Perturbation Of Global Carbon Cycle Research Articles

Astronomical constraints on the duration of the Early Jurassic Pliensbachian Stage and global climatic fluctuations

The Early Jurassic was marked by multiple periods of major global climatic and palaeoceanographic change, biotic turnover and perturbed global geochemical cycles, commonly linked to large igneous province volcanism. This epoch was also characterised by the initial break-up of the super-continent Pangaea and the opening and formation of shallow-marine basins and ocean gateways, the timing of which are poorly constrained. Here, we show that the Pliensbachian Stage and the Sinemurian–Pliensbachian global carbon-cycle perturbation (marked by a negative shift in δ13C of 2–4‰), have respective durations of ∼8.7 and ∼2 Myr. We astronomically tune the floating Pliensbachian time scale to the 405 Kyr eccentricity solution (La2010d), and propose a revised Early Jurassic time scale with a significantly shortened Sinemurian Stage duration of 6.9±0.4 Myr. When calibrated against the new time scale, the existing Pliensbachian seawater 87Sr/86Sr record shows relatively stable values during the first ∼2 Myr of the Pliensbachian, superimposed on the long-term Early Jurassic decline in 87Sr/86Sr. This plateau in 87Sr/86Sr values coincides with the Sinemurian–Pliensbachian boundary carbon-cycle perturbation. It is possibly linked to a late phase of Central Atlantic Magmatic Province (CAMP) volcanism that induced enhanced global weathering of continental crustal materials, leading to an elevated radiogenic strontium flux to the global ocean.

Micropaleontological evidence for redox changes in the OAE3 interval of the US Western Interior: Global vs. local processes

The Coniacian-Santonian interval has been proposed as the youngest of the Cretaceous ocean anoxic events (OAE3), but this designation has long been debated. OAE3 is associated with a long-lasting (∼3 myr) succession of black shales from the central and South Atlantic, Caribbean region, and the North American Western Interior; in the Western Interior it is characterized by an abrupt increase in total organic carbon (TOC) and corresponding trace metal indicators for anoxia. However, the modern concept of OAEs is predicated on detection of global carbon cycle perturbations as recorded by substantial carbon isotope excursions (CIE), and the protracted Coniacian-Santonian black shale interval does not have a large CIE. A more conservative definition of OAE3 might limit the event to the modest positive carbon isotope excursion restricted to the upper Coniacian Scaphites depressus Ammonite Zone. Trace metal proxies suggest that oxygen levels abruptly declined prior to the onset of this CIE in the Western Interior Sea (WIS), but it is unknown whether regional anoxic conditions were confined to sediments/pore waters, or how anoxia may have affected the biota. In an effort to characterize the oxygenation history of the WIS and to better understand the nature of the hypothesized OAE3, we present micropaleontological evidence of declining oxygen in bottom waters prior to the event using benthic foraminifera, which are sensitive to dissolved oxygen. Changes in benthic foraminiferal abundances suggest a decline in oxygen at least 1-myr prior to the CIE (including a nadir immediately below the start of the excursion), improving bottom water oxygen during the CIE, and re-establishment of persistent anoxia following the isotope excursion. Anoxia endured for nearly 3 myr in the central seaway, showing some signs of recovery toward the top of the Niobrara Formation. Our findings suggest that declining oxygen concentrations in the seaway eventually reached a tipping point, after which dissolved oxygen quickly dropped to zero.The late Coniacian CIE is an exception to the trend of declining oxygen in the WIS, and part of a larger pattern in the oxygenation history of the Niobrara Formation which suggests that it does not adhere to standard black shale models. Transgressive periods, including the Fort Hays Limestone and the lower limestone unit of the Smoky Hill Shale (which corresponds to the CIE) are relatively oxic, while periods of highstand (i.e., most of the Smoky Hill) correspond to deteriorating oxygen conditions. This contrasts with the standard black shale model for sea level and oxygen, where transgressions typically correlate with maximum TOC enrichment, interpreted to result from both sediment condensation and oxygen deficiency. The association of global carbon burial/anoxia (as indicated by carbon isotopes) with a regional increase in oxygen and decrease in organic matter preservation is reminiscent of the Cenomanian-Turonian Greenhorn Limestone, which contains OAE2. In both cases, the facies are not typical black shales, but instead have appreciable carbonate content. Western Interior redox trends support the rejection of the original concept of a protracted Coniacian-Santonian OAE3 because it is not a distinct “event.” Increasing local oxygen during the late Coniacian CIE also argues against a narrower OAE designation for this event, because the excursion can't be tied to anoxia here or anywhere else it has been described. Nevertheless, the Late Coniacian Event (as we prefer to call this CIE) still represents an important perturbation of the global carbon cycle. This is emblematic of the shift away from widespread, discrete anoxic events during the ongoing paleoceanographic reorganization of the Late Cretaceous, even as large carbon cycle perturbations continued.

