The National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) played an integral part in developing compound specific radiocarbon analysis (CSRA) in the late 1990’s (refs). The laboratory has remained at the forefront of the field and continues to explore the limits imposed on the measurements by both analytical preparation and analysis on the accelerator mass spectrometer (AMS). The greatest limitations on CSRA analyses are the challenges of obtaining a robust sample and of running very small samples on the accelerator. Marine sediments often have very low organic matter (SOC) concentrations (0.3-2% C by weight); the amount extractable for biomarker analysis represents a very small percentage of this SOC (5-10 %). Collection of a sample requires the use of preparative organic analytical techniques such as capillary gas chromatography and HPLC. Samples are then combusted to CO<sub>2</sub>, reduced to graphite and analyzed on the AMS system. The precision of radiocarbon analyses on the smallest samples (5-100 µg C) is limited not only by the magnitude and radiocarbon content of the process blanks but also by how well we know those quantities. NOSAMS ultra-small graphite preparation adds a blank of 0.28 ± 0.1 µg C with a fraction modern value of 0.43 ± 0.3. We have recently developed an ultra-clean combustion procedure that adds only nanograms of C to the sample. Radiocarbon results on samples prepared using this technique are indistinguishable from the results obtained on gas samples. NOSAMS operates two AMS systems, one that has a very high efficiency and one that has a very low blank. We are currently investigating the relative effects of counting statistics, measurement repeatability and machine background on precision and accuracy of ultra-small sample analysis.<br/><br/>Our most recent application of both ultra-small analysis and CSRA techniques is a study of terrestrial carbon cycling in the Arctic during past warming events. We coupled nano-blank combustion techniques with ultra-small graphite preparation to produce radiocarbon chronologies based on very small plant and insect macrofossils for two sediment cores from northern Alaskan lakes. CSRA of plant- and sphagnum moss-derived lignin and p-hydroxy phenols will be used to explore the age and source of the terrestrial carbon mobilized and delivered to the lake during past warming events.

Compound specific radiocarbon analyses at NOSAMS: Unlocking ocean and lake chronologies one molecule at a time

253rd National Meeting (2017)

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