δ13C Excursion Research Articles

Wide but not ubiquitous distribution of glendonite in the Doushantuo Formation, South China: Implications for Ediacaran climate

Glendonites are pseudomorphs of syndepositional or early authigenic ikaite (CaCO3·6H2O) that often forms at near-freezing temperatures. Silicified glendonite has been reported from inner-shelf deposits of the lower Ediacaran Doushantuo Formation at a single stratigraphic section in South China, where they are stratigraphically associated with the positive δ13C excursion EP1 but predate the negative δ13C excursion EN3 (=Shuram excursion), indicating a period of cool climate somewhere between ~609 Ma and ~551 Ma. This interpretation predicts a wider geographic distribution of Ediacaran glendonites in equivalent strata in the Yangtze Block of South China. To test this prediction, we conducted a regional survey of Doushantuo Formation and found that, although glendonites are not universally present, they occur in expected stratigraphic intervals and in association with EP1 at two new sections representing inner-shelf and intrashelf basin facies. The wide but not ubiquitous distribution of Doushantuo glendonites indicates that ikaite precipitation and glendonite formation was controlled by both regional climatic and local geochemical factors. Glendonites at the new localities are stellate clusters pseudomorphed by calcite spar and sometimes are rimmed with silica. The calcite spar is characterized by highly variable and mostly negative δ13C values as low as −37‰, indicating that diagenetic transformation of precursor ikaite to calcitic glendonite may be related to anaerobic oxidation of organic matter or methane in sediment. The new data suggest an early Ediacaran cold period prior to EN3 or the Shuram excursion and facilitation of glendonite/ikaite formation by both climatic and local geochemical conditions such as redox conditions and phosphate concentrations.

High-resolution paired carbon isotopic records from the Meishucun section in South China: Implications for carbon cycling and environmental changes during the Ediacaran-Cambrian transition

Several prominent negative carbon isotope (δ13C) excursions are observed during the Ediacaran-Cambrian (E-C) transition, reflecting large perturbations of the carbon cycle in the ocean–atmosphere system. These negative δ13C excursions have been linked to dramatic paleoenvironmental changes and been widely used as regional and global chemostratigraphic correlation tools. However, there is still much debate about their causes and mechanisms. In this study, we report high-resolution δ13C records of both carbonate (δ13Ccarb) and organic carbon (δ13Corg) from the Meishucun section in South China. Combined with lithological variations and intrabasinal correlations, we find that the negative δ13C excursions during the E-C transition in South China are tightly linked with transgressive events. The existence of a large vertical (surface-to-deep water) δ13C gradient in a stratified ocean, and episodic incursions of 13C-depleted anoxic/euxinic deep waters into shallow water environment through enhanced upwelling or shoaling of the chemocline, may have contributed to these negative δ13C excursions. In addition, our results also imply a large reduction in the vertical δ13Corg gradient during the E-C transition, which may reflect the ventilation and oxygenation of the deep ocean. The corresponding expansion of oxic seafloor conditions and a large increase in the extent of habitable space for animals may have facilitated the radiation of metazoans during the Cambrian explosion.

Pacing of the latest Ordovician and Silurian carbon cycle by a ~4.5 Myr orbital cycle

The cyclic eccentricity, precession and obliquity parameters of the Earth's orbit and axial motion have been demonstrated to have a large impact on climate on short (104 to 105 yr) timescales during the Cenozoic and Mesozoic. Amplitude modulation (AM) of these parameters on longer (106 yr) timescales induces “grand orbital cycles” that have also been detected for large parts of the Cenozoic and Mesozoic. However, the response of paleoclimatic variables to these cycles is unknown for the Paleozoic due to past chaotic transitions in the orbital motion of the inner planets. The Late Ordovician to Early Devonian remains an enigmatic period of geological time. During the Hirnantian, extensive Southern Hemisphere glaciation was associated with the second largest mass extinction in Earth's history. The subsequent Silurian was met with several high-amplitude (>4‰) carbon isotope (δ13C) excursions associated with equally erratic climatic, biotic and eustatic fluctuations. Here, I conduct spectral analysis of carbonate δ13C and conodont apatite oxygen isotope (δ18O) compilations for the Katian (~448 Ma) to mid-Lochkovian (~415 Ma). The resulting spectra document a prominent cyclicity of ~4.2 (3.7 to 4.8) Myr in the carbon and climate cycles, similar to dominant periods found in Cenozoic astronomical solutions related to the long-term AM of the 100-kyr eccentricity band, 400-kyr eccentricity band and axial obliquity. Variations in the ~4.5 Myr eccentricity and axial obliquity cycles respectively enhanced organic carbon burial or reservoir stability leading to fluctuations in the Paleozoic δ13C record. This process led to a decline in atmospheric CO2 to threshold levels, after which astronomical forcing triggered the onset of glaciation and extensive global cooling, associated with positive δ18O excursions during the Hirnantian, the early and late Llandovery, the Telychian-Sheinwoodian boundary, the Homerian, the late Ludfordian and the late Pridoli-early Lochkovian.

