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Alan J Kaufman - One of the best experts on this subject based on the ideXlab platform.

  • carbon and sulfur isotope chemostratigraphy of the neoproterozoic quanji group of the chaidam basin nw china basin stratification in the aftermath of an ediacaran glaciation postdating the shuram event
    Precambrian Research, 2010
    Co-Authors: Alan J Kaufman, Bing Shen, Shuhai Xiao, Chuanming Zhou, Xunlai Yuan
    Abstract:

    The Neoproterozoic Quanji Group in the Chaidam Basin, northwest China, consists of, in ascending order, the Mahuanggou, Kubaimu, Shiyingliang, Hongzaoshan, Heitupo, Hongtiegou, and Zhoujieshan formations. It is a siliciclastic-dominated sequence with only two carbonate units, the ∼300-m-thick Hongzaoshan dolostone and the ∼4-m-thick Zhoujieshan cap dolostone, the latter of which lies above the Hongtiegou glacial diamictite. In this study, we analyzed the δ13C and δ18O of the Hongzaoshan dolostone and Zhoujieshan cap dolostone, and the δ34SCAS, δ34Spy, and trace element compositions (Fe, Mn, Sr, and S) of the Zhoujieshan cap dolostone. δ13C profile of the Hongzaoshan dolostone shows a shift from −5‰ to 0‰. This shift could be correlated with a similar shift in the Shuiquan Formation (<615 ± 6 Ma) in the Quruqtagh area of the Tarim Block (NW China), and may be equivalent to negative δ13C excursions in the middle or uppermost Doushantuo Formation, the latter of which is widely believed to be correlative with the Shuram negative δ13C excursion. This correlation, together with available paleontological evidence (fragments of Redkinia from the Heitupo Formation, a genus that also occurs in ∼555 Ma successions in the East European Platform), suggests that the Hongtiegou diamictite may represent an Ediacaran glaciation postdating the Gaskiers glaciation or occurring near the Precambrian–Cambrian Boundary. The Zhoujieshan cap dolostone is characterized by positive δ13C values ranging from 0‰ to +1.7‰, in sharp contrast to cap carbonates overlying the Sturtian and Marinoan diamictites. δ34SCAS of the Zhoujieshan cap dolostone varies strongly between +13.9‰ and +24.1‰, probably due to relatively low-sulfate concentration in seawater. δ34Spy values, however, show a steady increase from +12.9‰ to +26.4‰. Thus, δ34SCAS and δ34Spy trends are decoupled in the Zhoujieshan cap dolostone, and the high δ34Spy values in the upper part of the cap dolostone indicate inverse sulfur isotope fractionations (δ34SCAS < δ34Spy). The decoupling of δ34SCAS and δ34Spy suggests that CAS and pyrite were derived from different sulfur pools (a possible scenario if the post-glacial Oulongbluq basin was restricted and stratified) and/or that aerobic oxidation of sulfide was intense in the post-glacial Oulongbluq basin.

  • carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient
    Earth and Planetary Science Letters, 2007
    Co-Authors: Ganqing Jiang, Alan J Kaufman, Nicholas Christieblick, Shihong Zhang
    Abstract:

    New isotope data obtained from relatively conformable, carbonate-rich strata of the Ediacaran Yangtze platform in South China reveal substantial δ 13 C variability. In platform sections, four negative δ 13 C anomalies with a nadir down to ≤−8‰ (PDB) are present in the interval between the cap carbonate level (∼635 Ma) and the Precambrian/Cambrian Boundary (∼542 Ma), while in slope and basinal sections, δ 13 C values are negative through the entire Doushantuo Formation (∼635–551 Ma). If these δ 13 C values are close to their primary seawater signature, they imply a strong (≥10‰) surface-to-deep ocean δ 13 C gradient that is consistent with long-term deep ocean anoxia and the presence of a large dissolved organic carbon (DOC) reservoir. The two prominent negative δ 13 C excursions within the Doushantuo Formation above the cap carbonate level are associated with shoaling and local exposure of the platform. The anomalies may thus record remineralization of a large oceanic DOC pool via sulfate reduction that transferred 13 C-depleted carbon from the oceanic DOC reservoir to the surface ocean during regression. Inconsistencies in Ediacaran δ 13 C profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in δ 13 C from an inferred average seawater signature.

