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High-resolution C-isotope chemostratigraphy of the uppermost Cambrian stage (Stage 10) in South China: implications for defining the base of Stage 10 and palaeoenvironmental change

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Abstract

The Wa'ergang section in South China has been proposed as a potential Global Stratotype Section and Point (GSSP) for the base of Stage 10, the uppermost stage of the Cambrian System. In this study, high-resolution C-isotopic compositions are reported and we identified three large negative δ ¹³ C excursions, namely N1, N2 and N3, at Wa'ergang. The N1 is located just above the First Appearance Datum (FAD) of Lotagnostus americanus , corresponding to the possible base of the Proconodontus posterocostatus conodont Zone. The N2 was identified within the Micragnostus chuishuensis trilobite Zone and the Proconodontus muelleri conodont Zone. The N3 is located in the lowermost part of the Leiagnostus cf. bexelli – Archaeuloma taoyuanense trilobite Zone or Eoconodontus conodont Zone. The N1 and N2 can be correlated with the negative δ ¹³ C excursions preceding the Top of Cambrian Carbon Isotope Excursion (TOCE) observed globally. The N3 can be correlated with the TOCE or the HEllnmaria–Red Tops Boundary (HERB) Event. The inter-basinal correlation of N1 and L. americanus strongly supports that the base of Stage 10 may be best defined by the FAD of L. americanus . We also used a box model to quantitatively explore the genesis of the negative δ ¹³ C excursions from South China. Our numerical simulations suggest that weathering of the organic-rich sediments on the platform, probably driven by intermittent sea level fall and/or the oxygenation of the Dissolved Organic Carbon (DOC) reservoir in seawater, may have contributed to the generation of the negative δ ¹³ C excursions observed in the Stage 10 at Wa'ergang in South China.

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... However, it is organic matter export and burial that drives most primary isotope variations within the marine system and governs the overall DIC composition of the oceans that, in turn, also affects the isotopic composition of buried marine organic matter (Cramer and Jarvis, 2020). During the last three decades, a rapid development of carbon isotope chemostratigraphy has led to a breakthrough in global correlation of the Furongian, especially carbonatedominated successions, resulting in clarification of many stratigraphic relationships (Ripperdan et al., 1992;Saltzman et al., 1998Saltzman et al., , 2000Saltzman et al., , 2004Zhu et al., 2006Zhu et al., , 2019Landing et al., 2011;Miller et al., 2011Miller et al., , 2014aAzmy et al., 2014;Azmy, 2019;Li et al., 2017;Scorrer et al., 2019;Wang et al., 2019Wang et al., , 2021. Until recently, Baltic successions have received much less attention, primarily because i) Baltica was located at high latitudes (~60 o S, Torsvik and Cocks, 2016) with very limited deposition of carbonates; ii) this old craton was geographically isolated from contemporary palaeocontinents and characterized by a comparatively endemic shelly fauna; and iii) sediment starvation resulted in condensed deposition with numerous breaks in inboard settings throughout the Cambrian and Ordovician (e.g. ...
... Powder samples (~2 g) devoid of visible macroscopic pyrite concretions, calcite veins and limestone intercalations were sampled using a Gerhardt and Gill (2016), others include Baltica Ahlberg et al., 2018), Australia (Ripperdan et al., 1992;Saltzman et al., 2011), Western Laurentia (Landing et al., 2011;Miller et al., 2011Miller et al., , 2014aMiller et al., , 2015, Eastern Laurentia (Ripperdan and Miller, 1995;Miller et al., 2011Miller et al., , 2014bAzmy et al., 2014;Azmy, 2019), South China (Li et al., 2017;Zuo et al., 2018) and North China (Ripperdan et al., 1993;Chen et al., 1995;Wang et al., 2019Wang et al., , 2021. (b) Present-day distribution of lower Palaeozoic strata in Baltoscandia (modified from Nielsen et al., 2020). ...
... However, no matching high-amplitude negative δ 13 C excursion has been observed in the lower Acerocarina Superzone of the studied sections (Fig. 4). It is of course theoretically possible that local environmental changes may have concealed a younger δ 13 C org excursion in Scandinavia, but we tentatively assume that the discussed excursions are synonymous, following Zhu et al. (2019Zhu et al. ( , 2020, Terfelt et al. (2014), Miller et al. (2015, Li et al. (2017) and Ahlberg et al. (2018), as well as the latest edition of the Geologic Time Scale volume . In the Grönhögen-2015 core, the onset of the excursion labelled as TOCE by Ahlberg et al. (2018) appears to start earlier than in the Håslöv-1 core described by Terfelt et al. (2014). ...
Article
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To evaluate petroleum resources, it is important to quantify the hydrocarbon generation and expulsion characteristics of source rock. The hydrocarbon generation potential method is an efficient model for calculating the characteristics of source rocks and evaluating the resource potential. The previous hydrocarbon generation potential model was mainly established by measured Rock-Eval pyrolysis data, and due to inappropriate preservation measures, an important component (volatile hydrocarbon content) of source rock may be significantly reduced. Prior methods may, therefore, not objectively reflect the properties of a source rock and cause unacceptable errors in the resource assessment. This study proposes an improved hydrocarbon generation potential method. The new method considers the initial hydrogen index (HI⁰) and transformation ratio (Tr) during the evolution of source rocks, which can more realistically reflect the hydrocarbon generation and expulsion history of source rocks. Therefore, the hydrocarbon evaporative loss (Qloss) is calibrated and the hydrocarbon generation and expulsion features of source rocks are more accurately evaluated. The proposed method is used to examine the effectiveness of the third member of the Upper Paleogene Shahejie Formation (Es3) in Nanpu Sag. By utilizing the improved hydrocarbon generation potential model, the hydrocarbon expulsion threshold of the Es3 source rock is positioned at Ro = 0.875%. The maximum hydrocarbon generation and expulsion potential (Qg and Qe) are 593.78 and 323.78 mg HC/g TOC, respectively, and the maximum hydrocarbon expulsion efficiency is 54%. The maximum hydrocarbon generation and expulsion intensities are 500 × 10⁴ t/km² and 250 × 10⁴ t/km², respectively, which locate source rock at the Laoyemiao and Gaoshangpu Structural Belts. The amounts of generated and expelled hydrocarbon are 13.7 × 10⁸ t and 6.9 × 10⁸ t, respectively, and the resource potential of the tight oil of the Es3 in the Nanpu Sag is 0.69 × 10⁴ t. Compared with the previous method, the improved hydrocarbon generation potential method limits the maximum Qg and calibrates the Qloss; therefore, the new method more objectively reflects the hydrocarbon generation and expulsion characteristics and resource prospect of the source rock.
... However, the lower boundary of the zone seems to be far down below the exposed section (Cooper et al. 2001). The conodont biozonation documented for the studied section at Martin Point ( Fig. 2) spans approximately the lower part of the Proconodontus muelleri Zone and reaches down to the Proconodontus posterocostatus Zone on the global conodont biozonation scheme including that of N. America (James & Stevens, 1986;Barnes, 1988;Miller et al. 2011;Li et al. 2017). The P. muelleri and P. posterocostatus zones have been documented below the Eoconodontus notchpeakensis Zone in the Lawson Cove and Sneakover Pass sections of Utah, USA (Miller et al. 2011), where the base of the notchpeakensis Zone is marked by the distinct HERB δ 13 C excursion (Miller et al. 2011;Li et al. 2017;Azmy, 2018). ...
... The conodont biozonation documented for the studied section at Martin Point ( Fig. 2) spans approximately the lower part of the Proconodontus muelleri Zone and reaches down to the Proconodontus posterocostatus Zone on the global conodont biozonation scheme including that of N. America (James & Stevens, 1986;Barnes, 1988;Miller et al. 2011;Li et al. 2017). The P. muelleri and P. posterocostatus zones have been documented below the Eoconodontus notchpeakensis Zone in the Lawson Cove and Sneakover Pass sections of Utah, USA (Miller et al. 2011), where the base of the notchpeakensis Zone is marked by the distinct HERB δ 13 C excursion (Miller et al. 2011;Li et al. 2017;Azmy, 2018). The HERB δ 13 C excursion has been documented in the GSSP section of Green Point (e.g. ...
... The P. muelleri and P. posterocostatus zones have not been documented yet in the Martin Point section but are based on the global scheme (e.g. Li et al. 2017) and also on that suggested for the Cow Head Group and North America (Barnes, 1988;Miller et al. 2011). The grey line highlights the main changes in the δ 13 C profile of the investigated carbonates. ...
Article
The δ ¹³ C profile from the lower interval of the Martin Point section in western Newfoundland (Canada) spans the Upper Cambrian (uppermost Franconian – lowermost Trempealeauan). The investigated interval (∼110 m) is a part of the Green Point Formation of the Cow Head Group and consists of the upper part of the Tucker Cove Member (topmost part of the Shallow Bay Formation) and the lowermost part of the Martin Point Member (bottom of the Green Point Formation). It is formed of rhythmites of marine carbonates alternating with shales and minor conglomeratic interbeds. Multiscreening petrographic and geochemical techniques have been utilized to evaluate the preservation of the investigated lime mudstones. The δ ¹³ C and δ ¹⁸ O values of the sampled micrites (−4.8 ‰ to +1.0 ‰ VPDB and −8.2 ‰ to −5.3 ‰ VPDB, respectively) have insignificant correlation (R ² = 0.01), as similarly do the δ ¹³ C values with their Sr counterparts (R ² = 0.07), which supports the preservation of at least near-primary δ ¹³ C signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ ¹³ C profile exhibits two main negative excursions: a lower excursion (∼4 ‰) that reaches its maximum at the bottom of the section and an upper narrow excursion (∼6 ‰) immediately above the boundary of the Tucker Cove/Martin Point members (Shallow Bay Formation – Green Point Formation boundary). The lower excursion may be correlated with the global SPICE event, whereas the upper excursion may match with a post-SPICE event that has been also recognized in profiles of equivalent sections on different palaeocontinents.
