Effects of Bioturbation on Carbon and Sulfur Cycling Across the Ediacaran

Vrije Universiteit Brussel

Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada Hantsoo, Kalev ; Kaufman, Alan ; Cui, Huan; Plummer, Rebecca; Narbonne, Guy

Published in: Canadian Journal of Earth Sciences

DOI: 10.1139/cjes-2017-0274

Publication date: 2018

Document Version: Final published version

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Citation for published version (APA): Hantsoo, K., Kaufman, A., Cui, H., Plummer, R., & Narbonne, G. (2018). Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada. Canadian Journal of Earth Sciences, 55(11), 1240–1252. https://doi.org/10.1139/cjes-2017-0274

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Effects of bioturbation on carbon and sulfur cycling across the Ediacaran–Cambrian transition at the GSSP in Newfoundland, Canada1 Kalev G. Hantsoo, Alan J. Kaufman, Huan Cui, Rebecca E. Plummer, and Guy M. Narbonne

Abstract: The initiation of widespread penetrative bioturbation in the earliest Phanerozoic is regarded as such a significant geobiological event that the boundary between Ediacaran and Cambrian strata is defined by the appearance of diagnostic trace fossils. While ichnofabric analyses have yielded differing interpretations of the impact of Fortunian bioturbation, the disruption of sediments previously sealed by microbial mats is likely to have effected at least local changes in carbon and sulfur cycling. To assess the geochemical effects of penetrative bioturbation, we conducted a high resolution chemostratigraphic analysis of the siliciclastic-dominated basal Cambrian Global Stratotype Section and Point (GSSP; Chapel Island Formation, Newfoundland, Canada). A positive ␦13C excursion in organic matter starts at the Ediacaran–Cambrian boundary and returns to stably depleted values near the top of member 2, while the ␦13C of carbonate carbon increases from strongly depleted values toward seawater values beginning near the top of member 2. Pyrite sulfur coincidently undergoes significant 34S depletion at the Ediacaran– Cambrian boundary. These isotope anomalies most likely reflect progressive ventilation and oxygenation of shallow sediments as a consequence of bioturbation. In this interpretation, sediment ventilation in the earliest Cambrian may have spurred a temporary increase in microbial sulfate reduction and benthic sulfur cycling under low-oxygen conditions. In the late Fortunian, local carbon cycling appears to have stabilized as reductants were depleted and more oxygenated conditions predominated in the shallow substrate. Overall, these data attest to the geochemical significance of the initiation of sediment ventilation by animals at the dawn of the Phanerozoic. Résumé : Le début de la bioturbation pénétrante répandue au tout début du Phanérozoïque est un évènement géobiologique jugé si important que la limite entre les strates édiacariennes et cambriennes est définie par l’apparition d’ichnofossiles diagnostiques. Si les analyses d’ichnofabriques ont produit des interprétations divergentes de l’incidence de la bioturbation fortunienne, la perturbation de sédiments préalablement scellés par des tapis microbiens a vraisemblablement entraîné des modifications locales, à tout le moins, des cycles du carbone et du soufre. Afin d’évaluer les effets géochimiques de la bioturbation pénétrante, nous avons réalisé une analyse chimiostratigraphique de haute résolution du point stratotypique mondial (PSM)

For personal use only. à dominance silicoclastique de la base du Cambrien (Formation de Chapel Island, Terre-Neuve, Canada). Une excursion positive du ␦13C dans la matière organique commence à la limite Édiacarien–Cambrien et revient à des valeurs uniformément appauvries près du sommet du membre 2, alors que, à partir des environs du sommet du membre 2, le ␦13C de carbonates passe de valeurs fortement appauvries à des valeurs qui tendent vers celles de l’eau de mer. Le soufre pyritique présente un appauvrissement concurrent signiﬁcatif en 34S à la limite Édiacarien–Cambrien. Ces anomalies isotopiques reﬂètent fort probablement l’aération et l’oxygénation progressives de sédiments peu profonds des suites de la bioturbation. Selon cette interprétation, l’aération de sédiments au tout début du Cambrien pourrait avoir provoqué une augmentation provisoire de la sulfatoréduction microbienne et une accélération du cycle du soufre benthique dans des conditions pauvres en oxygène. Au Fortunien tardif, le cycle du carbone à l’échelle locale semble s’être stabilisé avec l’épuisement des réducteurs et la prédominance de conditions plus oxygénées dans le substrat peu profond. Collectivement, ces données témoignent de l’importance géochimique du début de l’aération des sédiments par les animaux à l’aube du Phanérozoïque. [Traduit par la Rédaction]

1. Introduction 2011; Erwin and Valentine 2013), preserved in remarkable assem- The Ediacaran–Cambrian transition represents a ﬁrst-order fun- blages of trace and biomineralized body fossils. Some of these damental change in Earth system history. It records an unprece- animals were capable of penetrating the widespread microbial

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 dented increase in the diversity and disparity of soft-bodied and mats that had characterized the Ediacaran Period (McIlroy and shelly faunas representative of modern animal phyla (Erwin et al. Logan 1999; Carbone and Narbonne 2014; Buatois et al. 2014).

Received 21 December 2017. Accepted 11 May 2018. Paper handled by Special Editor Marc Laﬂamme. K.G. Hantsoo. Department of Geology, University of Maryland, College Park, MD 20742 USA; Department of Geosciences, The Pennsylvania State University, University Park, PA 16802 USA. A.J. Kaufman. Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742 USA. H. Cui. Department of Geology, University of Maryland, College Park, MD 20742 USA; NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706 USA. R.E. Plummer. Department of Geology, University of Maryland, College Park, MD 20742 USA. G.M. Narbonne. Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada. Corresponding author: Kalev G. Hantsoo (email: [email protected]). 1This paper is part of a Special Issue entitled “Geobiology of the Ediacaran-Cambrian Transition: ISECT 2017”. Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.

Can. J. Earth Sci. 55: 1240–1252 (2018) dx.doi.org/10.1139/cjes-2017-0274 Published at www.nrcresearchpress.com/cjes on 6 September 2018. Hantsoo et al. 1241

