PRECISION IN CARBON ISOTOPE STRATIGRAPHY—THE IMPORTANCE OF SAMPLING INTERVAL IN RECOGNIZING FORM AND RATES

Chronostratigraphy at high temporal resolutions (10⁰-10² k.y.) provides an opportunity to recognize known Earth processes in deep-time. Studies of the Cenozoic demonstrate the utility of large geochemical data sets for regional to global correlation and as proxy records of Earth system dynamics. However, to date, many studies of ancient sedimentary successions lack similarly robust data sets.

Centimeter-scale carbon isotope stratigraphy combined with high-precision CA-TIMS U-Pb zircon radioisotopic age determinations from ash-fall beds provides new insights into the form and rate of high-frequency variation apparent in a well-known carbon isotope excursion in deep-time ("GICE", Upper Ordovician). A new, robust carbon isotope data set, including a ~30 m profile for a drill core sampled at 5 cm-spacing, for Sandbian- to Katian-age strata throughout the upper Mississippi Valley (USA) rectifies inconsistencies in previously reported trends. These data show a long-term positive shift in values beginning much farther below the previously defined base of the excursion interval. Short-term variability in this record can be subdivided into high-frequency positive and negative excursions, as well as flat-line offsets and negative "spikes" associated with unconformity surfaces. Single-crystal U-Pb zircon radioisotopic ages for tephras bounding a high-frequency 3‰ negative excursion suggest a duration of 150 ± 50 k.y. Further, when compared with U-Pb zircon ages from the Katian GSSP (Atoka, Oklahoma), these new ages suggest that the "GICE" occurs within the Sandbian Stage and not the lower Katian as commonly reported. On a more controversial note, these new data sets suggest inconsistencies and time-transgressive trends between the correlation of North American conodont and graptolite biozonations at fine temporal resolution.

This study demonstrates the importance of robust data sets for defining C-isotope trends in ancient rock successions and shows that many high-frequency features of the C-isotope record may be obscured by low sample density. Integration of stable isotope records with high-precision radioisotopic age analysis provides a means to assess the tempo of deep-time Earth processes with the potential to resolve trends present in more recently deposited successions.