Climatic Control of Riverine and Seawater Uranium-Isotope Ratios

The large variation in the ratio of uranium-234 to uranium-238 (^(234)U/^(238)U) in rivers is not well understood, but may provide information about past weathering and rainfall and is important because it controls seawater (^(234)U/^(238)U). Here, we demonstrate the importance of physical weathering and rainfall for (^(234)U/^(238)U), using rivers from South Island, New Zealand. These data allow interpretation of an existing speleothem (^(234)U/^(238)U) record and suggest that New Zealand glacier advance 13,000 years ago was influenced by increased rainfall rather than by Younger Dryas–like cooling. A model of seawater (^(234)U/^(238)U) during glacial cycles indicates that rejection of corals based on modern (^(234)U/^(238)U) ± <0.01 is not merited and may reject the highest quality ages.