A multiproxy-based reconstruction of the mid- to late Holocene paleoenvironment in the Laptev Sea off the Lena River Delta (Siberian Arctic)

Land-shelf interactions and related environmental changes were reconstructed for the past 6 cal. kyrs interpreting multiproxy records from the two AMS14C-dated sediment cores from the sites located to the north and northeast of the Lena River delta. Proxies used include terrestrial and aquatic palynomorphs, benthic/planktic foraminifers and ostracods paired with benthic δ18O and δ13C records of Haynesina orbiculare. The study focused on unravelling the relation between river runoff and the regional climate changes on the one hand, and its imprint on the shelf sea environment on timescales beyond the instrumental records on the other.The palynomorph records show that the Lena River outflow largely determined the composition of species associations and the magnitude of terrestrial matter influx from land. Pollen assemblages of the inner Laptev Sea shelf reflect complex pollen contribution of the Arctic tundra and remote taiga zones drained by the Lena River and indicate a vegetation response to warmer-than-present climatic conditions between 6.0 and 4.5 cal. ka and a subsequent gradual cooling. Fluvial influence in the records is manifested by (1) increases in sedimentation rates; (2) high influxes of pollen/spores and freshwater chlorophycean algae, wood and plant remains; (3) negative δ13C excursions due to the introduction of dissolved inorganic carbon (DIC) from river water. Episodes of enhanced freshwater influence in the surface water layers (fluvial events) correlate with positive benthic δ18O excursions and increasing representation of river-distal species among benthics. Altogether, this points to an estuarine-like reversed (north to south) bottom current activity along the submarine paleovalleys on the shelf. The most pronounced fluvial events are recognized at 5.3–5.9, 1.5 and <0.5 cal. ka. The oldest fluvial event coincides with the final stage of mid-Holocene climate warming. A fluvial event at 1.5 cal. ka is specifically strong at the northeastern site thus marking a direction change of the Lena River outflow at this time and the progressive protrusion of the delta. During the past 500 years distinct negative δ13C values at the northeastern site reflect enhanced riverine influence. It is therefore concluded that the unprecedented change in the δ13C trend corroborates the other evidence for a principal diversion of the major Lena River outflows into its present-day, easterly direction.

Cenomanian–Turonian drowning of the Arabian Carbonate Platform, the İnişdere section, Adıyaman, SE Turkey

Abstract The Cenomanian–Turonian carbonate ramp in the Adıyaman Region of SE Turkey (Northern Arabian Platform) records an abrupt shift from benthic carbonate deposits to pelagic deposits near the Cenomanian–Turonian boundary event (CTBE) in the İnişdere stratigraphic section and surrounding borehole sections. A positive δ 13 C excursion of up to 2.15% is recorded in carbonate and organic carbon deposited around the CTBE and provides evidence of a direct link between the CTBE and oceanic anoxic events and the demise of the shallow carbonate production in the Derdere Formation. The microfacies analyses, biostratigraphic dating and palaeoenvironmental interpretations suggest that the platform was drowned near the CTBE as a result of changing environmental conditions. The microfacies indicating significant deepening show a contemporaneity to equivalent surfaces globally and thus strongly support an isochronous formation of Cenomanian–Turonian facies by eustatic sea-level changes. Anoxia spreading over the platform drastically reduced the carbonate production as observed in the studied sections and, therefore, resulted in a reduction in carbonate accumulation rates. Regional/local subsidence and a coeval sea-level rise during the late Cenomanian to early Turonian interval were the cause of the drowning of the platform, including regional anoxia at the northern Arabian platform linked to the Cenomanian–Turonian oceanic anoxic event (OAE2).