... This species has its first observed appearance in Laurentia at the base of the E. notchpeakensis Subzone of the Eoconodontus Zone (conodonts), equivalent to the middle of the Saukia Zone (Saukiella junia Subzone, polymerid trilobites). The FAD coincides with the onset of the TOCE (Top of Cambrian carbon isotope Excursion), a negative δ 13 C excursion alternatively referred to as the HERB Event (Ripperdan, 2002;Miller et al., 2006Miller et al., , 2011Miller et al., , 2015Li et al., 2017; but see Landing et al., 2011, who recognized two separate excursions/events). E. notchpeakensis has a long stratigraphic range, extending into the I. fluctivagus Zone of the Lower Ordovician. ...
... Carbon isotope (δ 13 C) and strontium isotope ( 87 Sr/ 86 Sr) chemostratigraphy of the Cambrian System and comparison to biotic events. The carbon isotope chemostratigraphy and comparison to biotic events is modified from Zhu et al. (2006), Miller et al. (2006, Howley and Jiang (2010), and Li et al. (2017). The strontium curve is a composite derived from curves for the upper Terreneuvian Series and lower Series 2 (Derry et al., 1994); upper Series 2 through the Miaolingian Series (Montañez et al., 2000); and much of the Furongian Series (Saltzman et al., 1995;Kouchinsky et al., 2008), except for the uppermost part of Stage 10 (Ebneth et al., 2001). ...
... The peak of the TOCE (Top of Cambrian carbon isotope Excursion; Zhu et al., 2006) is in a short interval below the base of the Ordovician, and its peak value is about 23.5m. The TOCE also has been referred to as the HERB (HEllnmariaÀRed Tops Boundary) Event (Ripperdan, 2002;Peng et al., 2012a;Li et al., 2017;Azmy, 2019), which is a junior synonym of TOCE as the term became available and valid later than TOCE. As recognized by Li et al. (2017), there are three successive δ 13 C negative excursions within Stage 10 (N1, N2, and TOCE) with the N1 excursion located just above the base of Stage 10. ...
Chapter
Appearance of metazoans with mineralized skeletons, “explosion” in biotic diversity and disparity, infaunalization of the substrate, occurrence of metazoan Konservat Fossil-lagerstätten, establishment of most invertebrate phyla, strong faunal provincialism, dominance of trilobites, generally warm climate but with possible glacial–interglacial cycles in the later part, opening of the Iapetus Ocean, progressive equatorial drift and separation of Laurentia, Baltica, Siberia, and Avalonia from Gondwana characterize the Cambrian Period.
... Although the La Flecha dolostones have not yielded diagnostic fauna, fossils from under-and overlying strata confirm a late Cambrian age and demonstrate the correlation utility of the HERB Event, even in unfossiliferous units. A similar negative carbon isotope shift also occurs at the base of E. notchpeakensis-bearing strata at Dayangcha, North China (Ripperdan et al. 1993), while other studies in Baltica and South China recognize the HERB Event but use the name TOCE for it (Terfelt et al. 2014;Li et al. 2017;Ahlberg et al. 2019; discussed in the following section). ...
... A fourth iteration of the TOCE correlation exists. By this correlation, Li et al. (2017) and Ahlberg et al. (2019) placed the TOCE excursion below the Prosaukia serotina Subzone and in the Saukiella junia Subzone, synonymized it with the earlier-named HERB Event, and abandoned the designation HERB Event. Landing et al. (2011, fig . ...
... While we agree with Terfelt et al. (2014) that there is only one large (>2‰) 13 C carb excursion in Stage 10, it is clear that the excursion should be identified properly as the HERB Event. Ahlberg et al. (2019) and Li et al. (2017) also documented a single strong negative carbon excursion within middle Stage 10 in the uppermost Cambrian of the Baltic and South China but, confusingly, use the designation TOCE, while equating and synonymizing this excursion with the HERB interval. The peculiar inattention to the appropriate name for this middle Stage 10 excursion extends to reports on its type area which correctly use HERB (Miller et al. 2006(Miller et al. , 2011(Miller et al. , 2014 but later, and without explanation, report TOCE for the same excursion at the same localities (Miller et al. 2015(Miller et al. , 2018. ...
Article
Chemostratigraphic units require consistent definitions and unambiguous names. So-called TOCE (Top of Cambrian Excursion) is used as an uppermost Cambrian δ ¹³ C carb negative excursion although it was proposed without documentation, is ambiguously defined, and variably correlated into four Laurentian trilobite zones. TOCE, a nihilartikel, is regularly substituted to the exclusion of the earlier named, precisely documented and geochronologically older HERB (Hellnmaria-Red Tops Boundary) Event. HERB allows late Cambrian global correlation; its onset is close to the lowest occurrence of the conodont Eoconodontus notchpeakensis at the base of a proposed replacement (Lawsonian Stage) of Cambrian Stage 10. TOCE must be retired from use and abandoned as a synonym of the HERB Event.
... Ripperdan, Magaritz & Kirschvink, 1993;Chen et al. 1995;G. S. Nowlan, unpublished report, 1995;Buggisch, Keller & Lehnert, 2003;Jing et al. 2008;Miller et al. 2011Miller et al. , 2014Azmy et al. 2014;Terfelt, Eriksson & Schmitz, 2014;Li et al. 2017). Similar δ 13 C excursions, caused by variations in primary productivity or organic preservation, have been documented in marine environments throughout the Earth's history (e.g. ...
... Chen et al. 1995;Jing et al. 2008;Landing, Westrop & Adrain, 2011;Miller et al. 2011Miller et al. , 2014Azmy et al. 2014;Terfelt, Eriksson & Schmitz, 2014, fig. 9;Li et al. 2017, fig. 4). ...
... 4). Barnes, 1988;Terfelt, Eriksson & Schmitz, 2014;Li et al. 2017). The peaks of the δ 13 C excursions are marked. ...
Article
The δ ¹³ C profile from an interval of the Martin Point section in western Newfoundland (Canada) spans the upper Furongian (uppermost Cambrian). The interval (~90 m) is a part of the Green Point Formation of the Cow Head Group and consists of the Martin Point (lower) and the Broom Point (upper) members. It is formed of slope marine carbonates alternating with shales (rhythmites) and conglomeratic interbeds. The preservation of the investigated micritic carbonates was meticulously evaluated by multiple petrographic and geochemical screening tools. The δ ¹³ C and δ ¹⁸ O values (−0.5 ± 0.8 ‰VPDB and −7.1 ± 0.3 ‰VPDB, respectively) exhibit insignificant correlation ( R² = 0.002) and similarly the correlation of δ ¹³ C values with their Sr and Mn counterparts, which supports the preservation of at least near-primary δ ¹³ C signatures that can be utilized to construct a reliable high-resolution carbon-isotope profile for global correlations. The δ ¹³ C profile exhibits two main negative excursions, a lower broad excursion (~3 ‰) that reaches its maximum at ~70 m below the Martin Point / Broom Point members boundary and an upper narrow excursion (~2.5 ‰) immediately below the same boundary. The lower excursion can be correlated with the global latest Furongian HERB event (TOCE), which is also recognized in the C-isotope profile of the GSSP boundary section at Green Point whereas the upper excursion matches with that of the Cambrian‒Ordovician boundary in the same section. The peak of the HERB δ ¹³ C excursion is correlated with positive shifts on the Th/U and Ni profiles (redox and productivity proxies).
... However, it is organic matter export and burial that drives most primary isotope variations within the marine system and governs the overall DIC composition of the oceans that, in turn, also affects the isotopic composition of buried marine organic matter (Cramer and Jarvis, 2020). During the last three decades, a rapid development of carbon isotope chemostratigraphy has led to a breakthrough in global correlation of the Furongian, especially carbonatedominated successions, resulting in clarification of many stratigraphic relationships (Ripperdan et al., 1992;Saltzman et al., 1998Saltzman et al., , 2000Saltzman et al., , 2004Zhu et al., 2006Zhu et al., , 2019Landing et al., 2011;Miller et al., 2011Miller et al., , 2014aAzmy et al., 2014;Azmy, 2019;Li et al., 2017;Scorrer et al., 2019;Wang et al., 2019Wang et al., , 2021. Until recently, Baltic successions have received much less attention, primarily because i) Baltica was located at high latitudes (~60 o S, Torsvik and Cocks, 2016) with very limited deposition of carbonates; ii) this old craton was geographically isolated from contemporary palaeocontinents and characterized by a comparatively endemic shelly fauna; and iii) sediment starvation resulted in condensed deposition with numerous breaks in inboard settings throughout the Cambrian and Ordovician (e.g. ...
... Powder samples (~2 g) devoid of visible macroscopic pyrite concretions, calcite veins and limestone intercalations were sampled using a Gerhardt and Gill (2016), others include Baltica Ahlberg et al., 2018), Australia (Ripperdan et al., 1992;Saltzman et al., 2011), Western Laurentia (Landing et al., 2011;Miller et al., 2011Miller et al., , 2014aMiller et al., , 2015, Eastern Laurentia (Ripperdan and Miller, 1995;Miller et al., 2011Miller et al., , 2014bAzmy et al., 2014;Azmy, 2019), South China (Li et al., 2017;Zuo et al., 2018) and North China (Ripperdan et al., 1993;Chen et al., 1995;Wang et al., 2019Wang et al., , 2021. (b) Present-day distribution of lower Palaeozoic strata in Baltoscandia (modified from Nielsen et al., 2020). ...
... However, no matching high-amplitude negative δ 13 C excursion has been observed in the lower Acerocarina Superzone of the studied sections (Fig. 4). It is of course theoretically possible that local environmental changes may have concealed a younger δ 13 C org excursion in Scandinavia, but we tentatively assume that the discussed excursions are synonymous, following Zhu et al. (2019Zhu et al. ( , 2020, Terfelt et al. (2014), Miller et al. (2015, Li et al. (2017) and Ahlberg et al. (2018), as well as the latest edition of the Geologic Time Scale volume . In the Grönhögen-2015 core, the onset of the excursion labelled as TOCE by Ahlberg et al. (2018) appears to start earlier than in the Håslöv-1 core described by Terfelt et al. (2014). ...