Chemostratigraphy of the Upper Albian to mid‐Turonian Natih Formation (Oman) – how authigenic carbonate changes a global pattern

AbstractThe Oman Mountains preserve a Cretaceous continental margin transect with the proximal Arabian carbonate shelf and the adjacent deep Hawasina Basin. Today, the sediments from the Arabian Platform outcrop in the Oman Mountains (Jabal Akdhar and Saih Hatat) and in the Adam Foothills. The western part of the Adam Foothills provides insight into the evolution of a Late Cretaceous intra‐platform basin with organic‐rich sediments in the central part of this basin. The aims of this study are (a) to establish a biostratigraphy and chemostratigraphy of the Natih Formation and (b) to reconstruct depositional conditions of organic‐rich sediments in an intra‐platform basin during Cenomanian–Turonian times. The hypothesis that local black shale formation is an expression of global perturbations of the global carbon cycle will be tested. Reconstruction of the depositional history of the Arabian Platform and its intra‐platform basin within a global palaeoclimatic framework requires an accurate time frame. The Upper Albian to mid‐Turonian biostratigraphy of the Natih Formation has resulted in controversial age models that will be integrated into a solid chemostratigraphic framework with additional biostratigraphic data. A major positive δ13C excursion (+4·6‰) has been identified as of Middle Cenomanian age, which is confirmed by an ammonite datum. A second positive δ13C excursion (+4·5‰) following a major negative excursion (−1·0‰) confirms the existence of the Cenomanian/Turonian Boundary Event. The accurate chemostratigraphy and biostratigraphy confirms that major source rocks in the Mishrif‐Natih Basin precede OAE2. Low δ13C values measured in the sediments of the Natih B member are considered a consequence of diagenetic alteration. Elevated organic carbon contents and argillaceous sediments alternating with limestones resulted in diagenetic conditions favouring formation of authigenic calcite depleted in C‐13.

Global carbon cycle perturbation across the Eocene‐Oligocene climate transition

AbstractThe Eocene‐Oligocene transition (EOT), ~34 Ma, marks a tipping point in the long‐term Cenozoic greenhouse to icehouse climate transition. Paleorecords reveal stepwise rapid cooling and ice growth across the EOT tightly coupled to a transient benthic δ13C excursion and a major and permanent deepening of the carbonate compensation depth (CCD). Based on biogeochemical box modeling, Merico et al. (2008) suggested that a combination of (1) glacioeustatic sea level fall‐induced shelf‐basin carbonate burial fractionation and (2) shelf carbonate weathering can account for the carbon cycle perturbation, but this finding has been questioned. Alternative proposed mechanisms include increased ocean ventilation, decreased carbonate burial, increased organic carbon burial, increased silicate weathering, and increased ocean calcium concentration. Here we use an improved version of the biogeochemical box model of Merico et al. (2008) to reevaluate these competing hypotheses and an additional mechanism, the expansion of “carbon capacitors” such as permafrost and peatlands. We find that changes in calcium concentration, silicate weathering, and carbonate or organic carbon burial each yield a response that is fundamentally at odds with the form and/or sign of the paleorecords. Shelf‐basin carbonate burial fractionation (CCD change), plus shelf carbonate weathering, sequestration of 12C‐enriched carbon into carbon capacitors, and possibly increased ocean ventilation (δ13C excursion), offers the best fit to the paleorecords. Further work is needed to understand why the EOT carbon cycle perturbation is so unique when the forcing mechanisms hypothesized to be responsible (cooling and ice growth) are not peculiar to this event.

Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran

Sedimentary successions across the Permian–Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C–O isotopes and Mn–Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C–δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of −1.2(±0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‘Boundary Clay’ in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by −2.1(±0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~50–80°C and water/rock ratios of <10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon at a global scale that has received relatively little attention to date.

The global Hangenberg Crisis (Devonian–Carboniferous transition): review of a first-order mass extinction

Abstract The global Hangenberg Crisis near the Devonian–Carboniferous boundary (DCB) represents a mass extinction that is of the same scale as the so-called ‘Big Five’ first-order Phanerozoic events. It played an important role in the evolution of many faunal groups and destroyed complete ecosystems but affected marine and terrestrial environments at slightly different times within a short time span of c. 100–300 kyr. The lower crisis interval in the uppermost Famennian started as a prelude with a minor eustatic sea-level fall, followed rather abruptly by pantropically widespread black shale deposition (Hangenberg Black Shale and equivalents). This transgressive and hypoxic/anoxic phase coincided with a global carbonate crisis and perturbation of the global carbon cycle as evidenced by a distinctive positive carbon isotope excursion, probably as a consequence of climate/salinity-driven oceanic overturns and outer-shelf eutrophication. It is the main extinction level for marine biota, especially for ammonoids, trilobites, conodonts, stromatoporoids, corals, some sharks, and deeper-water ostracodes, but probably also for placoderms, chitinozoans and early tetrapods. Extinction rates were lower for brachiopods, neritic ostracodes, bryozoans and echinoderms. Extinction patterns were similar in widely separate basins of the western and eastern Prototethys, while a contemporaneous marine macrofauna record from high latitudes is missing altogether. The middle crisis interval is characterized by a gradual but major eustatic sea-level fall, probably in the scale of more than 100 m, that caused the progradation of shallow-water siliciclastics (Hangenberg Sandstone and equivalents) and produced widespread unconformities due to reworking and non-deposition. The glacio-eustatic origin of this global regression is proven by miospore correlation with widespread diamictites of South America and South and North Africa, and by the evidence for significant tropical mountain glaciers in eastern North America. This isolated and short-lived plunge from global greenhouse into icehouse conditions may follow the significant drawdown of atmospheric CO 2 levels due to the prior massive burial of organic carbon during the global deposition of black shales. Increased carbon recycling by intensified terrestrial erosion in combination with the arrested burial of carbonates may have led to a gradual rise of CO 2 levels, re-warming, and a parallel increase in the influx of land-derived nutrients. The upper crisis interval in the uppermost Famennian is characterized by initial post-glacial transgression and a second global carbon isotope spike, as well as by opportunistic faunal blooms and the early re-radiation of several fossil groups. Minor reworking events and unconformities give evidence for continuing smaller-scale oscillations of sea-level and palaeoclimate. These may explain the terrestrial floral change near the Famennian–Tournaisian boundary and contemporaneous, evolutionarily highly significant extinctions of survivors of the main crisis. Still poorly understood small-scale events wiped out the last clymeniid ammonoids, phacopid trilobites, placoderms and some widespread brachiopod and foraminiferan groups. The post-crisis interval in the lower Tournaisian is marked by continuing eustatic rise (e.g. flooding of the Old Red Continent), and significant radiations in a renewed greenhouse time. But the recovery had not yet reached the pre-crisis level when it was suddenly interrupted by the global, second-order Lower Alum Shale Event at the base of the middle Tournaisian.