Three temporally overlapping revolutions in the infauna (Mángano 2. Geological background and Buatois 2017), the Cambrian Information Revolution (Plotnick et al. 2010), the Agronomic Revolution (Seilacher and Pflüger 1994; 2.1. Geological history Seilacher 1999), and the Cambrian Substrate Revolution (Bottjer The Chapel Island Formation (Hutchinson 1962) is situated et al. 2000) may have provided greater opportunities for early along the western end of the Burin Peninsula, which is located animals by expanding habitable ecosystems (McIlroy and Logan in southeastern Newfoundland, Canada (Fig. 1). It is part of the 1999; Knoll and Bambach 2000). Ecosystem expansion via biotur- Avalon Terrane, an island arc that developed off the northern bation might explain the macro-evolutionary lag between the di- margin of Gondwana and later accreted to Laurentia. U–Pb dating vergence of major animal clades during the Cryogenian and early of andesites suggests that the volcanic arc developed between 635 Ediacaran periods and their diversification during the Cambrian and 570 Ma, and that Avalonia either rifted from Gondwana be- Explosion (Erwin et al. 2011), which intensified at the end of the tween 585 and 540 Ma, or that a topographic barrier arose and Fortunian Age—the basal Cambrian subdivision constrained to as diverted Gondwanan drainage basins away from the Avalonian little as 10 to 12 million years (Kaufman et al. 2012; Lee et al. 2013). margin (Murphy et al. 2004). Paleogeographic reconstruction Numerous studies worldwide have documented an increase in the places Avalonia at a high latitude in the Ediacaran and Cambrian size, diversity, and architectural complexity of animal burrows (Landing et al. 2013b). The terrane and its shelf sediments later from the late Ediacaran to the early Cambrian (see Mángano and accreted to Laurentia by overthrusting during the Acadian Orog- Buatois 2016 for a recent compilation); however, there is less eny in Devonian time (Bradley 1983). agreement about the impact of these changes on sea floor bioturbation. Some local and global ichnological studies (Droser and 2.2. Lithostratigraphy and depositional facies Bottjer 1988; McIlroy and Logan 1999; Mángano and Buatois 2014, Sedimentary successions similar to the CIF occur discontinu- Mángano and Buatois 2016) document a stepwise increase in the ously at the base of Cambrian successions in Avalonian New- abundance and depth of bioturbation through the Cambrian, foundland, Nova Scotia, and New Brunswick (Geyer and Landing while others conclude that sediment mixing was not significant 2016). In Newfoundland, the CIF is subdivided into five informal until the late Cambrian (Tarhan and Droser 2014) or even the late members based on outcrops at Grand Bank Head (GBH), Fortune Silurian (Tarhan et al. 2015; Tarhan 2018). Head (FH), and Little Dantzic Cove (LDC) (Bengtson and Fletcher Geochemical markers of sedimentary ventilation, oxidation of 1983). The lower four members together constitute the Quaco reduced compounds, and mixing of oxidized products with over- Road Member formally defined in New Brunswick, while member 5 lying seawater are likely to have been recorded in sedimentary is equivalent to the Mystery Lake Member (Landing 1996). On the archives (Pawlowska et al. 2013). Indeed, the global sedimentary Burin Peninsula of Newfoundland, the 1 km thick CIF primarily record of carbon isotope variations reveals a prolonged interval of comprises fine-grained siliciclastic sediments deposited in storm- carbon cycle instability in carbonate-dominated Fortunian strata, dominated, shallow marine and deltaic environments ranging in with rapid oscillations between negative and positive extremes depth from intertidal to just below storm wave-base (Myrow 1992; (Knoll et al. 1995; Kaufman et al. 1996; Brasier and Sukhov 1998; Myrow and Hiscott 1993). Nodules of ferroan dolomite occur spo- Maloof et al. 2010). Similarly, analyses of sedimentary pyrite radically throughout the succession (Brasier et al. 1992; Strauss and carbonate associated sulfate in transitional Ediacaran to Cam- et al. 1992), but bedded limestones are restricted to a few brian successions record significant sulfur isotope variations (Fike decimetre-thick beds in member 4 (Myrow and Landing 1992). and Grotzinger 2008; Cui et al. 2016a, 2016b). Geochemical box At GBH the red-bed dominated Rencontre Formation grades

For personal use only. modeling suggests that sulfur remobilization through bioturba- conformably into CIF member 1, which includes red and green tion would have enhanced oxidative processes and increased sandstones interspersed with siltstones and shales. Lenticular, oceanic sulfate concentration (Canfield and Farquhar 2009). channel-fill sandstone beds, desiccation and syneresis cracks, In siliciclastic-dominated successions, the geochemistry of mud intraclasts, current ripples, and erosional features are pres- early Cambrian oceans has been more difficult to discern. This ent throughout the member, suggesting peritidal marginal ma- includes the Global Stratotype Section and Point (GSSP) for the rine environments. The upper half of the member contains fewer base of the Cambrian System, which lies within the Chapel Island red sandstones and more dark gray shales and siltstones; this Formation (CIF) in Newfoundland, Canada (Fig. 1; Narbonne et al. facies has been interpreted to reflect deposition near a low-energy, 1987; Brasier et al. 1994; Landing 1994; Landing et al. 2013a; Geyer muddy, micro- to mesotidal coastal plain (Myrow and Hiscott and Landing 2016). Chemostratigraphic analyses of sulfur and car- 1993). These gray-black mudstones continue to the top of the GBH bon have been applied to the CIF in prior research, but the data section and into the basal4moftheFHsection. They are overlain were deemed to have little interpretive utility due to low TOC by informal member 2, which is 430 m thick and consists of abundances, wide scatter in isotope data, and generally depleted homogeneous grey and green siltstones and mudstones with thin, 13C abundances in mudstone carbonate (Strauss et al. 1992). Al- storm-deposited sandstone interbeds. Deposition of member 2 though these constraints make it difficult to interpret the succes- occurred predominantly between wave-base and storm wave-base sion’s chemostratigraphy in terms of changes in global ocean in muddy deltaic and shelf environments (Myrow and Hiscott Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 chemistry, carbon and sulfur isotope analysis may nonetheless 1993). The basal Cambrian GSSP is located 2.4 m above the base of permit some useful inferences about benthic reworking of or- member 2 (Narbonne et al. 1987; Landing 1994; Brasier et al. 1994). ganic matter, as well as the degree of solute exchange between The LDC section contains the upper 135 m of member 3, compris- seawater and sediment pore water. Viewed through this lens, the ing thinly laminated silver-green siltstones that were deposited in CIF offers a suitable setting in which to test the chemical effects of a distal shelf setting below storm wave-base, and member 4 (85 m), bioturbation on sediments: it features a thick, homogeneous, and which consists of low energy inner-shelf mudstone with thin per- fine-grained siliciclastic succession of well-preserved sedimentary itidal carbonate caps that contain abundant small shelly fossils rocks that document the progressive increase in size, complexity, (SSFs; Myrow and Landing 1992; Myrow and Hiscott 1993). The and diversity of trace fossils from the terminal Ediacaran to Cam- member 4–5 boundary is a disconformity that reflects basin reor- brian Stage 2. To test the hypothesis that penetrative bioturbation ganization and progradation of a thick sandstone succession. altered the benthic biogeochemistry of this basin, we analyzed a high-resolution stratigraphic collection of CIF mudstones and silt- 2.3. Biostratigraphy stones tied to existing detailed sedimentary and ichnofabric ar- Uppermost Ediacaran deposits (in informal member 1 and the chitectures for their carbon and sulfur abundance and isotopic basal 2.4 m of member 2; Narbonne et al. 1987) contain impres- compositions. sions of Ediacaran soft-bodied fossils such as Harlaniella and

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Fig. 1. The locations of sampled sections of the Chapel Island Formation are labeled: GBH, Grand Bank Head (47.108°N, 55.770°W); FH, Fortune Head (47.075°N, 55.859°W); LDC, Little Dantzic Cove (46.950°N, 55.985°W). These outcrops of the Chapel Island Formation are situated at the western end of the Burin Peninsula, Newfoundland, Canada.

Palaeopascichnus (Fig. 2a); the latter is an index fossil for the upper taphonomic bias (Landing 1993, 2013a; Droser et al. 1999; Maloof Ediacaran worldwide (Antcliffe et al. 2011). Simple trace fossils, prin- et al. 2010). cipally millimetre-diameter burrows Planolites and Helminthoidichnites (Fig. 2f), range throughout the studied section and probably repre- 3. Methods sent horizontal grazing traces within the microbial mats (Buatois A systematic collection of 180 stratigraphic samples from the et al. 2014). Specimens of the organic-walled tube Sabellidites cambriensis wave-cut GBH, FH, and LDC exposures of grey and green mud- (Fig. 2b) range from below the GSSP to the top of the CIF. stones and siltstones through informal members 1 through 4 of Isolated specimens of Treptichnus and Gyrolithes occur in the up- the CIF was analyzed for the abundance and isotopic composition permost few metres of the Ediacaran (Gehling et al. 2001; Laing of carbonate, total organic carbon (TOC), and pyrite sulfur. We

