Global Carbon Isotope Excursion Research Articles

Multiple diachronous “Black Seas” mimic global ocean anoxia during the latest Devonian

Abstract The Devonian-Carboniferous boundary (DCB) Hangenberg extinction in Europe (ca. 359 Ma) and interval of black shale deposition has been considered a global anoxic event, the cause of which remains debated. However, the lack of a prominent global carbon isotope excursion and the duration of anoxia (3–4 m.y. at least) is inconsistent with other global anoxic events (e.g., Mesozoic ocean anoxic events) throughout the Phanerozoic. We examined geochemical and biostratigraphic data along with radiometric dating from purported Hangenberg event sections across the DCB in western Canada and demonstrate that the onset of anoxic pulses in Laurentian seas started before 363 Ma, diachronous with those in Baltica (Europe). Thus, DCB black shales do not record a single global anoxic event. We suggest instead that black shales near the DCB record multiple, but diachronous, Black Sea–like basins around the globe, promoted by the Late Devonian tectonic configuration. This drove a gradual biotic crisis in the form of regional punctuated benthic faunal extinction events.

U–Pb Geochronology and Stable Isotope Geochemistry of Terrestrial Carbonates, Lower Cretaceous Cedar Mountain Formation, Utah: Implications for Synchronicity of Terrestrial and Marine Carbon Isotope Excursions

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

Discovery of SPICE and carbon isotope stratigraphic correlation of the Cambrian Furongian Series in Tarim Craton, NW China

The Furongian Series of the Cambrian in the Tarim Craton (NW China) is mainly composed of crystalline dolostones, and is an important target for oil and gas exploration. The chronostratigraphic framework of the Furongian Series in the Tarim Craton has not yet been established due to the scarcity of fossils and the absence of the Steptoean Positive Carbon Isotope Excursion (SPICE), which is the primary global carbon isotope excursion of the Furongian Series. In this study, the SPICE was discovered by examining the carbon isotopes of the representative drilling wells in the West Platform and East Basin. A positive carbon isotope anomaly with δ13C values ranging from 1.5‰ to 1.9‰ (PDB), were found in the middle of the Xiaqiulitag Formation of the three drilling wells in the West Platform, and a positive excursion with δ13C values up to 4.0‰ (PDB), which is correlated with the SPICE, was found in the lower limestone of the Tuershaketag Formation in the East Basin. Based on the carbon isotopic data and chemostratigraphic correlation, the chronostratigraphic framework of the Furongian Series across different facies in the Tarim Craton is preliminarily established. The basal boundary of the Furongian Series was preliminarily defined by the occurrence of the SPICE. A weak positive anomaly of δ13C was observed at the base of the Penglaiba Formation of the three boreholes in the West Platform and the top of the Tuershaketag Formation of Tadong-2 well. We propose that the positive anomaly of the carbon isotope above the weak negative anomaly at the base of the Penglaiba Formation and the top of the Tuershaketag Formation can be used to define the Cambrian-Ordovician boundary in the Tarim Craton.

Archaeal lipid biomarker constraints on the Paleocene-Eocene carbon isotope excursion

A negative carbon isotope excursion recorded in terrestrial and marine archives reflects massive carbon emissions into the exogenic carbon reservoir during the Paleocene-Eocene Thermal Maximum. Yet, discrepancies in carbon isotope excursion estimates from different sample types lead to substantial uncertainties in the source, scale, and timing of carbon emissions. Here we show that membrane lipids of marine planktonic archaea reliably record both the carbon isotope excursion and surface ocean warming during the Paleocene-Eocene Thermal Maximum. Novel records of the isotopic composition of crenarchaeol constrain the global carbon isotope excursion magnitude to −4.0 ± 0.4‰, consistent with emission of >3000 Pg C from methane hydrate dissociation or >4400 Pg C for scenarios involving emissions from geothermal heating or oxidation of sedimentary organic matter. A pre-onset excursion in the isotopic composition of crenarchaeol and ocean temperature highlights the susceptibility of the late Paleocene carbon cycle to perturbations and suggests that climate instability preceded the Paleocene-Eocene Thermal Maximum.