Paleoenvironmental Changes at ODP Site 702 (South Atlantic): Anatomy of the Middle Eocene Climatic Optimum

AbstractThe Middle Eocene Climatic Optimum (MECO) was an unusual global warming event that interrupted the long‐term Eocene cooling trend ca. 40 Ma. Here we present new high‐resolution bulk and benthic isotope records from South Atlantic ODP Site 702 to characterize the MECO at a high latitude setting. The MECO event, including early and peak warming as well as recovery to background levels, had an estimated ~300 Kyr duration (~40.51 to ~40.21 Ma). Cross‐plots (δ18O vs. δ13C) suggest that the mechanisms driving coupled changes in O and C isotope values across the MECO were weaker or absent before the event. The paleoecological response has been evaluated by quantitative analysis of calcareous nannofossils and benthic foraminifera assemblages. We document a shift in the biogeographical distribution of warm and temperate calcareous nannoplankton taxa, which migrated toward higher latitudes due to increased temperatures during the MECO. Conversely, changes in the organic matter flux to the seafloor appear to have controlled benthic foraminifera dynamics at Site 702. Benthic phytodetritus exploiting taxa increased in abundance coinciding with a positive δ13C excursion, ~150 Kyr before the start of the δ18O negative excursion that marks the start of MECO warming. Our data suggest that paleoecological disturbance in the deep sea predates MECO δ18O excursion and that it was driven by changes in the type and/or amount of organic matter reaching the seafloor rather than by increased temperature.

Midlatitude Southern Hemisphere Temperature Change at the End of the Eocene Greenhouse Shortly Before Dawn of the Oligocene Icehouse

AbstractThe Eocene‐Oligocene transition (EOT) marked the initiation of large‐scale Antarctic glaciation. This fundamental change in Cenozoic climate state is recorded in deep‐sea sediments by a rapid benthic foraminiferal δ18O increase and appearance of ice‐rafted debris in the Southern Ocean. However, we know little about the magnitude of cooling associated with the EOT in shallow water environments, particularly at middle to high latitudes. Here we present new stratigraphic records of the C13r/C13n magnetochron boundary and the EOT in the clay‐rich Blanche Point Formation, South Australia. The Blanche Point Formation was deposited in a shallow shelf setting (water depths of &lt;100 m) at a paleolatitude of ~51°S. We present high‐resolution δ18O, δ13C, and Mg/Ca records of environmental change from well‐preserved benthic foraminifera of latest Eocene age at this site. A marked, negative δ13C excursion occurs immediately before EOT Step 1 and may be a globally representative signal. An ~2 °C cooling of shallow shelf seawater is evident from benthic foraminiferal Mg/Ca across Step 1. This cooling signal is both sufficient to account fully for the δ18O increase in our data and is of similar amplitude to that documented in published records for shallow shelf and upper water column open ocean settings, which suggests no obvious polar amplification of this cooling signal. Our results strengthen the evidence base for attributing EOT Step 1 to global cooling with little contribution from ice volume growth and contradict the mechanism suggested to explain the inferred northward migration of the intertropical convergence zone in the contemporaneous equatorial Pacific Ocean.

