Project

The Cambrian GSSP at Fortune Head, Newfoundland, Canada: a critical reassessment of the Ediacaran-Cambrian boundary

Goal: In 1992, the International Stratigraphic Commission ratified the Cambrian GSSP at the base of the Treptichnus pedum IAZ at Fortune Head, Newfoundland, Canada. Since then, important debate surrounds its validity and many researchers underscore the greater efficiency of other proxies (e.g. Small Shelly Fossils, BACE δ13Ccarb excursion). The aim of this project is to clarify our understanding of early complex metazoan activities and evolutionary patterns that led to the Cambrian explosion.

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Romain Gougeon
added a research item
The Chapel Island Formation (CIF) at the Burin Peninsula, Newfoundland, Canada, is a 1000+ m-thick siliciclastic succession that hosts the Cambrian GSSP (ca. 541 Ma) 2.4 m above the base of its member 2 (M2) at Fortune Head. In this section, the first appearance of Treptichnus pedum, the index fossil for the base of the Cambrian, but also of other burrows (e.g., Bergaueria, Gyrolithes) represent some of the earliest evidence of complex behavior and penetrative bioturbation made by metazoan-grade organisms. Although Fortune Head has been the focus of most previous work on the CIF, Ediacaran and early Cambrian strata are also reported in three other sections, namely Grand Bank Head, Lewin’s Cove, and Point May. Despite difficulties related to the quality of exposure of these other sections (e.g., gaps, faults, folds), compiling ichnologic datasets from these localities could help to test the robustness of the patterns observed at Fortune Head. Detailed information on sedimentary facies and trace fossils, the latter including trace fossil occurrences, trace fossil sizes and depths, and bioturbation intensities (Bioturbation Index [BI] and Bedding Plane Bioturbation Index [BPBI]), has been collected from the four sections. Integrating these datasets has allowed the reconstruction of Ediacaran and early Cambrian ichnofabrics for each section. In Fortune Head, Grand Bank Head and Lewin’s Cove, the Ediacaran is dominated by low ichnodiversity, mm-scale burrow size and depth, and low bioturbation intensities (BI = 0-1, BPBI = 1-2). Conversely, the basal Fortunian is marked by a burst in ichnodiversity and ichnodisparity, the appearance of cm-scale burrow size and depth, and a slight increase in bioturbation intensities (BI = 0-2, BPBI = 1-4). Point May presents some challenges, as the lack of bedding surface views hampers the quality of the trace fossil dataset. Nevertheless, higher bioturbation intensities (BI = 2) and vertical burrows (Bergaueria, Gyrolithes) appeared respectively 4.5 m and 17.5 m above the base of M2, therefore representing typical Fortunian ichnofabrics. Comparing the vertical distribution of trace fossils and ichnofabrics across Ediacaran and early Cambrian strata in the different localities at the Burin Peninsula underscores the robustness of the Fortune Head section as the GSSP and indicates that the patterns observed at the type section can be detected in the whole region as well.
