Weathering versus circulation-controlled changes in radiogenic isotope tracer composition of the Labrador Sea and North Atlantic Deep Water

Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciation, 2.7 Myr ago, and convection o f Labrador Sea Water (LSW) were caused by the closure of the Panama Gateway similar to 4.5 Myr ago. Time series data that have been obtained from stud ies of ferromanganese crusts from the northwestern Atlantic suggest that ra diogenic isotopes of intermediate ocean residence time (Pb and Nd) can serv e as suitable tracers to reconstruct these events. However, it has been unc lear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulat ion or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic is otope compositions are unlikely to be due to changes in convection in LSW b ut can be explained in terms of increases of erosion levels due to the glac iation of Greenland and Canada. Furthermore, we provide experimental eviden ce for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents an d accounts for the paradox that one of the areas of the world most deficien t in radiogenic Pb should provide such a rich supply of radiogenic Pb to th e oceans. An important general conclusion is that the compositions of radio genic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in rad iogenic tracers in seawater.