  • the effect of rising atmospheric oxygen on carbon and sulfur isotope anomalies in the neoproterozoic johnnie formation death valley usa
    Chemical Geology, 2007
    Co-Authors: Alan J Kaufman, Frank A Corsetti, Michael A Varni
    Abstract:

    Abstract Carbonates within the Rainstorm Member in the terminal Neoproterozoic Johnnie Formation of Death Valley, California record a remarkable negative carbon isotope anomaly – to a nadir of near − 11‰ – that accompanies a dramatic rise in trace sulfate abundance and fall of carbonate associated sulfate δ 34 S values. The carbonates, including the laterally extensive Johnnie Oolite, were deposited during marine flooding atop a sequence Boundary best observed in cratonward sections. The 12 C-rich alkalinity delivered to the seawater pool was likely associated with a dramatic rise in atmospheric O 2 , which resulted in 1) the oxidation of exposed continental shelf sediments rich in fossil organic matter and sulfides, and 2) the ventilation of the oceans. Similar carbon isotope anomalies are recorded in broadly equivalent successions that post-date known Marinoan glacial deposits and pre-date the Precambrian–Cambrian Boundary in Oman, India, China, Australia, and Namibia, supporting the view that the oxygenation event was global. Thus, the biogeochemical anomaly post-dates the last widely recognized Neoproterozoic ice age, but is nonetheless associated with sea level drawdown that is likely to be eustatic in origin. The radiation of a high diversity microbiota and the first appearance of large metazoan fossils (Ediacaran animals) follow this carbon isotope anomaly, suggesting that a critical threshold with respect to atmospheric O 2 had been crossed by this time. A negative excursion of similar magnitude occurs in overlying strata at the Precambrian–Cambrian Boundary, which likely reflects similar processes.

  • stratigraphic investigations of carbon isotope anomalies and neoproterozoic ice ages in death valley california
    Geological Society of America Bulletin, 2003
    Co-Authors: Frank A Corsetti, Alan J Kaufman
    Abstract:

    An unusual richness of biogeochemical events is recorded in Neoproterozoic‐ Cambrian strata of the Death Valley region, California, United States. Eight negative carbon isotope (d13C) excursions are found in carbonate units between 1.08 Ga and the Precambrian/Cambrian Boundary; four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial ‘‘cap carbonates’’ (including one or more of the following: laminite with rollup structures, apparent ‘‘tube rocks,’’ seafloor precipitates, and sheet-crack cements). However, only two of these units, the Sourdough limestone member of the Kingston Peak Formation and the Noonday Dolomite, rest directly upon glacial strata. The basal Beck Spring Dolomite and the Rainstorm Member of the Johnnie Formation each contain negative excursions and cap-carbonate‐like lithofacies, but do not rest on known glacial deposits. If the negative d13C excursions are assumed to record depositional processes, two equally interesting hypotheses are possible: (1) The Death Valley succession records four glacial pulses in Neoproterozoic time, but glacial units are not preserved at two stratigraphic levels. (2) Alternatively, other global oceanographic processes can cause negative excursions and cap-carbonate‐ like facies in addition to, or independent of, glaciation.

  • neoproterozoic variations in the c isotopic composition of seawater stratigraphic and biogeochemical implications
    Precambrian Research, 1995
    Co-Authors: Alan J Kaufman, Andrew H Knoll
    Abstract:

    The recent proliferation of stratigraphic studies of delta 13C variation in carbonates and organic C in later Neoproterozoic and basal Cambrian successions (approximately 850-530 Ma) indicates a strong oscillating trend in the C-isotopic composition of surface seawater. Alone, this trend does not adequately characterize discrete intervals in Neoproterozoic time. However, integrated with the vectorial signals provided by fossils and Sr-isotopic variations, C isotope chemostratigraphy facilitates the interbasinal correlation of later Neoproterozoic successions. Results of these studies are evaluated in terms of four stratigraphic intervals: (1) the Precambrian/Cambrian Boundary, (2) the post-Varanger terminal Proterozoic, (3) the late Cryogenian, and (4) the early Cryogenian. Where biostratigraphic or radiometric data constrain the age of Neoproterozoic sedimentary sequences, secular variations in C and Sr isotopes can provide a level of stratigraphic resolution exceeding that provided by fossils alone. Isotopic data place strong constraints on the chemical evolution of seawater, linking it to major tectonic and paleoclimatic events. They also provide a biogeochemical framework for the understanding of the initial radiation of macroscopic metazoans, which is associated stratigraphically, and perhaps causally, with a global increase in the burial of organic C and a concomitant rise of atmospheric O2.

Kevin J Peterson - One of the best experts on this subject based on the ideXlab platform.

