Steptoean Positive Carbon Isotope Excursion Research Articles

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.

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.

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.

No evidence for a volcanic trigger for late Cambrian carbon-cycle perturbations

Abstract The early Paleozoic was marked by several carbon-cycle perturbations and associated carbon-isotope excursions (CIEs). Whether these CIEs are connected to significant (external) triggers, as is commonly considered to be the case for CIEs in the Mesozoic and Cenozoic, or result from small carbon-cycle imbalances that became amplified through lack of efficient silicate weathering or other feedbacks remains unclear. We present concentration and isotope data for sedimentary mercury (Hg) and osmium (Os) to assess the impact of subaerial and submarine volcanism and weathering during the late Cambrian and early Ordovician. Data from the Alum Shale Formation (Sweden) cover the Steptoean positive carbon-isotope excursion (SPICE; ca. 497–494 Ma), a period marked by marine anoxia and biotic overturning, and several smaller CIEs extending into the early Ordovician. Our Hg and Os data offer no strong evidence that the CIEs present in our record were driven by (globally) elevated volcanism or continental weathering. Organic-carbon and Hg concentrations covary cyclically, providing further evidence of an unperturbed Hg cycle. Mesozoic and Cenozoic CIEs are commonly linked to enhanced volcanic activity and weathering, but similar late Cambrian–early Ordovician events cannot easily be connected to such external triggers. Our results are more consistent with reduced early Paleozoic carbon-cycle resilience that allowed small imbalances to develop into large CIEs.

Seawater carbon and strontium isotope variations in the eastern Tarim Basin during the Cambrian: Implications for connection with the open ocean

Cambrian was a critical period in Earth's history, marked by rapid diversification of early animals and significant changes in oceanic conditions. Marine sediments, particularly carbonates, have been used to study variations in the global carbon cycle, seawater redox, weathering input, etc. In search of new archives of Cambrian paleo-seawater chemistry, we conducted an integrated study of Cambrian C and Sr isotope stratigraphy using the carbonates of the Xingdi section in the Kruqtagh region of the eastern Tarim Basin. Based on reported paleontological data, the C isotope record is comparable to the Cambrian global seawater C isotope variation, providing a feasible stratigraphic framework for the Cambrian strata in the eastern Tarim Basin. A sharp δ13C increase at the top of the Xidasan Formation probably corresponds to the recovery of the Redlichiid–Olenellid Extinction C isotope Excursion. Additionally, a negative C isotope excursion in the lower part of the Mohuosan Formation and a positive C isotope excursion at the bottom of the Tuersaketagh Formation likely correspond to DrumIan Carbon isotope Excursion and Steptoean Positive Carbon Isotope Excursion, respectively. However, the Sr isotope curve in the eastern Tarim Basin decoupled from global seawater Sr isotope variations during Cambrian Series 2–3 until Furongian, indicating that the eastern Tarim Basin was not well connected with the open ocean until Furongian. It also highlights a significant impact of paleogeographical connections of basins with the open ocean on their seawater Sr isotopic signal.

Environmental and trilobite diversity changes during the middle-late Cambrian SPICE event

The Steptoean Positive Carbon Isotope Excursion (SPICE) event at ca. 497−494 Ma was a major carbon-cycle perturbation of the late Cambrian that coincided with rapid diversity changes among trilobites. Several scenarios (e.g., climatic/oceanic cooling and seawater anoxia) have been proposed to account for an extinction of trilobites at the onset of SPICE, but the exact mechanism remains unclear. Here, we present a chemostratigraphic study of carbonate carbon and carbonate-associated sulfate sulfur isotopes (δ13Ccarb and δ34SCAS) and elemental redox proxies (UEF, MoEF, and Corg/P), augmented by secular trilobite diversity data, from both upper slope (Wangcun) and lower slope (Duibian) successions from the Jiangnan Slope, South China, spanning the Drumian to lower Jiangshanian. Redox data indicate locally/regionally well-oxygenated conditions throughout the SPICE event in both study sections except for low-oxygen (hypoxic) conditions within the rising limb of the SPICE (early-middle Paibian) at Duibian. As in coeval sections globally, the reported δ13Ccarb and δ34SCAS profiles exhibit first-order coupling throughout the SPICE event, reflecting co-burial of organic matter and pyrite controlled by globally integrated marine productivity, organic preservation rates, and shelf hypoxia. Increasing δ34SCAS in the “Early SPICE” interval (late Guzhangian) suggests that significant environmental change (e.g., global-oceanic hypoxia) was under way before the global carbon cycle was markedly affected. Assessment of trilobite range data within a high-resolution biostratigraphic framework for the middle-late Cambrian facilitated re-evaluation of the relationship of the SPICE to contemporaneous biodiversity changes. Trilobite diversity in South China declined during the Early SPICE (corresponding to the End-Marjuman Biomere Extinction, or EMBE, of Laurentia) and at the termination of the SPICE (corresponding to the End-Steptoean Biomere Extinction, or ESBE, of Laurentia), consistent with biotic patterns from other cratons. We infer that oxygen minimum zone and/or shelf hypoxia expanded as a result of locally enhanced productivity due to intensified upwelling following climatic cooling, and that expanded hypoxia played a major role in the EMBE at the onset of SPICE. During the SPICE event, global-ocean ventilation promoted marine biotic recovery, but termination of SPICE-related cooling in the late Paibian may have reduced global-ocean circulation, triggering further redox changes that precipitated the ESBE. Major changes in both marine environmental conditions and trilobite diversity during the late Guzhangian demonstrate that the SPICE event began earlier than the Guzhangian-Paibian boundary, as previously proposed.

A 20-million-year reconstruction to decipher the enigmatic Cambrian extinction – Ordovician biodiversification transition

The late Cambrian extinction – Early Ordovician biodiversification represents one of the crucial lower Paleozoic biological changes. However, the mechanisms responsible for this transition remain poorly understood. Here, we reconstructed the paleoenvironmental changes based on a model that integrated the atmospheric-oceanic-biological inputs and provided the first detailed assessment of the Cambro-Ordovician biological turnover. The results show depositional environments evolved into extremely sulfidic conditions with lower nutrient inputs and more restricted water circulation from the Miaolingian to early Furongian, leading to the Steptoean Positive Carbon Isotope Excursion event. The intense volcanic activity in the early Jiangshanian appears to be responsible for the recurrent bio-calamity. Later in the mid-late Furongian (mid-Jiangshanian to Stage 10), enhanced terrestrial weathering contributed to the Earth's cooling and higher inputs of terrestrial nutrients, beneficial to the subsequent biological recovery. In the Early Ordovician and despite reduced terrestrial nutrient input, massive oceanic water upwelling alleviated sulfidic conditions and brought nutrients, laying the foundation for the Great Ordovician Biodiversification Event.

Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA

Abstract We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The forma tion consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meterscale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.

A pilot study of upper Yangtze shallow-water carbonates of the Paibian global marine euxinia: Implications for the late Cambrian SPICE event

The SPICE (Steptoean Positive Carbon Isotope Excursion) is a significant event in the Late Cambrian, recording global carbon cycle perturbations. Its driving mechanism is still controversial, but mainly believed to be related to an increase of organic carbon burial caused by ocean anoxia. The shallow-water platform carbonates of the Upper Cambrian Loushanguan Formation of Yankong outcrop (Upper Yangtze) spans the event interval. Their primary geochemical proxy signatures (trace elements, carbon and strontium isotopes), have been utilized to better understand the driving mechanism of the SPICE event. The variations of paleoenvironmental proxies in the upper Loushanguan Formation exhibit a rapid transformation of redox conditions similar to those of deep-water regions, thus reflecting an expansion of anoxic seawater to shallow-water column. The increase of 87Sr/86Sr, Al, and ∑REE during the SPICE suggests an enhancement in nutrient inputs from continental weathering that might have promoted primary productivity, which preferentially consumed light 12CO2 of surface waters, resulting in 13C-enriched DIC. The occurrence of PGMEE (Paibian global marine euxinia event), immediately before the enhancement of continental weathering, suggests that it was the onset-driving mechanism of the SPICE event and the increase in continental weathering might have promoted primary productivity to further intensify carbon burial and play a superimposed effect on that of the SPICE event.

