A depth profile of uranium-236 in the Atlantic Ocean

In this study the first two oceanic depth profiles of 236U sampled in the western equatorial Atlantic Ocean are presented. The measured 236U/238U ratios decrease from about 10−9 at the surface down to about 10−10. Even the lowest ratios measured below 4000 m depth are more than three orders of magnitude above the estimated natural 236U/238U level for the pre-anthropogenic ocean. This clearly indicates that anthropogenic 236U already has reached the deep Atlantic Ocean. Three different conceptual models are applied to identify the relevant processes capable of transporting significant amounts of 236U from the surface into the deep ocean. While the vertical transport of particulate U is excluded as a significant source, box model calculations suggest that North Atlantic Deep Water production with some minor contribution of 236U from nuclear reprocessing as the most likely source for 236U in the deep western equatorial Atlantic Ocean. Our results show that 236U has a large potential as a new, conservative, and transient tracer in Oceanography.