Article
Full-text available
Continuous and biostratigraphically well-constrained carbon isotope (δ¹³Corg) profiles through the upper Miaolingian (middle Cambrian) to Tremadocian (Lower Ordovician) part of the Alum Shale Formation in four drill cores from southernmost Scandinavia are described. The sections record seven Furongian (upper Cambrian) excursions/spikes, of which three appear valuable as auxiliary markers for identifying global stage boundaries. The Steptoean Positive Carbon Isotope Excursion (SPICE) event with a magnitude of up to 2.8‰ is located mainly within the Olenus Superzone. The onset, as specified here, coincides with the end-Marjuman extinction and is marked by a minor negative excursion (up to −1.6‰) right below the Miaolingian–Furongian boundary. The end of the SPICE event approximates the upper boundary of the Olenus Superzone and has potential for correlation of the Paibian–Jiangshanian boundary. The Top of Cambrian Excursion (TOCE) with a magnitude of about −1.6‰ straddles the Parabolina lobata–Parabolina heres megalops zonal boundary in Baltoscandia. This chemostratigraphic excursion, considered as a synonym of the Hellnmaria-Red Tops Boundary excursion (HERB), represents a secondary marker for correlating the base of Stage 10, if defined by the FAD of Eoconodontus notchpeakensis. A positive spike at the Cambrian–Ordovician boundary, here named COBS, recognized in the stratotype section at Green Point (Canada; δ¹³Ccarb magnitude +5.3‰), Scandinavia (δ¹³Corg magnitude up to +2.2‰) and elsewhere, appears as a valuable supplementary marker for recognizing this important boundary. The described δ¹³Corg profile from the Albjära-1 core provides a standard reference for Furongian carbon isotope chemostratigraphy in Baltica, refines the correlation between Baltica and other palaeocontinents, and indicates new possibilities for pinpointing international stage boundaries within the Furongian based on carbon isotope records.
... The TOCE excursion exhibits large spatial variations in δ 13 C values (4-1.5‰) (Miller et al., 2011;Li et al., 2017). It should be noted that the TOCE excursion has commonly been mixed with the HERB and SNICE excursions in the literature. ...
... No data are available in the interval from the upper Xiaotanian Stage to the basal Chiungchussuan Stage due to an absence of carbonate lithologies. The data of the middle-upper Chiungchussuan Stage are from archaeocyathid-bearing carbonates in central Guizhou, while the data from the Duyunian to the top of the Cambrian are from the continuous carbonate sequence which is characterized in slope facies of western Hunan (Saltzman et al., 2000;Zhu et al., 2004;Li et al., 2017). The Cambrian δ 13 C chemostratigraphic profile of South China exhibits global δ 13 C evolutionary patterns with records of major excursions and thus can be used as a practical tool for stratigraphic subdivision and correlation, particularly for the those strata devoid of fossils. ...
... Cambrian chronostratigraphy and timescale of South China. δ 13 C data are fromSaltzman et al. (2000),Zhu et al. (2004),Li et al. (2017) and unpublished data of authors. ...
Article
Full-text available
The Cambrian Period is the first period of the Phanerozoic Eon and witnessed the explosive appearance of the metazoans, representing the beginning of the modern earth-life system characterized by animals in contrary to the Precambrian earth-life system dominated by microbial life. However, understanding Cambrian earth-life system evolution is hampered by regional and global stratigraphic correlations due to an incomplete chronostratigraphy and consequent absence of a highresolution timescale. Here we briefly review the historical narrative of the present international chronostratigraphic framework of the Cambrian System and summarize recent advances and problems of the undefined Cambrian stage GSSPs, in particular we challenge the global correlation of the GSSP for the Cambrian base, in addition to Cambrian chemostratigraphy and geochronology. Based on the recent advances of the international Cambrian chronostratigraphy, revisions to the Cambrian chronostratigraphy of China, which are largely based on the stratigraphic record of South China, are suggested, and the Xiaotanian Stage is newly proposed for the Cambrian Stage 2 of China. We further summarize the integrative stratigraphy of South China, North China and Tarim platforms respectively with an emphasis on the facies variations of the Precambrian-Cambrian boundary successions and problems for identification of the Cambrian base in the different facies and areas of China. Moreover, we discuss stratigraphic complications that are introduced by poorly fossiliferous dolomite successions in the upper Cambrian System which are widespread in South China, North China and Tarim platforms.
... The global Cambrian δ 13 C profile exhibits distinct negative shifts documented from sections in basins on different paleocontinents (e.g., Brasier, 1993;Hurtgen et al., 2009;Fan et al., 2011;Miller et al., 2011;Woods et al., 2011;Li et al., 2017;Azmy, 2019a,b;Schmid, 2017;Pulsipher et al., 2021) and correlated with the Middle Cambrian (e.g., DICE, Drumian Carbon Isotope Excursion) and the base of the Upper Cambrian (e.g., SPICE, Steptoean Positive Carbon Isotope Excursion). The correlations of those C-isotope shifts were consistent with the global biostratigraphic schemes. ...
... The SPICE reflects a global oceanographic event characterized by a distinct enhancement of oxygenation of the Cambrian seawater, which (Veizer et al., 1999). affected the atmosphere (e.g., Brasier, 1993;Hurtgen et al., 2009;Fan et al., 2011;Miller et al., 2011;Woods et al., 2011;Li et al., 2017;Azmy 2019a,b) and also marked a change from solely microbial forms to taxonomically diverse multicellular organisms in the ocean water (Rowell and Brady, 1976;Maruyama et al., 2014). ...
Article
The examined interval of the March Point and Petit Jardin formations (total thickness ∼130 m) spans the Middle−Upper Cambrian (lower Port au Port Group) at the Belle Isle Strait, western Newfoundland. The deposits consist of dolomitized shallow-water carbonates ranging from microbial mudstones to oolitic grainstones. A multi-technique evaluation proved the petrographic and geochemical preservation of the investigated micritic carbonates. The δ¹³C and δ¹⁸O values (−1.2 ± 1.1 ‰ VPDB and −6.7 ± 1.3 ‰ VPDB, respectively) exhibit an insignificant correlation (R² = 0.16) and similarly the δ¹³C values with their Mn/Sr ratio (R² = 0.01). This supports the preservation of at least near-primary δ¹³C signatures that can be utilized to construct a high-resolution δ¹³C profile for reliable global correlations. The δ¹³C profile exhibits two major negative excursions, the lower excursion (∼4 ‰) immediately below the lower boundary of the March Point Formation (Hawke Bay/March Point boundary) and the upper excursion (∼2 ‰) almost at the lower boundary of the Petit Jardin Formation where these excursions are likely associated with short-term sealevel changes. The trilobite biozonation scheme suggests the correlation of the lower excursion with the DICE (Drumian Carbon Isotope Excursion) event and the upper excursion with the base of the SPICE (Steptoean Positive Carbon Isotope Excursion) counterpart and both events have been globally documented on different paleocontinents.
... The term has been adopted and discussed by numerous authors, including Landing and colleagues (e.g. Terfelt et al. 2014;Li et al. 2017;Westrop & Landing, 2017;Ahlberg et al. 2019;Geyer, 2019;Scorrer et al. 2019;Zhu et al. 2019), and it has been adopted for use in the Geologic Time Scale volumes (Ogg et al. , 2016Gradstein et al. 2012, in press). ...
... To summarize, the interpretation that the HERB Event and the TOCE excursion are separate δ 13 C carb entities (Landing et al. 2011(Landing et al. , 2020) is a misunderstanding, as discussed elsewhere (e.g. Peng et al. 2012, in press;Terfelt et al. 2014;Miller et al. 2015;Li et al. 2017;Ahlberg et al. 2019;Geyer, 2019;Harper et al. 2019;Scorrer et al. 2019;Zhu et al. 2019), and as elaborated here. Geyer (2019), in particular, noted that the HERB and TOCE represent the 'same fluctuation in ocean chemistry' and that 'no negative δ 13 C excursion is recorded above the HERB peak in North America' (Geyer, 2019). ...
Article
In a recent communication on carbon isotope chemostratigraphy of the uppermost Cambrian strata, it was claimed that the Top of Cambrian Excursion (TOCE) is (1) an undocumented negative δ13C carb excursion; (2) ambiguously defined; (3) deliberately fictitious or, in the authors' words, a 'nihilartikel'; and (4) not synonymous with the Hellnmaria-Red Tops Boundary (HERB) Event. As the authors who have been involved in much of the discussion surrounding the TOCE since its introduction and in subsequent clarification, we wish to emphasize that the recent communication overlooks the fact that the TOCE is in fact a well-documented and clearly defined negative δ13C carb excursion, and that the term 'HERB Event' was originally used informally, without definition or reference data, for a negative δ13C carb peak, a peak later shown to occur within the TOCE excursion. Nearly a decade after the TOCE was named, the concept of the HERB Event was modified from a negative δ13C carb peak to a negative δ13C carb excursion, making it conceptually synonymous with the TOCE excursion. The recently published commentary is misleading and replete with misconceptions, as we discuss here.
... Notably, like other sections worldwide, the magnitude of the SPICE ␦ 13 C carb excursion is approximately two times greater than the ␦ 13 C org excursion ). In addition, our new ␦ 13 C carb data from post-SPICE strata of the Cronin Formation constitutes one of the thickest and most expanded Furongian carbon isotope records to date (e.g., Saltzman et al. 2000Saltzman et al. , 2004Saltzman and Thomas 2012;Li et al. 2017;Miller 2019;Wang et al. 2019). Unfortunately, however, the current lack of detailed biostratigraphic data from these strata makes it challenging to perform detailed correlations with other global sections (Li et al. 2017). ...
... In addition, our new ␦ 13 C carb data from post-SPICE strata of the Cronin Formation constitutes one of the thickest and most expanded Furongian carbon isotope records to date (e.g., Saltzman et al. 2000Saltzman et al. , 2004Saltzman and Thomas 2012;Li et al. 2017;Miller 2019;Wang et al. 2019). Unfortunately, however, the current lack of detailed biostratigraphic data from these strata makes it challenging to perform detailed correlations with other global sections (Li et al. 2017). ...