Chemostratigraphy of Late Sinemurian – Early Pliensbachian shallow-to deep-water deposits of the Central High Atlas Basin: Paleoenvironmental implications

Shallow water carbonate rocks are especially prone to diagenetic alteration. As such they are sometimes problematic archives of past carbon cycle perturbations, casting doubt on the reliability of shallow-water carbonates carbon isotopes analyses for chemostratigraphic purposes. In this paper, bulk organic carbon isotopes (δ13Corg) is used as a robust replacement of the more sensitive carbonate carbon isotope systems for the establishment of a refined chronostatigraphic framework for the evolution of Sinemurian-Pliensbachian neritic carbonate systems in the Central High Atlas Basin of Morocco. These data show very similar patterns and characteristics in the organic carbon isotope curves of basinal and platform sections and thus illustrate the reliability of these archives. Simultaneously, we explore the expression of the global carbon isotope excursion at the Sinemurian-Pliensbachian boundary (SP event) in the study area in Morocco. This event has been previously described as a negative carbon isotope excursion of ∼2‰ associated with a large transgression in numerous basins. In Morocco and other neritic carbonate records, the SP event is better described as a return to previous values after a positive carbon isotope excursion coinciding with a major latest Sinemurian regression recognised on a global scale. Similar patterns are also found in other sections, although sometimes poorly expressed. However, it shows, that the SP event has ambiguous characteristics and questions the conventional hypothesis of it being linked to the massive injection of 13C-depleted carbon into the atmosphere-ocean system.

Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation

A hallmark of the rapid and massive release of carbon during the Palaeocene–Eocene Thermal Maximum is the global negative carbon isotope excursion. The delayed recovery of the carbon isotope excursion, however, indicates that CO2 inputs continued well after the initial rapid onset, although there is no consensus about the source of this secondary carbon. Here we suggest this secondary input might have derived partly from the oxidation of remobilized sedimentary fossil carbon. We measured the biomarker indicators of thermal maturation in shelf records from the US Mid-Atlantic coast, constructed biomarker mixing models to constrain the amount of fossil carbon in US Mid-Atlantic and Tanzania coastal records, estimated the fossil carbon accumulation rate in coastal sediments and determined the range of global CO2 release from fossil carbon reservoirs. This work provides evidence for an order of magnitude increase in fossil carbon delivery to the oceans that began ~10–20 kyr after the event onset and demonstrates that the oxidation of remobilized fossil carbon released between 102 and 104 PgC as CO2 during the body of the Palaeocene–Eocene Thermal Maximum. The estimated mass is sufficient to have sustained the elevated atmospheric CO2 levels required by the prolonged global carbon isotope excursion. Even after considering uncertainties in the sedimentation rates, these results indicate that the enhanced erosion, mobilization and oxidation of ancient sedimentary carbon contributed to the delayed recovery of the climate system for many thousands of years. Delivery of fossil carbon to the oceans strongly increased about 15 kyr after the onset of the Palaeocene–Eocene Thermal Maximum as a result of oxidation of sedimentary carbon, suggests an analysis of geochemical measurements with a biomarker mixing model.

Constraining the depositional age of an Upper Cretaceous non-marine and shallow marine siliciclastic succession, Kuji Group, northeastern Japan, based on carbon isotope stratigraphy and U−Pb radiometric dating

Determining detailed geologic ages for pre-Quaternary non-marine and shallow marine siliciclastic successions has generally been difficult due to the rare occurrence of age-diagnostic fossils. In this study, we present the first record of stable carbon isotope stratigraphy derived from detrital woods with U–Pb radiometric dating for felsic tuffs in the Upper Cretaceous Kuji Group, northeast Japan, for which the geologic age has been poorly constrained to date. The radiometric ages for each tuff, obtained by laser ablation–inductively coupled plasma mass spectrometry (LA–ICPMS) analyses, range from 92.6 ± 0.6 to 79.1 ± 1.2 Ma (95% confidence interval). The composite δ13Cwood curve in the Kuji Group is correlated to the Upper Cretaceous reference sections in the Northwest Pacific, North America and Northwest Europe. Based on our correlations, several global carbon isotope excursion events in the late Turonian–early Campanian interval were recognized, and the Upper Cretaceous stage boundaries in the Kuji Group were partially identified, which agree well with previous biostratigraphic assumptions. Further, the depositional age of the dinosaur-bearing bone bed in the Kuji Group was estimated to be restricted to the early–middle Santonian. Our detailed chronostratigraphic framework provides significant implications for sequence stratigraphy, regional tectonic reconstruction and further exploration into the terrestrial ecosystem of the Late Cretaceous East Asian continental margin.

