Dating Neogene Eolian Deposits by Ichthyolith Sr Isotope Stratigraphy
Abstract
Comparison of the 87^Sr/86^Sr isotopic ratio of fossil fish teeth (ichthyoliths) with the global seawater Sr curve should provide a highly accurate method for dating pelagic clay cores. This is potentially useful for deep sea red clay cores that are rich in ichthyolith material but barren of other fossils. The strontium component of the fish teeth is assumed to have grown in isotopic equilibrium with seawater. However, acquiring consistent, reproducible results from Sr isotopic analysis of ichytholiths has posed many problems in the past. These difficulties have been variably ascribed to 1) contamination from authigenic ferromanganese oxyhydroxide coatings, 2) Sr exchange during burial diagenesis, and 3) laboratory contamination from cleaning reagents. We have developed a rapid, efficient, low-blank method for removing authigenic Fe-Mn oxyhydroxide coatings and other potential contaminants including clays, organics, carbonate, opal and zeolites from all surfaces and internal cavities of ichthyoliths, as verified by SEM imagery. To test our method, we produced improved age-depth profiles for R/V Ewing cores 9709 PC01 and PC07 from the north central Pacific Ocean (see companion abstract by Johnson et al.). Average uncertainties in our dataset from age assignments using the Sr isotope LOWESS fit of McArthur et al. (2001) vary from <+/-1 m.y. for the 15 Ma to 40 Ma interval to +/-1-2 m.y. for the 0-15 Ma interval. Currently, we are limited by our blank in analyzing the smallest samples (<100 micron size fraction), but are working on an ultra low blank method that will eventually allow us to analyze very small individual teeth. At present, we can analyze single teeth if they are in the 50-100 microgram mass range, translating to 50-100 nanogram loads on mass spectrometer filaments. This Sr isotope-based chronology should provide a precise time scale for the pelagic clay sections and marine wind blown dust records we have been studying, and represents a significant improvement over previous efforts to date the red-clay record.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMPP42B0511G
- Keywords:
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- 4267 Paleoceanography;
- 4825 Geochemistry;
- 4860 Radioactivity and radioisotopes