Article
Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank (GACB), a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the GACB called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans late Cambrian (Furongian)–Middle Devonian (Eifelian) time, and new chemostratigraphic data record multiple global carbon isotopic excursions (CIEs), including the late Cambrian SPICE event, the Late Ordovician Guttenberg excursion (GICE), the Silurian Aeronian, Valgu, Mulde, Ireviken, and Lau events, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide a critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.
... A high-amplitude negative carbon-isotope excursion, known as the Herllnmaria -Red Tops Boundary (HERB) event (Ripperdan 2002), has been investigated in sections from Utah (USA), Queensland (Australia), and Newfoundland (Canada), among others (Ripperdan et al. 1992;Buggisch et al. 2003;Jing et al. 2008;Li et al. 2017;Miller et al. 2014aMiller et al. , 2015Azmy 2018). The HERB event is represented by a negative ␦ 13 C excursion that consists of several high-amplitude ␦ 13 C peaks in Upper Cambrian sections and can be used for global correlations. ...
... The large positive excursion (peak 14, Fig. 7) likely represents the beginning of the Ordovician System because the spike is similar to the spikes in the interval with planktonic graptolites of the Green Point section, western Newfoundland ( Fig. 8; see Cooper et al. 2001;Azmy et al. 2014;Stouge et al. 2017). The HERB event has been documented globally by several authors in many sections from different paleocontinents such as China (Chen et al. 1995;Jing et al. 2008;Li et al. 2017), Argentina (Buggisch et al. 2003), and Australia (Ripperdan et al. 1992). ...
Article
Carbon-isotope stratigraphy of the Furongian (stage 10; Upper Cambrian) and Tremadocian (lowermost Ordovician) reveals distinct variations from the carbonates of the Berry Head and Watts Bight formations of the East Isthmus Bay section that accumulated in a shallow-marine setting on the eastern Laurentian platform in a passive margin setting in western Newfoundland, Canada. The East Isthmus Bay δ ¹³ C values show insignificant correlation with their Sr (R ² = 0.04), Mn (R ² = 0.001) and Fe (R ² = 0.02) counterparts, implying preservation of at least near-primary C-isotope compositions. The investigated section is largely fossil poor, but the δ ¹³ C profile shows a pattern with distinct variations that can be matched with those of the western Laurentian Lawson Cove Auxiliary Boundary Stratigraphic Section and Point (ASSP) section, Utah, USA. Therefore, it was possible to reconstruct a conodont biozonal scheme by matching the δ ¹³ C profile with its counterpart from the Lawson Cove ASSP section. At the base of the East Isthmus Bay section, the δ ¹³ C profile exhibits a broad excursion (the top of the Herllnmaria – Red Tops Boundary), which can be matched with the base of the Eoconodontus Zone (mid-Furongian), followed by an enrichment trend through the Cordylodus intermedius Zone (top Furongian). A positive excursion (Hirsutodontus simplex spike) is recorded in the Cordylodus intermedius Zone (top Cambrian), and a prominent positive peak characteristic for the Cordylodus lindstromi Zone is recorded from the top of the investigated section. The δ ¹³ C values of the Newfoundland carbonates are generally ~1‰ Vienna Pee Dee Belemnite lower than those of Lawson Cove, which is likely attributable to a relatively higher productivity and (or) organic burial in the Utah region.
... Although it has been argued (e.g. Landing, Westrop & Adrain, 2011) (2014), Miller et al. (2015) and Li et al. (2017) in considering them as the same carbon isotopic excursion. ...
Article
The Grönhögen-2015 core drilling on southern Öland, Sweden, penetrated 50.15 m of Cambrian Series 3, Furongian and Lower–Middle Ordovician strata. The Cambrian succession includes the Äleklinta Member (upper Stage 5) of the Borgholm Formation and the Alum Shale Formation (Guzhangian–Tremadocian). Agnostoids and trilobites allowed subdivision of the succession into eight biozones, in ascending order: the uppermost Cambrian Series 3 (Guzhangian) Agnostus pisiformis Zone and the Furongian Olenus gibbosus , O. truncatus , Parabolina spinulosa , Sphaerophthalmus ? flagellifer , Ctenopyge tumida , C. linnarssoni and Parabolina lobata zones. Conspicuous lithologic unconformities and the biostratigraphy show that the succession is incomplete and that there are several substantial gaps of variable magnitudes. Carbon isotope analyses (δ ¹³ C org ) through the Alum Shale Formation revealed two globally significant excursions: the Steptoean Positive Carbon Isotope Excursion (SPICE) in the lower–middle Paibian Stage, and the negative Top of Cambrian Excursion (TOCE), previously referred to as the HERB Event, in Stage 10. The δ ¹³ C org chemostratigraphy is tied directly to the biostratigraphy and used for an improved integration of these excursions with the standard agnostoid and trilobite zonation of Scandinavia. Their relations to that of coeval successions in Baltoscandia and elsewhere are discussed. The maximum amplitudes of the SPICE and TOCE in the Grönhögen succession are comparable to those recorded in drill cores retrieved from Scania, southern Sweden. The results of this study will be useful for assessing biostratigraphic relations between shale successions and carbonate facies on a global scale.
... Whereas the Ediacaran-Cambrian transition witnessed the rapid diversification of the Cambrian evolutionary fauna, the Cambrian-Ordovician transition records the further radiation of the Cambrian evolutionary fauna into the Palaeozoic evolutionary fauna. In order to accomplish a detailed chronostratigraphy for the Cambrian-Ordovician transition, Li et al. (2017: this issue) report a high-resolution C-isotopic profile of the Wa'ergang section, western Hunan, South China. The Wa'ergang section has been proposed as a potential GSSP for the base of Cambrian Stage 10, the uppermost stage of the Cambrian System, and so these new data provide a chemostratigraphic reference point for global correlation of the Cambrian-Ordovician transition. ...
Article
The Neoproterozoic–Palaeozoic transition (NPT) around 600 Ma ago was a critical time interval when the Earth experienced fundamental change, manifested as climatic extremes – ‘snowball Earth’ – followed by the emergence and rapid diversification of animals – ‘Cambrian explosion’. How animals and environments co-evolved, and what caused these fundamental changes to the Earth system during the NPT, is a great scientific puzzle, which has been a rapidly developing frontier of interdisciplinary research between bio- and geosciences. South China preserves a complete stratigraphic succession of the NPT developed in various facies ranging from shallow to deep marine realms with extraordinarily well-preserved, successive fossil biotas in various taphonomic settings (Zhu, 2010; Fig. 1), making it a key area and global focus of studies in the field over recent decades. Indeed, the current narrative of early animal evolution has largely been based on the fossil biotas from South China. These include: (1) the world's oldest microscopic animal fossils with cellular details from the early Ediacaran Weng'an biota (Doushantuo Formation); (2) putative macroscopic animal fossils preserved as carbonaceous imprints from the early Ediacaran Lantian, Wenghui and Miaohe biotas (also Doushantuo Formation); (3) typical late Ediacaran faunas, preserved in dark limestone (Shibantan biota) and as large and poorly mineralized tubular animal fossils (Gaojiashan biota), both from the Dengying Formation; (4) phosphatized small shelly and soft-bodied animal fossils from the early Cambrian Meishucun and Kuanchuanpu faunas; and (5) Cambrian fossil Lagerstätten (Chengjiang, Guanshan and Kaili faunas) with typical Burgess Shale-type soft-bodied preservation.
... The investigated interval of Martin Point consists of slope deposits of rhythmites of mainly alternating thinly-bedded lime mudstones and shales that were deposited under dysoxic (suboxic) conditions (e.g., Azmy, 2018;Azmy et al., 2015;James & Stevens, 1986;Landing, 2013;Landing, Geyer, & Bartowski, 2002). The main objectives of this study are 1. to investigate the C-and N-isotope compositions of the organic matter in the lime mudstone interbeds to better understand the control of sea-level changes on their signatures as palaeoredox proxies during the Late Cambrian, particularly the HERB event (Azmy, 2018;Landing, Westrop, & Adrain, 2011;Miller, Evans, Freeman, Ripperdan, & Taylor, 2011), and reconstruct the global palaeoenvironmental changes (climatic and oceanographic) that prevailed during that time interval (Buggisch, Keller, & Lehnert, 2003;Li et al., 2017;Miller, Repetski, Nicoll, Nowlan, & Ethington, 2014;Ripperdan, Magaritz, & Kirschvink, 1993) and 2. to investigate the influence of sea-level changes on trace element geochemistry of the seawater and consequently those carbonates caused by changes in terrestrial inputs and primary productivity. Barnes, & Knight, 1989). ...
Article
The Martin Point section (western Newfoundland, Canada) spans the uppermost Cambrian Broom Point and Martin Point members of the Green Point Formation (upper Furongian). The investigated interval (~90 m) consists of rhythmites of thinly‐bedded marine carbonates (lime mudstones) alternating with green and black shale and thin conglomerate beds. Samples were extracted from the micritic carbonates, and their preservation was evaluated by multiple petrographic and geochemical examinations. The δ13Corg profile exhibits a positive shift (~2‰) associated with the globally well‐documented negative δ13Ccarb HERB (Hellnmaria–Red Tops boundary) excursion and correlated with similar distinct shifts in the Al, Si, and ∑REE components, thus suggesting contributions from detrital organic matter relatively depleted in 12C induced likely by a eustatic sea‐level drop and enhancement of terrigenous inputs. Similarly, the δ15Norg variations are consistent with the proposed sea‐level drop. This is also correlated with positive shifts on the Fe and Mn profiles reflecting the overprint of terrigenous inputs rather than redox conditions.
... Precambrian Research 349 (2020) 105503 ( Fig. 3A), which exclude the possibility of post-depositional fluid alteration. Moreover, even carbonate samples with extremely low δ 18 O values (< −10‰) may have preserved primary carbon isotope signatures, as has been widely reported in recent studies of Cambrian carbonate strata worldwide (Li et al., 2013;Ishikawa et al., 2014;Li et al., 2017). This can be attributed to the differing susceptibility of oxygen and carbon isotopes to diagenetic alteration during early diagenesis. ...
