Description: To document distinct sources of particulate organic carbon (POC) to the Río Bermejo, we collected 15 soil and 13 leaf litter samples from the local floodplain, and 10 bedrock (predominantly outcroppings of fine-grained sedimentary bedrock) and 2 soil samples from the Río Bermejo headwaters. Leaf litter and soil were oven-dried at 40°C for 〉48 hours. We shredded leaf litter in an industrial blender, homogenized soil samples in an agate mortar and manually removed root and plant debris 〉1 cm, and pulverized bedrock samples to 〈63 µm.

Description: These data were collected from the Río Bermejo in northern Argentina. To determine the seasonal variability in the particulate organic carbon composition of exported river sediment, we collected weekly suspended sediment samples (March 2016 to March 2018) at the Puente Lavalle (PLV) monitoring site, ~870 river km downstream of the mountain front (-25.655°S, -60.130°W). Surface water samples were collected from a bridge using a river-rinsed bucket and were filtered through a 0.22 µm polyethersulfone membrane. Samples were stored on site at ambient temperatures for up to one year, transferred to Germany and subsequently stored at ~4°C until processing. Suspended sediment was rinsed from filters into pre-combusted glass evaporating dishes using ultra-pure (18.2 M) water, oven-dried at 40°C for 〉48 hr, and homogenized in an agate mortar without crushing. Geochemical and grain size analyses required 0.8 g sediment; for samples 〈0.8 g, we combined consecutive weekly samples to create a new bulk sample of 〉0.8 g (Table S1). We split sediment samples into aliquots for grain size analysis via laser diffraction and geochemical analyses. Sediment particle size distributions were measured on ~0.2 g aliquots using a laser diffraction particle size analyzer (Retsch/Horiba LA-950V2). Aliquots for geochemical analyses were ground to 〈63 µm. The homogenized suspended sediment, bedrock, soil and leaf litter aliquots were further split for total nitrogen measurement (TN, wt%) and organic carbon analyses including total organic carbon (TOC, wt%), stable carbon isotope composition (δ13COC), and radiocarbon fraction modern (Fm). We decarbonated the aliquots for POC measurements using a liquid HCl leach following Galy et al., (2007). TOC and TN measurements were split between facilities at the German Research Centre for Geosciences (GFZ), Durham University, and University of Nevada Reno (UNR) using an elemental analyzer (EA). δ13COC was measured with a coupled EA-isotope ratio mass spectrometer (EA-IRMS). All isotopic compositions are reported using standard delta (δ) notation in per mil (‰) relative to Vienna PeeDee Belemnite (VPDB). Calibration and accuracy were monitored through analyses of in-house standards (Glutamic Acid, 40.82% C, 9.52% N at Durham; Boden3, HEKATECH at GFZ), which were calibrated against international standards (e.g., USGS 40, USGS 24, IAEA 600, IAEA CH3, IAEA CH7, IAEA N1, IAEA N2). Radiocarbon content was measured for a subset of 29 samples at ETH Zürich using a combined EA and accelerator mass spectrometer (EA-AMS) (Ruff et al. 2010; McIntyre et al., 2017). All 14C /12C ratios are reported as fraction modern (Fm, equivalent to F14C as defined by Reimer et al. (2004)) relative to 95% of the 14C activity of NBS Oxalic Acid II in 1950 (δ13COC = -17.8‰) and normalized to δ13COC = -25‰ of VPDB.\n\nThis geochemical dataset is supported by hydrologic measurements of daily water discharge at the El Colorado gauging station (river km 1086, SNIH, https://snih.hidricosargentina.gob.ar/) collected between 2016 and 2018.

