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43066-AC2
Warm Saline Deep Water Production in the Middle Eocene-Early Oligocene

Ellen E. Martin, University of Florida

The initial goal of this project was to use multiple proxies, including Nd and Pb isotopes to identify periods of warm, saline deep water production in the Southern Ocean during the Eocene and Oligocene. One of the primary challenges for this research has been technique development for the Pb isotopic work. Previous studies report Pb isotopic ratios recovered from ferromanganese crusts over Cenozoic timescales and extracted from dispersed Fe-Mn oxide coatings on bulk sediment from Pleistocene-Holocene samples. Thus, much of our focus to date has been modification and testing of extraction techniques to verify that Pb extracted from much older samples represents seawater. We are also evaluating whether fossil fish teeth, an archive commonly used for Nd isotopic studies in my lab, are robust archives for Pb as well. Funding from this grant has been crucial for Chandranath Basak, the PhD candidate working on the project, and has allowed my lab to expand into a new aspect of radiogenic isotopic studies.

After extensive evaluation of different size fractions, reagents and protocols we now have a technique that yields relatively reproducible ratios from an extraction of bulk sediment (Basak and Martin, 2007). The next step in the process will be to perform a sequential extraction procedure in order to gain a better understanding of Pb concentrations and isotopic ratios in each fraction of the sediment (carbonate vs. organics vs. oxide coatings vs. insoluble residue). This will allow us to evaluate the potential impact of contaminants during the extraction process. We are also starting to compile REE pattern data on the oxide extraction as another test of the purity of the seawater signal. Data from fossil fish teeth (bioapatite) indicate the teeth contain ~20 to 50 ppm Pb, meaning we can analyze samples as small as 50 μg. Analyses of teeth would eliminate the uncertainty generated by any extraction procedure.

Initial Nd and Pb isotopic data from the Oligocene at Site 1090 in the Southern Ocean produce a surprising inverse correlation between ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb vs. Ε _Nd (see “nugget”; Basak and Martin, 2007). In general, Nd isotopes record a mixed signal of water mass and local weathering that is dominated by the water mass signal, while Pb isotopes are dominated by local weathering inputs due to the short residence time of Pb in the oceans (50 – 200 yrs). Both the circulation and weathering signals represent a response to, and possibly forcing by, climate. A previous study of Nd isotopes at this site (Scher and Martin, in press) demonstrated that Ε _Nd variations record changes in the fluxes of Pacific and North Atlantic waters flowing opening of Drake Passage. The correlation between the Nd and Pb isotopes suggests the Pb isotopes are also recording changes in water mass abundance; an idea further supported by a ²⁰⁶Pb/²⁰⁴Pb - ²⁰⁷Pb/²⁰⁴Pb crossplot (see “nugget”) illustrating that the Site 1090 data plot on a mixing line between data from Oligocene Pacific and mid Atlantic ferromanganese crusts. Given the short residence time of Pb, we assumed the Pb signal would not propagate from the Pacific. Another exciting aspect of the data is that it supports the theory that a large shift in Nd isotopes at 41 Ma represents the initial opening of Drake Passage to shallow inflow (Scher and Martin, 2006). Specifically, the single data point prior to 41 Ma falls off the mixing line connecting Pacific and Atlantic values, indicating the presence of different water masses at this time. We plan to generate a higher resolution record at this site plus sites 689 and 690 to determine how the two isotopic systems respond to changes in climate and weathering inputs, as well as potential intervals of warm, saline deep water.

Based on our results this study has evolved to focus more on general changes in ocean circulation in the Southern Ocean during a major greenhouse to icehouse transition, the Eocene/Oligocene (E/O). The Pb isotopes are adding interesting constraints to the Nd isotopic data. Along these lines we recently ordered new samples from Site 738 on Kerguelen Plateau, where Nd isotopes record two negative shifts at the E/O boundary associated with short pulses in IRD. It is expected that the Pb isotopes may record an even more pronounced response to this purported local weathering input. In addition, analyses of the Pb isotopes in the IRD compared to the seawater signal should yield information about the sources and extent of physical (IRD) and chemical (seawater) weathering of Antarctica during the development of large ice sheets on this continent.

For the past year the development of the Pb technique has consumed most of our efforts. Currently we are posed to progress on the rest of the data for this proposal. We will continue this work under a one year no-cost extension.

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