The long-lived uranium decay products 230Th and 231Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230Th and 231Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230Th-normalization and to demonstrate enhanced 231Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231Pa is apparent. The 231Pa/230Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation.
|Original language||English (US)|
|Number of pages||13|
|Journal||Deep-Sea Research Part II: Topical Studies in Oceanography|
|State||Published - Jun 1 2015|
Bibliographical noteFunding Information:
Funding for ship time, sampling operations, and hydrographic data was provided by the U.S. National Science Foundation to the US GEOTRACES North Atlantic Transect Management team of W. Jenkins ( OCE-0926423 ), E. Boyle ( OCE-0926204 ), and G. Cutter ( OCE-0926092 ). Radionuclide studies were supported by NSF ( OCE-0927064 to L-DEO, OCE-0926860 to WHOI, OCE-0927757 to URI, and OCE-0927754 to UMN). LFR was also supported by a Marie Curie Reintegration Grant and the European Research Council . The crew of the R/V Knorr , the Ocean Data Facility team (Mary Johnson, Rob Palomares, Susan Becker, Melissa Miller and Courtney Schatzman), and the science team samplers for Niskin bottles and in situ pumps (Katharina Pahnke, Brett Longworth, Paul Morris, Daniel Ohnemus, Kuanbo Zhou, Sylvain Rigaud and Stephanie Owens) are all acknowledged for their critical roles in the success of these cruises. On-shore analysis efforts of Maureen Auro, Joanne Boudreau, and the WHOI Plasma Facility are greatly appreciated. Figs. 1, 3 and 7 were created using Ocean Data View ( Schlitzer, 2011 ). We thank Alex Thomas and an anonymous reviewer for improving the quality of the manuscript.
- North Atlantic Ocean