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 10Be to continent-sourced 9Be from two Fe–Mn crusts located in the northwest Atlantic in the vicinity of the origins of Northern Atlantic Deep Water (NADW). These initial 10Be/9Be ratios represent those of the ambient seawater at the time of crust deposition and are constant through the last 7 Ma. Previously published isotope records of radiogenic Nd and Pb isotopes in these crusts show a decrease in εNd and an increase in 206Pb/204Pb over the last 5 Ma. These changes are consistent with a gradual increase in the strength of NADW production, an increase in the Labrador Sea component of NADW, or an increased erosional input into the Labrador Sea. Since the residence time and thus the advective length scales of dissolved Be in deep water lie between those of Nd and Pb, records of 10Be/9Be should, contrary to what has been observed, vary in phase with those of Nd and Pb if these variations are accounted for by changes in paleo-circulation. The observed variations in radiogenic Nd and Pb isotopes are therefore most likely to be explained with an increase in the flux of terrigenous matter in the area of the Labrador Sea, which was possibly associated with ice rafting and selective leaching of Pb isotopes. An increase of the Labrador Sea component that contributes to NADW is also a possibility, but based on the 10Be/9Be data presented here large-scale changes in the thermohaline circulation can be discounted as cause for the radiogenic tracer variations.