Refining the Bulk: Ramped Pyrolysis Radiocarbon Dating Enhanced with Parallel Pyrolysis-GC-MS Analysis
Dr Catherine Ginnane1, Dr Sebastian Naeher1, Dr Jocelyn Turnbull1,2, Mr Andy Phillips1, Dr Albert Zondervan1
1GNS Science, Lower Hutt, New Zealand, 2CIRES, University of Colorado at Boulder, Lower Hutt, New Zealand
Ramped pyrolysis oxidation accelerator mass spectrometry (RPO-AMS) has been recently established at the Rafter Radiocarbon Laboratory. This technique improves on bulk radiocarbon measurements for difficult-to-date sediments and soils, including Antarctic detrital sediment, where the depositional age is often obfuscated by the presence of reworked carbon associated with detrital sediment. By exploiting the thermochemical stability of distinct organic carbon pools present in a sample, younger, more labile authigenic carbon corroborating sediment deposition can be separated from older, more refractory components characteristic of detrital material. Radiocarbon measurements of these carbon fractions provide a more reliable chronology of deposition.
RPO-AMS presents a step change in radiocarbon measurement, providing an intermediate technique between bulk and compound-specific radiocarbon analysis. More accurate information is obtained relative to bulk sediment dating without the cost and major technical challenges associated with compound-specific radiocarbon dating. The traditional RPO-AMS technique requires radiocarbon measurement of multiple CO₂ aliquots obtained at a series of pyrolysis temperatures. The youngest age obtained from the series is assumed to be the limiting age representative of deposition. The requirement for multiple radiocarbon measurements can still be cost prohibitive for comprehensive downcore dating. As the limiting age predominantly coincides with the most labile, lower temperature fractions, it has become more common practice to conservatively date only the lower temperature aliquot(s) to obtain a depositional age.
In order to provide evidence to confirm this assumption that the lower temperature aliquots represent deposition, we have coupled ramped pyrolysis radiocarbon dating with pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) as a novel application for carbon source identification. The Py-GC-MS is configured to mimic the conditions of the RPO-AMS system, thereby identifying the compound classes present in each aliquot taken at equivalent temperatures and allowing discernment of the desired fractions indicative of deposition. For example, in early results, the presence of short chain n-alkanes indicated marine carbon deposition in more labile, low temperature fractions. Higher temperature aliquots were comprised of older, diagenetically stable aromatic carbon structures that skewed the bulk age from the depositional age.
By combining Py-GC-MS identification with RPO-AMS, the most suitable temperature aliquot comprised of carbon compounds representative of deposition can be selected for radiocarbon dating, thereby shortening run times and reducing measurement costs to achieve more reliable, substantiated comprehensive downcore chronologies.
The development of these RPO-AMS and Py-GC-MS techniques is particularly useful in ongoing climate change research where determining accurate sediment chronologies can elucidate interpretations of polar ice movement and carbon cycle dynamics. Comparisons of RPO-AMS measurements with bulk sediment radiocarbon measurements in the Antarctic has improved chronologies by several hundreds to thousands of years, thereby clarifying inconsistencies in the timing and mechanism of deglaciation events.
Dr. Catherine Ginnane is a Senior Laboratory Technician in the Rafter Radiocarbon Laboratory at GNS Science in Lower Hutt, New Zealand. Cathy specializes in instrument and laboratory capability development, including elemental analysis combustion for radiocarbon analyses, graphitisation systems, and ramped pyrolysis oxidation (RPO)-AMS.