Proxy Applications of Pa/Th Investigated with Scavenging Chemistry in the North Atlantic

The natural radionuclides ²³¹Pa and ²³⁰Th have potential value as proxies of past biological productivity in the marine sediment record. In addition to its use as a circulation proxy, the particulate Pa/Th ratio has been suggested to monitor total particle flux and/or diatom productivity via processes related to the scavenging, or the adsorptive removal of these elements onto particles. We investigate the nature of scavenging using trans-Atlantic measurements from GEOTRACES of dissolved (<0.45 µm) and particulate (0.8-51 µm) ²³¹Pa and ²³⁰Th, together with major particle composition. We find widespread impact of intense scavenging by authigenic Fe/Mn oxides, in the form of hydrothermal particles emanating from the Mid-Atlantic ridge and particles resuspended from reducing conditions near the seafloor off the coast of West Africa. Biogenic opal was not a significant scavenging phase for either element, essentially because of its low abundance at the studied sites.

In the context of the paleo-record, the particulate Pa/Th ratio responds most significantly to scavenging intensity, caused by either biotic or abiotic processes. In the modern setting at least, the influence of North Atlantic Deep Water circulation on Pa/Th is apparently outweighed. The Pa/Th proxy, therefore, is best used in conjunction with other information to support the cause for past changes in scavenging intensity.