Dynamic oceanic redox conditions across the late Cambrian SPICE event constrained by molybdenum and uranium isotopes

The Steptoean Positive Carbon Isotope Excursion Event (SPICE) represents one of the largest carbon cycle perturbations in the Cambrian, which coincided with climatic changes and dramatic extinction of shallow-shelf faunas. This carbon perturbation has been linked to a global expansion of marine euxinia (SPICE-OAE), but the precise timing, duration and extent of changing redox conditions across the SPICE event, as well as its influences on the coeval biotic evolution, remain unclear. Here, we report paired δ98Mo–δ238U data across this event, constrained by a radioisotopically anchored astrochronology, based on Alum Shale samples from the Albjära-1 and Billegrav-2 drill-cores in Scandinavia. The δ98Mo profiles are largely invariant around 1.08±0.21‰ with short-term fluctuations parallelling changes in pyritization of iron, possibly as a result of locally varying [H2S]aq in the water column and/or fluctuating reactive iron delivery. The bottom water is interpreted to have been primarily anoxic, but with frequent local variations between oxic and sulfidic conditions in a moderately restricted basin. A novel analytic method based on variation of Mo concentrations at millimeter scale stratigraphic resolution was applied to distinguish between local and global redox signatures, revealing that cyclic redox changes are recorded in the Alum Shale sea with periods of ∼0.3 Myr, ∼0.9 Myr and ∼0.15 Myr before, during and after the SPICE-OAE, respectively. Stratigraphic profiles of Mo, U, Mo/TOC ratios and δ238U in the two cores show comparable trends, facilitating recognition of three chemically distinct stages across the SPICE event: 1) global expansion of marine euxinia (SPICE-OAE; 497.9–496.6 Ma), 2) global contraction of euxinia accompanied by enhanced carbonate weathering inputs (496.6–495.8 Ma) and 3) general global oxygenation (495.8–494.4 Ma). The first in situ correlation between the onset of the SPICE-OAE and the end-Miaolingian extinctions in Baltica corroborates that expansive anoxic and sulfidic water masses likely contributed to the biotic crisis. The time gap between the start of the extinction and the appearance of the new fauna in Baltoscandia is estimated to be ∼600 kyr.

Molecular record of the SPICE event in well YD2, Tarim Basin: Insights from catalytic hydropyrolysis of kerogens

The Steptoean Positive Carbon Isotope Excursion (SPICE) event is extensively documented in globally distributed carbonate and organic-rich successions of slope and platform environments during the late Cambrian (ca. 497–494 Ma). Despite numerous studies, its regional manifestation remains controversial because of the major uncertainty of the redox conditions in sedimentary paleoenvironments and its effect on the changes in the biota. Biogeochemical processes that occurred during the SPICE event are important for assessing whether the excursion is associated with the co-evolution of organisms and environments. In this study, we investigated the changes in plankton and paleoenvironment at the molecular level by combining kerogen elemental analysis, catalytic hydropyrolysis (HyPy), and gas chromatography-mass spectrometry (GC-MS) analyses of saturates and aromatics with published carbon isotopic data. The three fluorene series, gammacerane/C30hopane (G/C30H) and pristine/phytane (Pr/Ph) ratios demonstrated that the shallow shelf seawater changed significantly from relatively reducing to oxidizing and back to reducing conditions during the SPICE interval coupled with a regression-transgression cycle. The phytoplankton during the rise of the SPICE were mainly organic-walled, such as cyanobacteria, various acritarchs, and algae, as evidenced by terpanes, hopanes, and steranes. Significant decreases in phytoplankton diversity and abundance coincided with the peak of the SPICE, corresponding to the most oxidized and hypersaline periods during the SPICE event in the eastern Tarim Basin. Plankton assemblages diversified significantly with increasing living space due to the rise of sea-level during the post-SPICE period. Therefore, this study supports the idea that plankton groups changed dramatically with the environment, and its revolution followed the SPICE into the Great Ordovician Biodiversification Event.

Zinc isotopic evidence for enhanced continental weathering and organic carbon burial during the late Cambrian SPICE event

The globally recognized late Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE; ca. 497–494 Ma) is one of the most prominent carbon isotope excursions during the Phanerozoic eon, yet its primary cause remains debatable. To provide new insights into this issue, here we investigate zinc stable isotopes (expressed as δ66Zn) on two coeval carbonate successions spanning the Miaolingian-Furongian transition in North China. Similar stratigraphic δ66Zn trends are observed in the two sections, indicating a widespread perturbation to the marine Zn cycling. An abrupt decline of δ66Zn values by up to 0.4‰ and a concomitant increase of Zn/Ca ratios are observed at the pre-SPICE to the early SPICE interval, which can be best explained by the enhanced influx of isotopically light Zn from continental weathering. Enhanced continental weathering increased the inputs of terrestrial-sourced nutrients to the oceans, promoting the primary productivity and burial fluxes of organically bound Zn. At the middle stage of the SPICE, the coupled positive shifts of δ66Zn and δ13Ccarb indicate a phase of marine Zn cycling fluctuations driven by massive organic matter burial. Enhanced sequestration of organic carbon caused a rise in atmospheric oxygen levels, favoring the remobilization of previously buried Zn at shallower marine settings. With the development of muted continental weathering during the falling limb of the SPICE, the return of δ66Zn to initial background values marks the reestablishment of the stable seawater chemistry condition. Our results constrain the important roles of enhanced continental weathering and organic matter burial in micronutrient fluctuations during the SPICE, which were most likely the primary trigger for the SPICE.

Stratigraphic and volcanic signatures of Miaolingian-Late Ordovician rift pulses in the Alborz Mountains, northern Iran

• The Cambrian-Ordovician succession of the eastern Alborz Mountain Range displays uplift episodes associated with erosion. • The SPICE chemostratigraphic shift marks the Miaolingian–Furongian boundary. • Alkaline to subalkaline magmatic activities are separated by quiescent intervals of up to 35 m.y. • Intraplate (continental rift) volcanism reflects OIB-type mantle sources . • Plume-related rift pulses are linked to a large igneous province about 600 km across. Recent reconnaissance geological mapping, identification of unconformities and chronostratigraphic dating of Miaolingian to Ordovician strata in the eastern Alborz Mountains have led to recognition of synsedimentary uplift episodes associated with block tilting and partial erosion of the Cambrian Mila and Ordovician Simeh Kuh, Qumes and Lashkarak formations. A chronostratigraphic revision of the Miaolingian-Late Ordovician fossil content is presented, and the Steptoean Positive Carbon Isotope Excursion (SPICE) chemostratigraphic shift, marking the Miaolingian–Furongian boundary, is constrained close to the Members 2/3 contact of the Mila Formation. Upper Ordovician basaltic lava flows embedded in the Abarsaj Formation and Devonian sills and dykes intruded in the Mila and Simeh-Kuh formations were geochemically analysed and compared with the voluminous lava eruptions recognized in the Katian-Llandovery Soltan-Maidan and Gorgan Schists formations, and in the ?Emsian-Eifelian Mighan Formation. They represent successive magmatic pulses of alkaline to subalkaline activities, separated by background (quiescent) intervals of up to 35 m.y., characterizing an intraplate field (continental rift) affinity compatible with OIB-like (ΔNb greater than 0) mantle sources. Successive Miaolingian-Mid Ordovician (volcanic-free) uplift episodes were related to lithospheric doming preceding the onset of stepwise plume-related rift pulses, which shaped a large igneous province with transects of about 600 km, linking the Damghan and Alestan domains of the eastern Alborz Mountains with South Kopet-Dagh and some blocks of Central Iran.

Synchronous positive δ13Ccarb and δ13Corg excursions during 497–494 Ma: From a CO2 concentrating mechanism dominated photosynthesis?

Steptoean Positive Carbon Isotope Excursion (SPICE) event is featured particularly as the synchronous positive δ13Ccarb and δ13Corg excursions, which are recorded worldwide in the Late Cambrian (ca. 497–494 Ma). However, the cause of this event remains elusive. To better understand the underlying mechanism of the contemporaneous positive isotope excursions, here we present a series of geochemical data from two drilling wells of the Tarim Basin, China and some relevant data reported from worldwide sections. CO2 concentrating mechanisms (CCMs, e.g., utilized by cyanobacteria) developed in response to decrease in pCO2 and increase in pO2 during the SPICE event, in which the HCO3− enriched in 13C but depleted in 2H is utilized for photosynthesis, is proposed to result in the synchronous positive excursions of δ13Ccarb and δ13Corg coupled by a negative shift of δDorg as exemplified by the samples from the Tarim Basin. The SPICE event was followed by an increase in plankton, which could relate to triggering CCMs and maintaining the biodiversity of marine organisms during the event.

Linkage of the late Cambrian microbe-metazoan transition (MMT) to shallow-marine oxygenation during the SPICE event