Article
The middle Cambrian (Cambrian Series 3) Drumian carbon isotope excursion (DICE) represents a pronounced negative carbon isotopic excursion, and has been proposed as a key chemostratigraphic marker for identifying the base of Cambrian Drumian Stage and global correlation. However, the precise interbasinal correlation of the DICE to the South China remains unsolved and the driving mechanism of the DICE is to be further understood. In this study, we report new paired δ¹³Ccarb and δ¹³Corg data from the Wangcun section in South China. Both δ¹³Ccarb and δ¹³Corg profiles capture a large negative excursion (DICE) near the FAD of P. atavus. We suggest that the parallel δ¹³Ccarb and δ¹³Corg negative excursions during the DICE may have resulted from the shoaling of anoxic deep waters associated with the transgressive event. Shoreward and upward incursion of deep anoxic waters into oxygenated shallower water column during transgression could lead to the enhanced oxidation of dissolved organic carbon (DOC) and introduce additional ¹³C-depleted carbon to the surface water, resulting in the parallel δ¹³Ccarb and δ¹³Corg negative excursions. More importantly, we use a box model to quantify the flux of DOC oxidation needed to produce a carbon isotope excursion of the magnitude and duration of the DICE we observed. The model results indicate that oxidation of DOC during the DICE may require massive amounts of oxidants (i.e., sulfate and oxygen), probably causing further expansion of anoxia in the shallow marine environment. We suggest that shoaling of anoxic water during the DICE could have delayed the recovery of metazoan reef systems from the early–middle Cambrian mass extinctions.
... No negative δ 1 3 C excursion is recorded above the HERB peak in North America (Miller et al., 2006(Miller et al., , 2011. This is the N3 peak of Li et al. (2017) registered in South China. ...
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The Cambrian is the most poorly dated period of the past 541 million years. This hampers analysis of profound environmental and biological changes that took place during this period. Astronomically forced climate cycles recognized in sediments and anchored to radioisotopic ages provide a powerful geochronometer that has fundamentally refined Mesozoic–Cenozoic time scales but not yet the Palaeozoic. Here we report a continuous astronomical signal detected as geochemical variations (1 mm resolution) in the late Cambrian Alum Shale Formation that is used to establish a 16-Myr-long astronomical time scale, anchored by radioisotopic dates. The resulting time scale is biostratigraphically well-constrained, allowing correlation of the late Cambrian global stage boundaries with the 405-kyr astrochronological framework. This enables a first assessment, in numerical time, of the evolution of major biotic and abiotic changes, including the end-Marjuman extinctions and the Steptoean Positive Carbon Isotope Excursion, that characterized the late Cambrian Earth.
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Recurrent mass extinction events (at "biomere"-a biostratigraphic unit-boundaries) characterize the middle Cambrian to Early Ordovician (Tremadocian) time interval that is between the major Cambrian and Ordovician radiations of animal life. A role for anoxia in maintaining elevated extinction rates in the late Cambrian has been proposed based on coincidence of an extinction with positive excursions in delta C-13(carb) and delta S-34(CAS) (CAS-carbonate-associated sulfate). Here we examine an Early Ordovician extinction event at the base of the North American Stairsian Stage (upper Tremadocian), and demonstrate concurrent onset of positive excursions in delta C-13 and delta S-34 inferred to reflect enhanced organic matter burial under anoxic waters. Sea-level rise may have brought anoxic waters onto the shelf to initiate extinctions. The evidence for delta C-13 excursions and elevated extinction rates appears to wane in the Tremadocian, consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the Great Ordovician Biodiversification Event.
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A revised lithostratigraphy for Lower Paleozoic strata in New Mexico and west Texas was developed through detailed sedimentological study of the Bliss and Hitt Canyon Formations within a refined temporal framework assembled from precise biostratigraphic (trilobite and conodont) and chemostratigraphic (carbon isotope) data. Member boundaries within the Hitt Canyon now correspond with mappable and essentially isochronous horizons that represent major depositional events that affected sedimentation in basins throughout Laurentian North America. This trip is designed to examine these and other important intervals, such as the extinction horizons at the base and top of the Skullrockian Stage, and to demonstrate the utility of associated faunas and isotopic excursions for correlation within and beyond the region.
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Restudy of type and other material shows that the Cambrian (Sunwaptan, Furongian) agnostoid arthropod Lotagnostus americanus (Billings, 1860), far from being distributed globally, can be identified with confidence only at its type locality of Quebec. As such, it is ill-suited as an index for international correlation. Lotagnostus obscurus Palmer, 1955, from Nevada is not a synonym of L. americanus and is most similar to other effaced species from Australia. Sclerites from Avalonian Canada are represented by perhaps as many as two species, one of which is L. germanus (Matthew, 1901), and the other is similar to the type species, L. trisectus (Salter, 1864). However, loss of information due to compaction makes L. trisectus difficult to interpret in its type region of Avalonian Britain, and possible occurrences in other areas are therefore questionable at best. At the current state of knowledge, this species name should not be applied outside of Avalonian Britain. Aside from L. obscurus, which may possibly appear in slightly older strata, all Laurentian occurrences are confined to Upper Sunwaptan successions. Most are in the Illaenurus and Prosaukia pyrene zones, although review of material assigned to L. hedini (Troedsson, 1937) in western Newfoundland suggests that this species may occur, albeit questionably, only in the basal Skullrockian Phylacterus saylesi Fauna; indeterminate species occur in the Keithia schucherti Fauna in the latter region. Occurrences in Avalonian Canada are all from the upper Peltura Zone, and are likely somewhat younger (Saukiella junia Subzone and correlatives) than those in Laurentian successions.
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The concept of the agnostoid arthropod species Lotagnostus americanus (Billings, 1860), which has been reported from numerous localities in the upper Furongian Series (Cambrian) of Laurentia, Gondwana, Baltica, Avalonia, and Siberia, is reviewed with emphasis on morphologic and taphonomic information afforded by large collections from Hunan in South China, Xinjiang in Northwest China, and Zhejiang in Southeast China. Comparisons are made with type and topotype material from Quebec, Canada, as well as material from elsewhere in Canada, the USA, the United Kingdom, Sweden, Russia, and Kazakhstan. The new information clarifies the limits of morphologic variability in L. americanus owing to ontogenetic changes and variation within holaspides, including inferred microevolution. It also provides details on apparent variation of taphonomic origin. The Chinese collections demonstrate a moderately wide variation in L. americanus, indicating that arguments favoring restriction of Lotagnostus species to narrowly defined, geographically restricted forms are unwarranted. Species described as L. trisectus (Salter, 1864), L. asiaticus Troedsson, 1937, and L. punctatus Lu, 1964, for example, fall within the range of variation observed in L. americanus, and are regarded as junior synonyms. The effaced form Lotagnostus obscurus Palmer, 1955 is removed from synonymy with L. americanus. A review of the stratigraphic distribution of L. americanus as construed here shows that the earliest occurrences of the species in all regions of the world are nearly synchronous.
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The International Subcommission on Cambrian Stratigraphy (ISCS) has recommended a subdivision of the Cambrian System into four series and ten stages. The first appearance datum (FAD) of the cosmopolitan agnostoid trilobite Lotagnostus americanus has been chosen as a good indicator for the base of the uppermost Cambrian stage (Stage 10). The Khos-Nelege section located in the north-eastern Siberian Platform (Western Yakutia, Russia) is proposed as the Global Standard Stratotype Section and Point (GSSP) for the base of the 'Cambrian Stage 10'. The proposed GSSP coincides with the FAD of L. americanus at 339m above the base of the Ogon'or Formation. The section fulfills the geological and biostratigraphical requirements for a stratotype and the GSSP horizon can be constrained by a number of auxiliary marker taxa. The first appearance of L. americanus is an easily recognizable level in the Cambrian which can be recognized from all major Cambrian continents, including Baltica, Laurentia, Avalonia, South China and Australia.
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Abstract Since 1985, samples with a total weight of more than 14,000 kg, mainly from three key sections in western and northwestern Hunan, South China, have been processed for conodonts. In strata older than the late Late Cambrian paraconodonts have proved useful for stratigraphic subdivision and correlation. Thirteen conodont zones are proposed in the Middle Cambrian through lowermost Ordovician. The correlation between these zones and those of North China, western U. S.A., western Newfoundland, Canada, and Iran is discussed. In ascending order, these 13 zones are as follows: The Gapparodus bisulcatus‐Westergaardodina brevidens Zone, Shandongodus priscus‐Hunanognathus tricuspidatus Zone, Westergaardodina quadrata Zone, Westergaardodina matsushitai‐W. grandidens Zone, Westergaardodina lui‐W. ani Zone, Westergaardodina cf. calix‐Prooneotodus rotundatus Zone, Proconodontus tenuiserratus Zone, Proconodontus Zone, Eoconodontus Zone, Cordylodus proavus Zone, Cordylodus intermedius Zone, Cordylodus lindstromi Zone, and Cordylodus angulatus Zone (lower part). The Westergaardodina lui‐W. ani and Westergaardodina cf. calix‐Prooneotodus rotundatus Zones replace the Westergaardodina proligula and Westergaardodina cf. behrae‐Prooneotodus rotundatus Zones, respectively, in the lowermost Upper Cambrian. Two new species (Westergaardodina lui and Westergaardodina ani) and one conditionally identified species (Westergaardodina cf. calix) are described.
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The first appearance datum (FAD) of the cosmopolitan agnostoid trilobite Lotagnostus americanus is a widely used marker for defining the base of the uppermost stage of the Cambrian System. Detailed field study on the L. americanus-bearing interval of the Wa'ergang section, Hunan, China, reveals the FAD of L. americanus to be 29.65 m above the base of the Shengjiawan Formation. This horizon is close to the lowest known occurrences of the intercontinentally distributed polymerid trilobites Hedinaspis regalis and Charchaqia norini. The section has strong potential as a candidate for defining a global standard stratotype-section and point marking the base of provisional Stage 10.
