Description: Compound-specific radiocarbon dating often requires working with small samples of 〈 100 µg carbon (µgC). This makes the radiocarbon dates of biomarker compounds very sensitive to biases caused by extraneous carbon of unknown composition, a procedural blank, which is introduced to the samples during the steps necessary to prepare a sample for radiocarbon analysis by accelerator mass spectrometry (i.e., isolating single compounds from a heterogeneous mixture, combustion, gas purification and graphitization). Reporting accurate radiocarbon dates thus requires a correction for the procedural blank. We present our approach to assess the fraction modern carbon (F14C) and the mass of the procedural blanks introduced during the preparation procedures of lipid biomarkers (i.e. n-alkanoic acids) and lignin phenols. We isolated differently sized aliquots (6-151 µgC) of n-alkanoic acids and lignin phenols obtained from standard materials with known F14C values. Each compound class was extracted from two standard materials (one fossil, one modern) and purified using the same procedures as for natural samples of unknown F14C. There was an inverse linear relationship between the measured F14C values of the processed aliquots and their mass, which suggests constant contamination during processing of individual samples. We used Bayesian methods to fit linear regression lines between F14C and 1/mass for the fossil and modern standards. The intersection points of these lines were used to infer F14Cblank and mblank and their associated uncertainties. We estimated 4.88±0.69 μgC of procedural blank with F14C of 0.714±0.077 for n-alkanoic acids, and 0.90±0.23 μgC of procedural blank with F14C of 0.813±0.155 for lignin phenols. These F14Cblank and mblank can be used to correct AMS results of lipid and lignin samples by isotopic mass balance. This method may serve as a standardized procedure for blank assessment in small-scale radiocarbon analysis.

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.

Description: Collapse of permafrost coasts delivers large quantities of particulate organic carbon (POC) to arctic coastal areas. The objective of this study is to assess the pathways and fate of POC derived from thawing and eroding permafrost coasts in the nearshore zone, to better understand its impact on the carbon cycle and climate. We collected suspended particulate matter and sediment samples at the coastline and in the nearshore zone of Herschel Island - Qikiqtaruk (N 69.60°; W 139.00°) during a two‐week period in July‐August 2017. Locations were labelled A - H clockwise around the island, starting at the NE corner (Collinson Head). Samples were taken in transects perpendicular to the coast, at point locations offshore ('Monitoring point 1 - 3'), and from 'thaw streams', creeks carrying material from retrogressive thaw slumps and other abrupt thaw features, at various locations around the island. A total of 49 locations were visited; 12 at thaw streams, and 37 in the nearshore zone ranging from right at the shoreline to up to 20 m water depth (about 2 km offshore). We performed biogeochemical analyses (CN, δ13C, Δ14C) and sedimentological analyses (grain size, mineral surface area) in order to trace the pathway of permafrost material from its source to the nearshore water column and sediments.

Description: The Arctic is undergoing accelerated changes in response to ongoing modifications to the climate system, and there is a need for local to regional scale records of past climate variability in order to put these changes into context. The Mackenzie delta region in northern Canada is populated by numerous small shallow lakes. They are classified as no-, low- and high-closure (NC, LC, HC, respectively) lakes, reflecting varying degrees of connection to the river main stem, and have different sedimentation characteristics. This study examines sedimentological (mineral surface area, grain size), carbon isotopic (bulk and molecular-level) and inorganic isotopic (neodymium) characteristics of sediment cores from three lakes representing each class. We find that HC lake sediments exhibit strikingly different properties from the other lake sediments. Specifically, they are characterized by higher organic carbon loadings per unit mineral surface area and with relatively minor influence from allochthonous, petrogenic (rock-derived) organic carbon. In contrast, LC and NC lakes, have the potential to record basin-scale climatic changes at a high resolution by virtue of enhanced detrital sedimentation. Overall the delta have the capacity to bury about 2 MtC yr-1, with little changes since 200 years.