For personal use only. et al. 2018), but the first abundant small-scale penetrative burrows present our integrated stratigraphy relative to the height and of Treptichnus (Figs. 2g and 2h), Gyrolithes (Fig. 2i), and arthropod depth in metres above and below the GSSP datum, respectively. A scratch marks (Fig. 2m) first appear in the Treptichnus pedum Zone, complete description of the methods employed in geochemical which marks the basal strata of the Cambrian System (Crimes and analyses is included in Appendix A. Anderson 1985; Narbonne et al. 1987; Herringshaw et al. 2017). All of the taxa of the Treptichnus pedum Zone continue upwards into 3.1. Sampling strategy younger strata. Overlying strata of the Rusophycus avalonensis Zone The stratigraphic interval analyzed in this study ranges from (from 130 m above the GSSP to the top of the succession) contain near the base of informal member 1 to the top of member 4, a diverse trace fossil assemblage that indicates further sediment covering a stratigraphic interval that is analogous to the Quaco disruption. The R. avalonensis Zone contains more than 20 taxa, Road Member formally defined in New Brunswick (Landing 1996). including the sinuous to meandering horizontal feeding burrow Member 5 was not sampled because it overlies a sequence bound- Psammichnites (Figs. 2e and 2k), the trilobite resting burrow Rusophycus ary and features relatively coarse-grained lithologies. Sampling (Fig. 2j), and the spreiten feeding burrow Teichichnus (Fig. 2l). Speci- for carbon and sulfur analysis was limited to fine-grained green to mens of Rusophycus from the CIF are currently regarded as the world’s grey colored lithologies (mudstones, shales, and siltstones), and oldest unequivocal evidence for the high-level crown-groups Ecdyso- to the thin limestone beds in member 4. Sandstones and red bed zoa, Lobopodia, and Arthropoda (Benton et al. 2015; Wolfe et al. intervals (e.g., 99–155 m above GSSP) were excluded from sampling Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 2016). insofar as oxidized and coarse-grained lithologies are less likely to Mudstones near the top of member 3 record the first appear- preserve organic matter and pyrite. Samples were not selected ances of the early micro-molluscs Watsonella crosbyi and Aldanella from fault surfaces or areas of fault gouge. One carbonate nodule attleborensis (Fig. 2c). These fossils define the base of the Watsonella (GBH1 01.55C) was tested for organic matter abundance and isoto- crosbyi Zone, which is currently under consideration for the base pic composition. of Cambrian Stage 2 of the Terreneuvian Series (Fig. 3a). Member 3 The exposure of member 1 along the coast at GBH is character- also contains specimens of Platysolenites cooperi (McIlroy et al. 2001). ized by steeply inclined beds of alternating red, green, and grey to Peritidal limestones in member 4 contain a Ladatheca wackestone black mudstone. These outcrops are stratigraphically continuous, and new SSF species, including: Igorella ungulata, Coleoloides typicalis, with the exception of three narrow normal faulted intervals with Tiksitheca korobovi, Fomitchella infundabuliformi, Lapworthella ludvigensi, minor stratigraphic offsets. Where strata are continuous, we mea- Allatheca degeeri, and Protohertzina (?) canadia (Landing et al. 1989, sured exact stratigraphic positions relative to our datum; in 2013a; McIlroy and Szaniawski 2000). The preservation of this faulted zones the assigned positions reflect distance perpendicu- diverse assemblage in limestone event beds might be considered lar to dip and have not been corrected across fault surfaces. There- as recording the onset of the Cambrian Explosion, but may fore, we primarily compare the aggregate carbon and sulfur equally or better be regarded as an artifact of faunal migration or values of member 1 to values from demonstrably Cambrian strata,

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Fig. 2. Key soft-bodied fossils (a, b), small shelly fossils (c, d), and trace fossils (e–m) from the Chapel Island Formation at Fortune Head (a, b, e, i–m), Grand Bank Head (f–h), and Little Dantzic Cove (c, d). Stratigraphic ranges are those in the Chapel Island Formation, and are given relative to the basal Cambrian Global Stratotype Section and Point (GSSP). Scale bars represents 1 cm. (a) Palaeopascichnus delicatus Fedonkin, a soft-bodied Ediacara-type fossil impression, range from −80 to −0.2 m; (b) Sabellidites cambriensis Yanishevsky, an organic-walled small shelly fossil, range from −20 m to the top of the succession; (c) Aldanella attleborensis (Shaler and Foerste), pyrite steinkern of an originally aragonitic micromollusc, range from +550 m to the top of the succession (Watsonella crosbyi Zone); (d) Ladatheca cylindrica (Grabau), crushed conch of an originally aragonitic hyolith, range from member 3 or uppermost member 2 to the top of the succession (Ladatheca cylindrica and Watsonella crosbyi Zones); (e) Psammichnites gigas Torell, a feeding burrow, range from +130 m to above the top of the succession (Rusophycus avalonensis Zone); (f) Helminthoidichnites tenuis, a simple horizontal burrow that is common from −160 m to above the top of the succession (longranging taxon occurring in all trace fossil zones); (g, h) Treptichnus pedum (Seilacher), horizontal feeding burrow with vertical probes, range from −6 m to above the top of the succession (Treptichnus pedum and Rusophycus avalonensis Zones); (i) Gyrolithes polonicus Fedonkin, vertical spiral burrow, range from +0.5 to +400 m (Treptichnus pedum and Rusophycus avalonensis Zones); (j) Rusophycus avalonensis Crimes and Anderson, trilobite resting burrow, range from +130 m to above the top of the succession (Rusophycus avalonensis Zone); (k) Psammichnites, regularly meandering grazing burrow, range from +325 to +410 m (Rusophycus avalonensis Zone); (l) Teichichnus rectus Seilacher, a horizontal, spreiten, feeding burrow, range from +280 to above the top of the succession (Rusophycus avalonensis Zone); (m) Arthropod scratch marks, range from +2 m to above the top of the succession (Treptichnus pedum and Rusophycus avalonensis Zones). [Colour online.] For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18

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Fig. 3. Time-series paleontological and geochemical variations through the Chapel Island Formation, Newfoundland, Canada. Red lines represent 10-point moving averages. The grey shaded section represents Interval B (see Section 5.1.), which is stratigraphically deﬁned by the organic carbon isotope excursion; areas below and above the shaded section represent Intervals A and C, respectively. Dashed lines represent lithologic boundaries. (a) System, Stage, Location, Member, Lithology (bar width is qualitative indicator of average grain size), and FADs of key trace and body fossils; (b) mass percent total organic carbon (TOC) of bulk rock; (c) ␦13C of organic carbon, per mil relative to Vienna Pee Dee Belemnite; (d) mass percent carbonate of bulk rock; (e) ␦13C of carbonate carbon, per mil relative to Vienna Pee Dee Belemnite; (f) mass percent sulfur of bulk rock; (g) ␦34S of pyrite sulfur, per mil relative to Vienna Canyon Diablo Troilite. Full trace and body fossil names are as follows: Trp, Treptichnus; Sko, Skolithos; Mon, Monomorphichnus; Gyr, Gyrolithes; Cur, Curvolithus; Rus, Rusophycus; Pas, Psammichnites; Dim, Dimorphichnus; Hed, Helminthoida; Tei, Teichichnus; Hal, Halkieria; Wat, Watsonella; Igo, Igorella; Lap, Lapworthella. Lithologic column adapted from Myrow and Hiscott (1993). [Colour online.] For personal use only. Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18

but we have not interpreted the internal chemostratigraphy of plementary data (Table S12), and are systematically discussed be- member 1. The FH and LDC exposures are continuous and faults low. Based on the geochemical trends, we bin the data into three along the shoreline outcrops have been mapped in detail; samples distinct intervals (see A, B, and C in Figs. 3, 4, 6, and 7). For all collected from those sections are directly keyed to stratigraphic sampled facies, including the peritidal limestones, TOC abun- positions determined in an earlier sedimentological study of the dances range from 0.05% to 0.24% (Fig. 3b), and sulfur abundances CIF (Narbonne et al. 1987). from <0.05% to 1.36% (Fig. 3f). Carbonate abundances in the mudstones range from 3.6% to 42.2% (Fig. 3d). After stoichiometric 4. Results conversion, carbonate carbon constitutes 94.2 ± 2.7% of total car- Elemental and isotope abundances of all analyzed CIF samples bon when averaged for all samples in the succession. There is an along with their lithologic associations are reported in the sup- increase in carbonate abundance between Intervals A and B, rang-

2Supplementary data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjes-2017-0274.

