Updates to the classic small volume (2-4L) 234Th analysis protocol

Samantha Jade Clevenger1,2, Ken Buesseler1, Claudia R Benitez-Nelson3, Jessica Drysdale4 and Steven M Pike5, (1)Woods Hole Oceanographic Institution, Department of Marine Chemistry & Geochemistry, Woods Hole, United States, (2)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (3)University of South Carolina, School of the Earth, Ocean, and Environment, Columbia, United States, (4)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (5)Woods Hole Oceanographic Institute, Woods Hole, United States
234Th measurements have been widely used in ocean sciences to study carbon removal processes in the upper ocean. Comparing the disequilibrium between the particle-reactive 234Th (half-life = 24.1 days) and its parent, 238U (half-life = 4.468×109 years), with a defined element to 234Th ratio on sinking particles can provide insight into the amount of material, such as carbon, exported via the biological carbon pump. We focused on further optimization of the classic small volume (2–4 L) procedure in which 230Th is added as a yield monitor, 234Th is scavenged with a MnO2 precipitate, and the precipitate is measured immediately via beta counting, with a final background beta count after 6 half-lives (144 days). Further experiments elucidate external factors that influence beta count rates including filtration time, volume of precipitation reagents used, as well as background interference by high-energy beta emitters from the 238U decay chain. Updates to the 230Th yield-monitor recovery procedure are also discussed. Additionally, a comprehensive accounting of 234Th and 230Th measurements at different points in the sample processing protocol indicate where sample loss can be best minimized. High temporal resolution measurements of beta count efficiency using 238U standards show the extent of measurement variation and uncertainty attributed to beta counters. Together, these updates present the most optimized and up-to-date techniques for accurately gathering high resolution 234Th data.