U-Th chronology and paleoceanographic record in a Fe-Mn crust from the NE Atlantic over the last 700 ka

Hydrogenetic ferromanganese crusts (Fe-Mn crusts) provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes. In this respect, the acquisition of an absolute and reliable chronology in Fe-Mn crusts is a prerequisite. Here we combine four different and complementary chronometers (Be-10, Th-230(ex), Th-230(ex)/Th-232, U-234/U-238) in a Fe-Mn crust dredged at similar to 2000 m depth in the east Atlantic to first establish a reliable chronology over the Quaternary period. Then, we use EDS chemical analysis to look for correlation between major element chemistry and climate changes. (Th-230(ex)), (Th-230(ex)/Th-232), and Be data give very consistent growth rates. In particular, the good match between (Th-230(ex)) and (Th-230(ex)/Th-232) data indicates that at the location of crust 121DK, Th-230(ex) and Th-232 fluxes in the water column change simultaneously and suggests that the normalization of Th-230(ex) to Th-232 makes (Th-230(ex)/Th-232) a better chronometer. Our best-fit model suggests that crust 121DK experienced changes in growth rates at similar to 122 and 312 ka and a growth with a constant Th-230 initial flux. This chronology returns an age of 680 ka for the uppermost 1.5 mm. The (U-234/U-238) depth profile, however, was clearly affected by diffusion of U-234 in the porous crust and can therefore not be used to derive a reliable chronology. One part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity. On the basis of the (Th-230(ex)/Th-232) chronology, major element chemistry is shown to be linked to climate change. Mn/Fe variations compare well with those in a Fe-Mn crust from the Pacific, showing systematic maxima during glacial stages 2 and 4. High Mn/Fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods, resulting from higher bioproductivity. In addition we note that the surface (Th-230/Th-232) activity ratio of crust 121DK is entirely consistent with advection of deep water from the western toward the eastern Atlantic basin. Copyright (c) 2005 Elsevier Ltd.