Development of an improved ramped pyrolysis method for radiocarbon dating and application to Antarctic sediments
<p>Archives of the retreat history of the Antarctic Ice Sheet since the Last Glacial Maximum (~20,000 years ago) are preserved in marine sediment cores from around the margins of Antarctica, but accurate dating methods remain elusive in many areas. Radiocarbon dating of key lithofacies transitions indicative of grounding-line retreat is problematic due to pervasive reworking issues in glacimarine sediments. Bulk sediment material can be radiocarbon dated but yields ages which are not indicative of the time of sedimentation due to the presence of reworked carbon material from pre-Last Glacial Maximum times. Consequently, development of methods to date only the autochthonous carbon component of these sediments are required to date the retreat of the Last Glacial Maximum ice sheet in Antarctica. A new radiocarbon dating capability has been developed at Rafter Radiocarbon Laboratory (RRL), National Isotope Centre, GNS Science, Lower Hutt, in the course of this study. This has entailed designing, building and testing a ramped pyrolysis (RP) system, in which sedimentary material is heated from ambient to ~1000oC in the absence of oxygen (pyrolysed), with the carbon liberated during pyrolysis being combined with oxygen at a temperature of ~800oC to produce CO2. The amount of CO2 produced is measured by a gas analyser and the CO2 is captured in a vacuum line. The method exploits the thermochemical behaviour of degraded organic carbon. Organic carbon which has been least degraded with time breaks down earliest under pyrolysis, so CO2 captured from this fraction most closely approximates the time of deposition of the sediment. CO2 captured at higher temperatures represents more degraded carbon-containing fractions and yields older ages. The RP system includes a gas delivery system to deliver ultra-high purity He (carrier gas) and O2, a furnace system in which to pyrolyse sample material and oxidise the liberated carbon, a CO2 detection system to measure the CO2 produced and a vacuum line system to enable simultaneous collection and processing of CO2. The RRL system was based on the design developed by Dr Brad Rosenheim (University of South Florida (USF)), the originator of the first RP system at the National Ocean Sciences AMS Facility (Woods Hole Oceanographic Institution, Massachusetts, USA), who also provided guidance in this thesis. As part of the study, a visit to USF was undertaken, with sediment samples from Crystal Sound, Antarctic Peninsula being processed in the USF RP system. CO2 collected from RP processing was radiocarbon dated at RRL. The scope of this thesis was to develop and build the RRL RP system, and numerous tests were conducted during this process and are presented in this thesis. As part of this, sediment samples from Crystal Sound were also processed on the RRL RP system, and an interlaboratory comparison was conducted on the same materials processed independently through both the USF and RRL RP systems. In the development and testing of the RRL system, numerous issues were identified and a set of operating protocols developed. Due to time constraints and the scope of this thesis, interlaboratory comparisons were limited in number, but initial results show good reproducibility, and that ramped pyrolysis captured significantly younger carbon populations in both the USF and RRL RP systems than methods using bulk sediment dating alone. Within uncertainties, the ages of the youngest and oldest splits from RP processing of the same material on both systems were indistinguishable.</p>