  • Where's the glass? Biomarkers, molecular clocks, and microRNAs suggest a 200-Myr missing Precambrian fossil record of siliceous sponge spicules
    2020
    Co-Authors: E Sp A Erling, Jeffrey M Robinson, Kevin J Peterson
    Abstract:

    ABSTRACT The earliest evidence for animal life comes from the fossil record of 24-isopropylcholestane, a sterane found in Cryogenian deposits, and whose precursors are found in modern demosponges, but not choanoflagellates, calcareans, hexactinellids, or eumetazoans. However, many modern demosponges are also characterized by the presence of siliceous spicules, and there are no convincing demosponge spicules in strata older than the Cambrian. This temporal disparity highlights a problem with our understanding of the Precambrian fossil record -either these supposed demosponge-specific biomarkers were derived from the sterols of some other organism and are simply retained in modern demosponges, or spicules do not primitively characterize crown-group demosponges. Resolving this issue requires resolving the phylogenetic placement of another group of sponges, the hexactinellids, which not only make a spicule thought to be homologous to the spicules of demosponges, but also make their first appearance near the Precambrian ⁄ Cambrian Boundary. Using two independent analytical approaches and data sets -traditional molecular phylogenetic analyses and the presence or absence of specific microRNA genes -we show that demosponges are monophyletic, and that hexactinellids are their sister group (together forming the Silicea). Thus, spicules must have evolved before the last common ancestor of all living siliceans, suggesting the presence of a significant gap in the silicean spicule fossil record. Molecular divergence estimates date the origin of this last common ancestor well within the Cryogenian, consistent with the biomarker record, and strongly suggests that siliceous spicules were present during the Precambrian but were not preserved

  • where s the glass biomarkers molecular clocks and micrornas suggest a 200 myr missing precambrian fossil record of siliceous sponge spicules
    Geobiology, 2010
    Co-Authors: Erik A Sperling, Jeffrey M Robinson, Kevin J Peterson
    Abstract:

    The earliest evidence for animal life comes from the fossil record of 24-isopropylcholestane, a sterane found in Cryogenian deposits, and whose precursors are found in modern demosponges, but not choanoflagellates, calcareans, hexactinellids, or eumetazoans. However, many modern demosponges are also characterized by the presence of siliceous spicules, and there are no convincing demosponge spicules in strata older than the Cambrian. This temporal disparity highlights a problem with our understanding of the Precambrian fossil record ‐ either these supposed demosponge-specific biomarkers were derived from the sterols of some other organism and are simply retained in modern demosponges, or spicules do not primitively characterize crown-group demosponges. Resolving this issue requires resolving the phylogenetic placement of another group of sponges, the hexactinellids, which not only make a spicule thought to be homologous to the spicules of demosponges, but also make their first appearance near the Precambrian ⁄Cambrian Boundary. Using two independent analytical approaches and data sets ‐ traditional molecular phylogenetic analyses and the presence or absence of specific microRNA genes ‐ we show that demosponges are monophyletic, and that hexactinellids are their sister group (together forming the Silicea). Thus, spicules must have evolved before the last common ancestor of all living siliceans, suggesting the presence of a significant gap in the silicean spicule fossil record. Molecular divergence estimates date the origin of this last common ancestor well within the Cryogenian, consistent with the biomarker record, and strongly suggests that siliceous spicules were present during the Precambrian but were not preserved.

Tsuyoshi Komiya - One of the best experts on this subject based on the ideXlab platform.

  • irreversible change of the oceanic carbon cycle in the earliest cambrian high resolution organic and inorganic carbon chemostratigraphy in the three gorges area south china
    Precambrian Research, 2013
    Co-Authors: Tomoko Ishikawa, Tsuyoshi Komiya, Naohiro Yoshida, Yuichiro Ueno, Yong Li
    Abstract:

    The terminal Neoproterozoic and its transition into the Cambrian witnessed major evolutionary and geochemical changes. Large fluctuations in the carbon isotope ratio for inorganic carbon (δ13Ccarb) across the Precambrian/Cambrian Boundary (Pc/C Boundary) indicate a significant change in the oceanic carbon cycle at that time. This work presents the first high-resolution δ13Corg chemostratigraphy of drill core samples across the Pc/C Boundary in the Three Gorges area, South China, and discusses carbon cycles based on the relation between δ13Corg and δ13Ccarb. We found that changes of δ13Corg and δ13Ccarb values were decoupled during the significant negative excursion of δ13Ccarb across the Pc/C Boundary, suggesting the presence of a huge organic carbon pool (OCP). Previously, the large OCP was inferred to have been established in the terminal Proterozoic and to have disappeared at the end of the mid-Ediacaran Shuram excursion. In contrast, our study suggests that the large OCP lasted until the early Nemakit-Daldynian stage. On the other hand, in the middle Nemakit-Daldynian to Atdabanian, the δ13Ccarb and δ13Corg changed in parallel as observed normally in the Phanerozoic. In order to evaluate these scenarios quantitatively, we performed numerical simulations and calculated the time evolution of the δ13Ccarb, masses of the inorganic and organic carbon pools and the fluxes between them. As a result, we show that the observed negative δ13Ccarb excursion with steady δ13Corg can be produced by enhanced primary productivity and remineralization of the large OCP, if the rate constant for removal of organic carbon was considerably lower than that of today. The end of the decoupled negative δ13Ccarb excursion is attributable to an increase in the removal rates of inorganic and organic carbon from the ocean. The enhanced flux of organic carbon burials eliminated the large OCP, so that the modern-style carbon cycle was established by the early Nemakit-Daldynian stage. The inferred rapid increases of both organic and inorganic carbon burial rates in the middle Nemakit-Daldynian stage were likely caused by the appearance of planktonic metazoans with guts making fecal pellets and by the first skeletal animals. The biological innovations may have changed the oceanic carbon cycle irreversibly just after the Pc/C Boundary.

  • carbon isotope chemostratigraphy of a precambrian cambrian Boundary section in the three gorge area south china prominent global scale isotope excursions just before the cambrian explosion
    Gondwana Research, 2008
    Co-Authors: Tomoko Ishikawa, Tsuyoshi Komiya, Shigenori Maruyama, Yusuke Sawaki, Yuichiro Ueno, Yong Li, Naohiro Yoshida
    Abstract:

    Abstract Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) Boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ13C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ13C chemostratigraphy of a drill core sample across the Pc/C Boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C Boundary, the δ13Ccarb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to − 7‰ (negative excursion 1: N1). Subsequently, the δ13Ccarb increases continuously up to about + 5‰ at the upper part of the Nemakit–Daldynian stage. After this positive excursion, δ13Ccarb sharply decreases down to about − 9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ13C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ13C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ13C profile best represents the global change in seawater chemistry. The minimum δ13C of the N1 (− 7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13C-depleted carbon just across the Pc/C Boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ13C rise up to + 5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ13C (− 9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.

Martin D Brasier - One of the best experts on this subject based on the ideXlab platform.

  • giving the early fossil record of sponges a squeeze
    Biological Reviews, 2014
    Co-Authors: Martin D Brasier, Jonathan B Antcliffe, Richard H T Callow
    Abstract:

    Twenty candidate fossils with claim to be the oldest representative of the Phylum Porifera have been re-analysed. Three criteria are used to assess each candidate: (i) the diagnostic criteria needed to categorize sponges in the fossil record; (ii) the presence, or absence, of such diagnostic features in the putative poriferan fossils; and (iii) the age constraints for the candidate fossils. All three criteria are critical to the correct interpretation of any fossil and its placement within an evolutionary context. Our analysis shows that no Precambrian fossil candidate yet satisfies all three of these criteria to be a reliable sponge fossil. The oldest widely accepted candidate, Mongolian silica hexacts from c. 545 million years ago (Ma), are here shown to be cruciform arsenopyrite crystals. The oldest reliable sponge remains are siliceous spicules from the basal Cambrian (Protohertzina anabarica Zone) Soltanieh Formation, Iran, which are described and analysed here in detail for the first time. Extensive archaeocyathan sponge reefs emerge and radiate as late as the middle of the Fortunian Stage of the Cambrian and demonstrate a gradual assembly of their skeletal structure through this time coincident with the evolution of other metazoan groups. Since the Porifera are basal in the Metazoa, their presence within the late Proterozoic has been widely anticipated. Molecular clock calibration for the earliest Porifera and Metazoa should now be based on the Iranian hexactinellid material dated to c. 535 Ma. The earliest convincing fossil sponge remains appeared at around the time of the Precambrian-Cambrian Boundary, associated with the great radiation events of that interval.

