An excess Ra-226 chronology for deep-sea sediments from Saanich Inlet, British Columbia

To further explore the efficacy of 226Ra(excess) dating for deep-sea sediments, previously dated varve sediments from Saanich Inlet were investigated. Ages obtained using 226Ra(excess) are comparable to the varve ages in the upper 20-25 m of the sedimentary record, but radiometric ages for those sediments older than c. 4000 yr BP are significant underestimates. This results from major changes in sedimentation within Saanich Inlet around 4000 yr BP linked to rising sea levels, with younger sediments characterised by a higher biogenic contribution resulting from the establishment of an anoxic fjord environment. The older sediments were deposited in a shallow water inlet characterised by variable Ra mass balance and non-radiogenic losses. Therefore, while 226Ra(excess) can produce reliable dates, its application may be limited where the relative significance of authigenic and allogenic input and bottom water anoxia have been variable and where closed-system behaviour is compromised.