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Fig. 4. Stratigraphic Intervals A, B, and C are defined by a positive excursion in organic carbon isotopes. Chemostratigraphic trends through the Chapel Island Formation (CIF) are expressed here in box and whisker plots. The left and right sides of each box correspond to the lower and upper quartile, respectively, of the data bin. The line in the middle of each box represents the median value of the bin. Whiskers on the left and right of each box represent the total range of values in the bin. Diagonal notches on each box represent the comparison interval at ͌ 5% significance for each median, where the notch width (w) is set by the function w = q2 ± 1.57(q3 – q1)/ n, where q1 is the lower quartile, q2 is the median, q3 is the upper quartile, and n is the number of points in the bin. If two comparison intervals do not overlap between two bins, there is a 95% probability that the median values of those bins are significantly different. For personal use only.

ing from a mean of 11.74% (median = 9.05%) to 13.46% (median = returns to stable pre-excursion values at a height of 378 m above 13.30%). There is also a decrease in sulfur abundance between the GSSP. Intervals A and B, from a mean of 0.36% (median = 0.24%) to 0.16% Conversely, the time-series carbon isotope composition of the ␦13 ␦13 (median = 0.11%). Mean and median TOC abundance shows little carbonate fraction ( Ccarb) reveals an antithetic negative C stratigraphic variation except for a decrease from Interval B to Inter- shift of ϳ2‰ also starting at the base of the Fortunian Stage val C, from a mean of 0.10% (median = 0.09%) to 0.08% (median = (Fig. 3e). These carbonates are strongly depleted in 13C, reaching a 0.07%). stable minimum near −27‰ VPDB in the lower R. avalonensis Zone, 13 ␦13 Organic C abundances ( Corg) in Ediacaran strata (member 1 ϳ ␦13 some 150 to 300 m above the GSSP. Above 300 m, Ccarb values and the basal 2.4 m of member 2; i.e., Interval A) range narrowly, in mudstone samples begin to increase, reaching values in the ␦13 ␴ with a mean Corg of −28.1 ± 1.3‰ VPDB (1 reported for all range of −10‰ to −20‰ VPDB at the top of the measured succes- standard deviations). Within1moftheEdiacaran–Cambrian sion. A local maximum of ␦13C is superimposed on this positive boundary, however, there is a 13C enrichment in organic carbon carb Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 trend and peaks at 565.5 m above the GSSP with a value of −6.85‰ that is sustained over the overlying 378 m of section (i.e., Interval B; VPDB. Figs. 3c and 4a). A 10-point running average line applied to the ϳ ␦34 There is a 60‰ range in Spyr values through the measured organic carbon isotope data changes slope at 0.3 m above the GSSP 34 ␦13 ␦13 interval (Fig. 3g), with a broad (albeit noisy) trend toward lower S datum. In the Corg excursion interval, the range of Corg values is larger than in Interval A (i.e., the pre-excursion interval) and abundances up section. A 10-point moving average line applied ␦34 the mean is −24.4 ± 2.2‰ VPDB, almost 4‰ enriched from Ediac- to the Spyr data set reaches a maximum and begins to dec- ␦13 line between 10 and 20 m below the GSSP, but the slope of the line aran samples of similar facies in (Fig. 4a). The change in Corg values is not correlated with stratigraphic changes in TOC (wt%) steepens sharply less than 1 m above the GSSP datum. The decline ␦13 in ␦34S is coupled with a net decrease in pyrite abundances (Fig. 4d), nor do the Corg values of individual samples appear to pyr vary as a function of TOC (wt%) (R2 = 0.0287; Fig. 5a). Above 378 m (Fig. 4f), including a largely sulfur-barren interval (<0.05% mass for ␦13 all but one sample) from 200 to 400 m above the GSSP in upper from the GSSP, the variance of the Corg data decreases and the ␦34 mean value falls to −28.8 ± 1.0‰ VPDB, similar to the Ediacaran member 2 (Fig. 3f). However, a scatter plot of Spyr versus sulfur mean. Overall, these organic carbon isotope data deﬁne a broad abundance for individual samples does not indicate correlation ϳ ␦34 2 4‰ positive excursion that initiates at the Ediacaran–Cambrian between Spyr and sulfur abundance (R = 0.0678; Fig. 5c). Al- ␦34 GSSP, persists through the FADs of diversifying trace fossils, and though Spyr values in members 3 and 4 are generally depleted,

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Fig. 5. Scatter plots comparing isotopic signatures to elemental sitions. The organic and carbonate carbon isotope trends continue abundance. (a) ␦13C of organic carbon (per mil relative to Vienna Pee between FH and LDC without discernible offsets. On the other Dee Belemnite) versus mass percent total organic carbon (TOC) of hand, the increase in sulfur abundance from upper FH to lower bulk rock; (b) ␦13C of carbonate carbon (per mil relative to Vienna LDC is most likely a facies effect because the sediments of the LDC Pee Dee Belemnite) versus mass percent carbonate of bulk rock; section were deposited in a more distal shelf setting than those in (c) ␦34S of pyrite sulfur (per mil relative to Vienna Canyon Diablo the FH section (Myrow 1992). Even though the overall trend of Troilite) versus mass percent sulfur of bulk rock. sulfur isotope depletion is clear between all three exposures (Fig. 4c), the facies dependence of sulfur abundance makes it dif- ﬁcult to attempt more reﬁned interpretations of the sulfur isotope chemostratigraphy. To minimize isotopic shifts that may have occurred as a result of lithologic changes, sampling was restricted to green and gray shale–mudstone–siltstone intervals, as well as the limestone beds of member 4. Although the member 1–2 boundary marks a lithologic transition from gray-black peritidal shale to green-gray, thinly laminated subtidal siltstone, the organic isotope excursion initiates 0.3 m above the GSSP—i.e., 2.7 m above the base of member 2—as indicated by a 10-point moving average line (Fig. 3c). Additionally, the abrupt termination of the organic isotope excursion occurs in homogeneous mudstones of upper member 2, nearly 40 m below the top of the FH section, and the stabilized ␦13 Corg signal of Interval C persists through mudstone, siltstone, and peritidal limestone facies.

5.1.2. Thermal alteration X-ray diffraction analysis of illites from the CIF indicates maxi- mal burial temperatures of 140–200 °C (Strauss et al. 1992). Gen- erally consistent with this estimate, kerogen and microfossil colors lie in the range of 3–5 on the Thermal Alteration Index (Tissot and Welte 1984), suggesting maximum heating between 150 and 250 °C. This temperature range falls into the zeolite or prehnite–pumpellyite metamorphic facies (Strauss et al. 1992). Insofar as these chemostratigraphic signals are preserved in rapidly accumulating, monotonous, ﬁne-grained siliciclastic facies and the metamorphic grade of the succession is low, it is unlikely that thermal effects (Hayes et al. 1983) were the cause of the carbon isotope excursions or the low overall TOC and high carbonate

For personal use only. abundance in the CIF mudstones. It is also important to note that although the carbonate fraction of the mudstones is clearly not reflective of an oceanic alkalinity signal, the peritidal limestones ␦13 in member 4 record Ccarb values of −3.1‰, −1.2‰, and 0.6‰ VPDB (Strauss et al. 1992), suggesting minimal thermal alteration. Loss of organic carbon via thermal alteration can enrich refrac- ␦34 tory carbon by ϳ6‰ for every 10-fold decrease in carbon abun- there is a local maximum in Spyr that peaks 2.9 m above the FAD of Watsonella crosbyi at a value of 7.52‰ VCDT. dance (Hayes et al. 1983); however, mean and median TOC (wt%) in A Gaussian mixture model was used to sort the data from the the CIF remain nearly uniform over the sampled intervals despite ␦13 ␦34 wide variance (Figs. 3b and 4d), and there is no apparent correla- scatter plot of Corg versus Spyr into three bins (Fig. 6). The ␦13 resulting clusters are nearly identical to Intervals A, B, and C, tion between TOC (wt%) and Corg values (Fig. 5a). Furthermore, which are stratigraphically defined by the initiation and termina- low average TOC abundances and wide variance in TOC (wt%) have ␦13 tion of the organic carbon isotope excursion (see Section 5.2.). Five not affected the Corg signal. This is evident from comparing out of 69 points receive divergent classifications between the Interval B to Interval C: both have similar amounts of variance in ␦13 model-generated set and the stratigraphically bounded set, of TOC (wt%), but Interval C has substantially less variance in Corg (Fig. 4).

