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Dr. Benjamin Dattilo, Anne Argost
Department of Geosciences
Indiana University – Purdue University Fort Wayne
Sequence- and bio- stratigraphic techniques are well-established tools for correlating sediments globally and regionally. More recently Carbon isotopes have been used to make higherresolution global correlations of thin stratigraphic units, based on the assumption that carbon is evenly mixed in the atmosphere and the oceans have uniform isotopic composition with respect to Carbon. In a recent attempt to utilize Carbon isotopes for correlating 450 million-year-old meter-scale units between Kentucky, Ohio, and Indiana, apparently anomalous isotopic readings were found in tidal-flat sediments (the Tate Member of the Ashlock Formation) at Point Leavell, KY. These values do not match values reported from other sections of similar age. Since tidal flat sediments are fossil poor, and since fossils are necessary to calibrate global isotopic correlations, tidal flat sediments were not represented in previous studies. Furthermore, tidal flat sediments are richer in the mineral dolomite whereas subtidal sediments are richer in calcite. Therefore there are two possible explanations for the anomalous readings: 1) the dolomitic tidal-flat sediments could have biased the isotopic composition, or 2) the extremely shallow-water environments could have preserved sediments at a time when other sediments are not preserved.
To test the hypothesis that isotopic values were biased in sediments containing dolomite, XRD techniques were employed on a selection of samples from the Point Leavell outcrop including samples whose isotopes were anomalously enriched in 13C (carbon 13). The isotopic composition is not correlated with calcite composition indicating that the relative proportion of calcite and dolomite (hence subtidal vs intertidal environments) in sediments did not bias isotopic composition. This implies that restricted shallow water circulation may not have affected isotopic composition in any particular way, but rather that tidal flat deposits accumulated at different times from subtidal deposits, thereby preserving different isotopic signatures. Intertidal sediments are rarely considered in global correlations. In the past, when fossiliferous subtidal sediments were the focus, the ages of the tidal flats were assumed to be the same as the subtidal, and this study reveals that the ages are in fact, different. They represent “time slices” not previously considered in reconstructing earth history.
Earth Sciences | Physical Sciences and Mathematics
Yeater, Ross; Freeman, Rebecca; Argast, Anne S.; and Fisher, Sarah, "Are Intertidal Sediments Biased With Respect to Carbon Isotopes?" (2015). 2015 IPFW Student Research and Creative Endeavor Symposium. 74.