Pb Isotopes from Paleogene Fe-Mn Oxide Coatings from the Southern Ocean as a Proxy for Deep Water Circulation and Weathering Intensity.
Pb isotopes extracted from Fe-Mn oxide coatings on bulk sediment are investigated as proxies for seawater Pb isotopes on Cenozoic time scales. Previous studies investigating long-term variations in weathering inputs and circulation patterns used Pb isotopes derived from Fe-Mn crust, but these crusts grow very slowly, are difficult to date, and have limited spatial resolution. Due to the slow growth rates they record a signal that is averaged over ~ 104-105 years, limiting the application to studies of long term changes. Initial results suggest that Pb isotopes extracted from Fe-Mn oxide coatings may be a proxy for more rapid variations in weathering and circulation. Comparison between the Pb isotopic values derived from these coatings and from fossil fish teeth for ~25 Ma old samples from ODP Site 1090 on the Aghulus Ridge (Southern Ocean) shows similar values, although there is a small systematic offset when plotted in 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs.206Pb/204Pb space. This may be due to initial Pb concentrations in the hydroxyapatite of fossil fish teeth. Initial data from oxide coatings indicate a well-defined inverse correlation between values for ∊Nd compared to 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb for samples ranging from 15-45 Ma at ODP Site 1090. Previous studies argue that the Nd isotopic variations at this site are largely driven by circulation, specifically the opening of Drake Passage followed by the influx of North Atlantic waters in response to development of the Antarctic Circumpolar Current (ACC) (Scher and Martin 2004, 2006 and in review). This correlation between the two isotopic systems suggests the Pb isotopic record is also dominated by a circulation signal. A 207Pb/204Pb vs. 206Pb/204Pb plot illustrates that Pb isotopic values varied over this interval as a function of the relative influx of Atlantic and Pacific waters, with the introduction of less radiogenic Pacific values indicating opening of Drake Passage, followed by the introduction of more radiogenic Atlantic waters. It is intriguing that Atlantic Pb signals are carried as far as the Southern Oceans given the short residence time of Pb (50-200yrs). Increased variability of Pb isotopic data and a decreased correlation to ∊ Nd values from 20 Ma and younger may record variations in more local weathering inputs.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 4825 Geochemistry;
- 4860 Radioactivity and radioisotopes