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 which only one point, 10.69 m above the GSSP (i.e., FH 10.69), is situated near an interval boundary. It is evident that the low TOC (wt%) values in the CIF do not result from thermal diagenesis; instead, we suggest that they re- 5. Discussion flect microbial remineralization and the production of authigenic ␦13 carbonate. This is corroborated by depleted values of Ccarb in 5.1. Controlling factors of the isotopic variations mudstone cements through most of the sampled interval that ␦13 5.1.1. Paleobathymetry and lithology give way to unaltered seawater Ccarb values in the peritidal Organic carbon isotope variations in the CIF have been attrib- limestone beds. The observation that the carbon and sulfur anom- uted to paleobathymetric variations between peritidal and distal alies coincide with paleontological evidence for penetrative bio- shelf settings (Strauss et al. 1992). However, the interval that be- turbation also suggests that the measured geochemical changes ␦13 gins 378 m above the GSSP features uniform Corg values despite resulted from sediment ventilation by earliest Cambrian animals, paleobathymetric deepening from middle (upper member 2) to with the additional possibly of changes in water column carbon distal (member 3) shelf environments, followed by shallowing to cycling. Thus, while the isotope chemostratigraphy of the CIF inner shelf and peritidal (member 4) settings regardless of the might not offer a reliable imprint of primary oceanic signals, it sampling locality (Fig. 3c). These observations indicate that water does likely reflect changes in the exchange of fluids and gases ␦13 column depth did not exert a strong influence on Corg compo- across the sediment–water interface and the biological reworking

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Fig. 6. A comparison of the stratigraphic Intervals A, B, and C, which are defined by the positive organic carbon isotope excursion, to the ␦13 ␦34 result of a Gaussian Mixture Model that sorted the same data set in Corg – Spyr space. (a) A Gaussian Mixture Model was applied to the ␦13 ␦34 69 data points for which both Corg and Spyr data were available. The maximum number of iterations of the expectation-maximization algorithm was set to 1000, and the number of data groups was prescribed at three. The model was generated in Matlab by applying the ‘cluster’ function to the output of the ‘fitgmdist’ function. The code used to generate this subplot is included in the supplementary data (File S12). (b) The same set of 69 points is plotted in the same space but grouped according to the stratigraphic Intervals A, B, and C. Five points fall into groups that are different from the clusters generated by the Gaussian Mixture Model, of which one point (FH 10.69) is relatively close to an Interval boundary. These five points are circled and labeled. The other 64 points are sorted identically between the two sets. For personal use only. of sedimentary organic matter associated with penetrative biotur- 5.2.2. Interval B (Early Fortunian) ␦13 ␦13 ␦34 bation. In short, we interpret the Corg, Ccarb, and Spyr Although some Ediacaran organisms may have supplied oxygen signals as indicators of sedimentary ventilation by animals and to the sediments beneath their own bodies via simple diffusion the microbial response to changes in the oxidation state and sul- (Dufour and McIlroy 2016), evidence for more vigorous sediment fate abundance of pore waters (Fig. 7). ventilation by animals starts at the base of the Treptichnus pedum Zone defining the Ediacaran–Cambrian boundary (Narbonne et al. 5.2. Biogeochemical reconstruction: Intervals A, B, and C 1987; Bottjer et al. 2000; Herringshaw et al. 2017). This mode of bioturbation may have been a response to an increase of organic 5.2.1. Interval A (Terminal Ediacaran) matter export from the water column to the sediments due to the Ediacaran strata of the CIF are distinguished by a combination efficient packaging into fecal pellets associated with the advent of of enriched pyrite sulfur isotopes and depleted organic carbon coelomate animals (Logan et al. 1995; McIlroy and Logan 1999), or isotopes (Fig. 6). Carbonate carbon isotopes are strongly depleted to the evolution of larger phytoplankton in response to increased and are probably the authigenic products of anaerobic microbial zooplankton predation (Butterfield 2009). Coelomate-grade ani- respiration. These isotopic signals were generated in part by the mals may also have sought shelter from predation in the sedi- sedimentary sealing that characterized the Ediacaran sea floor. ments. Cambrian-style bioturbation probably increased solute Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 Matground biofilms inhibited diffusion of seawater alkalinity and exchange between seawater and pore water, especially if biotur- sulfate into the sediments, as well as the diffusion of methane and bating animals established advective flow pathways in pore water sulfide out of them. Additionally, the burial flux of organic carbon via bio-irrigation (Herringshaw et al. 2017). may have been degraded and refractory as a result of inefficient A temporal buildup of sulfate in the Chapel Island basin is 34 export and long residence times in the water column (McIlroy and suggested by the 60‰ decrease of S abundances of pyrite in the mudstone samples from the base to the top of the measured sec- Logan 1999). Matgrounds would have attenuated oxidative pro- tion. We attribute this large shift to progressively greater degrees cesses in the sediments, promoting the formation of authigenic 13 34 of sulfur isotope fractionation during open system MSR (Sim et al. C-depleted carbonate (Sun and Turchyn 2014) and S-enriched 2011; Zhelezinskaia et al. 2014), made possible through the venti- sulfide through closed-system microbial sulfate reduction (MSR). lation of sediments. Ventilation spurred the conversion of H Sto 34 2 The S enrichment observed in Ediacaran CIF samples matches sulfate or intermediate sulfur compounds by way of chemolitho- the pattern of high ␦34S values noted in other terminal Ediacaran trophic sulfide oxidation. Reoxidative sulfur cycling in shallow strata (cf. Cui et al. 2016a, 2016b), which has been interpreted to sediments had become globally significant by the early Ediacaran represent the efficient burial and sequestration of pyrite (Fike and (Kunzmann et al. 2017), and even in low-oxygen bottom waters, Grotzinger 2008; Tostevin et al. 2017). bioturbation would have intensified reoxidative sulfur cycling via

