1. The TOCE is a well-documented, negative δ¹³C_carb excursion with explicit definition

The assertion by Landing et al. (Reference Landing, Ripperdan and Geyer2020) that the Top of Cambrian Excursion (TOCE) was named ‘without stratigraphic or laboratory data or reference to previous studies or a representative section, and these data have never been provided’ is demonstrably incorrect. The TOCE was originally defined (Zhu et al. Reference Zhu, Babcock and Peng2006, p. 220) as ‘a large negative excursion close to the end of the Cambrian System (uppermost part of Stage 10 and the Furongian Series)’ in a synthetic paper that presented the first composite Cambrian δ¹³C_carb chemostratigraphic profile (Zhu et al. Reference Zhu, Babcock and Peng2006). As Zhu et al. (Reference Zhu, Babcock and Peng2006) explained in the caption to their figure 1 and in the reference list, the composite Cambrian carbon isotope excursion was based on various published δ¹³C_carb datasets. Specifically, the TOCE was based on the suggestion of Buggisch et al. (Reference Buggisch, Keller and Lehnert2003, fig. 8) that prominent δ¹³C_carb negative excursions identified from the Eoconodontus Conodont Zone of the Argentine Precordillera, the Black Mountain section of Australia (Ripperdan et al. Reference Ripperdan, Magaritz, Nicoll and Shergold1992), and the Lawson Cove section of Utah, USA (Ripperdan & Miller, Reference Ripperdan, Miller and Cooper1995) were synchronous and could be used for global correlation of Cambrian–Ordovician boundary sections worldwide. As that level is close to the Cambrian–Ordovician boundary, it was designated as the TOCE.

The TOCE was not tied to biozonation when named (Zhu et al. Reference Zhu, Babcock and Peng2006), and not tied precisely to the trilobite and conodont zonation available at the time (summarized in Peng & Babcock, Reference Peng, Babcock, Ogg, Ogg and Gradstein2008). Also, robust geochronologic constraints were not available for biozones in the uppermost Cambrian strata at that time. Although the definition of the TOCE has not changed since its original publication, subsequent attempts to tie its position 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’ (Landing et al. Reference Landing, Ripperdan and Geyer2020). To minimize ambiguity in its recognition, Peng et al. (Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012) identified the TOCE as a negative δ¹³C_carb excursion within the Eoconodontus Conodont Zone. Its onset was regarded as occurring about halfway through provisional Stage 10, with the return to positive values being a little below the Ordovician base (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, p. 454–5). Further means of recognition were provided in a correlation chart showing the stratigraphic position of the TOCE in relation to the trilobite zonation in South China, Siberia, Australia and Laurentia (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, fig. 19.3).

Since its introduction in 2006, the TOCE has been a widely used chemostratigraphic term among the Cambrian community. The term has been adopted and discussed by numerous authors, including Landing and colleagues (e.g. Terfelt et al. Reference Terfelt, Eriksson and Schmitz2014; Li et al. Reference Li, Zhang, Chen, Zhang, Chen, Huang, Peng and Shen2017; Westrop & Landing, Reference Westrop and Landing2017; Ahlberg et al. Reference Ahlberg, Lundberg, Erlström, Calner, Lindskog, Dahlqvist and Joachimski2019; Geyer, Reference Geyer2019; Scorrer et al. Reference Scorrer, Azmy and Stouge2019; Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2019), and it has been adopted for use in the Geologic Time Scale volumes (Ogg et al. Reference Ogg, Ogg and Gradstein2008, Reference Ogg, Ogg and Gradstein2016; Gradstein et al. Reference Gradstein, Ogg, Schmitz and Ogg2012, Reference Gradstein, Ogg, Schmitz and Oggin press).

