HR: 1340h
AN: PP33A-0914    [Abstracts]
TI: An Excess $^{226}$Ra Geochronology for Saanich Inlet
AU: * Grayson, R P
EM: r.p.grayson@liverpool.ac.uk
AF: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT United Kingdom
AU: Plater, A J
EM: gg07@liverpool.ac.uk
AF: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT United Kingdom
AU: Boyle, J F
EM: jfb@liverpool.ac.uk
AF: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT United Kingdom
AU: Appleby, P G
EM: Appleby@liverpool.ac.uk
AF: Department of Geography, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZT United Kingdom
AB: When examining past environmental and climatic change the ability to date both freshwater and marine sediments is essential. Numerous methods are currently available, however where present techniques fail, novel dating techniques are required to overcome these obstacles. In particular the use of $^{14}$C to date sediments over the Holocene period can be severely limited where there are significant hard-water and reservoir effects and where reworking of organic matter occurs. The use of unsupported or excess $^{226}$Ra offers a potential dating tool in such depositional contexts over the Holocene time period as a result of its half-life of 1600 years. Indeed, the technique has been applied to a number of environments since first being proposed by Koide et al. (1976). However, numerous question marks still surround the application of the technique, particularly with reference to the dating of sub-aqueous sediment deposits. As part of wider research into the applicability of excess $^{226}$Ra to date lake sedimentary environments, deep-sea samples from Saanich Inlet were obtained from the ODP (Leg 169S) in order to validate the dating technique. Saanich Inlet is an anoxic fjord located on the coastline of Vancouver Island, British Colombia, the sediments of which have been shown to be comprised of lamina-scale alternations between diatom-rich and diatom-poor clays and silts representing annual. Using digital sediment colour analysis Nederbragt and Thurow (2001) constructed a varve chronology for the last 6000 years. Alpha-spectrometry was successfully used to measure $^{226}$Ra, $^{230}$Th and $^{234}$U in sediment samples collected from Saanich Inlet during ODP Leg 169S. These activities were used to determine an excess $^{226}$Ra geochronology for Saanich Inlet. This radiometric chronology compares well with dates obtained from the varve record, and spans two different phases of sediment accumulation.
DE: 4860 Radioactivity and radioisotopes
DE: 4267 Paleoceanography
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting