Positive Carbon Isotope Excursion Research Articles

Sedimentary facies controlled biogeochemical process of biotic extinction and turnover across the Cambrian SPICE event

The Steptoean positive carbon isotope excursion (SPICE) event, one of the largest carbon cycle perturbations in the Cambrian, coincides with shallow-shelf-fauna extinction and plankton revolution (critical transition of plankton). The event is globally documented, but biogeochemical responses of these biotic evolutions in varying facies environments are not well understood. Here high-resolution paired δ18Ocarb, δ13Ccarb and δ13Corg datasets from varied paleodepth environments in the Tarim Basin, NW China reveal facies-dependent signatures of the event, with globally synchronous patterns but notable intra-basinal variability. Shallow marine facies record the end-Marjuman extinction with a distinct negative δ13Corg excursion prior to the event, while the transitional facies region marks twice positive δ13C excursions corresponding to an asynchronous plankton revolution from shallow and deep areas during the event. The varying isotope responses are interpreted in the context of primary productivity and redox conditions, with deeper basins recording more 13C enriched signals (i.e., higher δ13C) due to greater organic matter preservation under anoxic conditions, compared to the platform area. The biotic extinction, the planktonic revolution and the interaction of organisms along the shallow to deep marine depth gradient were reflected by the significant isotopic shifts recorded during the event, suggesting depth-dependent biogeochemical processes that shaped marine ecosystems.

Timing and origin of the Lomagundi-Jatuli Event: Insights from U[sbnd]Pb geochronology, C-O-Fe isotopes and REE compositions from the Jingshan Group, North China Craton

The Lomagundi-Jatuli Event (LJE) represents perhaps the largest and longest positive carbon isotope excursion of Earth history. However, the synchroneity, scale, and linkage of the LJE to Earth's early history of atmospheric oxygenation remain controversial. Strata of the Paleoproterozoic Jingshan Group of the North China Craton that preserve the isotopic record of the LJE excursion in marble layers and abundant graphite deposits provide an opportunity to elucidate the significance of the LJE. Carbon isotopic values (δ13CV-PDB) of the LJE based on analysis of 20 samples of the Lugezhuang Formation, lower Jingshan Group, range from −0.8 to +9.6 ‰ and display positive co-variance with stable oxygen isotope values (δ18OV-PDB). The positive carbon isotope excursion is constrained to 2140.6 ± 8.5 Ma (MSWD = 0.82, n = 25) based on magmatic zircon UPb geochronology of biotite granulite. Variation of facies-dependent carbon isotope values, the presence of graphite deposits of the Douya Formation, upper Jingshan Group, and the absence of a Ce anomaly in PAAS normalized REE patterns of marble samples suggest that the positive carbon isotope excursion is not linked to a marked increase of organic carbon burial and associated significant atmosphere oxygenation. Elevated concentrations of iron and PAAS-normalized middle REE enrichment of analyzed Jingshan Group marble samples point to anoxic and ferruginous oceanic conditions during accumulation of Jingshan Group carbonate. A positive Eu anomaly (average = 1.58), low La (average = 0.23), (Nd/Yb)N (average = 1.27), and Y/Ho (average = 36.6) anomalies, and negative iron isotope values (average δ56Fe = −0.12 ‰) are consistent with accumulation of Paleoproterozoic Jingshan Group carbonate in a restricted marine setting that was affected by high temperature hydrothermal fluids. Enrichment of the studied samples in heavy carbon isotope suggests elevated bio-productivity in the restricted, redox stratified marine setting in which Jingshan Group carbonate accumulated. Thus, it is likely that the positive stable carbon isotope excursion associated with Jingshan Group strata as well as other contemporaneous isotope excursions are local signals that are linked to a global perturbation of the carbon cycle.

Terrestrial response to the Early Cretaceous Weissert Event: Insights from carbon isotope records of organic matter and leaf wax n-alkanes in an inland East Asian lake

The Weissert Event (WE), a significant Early Cretaceous environmental disruption, is recognized globally in marine carbonate records for its positive carbon isotope excursion (CIE), organic matter (OM) enrichment, and widespread anoxia. However, records of its terrestrial impact are sparse. This study examines carbon isotope compositions (δ13C) from the lacustrine facies of North China's Dabeigou Formation (Yushuxia section, Luanping Basin). The synchronous positive CIEs of total organic carbon (TOC) and C27–C31n-alkanes within Members 2 to 3 of the Dabeigou Formation corroborate the Weissert Event, marking its first terrestrial observation in North China. Ratios of the isoprenoids pristane and phytane (Pr/Ph), Corg/P ratios and enrichment factors (EFs) of redox-sensitive trace metals (Mo and U) indicate transient anoxic conditions within a mainly oxic-suboxic setting from the late Valanginian to early Hauterivian, which were unfavorable for OM preservation. This highlights that although a global positive CIE occurred during the Weissert Event, anoxia and/or high TOC deposition in inland lakes were not inherent features.

Re-Os geochronology and geochemical evolution of late Cambrian to Middle Ordovician Alum and Tøyen shales, Sweden

The limited number of accurate and precise radiometric ages through the ∼100 Myr span of the Cambrian and Ordovician impedes reliable age determinations for stage boundaries in these periods. Here, we fill significant gaps in the early Paleozoic chronostratigraphy by providing precise Re-Os time-pins. Sample selection is linked to a firm biostratigraphic framework built on the appearance and distribution of trilobites, graptolites, and conodonts. A Furongian (upper Cambrian) Alum Shale section (Andrarum-3 drill core, Scania, Sweden) at the onset of the Steptoean Positive Carbon Isotopic Excursion (SPICE) yields highly non-isochronous Re-Os isotopic data from a section with wildly fluctuating δ13Corg; however, selected data from a narrow sediment band with steady carbon isotope stratigraphy provides an imprecise Re-Os age of 497 ± 28 Ma (2σ; Model 3; n = 3), with an initial 187Os/188Os ratio (Osi) of 0.74 ± 0.05. Organic-rich Alum Shale (Tomten-1 drill core, Västergötland, Sweden) from ∼120 cm below the Cambrian-Ordovician boundary yields a Model 1 age of 488.6 ± 5.1 Ma (2σ; MSWD = 1.5; n = 25) and an Osi of 0.82 ± 0.04 for Stage 10, uppermost Cambrian. Biostratigraphic data indicate the dated Alum Shale is from an interval slightly below the Top Of Cambrian Excursion (TOCE) and slightly above the First Appearance Datum (FAD) of the agnostoid Lotagnostus americanus. Organic-rich Tøyen Shale (Lerhamn drill core, Scania, Sweden) yields a precise Model 1 Re-Os age of 469.7 ± 1.4 Ma (2σ; MSWD = 1.0; n = 10) and Osi of 0.802 ± 0.002 for the maximum age of the Floian–Dapingian stage boundary (Lower–Middle Ordovician boundary). The Os isotopic composition of seawater from the latest Ediacaran through the Cambrian to Early-Middle Ordovician hovers around 0.8 but falls to 0.54 by early Silurian. This significant decrease in seawater 187Os/188Os is consistent with reduced chemical weathering and cooler seawater temperatures through the Middle–Late Ordovician. Overall, Redox Sensitive Element (RSE; Re, Os, Mo, U) abundances correlate positively with Total Organic Carbon (TOC), suggesting efficient removal of these elements from an anoxic water column by organic matter. However, these relationships break down for high TOC (>10%) shales depositing under euxinic conditions. The RSE-TOC relationship breakdown supports enhanced metal drawdown from the water column with local pyrite accumulation. Geochemical data suggest the deposition of Alum and Tøyen shales under hydrographically restricted settings with increased primary productivity along the Baltica's margin during the latest Cambrian to Early-Middle Ordovician.

The origin and fate of organic carbon in graphite–manganese bearing rocks and implications for the Lomagundi–Jatuli Event

Our study helps to unravel the complexity of the Lomagundi–Jatuli event, the largest and longest positive carbon isotope excursion ever recorded on the Earth’s surface, by providing a unique view of Paleoproterozoic graphitic rocks from the Borborema province of Northeastern Brazil. Through detailed mineralogical, textural, chemical and isotopic analyses, we bring a new perspective that provide support to elevated primary productivity and large-scale organic carbon burial during the Lomagundi–Jatuli event. Graphite crystals with distinctive textural features occur in association with silicate and oxidised manganese ores, manganese quartzites, garnetites, and gneisses. The graphites were crystallised at temperatures up to 634 °C, consistent with amphibolite facies metamorphism, according to Raman thermometry. An average total carbon content of 2.1 wt%, with δ13C values ranging from − 15.0 to − 21.5‰, is indicated by whole-rock geochemistry and carbon isotopic composition, respectively. Based on these results, our study proposes that these graphitic rocks may represent remnants of organic matter, possibly derived from bacterial biomass associated with manganese-rich sediments, preserved under reducing environmental conditions in a redox-stratified marine setting. Biological mediation on the origin of silicates is suggested by the close relationship between reduced manganese silicates and graphite. These constraints indicate that Paleoproterozoic graphite-rich rocks represent an important but overlooked reservoir of organic carbon that was partially degassed during the metamorphism of organic-rich sequences. Overall, this research provides new insights for the enigmatic emergence of the Lomagundi–Jatuli event, highlighting the intricate interplay among organic carbon, manganese-rich rocks and Earth's evolutionary processes during this period.

Stable carbon isotope chemostratigraphy of the base of the Callovian in Greenland

D’Orbigny named the Callovian stage after Kellaways in Wiltshire, UK, in the 1850 s. However, agreement on its boundaries and, more recently, on the position and location of the Global Boundary Stratotype Section and Point (GSSP) for the base of the Callovian has proven difficult for the last 170 years. This is mainly due to the lack of agreement on the appropriate index fossil and its regional and global correlations, as well as the location of the stratotype section. Stable carbon isotope chemostratigraphy and event stratigraphy are now essential tools for aiding in the definition of GSSPs. In this study, stable carbon isotopic analysis of 91 samples from east Greenland’s Middle Jurassic shallow marine sandstones of the Pelion Formation (Store Koldewey and Hold with Hope) and correlative and well-dated offshore siltstones and mudstones of the Fossilbjerget Formation (Jameson Land) is used to discriminate several isotopic events previously observed in other European basins and propose the early Callovian (δ13CTOC) positive carbon isotopic excursion as a secondary marker for defining the Callovian GSSP. The early Callovian (δ13CTOC) positive carbon isotopic excursion is a fundamental tool for superregional correlation between candidate GSSPs and Standard Auxiliary Boundary Stratotypes. It can also help establish a chronological order (synchronous vs diachronous) of species occurrences between different locations.

Carbon isotope stratigraphy and biostratigraphy, and associated organic carbon deposition during the Late Cenomanian to Early Turonian in the Tarfaya Basin, Morocco

The Cenomanian-Turonian (C/T) boundary event, also known as Oceanic Anoxic Event 2 (OAE2), is characterised by significant organic carbon preservation and a positive carbon isotope excursion that is recorded globally in sedimentary strata. In this study of the Tarfaya Basin, Morocco, three outcrop sections were logged - at Lagguing East, Tazra, and En-Naila, dated from Late Cenomanian to Early Turonian. New dating, based on ammonites and planktic foraminifera, combined with high-resolution δ13C isotope stratigraphy, yields a more precise delineation of the C/T boundary in the studied section. A total of 99 samples were collected for combined sedimentological, mineralogical, and geochemical analyses to allow a better understand of the palaeoenvironments of deposition and interpret controlling factors on organic enrichment. Five lithofacies have been identified, all indicative of a relative deep marine setting. Variations in lithofacies can be correlated with relative sea level changes, which align with the global eustatic curve.The OAE2 interval comprises black mudstones that signal anoxic conditions, leading to significant organic matter (OM) preservation, even though they appear weathered in the studied section. Notably, organic matter-rich mudstones were evident during the post-OAE2 Early Turonian maximum marine transgression. Trace element enrichment in these organic-rick mudstones indicates that organic matter accumulation was accompanied with increased sea surface productivity and associated with oxygen-depleted bottom water conditions, which facilitated organic matter preservation and enrichment. Upwelling has also been previously described as a process to deliver nutrients to the shelf, enhancing productivity. Some units with depleted organic content are the result of dilution from continental derived clastics. Others may originally have had high TOC, but were influenced by weathering or diagenetic process, changing from the typical black mudstones to yellow in colour. The identified carbon isotopic values and organic carbon enrichment in the C/T strata of the Tarfaya Basin reflect signals of global perturbations in the carbon cycles.

Identification of carbonates with high positive carbon isotope excursion from the Liaohe Group in the northeastern North China Craton and implications for the Lomagundi-Jatuli Event

Identification of carbonates with high positive carbon isotope excursion from the Liaohe Group in the northeastern North China Craton and implications for the Lomagundi-Jatuli Event

Palaeoenvironmental and palaeoclimatic study of the Olazagutia (Spain) and Ten Mile Creek-Arbor Park (USA) sections during the Coniacian–Santonian interval

The mechanisms and palaeoenvironmental conditions leading to and occurring through the Coniacian–Santonian Ocean Anoxic Event 3 (OAE3) are poorly known, particularly with regard to the marine phosphorus cycle and the climatic conditions in general. To explore these issues further, two famous locations have been studied, Olazagutia (Spain) and Ten Mile Creek–Arbor Park (TX, USA), which are located in different palaeogeographical areas and deposited at different palaeodepths. The study of these two locations is based on mineralogy (bulk and clay), geochemistry (stable isotopes, organic matter, phosphorus and major element) and high-resolution biostratigraphy (nannofossils) to infer changes in climate and primary productivity across OAE3. The investigated sections were both previously proposed as candidates for the base Santonian global boundary stratotype section and point (GSSP). The Olazagutia section was ratified in 2013, where the base of the Santonian was defined by the first appearance datum of the inoceramid Platyceramus undulatoplicatus . Both sections, deposited in oxygenated conditions, record the δ 13 C patterns which characterized the Coniacian–Santonian OAE3 interval, in particular the positive carbon-isotope excursions comprising the K2, Michel Dean and Bedwell events. New nannofossil biostratigraphy for both sections is presented including new occurrences. Significant diachronism of marker taxa is observed between the Spanish and Texan sections. Based on a weathering index and mineralogy, similar climate changes are observed in all sections. The climate shifted synchronously from relatively drier to warmer and wetter conditions above the Coniacian–Santonian boundary from the Michel Dean event to above the Bedwell event during the early Santonian. Fluctuations in total phosphorus contents appear mainly to have been driven by changes in detrital input and consequently by climate change in Spain and Texas. Several bentonite layers were observed in Texas close to the Coniacian–Santonian boundary, but only one located 7 m above the Coniacian–Santonian boundary provided sufficient well-preserved zircon minerals, and gives an age of 86.24 ± 0.12 Ma based on U–Pb geochronology, consistent with an early Santonian age.

Shallow ocean deoxygenation drove trilobite turnover during the late Cambrian SPICE event

The spread of marine anoxia is believed to have played a key role in the development of the SPICE (Steptoean Positive Carbon Isotope Excursion) event and the end-Marjuman extinction in the late Cambrian (∼497.5 m.y. ago), but their cause-and-effect relationship is poorly constrained. Here we present an integrated analysis of carbonate δ13C, cerium anomalies (Ce/Ce*), and genus-level diversity data of trilobites from the North China Platform. Our results show tightly coupled changes between the SPICE, an increase in Ce/Ce*, and a trilobite turnover event, which we interpret as indicating enhanced productivity and organic remineralization, leading to the development of low-oxygen conditions in shallow-water settings. This study therefore establishes a direct link between local ecological stress and trilobite turnover during the global SPICE event. Furthermore, the presence of low-oxygen rather than fully anoxic conditions during the peak of the SPICE event could explain the nature of the end-Marjuman crisis, which was characterized by the replacement of shallow-water fauna by deeper-water counterparts that were potentially more tolerant of hypoxia.

Phytoplankton and zooplankton paleocommunity change before and during the onset of the Lau Extinction Event (Ludlow, Silurian)

The Ludlow epoch of the Silurian period was a time of significant geobiological perturbations, the most significant being the mid-Ludfordian Lau Event and associated huge positive carbon isotopic excursion. On the other hand, the impact of the Lau Event on ecosystems is far from understood, with the majority of studies being concentrated on conodonts, graptolites, and brachiopods. Therefore, here we present the high-resolution Gorstian to mid-Ludfordian Baubliai-2 core section record of the phytoplankton (acritarch and green algae) change from the deep shelfal facies belt of the Silurian Baltic Sedimentary Basin. Seven distinct phytoplankton assemblages are distinguished. The general feature of the assemblage change is an overarching trend toward higher relative abundances of the green algae disaster taxon Leiosphaeridia spp., which peaked during the Lau Event. The absolute abundance variability of the various components of the plankton including the hyper-abundant Leiosphaeridia spp. and Tasmanites spp. revealed the presence of consistent 0.93 Ma cyclicity. The diversity of phytoplankton is directly correlated with graptolite species diversity, which suggests common driving mechanisms of evolution for different components of the plankton community. The recurrence and joint recurrence plots and recurrence quantification analysis revealed a coordinated evolution of phytoplankton and graptolite diversities. The Lau Event interval stood out as a distinct coordinated low diversity state in both phytoplankton and in graptolites. Similar anomalous community states were previously detected in benthic communities (brachiopods), and in communities of pelagic predators (conodonts). Therefore, the current evidence suggests that the Lau Event had a significant impact across the whole range of communities and ecosystems.

Paleoenvironmental variations during the Late Cambrian: Implications from Zn isotopes, I/(Ca+Mg) ratios, and other elemental proxies

The Upper Cambrian at the Martin Point section (∼110 m-thick) in western Newfoundland is a part of the Cow Head Group and partially spans the coeval Tuckers Cove and the Martin Point members of the Shallow Bay and Green Point formations, respectively. These units comprise alternating shale, minor conglomerate beds, and limestone rhythmites of a toe-of-slope apron. Earlier studies of the C-isotope profile, based on the lime mudstone interbeds, documented global negative δ13Ccarb shifts (from old to young: NL1, NL2, HERB) correlated with the lowermost Proconodontus posterocostatus, Proconodontus muelleri and base of Eoconodontus notchpeakensis conodont zones, respectively (post-Steptoean Positive Carbon Isotope Excursion NL1 and NL2 and Hellnmaria-Red Tops Boundary-HERB). Samples were extracted from the most preserved spots of micritic to near-micritic grain size (≤4 μm−10 μm). Fabric retention confirms petrographic preservation and insignificant correlations (R2 ≤ 0.1) of diagenetic proxies (e.g., Sr) with their environmental counterparts support the preservation of at least near-primary geochemical signatures. The paleoenvironmental proxies (TOC, Zn, P, Cu, Al/Ti, Mn, Fe, V, Mo) support dominant dysoxic conditions. The δ66Zn values (0.09–0.73 ‰ JMC Lyon) and (I/(Ca + Mg)) ratios (0.02–0.48 μmol/mol) vary consistently throughout the studied events. The minor decreases in δ66Zn values seem to denote inhibition of bioproductivity in a dysoxic water column, consistent with warm and humid climate during general transgressive settings and shoaling of organic-rich water into the shallow environment. The low I/(Ca + Mg) ratios (<0.5 μmol/mol) reinforce this interpretation, suggesting general dysoxic settings throughout the entire section.

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

The uppermost Cambrian succession (Stage 10) of the Wa'ergang section (∼300 m of rhythmites, South China) records three globally correlated negative δ13Ccarb excursions (N1, N2, and the HERB- Helenmaria – Red Tops Boundary), overlying the SPICE event (Steptoean Positive Carbon Isotope Excursion), indicating perturbations in the global carbon cycle. The trigger and process of these events still remain elusive. The petrographic and chemical screening of the investigated carbonates proves well preservation of sedimentary fabrics and at least near-primary geochemical signatures. The paleoredox proxy (UEF/TOC, VEF/TOC, and MoEF/TOC) and δ15Norg profiles exhibit positive excursions that correlate with the negative δ13Ccarb excursions, indicating pulses of expanded oceanic anoxia potentially resulted from anoxic deep-water upwelling. The correlated negative shifts in TOC and paleoproductivity proxy (NiEF and CuEF) profiles suggest suppressed primary productivity, possibly by upwelling-induced oceanic toxic events, aligning with mass extinctions particularly during the HERB event. Independently, the paired δ13Ccarb and δ13Corg (Δ13C) variations reveal decreased biological fractionation that aligns with the δ13Ccarb excursions, implying pulses of declining atmospheric O2 levels, which are consistent with the suggested expanded oceanic anoxia and suppressed primary productivity. The peaks of the negative δ13Ccarb excursions are correlated with the base of upward-shallowing sequences, consistent with the evidence from TOC, δ13Corg, δ15Norg, and trace element proxies, support a scenario of upwelling deep toxic waters during sea-level rise that likely resulted in expanded oceanic anoxia, and ultimately led to the negative excursions in δ13Ccarb values and mass extinction.

Enhanced primary productivity and perturbation of marine nitrogen cycling during the onset of the Cambrian SPICE event

New high-resolution paired carbonate carbon (δ13Ccarb) and nitrogen (δ15Nbulk) isotope records of the Steptoean Positive Carbon Isotope Excursion (SPICE) from the Laurentian epicontinental platform reveal a transient negative excursion in δ15Nbulk that closely aligns with the onset of the SPICE event and the initiation of the positive δ13Ccarb excursion. The negative excursion in δ15Nbulk identified here likely demonstrates an increase in microbially fixed nitrogen by diazotrophs, or cyanobacteria capable of N2 fixation, in the marine system due to a reduction in bioavailable recycled N through several processes at the onset of the biogeochemical event. Expansion of reducing environments during the initiation of the SPICE event would have liberated phosphorous previously bound to organic matter and/or marine iron oxy-hydroxides and simultaneously promoted an expansion of denitrification in oxygen-poor settings that would have reduced bioavailable N in the marine system. The enhanced delivery of phosphorous to the photic zone would have stimulated primary productivity, which would have also put additional pressure on the marine bioavailable N budget. These feedback systems would have combined to lower the marine N:P ratio such that diazotrophs would become a significant contributor to the total bioavailable N in the marine realm. The data presented here are the first demonstration of this significant perturbation to local marine nutrient cycling during the onset of the SPICE event and provide new insight into what may ultimately be a consistent set of biogeochemical triggers for major Paleozoic biogeochemical events.

The dynamic ocean redox evolution during the late Cambrian SPICE: Evidence from the I/Ca proxy

The late Cambrian Steptoean positive carbon isotope excursion (SPICE) is a distinct chemostratigraphic feature of the Paleozoic, marked by a 4–5‰ shift in carbonate δ13C that has been recognized across the globe during the Paibian Stage. The SPICE may be related to enhanced burial of organic matter and pyrite during the expansion of marine euxinia, which as a source of O2 also results in a pulse of atmospheric oxygen. However, geochemical proxies have not clearly illustrated how the ocean redox evolved with atmospheric oxygen changes during the SPICE. This study presents new carbonate I/Ca data, a redox proxy for the upper ocean, from three basins. I/Ca values are low at Great Basin and South China from early into the peak of SPICE, indicating generally anoxic conditions in shallow waters. The overall increasing trend in I/Ca through the peak and recovery phase of the SPICE roughly correlates with the previously modeled rise in atmospheric oxygen. Spatially, the Georgina Basin (Mt. Whelan) might have recorded a relatively more oxic upper ocean compared to the Great Basin and South China. Earth system model simulations also demonstrate the importance of paleogeographic and oceanographic settings on local redox conditions, highlighting the redox heterogeneity during the SPICE.

Devonian–Carboniferous transition in various facies of Northeast Laurussia (North Urals)

This study analyzed 14 stratigraphic sections exposing the Devonian–Carboniferous boundary interval in the North Urals, representing different facies from shallow-water carbonate platform to bathyal, in order to assess the possibility of recognizing regional levels corresponding to the base of the conodont zones S. sulcata and Protognathodus kockeli. The base of the S. sulcata Zone is regionally recognized by the first appearances of the conodonts Siphonodella sulcata (Huddle), S. semichatovae Kononova and Lipnjagov and Patrognathus crassus Kononova and Migdisova, roughly correlated with the first appearance of the foraminifer Tournayellina pseudobeata Reitlinger and Kulagina, and with the first appearances of the ostracods Cribroconcha primaris Kochetkova, Chamishaella grekoffi Tchizhova, Glyptolichvinella spiralis (Jones and Kirkby), Armilla uralica Sobolev, Calcarofera ex gr. media Schornikov, Compositocostata cumina Sobolev, Spinoalacia tschigovae Sobolev, Richterina latior Rabien. The base of the S. sulcata Zone occurs in the interval between two positive inorganic carbon isotope excursions. The base of the Protognathodus kockeli Zone is marked by the first appearance of Protognathodus kockeli (Bischoff) in a very narrow facies belt on the slope of the carbonate platform, recognized regionally by the onset of the last phase of the positive inorganic carbon isotope excursion, the beginning of the transgression after the terminal Famennian regression, the beginning of the unilocular foraminifera Bisphaera bloom and probably the appearance of the ostracods Monoceratina sp. 1. The level corresponding to the base of Pr. kockeli Zone is poorly supported biostratigraphically, but is characterized by clear isotope-stratigraphic and cyclostratigraphic data, although the eustatic signal may be obscured in places by local tectonics.

Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C37 alkenones expressed as UK'37 is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the UK'37 proxy for assessment of Cretaceous paleoclimates. Herein, we report a C40 alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C40 alkadienone (tetraconta-16E, 23E-dien-3-one; C40:2 Et) allowed calculation of the UK’40 unsaturation index, comparable to UK’37. Stratigraphic variations in the δ13C of C40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO2. The UK’40 profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO2 during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

Redox conditions across the Upper Cambrian of eastern Laurentia: Implications from geochemical proxies

The lower part of the Martin Point section (∼ 110 m - rhythmites) spans the uppermost Shallow Bay and lowermost Green Point formations (uppermost Franconian–lowermost Trempealeauan) of the Cow Head Group (western Newfoundland, Canada). It records two post-SPICE (Steptoean Positive Carbon Isotope Excursion) events marked by negative δ13Ccarb excursions (NL1 and NL2) that were also documented in South China. The petrographic and chemical screening of the investigated carbonate samples proves the preservation of at least near-primary geochemical signatures. The proxies of weathering (e.g., Mn, Fe, Al, and ∑REE), bioproductivity (e.g., δ13Corg, TOC, P, Ni, Zn, and Cu), and paleoredox (e.g., Cr, Th/U, Ce/Ce*) have been utilized to reconstruct the paleoenvironmental conditions during the post-SPICE events. The TOC, P, Ni, Cu, Cr, ∑REE, Mn, Fe, and U Ca-normalized profiles exhibit a positive pulse at the base of NL2 (upper negative δ13Ccarb excursion), followed by a drop. This might have resulted from shoaling organic-rich anoxic waters into an oxygenated shallow environment during transgression, which led to a negative shift in the δ13Ccarb and a decrease in TOC contents as well as an increase in the δ13Corg. The correlated enrichment in Zn and δ15Norg values and the Ce/Ce* around unity are consistent with the shoaling scenario. A similar scenario seems to be consistent with NL1 (lower negative δ13Ccarb excursion) although the lower resolution of correlated proxy variations makes the interpretations based on a few points and to be therefore taken with caution.

An overview of the Cretaceous oceanic anoxic events in Egypt, southern Tethys

Abstract The Cretaceous Period ( c. 143–66 Ma) attested to successive phases of regional tectonic activities and large-scale submarine volcanism resulting in increased atmospheric CO 2 concentrations, enhanced greenhouse climate and global warming, surface water bioproductivity and well-developed pore and bottom water anoxia, especially during the mid-Cretaceous (Aptian to Turonian). These conditions triggered widespread deposition of organic carbon (OC)-rich black shales during episodes of severe oxygen exhaustion, commonly known as oceanic anoxic events (OAEs). Oceanic anoxic events had many important consequences, including the widespread mass extinction of marine and terrestrial organisms. Several intervals of enhanced oxic ventilation and deposition of OC-poor pelagic–hemipelagic red carbonates and claystone interrupted Cretaceous OAEs, which are commonly known as Cretaceous oceanic red beds (CORBs). The mid-Cretaceous black shales from many parts of the world and the corresponding global carbon cycle changes indicated that OAE1 (Selli Level) and OAE2 (Bonarelli Event) are the most widespread and well-defined OAEs compared with the more regional OAE3 and Valanginian–Hauterivian black shales (Weissert Event). These appear to have been more restricted to the Atlantic and adjacent areas, a result of more regional conditions rather than being forced by global environmental perturbations. In the course of this review, the Egyptian territory in the southern Tethys shelf is taken as a case study area for the development, evolution and regional expression of mid-Cretaceous OAEs. The region is characterized by a thick Cretaceous sedimentary succession of OC-rich black shale and OC-poor carbonates and mudstone interbeds. The late Albian OAE1d was reported in the North Western Desert (upper Kharita Formation) based on a positive carbon isotope excursion and moderate total sulfur, total organic carbon and redox-sensitive elements. The end-Cenomanian OAE2 was reported in several regions of Egypt based on positive δ 13 C excursions. However, differential deposition between OC-rich and OC-poor facies took place, especially in marginal to shallow marine settings where black shales are absent, such as in the Eastern Desert and Sinai. During the Coniacian–Santonian, OC-poor limestone, calcareous shale and red claystone were deposited under enhanced water column respiration in Egypt. This indicates that enhanced oxygenated ocean circulation controlled organic matter decomposition during weak continental weathering and enhanced carbonate production, all of which led to the deposition of OC-poor CORBs in this region.

The upper Cambrian SPICE carbon isotope excursion from the Alborz Ranges, northeastern Iran

Seawater chemistry has been highly variable during Earth's history. Whereas variations in the ratio of organic relative to inorganic carbon burial in oceanic basins are considered to trigger positive shifts in global seawater δ13C values, those in δ18O are related to Earth's climate fluctuations. Positive shifts in the carbon isotope record known from Cambrian marine carbonates on a global scale have provided invaluable stratigraphic insights. Carbon and oxygen isotope data from Middle-Upper Cambrian shallow marine carbonates of the Mila Formation, eastern Alborz Ranges, Iran are presented herein. Accordingly, the Furongian SPICE (Steptoean Positive Carbon Isotope Excursion) event is documented from the Qol-Qol Member of the Mila Formation at two stratigraphic exposures where this prominent and globally recorded isotopic event is detected in micrites/skeletal limestone with a maximum δ13C value of +4‰ at both localities. The positive shift initiates at the base, reaches the climax in the middle, and then declines in upper part of the Qol-Qol Member. Therefore, the base of the latter denotes the base of Furongian Series/Paibian Stage in the Alborz Ranges. The event occurred in the Palaeadotes–Torifera and Prochuangia trilobite Zones but detailed biostratigraphic studies are yet to be undertaken. The occurrence of glauconite and abundant benthic fauna in some intervals within the rock unit can be interpreted as recurring slightly-reduced and oxygenated sea floor circumstances during deposition of the Qol-Qol Member of the Mila Formation.