2. The HERB Event is synonymous with the TOCE

The term HERB (later defined as Hellnmaria–Red Tops Boundary) first appeared in a Geological Society of America (GSA) meeting abstract as a Sunwaptan-aged δ¹³C event recognized in Australia, Newfoundland and western North America, but it appeared without reference data and explanation of the acronym (Ripperdan, Reference Ripperdan2002). Its first appearance in a formal publication, again without explanation of the acronym, was in a paper by Miller et al. (Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, p. 400) who defined the HERB Event as a ‘distinctive negative peak at point B’ in the Lawson Cove section of Utah (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, fig. 12). The peak occurs within a broader negative δ¹³C_carb excursion. Miller et al. (Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, fig. 12) correlated the HERB Event to an equivalent negative δ¹³C_carb peak in the Black Mountain, Australia, section. The δ¹³C_carb dataset used to define the HERB Event was published earlier by Ripperdan et al. (Reference Ripperdan, Magaritz, Nicoll and Shergold1992) and Ripperdan & Miller (Reference Ripperdan, Miller and Cooper1995). The dataset is the same as that cited by Buggisch et al. (Reference Buggisch, Keller and Lehnert2003) and subsequently used for defining the TOCE. Under the definition of Miller et al. (Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006), the HERB Event represents a nadir within the TOCE excursion.

The acronym HERB was defined by Miller et al. (Reference Miller, Evans, Freeman, Ripperdan and Taylor2014, Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015), 12 years after the term was first used by Ripperdan (Reference Ripperdan2002). The Hellnmaria and Red Tops members belong to the Notch Peak Formation. In their 2015 paper, Miller et al. (Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015) modified the concept of the HERB Event, redefining it as ‘a prominent excursion that begins slightly above the boundary between the Hellnmaria and Red Tops Members and continues into the Lava Dam Member’ in Utah. Under the modified definition, the HERB was expanded across a rather lengthy stratigraphic interval. Although predominantly a negative excursion (δ¹³C_carb) interval, the new definition encompasses an interval having large δ¹³C_carb variability, including stratigraphic levels with distinctly positive values, in four Utah sections (Miller et al. Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015, fig. 8). The most consistently negative part of the redefined excursion is nearly restricted to an interval within the Red Tops Member.

Miller et al. (Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015) described the HERB as a prolonged episode, characterized predominantly by a negative δ¹³C_carb profile. As such, it is a better match conceptually to the definition of the TOCE. Largely for this reason, the HERB Event has commonly been referred to as synonymous with the TOCE in Cambrian stratigraphy (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, Reference Peng, Babcock, Ahlberg, Gradstein, Ogg, Schmitz and Oggin press; Terfelt et al. Reference Terfelt, Eriksson and Schmitz2014; Li et al. Reference Li, Zhang, Chen, Zhang, Chen, Huang, Peng and Shen2017; Ahlberg et al. Reference Ahlberg, Lundberg, Erlström, Calner, Lindskog, Dahlqvist and Joachimski2019; Harper et al. Reference Harper, Topper, Cascales-Miñana, Servais, Zhang and Ahlberg2019; Geyer, Reference Geyer2019; Scorrer et al. Reference Scorrer, Azmy and Stouge2019; Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2019).

Landing et al. (Reference Landing, Westrop and Adrain2011) added confusion to discussions of uppermost Cambrian stratigraphy by erroneously recognizing the HERB Event and the TOCE as two separate phenomena within Stage 10 (Landing et al. Reference Landing, Westrop and Adrain2011, fig. 1). According to their illustration, the HERB Event preceded the onset of the TOCE. We never intended to recognize the HERB and TOCE as separate, successive excursions, and we know of no dataset supporting the isotopic profile for the uppermost Cambrian strata presented by Landing et al. (Reference Landing, Westrop and Adrain2011, fig. 1). In their recent communication, Landing et al. (Reference Landing, Ripperdan and Geyer2020) stated that they now consider the ‘credulously accepted existence’ of the TOCE as an excursion above the HERB (Landing et al. Reference Landing, Westrop and Adrain2011) to be a ‘nihilartikel’. In their view, the TOCE excursion is not synonymous with the HERB Event because of its ‘ever-shifting meaning’ (Landing et al. Reference Landing, Ripperdan and Geyer2020, fig. 1). This misunderstanding by Landing and colleagues seems to owe its origin to a combination of factors, as follows.

