Calibration of a conodont apatite-based Ordovician⁸⁷Sr/⁸⁶Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

Abstract

The Ordovician 87 Sr/ 86 Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in 87 Sr/ 86 Sr is estimated at ~5.0-10.0 × 10 -5 per m.y. Variable preservation of conodont elements limits the precision for individual stratigraphic horizons. Replicate conodont analyses from the same sample differ by an average of ~4.0 × 10 -5 (the 2σ standard deviation is 6.2 × 10 -5 ), which in the best case scenario allows for subdivision of Ordovician time intervals characterized by the highest rates of fall in 87 Sr/ 86 Sr at a maximum resolution of ~0.5-1.0 m.y. Links between the increased rate of fall in 87 Sr/ 86 Sr beginning in the mid-late Darriwilian (Phragmodus polonicus to Pygodus serra conodont zones) and geologic events continue to be investigated, but the coincidence with a long-term rise in sea level (Sauk-Tippecanoe megasequence boundary) and tectonic events (Taconic orogeny) in North America provides a plausible explanation for the changing magnitude and 87 Sr/ 86 Sr of the riverine Sr fl ux to the oceans.

The Hirnantian (Late Ordovician) and end-Guadalupian (Middle Permian) mass-extinction events compared

“…0.7080 in the Katian-Hirnantian for the Ordovician (e.g. Saltzman et al 2014), and at ca. 0.7068 in the Capitanian for the Permian (e.g.…”

“…Saltzman et al . ); nonetheless, active tectonics/orogeny is always in operation somewhere in the world as long as plates keep moving, and the effects of regional tectonism remain locally not in global context.…”

“…2). By the Capitanian extinction, various marine sessile benthonic groups with passive respiratory systems, that is, those with no gills, weak internal circulation and low metabolic rate, such as cnidarians, echinoderms and articulate brachiopods, were selectively eliminated (Knoll et al 1996), whereas free Korte et al 2006;Isozaki et al 2007;Saltzman et al 2014;Kani et al 2013;Pavlov et al 2012;Kirschvink et al 2015). Note the remarkable similarities in the pattern of sea-level change (ca.…”

“…Previous researchers struggled to explain this discrepancy and often blamed temporary tectonic episodes for the cause (e.g. Saltzman et al 2014); nonetheless, active tectonics/orogeny is always in operation somewhere in the world as long as plates keep moving, and the effects of regional tectonism remain locally not in global context.…”

Gradients in the carbon isotopic composition of Ordovician shallow water carbonates: A potential pitfall in estimates of ancient CO2 and O2

“…Therefore, success in correlating  13 C curves is dependent on the rate of change in the seawater 87 Sr/ 86 Sr curve (Woodfine et al, 2008;Huck et al, 2011;Di Lucia et al, 2012;Frijia et al, 2015;Navarro-Ramirez et al, 2015) as well as the uncertainty associated with measuring ancient 87 Fig. S1; McArthur et al, 2012;Saltzman et al, 2014). Although we find evidence for strong coupling between  13 C trends in restricted carbonate platform and open marine deposits that is consistent with global forcing (e.g., a change in the fraction of carbon buried as organic matter, or f org ), we also observe potential environmental effects from offsets in  13 C as large as ~ 2‰ during relative sea level fall.…”