Dynamic carbon and sulfur cycling in the aftermath of the Lomagundi-Jatuli Event: Evidence from the Paleoproterozoic Hutuo Supergroup, North China Craton

The unprecedented positive δ13C excursion in carbonates deposited between 2.2 and 2.0 Ga, known as the Lomagundi-Jatuli Event (LJE), has been documented globally and linked to the rise of atmospheric oxygen. Increasing oxidation inevitably changed the atmosphere-hydrosphere system, but few chemostratigraphic or quantitative constraints for the aftermath of this event exists. Here, we describe a ~200 m-thick carbonate succession in the Huaiyincun Formation, Hutuo Supergroup, ~2.0–1.9 Ga, from the North China Craton. There is a lithological transition from pink-purple dolostones to grey dolostones at ca. 91.6 m above the base of the Huaiyincun Formation. The former are more enriched in hematite and detrital minerals, whereas the latter contain more organic matter but almost no detrital phases. Meanwhile, the frequent occurrence of tempesite structures, along with the abrupt decline of stromatolites in the upper Huaiyincun Formation, suggest a storm-dominated environment. These distinct features within the Huaiyincun Formation reveal increased water depth during a transgression event. Two types of Raman spectra of organic matter were found in the lower and upper Huaiyincun Formation, respectively, which is proposed to be the result of variable oxidation.At 65.6 m, 26 m below the lithological transition (~91.6 m), remarkable decreases in both δ13Ccarb and δ34SCAS are observed. This discordance between C-S isotopic excursions and sedimentological and mineralogical variations argues against a seawater depth gradient effect of the δ13Ccarb and δ34SCAS curves. Instead, the decline of δ13Ccarb rather correlates with the negative δ13Ccarb excursions in ca. 2.0 Ga carbonates from Gabon and Russia, known as the Shunga-Francevillian Event (SFE). The result of the quantitatively constrained paleo-seawater [SO42−]sw suggests a crash of the seawater sulfate reservoir compared with that during the preceding LJE. However, the decreased δ34SCAS and increased CAS concentration towards the top of the study unit represent the recovery of seawater sulfate reservoir. The coordinated decline in δ34SCAS and δ13Ccarb values is likely related to enhanced oxidation of continental pyrite and organic matter in the aftermath of the LJE. The Huaiyincun Formation therefore represents a critical interval that recorded dynamic carbon and sulfur cycles after the LJE.

CO2 Release From Pockmarks on the Chatham Rise‐Bounty Trough at the Glacial Termination

AbstractSeafloor pockmarks of varying size occur over an area of 50,000 km2 on the Chatham Rise, Canterbury Shelf and Inner Bounty Trough, New Zealand. The pockmarks are concentrated above the flat‐subducted Hikurangi Plateau. Echosounder data identify recurrent episodes of pockmark formation at ~100,000‐year frequency coinciding with Pleistocene glacial terminations. Here we show that there are structural conduits beneath the larger pockmarks through which fluids flowed upward toward the seafloor. Large negative Δ14C excursions are documented in marine sediments deposited next to these subseafloor conduits and pockmarks at the last glacial termination. Modern pore waters contain no methane, and there is no negative δ13C excursion at the glacial termination that would be indicative of methane or mantle‐derived carbon at the time the Δ14C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large Δ14C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the Δ14C anomalies and pockmarks to release of 14C‐dead CO2 and carbon‐rich fluids from subsurface reservoirs, the most likely being dissociated Mesozoic carbonates that subducted beneath the Rise during the Late Cretaceous. Because of the large number of pockmarks and duration of the Δ14C anomaly, the pockmarks may collectively represent an important source of 14C‐dead carbon to the ocean during glacial terminations.

Middle Permian organic carbon isotope stratigraphy and the origin of the Kamura Event

Large carbon cycle perturbations associated with the Middle Permian (Capitanian) mass extinction have been widely reported, but their causes and timing are still in dispute. Low resolution carbon isotope records prior to this event also limit the construction of a Middle Permian chemostratigraphic framework and global or local stratigraphic correlation, and hence limit our understanding of carbon cycle and environmental changes. To investigate these issues, we analyzed the 13Corg values from the Middle Permian chert-mudstone sequence (Gufeng Formation) in the Lower Yangtze deep-water basin (South China) and compared them with published records to build a chemostratigraphic scheme and discuss the underlying environmental events. The records show increased δ13Corg values from late Kungurian to early Guadalupian, followed by a decrease to the late Wordian/early Capitanian. The early-mid Capitanian was characterized by elevated δ13Corg values suggesting the presence of the “Kamura Event”: an interval of heavy positive values seen in the δ13Ccarb record. We propose that these heavy Capitanian δ13C values may be a response to a marked decline in chemical weathering rates on Pangea and associated reduction in carbonate burial, which we show using a biogeochemical model. The subsequent negative δ13C excursion seen in some carbonate records, especially in shallower-water sections (and in a muted expression in organic carbon) coincide with the Capitanian mass extinction may be caused by the input of isotopically-light carbon sourced from the terrestrial decomposition of organic matter.

