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Discussion of ‘Reply to “Uppermost Cambrian carbon chemostratigraphy: the HERB and undocumented TOCE events are not synonymous”’

Published online by Cambridge University Press:  02 November 2021

ED Landing Open the ORCID record for ED Landing [Opens in a new window] ,
Robert L. Ripperdan  and
Gerd Geyer Open the ORCID record for Gerd Geyer [Opens in a new window]
ED Landing*
Affiliation:
New York State Museum, 222 Madison Avenue, Albany, NY 12230, USA Montgomery County Public Schools, Montgomery City, MO 63361, USA
Robert L. Ripperdan
Affiliation:
Montgomery County Public Schools, Montgomery City, MO 63361, USA
Gerd Geyer
Affiliation:
Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für Geographie und Geologie, Bayerische Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
*
Author for correspondence: Ed Landing, Email: ed.landing@nysed.gov
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Abstract

No basis for controversy exists in the naming of a global, strongly negative, uppermost Cambrian carbon isotope (δ13C) excursion. The HERB Event (HERB) has met the standards for chemostratigraphic units (i.e. consistent biostratigraphic brackets, content and concept) since 1992. By comparison, the TOCE excursion morphed through four temporally distinct δ13C events with spike-like nadirs that shifted temporally through the uppermost Cambrian until its synonymization with HERB (2006–12). In 2018, TOCE became a prolonged interval with very early onset and enveloped HERB – meaning five TOCE homonyms have been unambiguously defined and figured. HERB lies in the high-diversity ptychaspid biomere (trilobites) and below the ptychaspid extinction. But, data on it were used in TOCE’s 2006 proposal and in later iterations (2008, 2012) to show it (1) higher, both at and above the ptychaspid extinction; (2) at the level of HERB (2012, 2018); and (3) even extending well below HERB (2018). TOCE fails the recommendations for a formal chemostratigraphic unit. Its relationship to latest Cambrian biotic turnover includes equation with extinction and high-diversity intervals. One TOCE homonym is a synonym, albeit junior, of HERB.

Keywords

Cambriancarbon isotope excursionsHERB excursion“TOCE”
Type
Discussion - Reply
Information
Geological Magazine , Volume 159 , Issue 1 , January 2022 , pp. 173 - 176
Copyright
© The Author(s), 2021. Published by Cambridge University Press

1. Introduction

Variations in geochemical properties in sedimentary successions (i.e. rock (including ice), soil) contribute to geologic synthesis and temporal correlation. Chemostratigraphic excursions or events are physical stratigraphic units that reflect environmental changes, and require a formal methodology and standardized terminology just like litho- and biostratigraphic units in the North American and international stratigraphic codes (e.g. Ramkunar, Reference Ramkumar and Ramkumar2015). (The following acronyms are used below: HERB, Hellnmaria–Red Tops Boundary (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006), and TOCE, Top of Cambrian Excursion (Zhu et al., Reference Zhu, Babcock and Peng2006).)

Chemostratigraphic units require documentation of geochemical changes through a lithosequence, consistent ranges shown by independent markers (bio-, magnetostratigraphic, etc.) and reference sections (Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020). Use of a chronostratigraphic unit must be consistent with its original concept and not replace a pre-existing unit as a synonym (Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020). These considerations emphasize HERB’s utility as an Upper Cambrian correlation tool. (In this report, the Cambrian is divided into three subsystems and subperiods (Lower/Early, Middle/Middle, Upper/Late); the undefined divisions ‘lower/early’, ‘middle/middle’ and ‘upper/late’ are not used; Landing et al. Reference Landing, Geyer, Schmitz, Wotte and Kouchinsky2020 b).

2. Multiple TOCE homonyms

Discussion of the HERB and TOCE events allows application of Scott et al.’s (Reference Scott, Brett, Fluegeman and Pratt2020) recommendations for defining chemostratigraphic units. It also emphasizes the problem with the creation of multiple homonyms of chemostratigraphic events in geological correlation.