Romain Gougeon
added a research item
The Chapel Island Formation (CIF) of Newfoundland, Canada, is a 1000+ m-thick siliciclastic succession that hosts the Cambrian GSSP (ca. 541 Ma) 2.4 m above the base of its member 2 (M2) in Fortune Head. Although the first appearance of Treptichnus pedum was considered as the marker of the base of the Cambrian, other burrows typical of the Fortunian (e.g. Bergaueria, Gyrolithes) made their first appearance close to the base of M2 as well. Fortune Head has been the focus of most studies in the area, but Ediacaran and Cambrian strata are also recorded in three additional sections of the CIF. Grand Bank Head, located ⁓ 6 km northeast of Fortune Head, consists of a thick Ediacaran interval (member 1, or M1), and a good exposure of M2; however, the beginning of the Fortunian is located on a faulted zone with difficult access. The succession at Lewin’s Cove, located ⁓ 45 km east of Fortune Head, includes strata from M1 up to the middle of M2; however, rock exposure is hampered by discontinuities and vegetation cover. Finally, Point May is located ⁓ 17 km southwest of Fortune Head and holds a record of the top of M1 to M2 with a fault affecting the M1/M2 contact. Detailed information on sedimentary facies and trace fossils, the latter including trace fossil occurrences, trace fossil sizes and depths, and bioturbation intensities [Bioturbation Index (BI) and Bedding Plane Bioturbation Index (BPBI)], has been collected from Fortune Head, Grand Bank Head, Lewin’s Cove, and Point May. At the former three sections, the Ediacaran is dominated by low ichnodiversity, mm-scale burrow size and depth, and low bioturbation intensities (BI = 0-1, BPBI = 1-2). Conversely, the basal Fortunian is marked by a burst in ichnodiversity and ichnodisparity, the appearance of cm-scale burrow size and depth, and a slight increase in bioturbation intensities (BI = 0-2, BPBI = 1-4). However, a different situation is apparent at Point May, with trace fossil diversity and bioturbation intensities remaining relatively low through M2. Facies analysis at Point May shows that depositional environments fluctuated between the lower and upper offshore, which are also common at the three other localities. However, the paucity of bedding plane exposures, which are the most favorable setting to detect predominantly horizontal Cambrian trace fossils (e.g. Psammichnites, Rusophycus, Treptichnus), may have impacted negatively on trace fossil identification. This may be the reason why the Point May section does not provide confident examples of Treptichnus pedum. Therefore, other ichnologic criteria had to be employed to delineate the basal Fortunian, such as the first appearance of vertical burrowing (Bergaueria, Gyrolithes) and slightly higher bioturbation intensities (BI = 2). These results demonstrate that robust sedimentological analyses coupled with an assessment of outcrop quality are essential procedures for any ichnological studies focussing on this critical time of the history of life.
Romain Gougeon
added 2 research items
The Cambrian GSSP (ca. 540 Ma) is marked by the appearance of complex trace fossil from the Treptichnus pedum Ichnofossil Assemblage Zone in the Chapel Island Formation (CIF) at Fortune Head, Newfoundland in Canada (Brasier et al., 1994). The CIF is a 1000+ m-thick, mostly continuous siliciclastic succession that ranges from the late Ediacaran to Cambrian Stage 2. Our present study has relogged this section in detail using the most up to date ichnotaxonomy and facies interpretation. Ichnologic work also focussed on reporting bioturbation index (BI; sensu Taylor & Goldring, 1993) and bedding plane bioturbation index (BPBI; sensu Miller & Smail, 1997) and measuring trace fossil width and depth. Tiering and ecospace utilization were evaluated based on this data. Similar depositional environments reoccur through the Fortunian and Cambrian Stage 2 intervals of the CIF, allowing comparisons of ichnofaunas formed in similar environmental settings at different times. The Fortunian is then characterised by surficial to shallow-tier, diverse assemblages, whereas the Cambrian Stage 2 displays shallow- to deep-tier, moderately to poorly diverse assemblages. Ichnologic analysis through the CIF indicates a temporal increase in burrow size, BI, BPBI and depth of penetration (Gougeon et al., 2018, 2019). Change in ecospace utilization results from the ecologic engineering effects related to the appearance of large sediment bulldozers and deep-tier suspension feeders during the Fortunian-Cambrian Stage 2 transition (Mángano and Buatois, 2014; Gougeon et al., 2018, 2019). The bioturbation by deposit feeders and the bioadvective effects of suspension feeders significantly altered the benthic habitat, causing modifications of the sediment structure, sediment stability and increased pore water and solute movements deeper within the sediment.