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In post-Cambrian time, five events—the end-Ordovician, end-Frasnian in the Late De-vonian, end-Permian, end-Triassic, and end-Cretaceous—are commonly grouped as the ''big five'' global intervals of mass extinction. Plotted by magnitude, extinction intensities for all Phanerozoic substages show a continuous distribution, with the five traditionally recognized mass extinctions located in the upper tail. Plotted by time, however, proportional extinctions clearly divide the Phan-erozoic Eon into six stratigraphically coherent intervals of alternating high and low extinction in-tensity. These stratigraphic neighborhoods provide a temporal context for evaluating the intensity of extinction during the ''big five'' events. Compared with other stages and substages in the same neighborhood, only the end-Ordovician, end-Permian, and end-Cretaceous extinction intensities appear as outliers. Moreover, when origination and extinction are considered together, only these three of the ''big five'' events appear to have been generated exclusively by elevated extinction. Low origination contributed more than high extinction to the marked loss of diversity in the late Fras-nian and at the end of the Triassic. Therefore, whereas the ''big five'' events are clearly times when diversity suffered mass depletion, only those at the end of the Ordovician, Permian, and Cretaceous periods unequivocally qualify as globally distinct mass extinctions. Each of the three has a unique pattern of extinction, and the diversity dynamics of these events differ, as well, from the other two major diversity depletions. As mass depletions of diversity have no common effect, common cau-sation seems unlikely.
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The Furongian Series (Cambrian) biostratigraphy of Scandinavia is revised. We propose a two-fold trilobite zonation based on agnostoids and polymerids respectively. The agnostoid zonation includes four zones, in ascending order: the Glyptagnostus reticulatus, Pseudagnostus cyclopyge, Lotagnostus americanus, and the Trilobagnostus holmi Zone. The polymerid zonation includes 28 zones, in ascending order: the Olenus gibbosus, O. truncatus, O. wahlenbergi, O. attenuatus, O. dentatus, O. scanicus, Parabolina brevispina, P. spinulosa, Leptoplastus paucisegmentatus, L. raphidophorus, L. crassicornis, L. ovatus, L. stenotus, L. neglectus, Ctenopyge postcurrens, C. flagellifera, C. similis, C. spectabilis, C. tumida, C. affinis, C. bisulcata, C. linnarssoni, Parabolina lobata, Peltura paradoxa, P. transiens, P. costata, Westergaardia scanica, and the Acerocare ecorne Zone. Traditional subzones are elevated to zonal status and modified by definition to become interval-zones. The traditional 'superzones' are abandoned. Each of the biozones, as now defined, is delimited at the base by the first appearance of the eponymous species, and delimited at the top by the base of the overlying zone.
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The best definition for the base of the terminal Cambrian (Stage 10) is the conodont Eoconodontus notchpeakensis FAD ± onset of the HERB carbon isotope excursion. These horizons allow precise intercontinental correlations in deep ma-rine to peritidal facies. The agnostoid Lotagnostus americanus (Billings, 1860) FAD has been suggested as a Stage 10 base, but restudy of types and typotypes shows that the species occurs only in Late Cambrian (Sunwaptan) debris flow boulders in Quebec (Westrop et al., this volume). Non-Quebec reports of "L. americanus" are an amalgum of small, often poorly documented specimens with effaced–highly furrowed cephala and pygidia and with or without a highly tri-sected pygidial posteroaxis. Many of these occurrences have local species names, but no evidence suggests that they re-cord intraspecific variation of a globally distributed taxon. They are not synonyms of L. americanus. Lotagnostus, largely a dysoxic form, does not allow precise correlation into oxygenated platform facies. Another proposal used the conodont Cordylodus andresi FAD as a Stage 10 base, but other work shows this FAD is diachronous. An unrealistic ap-proach to L. americanus' systematics and the correlation uncertainty of C. andresi are overcome by defining a Stage 10 base at the globally recognizable E. notchpeakensis FAD, with the C. andresi FAD a useful proxy on cool-water conti-nents. The "Lawsonian Stage", named for Lawson Cove in western Utah, has a basal GSSP at the E. notchpeakensis FAD and replaces informal Stage 10. The Lawsonian, ~150 m-thick in western Utah, underlies the basal Ordovician Iapetognathus Zone. •
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Carbon isotope and magnetic polarity stratigraphic results from the Cambrian-Ordovician Boundary section at Xiaoyangqiao, near Dayangcha, Jilin Province, China, in comparison to a contemporaneous section at Black Mountain, Australia, indicate strata equivalent to major portions of the Australian sequence are either absent or are restricted to highly condensed intervals. These intervals are correlative with regressive sea level events identified in Australia and western North America, suggesting regional or eustatic sea level changes strongly influenced deposition of the Xiaoyangqiao sequence. These results also suggest the Xiaoyangqiao section is unfavourable as the site of the Cambrian-Ordovician Boundary Global Stratotype Section and Point.
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An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallow-marine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust'-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing δ 13 C positive values as high as +4.6 ‰ and least-altered 87 Sr/ 86 Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between the Orakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals.
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Exposed in natural outcrops near the Duibian Village, Jiangshan County, Zhejiang Province, China, the Duibian B section is proposed as the boundary stratotype for the base of an unnamed stage termed provisionally Cambrian Stage 9. The proposed position of the GSSP is 108.12 m above the base of the Huayansi Formation, at a horizon coinciding with the first appearance of the cosmopolitan agnostoid trilobite Agnostotes orientalis. This horizon coincides also with the first appearance of the cosmopolitan polymerid trilobite Irvingella angustilimbata. The section fulfills all the requirements for a GSSP, and the horizon can be constrained not only by the primary stratigraphic marker (A. orientalis) but also with secondary biostratigraphic, chemostratigraphic, and sequence-stratigraphic correlation tools. The first appearance of A. orientalis is one of the most readily recognizable levels in Cambrian, and can be correlated with precision to all paleocontinents.
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Biostratigraphically constrained sequences at the Wushi Yingshan and Kalpin Cement Plant sections (Kalpin Region; Tarim Basin) were densely sampled for geochemical studies. Carbonates across the Cambrian-Ordovician boundary of both sections mainly record negative carbon isotope values. Stable isotope curves show four negative and four positive excursions appearing alternately at the Wushi Yingshan section and five negative alternating with five positive excursions at the Kalpin Cement Plant section. The carbon isotope logs of these two sections are correlated with the international Cambrian-Ordovician boundary key sections: (1) Dayangcha section in China, (2) Green Point section in Canada, (3) Black mountain section in Australia and (4) Lowson Cove section in USA. These correlations suggest that the Cambrian-Ordovician boundary of the Wushi Yingshan section and the Kalpin Cement Plant section can be placed within a particular horizon that also corresponds to the observed biostratigraphic units.
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The Cambrian–Ordovician transition of the western Mediterranean region (NW Gondwana) is characterized by the record of major erosive unconformities with gaps that range from a chronostratigraphic stage to a series. The hiatii are diachronous and involved progressively younger strata along the Gondwanan margin, from SW (Morocco) to NE (Montagne Noire). They can be related to development of a multi-stage rifting (further North), currently connected to the opening of the Rheic Ocean, and concomitant erosion on southern rift shoulders. The platforms of this margin of Gondwana occupied temperate-water, mid latitudes and were dominated by siliciclastic sedimentation, while carbonate factories were only episodically active in the Montagne-Noire platform.
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The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ13CCaCO3 variability from Siberia, Mongolia, and China onto a Moroccan U/Pb-δ13CCaCO3 age model constrained by five U/Pb ages from inter bedded volcanic ashes. The δ13CCaCO3 correlations ignore fossil tie points, so we assume synchroneity in δ13C trends rather than synchroneity in first appearances of animal taxa. We pre sent new δ13Corg, 87Sr/86Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan δ13CCaCO3 curve. The result is a new absolute time line for first appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540-538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534-530 Ma), and a moderate pulse in the Tommotian (ca. 524-522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydro-thermal flux of Sr.
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A rise in atmospheric O(2) has been linked to the Cambrian explosion of life. For the plankton and animal radiation that began some 40 million yr later and continued through much of the Ordovician (Great Ordovician Biodiversification Event), the search for an environmental trigger(s) has remained elusive. Here we present a carbon and sulfur isotope mass balance model for the latest Cambrian time interval spanning the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) that indicates a major increase in atmospheric O(2). We estimate that this organic carbon and pyrite burial event added approximately 19 × 10(18) moles of O(2) to the atmosphere (i.e., equal to change from an initial starting point for O(2) between 10-18% to a peak of 20-28% O(2)) beginning at approximately 500 million years. We further report on new paired carbon isotope results from carbonate and organic matter through the SPICE in North America, Australia, and China that reveal an approximately 2‰ increase in biological fractionation, also consistent with a major increase in atmospheric O(2). The SPICE is followed by an increase in plankton diversity that may relate to changes in macro- and micronutrient abundances in increasingly oxic marine environments, representing a critical initial step in the trophic chain. Ecologically diverse plankton groups could provide new food sources for an animal biota expanding into progressively more ventilated marine habitats during the Ordovician, ultimately establishing complex ecosystems that are a hallmark of the Great Ordovician Biodiversification Event.