High-precision U–Pb zircon age constraints on the duration of rapid biogeochemical events during the Ludlow Epoch (Silurian Period)

Precise determinations of the rates and durations of Palaeozoic biogeochemical events are largely unavailable. Here, we present two new high-precision U–Pb (zircon) dates from volcanic ash deposits from the Ludlow Series (Silurian System) of Podolia, Ukraine, that yielded weighted mean 206 Pb/ 238 U dates of 424.08 ± 0.20 (0.29) [0.53] Ma and 422.91 ± 0.07 (0.21) [0.49] Ma (analytical, tracer and total uncertainties). These new dates bracket the largest post-Cambrian global carbon cycle perturbation (Lau Excursion) and constrain the ‘Ludlow Rise’ in 87 Sr/ 86 Sr. These chronostratigraphically well-controlled dates improve the calibration of the Silurian time scale and provide the first determinations of the rates of biogeochemical change during the Ludlow Epoch. Supplementary material: U–Pb geochemical methods, data and CL imagery are available at http://www.geolsoc.org.uk/SUP18798 .

Response and recovery of the Comanche carbonate platform surrounding multiple Cretaceous oceanic anoxic events, northern Gulf of Mexico

The ubiquity of carbonate platforms throughout the Cretaceous Period is recognized as a product of high eustatic sea-level and a distinct climatic optimum induced by rapid sea-floor spreading and elevated levels of atmospheric carbon-dioxide. Notably, a series of global oceanic anoxic events (OAEs) punctuate this time-interval and mark periods of significantly reduced free oxygen in the world's oceans. The best records of these events are often from one-dimensional shelf or basin sections where only abrupt shifts between oxygenated carbonates and anoxic shales are recorded. The Comanche Platform of central Texas provides a unique opportunity to study these events within a well-constrained stratigraphic framework in which their up-dip and down-dip sedimentologic effects can be observed and the recovery of the platform to equilibrium states can be timed and understood. Stable isotope data from whole cores in middle Hauterivian through lower Campanian mixed carbonate-siliciclastic strata are used to construct a 52-myr carbon isotope reference profile for the northern Gulf of Mexico. Correlation of this composite curve to numerous global reference profiles permits identification of several anoxic events and allows their impact on platform architecture and facies distribution to be documented. Oceanic anoxic events 1a, 1b, 1d, and 2 occurred immediately before, after, or during shale deposition in the Pine Island Member, Bexar Member, Del Rio Formation, and Eagle Ford Group, respectively. Oceanic anoxic event 3 corresponds to deposition of the Austin Chalk Group. Platform drowning on three occasions more closely coincided with globally recognized anoxic sub-events such as the Fallot, Albian-Cenomanian, and Mid-Cenomanian events. This illustrates that the specific anoxic event most affecting a given carbonate platform varied globally as a function of regional oceanographic circumstances.Using chemo- and sequence-stratigraphic observations, a four-stage model is proposed to describe the changing facies patterns, fauna, sedimentation accumulation rates, platform architectures, and relative sea-level trends of transgressive-regressive composite sequences that developed in response to global carbon-cycle perturbations. The four phases of platform evolution include the equilibrium, crisis, anoxic, and recovery stages. The equilibrium stage is characterized by progradational shelf geometries and coral-rudist phototrophic faunal assemblages. Similar phototrophic fauna typify the crisis stage; however, incipient biocalcification crises of this phase led to retrogradational shelf morphologies, transgressive facies patterns, and increased clay mineral proportions. Anoxic stages of the Comanche Platform were coincident with back-ground deposition of organic-rich shale on drowned shelves and heterotrophic fauna dominated by oysters or coccolithophorids. Eustatic peaks of this stage were of moderate amplitude (∼30 m), yet relative sea-level rises were greatly enhanced by reduced sedimentation rates. In the recovery stage, heterotrophic carbonate factories re-established at the shoreline as progradational ramp systems and sediment accumulation rates slowly increased as dysoxia diminished. Full recovery to equilibrium conditions may or may not have followed. Geochemical and stratigraphic trends present in the four stages are consistent with increased volcanism along mid-ocean ridges and in large-igneous provinces as primary drivers of Cretaceous OAEs and the resulting transgressive-regressive composite sequences.