1. (1) Failure to recognize that the TOCE was adequately defined, and was supported by published data (Buggisch et al. Reference Buggisch, Keller and Lehnert2003), as explained above.

2. (2) Controversy over the level of the base of provisional Stage 10, which is pending ratification. The International Subcommission on Cambrian Stratigraphy voted to accept the first appearance datum (FAD) of the agnostoid Lotagnostus americanus as the marker for the base of Stage 10 (Babcock et al. Reference Babcock, Peng, Geyer and Shergold2005; see also Peng & Babcock, Reference Peng and Babcock2005, Reference Peng, Babcock, Ogg, Ogg and Gradstein2008; Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012). Landing and others (Landing et al. Reference Landing, Westrop and Adrain2011; Miller et al. Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015) subsequently indicated support for a much higher level, identified by the FAD of a conodont, Eoconodontus notchpeakensis. In proposing an alternative level for the base of the stage, and simultaneously introducing the idea that the TOCE and HERB represent successive, unrelated chronostratigraphic markers, Landing et al. (Reference Landing, Westrop and Adrain2011) squeezed the TOCE excursion into a narrowed stratigraphic interval, erroneously placing it closer to the ratified base of the Ordovician System. Recognizing the HERB as an earlier event, an event whose stratigraphic record is close to the level of E. notchpeakensis, Landing et al. (Reference Landing, Westrop and Adrain2011) significantly compounded the error, with the result being further misunderstanding of the criteria under consideration for defining the base of the stage.

3. (3) A suggested link between the TOCE and the mass extinction associated with the Ptychaspid Biomere illustrated by Zhu et al. (Reference Zhu, Babcock and Peng2006, fig. 1). In their recent communication, Landing et al. (Reference Landing, Ripperdan and Geyer2020) stated that the TOCE ’seems to be correlated into the Eurekia apopsis Zone (trilobites) of Laurentia’ because the Ptychaspid Biomere extinction has had a number of different definitions (Taylor, Reference Taylor2006). It is true that differing biostratigraphic levels have been proposed for the top of the Ptychaspid Biomere. Despite differences in interpretation about the position of the biomere top, the timing of extinction is clearly linked to the TOCE excursion. The extinction level was recently placed at the base of the Cordylodus proavus Zone, that is, near the top of the TOCE by Miller et al. (Reference Miller, Evans, Freeman, Loch, Ripperdan and Taylor2018). It should be emphasized that the TOCE excursion was shown to be within the Eoconodontus Conodont Zone, equivalent to the middle of the Saukia Zone (Saukiella junia Polymerid Trilobite Subzone) in Laurentia (Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, p. 454). The argument that the inferred stratigraphic position of the TOCE has shifted through four biotic zones (Landing et al. Reference Landing, Ripperdan and Geyer2020) is simply incorrect.

To summarize, the interpretation that the HERB Event and the TOCE excursion are separate δ¹³C_carb entities (Landing et al. Reference Landing, Westrop and Adrain2011, Reference Landing, Ripperdan and Geyer2020) is a misunderstanding, as discussed elsewhere (e.g. Peng et al. Reference Peng, Babcock, Cooper, Gradstein, Ogg, Schmitz and Ogg2012, Reference Peng, Babcock, Ahlberg, Gradstein, Ogg, Schmitz and Oggin press; Terfelt et al. Reference Terfelt, Eriksson and Schmitz2014; Miller et al. Reference Miller, Ripperdan, Loch, Freeman, Evans, Taylor and Tolbart2015; Li et al. Reference Li, Zhang, Chen, Zhang, Chen, Huang, Peng and Shen2017; Ahlberg et al. Reference Ahlberg, Lundberg, Erlström, Calner, Lindskog, Dahlqvist and Joachimski2019; Geyer, Reference Geyer2019; Harper et al. Reference Harper, Topper, Cascales-Miñana, Servais, Zhang and Ahlberg2019; Scorrer et al. Reference Scorrer, Azmy and Stouge2019; Zhu et al. Reference Zhu, Yang, Yuan, Li, Zhang, Zhao, Ahn and Miao2019), and as elaborated here. Geyer (Reference Geyer2019), in particular, noted that the HERB and TOCE represent the ‘same fluctuation in ocean chemistry’ and that ‘no negative δ¹³C excursion is recorded above the HERB peak in North America’ (Geyer, Reference Geyer2019).