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Rich mid-Cambrian trilobite faunas were recovered from limestone boulders in conglomerates that form the lowest exposures of the Cow Head Group (Shallow Bay Formation) near Broom Point in western Newfoundland. These boulders were derived from various shelf-edge and upper slope environments along the margin of Laurentia. Twenty-nine species representing twenty genera are described. Three trilobite biofacies are defined based on the abundance of the genera in separate boulders: the Zacanthoidid-Pagetia, Bathyuriscus, and Onchocephalites biofacies. The association of Zacanthoides gilberti, Parkaspis caboti, Peronopsis interstricta, Pagetia rasettii, Elrathia kindlei, and Bathyuriscus richardsoni in the Zacanthoidid-Pagetia Biofacies is assigned to a Zacanthoides gilberti Fauna, which correlates with the Ptychagnostus gibbus. Zone of western North America. -from Authors
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Late Cambrian trilobite faunas are described from the Bitiao and Shenjiawan Fmns in the Cili-Taoyuan area, northwestern Hunan. From two measured sections, 131 species and subspecies are assigned to 71 genera and subgenera, among which three genera and subgenera and 41 species and subspecies are new. The new subfamilies, Quadrahomagnostinae, Ciliinae and Hedinaspidinae, are established. An Upper Cambrian biostratigraphy is proposed with 15 assemblage zones and two assemblage subzones for slope facies sequences in southern China. Mixed platform and slope faunas of the Jiangnan Slope Belt are shown to be highly significant in correlations between biofacies belts in China and internationally. -from Author
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The lowest occurrence of the conodont Eoconodontus notchpeakensis (Miller, 1969) has been proposed as the base of Cambrian Stage 10. The horizon is recognized in three sections in the House Range, western Utah, USA in the lower part of the Red Tops Member of the Notch Peak Formation. This horizon fits within a tightly integrated framework that includes conodont, trilobite, and brachiopod biozonations, as well as carbon-isotope stratigraphy and sequence stratigraphy. The proposed horizon is the base of the Eoconodontus conodont Zone. This horizon is in the lower part of the Saukiella junia Subzone of the Saukia trilobite Zone and is near the top of the Billingsella brachiopod Zone. The HERB Event is a high-amplitude, negative carbon-isotope excursion that has been identified in Laurentia, Australia, China, and Argentina. The start of the excursion is at a negative carbon-isotope peak that is less than half a metre above the base of the E. notchpeakensis Subzone, and the highest-amplitude peak of the HERB Event is near the middle of that relatively thin subzone. The HERB Event has been identified in strata with minimal faunal data, providing the possibility of identifying the base of Stage 10 in nearly unfossiliferous strata. The Notch Peak Formation has been divided into a detailed sequence-stratigraphic framework within a lithostratigraphic context, and some of the sequence boundaries have been identified in Australia and China. Conodonts diagnostic of the Eoconodontus Zone have been identified at 54 localities around the world, including in a succession of Cambrian deep-ocean radiolarian cherts. The Utah conodont zonation has been identified across Laurentia and in other parts of the world in facies ranging from continental slope to nearshore sandstone deposits. The variety of correlation tools and the integration of diverse data produce a superior framework for correlation of the proposed base of Stage 10 and for correlation of many horizons within Stage 10.
Chapter
: Appearance of metazoans with mineralized skeletons, “explosion” in biotic diversity and disparity, infaunalization of the substrate, occurrence of metazoan Konservat Fossil-Lagerstatten, establishment of most invertebrate phyla, strong faunal provincialism, dominance of trilobites, globally warm climate (greenhouse conditions), opening of the Iapetus Ocean, progressive equatorial drift and separation of Laurentia, Baltica, Siberia, and Avalonia from Gondwana all characterize the Cambrian Period.
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The developing high-resolution chronostratigraphy of the Cambrian provides an updated age model for various geologic and biotic events that occurred during this critical period of Earth history. Broad, time-specific patterns of lithofacies, such as organic-rich deposits, and biofacies appear to be consistent across all Cambrian paleocontinents. Records of important evolutionary events including first appearances of certain metazoan taxa, migrations, and extinctions, tend to coincide with changes in eustatic sea level, as do the positions of many Konservat-Lagerstätten, concretion horizons, agnostoid-rich beds, and other sedimentary features. Most of these events or horizons also show a relationship to perturbations in the global carbon cycle. The positions of organic-rich deposits bear strong relationship to both paleogeographic position and sea level history. Cambrian strata show evidence of cyclicity at multiple scales. Synchronous or near-synchronous global cyclicity is inferred to be associated with oceanographic and climatic cycles characteristic of glacial expansion and deglaciation.
Article
The name Lawsonian Stage is proposed as the highest stage of the Cambrian System (Stage 10). The base of the proposed Lawsonian Stage is at the First Appearance Datum (FAD) of the euconodont Eoconodontus notchpeakensis (Miller, 1969). That horizon, the base of the E. notchpeakensis Subzone of the Eoconodontus Zone, is 3m above the base of the Red Tops Member of the Notch Peak Formation at the Steamboat Pass section in the House Range, western Utah, USA. The conodont fauna of the Eoconodontus Zone is widespread in North America and occurs in Asia, Australia, Europe, and South America in facies that include cratonic nearshore sandstones, shallow and intermediate-depth platform carbonates, deep-water ramp, continental slope deposits, and deep-ocean radiolarian chert. This horizon lies at or slightly below the top of the Illaenurus trilobite Zone of western North America and within the lower part of the Saukiella junia Subzone of the Saukia Zone in areas east of the Rocky Mountains. These trilobite and conodont faunas can be traced into slope deposits containing cosmopolitan trilobites. The horizon is nearly coincident with the boundary between the Billingsella and the Finkelnburgia calcitic brachiopod Zones; ranges of organophosphatic brachiopods also characterize the horizon. A high-amplitude, negative, carbon-isotope excursion, the HERB Event, occurs in the Eoconodontus notchpeakensis Subzone. This distinctive geochemical signal is known in western Utah, Australia, China, and in poorly fossiliferous slope deposits in Newfoundland, Canada. The proposed boundary fits between two closely spaced sequence-stratigraphic boundaries described from Utah and Texas, USA.
Article
We report the occurrence of important carbon isotope excursions in early Paleozoic formations of the Eastern and Central Argentine Precordillera. The Steptoean positive isotope carbon excursion (SPICE) is known from North America, Kazakhstan, South China, Australia and South America, and the negative isotope carbon excursion (SNICE) has been described for the first time in South America. We report here the record of the SPICE and SNICE in a single section in the Eastern Precordillera. In the Central Precordillera, a minor middle Darriwilian positive carbon isotope excursion (MDICE) and a late Sandbian positive isotope carbon excursion, the GICE (~ + 3‰; C. bicornis biozone) are reported from two sections. One pre-GICE positive carbon-isotope excursion (Sandbian Sa1, N. gracilis biozone) in the Central Precordillera with a δ13C peak of ~ + 2‰ is, perhaps, equivalent to the positive Spechts Ferry δ13C excursion of North America. A positive δ13C excursion (~ 6‰; N. persculptus biozone) recorded at the base of the late Hirnantian La Chilca Formation probably corresponds to HICE.
Article
The Cambrian–Ordovician boundary interval in Scandinavia is characterized by largely endemic trilobite species and fossil-meager intervals within the Alum Shale Formation. Previous investigations of this interval in Scandinavia, based on drill cores, are rather sketchy. In order to characterize the faunal signature in a largely dysoxic setting during this time interval, as well as providing biostratigraphic and chemostratigraphic data valuable for intercontinental correlation, a small strip in the outskirts of the village Södra Sandby in Scania, southern Sweden, was excavated. Nearly 5 m of the Cambrian–Ordovician boundary strata, largely represented by alum shale, were exposed and the profile was meticulously investigated for fossil content and lithological characteristics and sampled for δ13Corg analyses. The uppermost Cambrian in Sweden has previously been described as barren of fossils; however, the present study reveals a rather diverse fauna, including lingulid brachiopods, trilobites, protoconodonts, paraconodonts and euconodonts in the uppermost 1.6 m of the Furongian. The first appearance datum of planktic graptolites is represented by a single shale surface covered by specimens of Rhabdinopora flabelliforme parabola at 1.74 m above the base of the section and roughly corresponds to the Cambrian–Ordovician boundary. The conodont fauna includes several cordylodids important for intercontinental correlation. The Södra Sandby section δ13Corg data were coupled with isotope data from two Scanian drill cores, Håslöv-1 and Tosterup-2, in order to compile a composite isotope curve spanning the uppermost Ctenopyge linnarssoni Trilobite Zone in the Furongian to the upper R. flabelliforme parabola Graptolite Zone in the Lower Ordovician (Lower Tremadocian). Two isotope shifts from baseline values, observed at the base of the Peltura paradoxa Trilobite Zone and the lower part of the Peltura transiens Trilobite Zone, respectively, can be correlated with contemporaneous shifts in other parts of the world. The former, a negative shift of approximately 0.4–0.7‰, corresponds to the widely documented Top of Cambrian carbon isotope Excursion (TOCE) and the latter, a positive shift of approximately 1‰, corresponds to an as-yet-unnamed excursion at the base of the Cordylodus proavus Conodont Zone. In terms of faunal content and isotopic signals, the present study represents the first detailed description of the Cambrian–Ordovician boundary interval in Baltica. The relatively diverse fauna recorded suggests that the dysoxic environment was not a serious inhibitor for marine life. Globally, no isotope values significantly different from background values have been reported at the Cambrian–Ordovician boundary. This is confirmed by the present study; however, as the base of the Ordovician GSSP lacks meaningful isotope data, global correlation of this important boundary is problematic.
Article
We propose a candidate for the Global Standard Stratotype-section and Point (GSSP) for the base of the highest stage of the Furongian Series of the Cambrian System. The section is at Lawson Cove in the Ibex area of Millard County, Utah, USA. The marker horizon is the first appearance datum (FAD) of the conodont Cordylodus andresi Viira et Sergeyeva in Kaljo et al. [Kaljo, D., Borovko, N., Heinsalu, H., Khazanovich, K., Mens, K., Popov, L., Sergeyeva, S., Sobolevskaya, R., Viira, V., 1986. The Cambrian–Ordovician boundary in the Baltic–Ladoga clint area (North Estonia and Leningrad Region, USSR). Eesti NSV Teaduste Akadeemia Toimetised. Geologia 35, 97–108]. At this section and elsewhere this horizon also is the FAD of the trilobite Eurekia apopsis (Winston et Nicholls, 1967). This conodont characterizes the base of the Cordylodus proavus Zone, which has been recognized in many parts of the world. This trilobite characterizes the base of the Eurekia apopsis Zone, which has been recognized in many parts of North America. The proposed boundary is 46.7m above the base of the Lava Dam Member of the Notch Peak Formation at the Lawson Cove section. Brachiopods, sequence stratigraphy, and carbon-isotope geochemistry are other tools that characterize this horizon and allow it to be recognized in other areas.
