Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: Proxies for trace metal paleosources and paleocean circulation

Abstract

We report high precision Pb isotopic data (2σ ≤ 100 ppm) together with Nd isotopes on depth profiles from two Fe–Mn crusts from the eastern Atlantic basin. The profiles provide a 13 Ma record of changes in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean Outflow Water (MOW). Pb isotope ratios in the two records display gradual and systematic changes, tracking each other through time. The highly precise Pb isotopic data also resolve, for the first time, changes in all three Pb isotope ratios in a single Fe–Mn crust, and exhibit well-defined binary mixing lines in Pb isotope space. The Pb isotopic record of the Tropic Seamount crust shows that eastern Atlantic Pb has been dominated by binary mixing throughout the last 13 Ma. The Pb binary mixing lines further demonstrate that a change in Pb provenance to the eastern Atlantic occurred at 8 and 4 Ma. This is shown by the distinctive Pb isotopic compositions of the mixing components in the time intervals 4–0 Ma and 13–8 Ma. Consideration of Pb and Nd isotope systematics show that from 4 Ma to the present, the two endmember components correspond to NADW and Southern Component Water (SCW). The quasi-cyclic character of 206Pb/ 204Pb variations, as well as age progressive changes in the Pb mixing proportions of NADW and SCW endmembers, appear to reflect changes in deep water circulation. In particular, strengthening of the Pb and Nd isotopic signal associated with NADW since 3–4 Ma in both the eastern and western Atlantic basins implies that NADW advection from the western into the eastern Atlantic has been in operation over the past 4 Ma. During the period from 13 to 8 Ma, two entirely different sources of Pb and Nd existed in the eastern Atlantic. The radiogenic Pb and unradiogenic Nd component may have originated in the Norwegian–Greenland seas and the low 206Pb/ 204Pb—high ε Nd endmember component in the Southern ocean or Tethys. This suggestion is supported by evidence from paleoceanographic studies indicating the importance of Greenland–Scotland ridge activity in Miocene deep water production, and also the presence of a distinctive high δ 13C water mass in the Southern Ocean, derived either locally or flowing in via Tethys. The 8 Ma record of the Lion Seamount crust (65GTV) demonstrates that Pb and Nd sources in MOW switched from predominantly internal, Mediterranean (European) sources prior to 4 Ma, to mainly external (Saharan) sources after 4 Ma. The gradual increase of Pb isotope ratios seen following the end of the Messinian reflects enhanced input of Saharan dust into the water column in the eastern Atlantic. The strengthening of the “Saharan” isotopic signal from about 4 Ma also matches the documented increase in aridity and dustiness in the Saharan and sub-Saharan regions and coincides with the re-establishment of water exchange between the Atlantic and Mediterranean after the Messinian. Although Pb and, to a lesser extent, Nd isotope ratios are distinct during the Messinian (6.5–5 Ma), there is no clear evidence for either a shutdown of MOW or a stronger North Atlantic signal during this period. The fact that a similar isotopic signal is observed, with an even higher amplitude in the ENADW record, shows that this signal is a feature of the whole eastern Atlantic. From 3 to 4 Ma ago, a source of radiogenic Pb and unradiogenic Nd appears to have dominated not only the eastern Atlantic but the world oceans, since it is seen ubiquitously in other Fe–Mn crusts at this time. This isotopic signal must be conveyed around the globe via the ocean circulation or the atmosphere. While the closure of the Panama gateway may have played an important role in these global changes in source(s) and/or fluxes of Pb and Nd to the oceans 3–4 Ma ago other events, in conjunction, such as the emergence of the Greenland–Scotland ridge, were probably also as influential.