Romain Gougeon
added a research item
The mixed layer of modern oceans is a zone of fully homogenized sediment resulting from bioturbation (Berger et al., 1979; Teal et al., 2008). The mixed layer is host to complex biogeochemical cycles that directly impact ecosystem functioning, affecting ocean productivity and marine biodiversity. However, the timing of appearance of a mixed zone at the sediment-water interface remains uncertain within a 100 million year interval stretching from the early Cambrian to the late Silurian (Mángano & Buatois, 2014; Tarhan et al., 2015), hindering palaeontological and geochemical studies of this key milestone in Earth evolution. In our study (Gougeon et al., 2018) we provide detailed evidence from the Global Stratotype Section and Point (GSSP) of the basal Cambrian in the Burin Peninsula of Newfoundland, Canada, demonstrating that a mixed layer of similar structure to that of modern ocean was well established in shallow marine settings by the early Cambrian (approximately 529 my ago). In contrast with modern seafloors, Ediacaran marine sediment surfaces were pervasively coated with resistant microbial mats that acted as geochemical filter between the underlying sediment and overlying seawater (Seilacher, 1999). Significant reductions in the prevalence of microbial mats coupled with widespread bioturbation and associated bioirrigation would have permitted increasingly free-interchange between surface sediments and the water column, dramatically changing fluid chemistry of both of these regimes (Canfield & Farquhar, 2009; Seilacher, 1999). Our analysis of bioturbation in this section shows that substrate exploitation in the Ediacaran was restricted to non-penetrative horizontal grazing of microbial mats, whereas a mixture of superficial mat grazing trails, undermat-mining burrows and small penetrative burrows characterize the Fortunian and the establishment of the marine sediment mixed layer is the main innovation of Cambrian Age 2. During the Fortunian, a drastic change in the ecosystem is typified by the appearance of Psammichnites, a large, backfilled horizontal burrow of the shallow-tier produced by sediment bulldozers. At first fairly scarce, the predominance of Psammichnites higher up in the section is coincidental with the general increase in sediment churning, reaching a critical point close to the Cambrian Age 2 where a sediment mixed layer has been identified. These findings imply that an accurate description of early Cambrian ichnocoenoses is a robust method to properly decipher major changes in the sediment sea-floor at this critical time.
Brittany Laing
added a research item
The transition between the seemingly disparate Ediacaran and Cambrian faunas is both enigmatic and body-fossil poor. The Chapel Island Formation on the Burin Peninsula, Newfoundland, Canada, contains a rich diversity of ichnofossils, providing new insight into the nature of the Ediacaran-Cambrian transition and early Fortunian ecosystems. Five ichnoguilds are recognized within the Treptichnus pedum zone. Ichnologic data are analysed from an ecospace perspective, revealing a more protracted transition between Ediacaran and Cambrian ecosystems. Our analysis documents the appearance of limbs, vertical burrows and uncontroversial equilibrium structures, as well as the retention of 'other' feeding styles, such as microbial grazing and chemosynthesis.
Romain Gougeon
added 2 research items
The Ediacaran-Cambrian transition is the only boundary defined by the first appearance datum (FAD) of a trace fossil (Treptichnus pedum). Consequently, the placement of the Global Boundary Stratotype Section and Point (GSSP) at Fortune Head, Burin Peninsula, Newfoundland, has recently resurfaced as a topic of interest. Reluctance to the use of ichnofossils as biostratigraphic markers is based upon the idea that ichnofossils are facies controlled. While the broad environmental tolerance of T. pedum has been demonstrated, its suitability as an index fossil remains questioned by some. Redefining the boundary by using an ichnoassemblage spanning a broader range of depositional environments may address these queries. Ongoing research in Fortune Head and a global compilation of the ichnology of the Ediacaran-Cambrian boundary emphasizes the importance of re-evaluating previous ichnotaxonomic determinations worldwide. In addition to T. pedum, other elements of this Ichnofossil Assemblage Zone (IAZ) are Gyrolithes, Allocotichnus, Diplichnites and Streptichnus. Re-examination of the interval containing the Ediacaran-Cambrian boundary and lowermost Fortunian strata may benefit from incorporating conceptual and methodological tools from the emerging field of stratigraphic paleobiology. One approach would be to re-examine and categorize the ichnofossils in these strata based on their complexity. In turn, a hierarchical classification of the ichnofossils based upon evolutionary innovations would be created, assisting in the definition of an IAZ. This more robust boundary would be less dependent on discrete depositional environments. Rather, it would be strongly based upon evolutionary innovations, following in the spirit of the original decision by the International Commission on Stratigraphy (ICS).
