Improved Geochronologic Accuracy and Precision for the ICS Chronostratigraphic Charts: Examples from the late Cambrian–Early Ordovician

Abstract

The utility of ICS International Chronostratigraphic Charts (ICS Charts) relies on the accuracy and precision of reported geochronologic dates and their relationship to the bases of global chronostratigraphic units. However, an examination of some early Paleozoic dates (i.e., Cambrian-Ordovician boundary and lowest Ordovician) on the ICS Charts shows that they differ from originally reported and late recalculated dates. Thus, they record a numerical precision that differs from these revised dates. The dates assigned to the bases of formal early Paleozoic chronostratigraphic units may be interpreted by the lay reader as precise determinations, although they are almost all estimates based on dated horizons that stratigraphically bracket a chronostratigraphic unit's base. The bases of informal Cambrian chronostratigraphic units (e.g., Stage 10, ~489.5 Ma) are also assigned relatively precise ages although consensus does not exist on the horizon that should define an informal unit's base, and several alternative GSSP horizons with differing ages may exist. The uncertainties of the ICS Chart's dates on stage bases within the traditional Upper Cambrian are so great that the tilde (e.g., "~500.5") indicating "about" might be replaced by question marks or an abbreviation for "estimated." A preferred alternative is to replace the age estimates on the bases of informal and almost all formal chronostratigraphic divisions in the Phanerozoic with precise dates that bracket the bases of the divisions. The ICS Chart should include (perhaps on-line) brief discussions on and references to the history and procedures of age determination, accuracy, and precision for each dated horizon. An even more accurate, precise, and informative ICS Chart would result from review by a new ICS subcommission on geochronology. The subcommission could evaluate proposed dates for the ICS Chart for accuracy, precision, and suitability for determining/bracketing the ages of chronostratigraphic unit bases and provide updated reviews of the procedures used to determine all ICS Chart dates.

Uppermost Cambrian carbon chemostratigraphy: the HERB and undocumented TOCE events are not synonymous

“…Both the 2006 and 2008 reports do not illustrate the previously named, documented and strong HERB Event below the purported TOCE excursion. A re-evaluation of latest Cambrian geochronology (Landing et al 2015) suggests that the HERB Event and middle Saukiella junia Subzone is c. 2 Ma older than the correlation of TOCE by Peng & Babcock (2008).…”

“…1) credulously accepted existence of an uppermost Cambrian TOCE excursion based on the Zhu et al (2006) and Peng & Babcock (2008) papers, and illustrated both the documented HERB and asserted TOCE excursions; this meant there were two strong, global uppermost Cambrian negative excursions separated by c. 2 Ma ( Fig. 1; see refined chronology of Landing et al 2015). Because Zhu et al (2006) had shown the negative TOCE excursion at the 'Mass Extinction (Ptychaspid Biomere)', this was interpreted (by EL in Landing et al 2011) to mean that TOCE actually existed and should be positioned at the bases of the coterminant Cordylodus proavus Zone (conodonts) and Laurentian Eurekia apopsis Zone (trilobites).…”

(Re)proposal of three Cambrian Subsystems and their Geochronology

“…Similarly, Babcock and Peng's (2007, p. 65) "third Cambrian series will correspond roughly to the Middle Cambrian of some regional correlation schemes" and thus is similar to the regional Avalonian Middle Cambrian Subsystem (Cowie et al, 1972;Landing, 1998a, b) and corresponds precisely to the global Middle Cambrian Subsystem of Landing et al (2013aLanding et al ( , 2018. Finally, "the uppermost series (the Furongian Series) corresponds to the … [Upper Cambrian] as used in South China … and … Kazakhstan" (Peng and Babcock, 2007, p. 65) and to the proposed global Upper Cambrian Subsystem of Landing et al (2015Landing et al ( , 2018.…”

“…The source of the 497 Ma date for the base of the Upper Cambrian Subsystem and Paibian Series (Peng and Babcock, 2008;Zhu et al, 2018) has long remained unknown, although it is repeated by Cohen et al (2013 and updates). The available evidence indicates the Middle-Upper Cambrian Subsystem boundary can only be bracketed by U-Pb zircon dates of 494.4 ± 3.5 Ma in theDrumian (Landing et al, 2015a) and 488.71 ± 1.17 Ma just below the proposed Lawsonian Stage (see Landing et al, 2010aLanding et al, , 2011Landing et al, , 2015.…”

Geomagnetic reversal rates following Palaeozoic superchrons have a fast restart mechanism

“…The late Cambrian and earliest Ordovician have a paucity of radiometric dates 52 , and only two can be directly related to the available magnetostratigraphy ( Fig. 7a and Supplementary Table 4 ).…”