Antarctic sediment chronology by programmed‐temperature pyrolysis: Methodology and data treatment

Abstract

[1] We report a detailed programmed-temperature pyrolysis/combustion methodology for radiocarbon ( 14 C) dating of Antarctic sub-ice shelf sediments. The method targets the autochthonous organic component in sediments that contain a distribution of acid-insoluble organic components from several sources of different ages. The approach has improved sediment chronology in organic-rich sediments proximal to Antarctic ice shelves by yielding maximum age constraints significantly younger than bulk radiocarbon dates from the same sediment horizons. The method proves adequate in determining isotope ratios of the pre-aged carbon end-member; however, the isotopic compositions of the low-temperature measurements indicate that no samples completely avoided mixing with some proportion of pre-aged organic material. Dating the unresolved but desired young end-member must rely on indirect methods, but a simple mixing model cannot be developed without knowledge of the sedimentation rate or comparable constraints. A mathematical approach allowing for multiple mixing components yields a maximum likelihood age, a first-order approximation of the relative proportion of the autochthonous component, and the temperature at which allochthonous carbon begins to volatilize and mix with the autochthonous component. It is likely that our estimation of the cutoff temperature will be improved with knowledge of the pyrolysis kinetics of the major components. Chronology is improved relative to bulk acid-insoluble organic material ages from nine temperature interval dates down to two, but incorporation of inherently more pre-aged carbon in the first division becomes more apparent with fewer and larger temperature intervals.

Ultra-Small Graphitization Reactors for Ultra-Microscale 14C Analysis at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility

“…Keigwin and Guilderson 2009), and it will allow for the 14 C analysis of finer fractions of volatiles and CO 2 evolved during ramped pyrolysis experiments (e.g. Rosenheim et al 2008;Rosenheim and Galy 2012).…”

Technical note: An inverse method to relate organic carbon reactivity to isotope composition from serial oxidation

“…In this study, we relate OC thermal recalcitrance to its corresponding δ 13 C and Fm values using ramped-temperature pyrolysis or oxidation (RPO). This method involves heating OC at a controlled rate while continuously quantifying and collecting evolved CO 2 , which is binned over user-defined time intervals (termed "fractions") and analyzed for δ 13 C and Fm (Rosenheim et al, 2008;Hemingway et al, 2017). We describe non-isothermal OC decay rates as a function of E, the Arrhenius activation energy, using a novel inverse solution to the distributed activation energy model (Braun and Burnham, 1987;Burnham and Braun, 1999;Cramer, 2004;White et al, 2011).…”

“…RPO analysis has been described in detail previously (Rosenheim et al, 2008;Hemingway et al, 2017). In summary, a solid sample containing ≈ 150-250 µg C is loaded into a precombusted (850 • C, 5 h) quartz reactor and placed into a twostage oven, as shown in Fig.…”

“…Such analyses separate carbon within a bulk sample based on its susceptibility to decomposition by chemical hydrolysis (Helfrich et al, 2007), UV light (Beaupré et al, 2007;Follett et al, 2014), heat (Rosenheim et al, 2008), or microbial respiration (Beaupré et al, 2016) and measure the stable carbon ( 13 C / 12 C, expressed as δ 13 C) and radiocarbon ( 14 C / 12 C, here expressed as fraction modern or Fm) content of evolved CO 2 . By separating CO 2 into multiple lability intervals, isotope ratios are obtained for carbon atoms exhibiting similar physical and/or chemical properties.…”

“…During analysis, the oven surrounding the sample is programmed to heat at a user-defined ramp rate, termed β (see Table 1 for symbol descriptions). Instantaneous temperature within the oven is measured using two thermocouples separated by ≈ 1 cm to monitor temperature heterogeneity, which is typically < 5 • C. Following standard practice (Rosenheim et al, 2008), a ramp rate of 5 • C min −1 was used for all experiments in which CO 2 gas was collected for isotope analysis. In the second (downstream) oven, eluent gas is passed over a Cu, Pt, and Ni catalyst wire held at 800 • C to facilitate oxidation of reduced carbon-containing gases to CO 2 .…”