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

Friday, 19 December 2014
Martin Q Fleisher1, Christopher T Hayes2, Robert F Anderson1, Phoebe J Lam3, Daniel Ohnemus4, Kuo-Fang Huang5, Laura F Robinson6, Yanbin Lu7, Hai Cheng7,8, R. Lawrence Edwards7 and S Bradley Moran9, (1)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)Woods Hole Oceanographic Inst, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (4)Bigelow Lab for Ocean Sciences, East Boothbay, ME, United States, (5)Woods Hole Oceanographic Inst, Woods Hole, MA, United States, (6)University of Bristol, Bristol, United Kingdom, (7)University of Minnesota, Minneapolis, MN, United States, (8)Xi'an Jiaotong University, Xian, China, (9)University of Rhode Island, Kingston, RI, United States
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.