped with polyethersulfone (PES) filters (pore size: 0.2 μm, Sartorius) and directly transferred from filters into pre-combusted glass vials using filtered river water. River bank sediment samples were taken, when possible, the morning following suspended load sampling, when the water level dropped slightly and river bank sediment was exposed. Bank samples were taken with a stainless-steel shovel and then wrapped in combusted aluminum foil and plastic bags.

In the Tibetan part of the catchment, bedrock from the Tethyan Sedimentary Sequence (Lupra, Bagung and Jomsom Formation) was sampled within the main river valley (n=11). Paleosol samples (n=21) were collected in the central valley from visibly organic rich horizons of paleosol sections and remnants (Menges et al., 2019). Vegetated topsoils were sampled (at 0 - 10 or 20 cm depth) at the fringes of the valley and downstream of Jomsom (2500 – 4000 m asl., n=8).

In the Himalayan part of the catchment, slates of the High and Lesser Himalayan Sequence (n=6) were sampled because of their potentially higher OC contents compared to other metamorphic bedrock. Modern soil samples were taken along the KG River (550 - 2500 m asl., n=17) including a wide range of different soil types. Topsoils (0 - 10 or 20 cm depth) were sampled after the removal of the litter/humus layer and if present, deeper soil horizons (ranging from 15 to 40 cm below ground). At seven locations the litter layer was sampled separately. Soil and litter samples were stored in paper bags and pre-dried in the sun. Rock and sediment samples were stored in aluminum foil. Samples were shipped to GFZ Potsdam within 1-2 weeks after sampling. Rock, soil and litter samples were dried immediately after arrival in the lab, in a drying oven at 60 °C (Memmert, UF450). Sediment samples were stored at -18°C until drying at 60 °C in the drying oven.

For all geochemical analyses, soil and sediment samples were ground in a vibratory disk mill using a steel grinding set. Rock samples were packed in clean aluminum foil and crushed with a hammer to a size suitable for milling. One very hard sample (NPCK-4) was prepared in a rock crusher. Litter samples were ground with a ball mill at MPI for Biogeochemistry Jena.

Total organic carbon (TOC) content and stable carbon isotopic composition were measured on in-situ decalcified samples at GFZ Potsdam. Subsamples were placed into a silver capsule (5 x 9mm) and treated in-situ first with 3 % and then 20 % liquid HCl until complete carbonate removal. Samples were then heated for 3 h at 75°C. After wrapping, TOC was measured on an elemental analyzer (Euro EA3000 Fa. EuroVector) and δ13Corg on an elemental analyzer (Carlo-Erba NC2500) coupled to an isotope ratio mass spectrometer (DELTAplusXL ThermoFisher). The machines were calibrated using certified elemental standards and blanks were processed with the samples. The analytical precision was below 0.2‰ for δ13Corg and below 0.1 % for TOC measurements. To account for sample heterogeneity and to test for complete carbonate removal all samples were analyzed at least in duplicate. Mean standard deviation of repeat measurements was 0.3 ‰ for δ13Corg. δ13Corg of leaf litter samples as well as total nitrogen (TN) content and nitrogen isotopic composition of all samples were measured on ground and homogenized bulk aliquots at MPI for Biogeochemistry Jena on an elemental analyzer (NA 1110, CE Instruments, Milan, Italy) coupled with Delta+XL IRMS (Thermo Finnigan, Bremen, Germany). The analytical precision was 0.2‰ for δ15N and 0.3 % for TN measurements. δ15N measurements with a signal intensity below 200 mV were considered below the measurement limit and not processed further. Samples were measured at least in duplicate to account for sample heterogeneity. Mean standard deviation of repeat measurements was 0.4 ‰ for δ15N measurements. As samples were homogenized carefully prior to subsampling for individual measurements, we assume a normal distribution of the errors and report mean values and standard errors. 13C/12C ratios are reported in δ13C notation relative to the Vienna Pee Dee Belemnite (VPDB) standard and 15N/14N ratios as δ15N relative to AIR.

Radiocarbon (14C) content of samples was measured by accelerator mass spectrometry (AMS) at the Laboratory for Ion Beam Physics at ETH Zürich. Subsamples ranging between 6 mg and 50 mg depending on the TOC content (with most river sediment and rock samples between 39 and 42 mg) were placed into combusted silver boats. Subsequently, they were decarbonated in a dessicator under HCl vapor (37 %) for 3 days at 60 °C and then neutralized with NaOH for 4 days at 60 °C. The samples and silver boats were then wrapped in tin capsules and measured on an online EA-IRMS-AMS system (MICADAS) (Wacker et al., 2010; McIntyre et al., 2017). We used in-house shale (radiocarbon dead) and soil (modern) reference material to be able to monitor and correct for the possible contamination during fumigation and combustion. Blanks were regularly measured and subtracted from sample measurements. 14C content is reported as fraction modern (Fm).