Oceanic organic carbon as a possible first-order control on the carbon cycle during the Bathonian–Callovian

Abstract Oceans are the largest, readily exchangeable, superficial carbon reservoir; a current challenge in investigating past and present environments and predict future evolution relates to the role of oceanic carbon in regulating Earths' carbon cycle and climate. At least one paired δ13Ccarb-TOC decoupling event is noted in the Late Bathonian–Early Callovian. Provokingly, we suggest that this decoupling event and other carbon isotopic events in the Bathonian–Callovian resulted from the addition and removal of carbon from the oceanic organic carbon (OOC) reservoir, likely dominated by dissolved and fine particulate oceanic organic carbon. The decoupling event is characterised by a mainly invariant δ13CTOC record and increasingly more positive δ13Ccarb values. The δ13Ccarb-TOC decoupling is tentatively explained by the expansion of the OOC reservoir, which increased the residence time of carbon in the oceans and buffered (slow equilibrium) δ13COOC (approximated by δ13CTOC) to changes in δ13C of oceanic inorganic carbon, comprised mainly of dissolved inorganic carbon (approximated by δ13Ccarb). Reconversion of OOC to CO2 may have resulted in negative δ13C excursions and increased atmospheric pCO2, whereas the change from accumulation of OOC to export of organic carbon to sediments may have resulted in positive δ13C excursions and widespread accumulation of organic matter. It is speculated that other small-scale (~ 1‰) δ13C excursions in the Mesozoic record may have resulted from similar episodes of OOC accumulation and oxidation. Our study calls attention to the potential of the OOC reservoir to act as a critical driver for planetary-scale changes over short geological time intervals.

The lead-up to the Sturtian Snowball Earth: Neoproterozoic chemostratigraphy time-calibrated by the Tambien Group of Ethiopia

AbstractThe Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations—the Sturtian “Snowball Earth.” Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0–2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ13C and 87Sr/86Sr data and U-Pb ID-TIMS ages from the Tambien Group are used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. Tambien Group δ13C data and U-Pb ID-TIMS ages reveal that a pre-Sturtian sharp negative δ13C excursion (referred to as the Islay anomaly in the literature) precedes the Sturtian glaciation by ∼18 m.y., is synchronous in at least two separate basins, and is followed by a prolonged interval of positive δ13C values. The composite Tonian 87Sr/86Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater 87Sr/86Sr rose ca. 880–770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, suggest that the 87Sr/86Sr increase was influenced by increased subaerial weathering of radiogenic lithologies as Rodinia rifted apart at low latitudes. The following 87Sr/86Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO2 and contributing to the initiation of the Sturtian glaciation.

The δ13C chemostratigraphy of Ordovician global stage stratotypes: geochemical data from the Floian and Sandbian GSSPs in Sweden

ABSTRACTThe δ13C chemostratigraphy of five of the seven Ordovician global stages has been published previously but no such data have been available from the Floian GSSP and most of the Sandbian GSSP in Sweden. This lack of information has now been remedied by isotope data obtained from series of closely spaced shale samples collected from the Floian stratotype at Diabasbrottet in Västergötland and the Sandbian stratotype at Fågelsång in Scania. Although the bases of these stages cannot be precisely tied to levels of conspicuous δ13C excursions, that of the Floian Stage, which is marked by the appearance of the graptolite Tetragraptus approximatus, is between the closely spaced excursions named LTNICE and BFICE. The base of the Sandbian Stage, which is defined as the appearance level of the graptolite Nemagraptus gracilis, is just below a negative excursion previously known as the “Upper Kukruse Low”, which is nowadays known as the LSNICE. The relations between chemostratigraphy and graptolite and conodont biostratigraphy in the Swedish GSSPs and some coeval key sections in Baltoscandia, China, and America are briefly discussed. It is concluded that the data at hand indicate that there is good regional agreement in these relations.

