Usefulness of Stable Polycyclic Aromatic Carbon for dating lake sediments.
Mr William Reynolds1, Dr Timothy Cohen1, Dr Samuel Marx1
1University Of Wollongong/CABAH, Wollongong, Australia
Lake sediments are useful for reconstructing past changes in the environment and climate. Analysis of a one meter sediment core from Table Top Swamp near Darwin showed conditions 35,000 to 18,000 years ago were perhaps half as dry as today. Rising sea levels at 9,000 years ago caused rainfall at the site to increase (as the coastline and moisture source moved closer to the study site). As a result the swamp became semi-permanent, the dominance of trees increased and fire became more frequent.
In addition to providing this broad story of rainfall variability over the past 35,000 years, the study site has the potential to provide finer scale information. However the ability to tease out more information is limited by the resolution of the age model. One of the barriers to deriving a more accurate chronological framework is the behavior of materials previously used for radiocarbon dating. Ages derived from charcoal and humins show these materials move around in the sediment post-deposition. They are also liable to post-depositional alteration due to weathering and biological activity which may affect their 14C concentration. Plant macro-fossils and lumps of charcoal are often the preferred materials for radiocarbon dating. But these were absent from our sediment. So we explored the alternative method of isolating black carbon from sediment by hydrogen pyrolysis.
In hydrogen pyrolysis samples are mixed with a molybdenum catalyst then heated to 500° C in the presence of hydrogen pressurized to ten Mpa. This gets rid of the younger, more labile organic components leaving behind black carbon or Stable Polycyclic Aromatic Carbon (SPAC). SPAC is composed of clusters of eight or more aromatic carbon rings. Because it is highly chemically stable it is presumed to give radiocarbon ages which are close to the age of the sediment. With the support of a generous AINSE grant we dated SPAC from a variety of depths.
Disappointingly, all the results clustered in a tight range around 7,000 years.
This means the SPAC itself was moving up and down through the sediment and mixing in the process returning an average age for the sediment. This pattern contrasts with the increase in age with depth shown by OSL techniques. The alternating wet and dry seasons experienced at Table Top Swamp cause the water table to fluctuate with the seasons. The evidence suggests SPAC is being moved through the sediment profile as the water table fluctuates. The take-home message is that the SPAC technique of isolating charcoal for radiocarbon dating may not be appropriate to apply to sites where the water table fluctuates seasonally.
After completing a B. Sc. at the Australian National University Will pursued a successful career in small business management. Returning to academia Will completed a M. Phil at University of Wollongong. Will’s M. Phil research revealed changes in the environment and climate at a site near Darwin over the last 35,000 years. Building on his interest in the past climate variation Will is currently investigating how the climate of northern Australia varied over the last 130,000 years as his PhD project.