Advection and mixing of deep water masses in the tropical and western North Atlantic Ocean from thorium and protactinium isotopes

• Applicant: Dr. Sven Kretschmer
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2011 to 2014
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 199971746

Natural radionuclides are powerful tracers to identify and quantify transport processes of particles and of water masses in the ocean. The distribution of the isotope pair 231Pa/230Th in Atlantic sediments has been widely used to trace past deep water ventilation. The formation area of deep water in the NW Atlantic has been considered the best example where the sediment record of this tracer can help unravel the history of Atlantic Meridional Overturning Circulation (AMOC). It came as a surprise that recent findings showed a discrepancy between present day ventilation and the 231Pa/230Th signature in surface sediments, which may be due to an unexpectedly large influence of particle composition on the differential removal of the two isotopes to the sediments. The NW Atlantic thus is now a test case for the applicability of 231Pa/230Th as paleocirculation tracer. Here I propose to study the scavenging behaviour of Th and Pa during the early stages of the formation of North Atlantic Deep Water and during its distribution in the tropical Atlantic by analyzing a comprehensive set of dissolved and particulate samples collected in 2010 along the sections of two cruises executed with RV Meteor (M81/1) (GEOTRACES Cruise A11, Canaries – Port of Spain, Trinidad and Tobago in February/March 2010) and during RV Pelagia expedition 64PE319/321 (GEOTRACES Cruise A02, Scrabster, Scotland – Fortaleza, Brasil, April - July 2010) in the tropical and western North Atlantic. These cruises were official GEOTRACES expeditions uniquely focused on the use of ultra trace metal clean sampling facilities. The high-resolution distribution of a large suite of other trace elements and their isotopes (TEIs) will provide the best achievable background to understand the possible influences controlling the scavenging process of trace metals and to investigate possible correction procedures to improve the 231Pa/230Th AMOC ventilation proxy.

• DFG Programme: Research Grants