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Fig. 7. Schematic diagram of interpreted geochemical effects of explanation for the low pyrite abundance through much of mem- bioturbation through the Chapel Island Formation, based on ber2(van de Velde and Meysman 2016). interpretation of carbon and sulfur chemostratigraphy. Intervals A, Carbon isotope variations in Interval B may also result from B, and C are defined by the positive excursion in organic carbon MSR reactions in a regime of intensified sulfur cycling. The posi- ␦13 isotopes. (a) The late Ediacaran regime featured slow particle tive Corg excursion in refractory organic carbon from basal settling, limited diffusion of sulfate into sediments, and microbial Cambrian strata of the CIF is an unusual feature; the only other reworking of relatively degraded organic matter. (b) The early documentation of a potential positive excursion in organic carbon Fortunian featured more rapid particle sinking and penetrative in basal Cambrian strata is from a similar siliciclastic-dominated bioturbation. In a regime of more rapid sulfur cycling and lower succession on the East European Platform (Strauss et al. 1997). We benthic oxygen, microbial sulfate reduction (MSR) increased. hypothesize that the organic carbon isotope excursion docu- (c) By the late Fortunian, more thorough sediment ventilation by mented in the CIF resulted from enhanced burial of labile organic bioturbating animals resulted in more complete oxygenation of the matter. This would occur as a result of benthic oxygen decline, shallow substrate as its reservoir of reduced compounds was which could have resulted from increasingly efficient export of depleted. Aerobic respiration replaced anaerobic respiration as the organic matter (McIlroy and Logan 1999; Butterfield 2009), or dominant mode of benthic microbial heterotrophy, which, in from a decline in primary production spurred by more efficient concert with increased seawater mixing, led to the decline of phosphorus burial through bioturbation (Boyle et al. 2014). In isotopically depleted authigenic carbonates in mudstone cements. either case, bottom water deoxygenation would confer an advan- tage to anaerobic respiration in the shallow substrate. Consump- tion of organic matter by MSR is slower and less quantitative than aerobic respiration, and the anaerobic process notably increases pore water alkalinity (LaRowe and van Cappellen 2011). In either oxic or anoxic sediments, refractory organic compounds tend to be enriched in 13C by 3‰–4‰ relative to their more labile coun- terparts (McArthur et al. 1992; Böttcher et al. 1998; Freudenthal et al. 2001). With these observations in mind, Interval B is viewed as a period of low-oxygen conditions in which MSR generated strongly 13C-depleted carbonates and left behind refractory organic matter enriched in 13C(Fig. 7). As a whole, this biogeochemical scenario would explain the increase in carbonate abundance ␦13 ␦13 and Corg values along with the decrease in Ccarb, %S, and ␦34 Spyr values above the Ediacaran–Cambrian boundary. In short, a combination of oxygen depletion in bottom waters and increased sedimentary sulfur cycling—both potentially spurred by bioturbation—can account for the isotopic shifts in carbon and sulfur seen in Interval B.

5.2.3. Interval C (Late Fortunian and Early Stage 2)

For personal use only. ␦13 The Corg signal returns to pre-excursion values 378 m above the GSSP. We propose that this reflects a stabilization of local carbon and oxygen cycling, at which point MSR was outcompeted by metabolically more efficient aerobic respiration in shallow sed- ␦13 iments (Fig. 7). This is attested by the end of the Corg excursion, ␦13 the decrease in TOC (wt%), and the progressive increase of Ccarb. The shift toward aerobic respiration in the shallow substrate would have hindered benthic production of authigenic carbonate, ␦13 thus driving Ccarb toward more positive values as MSR was forced into deeper sediments. Greater infiltration of seawater alkalinity into ventilated sediments likewise would have driven an ␦13 increase in Ccarb values. Reorganization of the local carbon cycle in the late Fortunian might be related to water column oxygenation, which likely increased during this interval (Chen et al. 2015). Near the top of the analyzed section at LDC, diagnostic

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 SSFs of Cambrian Stage 2 appear in mudstone and carbonate fa- ␦13 cies. In this interval, mudstone Ccarb compositions rise toward seawater values, which are preserved in the near 0‰ values of the bedded limestones (Brasier et al. 1992; Strauss et al. 1992). This ␦13 shift in Ccarb is in line with model results that imply that Pha- nerozoic stabilization of the carbon isotope record, including the decline of authigenic carbonate formation, may have resulted bacterial sulfur disproportionation. As long as oxygen was a lim- from bioturbation (Boyle et al. 2018). Values of ␦13C remain iting reagent, MSR in shallow sediments would have increased in org stably depleted through most of Interval C, while pyrite sulfur a regime of more rapid sulfur cycling. As implied by the sulfur- isotope abundances are variable but strongly depleted in 34S rela- barren interval at FH (Fig. 3f), progressive substrate oxygenation tive to underlying strata. ultimately depleted shallow sediments of sulfides, after which MSR reactions were pushed deeper into the substrate and lost 5.2.4. Summary of biogeochemical interpretations their prominent role in remineralization reactions. Reactive Our Interval zonation is defined by the stratigraphic boundaries ␦13 transport modeling also demonstrates that bio-irrigation tends to of the positive excursion in Corg. In our interpretation, Interval flush sulfur compounds out of sediments, which offers further A (−128 to 0 m) reflects an Ediacaran regime of sealed substrates

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and negligible solute exchange with seawater, with deposition of back processes, or reservoir sizes that act as inputs for a second refractory organic carbon and Rayleigh distillation of sulfur iso- generation of models. For example, the sulfur-based box model of topes. Interval B (0 to 378 m) reflects initial sediment ventilation Canfield and Farquhar (2009), also adapted by Tarhan et al. (2015), by bioturbating animals, probably concurrent with low-oxygen includes a coefficient (x) for the fraction of MSR-generated sulfide conditions. MSR would be expected to increase if bioturbation that escapes burial (in this case as a result of bio-irrigation or the

initially lowered water column pO2 (Boyle et al. 2014), or if more lack of ferrous iron to produce pyrite), as well as an exponent (y) efficient export productivity shifted oxygen demand toward the that modulates the response of MSR to increases in oceanic sulfate seafloor (McIlroy and Logan 1999). In either case, penetrative bio- concentration: turbation and bio-irrigation would have promoted the oxidative sulfur cycle in sediments, which would explain the precipitous F Ϫ E 1/y ϭ ͫ in ͬ ␦34 (1) [SO4]ocean drop in Spyr values above the GSSP (Canfield and Farquhar axOC 2009). Inefficient consumption of organic matter by MSR (LaRowe and van Cappellen 2011) would have led to burial of more refrac- where F is the flux of sulfur into the ocean, E is the flux of sulfur tory organic carbon, which would account for the positive excur- in into evaporites, a is a constant of proportionality, and OC is the sion in ␦13C compositions (Freudenthal et al. 2001). Interval B org average sedimentary concentration of labile organic matter. The ended as the oxygen buildup depleted reductants from the shal- exponent y would equal unity if the rate of MSR increased as a low substrate, forcing MSR deeper into the sediments. Interval C linear function of oceanic sulfate concentration, but the dimin- (378 to 626 m) in turn likely featured ventilation and oxygenation ishing concentration of labile organic matter in deeper sediments of sediments along with more efficient aerobic respiration of the typically limits MSR rates even as sulfate penetration increases, benthic detrital flux. Notably, oxygenation of the shallow sub- thus reducing the value of y. strate as suggested by this biogeochemical model was followed by In their models, both Canfield and Farquhar (2009) and Tarhan the local first appearance of biomineralizing organisms of Cam- et al. (2015) lower the value of x to represent the overall influence brian affinity. of bioturbation, but hold y constant through each simulation. The 5.3. Sediment disruption versus sediment ventilation reactive transport model of van de Velde and Meysman (2016), Previous studies of the CIF have shown an abrupt appearance of however, implies that both x and y are sensitive to the mode of penetrative burrows near the base of member 2 (Narbonne et al. bioturbation. These authors suggest that bio-mixing increases y by 1987; Gehling et al. 2001; Laing et al. 2018) with increased depth of advecting labile carbon downward from surface sediments, while burrowing and intensity of bioturbation upward through the bio-irrigation decreases y by oxygenating sedimentary pore wa- formation (McIlroy and Logan 1999; Gougeon et al. 2018). Our ters and thereby limiting the activity of sulfate reducers despite geochemical data suggest that this earliest Cambrian infaunal greater sulfate penetration. Both modes of bioturbation are ex- activity was sufficient to affect geochemical cycling on at least a pected to decrease values of x, but bio-irrigation decreases it more localized scale. Furthermore, a regime of limited sediment dis- effectively via sulfide flushing. In short, by lowering both x and y, ruption does not preclude infaunal generation of the isotopic bio-irrigation would have been responsible for a larger flux of changes documented in the CIF, especially in light of the fact that sulfur to the water column than bio-mixing, and it may have bio-irrigation (Herringshaw et al. 2017) and agrichnia (i.e., the induced significant chemical fluxes even if bio-mixing was not stimulation of microbial growth in burrows via accelerated pervasive. Of course, sulfur fluxes would have been sensitive to