TOCE of Zhu et al. (Reference Zhu, Babcock and Peng2006) was based on Buggisch et al.’s. (Reference Buggisch, Keller and Lehnert2003) synthesis, not their field and laboratory work. Buggisch et al. (Reference Buggisch, Keller and Lehnert2003) reported δ13C results from Argentina. They identified a strong uppermost Cambrian negative δ13C excursion known in Laurentia (Utah) and Australia (Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992; Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995; Dattilo et al. Reference Dattilo, Hlohowsky, Ripperdan, Miller and Shapiro2004; Fig. 1) in the unfossiliferous lower La Silla Formation. The Utah and Australian areas show a strong negative excursion in the Upper Cambrian Saukiella junia Subzone (trilobites) and equivalents, and its onset above the Eoconodontus Zone base (conodonts). It is not accurate to say that TOCE was ‘not tied precisely to the trilobite and conodont zonation available at the time’ (Zhu et al. Reference Zhu, Babcock, Peng and Ahlberg2020). Indeed, TOCE was clearly described as identical to the uppermost Cambrian negative excursion in Laurentia, Australia and Argentina using Buggisch et al. (Reference Buggisch, Keller and Lehnert2003). However, it was figured at a much higher interval than the S. junia Subzone and tied to the ‘Mass Extinction (Ptychaspid Biomere)’ horizon (Zhu et al. Reference Zhu, Babcock and Peng2006, fig. 1, their quotation marks).

Portrayal of TOCE as a negative δ13C spike above the Saukiella junia Subzone was not correct (see strong excursion and nadir in the S. junia Subzone in Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992; Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995; Datillo et al. Reference Dattilo, Hlohowsky, Ripperdan, Miller and Shapiro2004; Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006). This placement may follow a belief that carbon isotope excursions ‘seem to coincide with important biotic events’ (Zhu et al. Reference Zhu, Babcock and Peng2006), but Laurentia and Australia show the event in a high-diversity, not extinction, interval (e.g. Taylor, Reference Taylor2006).

The reason for the upward migration of TOCE and its nadir higher into the Laurentian Missisquoia depressa Subzone – Symphysurina brevispicata Zone (Peng & Babcock, Reference Peng, Babcock, Ogg, Ogg and Gradstein2008, fig. 4.4; Fig. 1) was unexplained. This interval does not have a strong negative δ13C excursion (e.g. Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995; Fig. 1), although it actually corresponds to an ‘important biotic event’ interval that ‘should’ show a δ13C excursion by the biotic turnover thesis in Zhu et al. (Reference Zhu, Babcock and Peng2006; see ptychaspid biomere review in Landing et al. Reference Landing, Ripperdan and Geyer2020 a). The 2006 and 2008 reports established biostratigraphically distinct TOCE homonyms 1 and 2.

TOCE was later ‘dropped down’ into the ptychaspid biomere with a sharp nadir in the Saukiella serotina Subzone (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, p. 440, fig. 19.3; i.e. homonym 3). The latter report said this event was ‘previously referred to as the HERB Event’, with Ripperdan et al. (Reference Ripperdan, Magaritz, Nicoll and Shergold1992) given as the source. This purported reference is 10 years before ‘HERB Event’ was first used (Ripperdan, Reference Ripperdan2002; Dattilo et al. Reference Dattilo, Hlohowsky, Ripperdan, Miller and Shapiro2004). In addition, there is no Ripperdan et al. (Reference Ripperdan, Magaritz, Nicoll and Shergold1992) report, and the HERB Event always was in the older S. junia Subzone (e.g. Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992). In the same report, Peng et al. (Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, p. 454) created homonym 4 by putting the TOCE onset at the ‘base of the E. notchpeakensis Subzone … equivalent to the … Saukiella junia Subzone …’

