Sediment Signatures of U- and Th-Series Nuclides and their Application as Paleoceanographic Tracers

This chapter discusses the long-lived U-series nuclides in sediments with an emphasis on their paleoceanographic applications. The chapter briefly discusses the short-lived nuclides and their application to modern sedimentation. The use of authigenic U as a proxy for sediment redox state, the applications of ²³⁰Th as a means of reconstructing the vertical rain of sediment, corrected for bias because of lateral sediment focusing and winnowing, and the use of ²³¹Pa/²³⁰Th ratios as tracers of particle flux and ocean circulation are presented subsequently. The U-series nuclides provide a critical set of tools for paleoceanographic research. They are used both for dating marine archives as well as understanding past particle flux, particle composition, and circulation in the ocean. The short-lived U-series nuclides ²¹⁰Pb and ²³⁴Th are used extensively in studies of contemporary sedimentation, specifically in determining rates of sediment accumulation and rates of sediment mixing. The tendency for uranium to precipitate in marine sediments under reducing conditions has led to the use of authigenic U as a proxy of sediment redox state. An important application of ²³⁰Th in paleoceanography is its use as a “constant flux proxy” to enable the reconstruction of sediment accumulation rates resulting from vertical deposition of material. As with all paleo-proxies, a thorough understanding of their behavior in the modern ocean is needed to properly interpret changes through time. At the same time, the generation of more paleo-records, particularly multi-proxy records, not only brings people closer to understanding Earth's climate on a variety of timescales, but can also reveal inconsistencies between tracers. Such inconsistencies prompt further investigation into the tracer's modern behavior.