An expanded lower Eocene shelf sequence from the eastern Aquitaine Basin, SW France: biostratigraphy, biofacies, and stable carbon and oxygen isotopes

The early Eocene is characterized by a succession of orbitally-controlled global stable carbon isotope excursions, with some being linked to climatic and related biotic perturbations. The impact of these isotopic excursions has been primarily studied in deep-sea sections under comparably stable conditions. In order to investigate the impact of global post-PETM isotopic signals on shallow marine settings, the Ypresian neritic ʻBlue Marls’ of the Corbieres (SW France) were investigated. High-resolution records of microfossil biota and stable carbon and oxygen isotopes pinpoint biostratigraphic, paleoecologic and geochemic constraints. Calcareous nannofossil biostratigraphy positions the sequence in the upper part of zone NP11, possibly ranging into basal NP12, which is conformable with larger benthic foraminifera data indicating shallow benthic zone SBZ8. This implies a time span of about 0.4 Myr and high overall sedimentation rates of about ~ 32 cm/kyr for the section. A shallowing upward trend from outer neritic to coastal settings is observed in the development of the lithostratigraphy and the microfossil assemblage. The assemblages can be subdivided in seven larger biofacies and four ostracod assemblage zones. The lower third of the section is characterized by strongly fluctuating and partly high plankton/benthos-ratios for neritic settings. A final pronounced peak in plankton occurrence is associated with strong decrease of benthic biota, suggesting anoxic conditions in the outer neritic environment. Several local negative δ13C- and δ18O-excursions can be identified in the section. The upper, most pronounced and consistent negative δ13C excursion is tentatively linked to global carbon isotope excursion K (ETM3) based on the biostratigraphic constraints.

Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges, South China

The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that δ15NTN values were high (~+6‰) until middle Ediacaran, gradually dropping down to −1‰ at the earliest Cambrian, then rising back to +4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the δ15NTN and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high δ15N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the 15N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both δ13Ccarb and δ15NTN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO2 with recycled organic nitrogen. After the Shuram event, the δ15N value continued to decrease despite that δ13Ccarb rose back to +4‰. The continued δ15N drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ13Ccarb event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative δ13Ccarb excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.

Recognition of Early Eocene global carbon isotope excursions using lipids of marine Thaumarchaeota

The Paleocene–Eocene Thermal Maximum (PETM; ∼56Ma) and Eocene Thermal Maximum 2 (ETM2; ∼53Ma) are geological short (<200,000 years) episodes of extreme global warming and environmental change. Both the PETM and ETM2 are associated with the injection of 13C-depleted carbon into the ocean–atmosphere system as revealed through a globally recognized carbon isotope excursion (CIE) and massive dissolution of deep sea carbonate. However, the magnitude of these CIEs vary with the type of fossil matter, i.e. multiple carbonate phases, bulk organic matter, and terrestrial and marine biomarker lipids, making it difficult to constrain the actual CIE in atmospheric and oceanic carbon pools. Here we analyzed the stable carbon isotopic composition (δ13C) of glycerol dibiphytanyl glycerol tetraether lipids (GDGTs) derived from marine Thaumarchaeota in sediments deposited during the PETM in the North Sea Basin and ETM2 in the Arctic Ocean. The δ13C values of these lipids are potentially directly recording variations in δ13C dissolved inorganic carbon (DIC) and can thus provide a record of marine δ13C DIC across both these Eocene carbon cycle perturbations. Reconstructed pre-CIE δ13CDIC values are slightly lower (0.5–1‰) than modern day values, possibly because Thaumarchaeotal lipids are not only derived from surface waters but also from 13C-depleted subsurface waters. Their values decrease by ∼3.6 (±0.3) ‰ and ∼2.5 (±0.7)‰ during the PETM and ETM2, respectively. The CIE in crenarchaeol for ETM2 is higher than that in marine calcite from other locations, possibly because of the admixture of deep water 13C-depleted CO2 generated by the euxinic conditions that developed occasionally during ETM2. However, the reconstructed PETM CIE lies close to the CIE inferred from marine calcite, suggesting that the δ13C record of crenarchaeol may document changes in marine DIC during the PETM in the North Sea Basin. The δ13C of thaumarchaeotal lipids may thus be a novel tool to reconstruct the δ13C of DIC in sediments that are devoid of carbonates, but relatively rich in organic matter, such as shallow marine coastal settings.

Coring project in Bighorn Basin: Drilling phase complete

Scientists have begun a new project to recover and study sediment cores in the Bighorn Basin of Wyoming to investigate the high‐frequency climatic and biotic variability of a continental depositional system during past episodes of greenhouse conditions. The project, called the Bighorn Basin Coring Project (BBCP), focuses on the early Paleogene (∼50–65 million years ago), the most recent interval of Earth history characterized by greenhouse climate conditions. During the early Paleogene, global mean annual temperatures were up to 10°C higher than today, and carbon dioxide (CO2) concentrations may have been more than 1000 parts per million [Zachos et al., 2008] compared with the roughly 390 parts per million today. Superimposed on this background greenhouse climate state were a series of short‐term extreme warming events called hyperthermals. The best known early Paleogene hyperthermal event is the Paleocene‐Eocene Thermal Maximum (PETM; ∼56 million years ago), which is characterized by a large global carbon isotope excursion and coincides with major changes to marine and continental ecosystems [McInerney and Wing, 2011]. At present, the causes of these hyperthermal events remain unknown, although several hypotheses exist.