Microbe-metazoan transitions (MMTs), representing a switch from microbe-mediated to metazoan-mediated carbonate production, have been linked to major changes in Earth-surface conditions. The ‘late Cambrian MMT’ (nomen novum), during which microbial reefs were replaced by maceriate and lithistid sponge reefs, coincided with a sharp rise in atmospheric O2 levels attributed to the Steptoean Positive Carbon Isotope Excursion (SPICE) at ~497–494 Ma. However, relationships between atmospheric oxygenation, marine redox conditions, and the MMT have not been thoroughly investigated to date. Here, we conducted paired analyses of carbonate carbon isotopes (δ13Ccarb), sulfur isotopes of carbonate-associated sulfate (δ34SCAS) and pyrite (δ34SPy), pyrite morphologies, and fossil assemblages of two upper Cambrian shallow-marine sections on the North China Platform (Tangwangzhai and Huangyangshan). The sulfur isotopic composition of framboidal pyrite (δ34SFrPy) was calculated from that of total pyrite (δ34STPy) assuming a varied fractionation with euhedral pyrite based on pyrite morphological analyses. Modelling of S-isotope fractionations (Δ34SCAS-FrPy) indicates that contemporaneous seawater had low sulfate concentrations (i.e., [SO42––]sw ~4–6 mM) with locally high spatial heterogeneity during the Rising SPICE (early to middle Paibian), which were the product of massive burial of pyrite (together with organic matter) linked to extensive oceanic anoxia. Immediately following the SPICE, a reduced pyrite burial rate and an increased flux of terrestrially sourced, 34S-depleted sulfate to shallow-marine environments resulted in lower δ34SCAS values. [SO42–]sw rose to a maximum of ~9–16 mM and became more uniform during the Post-SPICE (middle to late Jiangshanian), suggesting a more oxidized condition of seawater and the atmosphere. Rising O2 levels in both the atmosphere and marine environments triggered the late Cambrian MMT and set the stage for the subsequent Great Ordovician Biodiversity Event.

Synchronizing rock clocks in the late Cambrian

The Cambrian is the most poorly dated period of the past 541 million years. This hampers analysis of profound environmental and biological changes that took place during this period. Astronomically forced climate cycles recognized in sediments and anchored to radioisotopic ages provide a powerful geochronometer that has fundamentally refined Mesozoic–Cenozoic time scales but not yet the Palaeozoic. Here we report a continuous astronomical signal detected as geochemical variations (1 mm resolution) in the late Cambrian Alum Shale Formation that is used to establish a 16-Myr-long astronomical time scale, anchored by radioisotopic dates. The resulting time scale is biostratigraphically well-constrained, allowing correlation of the late Cambrian global stage boundaries with the 405-kyr astrochronological framework. This enables a first assessment, in numerical time, of the evolution of major biotic and abiotic changes, including the end-Marjuman extinctions and the Steptoean Positive Carbon Isotope Excursion, that characterized the late Cambrian Earth.

High mercury enrichments in sediments from the Baltic continent across the late Cambrian: Controls and implications

The late Cambrian witnessed significant biological and environmental changes. However, the main triggers responsible for that formidable turnover remain largely unknown. This study investigates mercury (Hg) enrichments from the late Miaolingian to Furongian section in a unique Alum Shale drilled core obtained from southeast Sweden. Our results show that the background level of Hg is relatively high because of the presence of reducing conditions and continuous Hg supply from water mass. However, our most spectacular findings are the presence of four Hg anomalies. The most pronounced Hg anomalies are characterized by the high values in total Hg (~900 ppb) and Hg/TOC ratio (~80), as well as specific signatures of mass independent fractionation of Hg (MIFs; Δ199Hg, Δ201Hg, and Δ200Hg) that we attribute to volcanic-derived Hg. This Hg loading event occurred immediately after the globally recognized Steptoean Positive Carbon Isotope Excursion (SPICE) event and could be attributed to volcanic activities during that period.

Re-Os geochronology for the Cambrian SPICE event: Insights into euxinia and enhanced continental weathering from radiogenic isotopes

AbstractThe late Cambrian Steptoean positive carbon isotope excursion (SPICE) represents a major perturbation to the global carbon cycle and was associated with trilobite extinctions and expansion of anoxic and/or euxinic water masses during episodes of eustatic sea-level change. We present a new Re-Os age together with Os and Nd isotope stratigraphy and major- and trace-element data from the Alum Shale Formation (Scania, Sweden). The Re-Os age of 494.6 ± 2.9 Ma is from the interval of peak δ13Corg values, providing the first radiometric age constraint for this Cambrian carbon isotope excursion, interpreted as a possible pre-Mesozoic ocean anoxia event, and the timing of biomere-level extinctions. The Os isotope chemostratigraphic profile can be explained by an increase in terrigenous weathering prior to the SPICE, potentially driven by sea-level fall, and in agreement with enhanced nutrient supply, primary productivity, and organic matter burial as the driver of the SPICE event. Post-SPICE, the Os isotopes become increasingly unradiogenic; however, invariant εNd(t) values argue against a change in provenance and instead support a decrease in the continental weathering flux, possibly related to eustatic sea-level rise.