T256 Oceans and Climates through Earth History: From Proxy Reconstructions to Model Assessments (Posters)

Wednesday, 7 November 2018
9:00 AM - 6:30 PM
Halls J-K

REFINING THE TIMING OF THE MIDDLE-LATE ORDOVICIAN INFLECTION POINT IN SEAWATER 87SR/86SR USING CONODONT APATITE

Paper number: T256-3
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Strontium isotope (87Sr/86Sr) stratigraphy is a useful method for tracking changes in the Earth’s geologic history. Due to its residence time, changes in the marine 87Sr/86Sr value are assumed to reflect global changes in the Earth’s silicate weathering patterns.

Seawater 87Sr/86Sr decreases at a steady rate through the Early Ordovician and most of the Middle Ordovician. The rate of decline increases—and 87Sr/86Sr drops sharply—during an inflection point in the late Darriwilian and early Sandbian. This increased rate of change has been interpreted as weathering of volcanic rocks from island-arc settings in the Taconic orogeny in eastern Laurentia. This would act as a sink for atmospheric pCO2 and initiate the icehouse transition observed in the Katian and Hirnantian. However, the timing of this shift does not fit a clear cause-and-effect relationship with δ18O, i.e. seawater temperature. Improved resolution of changes in 87Sr/86Sr and δ18O can help address whether a shift in weathering patterns influenced temperature or vice-versa. In addition, lower sedimentation rate could create the illusion of a sharper decrease in 87Sr/86Sr in the measured sections. In order to rule out a role for reduced sedimentation rate, data must be collected at high resolution from multiple well-dated stratigraphic sections.

The Middle to Late Ordovician inflection point is apparent in the 87Sr/86Sr values of conodont elements, the bioapatitic remains of a now-extinct marine chordate. Our samples come from the I-35 section of the Simpson Group in Arbuckle Mts, OK; the inflection point appears to occur within the upper Tulip Creek and lower Bromide formations. This study seeks to better constrain the 87Sr/86Sr shift by increasing resolution within the ~458 to 463 Mya period represented by part of the Simpson Group. Furthermore, this study differs from previous work in the Simpson Group (Saltzman et al., 2014, GSA Bulletin, v. 126) in that the conodont elements are processed via leaching in acetic acid and analysed for 87Sr/86Sr on a new generation TIMS instrument at Ohio State with higher external reproducibility (SRM 987 yields 2SD = 1.2x10-5; n=52). Additional data will be collected from the Antelope Range, NV in an effort to rule out the possibility that the observed inflection point is an artefact of changing sedimentation rates.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-324331

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