Excess protactinium in phosphate nodules from a coastal upwelling zone

Measurements of 231Pa/ 235U and 230Th/ 234U activity ratios have been made for a suite of Quaternary age phosphate nodules from the upwelling zone off Peru and Chile. Many of these samples contain more 231Pa than would be predicted based on closed-system 230Th/ 234U ages unlike samples from non-upwelling environments which have been shown to have concordant 231Pa and 230Th ages. Peru/Chile phosphate nodules apparently pick up 231Pa preferentially to 230Th. This is most likely a result of prior fractionation of protactinium and thorium in the open ocean. Highly productive coastal upwelling zones may be added to the growing list of localities where fractionation between thorium and protactinium isotopes has been observed in marine sediments.