The Ediacaran–Cambrian boundary in the Tarim Basin, NW China: Geological data anomalies and reservoir implication

Abstract In this study, unconformities (T90) formed during the succession from the Ediacaran to the Cambrian were identified based on five types of data, namely, field outcrops, C and O isotopes, major and trace elements, well logs and seismic data. The outcrop data show notable lithological changes above and below the T90 interface. The C and O isotope data show significant negative δ13C and δ18O excursions at the T90 interface. The negative δ13C excursion at the T90 interface occurred concurrently with the negative δ13C excursions that occurred globally during the same period. The average values of Ca, Mg, Nd, Pr, Ce and La were higher below the T90 interface than that above the T90 interface and that the average mass fractions of Zr, Sr, Zn, Ba, Mn, S, Si, Ni, Fe, Al, Rb, K, Cl and P were lower below that interface than that above it. The well log data show that at the T90 interface, the concentrations of U and Th and the mass fraction of K increased suddenly (from 97.66 to 104.308 ppm, from 210.005 to 232.086 ppm and from 11.609% to 12.99%, respectively) and the gamma ray value increased significantly, reflecting a brief land exposure of the interface. Onlap and truncation patterns were visible on the seismic sections above and below the T90 interface. Various types of unconformities have developed at the T90 interface in various areas of the Tarim Basin. Paraconformities are mainly distributed in the Kuluketage and Kalpin outcrop areas as well as the depressions in the northern Tarim Basin and a large area north of the Tarim Basin. Angular unconformities are distributed near the north wing of the central uplift belt. The paleogeomorphology of the T90 unconformity interfaces limited the distribution of sedimentary facies in the overlying strata, thereby affecting the distribution of the hydrocarbon source rocks of the Yurtus Formation, and controlled the development of karst reservoirs in the underlying strata.

Early Cambrian ocean mixing recorded by phosphorite successions in the Nanhua Basin, South China

Phosphorite successions in the nearshore Meishucun section and the offshore Gezhongwu section of the Nanhua Basin, South China, were investigated to decipher the circulation pattern of the early Cambrian ocean. The 539–535 Ma phosphorites from the Meishucun section contain authigenic phosphorus-rich mineral aggregates (collophanite grains) with much lower bio-limiting elements, Zn, Cd and Ni, than those from the Gezhongwu section. In contrast, collophanite grains in the 535–526 Ma phosphorites from both sections have similar concentrations of these elements. This change indicates that the ocean likely transformed from a chemically stratified state to a homogeneous state at ~535 Ma. Phosphorites from both sections have constant 87Sr/86Sr ratios and εNd(t) values, supporting the interpretation of a relatively constant terrestrial input into the Nanhua Basin between 539 Ma and 526 Ma. Instead, negative δ13C excursions and similar 208Pb/204Pb ratios of the 535–526 Ma phosphorites are indicative of an extensive upwelling of the deep oceanic water. In combination with previous studies of deep ocean oxygenation, our work provides evidence for the vertical mixing of the ocean between 535 Ma and 526 Ma, which introduced O2-rich water at the ocean bottom to promote early Cambrian biodiversity.

Astronomical time scale for the middle-upper Doushantuo Formation of Ediacaran in South China: Implications for the duration of the Shuram/Wonoka negative δ13C excursion

