Basak, C. et al. (2011): Sea water Pb isotopes extracted from Cenozoic marine sediments

ODP 177
ODP 177 1090


Basak, C.
University of Florida, Department of Geological Sciences, Gainesville, FL, United States

Martin, E. E.

Kamenov, G. D.

Sea water Pb isotopes extracted from Cenozoic marine sediments
Chemical Geology
Elsevier, Amsterdam, Netherlands
Nd and Pb isotopic compositions extracted from bulk deep sea sediments have been shown to be robust proxies for deep water circulation as well as weathering provenance and intensity over geologically young time scales. In this study we evaluated ten deep sea samples from Ocean Drilling Program (ODP) site 1090 ranging in age from mid Eocene to early-Miocene to test whether Pb isotopic compositions extracted from geologically older sediments record reliable seawater isotopic ratios and to evaluate the source of the extracted Pb. The sequential extraction protocol used in this study is similar to protocols reported for previous studies and produces acetic acid, hydroxylamine hydrochloride (HH) and residue fractions. Each extracted fraction was analyzed for Pb isotopes, rare earth elements (REEs), and a suite of major elements. Similar (super 206) Pb/ (super 204) Pb, (super 207) Pb/ (super 204) Pb, and (super 208) Pb/ (super 204) Pb ratios are recorded from the acetic acid and HH fractions for nearly equal 70-80% of the samples, suggesting that either the acetic acid dissolves Fe-Mn oxides or multiple phases are recording the same seawater isotopic value. Several indirect tests, such as Al mass balance, comparison of Sr isotopes in HH extracts to contemporaneous seawater Sr isotopes, and comparison of Nd isotopic compositions in HH extracts to published fossil fish teeth values, provide evidence that Pb isotopic compositions measured in our bulk HH extracts record bottom water values. The relationship between Pb, Mn and Ca concentrations in HH fractions indicates that Fe-Mn oxides and a Mn-bearing carbonate are the dominant phases contributing seawater Pb. Comparison of REE patterns derived from the HH fraction and total digestions of Fe-Mn nodule standards reveals that the trivalent REEs exhibit patterns consistent with the parent archive, but Ce can be fractionated during extraction. Ratios of REEs also produce unique fields for each fraction and can be used to test the purity of the seawater signal of the extraction protocol. Finally, an initial evaluation of Pb isotopic compositions in fossil fish indicates that this archive is not suitable for bottom water Pb isotope studies. Abstract Copyright (2011) Elsevier, B.V.
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Isotope geochemistry; Oceanography; Atlantic Ocean; bones; carbonates; chemical composition; Chordata; crystal chemistry; geochemistry; hydrochemistry; ICP mass spectra; iron oxides; isotope ratios; isotopes; lead; Leg 177; manganese oxides; marine sediments; mass spectra; metals; Nd-144/Nd-143; neodymium; Ocean Drilling Program; ODP Site 1090; oxides; Pb-206/Pb-204; Pb-207/Pb-204; Pb-208/Pb-204; Pisces; radioactive isotopes; rare earths; sea water; sediments; sequential extraction; South Atlantic; Southern Ocean; spectra; stable isotopes; teeth; Vertebrata;