  • new u pb zircon dates for the neoproterozoic ghubrah glaciation and for the top of the huqf supergroup oman
    Geology, 2000
    Co-Authors: Martin D Brasier, Gretta Mccarron, Robert D Tucker, Jonathan Leather, Philip A Allen, Graham A Shields
    Abstract:

    The Huqf Supergroup, Oman, contains volcanic horizons with the potential to calibrate Neoproterozoic events and chemostratigraphy using U-Pb zircon geochronology. A tuffaceous bed near the base, within the lower (Ghubrah) diamictite and beneath a lower cap carbonate, provides the first U-Pb zircon date obtained from within a Neoproterozoic glacial deposit. This date of 723 +16/−10 Ma suggests a Sturtian age. Diamictites about 1 km higher in the section are overlain by the Hadash cap dolomite, which compares with cap dolomites above Marinoan glacial units elsewhere. A U-Pb zircon age of 544.5 ± 3.3 Ma from ignimbrites in the Fara Formation, near the top of the supergroup, is just beneath that of the Precambrian-Cambrian Boundary (ca. 543 Ma), consistent with the presence of the Neoproterozoic skeletal-fossil Cloudina in correlative subsurface rocks. This revised chronology confirms the existence of four negative δ13C excursions between ca. 723 and 543 Ma, the lower two of which are clearly associated with glaciations.

Naohiro Yoshida - One of the best experts on this subject based on the ideXlab platform.

  • irreversible change of the oceanic carbon cycle in the earliest cambrian high resolution organic and inorganic carbon chemostratigraphy in the three gorges area south china
    Precambrian Research, 2013
    Co-Authors: Tomoko Ishikawa, Tsuyoshi Komiya, Naohiro Yoshida, Yuichiro Ueno, Yong Li
    Abstract:

    The terminal Neoproterozoic and its transition into the Cambrian witnessed major evolutionary and geochemical changes. Large fluctuations in the carbon isotope ratio for inorganic carbon (δ13Ccarb) across the Precambrian/Cambrian Boundary (Pc/C Boundary) indicate a significant change in the oceanic carbon cycle at that time. This work presents the first high-resolution δ13Corg chemostratigraphy of drill core samples across the Pc/C Boundary in the Three Gorges area, South China, and discusses carbon cycles based on the relation between δ13Corg and δ13Ccarb. We found that changes of δ13Corg and δ13Ccarb values were decoupled during the significant negative excursion of δ13Ccarb across the Pc/C Boundary, suggesting the presence of a huge organic carbon pool (OCP). Previously, the large OCP was inferred to have been established in the terminal Proterozoic and to have disappeared at the end of the mid-Ediacaran Shuram excursion. In contrast, our study suggests that the large OCP lasted until the early Nemakit-Daldynian stage. On the other hand, in the middle Nemakit-Daldynian to Atdabanian, the δ13Ccarb and δ13Corg changed in parallel as observed normally in the Phanerozoic. In order to evaluate these scenarios quantitatively, we performed numerical simulations and calculated the time evolution of the δ13Ccarb, masses of the inorganic and organic carbon pools and the fluxes between them. As a result, we show that the observed negative δ13Ccarb excursion with steady δ13Corg can be produced by enhanced primary productivity and remineralization of the large OCP, if the rate constant for removal of organic carbon was considerably lower than that of today. The end of the decoupled negative δ13Ccarb excursion is attributable to an increase in the removal rates of inorganic and organic carbon from the ocean. The enhanced flux of organic carbon burials eliminated the large OCP, so that the modern-style carbon cycle was established by the early Nemakit-Daldynian stage. The inferred rapid increases of both organic and inorganic carbon burial rates in the middle Nemakit-Daldynian stage were likely caused by the appearance of planktonic metazoans with guts making fecal pellets and by the first skeletal animals. The biological innovations may have changed the oceanic carbon cycle irreversibly just after the Pc/C Boundary.

  • carbon isotope chemostratigraphy of a precambrian cambrian Boundary section in the three gorge area south china prominent global scale isotope excursions just before the cambrian explosion
    Gondwana Research, 2008
    Co-Authors: Tomoko Ishikawa, Tsuyoshi Komiya, Shigenori Maruyama, Yusuke Sawaki, Yuichiro Ueno, Yong Li, Naohiro Yoshida
    Abstract:

    Abstract Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) Boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard δ13C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ13C chemostratigraphy of a drill core sample across the Pc/C Boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C Boundary, the δ13Ccarb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to − 7‰ (negative excursion 1: N1). Subsequently, the δ13Ccarb increases continuously up to about + 5‰ at the upper part of the Nemakit–Daldynian stage. After this positive excursion, δ13Ccarb sharply decreases down to about − 9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ13C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ13C profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our δ13C profile best represents the global change in seawater chemistry. The minimum δ13C of the N1 (− 7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13C-depleted carbon just across the Pc/C Boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent δ13C rise up to + 5‰ (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO2 and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low δ13C (− 9‰) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.