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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 ( 10 Be, 230 Thex, 230 Thex/ 232 Th, 234 U/ 238 U) in a Fe-Mn crust dredged at 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. ( 230 Thex), ( 230 Thex/ 232 Th), and Be data give very consistent growth rates. In particular, the good match between ( 230 Thex) and ( 230 Thex/ 232 Th) data indicates that at the location of crust 121DK, 230 Th and 232 Th fluxes in the water column change simultaneously and suggests that the normalization of 230 Thex to 232 Th makes ( 230 Thex/ 232 Th) a better chronometer. Our best-fit model suggests that crust 121DK experi- enced changes in growth rates at 122 and 312 ka and a growth with a constant 230 Th initial flux. This chronology returns an age of 680 ka for the uppermost 1.5 mm. The ( 234 U/ 238 U) depth profile, however, was clearly affected by diffusion of 234 U 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 ( 230 Thex/ 232 Th) 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 ( 230 Th/ 232 Th) activity ratio of crust 121DK is entirely consistent with advection of deep water from the western toward the eastern Atlantic basin. Copyright © 2005 Elsevier Ltd |
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