9404614 KU The pair of radionuclides 230Th (half-life 75,200 years) and 231 Pa (half-life 32,500 years) produced by the decay of their respective U parents (234U and 235U) in seawater provide important information about the particle flux and productivity in the oceans. They have certain advantages over other proxy indicators of paleoproductivity: 1) The two nuclides have a constant initial production ratio. (2) They are little affected by post-depositional dissolution and mobilization. (3) When used as ratio 230Th/231Pa, they are relatively insensitive to sediment focusing or redistribution. On the other hand, this 230Th/231Pa ratio proxy is still a new tool needing sharpening and refinement. Its limitations and full potential remain to be defined and explored. The primary objective of this project is to conduct an in-depth investigation on this promising and important paleoceanographic proxy. The scavenging of 230Th and 231Pa will be studied in terms of particle composition which may affect the 230Th/231 Pa ratio in a sediment. A new approach will be employed to assess the distributions of 230Th and 231Pa between seawater and individual carrier phases (clay, opal, Mn02, etc.). The ratios in the individual phases should serve as a more effective index than those of a total sediment for deciphering the sedimentary record of productivity changes in the past. The study will be carried out in three oceanic areas: western Pacific-Okinawa Trough, the Arabian Sea, and regions under the California Current. The relationship between 230Th/231 Pa and the oxygen consumption rate in will be examined and the possibility of using the relationship as an empirical way of studying paleo-redox conditions will be explored. It is anticipated that the study will contribute to our understanding of the carbon cycles in the ocean and their response to climate change.