A perturbed hydrological cycle during Oceanic Anoxic Event 2

Research Article| February 01, 2014 A perturbed hydrological cycle during Oceanic Anoxic Event 2 Niels A.G.M. van Helmond; Niels A.G.M. van Helmond * 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands *E-mail: n.vanhelmond@uu.nl. Search for other works by this author on: GSW Google Scholar Appy Sluijs; Appy Sluijs 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Gert-Jan Reichart; Gert-Jan Reichart 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Jaap S. Sinninghe Damsté; Jaap S. Sinninghe Damsté 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Caroline P. Slomp; Caroline P. Slomp 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Henk Brinkhuis Henk Brinkhuis 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Search for other works by this author on: GSW Google Scholar Author and Article Information Niels A.G.M. van Helmond * 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Appy Sluijs 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Gert-Jan Reichart 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Jaap S. Sinninghe Damsté 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands Caroline P. Slomp 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands Henk Brinkhuis 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands2NIOZ (Royal Netherlands Institute for Sea Research), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands *E-mail: n.vanhelmond@uu.nl. Publisher: Geological Society of America Received: 11 Jul 2013 Revision Received: 19 Oct 2013 Accepted: 22 Oct 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (2): 123–126. https://doi.org/10.1130/G34929.1 Article history Received: 11 Jul 2013 Revision Received: 19 Oct 2013 Accepted: 22 Oct 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Niels A.G.M. van Helmond, Appy Sluijs, Gert-Jan Reichart, Jaap S. Sinninghe Damsté, Caroline P. Slomp, Henk Brinkhuis; A perturbed hydrological cycle during Oceanic Anoxic Event 2. Geology 2014;; 42 (2): 123–126. doi: https://doi.org/10.1130/G34929.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Late Cretaceous Oceanic Anoxic Event 2 (OAE2; ca. 94 Ma) was one of the largest global carbon cycle perturbations during the Phanerozoic. OAE2 represents an important, although extreme, case study for modern trends because widespread anoxia and enhanced organic carbon burial during OAE2 were linked to exceptionally warm climates and high atmospheric CO2 concentrations. However, the consequences of this warmth for the hydrological cycle remain poorly understood, hampering our understanding of its impact on biogeochemical cycles during this greenhouse episode. Here we show evidence for changes in the hydrological cycle during OAE2 based on combined geochemical and palynological data for the stratigraphically expanded coastal OAE2 succession at Bass River, located on the New Jersey shelf (Ocean Drilling Program Leg 174AX), eastern United States. Paleothermometry, based on TEX86, indicates sea-surface warming at the onset of OAE2 and a subsequent pronounced cooling event. Palynological data show that these changes in temperature were associated with strong variations in precipitation and runoff. We suggest that an acceleration of the hydrological cycle during OAE2 played a key role in supplying nutrients to coastal waters and enhancing stratification, thus contributing to the development of ocean anoxia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Facies and climate changes across the Permian–Triassic boundary in the North German Basin: insights from a high-resolution organic carbon isotope record

Abstract The transition from the uppermost Permian Zechstein Group to the lowermost Triassic Lower Buntsandstein Subgroup in the North German Basin is characterized by a shift from sabkha to playa and to fluviatile facies indicative of a shift to a more humid climate and associated with a major reorganization of the sedimentary system. These changes are recorded in a high-resolution carbon isotope profile of bulk organic matter spanning from the upper Zechstein Group of Changhsingian age to the Calvörde Formation of the Lower Buntsandstein Subgroup of Changhsingian–Indusian age. The organic carbon isotopes reflect a varying composition of the organic matter. Dependent on the facies type, which is mainly controlled by humidity, the proportions of cyanobacterial, phytoplankton and land-plant-derived organic matter with differing organic carbon isotope values change in the samples. A negative organic carbon isotope excursion shortly below the supposed Permian–Triassic Boundary (PTB) is interpreted as a horizon rich in isotopically light phytoplankton, representing a perennial playa lake stage. Although certain effects of global carbon cycle perturbations around the PTB on the analysed organic carbon isotope profile cannot be excluded, the facies and climatic changes associated with the PTB events are thought to be more important.