Zhu et al. (Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018, figs 2, 4) do not record TOCE in δ13C data from South China. They show a protracted ‘global’ TOCE (homonym 5) that extends very low to the base of informal Cambrian Stage 10 as they define it (i.e. Zhu et al. Reference Zhu, Babcock and Peng2006). This is below the Saukiella junia Subzone and equivalents (Fig. 1). TOCE homonym 5 extends into middle Stage 10 with a major negative δ13C excursion cap, likely the traditional HERB Event. Homonym 5 excludes the higher TOCE homonym 2 of Peng & Babcock (Reference Peng, Babcock, Ogg, Ogg and Gradstein2008). Two strong negative δ13C nadirs low in the TOCE band (Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018) are unknown in coeval carbon isotope excursions, and their basis is unknown to us. The 2018 TOCE brackets a prolonged ‘Mass Extinction (Ptychaspid Biomere)’ (Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018, their quotation marks). This relationship of biotic change to chemostratigraphic events follows Zhu et al. (Reference Zhu, Babcock and Peng2006; Zhu et al. Reference Zhu, Babcock, Peng and Ahlberg2020, p. 3) in that ‘the timing of the extinction is clearly linked to the TOCE’, but it must be noted that TOCE homonym 5 is below the Eurekia apopsis Zone extinction and in a high-diversity interval (Taylor, Reference Taylor2006; Landing et al. Reference Landing, Ripperdan and Geyer2020 a).

Figure 1. HERB Event with multiple TOCE nadirs at asterisks in E. apopsis Subzone (Zhu et al. Reference Zhu, Babcock and Peng2006, fig. 6); “M.” depressa Subzone – S. brevispicata Zone (Peng & Babcock, Reference Peng, Babcock, Ogg, Ogg and Gradstein2008); Saukiella serotina and S. junia subzones (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012; Terfelt et al. Reference Terfelt, Eriksson and Schmitz2014). Vertical bar is TOCE band (Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018) with anomalously early onset (cf. onset in S. junia Subzone in Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012) and top of TOCE not defined in Zhu et al. (Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018). After Landing et al. (Reference Landing, Westrop and Adrain2011, figs 1, 4). Abbreviations: E., Eurekia; “M.”, ‘Missisquoia;’ P., Prosaukia; Sz., Subzone; Z., Zone. Informal Stage 10 base at lowest occurrence of problematical ‘Lotagnostus americanus’ (i.e. Landing et al. Reference Landing, Westrop and Adrain2011).

3. Utility of HERB event

Since a strong negative Upper Cambrian δ13C event was first reported (Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992) and termed an ‘excursion’ (Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995; Miller et al. Reference Miller, Evans, Freeman, Ripperdan and Taylor2011) or ‘event’ (Ripperdan, Reference Ripperdan2002; Dattilo et al. Reference Dattilo, Hlohowsky, Ripperdan, Miller and Shapiro2004), it has been recorded globally in the Saukiella junia Subzone or equivalents. It should be noted that no consistent distinction exists between chemostratigraphic ‘events’ and ‘excursions’,, with many ‘events’ (e.g. GICE, OAB 1–3) having long durations (Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020).

Rather than being tightly and consistently bracketed, TOCE has ranged through seven trilobite subzones and zones (Landing et al. Reference Landing, Ripperdan and Geyer2020 a; Fig. 1). Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020, pp. 1, 2) claim ‘subsequent attempts to tie [TOCE] into an evolving biostratigraphic and geochronologic framework have seemingly led to the erroneous statement that the TOCE “has had its biostratigraphic and geochronologic position changed in successive publications”’; but the syntheses by Zhu and colleagues since 2006 void this argument (Landing et al. Reference Landing, Ripperdan and Geyer2020 a; Fig. 1). An ‘evolving biostratigraphic and geochronologic framework’ leading to disparate TOCE definitions (Zhu et al. Reference Zhu, Babcock, Peng and Ahlberg2020) does not explain its changing position: the Queensland and Utah biostratigraphies are virtually unchanged since the first work on their δ13C profiles (Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992).

A forced synonymy of names, not biostratigraphically bracketed chemostratigraphic units, led to TOCE’s later representation as a sharp nadir in the Saukiella junia Subzone and equivalents (Terfelt et al. Reference Terfelt, Eriksson and Schmitz2014; Ahlberg et al. Reference Ahlberg, Lundberg, Erlström, Calner, Lindskog, Dahlqvist and Joachimski2019; homonym 4) or a broad uppermost Cambrian band (Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2018; TOCE homonym 5) that envelops the earlier-documented, biostratigraphically unvarying HERB (Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992; Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995; Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006). Thus, only TOCE homonym 4 is a synonym (albeit junior) of HERB.

Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020) argue that HERB is not legitimate as the acronym was not explained when proposed, although named chemostratigraphic events, if acronyms, are often not explained (Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020). Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020) thus assert a well-defined acronym has priority over the detailed biostratigraphy of the HERB Event in the same volume where TOCE was proposed based on data from the literature (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, p. 400, fig. 12; Zhu et al. Reference Zhu, Babcock and Peng2006). In Zhu et al. (Reference Zhu, Babcock and Peng2006), TOCE’s brackets are ambiguous, unrelated to a stratigraphic section, and its lower part is referable to three possible ‘Ptychaspid Biomere Extinction Interval’ horizons (Landing et al. Reference Landing, Ripperdan and Geyer2020 a).

Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020) noted HERB was not named in a formal publication (i.e. Ripperdan, Reference Ripperdan2002) as required by the North American Commission on Stratigraphic Nomenclature (2005), although no recommendations existed for chemostratigraphic unit names until recently (Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020). TOCE’s changing biostratigraphic position is shown in reports since 2006 (Fig. 1), but HERB’s bracketing and correlation across Laurentia, Baltica, Gondwana (Australia, Argentina), North China (Dayancha) and Kazakhstan are unchanged for almost 30 years (Landing et al. Reference Landing, Ripperdan and Geyer2020 a).

4. Discussion

Chemostratigraphic units require consistent definition (i.e. Ramkumar, Reference Ramkumar and Ramkumar2015). Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020, p. 1) claim TOCE is ‘a well-documented and clearly defined δ13Ccarb excursion, and that the term “HERB Event” was originally used informally, without definition or reference data, for a negative δ13Ccarb peak, a peak later shown to occur within the TOCE excursion’.’ This ignores the changing TOCE concepts since 2006 – variously a sharp negative event younger than or equated with and finally expanded to envelop HERB, and figured through seven Laurentian biostratigraphic intervals (zones and subzones). That ‘the stratigraphic position of the TOCE has shifted through four biotic intervals is simply incorrect’ (Zhu et al. Reference Zhu, Babcock, Peng and Ahlberg2020) is itself incorrect is shown by historical review (Landing et al. Reference Landing, Ripperdan and Geyer2020 a; Fig. 1). Zhu et al. (Reference Zhu, Babcock, Peng and Ahlberg2020) claimed Landing et al. (Reference Landing, Ripperdan and Geyer2020 a) were ‘misleading and replete with misconceptions’ and ‘misunderstood’ the relationship of HERB to TOCE – although Landing et al.’s discussion follows recommendations consistent with determining priority in stratigraphic nomenclature (i.e. North American Commission on Stratigraphic Nomenclature, 2005; Scott et al. Reference Scott, Brett, Fluegeman and Pratt2020; Fig. 1). Landing et al. (Reference Landing, Westrop, Miller, Fatka and Budil2010, Reference Landing, Westrop and Adrain2011) proposed the end-Cambrian Lawsonian Stage with a base at the lowest Eoconodontus notchpeakensis just below the HERB onset (Fig. 1). This proposal makes the unchanging definition and correlation of HERB particularly important.

Acknowledgements

The constructive comments of the reviewers are appreciated.

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Figure 1. HERB Event with multiple TOCE nadirs at asterisks in E. apopsis Subzone (Zhu et al. 2006, fig. 6); “M.” depressa Subzone – S. brevispicata Zone (Peng & Babcock, 2008); Saukiella serotina and S. junia subzones (Peng et al. 2012; Terfelt et al. 2014). Vertical bar is TOCE band (Zhu et al. 2018) with anomalously early onset (cf. onset in S. junia Subzone in Peng et al. 2012) and top of TOCE not defined in Zhu et al. (2018). After Landing et al. (2011, figs 1, 4). Abbreviations: E., Eurekia; “M.”, ‘Missisquoia;’ P., Prosaukia; Sz., Subzone; Z., Zone. Informal Stage 10 base at lowest occurrence of problematical ‘Lotagnostus americanus’ (i.e. Landing et al. 2011).