3. The HERB Event is an invalid synonym of the TOCE

Landing et al. (Reference Landing, Ripperdan and Geyer2020) commented that there is no formal procedure for the proposal, naming, description and definition of chemostratigraphic units in the International Stratigraphic Code. They claimed that ‘At the Geological Society of America 2019 meeting in Phoenix, the NACSN (North American Commission on Stratigraphic Nomenclature) confirmed its commitment to developing consistent stratigraphic and nomenclatural principles and practices for formal chemostratigraphic units’ (no formal document is provided). However, the NACSN is undoubtedly aware that any guidelines for global stratigraphic nomenclature must be formulated and approved by the International Commission on Stratigraphy. The current Code makes clear that, for priority considerations to take effect, any formally established stratigraphic unit-term must be published in a suitable scientific medium. The Code states specifically that ‘Abstracts, most fieldtrip guidebooks, dissertations, company reports, open file reports and similar media generally do not meet this requirement’ (see Murphy & Salvador, Reference Murphy and Salvador1999). By way of comparison, Landing et al. (Reference Landing, Ripperdan and Geyer2020) also commented that ‘it is impossible that a formal taxon name would be accepted without diagnosis or reference to existing work, or later used to supplant an existing name despite knowledge of the original taxon. Such a proposal would not be sanctioned under existing International Commission on Zoological Nomenclature and/or International Code of Botanical Nomenclature rules and regulations.’ This statement is misleading, however, as any new taxon mentioned in an informal medium such as an abstract would not be considered available (validly described) according to the International Code of Zoological Nomenclature (1999).

The name ‘Top of Cambrian Excursion’, with the acronym TOCE, was defined and published, with reference to supporting isotopic data (Buggisch et al. Reference Buggisch, Keller and Lehnert2003), as a negative δ¹³C excursion near the top of the Cambrian System in a refereed journal article (Zhu et al. Reference Zhu, Babcock and Peng2006, p. 220). In contrast, the acronym HERB first appeared in a GSA meeting abstract (Ripperdan, Reference Ripperdan2002) with neither definition nor referenced data. As this mention does not meet the most basic requirement for publication of a formal stratigraphic term, its usage in this sense should be considered informal. The term ‘HERB Event’ was first used in a formal journal article which appeared (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, p. 400) later in the same volume that the TOCE was named (Zhu et al. Reference Zhu, Babcock and Peng2006, p. 220). Where it first appeared (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, p. 400), it was defined as a negative isotopic peak at a specific point in section, but there was no explanation of the meaning of the acronym. That point was shown (Miller et al. Reference Miller, Ethington, Evans, Holmer, Loch, Popov, Repetski, Ripperdan and Taylor2006, fig. 12) to lie within the negative isotopic excursion defined earlier in the volume (Zhu et al. Reference Zhu, Babcock and Peng2006, p. 220) as the TOCE. The HERB is unequivocally a synonym, and arguably a junior synonym by reason of priority, of the TOCE. For this reason, TOCE, not HERB, was adopted for use in the Geologic Time Scale volumes (Ogg et al. Reference Ogg, Ogg and Gradstein2008, Reference Ogg, Ogg and Gradstein2016; Gradstein et al. Reference Gradstein, Ogg, Schmitz and Ogg2012, Reference Gradstein, Ogg, Schmitz and Oggin press).

As a final note, Sial et al. (Reference Sial, Peralta, Ferriera, Toselli, Aceñolaza, Parada, Gaucher, Alonso and Pimental2008, p. 438) recognized a δ¹³C_carb excursion in the uppermost Cambrian strata of the Argentine Precordillera, which they named ‘SNICE’ (Sunwaptan negative C-isotope excursion). This name of this excursion, which is apparently the same as the TOCE, was introduced without knowledge of earlier names. As a junior synonym, the SNICE has not received broad recognition.