Acquisition of the neodymium isotopic composition of the North Atlantic Deep Water

[1] The North Atlantic Deep Water (NADW) neodymium isotopic composition (Nd IC) is increasingly used in oceanography and paleoceanography to trace large-scale circulation and weathering processes, notably to investigate past variations of the global thermohaline circulation. Although the present-day NADW Nd IC is well characterized at epsilon(Nd) = - 13.5, the acquisition of this isotopic signature ( in other words, the causes of this value) has so far been very sparsely documented. Such an understanding is, however, fundamental to the interpretation of paleo records. Nd IC and rare earth element concentrations were measured at 9 stations within the North Atlantic Subpolar Gyre ( SIGNATURE cruise, summer 1999). The comparison of this data set with our understanding of water mass circulation provides a description of how the three layers constituting the NADW, the Labrador Sea Water (LSW, epsilon(Nd) = - 13.9 +/- 0.4), North East Atlantic Deep Water (NEADW, epsilon(Nd) - 13.2 +/- 0.4), and North West Atlantic Bottom Water ( NWABW, epsilon(Nd) - 14.5 +/- 0.4), acquire their Nd IC through distinct water mass mixings and lithogenic inputs. These different mechanisms, acting upon water masses from very diverse sources, seem to bring the Nd IC of the three NADW layers to values close together and similar to that of the NADW. It is suggested that sediment/ seawater interactions significantly lower the NEADW and NWABW Nd IC along the South East Greenland margin. Since these interactions do not significantly modify the Nd content of these water masses, sediment remobilizations leading to the Nd IC variations are probably associated with Nd removal fluxes from the water mass toward the sediment, a process called boundary exchange. On the other hand, LSW seems to acquire its Nd IC from the Subpolar Mode Waters from which it is formed by deep convection, and no other mechanism needs to be invoked. Its unradiogenic signature could ultimately be linked to fresh water runoff from the Canadian Shield. These conclusions should allow more precise interpretations of paleoceanographic Nd IC records, taking into account the distinct histories of the three NADW layers, including distinct water mass mixings and distinct lithogenic inputs.