Dating of sedimentary rock intervals using visual comparison of carbon isotope records: a comment on the recent paper by Bergström et al . concerning the age of the Winneshiek Shale

Abstract

The recently published Lethaia paper by Bergström et al. (https://doi.org/10.1111/let.12269) on the age of the Ordovician Winneshiek Shale (Iowa, USA), and the impact that formed the Decorah crater which hosts this rock unit, is an interesting scientific contribution, although there are a number of problems with the interpretations and data presentation that merit comment. Due mainly to a lack of adequate critical assessment of δ13C data and biostratigraphical control, we contend that the conclusions of Bergström et al. are poorly founded and should not be cursorily accepted and propagated in future scientific literature.

The Late Ordovician (Sandbian) Glasford structure: A marine‐target impact crater with a possible connection to the Ordovician meteorite event

“…This assumption is admittedly untested for most of the craters in question, and geochemical evidence suggests that a connection to the LCPB breakup can already be ruled out for one of them (Brent; Goderis et al. ; Lindskog and Young ). Similarly, in the absence of geochemical evidence of an L‐chondrite impactor, it is not possible to establish an unambiguous connection between the Glasford impact event and the LCPB breakup.…”

“…(), in a paper dating the Decorah structure to ~464–467 Ma, listed 15 craters (including Decorah, but not Glasford) that purportedly fall within this time interval. However, Lindskog and Young () argued that several of these craters are either “very roughly and/or ambiguously dated” (Clearwater East, Couture, Hummeln, and Decorah) or demonstrably too young (Slate Islands), implying a more conservative estimate of ~10 craters (not including Glasford) which can be confidently attributed to this age range. Thus, the addition of Glasford raises the number of known craters formed ~440 to 470 Ma to ~11 to 15.…”

“…This is in keeping with models (Zappal a et al 1998) that predict an enhanced influx of larger meteors to the Earth within 30 Ma of the~470 Ma breakup of the LCPB, and craters within this age interval are often interpreted as being related to that event (e.g., Schmitz et al 2018). This assumption is admittedly untested for most of the craters in question, and geochemical evidence suggests that a connection to the LCPB breakup can already be ruled out for one of them (Brent; Goderis et al 2010;Lindskog and Young 2019). Similarly, in the absence of geochemical evidence of an L-chondrite impactor, it is not possible to establish an unambiguous connection between the Glasford impact event and the LCPB breakup.…”

“…The 455 AE 2 Ma age groups the Glasford impact with several others that have been dated to~440-470 Ma, including two in the Midwestern United States, Rock Elm (French et al 2004), and Decorah French et al 2018). Owing to uncertainty in some of these dates, the exact number of impact craters that can be attributed to this time period is open to debate (Orm€ o et al 2014;Schmieder et al 2015;Bergstr€ om et al 2018Bergstr€ om et al , 2019Schmitz et al 2018;Lindskog and Young 2019). Bergstr€ om et al (2018), in a paper dating the Decorah structure to~464-467 Ma, listed 15 craters (including Decorah, but not Glasford) that purportedly fall within this time interval.…”

Miaolingian (Cambrian) trilobite biostratigraphy and carbon isotope chemostratigraphy in the Tingskullen drill core, Öland, Sweden

“…Although δ 13 C chemostratigraphy has emerged as an increasingly powerful tool for regional and global correlation of Cambrian strata, it has become evident that the stratigraphical pattern of carbon isotope curves can vary laterally (e.g. Edwards & Saltzman 2016;Henderson et al 2018;Lindskog & Young 2019) and δ 13 C events may be facies dependent and diachronous (Schiffbauer et al 2017;Barili et al 2018). Zhu et al (2006) compiled previous studies on Cambrian δ 13 C chemostratigraphy and provided a composite δ 13 C curve with ten distinct and named excursions, most of which seem to coincide with im portant biotic events (see also Babcock et al 2015).…”