For personal use only. redox cycling; Laing et al. 2018), are attributed to key fossils of other changes such as depth variations of the sedimentary redox- the Treptichnus pedum Zone within the CIF. These behaviors would cline, changes in microbial ecology, and the oxygenation of the have increased rates of physical and biogeochemical cycling, even water column; nonetheless, the geochemical profile of the CIF in the absence of more physically disruptive behaviors such as indicates that bio-irrigation may have been a significant factor in sediment bulldozing. Additionally, lateral permeability within establishing early Cambrian fluxes of sulfur between the sedi- upper matground sediments tends to be significantly higher than ment and the water column. permeability across the matground–water interface, allowing for lateral fluid transport within matground sediments (Wieland 6. Conclusions et al. 2001). To investigate the biogeochemical effects of penetrative biotur- This raises an important question about the relationship be- bation across the Ediacaran–Cambrian boundary, we conducted a tween physical disruption of sediments and chemical cycling in chemostratigraphic analysis of the elemental and isotopic abun- sediments: how abruptly did rates of chemical cycling and remo- dances of carbon and sulfur at the Ediacaran–Cambrian GSSP ␦13 bilization respond to incipient penetrative bioturbation, and how (Chapel Island Formation, Newfoundland). Trends in Ccarb, ␦13 ␦34 sensitive were these rates to varying modes of burrowing behavior? Corg, and Spyr from Ediacaran to Cambrian strata suggest The reactive transport model of van de Velde and Meysman (2016) that penetrative burrowing at the GSSP coincided with geochem- distinguishes bio-mixing (i.e., advection of sediment), which in- ical changes in the water column and (or) in sediments. Isotopic Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18 creases rates of MSR by forcing labile organic carbon down into depletion in pyrite sulfur above the GSSP suggests that burrowing sediments, from bio-irrigation (i.e., advection of fluid), which ef- increased the availability of sulfate to the sediments through pro- ficiently flushes sulfur from the sediments into the water column, gressive ventilation that stimulated the oxidative sulfur cycle. thereby depleting the pore water sulfide reservoir. Notably, the This reordering of the benthic sulfur cycle may have played a role model also demonstrates that incipient, low-intensity bio-mixing in the increase of marine sulfate concentrations, particularly if rapidly increases the rate of MSR, which then levels off as the bio-irrigation was a common tracemaking behavior. ␦13 sediments are more intensely stirred. These results are consistent We argue that the Corg excursion and the accompanying with the isotopic imprint of strengthening MSR in Interval B fol- increase in carbonate abundance documented in the lower Fortu- ␦13 lowed by weakening MSR in Interval C, as attested by Ccarb and nian strata of the CIF reflect increased rates of MSR under tran- ␦13 Corg trends. Additionally, the sulfide flushing effect of bio- sient low-oxygen benthic conditions. A decline in bottom water irrigation may explain the large pyrite–barren interval in member 2. pO2 in the earliest Cambrian could have resulted from more effi- Our understanding of the effects of bio-mixing and bio-irrigation cient biological pumping, although it is possible that bioturbation

on Cambrian geochemistry will beneﬁt from reconciling and re- itself may have lowered water column pO2 by burying phosphorus ﬁning the parameters of different numerical models, especially if and inhibiting primary production. A decline in bottom water pO2 model outputs are used to constrain divergent responses, feed- in concert with an increased supply of sulfate and labile organic

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matter would have spurred an increase in shallow sedimentary isotope stratigraphy of Early Cambrian carbonates in southeastern New- MSR, which in turn may have generated the organic carbon iso- foundland and England. Geological Magazine, 129(3): 265–279. doi:10.1017/ S001675680001921X. tope excursion that terminates in upper Fortunian strata. The end Brasier, M., Cowie, J., and Taylor, M. 1994. Decision on the Precambrian–Cambrian of the organic carbon isotope excursion implies stabilization of boundary stratotype. Episodes, 17:3–8. carbon and oxygen cycling in the shallow substrate, potentially Buatois, L.A., Narbonne, G.M., Mángano, M.G., Carmona, N.B., and Myrow, P. related to a balance between the production, export, and remin- 2014. Ediacaran matground ecology persisted into the earliest Cambrian. eralization of organic matter and (or) to a stabilization of water Nature Communications, 5: 3544. doi:10.1038/ncomms4544. Butterfield, N.J. 2009. Oxygen, animals and oceanic ventilation: an alternative ␦13 column pO2. Meanwhile, the Ccarb profile exhibits a strong au- view. Geobiology, 7: 1–7. doi:10.1111/j.1472-4669.2009.00188.x. thigenic imprint that weakens considerably up section, suggest- Canfield, D.E., and Farquhar, J. 2009. Animal evolution, bioturbation, and the ing an increase of seawater mixing and a decline in the activity of sulfate concentration of the oceans. Proceedings of the National Academy of MSR by the late Fortunian. In short, the isotopic signals in the CIF Sciences, 106: 8123–8127. doi:10.1073/pnas.0902037106. indicate ventilation and ultimately oxygenation of the shallow Carbone, C., and Narbonne, G.M. 2014. When life got smart: The evolution of behavioral complexity through the Ediacaran and early Cambrian of NW substrate, followed by the appearance of biomineralized Cam- Canada. Journal of Paleontology, 88: 309–330. doi:10.1666/13-066. brian fossils. Our findings suggest that bioturbation induced Chen, X., Ling, H.F., Vance, D., Shields-Zhou, G.A., Zhu, M., Poulton, S.W., et al. geochemical changes in the earliest Cambrian sediments of New- 2015. Rise to modern levels of ocean oxygenation coincided with the Cam- foundland, and that oxygenation of the shallow substrate was at brian radiation of animals. Nature Communications, 6: 7142. doi:10.1038/ ncomms8142. least a localized precursor to the appearance of biomineralizing Crimes, T.P., and Anderson, M.M. 1985. Trace fossils from Late Precambrian- animals of Cambrian affinity. Early Cambrian strata of southeastern Newfoundland (Canada): temporal and environmental implications. Journal of Paleontology, 59(2): 310–343. Acknowledgements Cui, H., Grazhdankin, D.V., Xiao, S., Peek, S., Rogov, V.I., Bykova, N.V., et al. The authors acknowledge Richard Thomas and Parks and Nat- 2016a. Redox-dependent distribution of early macro-organisms: Evidence from the terminal Ediacaran Khatyspyt Formation in Arctic Siberia. Palaeo- ural Areas, Newfoundland and Labrador for providing a Scientific geography, Palaeoclimatology, Palaeoecology, 461: 122–139. doi:10.1016/j. Research Permit for the study; Ryan Abrams, Thomas Boag, and palaeo.2016.08.015. Brittany Laing for assistance in sample collection; Ernest Follett Cui, H., Kaufman, A.J., Xiao, S., Peek, S., Cao, H., Min, X., et al. 2016b. Environ- for logistical assistance; and Elizabeth Lee and Jake Gearon for mental context for the terminal Ediacaran biomineralization of animals. help with geochemical measurements. This research is funded by Geobiology, 14(4): 344–363. doi:10.1111/gbi.12178. Droser, M.L., and Bottjer, D.J. 1988. Trends in depth and extent of bioturbation in NASA Exobiology (NNX12AR91G to A.J.K.), NSF Sedimentary Geol- Cambrian carbonate marine environments, western United States. Geology, ogy and Paleontology (EAR0844270 to A.J.K.), a Natural Sciences 16(3): 233–236. doi:10.1130/0091-7613(1988)016<0233:TIDAEO>2.3.CO;2. and Research Council of Canada (NSERC) Discovery Grant and a Droser, M.L., Gehling, J.G., and Jensen, S. 1999. When the worm turned: Concordance Queen’s University Research Chair to G.M.N., the Society of Eco- of Early Cambrian ichnofabric and trace-fossil record in siliciclastic rocks nomic Geologists (SEG) Student Research Grant and the Explorers of South Australia. Geology, 27: 625–628. doi:10.1130/0091-7613(1999)027 <0625:wtwtco>2.3.co;2. Club Exploration Fund Grant to H.C. H.C. wishes to thank the Dufour, S.C., and McIlroy, D. 2016. Ediacaran pre-placozoan diploblasts in the NASA Astrobiology Institute in UW–Madison for support, and Avalonian biota: the role of chemosynthesis in the evolution of early animal K.G.H. thanks the Penn State University Department of Geosci- life. Geological Society, London, Special Publications, 448, SP448-5. ences. The authors jointly wish to thank Canadian Journal of Earth Erwin, D.H., and Valentine, J.W. 2013. The Cambrian explosion: the construction of animal biodiversity. Genwodd Village, Colorado: Roberts and Company. Sciences editor Ali Polat and associate editor Marc Laflamme for Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., and their assistance, as well as two anonymous reviewers whose input Peterson, K.J. 2011. The Cambrian conundrum: Early divergence and later For personal use only. benefited the manuscript. Finally, the authors thank the organiz- ecological success in the early history of animals. Science, 334: 1091–1097. ers of the ISECT 2017 special issue of Canadian Journal of Earth doi:10.1126/science.1206375. ␦34 Sciences. The authors declare that there are no conflicts of interest Fike, D.A., and Grotzinger, J.P. 2008. A paired sulfate–pyrite S approach to understanding the evolution of the Ediacaran–Cambrian sulfur cycle. regarding this manuscript. Geochimica et Cosmochimica Acta, 72: 2636–2648. doi:10.1016/j.gca.2008.03. 021. References Freudenthal, T., Wagner, T., Wenzhöfer, F., Zabel, M., and Wefer, G. 2001. Early Antcliffe, J.B., Gooday, A.J., and Brasier, M.D. 2011. Testing the protozoan hypoth- diagenesis of organic matter from sediments of the eastern subtropical At- esis for Ediacaran fossils: A developmental analysis of Palaeopascichnus. Palae- lantic: evidence from stable nitrogen and carbon isotopes. Geochimica et ontology, 54: 1157–1175. doi:10.1111/j.1475-4983.2011.01058.x. Cosmochimica Acta, 65(11): 1795–1808. doi:10.1016/S0016-7037(01)00554-3. Bengtson, S., and Fletcher, T.P. 1983. The oldest sequence of skeletal fossils in the Gehling, J.G., Jensen, S., Droser, M.L., Myrow, P.M., and Narbonne, G.M. 2001. Lower Cambrian of southeastern Newfoundland. 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trap and separated by a gas chromatography (GC) column prior to above) using measured NBS-127 sulfur abundances and carbonate

