Response of beryllium and radiogenic isotope ratios in Northern Atlantic Deep Water to the onset of northern hemisphere glaciation

We present time-corrected ratios of cosmogenic Be-10 to continent-sourced B e-9 from two Fe-Mn crusts located in the northwest Atlantic in the vicinity of the origins of Northern Atlantic Deep Water (NADW). These initial Be-10 /Be-9 ratios represent those of the ambient seawater at the time of crust d eposition and are constant through the last 7 Ma. Previously published isot ope records of radiogenic Nd and Pb isotopes in these crusts show a decreas e in epsilon(Nd) and an increase in Pb-206/Pb-204 over the last 5 Ma. These changes are consistent with a gradual increase in the strength of NADW pro duction, an increase in the Labrador Sea component of NADW, or an increased erosional input into the Labrador Sea. Since the residence time and thus t he advective length scales of dissolved Be in deep water lie between those of Nd and Pb, records of Be-10/Be-9 should, contrary to what has been obser ved, vary in phase with those of Nd and Ph if these variations are accounte d for by changes in paleo-circulation. The observed variations in radiogeni c Nd and Pb isotopes are therefore most likely to be explained with an incr ease in the flux of terrigenous matter in the area of the Labrador Sea, whi ch was possibly associated with ice rafting and selective leaching of Pb is otopes. An increase of the Labrador Sea component that contributes to NADW is also a possibility, but based on the Be-10/Be-9 data presented here larg e-scale changes in the thermohaline circulation can be discounted as cause for the radiogenic tracer variations. (C) 1999 Elsevier Science B.V. All ri ghts reserved.