Article
This paper provides a brief outline of the current, detailed inter-disciplinary work on the Xiaoyangquiao section, trying to expose all the aspects for reference tied to the Global Single Stratigraphic Point (GSSP) Concept for defining the Cambrian–Ordovician Boundary. The 45 m critical interval of this section outcrops very well along the steep bank of a stream and is free from folding, faulting, intrusions, and has not been affected by weathering. Colour alternation of conodonts and acritarchs, and crystallinity indices of illite all indicate a maximum thermal grade of 100 °C. The lithofacies, being of great lateral persistence for a hundred kilometres, consists mainly of a rhythmical sequence of lime mudstone and shales deposited in a moderately deep outer shelf environment of quiet water, well below the normal storm wave base. Chemical investigation of the rocks demonstrates strong positive correlation between A1 2 O 3 content and those of K 2 O, MgO, Fe 2 O 3 , TiO 2 , Be, Cr, Co, Ni, Zn and Ba, indicating these components are tied to the clay fractions. A stable depositional environment is demonstrated by the uniform chemistries through the boundary interval. Close to the boundary itself P 2 O 5 contents are low, indicating continuous sedimentation at fairly substantial rates. The major biological events, the biostratigraphic framework, and the stratigraphic range of conodont, graptolite, trilobite, and acritarch taxa are illustrated briefly with diagrams. Following the majority views of the Calgary Plenary Session, the boundary level is to be chosen at a point marked by the First Appearance Datum (FAD) of the selected conodont taxon or taxa in the vicinity of the level close to, but below, the first influx of nematophorous graptolites. The following four points marked by the incoming of conodont taxon or taxa are recommended for consideration of the ‘Golden Spike'‘: (1) FAD of Cordylodus intermedius at 5.28 m below the first influx of nematophorous graptolites; (2) FAD of Hirsutodontus simpler–Cordylodus drucei–Albiconus postcostatus at 5.23 m; (3) FAD of Semiacontiodus lavadamensis–Utahconus utahensis–Monocostodus sevierensis at 3.85 m; (4) FAD of Cordylodus lindstromi at 2.23 m. For the following reasons the FAD of H. simplex–C. drucei–A. postcostatus is favoured: (1) the taxa are all distinct and widely dispersed; (2) intensive evolutionary change took place in conodonts, graptolites, trilobites and acritarchs prior to or after this point; all the fossil groups occur together, providing correlation with many regions throughout the world; (3) the point is in a position between the previously widely accepted boundary levels based on graptolites and trilobites; (4) the proposed point lies within a thin, laterally persistent, rhythmical sequence. The FAD of Cordylodus lindstromi is also a favourable point, sharing many advantages mentioned above. But this point is less satisfactory in being defined by the FAD of a single taxon C. lindstromi which also has an extremely small population size. An isochron age of 500.7 ± 7.4 Ma is determined from clay fractions of mudstones 8.5 m below the proposed point by means of the Rubidium–Strontium method. The ɛND signature determined from conodonts, trilobites hyolithids and acrotretid brachiopods has a mean value of −6.7, comparable with that of the coeval oceanic water mass occupying southeastern North America and Europe, and indicating that northeastern China bordered the same ocean. The mean Tdm model age determined was 1.26 Ga at the time of sedimentation, compatible with the mean Tdm model age of approximately 1.1 Ga for the Pacific Ocean today. The relatively low value of the Tdm model age indicates a substantial input from young orogenic volcanic island arcs and terranes.
Article
Carbon isotope profiles of marine carbonate units within the Cambrian-Ordovician boundary section at Black Mountain, western Queensland, Australia, reveal cyclical changes in δ13C within the Georgina basin during the boundary interval. Minimum values in the δ13C profile are found in strata bearing lithostratigraphic evidence for sea-level drop and also correlate strongly with conodont assemblage zone boundaries. These relations indicate that direct mechanistic links existed between carbon cycles, changes in sea level, and conodont evolution.
The evolution of early Palaeozoic conodont faunas shows a clear connection to sea-level changes. One way that this connection manifests itself is that thick successions of carbonates are missing beneath major sequence boundaries due to karstification and erosion. From this observation arises the question of how many taxa have been lost from different conodont lineages in these incomplete successions. Although many taxa suffered extinction due to the environmental stresses associated with falling sea-levels, some must have survived in these extreme conditions. The number of taxa missing in the early Palaeozoic tropics always will be unclear, but it will be even more difficult to evaluate the missing record in detrital successions of higher latitudes. A common pattern in the evolution of Cambrian-Ordovician conodont lineages is appearances of new species at sea-level rises and disappearances at sea-level drops. This simple picture can be complicated by intervals that consistently have no representatives of a particular lineage, even after extensive sampling of the most complete sections. Presumably the lineages survived in undocumented refugia. In this paper, we give examples of evolution in Cambrian-Ordovician shallowmarine conodont faunas and highlight problems of undiscovered or truly missing segments of lineages.
Article
The high-temperature and medium-pressure part of the PT-diagram of the system SiO2-H2O has been investigated experimentally. The equilibrium diagram is discussed in the light of Schreinemakers general theory of PT-diagrams. The triple invariant point cristobalite + tridymite + quartz lies at 1190°C and 1430 atm. Neither cristobalite nor tridymite are stable at high pressure. Quartz may precipitate from the melt at a very high temperature (1360°C and higher), if the pressure is great enough, and if the water content is low. Using new experimental and published data a PT-diagram of the system SiO2-H2O in the large PT-region is given.
Article
Terfelt, F., Ahlberg, P. & Eriksson, M.E. 2011: Complete record of Furongian polymerid trilobites and agnostoids of Scandinavia – a biostratigraphical scheme. Lethaia, Vol. 44, pp. 8–14. So far, 112 polymerid trilobite species/subspecies and 13 agnostoid species/subspecies have been recorded from the Furongian (upper Cambrian) of Scandinavia. For the first time, their zonal occurrences are summarized in a biostratigraphical scheme serving as a practical synopsis for students of this interval in time. Ninety-six of the recorded polymerid trilobite species/subspecies belong to the family Olenidae whereas the remaining 16 are distributed across eight other families. Levels of increased speciation and low diversity (including stratigraphical range gaps) are conspicuous and these may be correlated with recorded physical and chemical anomalies. □Agnostoids, biostratigraphy, Cambrian, Furongian, polymerids, Scandinavia, trilobites.
Article
Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.At Quebrada de La Angostura, upper part of the La Flecha Formation, δ13Ccarb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ13C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.At the Cerro La Silla section, the Zonda Formation exhibit δ13C values of ∼ − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.At the Quebrada de La Flecha, 87Sr/86Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of 87Sr/86Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).
Article
The environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly 12C-enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight 18O-enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine-meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose-phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions. Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian. These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations.
Article
The systematics, stratigraphic distribution and geographic distribution of two widely distributed agnostoid species from the Furongian Series of the Cambrian are reviewed. The speciesAgnostotes orientalis (Kobayashi, 1935) andLotagnostus americanus (Billings, 1860) have been described from around the world under a variety of species and subspecies names, and have been assigned to a variety of genera. Nomenclatural problems have hindered to some extent a full under standing of the value of these two forms for intercontinental correlation. Both species have relatively narrow stratigraphic ranges and intercontinental distributions in open-shelf lithofacies. These conditions make these species excellent candidates for defining the bases of stage boundaries within the Furongian Series.
Article
Screening of sampling and analytical data is crucial for the detection of real global shifts in marine δ13C, against the noise of diagenesis. The real global shifts of 13C are of two types: (1) long-term secular shifts caused by changes in the fractional burial of organic carbon, and (2) transient shifts caused by abrupt changes of biological productivity in the surface photic zone. Both are applicable to stratigraphic correlation, but differ in their application to modelling of changes in oceanographic or atmospheric composition. A recent compilation found 34 carbon isotope events in the Phanerozoic and Neoproterozoic, equally distributed among positive and negative shifts. This in itself attests to a variety of proximate causes of the shifts.
Article
The Steptoean positive carbon isotope excursion (SPICE) marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites. The SPICE excursion is documented from sections in Laurentia, Kazakhstan, China, and Australia where it begins with the mass extinction at the base of the Pterocephaliid Biomere (Steptoean Stage) in Laurentia and at coeval extinction horizons in Gondwana and periGondwana terranes. The peak of SPICE (+5‰) coincided with a time of maximum regression in Laurentia. SPICE is similar in this regard to excursions that coincide with glacio-eustatic falls, such as in the Late Ordovician. A plausible scenario involves the transformation of ocean circulation between two states, which led to enhanced coastal upwelling and benthic extinctions. The lack of evidence for glaciation indicates that the coeval sea level fall (Sauk II–Sauk III event) resulted from tectonic or hydrologic changes that remain poorly understood at this time.
Article
Variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon. Paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time, but changes in atmospheric pCO2 as well. Interpretations of these types of data are typically rather qualitative; a quantitative basis is required to develop a better understanding of changes in the carbon cycle. For this purpose, we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations, each lasting 500 ky, i.e., much longer than the residence times of carbon and phosphorus in the ocean–atmosphere system. In addition to standard considerations of carbon mass and isotopic fluxes to the ocean–atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate–carbon to sediments, the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pCO2. The inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial. A series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon (ΔB). With constant ΔB a 50% increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions. However, the same simulation with ΔB responsive to pCO2 changes produces an initial parallel increase, but this is followed by an even greater 13C-enrichment in organic carbon because pCO2 falls in response to increased organic carbon burial. The counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in . In addition, we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50% increase in volcanic CO2 outgassing, a 50% increase in weathering rate (with coupled phosphate and riverine carbon flux responses), a 50% decrease in shale-associated organic carbon weathering, a 50% decrease in silicate weathering rate, and the possible effects of the rise in abundance of C4 plants in the late Miocene to Recent. We compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate- and organic-carbon records to illustrate how these records might be interpreted in light of the model response.