43066-AC2
Warm Saline Deep Water Production in the Middle Eocene-Early Oligocene
Over the past year we have continued to work on a clean extraction procedure for seawater Pb isotopes preserved in marine sediments. This is a tricky problem, but Chandranath Basak, the graduate student working on the project, has been willing to tackle the problem meticulously, and is getting interesting and intriguing results that will form the basis of his PhD dissertation. In the last year of a no-cost extension, the grant continues to support Chandranath and most of his analytical costs. Derrick Newkirk, another graduate student in the lab, was partially supported by and contributed to the grant this summer.
Results from a sequential extraction procedure on eight Ocean Drilling Program (ODP) samples designed to determine the potential contributions of Pb and Nd from Fe-Mn oxides, carbonate, organics, and insoluble residues illustrated that approximately 76% of Nd resides in the residue, 12% is in the Fe-Mn oxide leachate, and 10% in the carbonate. In comparison, approximately 82% of the Pb resides in the residue, with most of the remainder in the carbonate (15%). These results indicate that carbonates contain the largest fraction of the easily extractable Pb. Our initial procedure to extract Pb from authigenic Fe-Mn oxides followed a published protocol applied to Pleistocene sediments (Gutjahr et al., 2007); however, we discovered that optima-grade sodium acetate used in a buffered acetic step to remove the carbonate contained a high blank, leading to results with poor reproducibility. An expensive, time consuming chelating step to reduce the Pb blank eventually produced reproducible results from ODP Site 1139. In the interim, Gourlan et al. (2008) published a paper using 1N acetic acid dissolution of bulk sediments to produced deep-sea Nd isotopic values. They suggested this procedure might also yield deep-water Pb isotopic data. It is easy and inexpensive to produce clean 1N acetic acid, therefore we reran samples using this protocol and discovered that the bulk acetic leach accurately reproduced the results from the super-clean reagent extraction at Site 1139. In addition, all measured “seawater” values were more radiogenic than contemporaneous residues, as expected based on work by Harvlan et al. (1998) and Foster and Vance (2006).
Despite these promising results, we wanted to verify that dissolution of a bulk sample in weak acid would produce a deep-water signal, given that the soluble carbonate fraction is dominated by planktonic foraminifera with a surface water signal. One of the most difficult aspects of developing and testing techniques to acquire seawater Pb isotopes is that there are so few data available from the modern ocean to ground truth results because the concentration of Pb is so low in seawater and the surface ocean is contaminated by anthropogenic Pb. Therefore, variations in Pb isotopes from the surface to the seafloor are unknown. It is feasible that Pb isotopes may be poorly stratified in the ocean because Pb is very particle reactive and is predominantly sourced to the ocean through surface inputs. We are currently comparing Nd and Pb isotopes of cleaned planktonic foraminifera (surface), benthic foraminifera (deep), and fossil fish teeth (deep) separates. The Nd isotopes should yield distinct surface and deep values (Vance and Burton, 1999; Klevenz et al., 2008), thereby that the water column is homogeneous with respect to Pb.
Based on the reproducibility of the Pb isotopic data from the 1N acetic acid dissolution, we also ran a test study on ODP Site 738 on the Kergulen Plateau. We’ve been anxious to collect Pb isotopic data from this site for comparison to a transient shift in Nd isotopes documented by Scher et al. (2007). Preliminary data illustrate that secular variations in Pb isotopes track the Nd isotopic excursion (see TOC), which is believed to represent a weathering pulse as ice sheets developed on Antarctica. A 206Pb/204Pb versus 207Pb/204Pb crossplot (reproduced in the “nugget”) yields very promising results suggesting a change in the Pb source during the transient event.
Chandranath Basak is in the third year of his PhD program and is anxious to start getting publications out. Results from this research will be the basis for three papers. The first will be a technical report on the extraction of a pure, reproducible seawater Pb isotope signal from marine sediments, the second will provide data on the vertical structure of the Pb isotopes in the water column, and the third will be a study of the weathering pulse at Site 738.
References:
Foster and Vance (2006) Nature, 444, 918-921.
Gourlan et al (2008) Earth and Planet. Sci. Letts. 267(1-2), 353-364.
Gutjahr et al. (2007) Chem. Geol. 242, 351-370.
Scher et al. (2007) 9th ICP, Shanghai, P 4-58. Vance and Burton (1999) Earth and Planet. Sci. Letts., 173, 365-379.