The Ediacaran Period (635–541 Ma) of the Neoproterozoic records the advent of multi-cellular life on Earth. The Ediacaran Doushantuo Formation (DST Fm) contains records of some phases of this bio-evolution and profound perturbations to the global carbon cycle, but the timing and the duration of these events are still unclear. The Jiulongwan (JLW) roadcut in South China is a well-studied reference section for these Ediacaran events, including the major EN3 (Shuram/Wonoka) negative carbon-isotope excursion. Spectral analysis of geochemical data through the middle-upper DST Fm indicates that the sedimentary cycles correspond to 405-kyr long-eccentricity and ~100 kyr short-eccentricity periods. Tuning the DST Fm at JLW section to the 405-kyr long-eccentricity cycles yielded a ~30 Myr-long astronomical time scale. Member DST-4 (black shale) of the uppermost DST Fm spans 7.5 Myr and the negative δ13C excursion (EN3/Shuram/Wonoka) has a duration of~20 Myr. Relative to a UPb date of 551.1 ± 0.7 Ma near the top of the DST Fm, the onset of Member DST-4 is at 557.8 ± 0.8 Ma, and the projected basal age of Member DST-3 is 579.3 ± 0.8 Ma. The onsets of positive EP2 and negative EN3 δ13C excursions were ca. 578.1 Ma and 571.1 Ma, respectively; therefore, both excursions occurred after the termination of the Gaskiers glaciation (~581 Ma). Thereby, these ages and astronomical time scale provide important new constraints on the subdivision of Ediacaran strata, and have implications for our understanding the character of the δ13C excursions. Orbital forcing may be important to the co-evolution of life, the global carbon cycle and sedimentary environments.

Stratigraphy and isotope geochemistry of the pre-Sturtian Ugab Subgroup, Otavi/Swakop Group, northwestern Namibia

The Islay carbon isotope anomaly is a late Tonian negative δ13C excursion that has been documented in marine carbonates globally. It has in the past been thought to be causally linked to the onset of Cryogenian glaciation ca. 717 Ma. However, recent work suggests that this anomaly is instead closer to ca. 739–735 Ma and that it may be one of two distinct late Tonian carbon isotope anomalies. Here we present the litho- and chemo-stratigraphy of the late Tonian Ugab Subgroup of the Otavi/Swakop Group in northwestern Namibia. Carbon isotope data from two separate outcrop belts, the Summas Mountains and the Vrede Domes, indicate that the Ugab Subgroup strata exposed in the former inlier are older than those of the latter, with only approximately 100 m of stratigraphic overlap. We use detailed measured sections to develop a sequence stratigraphic framework for the Ugab Subgroup in each outcrop belt, and correlate strata across the two exposures to construct a composite carbonate δ13C record for the late Tonian in Namibia. This carbon isotope profile of the Ugab Subgroup shows two separate pre-Sturtian negative anomalies, consistent with findings in northern Canada, Svalbard, Scotland, and Ethiopia. Our results are an important contribution to the pre-Cryogenian carbon isotope record, as these data may serve as the first definitive documentation of both anomalies within a single basin.

Biochemostratigraphy of an upper Albian–Turonian succession from the southeastern Neo-Tethys margin, SW Iran

A Neo-Tethyan upper Albian–Turonian (hemi-)pelagic carbonate succession (Sarvak Formation), accessible in outcrop in the Zagros, SW Iran, was investigated using sedimentology, microfacies analysis, planktonic foraminifera biostratigraphy, and carbon-isotope stratigraphy. The succession contains six biozones including Thalmanninella (Th.) appenninica, Th. globotruncanoides, Rotalipora cushmani, Whiteinella archaeocretacea, Helvetoglobotruncana Helvetica, and Marginotruncana schneegansi. Some of these biozones show variable stratigraphic positions throughout the Tethyan and Boreal realms that hinder correlation of the studied Iranian section with European reference sections. Therefore, the correlation of the existing biostratigraphic framework was complemented by carbonate carbon-isotope stratigraphy based on bulk carbonate material. Despite the low resolution of the presented carbon-isotope record, pronounced global events including the late Turonian Events, Cenomanian/Turonian Boundary (CTB) Event (OAE 2), and Albian/Cenomanian Boundary (ACB) Event can be distinguished. The δ13C excursions provide global synchronous time-lines for correlation. The distinguished biozones were compared among the sections regarding their stratigraphic positions relative to these time-lines. The correlation tentatively suggests that the biostratigraphic datum marking the ACB is diachronous with an SE–NW trend. First appearance datum (FAD) of Th. appenninica varies from the upper Albian to the lower Cenomanian. The FAD of Th. globotruncanoides is also variable from the upper Albian to the lower Cenomanian. However, the middle Cenomanian–Turonian biozones including Rotalipora cushmani and Whiteinella archaeocretacea zones are synchronous. Elevated pCO2 contributed to the gradual increase of the SST (sea surface temperature) during the Cenomanian, and the dramatic climax at the earliest Turonian. This trend closely correlates with the change from the diachronous ACB-biozones to synchronous CTB-biozones. Possibly, increasing SSTs provided temperature-tolerance range for the species throughout different latitudes, and assisted their rapid distribution. Moreover, the warming raised the sea level from the latest Albian to Turonian that inundated landbridges, resulting in an enhanced marine circulation and faster distribution of planktonic species throughout the Neo-Tethys Ocean.