The mixed layer of modern oceans is a zone of fully homogenized sediment resulting from bioturbation. The mixed layer is host to complex biogeochemical cycles that directly impact ecosystem functioning, affecting ocean productivity and marine biodiversity. The timing of origin of the mixed layer has been controversial, with estimates ranging from Cambrian to Silurian, hindering our understanding of biogeochemical cycling and ecosystem dynamics in deep time. Here we report evidence from the Global Stratotype Section and Point (GSSP) of the basal Cambrian in the Burin Peninsula of Newfoundland, Canada, showing that a well-developed mixed layer of similar structure to that of modern marine sediments was established in shallow marine settings by the early Cambrian (approximately 529 million years ago). These findings imply that the benthos significantly contributed to establishing new biogeochemical cycles during the Cambrian explosion.
Romain Gougeon
added a project goal
In 1992, the International Stratigraphic Commission ratified the Cambrian GSSP at the base of the Treptichnus pedum IAZ at Fortune Head, Newfoundland, Canada. Since then, important debate surrounds its validity and many researchers underscore the greater efficiency of other proxies (e.g. Small Shelly Fossils, BACE δ13Ccarb excursion). The aim of this project is to clarify our understanding of early complex metazoan activities and evolutionary patterns that led to the Cambrian explosion.
 
Romain Gougeon
added a research item
The appearance of complex organisms on Earth is a critical event that has led to profound, irreversible changes of the seafloor. In particular, the base of the Cambrian (541.0 ± 1.0 Ma) is marked by the appearance of the complex trace fossil Treptichnus pedum 2.4 m above the base of member 2 of the Chapel Island Formation at Fortune Head, Burin Peninsula, Newfoundland in Canada. Unsurprisingly, recent ichnological studies have focused on the boundary itself rather than on the whole formation. Most of the ichnotaxonomic identifications pertaining to the whole Chapel Island Formation had been performed in the late eighties, a time where the study of Ediacaran and early Cambrian trace fossils was still in its infancy. Recent progress in ichnotaxonomy including the use of ichnotaxobases and the availability of conceptual and methodological tools to assess the role of bioturbation is prompting re-evaluation of the Chapel Island Formation ichnofauna. In particular, reevaluation of key specimens and assessment of synonymies will help to refine ichnostratigraphic schemes and to calibrate ichnodiversity and ichnodisparity changes during this critical time in the history of the biosphere. Notably, many Ediacaran fossils that were previously regarded as trace fossils are now considered body fossils (e.g. Harlaniella, Intrites, Palaeopascichnus). Others, like Monomorphichnus, can be easily confused with tool marks, and careful examination and evaluation of biogenicity should be done in a case-bycase basis. Finally, many of previously defined ichnotaxa can be put in synonymy with other ichnogenera on the basis of recent ichnotaxonomic reviews (e.g. Buthotrephis, Helminthoida, Hormosiroidea, Neonereites, Taphrhelminthopsis). Our analysis indicates that the earliest Cambrian is characterized by secular changes, including an increase in trace fossil diversity and disparity, degree of morphologic complexity, and depth and extent of bioturbation. These dramatic changes started well below the appearance of the earliest body fossils in the succession and argue in favor of the current placement of the Cambrian GSSP based on ichnologic evidence.
Brittany Laing
added 2 research items
The beginning of the Cambrian explosion is characterized by the onset of infaunalization and the appearance of systematic patterns of burrowing. The trace fossil Gyrolithes is common in the Ediacaran-Cambrian Global Stratotype Section and Point, where it shows a higher diversity and burrow depth than previously reported from any Cambrian spiral vertical burrows. Two ichnospecies are present: G. scintillus isp. nov. exhibits a small burrow radius to whorl radius ratio, whereas G. gyratus (Hofmann) exhibits an exceptionally large ratio that formerly led to its identification as an aberrant ichnospecies of Skolithos. The helical morphology of these Cambrian Gyrolithes is interpreted as having served two purposes: as an anchor in a relatively high-energy environment, and as an optimal shape for maximizing surface area for bacterial gardening. These shallow-marine gardening burrows share a similar feeding strategy as graphoglyptids, suggesting that shallow early Cambrian sediments may have been relatively poor in organics and thus further supporting the model for onshore-offshore migration of gardening burrows.