elemental and isotopic analysis. Two urea standards (CH4N2O; 20% percentages determined by acidification was used to correct sam- C abundance with ␦13C = −29.39‰) were measured between every ple values. 10 samples to monitor the accuracy and precision of measure- Attempts at CO2 generation via phosphoric acid digestion of ments. Organic carbon isotope measurements are reported as ‰ minor and trace mudstone carbonate with our on-line system deviations (±0.1‰, 1␴) from the Vienna Pee Dee Belemnite (VPDB) were hindered by incomplete reactions. This was probably due to standard, and abundances are reported as TOC (wt%) (weight per- the presence of trace siderite, ankerite, or other insoluble late cent total organic carbon) relative to the pre-acidified mass of diagenetic carbonates in the CIF mudstones. These minerals resist sample powder. TOC abundances were calculated by first deter- dissolution in phosphoric acid at standard temperatures and re- mining the TOC (wt%) of acidified residues with action times. X-ray diffraction analyses of several representative mudstone samples did not yield carbonate mineral abundances because of interference from the high abundance of silicates. ϭ 20.0 (A1) %TOCresidue ͫ ͬ%TOCmeasured Therefore, in lieu of acid digestion, total carbon abundance and %Cavg.urea isotope compositions of non-acidified bulk sample powders were measured by combustion in the same manner as acidified resi- where 20.0 is the known mass percentage of carbon in urea; dues. The bulk sample powders were analyzed along with a house 13 %Cavg.urea is the mean mass percentage of carbon measured in the standard carbonate (JTB-1; 12.0% C abundance with ␦ C = −1.78‰ 13 batch urea samples; and %TOCmeasured is the uncorrected mass VPDB) using the combustion system. Carbonate ␦ C values ␦13 ␦13 ␦13 percentage of carbon measured in an acidified sample. The TOC ( Ccarb) were then calculated from Corg and Cbulk values (wt%) of bulk rock was then calculated by factoring in the fraction using the mass balance equation of bulk powder lost during acidification: ␦13 (A4) Ccarb %CO ( 13C␦ ء 13C Ϫ (%TOC␦ ء [(0.120 ء A2) %TOC ϭ %TOC ͫ1 Ϫ 3ͬ [%TOC ϩ (%CO) bulk residue 100 ϭ bulk 3 bulk bulk org ء %CO3 0.120

where %CO3 is the mass percentage of carbonate in the bulk sam- ␦13 ␦ ple as determined by acidification. Determining percent carbon- where Cbulk is the -value of total carbon in a non-acidified ␦13 ␦ ate via acidification can yield inaccurate results if the rock sample; Corg is the -value of the organic carbon in the sample; contains an appreciable amount of salts, but there is no indication and 0.120 is the molar fraction of carbon in calcium carbonate. that the CIF was deposited in an evaporative facies. Corrected Carbonate carbon isotope measurements are reported as ‰ devi- ␴ ations from the Vienna Pee Dee Belemnite (VPDB) standard. Error standard deviations ( corr) for %TOCbulk were calculated with ␴ ␦13 values (1 ) for Ccarb measurements range from 0.2‰ to 0.5‰ %TOC (Table S12, Supplementary Material); these values derive from er- ␴ ϭ ͫ bulkͬ␴ (A3) corr urea ror analyses of the JTB-1 carbonate standards combusted for each 20.0 ␦13 batch of Cbulk analyses. Assuming the CIF carbonate fraction includes trace siderite, ankerite, or other late diagenetic carbon- ␴ where 20.0 is the mass percentage of carbon in urea and urea is ates, we evaluated whether the presence of these minerals would

For personal use only. ␦13 the standard deviation of all measured carbon mass percentages have an effect on Ccarb values determined by mass balance of the urea standards in a batch. calculations. For example, changing the molar fraction term to ϳ For sulfur analyses, 0.3 mg of V2O5 was added to 1–8 mg of 0.104 (i.e., assuming that 100% of carbonate minerals are siderite) ␦13 powdered residues to ensure quantitative combustion at 1030 °C would change resulting values of Ccarb by only 0.02 ± 0.03‰. It in a different quartz combustion column filled with high purity is further possible that a fraction of very fine clay material was lost copper wire. To determine the accuracy and precision of sulfur from the samples during acidification and leaching, which would

measurements, two NBS-127 barite standards (BaSO4; 12.74% S introduce error into the %CO3 term in Eqs. (A2) and (A4). However, ␦34 with S = +20.3‰) and two IAEA-S-1 standards (Ag2S; 12.94% S even if it is assumed that each sample lost 3% of its bulk mass to ␦34 with S = −0.3‰) were measured between each set of 10 samples. silicate dissolution, the resulting reduction in %CO3 would change ␦ ␦13 Sulfur isotope data are expressed with the notation as per mil the calculated Ccarb values by a mean of only 0.17‰ (median of (‰) deviations (±0.3‰, 1␴) from the Vienna Canyon Diablo Troilite 0.08‰); in all but nine samples, the absolute value of the error (VCDT) international standard. For total sulfur (TS) abundance of would be less than 0.5‰. the samples, a calculation similar to those used for TOC (see Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Queens University on 11/01/18

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