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Work for this proposal focused on development of a chemical extraction technique to measure seawater Nd and Pb isotopes from bulk marine sediments in the Southern Ocean.  These records then allowed us to study changes in continental weathering intensity and style associated with the greenhouse-to-icehouse transition at the Eocene-Oligocene Boundary (EOB), when continental-scale ice sheets were established on Antarctica. 

A comparison between Nd isotopes derived from fossil fish teeth at Ocean Drilling Program sites 689 and 690 and data from chemical extractions of Eocene to Oligocene sediments from contemporaneous samples illustrated excellent agreement between the two archives when the bulk sediment was treated with a buffered acetic to remove the carbonate and then leached in 0.02 M hydroxylamine hydrochloride (HH) for two hours.  These results plus studies of rare earth element (REE) patterns from sequential extractions of bulk sediments: fish teeth, carbonate (acetic) fractions, oxides (HH fraction), and detrital or residual fractions, represent an important contribution to a paper titled “Extraction of Nd isotopes from bulk sediment for paleoceanographic studies on Cenozoic time scales” submitted to Chemical Geology.  Support from this grant is acknowledged in the paper, which is currently out for review.

Verifying that Pb isotopes from the chemical extraction represent seawater has been a trickier task, in part because there are no other archives known to record seawater values, thus there is no baseline for comparison.  In addition, Pb isotopes are easily contaminated by various chemicals used in the extraction procedure.  After much experimentation we have developed a technique using chelex 100-cleaned reagents that produces reproducible Pb isotopic results. 

Sequential extraction techniques allowed us to evaluate potential contributions of Nd and Pb from various components in the marine sediment.  Initial results indicate that ~80% of the Nd is derived from the HH fraction.  In contrast, ~55% of the Pb is found in the residue, ~35% in the carbonate fraction, and 10% in the HH fraction.  REE patterns from each fraction verify that the HH extraction technique does not attack the residue fraction.

We also tested phosphatic fossil fish teeth as archives for Pb.  The advantage of this archive is that we would know the exact source of the Pb, also clean acids can be used for dissolution.  The disadvantage is that in contrast to Fe-Mn oxides, the U/Pb ratio in phosphates is high enough to require a correction for in-growth of radiogenic Pb for early Cenozoic sediments.  A study of ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb for Eocene to Oligocene sediments from Site 1090 in the Southern Ocean (Atlantic sector) illustrated that extracted oxide samples formed a tight cluster, indicating a constant Pb source.  In comparison, contemporaneous fish teeth samples corrected for radiogenic in-growth showed similar values, but greater scatter.  We are currently investigating whether the scatter is due to initial common Pb in the teeth or diagenesis.

Recently Gourlan et al. (2008) reported that Nd isotopes acquired by leaching bulk sediments in 1N acetic yielded seawater isotopes and suggested the same technique might work for Pb.   This is potentially a very clean method for extracting Pb isotopes, but surface water Nd and Pb isotopes from the carbonate (largely planktonic foraminifera) are likely to alter the deep water signature from the oxide coatings.  Mass balance calculations indicate that this contamination will be a more significant problem for Pb.  Our experiments demonstrate that reductive cleaning of planktonic foraminifera tests reduces the concentrations of Pb and Nd in the sample, and at some sites cleaned planktonic foraminifera record Pb isotopic values that are clearly distinct from uncleaned planktonic and benthic foraminifera.  But at some sites cleaned and uncleaned planktonics and benthics all record the same value, suggesting that either the water column is not stratified in these areas, or the cleaning does not effectively remove oxides from the carbonate.  A paper to Geochimica Cosmochimica Acta acknowledging the support of ACS-PRF is currently in preparation outlining the techniques required to produce uncontaminated Pb isotope values from chemical extractions of Cenozoic sediments, the distribution of Pb in these samples, and the potential for using a 1N acetic leach as a preparation technique or fish teeth as additional archives.

Establishment of a clean technique for extracting deep water Pb isotopes meant we could start investigating changes in Pb isotopes across documented excursions in Nd isotopes and ice-rafted debris (IRD) at the EOB in Site 738 in the Southern Ocean (Indian Sector).  The Nd excursion has been attributed to a mechanical weathering pulse related to glaciation of Antarctica (Scher et al., 2007).  The ²⁰⁶Pb/²⁰⁴Pb record demonstrates greater variability during the two Nd excursions and IRD pulses. Surprisingly, much larger shifts in Pb isotopes are associated with the smaller of the two Nd anomalies (upper nugget figure). 

A comparison of ²⁰⁶Pb/²⁰⁴Pb values from the HH extraction (seawater) and residues (detrital fraction) corroborated our theory that differences between these two archives might record continental weathering styles.  This theory is based on the idea that mechanical erosion leads to incongruent weathering of Pb, with preferential release of radiogenic Pb (Erel et al., 1994; Harlavan et al., 1998) that is transferred to local seawater, accounting for distinct Pb isotopes in the seawater and detrital fractions.  In contrast, congruent, chemical weathering should transfer the whole rock signal to the ocean, producing similar seawater and detrital isotopes.  Initial results (lower nugget figure) illustrate for the first time that the two fractions do record the same value during the late Eocene greenhouse and distinct values after the Antarctic ice sheet reached the ocean.  We plan to present these results at the Fall American Geophysical Union meeting (December 2009).   In addition, these preliminary results helped us secure three years of funding (2009-2012) from NSF.  We are currently working to produce more compete records and evaluate the relationship between weathering style and atmospheric CO₂ in preparation for submission to Earth and Planetary Science Letters.

Erel Y., et al. (1994) GCA 58, 5299–5306.
Harlavan Y., et al. (1998) GCA 62, 33–46.
Scher H.D., et al. (2007) 9th Int. Conf. Paleoceanography, Shanghai, China, P-4-58.