Glacio‐eustasy and δ13C across the Mississippian–Pennsylvanian boundary in the eastern Paleo‐Tethys Ocean (South China): Implications for mid‐Carboniferous major glaciation

A positive carbonate δ13C excursion (by ~1.5–3.0‰) has been reported across the Mississippian–Pennsylvanian boundary (MPB) from Euramerican epicontinental seas, implying a coeval atmospheric pCO2 decrease and Gondwanan glaciation at that time. This excursion was mainly observed in carbonate platforms which experienced repeated subaerial exposure. The South China Block was located in the middle of the Paleo‐Tethys Ocean and the Panthalassic Ocean during the Carboniferous and contains well‐preserved carbonate slope strata across the MPB. Four lithofacies were defined in the studied Naqing, Narao, and Dianzishang sections, including thin‐bedded lime mudstones, laminated wackestones to packstones, normally‐graded packstones, and slumped limestones. Immediately above the MPB, a bed of normally‐graded packstones or slump masses occurs in the studied sections, which together with coeval subaerial‐exposure features in shallow‐water carbonate platform of South China, suggests a significant sea‐level fall across the MPB. Carbonate δ13C records show a consistent value of ~3.0‰ below the MPB in the Naqing and Narao sections, and both sections show an obvious rise in δ13C across the MPB (by ~0.5–1.0‰). The relatively small‐magnitude rise in δ13C would have resulted from well‐mixed seawater induced by intensified upwelling in the eastern Paleo‐Tethys Ocean during the glacial peak. Hence, the carbonate δ13C values recorded in the South China Block may represent a mean δ13C of the dissolved inorganic carbon in global ocean water at that time. Correlation between carbonate δ13C and previously published conodont δ18O and 87Sr/86Sr records suggests that the MPB glaciation was mainly driven by enhanced continental weathering, rather than increased organic carbon burial, although further quantitative simulation is required to better understand the interlinked processes during the Earth's penultimate icehouse.

Carbonate chemostratigraphy of the Vazante Group, Brazil: A probable Tonian age

The thick carbonate-siliciclastic succession of the Vazante Group was deposited on the western border of the São Francisco craton, central Brazil, which was deformed during the Brasiliano Orogeny that ended at 540 Ma. In this research, we studied five drill cores and outcrops to obtain integrated isotope data (δ13Ccarb, δ18Ocarb, and 87Sr/86Sr) as well as U-Pb ages of detrital zircons to understand the chronostratigraphic relationships between the Tonian and Cryogenian periods. The Vazante Group forms a succession of more than 3 km thick of limestones, dolostones, shales and rare sandstones. These rocks were deposited on a marine shelf that shows distinct δ13C variations throughout the stratigraphic succession. The lower limestone of this group (Lagamar Formation) has low positive δ13C values (0.3–1.0‰) and 87Sr/86Sr ratios of 0.70646–0.70692 (with 423–1,719 ppm Sr), and possible vase-shaped microfossils (VSMs) that may be equivalent to those found in Huaibei Group in northern China (lower Tonian) or has stratigraphic correlations to the VSMs-bearing units of the upper Tonian. The Serra do Poço Verde Formation has a plateau of moderate δ13C positive values (0.0–4‰) followed by dolostones of the Morro do Calcário Formation, which is a 50-m-thick interval with negative values (as low as −4‰). The upper unit of the Vazante Group (Serra da Lapa Formation) begins with a sharp negative δ13C excursion locally overlying a few layers of diamictite, which may be the equivalent of the Sturtian glacial age. Restrictions on establishing the depositional age of the Vazante Group continue; however, the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a Tonian age for the Vazante Group.