Abouchami and Goldstein (1995) showed that Pb isotopes in Circum-Antarctic Fe-Mn sediments display systematic geographical variations. Based on the consistency of Nd and Pb isotopic variations with present-day ocean circulation, they suggested that Pb isotopes could be a useful paleo-oceanographic tracer. Indeed, major reorganisations of the ocean-atmosphere system along with climatic changes during the Pleistocene are indicated by the 18O record of foraminifera. Short-term fluctuations in circulation patterns would lead to changes in the fluxes, sources, and distributions of elements in the oceans. Our goal is to evaluate these effects using the Pb isotopic record of a Fe-Mn crust from the Central Pacific dated at high resolution.
Hydrogenous Fe-Mn crusts grow slowly (few mm/Ma), mainly through the supply of Fe and Mn from the water column, which are related to environmental factors (biological productivity, CCD, O2 content...). Therefore, only the uppermost millimeter would correspond to the record of the late Quaternary.
The Mn crust VA13-2 (146°W, 9°25'N, 4830m) is unique. It has been dated by 10Be (Segl, 1984), 230Th (Eisenhauer et al., 1992), and TIMS U-Th (Eisenhauer et al., in prep.). The TIMS U-Th ages cover the last three glacial-interglacial intervals. To achieve a time resolution of ~4 ka, Pb isotopic measurements were performed on samples collected at 0.02 mm depth intervals over 1.5 mm. Pb isotope ratios and concentrations were measured simultaneously using a 205Pb spike.
Resolvable 206Pb/204Pb isotopic variations are observed from 200 ka to present-day whereas no detectable Pb isotopic change can be seen from 400 to 200 ka. There is also a secular decrease of 206Pb/204Pb ratios from ~72 ka to ~18 ka and decreases of Pb isotope ratios are observed toward the end of interglacial stages, at 90 ka (stage 5) and at ~200 ka (stage 7). At 90 ka, it coincides with the transition between fast (6.4 mm/Ma) and slow (3 mm/Ma) growth rate periods as reported by (Eisenhauer et al., in prep.). Long-term changes in Pb provenance are resolvable over the last 70 ka, whereas short-term excursions occur near interglacial-to-glacial transitions.
Large variations of Pb concentrations occur through time, with a general increase from ~200 ka to ~18 ka, and comparatively smaller variations from 400 to 200 ka. Sharp decreases of Pb content occur near interglacial-to-glacial boundaries, except between stages 3 to 2, where the Pb content reaches a maximum. The variations of Pb concentrations at these transitions might be related to changes in the Pb provenance or flux as a consequence of changing climate. The increase during glacial stages, following the initial drop of Pb content, might reflect the readjustment of the oceans to new climate conditions.
Mn and Fe have been analysed on a different drill-hole from the same piece of crust, with a depth resolution of 0.03mm (Eisenhauer et al., 1992). Pb variations patterns match the Mn and Fe patterns if the depths are offset by 0.06mm which corresponds to an age correction of 12 ka. The striking similarity between the three profiles suggest that Pb, Mn and Fe behaviours are strongly coupled in the oceans. The constant offset reflects a small difference in the zero-point of the Mn and Fe drill-cores relative to the Pb core and it is neither due to inhomogeneity of the layer thicknesses in the two cores or to a difference in the response time of Pb compared to Mn and Fe, because it is unlikely that Pb would respond faster than Mn and Fe to changing climate.
We conclude that the secular decrease of Pb isotope ratios over the last 70 ka and the short-term excursions during interglacial periods indicate a change in Pb provenance. The decrease of Pb near interglacial-to-glacial stages is coupled with a decrease of Mn and Fe. This is consistent with a decrease of MnO2 precipitation at these times (Mangini et al., 1990) and/or control of trace metals in the oceans by Mn-oxide phases (Sherrell and Boyle, 1992).
Abouchami, W. & Goldstein, S.L., Geochim. Cosmochim. Acta 59, 1809-1820 (1995).
Eisenhauer, A., Gögen, K., Pernicka, E. & Mangini, A., Earth Planet. Sci. Lett. 109, 25-36 (1992).
Eisenhauer, A., Chen, J.H. & Wasserburg, G.J., (in prep.).
Mangini, A., Segl, M., Glasby, G.P., Stoffers, P. & Plüger, W.L., Marine Geol. 94, 97-107 (1990).
Segl, M. et al., Nature 309, 540-543 (1984).
Sherrell, R.M. & Boyle, E.A., Earth Planet. Sci. Lett. 111, 155-174 (1992).