Lower Carboniferous carbon isotope stratigraphy in South China: Implications for the Late Paleozoic glaciation

Studies on carbon isotopes of bulk carbonates from Longan and Baping sections of Lower Carboniferous in Guangxi of China show that the stable carbon isotope compositions in carbonate rocks of the isolated platform and deep slope facies were resistant to the influence of early meteoric diagenesis and late burial diagenesis. Three major positive carbon isotope excursions have been recognized in Lower Carboniferous in South China. The first major positive δ 13 C shift of 4.19‰ occurred in the middle part of Siphonodella isosticha-upper Siphonodella crenulata zone (Tournaisian); the second with an amplitude of 4.65‰ occurred near the Tournaisian/Visean boundary; and the third of 2.23‰ in the lower part of Gnathodus bollandensis zone. The three positive shifts of δ 13 C can be correlated with global carbon isotope excursions and are consistent with the fall in global sea level, indicating that abundant organic carbon burial, lowering of atmospheric CO 2 , and glaciation may have occurred during these time intervals.

Oldest North American primate

Primates appear at or immediately after the beginning of the Eocene, in the global carbon isotope excursion (CIE) marking the Paleocene–Eocene Thermal Maximum. We presented evidence for dispersal of Teilhardina from Asia to Europe to North America in the first 25 kyr of the CIE (first 25 kyr of the Eocene; ref. 1 …

Discussion on the Late Palaeocene–Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales, associated environmental change, causes and consequences Journal , Vol. 164, 2007, 1093–1108

R. J. Bailey and D. G. Smith write: Cohen et al . (2007) review the major changes in ocean chemistry in the Early Jurassic (Toarcian) and Late Palaeocene–Early Eocene, the latter corresponding to the Palaeocene–Eocene Temperature Maximum (PETM). These changes involved global carbon isotope excursions (CIEs), temperature increases, marine anoxia and biospheric crises. Their preferred explanation is the massive thermal dissociation of methane hydrates, releasing isotopically light carbon. What is problematical is their view that the isotopic data series from the Toarcian and Palaeocene–Eocene intervals (fig. 1a and b) record not only the amounts of change in the carbon isotopes, but also the rates of change. In practice, this claim is reliable only if the samples analysed derive from intervals characterized by unbroken accumulation at an unchanging rate. For the Toarcian, its justification lies in an earlier paper (Kemp et al . 2005) and involves: (1) radiometric dates that place the Toarcian CIE within a c . 2.2 Ma interval; (2) the assumption that this interval was characterized by continuous accumulation at a constant rate; (3) spectral analysis of lithological data series suggesting 81 cm cycles in carbon isotope and biogenic carbonate abundance in the laminated, unbioturbated, sediments accumulated during the CIE; (4) use of the estimated 2.2 Ma steady rate of accumulation to calibrate the cycles, and the duration of the CIE, in terms of a 21.46 ka periodicity; (5) identification of this periodicity with precession-related, c . 21 ka, M-orbital forcing of terrestrial insolation, which is presumed to have triggered the abrupt shifts in δ13C. (The origin of this solution of the quasi-period of Early Jurassic precession is not indicated.) The concordance between the time calibration of the spectrally determined cycles and the estimated period of Early Jurassic orbital precession, somewhat circularly, supports the assumed constant …

Duration of the Early Toarcian carbon isotope excursion deduced from spectral analysis: Consequence for its possible causes

