T03_04

Refining the Bulk: Utilizing Complementary Pyrolysis-Gas Chromatography-Mass Spectrometry to Focus and Inform Ramped Pyrolysis Radiocarbon Analyses

Ginnane C¹, Turnbull J^1,2, Phillips A¹, Zondervan A¹, Naeher S¹

¹GNS Science, Lower Hutt, New Zealand, ²CIRES, University of Colorado at Boulder, Boulder, USA

Ramped pyrolysis oxidation-accelerator mass spectrometry (RPO-AMS) has been established at Rafter Radiocarbon Laboratory at GNS Science, NZ. RPO-AMS has a niche in Antarctic sediment chronology where traditional radiocarbon analyses are unsuitable. In this depositional environment, carbonates are not well-preserved, and autochthonous carbon is commingled with detrital carbon. RPO-AMS improves on bulk radiocarbon measurements for these challenging environments by partitioning the sedimentary organic carbon pool according to thermochemical stability. More labile, predominantly younger depositional carbon can be separated from older, refractory detrital carbon that skews the bulk radiocarbon measurement. 

RPO-AMS presents a step change in radiocarbon measurement, providing an intermediate technique between bulk and compound-specific radiocarbon analysis (CSRA). More accurate information is obtained relative to bulk sediment dating without the cost and major technical challenges associated with CSRA. By combining pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) with RPO-AMS, a chemical fingerprint is determined for each RPO split partitioned along the thermal gradient, informing the composition of the carbon source. Aliquots containing carbon compounds indicative of deposition can be identified prior to selecting samples for radiocarbon analysis. Focused analyses shorten run times and reduce measurement costs to achieve more accurate and comprehensive chronologies. In addition to targeted sample selection, the Py-GC-MS fingerprints can be used to interpret detrital depositional environments and sedimentary processes in tandem with RPO-AMS data.