Description: These data were collected from the Río Bermejo in northern Argentina. To determine the seasonal variability in the particulate organic carbon composition of exported river sediment, we collected weekly suspended sediment samples (March 2016 to March 2018) at the Puente Lavalle (PLV) monitoring site, ~870 river km downstream of the mountain front (-25.655°S, -60.130°W). Surface water samples were collected from a bridge using a river-rinsed bucket and were filtered through a 0.22 µm polyethersulfone membrane. Samples were stored on site at ambient temperatures for up to one year, transferred to Germany and subsequently stored at ~4°C until processing. To document distinct sources of particulate organic carbon (POC) to the Río Bermejo, we collected 15 soil and 13 leaf litter samples from the local floodplain, and 10 bedrock (predominantly outcroppings of fine-grained sedimentary bedrock) and 2 soil samples from the Río Bermejo headwaters. Suspended sediment was rinsed from filters into pre-combusted glass evaporating dishes using ultra-pure (18.2 M) water, oven-dried at 40°C for 〉48 hr, and homogenized in an agate mortar without crushing. Leaf litter and soil were oven-dried at 40°C for 〉48 hours. We shredded leaf litter in an industrial blender, homogenized soil samples in an agate mortar and manually removed root and plant debris 〉1 cm, and pulverized bedrock samples to 〈63 µm. Geochemical and grain size analyses required 0.8 g sediment; for samples 〈0.8 g, we combined consecutive weekly samples to create a new bulk sample of 〉0.8 g (Table S1). We split sediment samples into aliquots for grain size analysis via laser diffraction and geochemical analyses. Sediment particle size distributions were measured on ~0.2 g aliquots using a laser diffraction particle size analyzer (Retsch/Horiba LA-950V2). Aliquots for geochemical analyses were ground to 〈63 µm. The homogenized suspended sediment, bedrock, soil and leaf litter aliquots were further split for total nitrogen measurement (TN, wt%) and organic carbon analyses including total organic carbon (TOC, wt%), stable carbon isotope composition (δ13COC), and radiocarbon fraction modern (Fm). We decarbonated the aliquots for POC measurements using a liquid HCl leach following Galy et al. (2007, doi:10.1111/j.1751-908X.2007.00864.x)). TOC and TN measurements were split between facilities at the German Research Centre for Geosciences (GFZ), Durham University, and University of Nevada Reno (UNR) using an elemental analyzer (EA). δ13COC was measured with a coupled EA-isotope ratio mass spectrometer (EA-IRMS). All isotopic compositions are reported using standard delta (δ) notation in per mil (‰) relative to Vienna PeeDee Belemnite (VPDB). Calibration and accuracy were monitored through analyses of in-house standards (Glutamic Acid, 40.82% C, 9.52% N at Durham; Boden3, HEKATECH at GFZ), which were calibrated against international standards (e.g., USGS 40, USGS 24, IAEA 600, IAEA CH3, IAEA CH7, IAEA N1, IAEA N2). Radiocarbon content was measured for a subset of 29 samples at ETH Zürich using a combined EA and accelerator mass spectrometer (EA-AMS) (Ruff et al. (2010, doi:10.1017/S003382220005637X); McIntyre et al. (2017, doi:10.1017/RDC.2016.68)). All 14C /12C ratios are reported as fraction modern (Fm, equivalent to F14C as defined by Reimer et al. (2004)) relative to 95% of the 14C activity of NBS Oxalic Acid II in 1950 (δ13COC = -17.8‰) and normalized to δ13COC = -25‰ of VPDB. This geochemical dataset is supported by hydrologic measurements of daily water discharge at the El Colorado gauging station (river km 1086, SNIH, https://snih.hidricosargentina.gob.ar/) collected between 2016 and 2018.

Description: To study the transformation of organic carbon through long distance transport in rivers, we measured the composition of bulk organic carbon in river suspended sediment of the Rio Bermejo (northern Argentina). This river has a ~1300 km lowland flowpath with no significant tributaries. We collected fluvial suspended sediment in vertical depth profiles at five sampling locations along the length of the Rio Bermejo (northern Argentina) during near-bankfull conditions, when discharge varied between 675 and 1080 m3/s and banks were actively eroding. Additionally, we collected one depth profile from the Rio San Francisco (RSF) and one from the Rio Bermejo 10 km upstream of the RSF confluence. Combining these profiles and weighting them by the relative proportions of their total sediment load input to the mainstem Bermejo serves as a depth profile representing the headwaters. At each depth profile location, we collected water and suspended sediment from the channel thalweg by boat. We used a weighted 8-liter horizontal sampling bottle (Wildco Beta Plus bottle) with an attached pressure transducer to measure sampling depth. We separated sediment from the water using a custom-built 5-liter pressurized filtration unit with a 293 mm diameter, 0.2 µm polyethersulfone filter. In the laboratory, we rinsed sediment off the filters directly into an evaporating dish with ultrapure 18.2 MΩ water (pH~7). Samples were dried in an oven at 40ºC, and subsequently homogenized. Sediment particle size distributions were measured on ~10 mg aliquots using a laser diffraction particle size analyzer (Horiba LA-950). Specific surface area (SSA) of bulk sediment samples was measured on ~4 g aliquots using a Quantachrome NOVAtouch LX gas sorption analyzer and the Brunauer, Emmett, and Teller (BET) theory (Brunauer et al., 1938). Aliquots for organic carbon measurements were first treated with 4% HCl solution to remove inorganic carbon, following Galy et al. (2007, doi:10.1111/j.1751-908X.2007.00864.x). Total organic carbon (TOCPOC) and δ13C of POC was measured in duplicate at Durham University using a Costech elemental analyzer (EA) coupled to a CONFLO III and Thermo Scientific Delta V Advantage isotope ratio mass spectrometer (IRMS). Radiocarbon content was measured using an EA coupled to an accelerator mass spectrometer (EA-AMS) at ETH Zurich. We report 14C content as fraction modern (F14C), by normalizing measurements to 95% of the 1950 NBS Oxalic Acid II standard (δ13C = -17.8‰) and correcting for mass-dependent fractionation using a common δ13C value of -25‰. OC loading is the mass of organic carbon in a sample normalized by the sample's specific surface area (SSA). Reactive metals in the amorphous oxyhydroxide and crystalline oxide grain coatings, were extracted from the sediment samples using a procedure adapted from Wittmann et al. (2012, doi:10.1016/j.chemgeo.2012.04.031). The extracted oxyhydroxides and oxides were dried down and diluted in 3M HNO3. A 100 μl aliquot was taken for measurement of metal concentrations. Al, Fe, Mg, and Mn concentrations were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES). Uncertainty of ICP-OES measurements was 〈5%. All depth-integrated values are calculated as a function of the suspended sediment concentration relative to the depth-averaged suspended sediment concentration.