The marked 3–8‰ negative carbon isotope excursion associated with the Early Toarcian oceanic anoxic event (OAE; ~ 183 myr ago) in the Early Jurassic period is thought to represent one of the most important perturbations of the C-cycle in the last 200 myr. However, the origin of this excursion remains strongly debated, primarily due to uncertainties in the estimation of its duration, which ranges from ~ 200 kyr to 1 myr. Here we present a new orbital calibration of the Early Toarcian carbon isotope excursion, based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from Portugal (Peniche) and SW Germany (Dotternhausen), in order to better constrain the timescale and hence the origin of this excursion. These analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in Portugal and on the greyscale of black shales in Germany, which allow us to propose a duration of ≥ 1.9 myr for the Early Toarcian and of ~ 900 kyr for the entire carbon isotope excursion. The shift towards lower carbon isotope values lasted ~ 150 kyr, and carbon isotope values remained low for ~ 450 kyr; the subsequent increase of carbon isotope values lasted ~ 300 kyr. This calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over ~ 600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates. In the light of our new cyclostratigraphic timescale, the massive input of isotopically light carbon associated with the emplacement of the Karoo–Ferrar basaltic province appears as the most likely cause of the Toarcian global carbon isotope excursion. We also show that the C-isotope perturbation coincided with a transition from precession–eccentricity-dominated cycles to obliquity–eccentricity-dominated cycles, suggesting that the OAE was marked by a fundamental change in the response of the climate system, which allowed the obliquity signal, normally better recorded at high latitudes, to be a dominant forcing factor of short-term sedimentary cycles at tropical latitudes.

High-resolution carbon isotope stratigraphy and mammalian faunal change at the Paleocene-Eocene boundary in the Honeycombs area of the southern Bighorn Basin, Wyoming

The Paleocene-Eocene boundary is defined chemostratigraphically by the onset of a 100 kyr global carbon isotope excursion (CIE) that is recognized in marine and continental strata in both carbonate and dispersed organic carbon. The CIE is important in continental sections as a proxy for environmental change during the Paleocene-Eocene thermal maximum (PETM), which clearly affected faunal succession in Paleocene-Eocene mammals. At Polecat Bench in the northern Bighorn Basin, Wyoming, the CIE is in the Willwood Formation, where it spans four mammalian biozones from part of latest Clarkforkian Cf-3, through early Wasatchian Wa-M (<b><i>Meniscotherium</i></b>), Wa-0, and part of Wa-1. Here we report a new high-resolution study of the CIE recorded in dispersed organic carbon of the basal Willwood Formation in the Honeycombs area of the southern Bighorn Basin. In the Honeycombs area, 58 isotope sample sites span a vertical section of 71 meters, including the Wa-M locality ‘Halfway Hill North’ with both <b><i>Meniscotherium</i></b> and a <b><i>Heptodon</i></b>-like perissodactyl, and the richly fossiliferous Wa-0 wash site ‘Castle Gardens’ yielding a diverse mammalian fauna. Total organic carbon in the Honeycombs section is in the range of 0.1 to 0.4 percent by weight, and CaCO₃ is notably lacking except in the Castle Gardens interval where it reaches ca. 3 percent. Pre- and post-excursion values of δ¹³C_org (PDB) range from −26.2 to −24.1 permil. The Honeycombs CIE is about 42 m thick, and CIE excursion values range from −28.8 to −26.2 permil. The Honeycombs area is important because the Halfway Hill North locality is the first to yield a perissodactyl from the Wa-M <b><i>Meniscotherium</i></b> zone, and the Castle Gardens wash site is the first to yield abundant evidence of Wa-0 zone microvertebrates. As at Polecat Bench, the <b><i>Meniscotherium</i></b> zone in the Honeycombs area is in the lower part of the CIE, and Castle Gardens and other Wa-0 sites are in the longer middle and upper parts, starting near or after the maximum negative excursion. The CIE at Polecat Bench includes a series of 5 to 6 negative-excursion landmarks that are spaced, on average, about 8.5 m apart. These are tentatively interpreted as 21-kyr orbital precession cycles like those inferred for paleosol color. Negative-excursion landmarks are present in the Honeycombs CIE as well, but these are fewer and part of the Honeycombs section may be condensed or missing. Full understanding of the Paleocene-Eocene biotic transition will require investigation at even finer scales of temporal resolution, and results reported here suggest that finer-scale studies should be possible.

A global carbon isotope excursion (SPICE) during the Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level

The Steptoean positive carbon isotope excursion (SPICE) marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites. The SPICE excursion is documented from sections in Laurentia, Kazakhstan, China, and Australia where it begins with the mass extinction at the base of the Pterocephaliid Biomere (Steptoean Stage) in Laurentia and at coeval extinction horizons in Gondwana and periGondwana terranes. The peak of SPICE (+5‰) coincided with a time of maximum regression in Laurentia. SPICE is similar in this regard to excursions that coincide with glacio-eustatic falls, such as in the Late Ordovician. A plausible scenario involves the transformation of ocean circulation between two states, which led to enhanced coastal upwelling and benthic extinctions. The lack of evidence for glaciation indicates that the coeval sea level fall (Sauk II–Sauk III event) resulted from tectonic or hydrologic changes that remain poorly understood at this time.