Holocene evolution of deep circulation in the northern North Atlantic constrained by sedimentary radiogenic tracers

In this study we measure the Sm, Nd and Pb isotope signatures of the clayey fraction of deglacial and Holocene sediments from 4 deep North Atlantic cores. Radiogenic isotopes are use to trace the particle provenance. Temporal changes in the relative contribution of the different source-areas bring information on the deep circulation pathways (e.g., Fagel et al., 2004, Paleoceanography and references therein). Our aim is to reconstruct the main deep circulation changes over the Holocene in the northern North Atlantic. We compare the isotopic signatures at the inlet of the Labrador Sea (MD99-2227, 3460 m) with two sediment cores retrieved along the gyres of North Atlantic Deep Water (NADW) components. Cores were collected along fractures zones on the Mid-Atlantic Ridge: MD99-2254 on the Eastern side of the Bight Fracture Zone (2440 m) and HU91-045-080 on the western side of the Charlie Gibbs Fracture Zone (CGFZ, 3024 m). An additional core HU91-045-091 (3870 m) was taken on the Western side of the CGFZ, in the Labrador Sea but out of the Present deep circulation gyre. Core HU91-045-091 is chatacterized by a stable Sm, Nd and Pb composition. Its location is under the influence of proximal supplies from Labrador Sea margins. We conclude that the core HU91-045-091 remains sheltered from the deep circulation gyre over the Holocene. In contrast, significant shifts in the radiogenic composition of the sedimentary fractions are evidenced during the last 6 kyr for the other sites. Cores HU91-045-080 et MD99-2254 from the Eastern Atlantic basins are both characterised by a sharp increase in the chemical Sm/Nd ratio and the Nd composition at ca. 6 kyr BP. Those geochemical changes reflect a different composition of the particles driven by the deep currents through time. They imply a reorganisation of the deep North Atlantic components in the Eastern North Atlantic basins. A similar shift was observed but later (at 3 kyr BP) in core MD99-2227. Those compositional shifts all record a higher contribution of volcanic-derived material relative to crustal supplies, suggesting higher relative contributions from water masses that follows or crosses the Mid-Atlantic Ridge topography, i.e., the North East Atlantic Deep water (NEADW) components, in the deep circulation gyre. Likely Pb isotope signatures of deep sediments significantly evolve during the Holocene. The sedimentary mixings record changes in the relative contribution of the crustal material from the northern North Atlantic margins. The Eastern supplies from Western European margin are progressively diluted by more contribution from northern Greenland or Scandinavian margins. The observed spatial and temporal changes in the Nd and Pb isotopic composition of the northern North Atlantic sediments confirm the variability of the deep circulation over the Holocene, with the inception of the Present deep circulation gyre during the last 3 kyr.

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