Origin of giant wave ripples in snowball Earth cap carbonate

The most extreme climate transitions in Earth history are recorded by the juxtaposition of Neoproterozoic glacial deposits with overlying cap carbonate beds. Some of the most remarkable sedimentary structures within these beds are sharp-crested (trochoidal) bedforms with regular spacing of as much as several meters that are often interpreted as giant wave ripples formed under extreme wave conditions in a nonuniform postglacial climate. Here we evaluate this hypothesis using a new bedform stability diagram for symmetric oscillatory fl ows that indicates that the fi rst-order control on the formation of trochoidal rather than hummocky bedforms is sediment size, not wave climate. New measurements of bedform wavelengths and particle sizes from the ca. 635 Ma Nuccaleena Formation, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand; most are within the trochoidal bedform stability phase space for normal wave climates. Moreover, numerical simulations of flover fi xed bedforms show that symmetric trochoidal ripples with a nearly vertical angle of climb may be produced over long time periods with variable wave climates in conjunction with rapid seabed cementation. These data reveal that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during a global carbon cycle perturbation unprecedented in Earth history.

Chemostratigraphy of Early Aptian carbonates offshore Abu Dhabi, United Arab Emirates: Interactions between global carbon-cycle perturbations and paleoenvironmental changes across Oceanic Anoxic Event 1a

Chemostratigraphy of Early Aptian carbonates offshore Abu Dhabi, United Arab Emirates: Interactions between global carbon-cycle perturbations and paleoenvironmental changes across Oceanic Anoxic Event 1a

Early Toarcian (Early Jurassic) oceanic anoxic event recorded in the shelf deposits in the northwestern Panthalassa: Evidence from the Nishinakayama Formation in the Toyora area, west Japan

The early Toarcian oceanic anoxic event (T-OAE) was marked by the worldwide burial of organic-rich black shales and a coincident pronounced negative carbon-isotope excursion of carbonates, organic matter, and fossil wood. This characteristic carbon-isotope perturbation and other geochemical records have been recognized in many sections in the European domain and in selected sections in the central and eastern Panthalassa, but not in the western Panthalassa. We present herein, for the first time, data of carbon isotope ratios of organic carbon, total organic carbon (TOC) contents, and sedimentary fabrics from the uppermost Pliensbachian to the lower Toarcian sequence in the Sakuraguchi-dani Valley, Toyora area, west Japan, that was deposited in the shelf basin of the northwestern Panthalassa. The carbon-isotope profile displays a similar trend to those in contemporaneous strata in the Tethys and in the eastern and central Panthalassic regions, which is characterized by a distinct negative excursion around the Tenuicostatum/Falciferum Zone boundary in the early Toarcian. This evidence suggests that the widely recognized negative carbon-isotope excursion represents a nature of the global carbon-cycle perturbation. Analyses of total organic carbon contents and sedimentary fabrics in the study section reveal that accumulation of organic carbon occurred and anoxic bottom-water was prevalent during the early Toarcian. The most organic-rich strata and bottom-water anoxia described in the study section, however, exist in the interval showing the carbon-isotope positive recovery after the abrupt negative shift, and this fact suggests that the accumulation of organic matters and development of marine anoxia were globally diachronous, even though the negative CIE was interpreted as a synchronous event. Our study also demonstrates that the early Toarcian anoxia in the northwestern Panthalassic basin was not a single and continuous anoxic event but was interrupted by episodic short-term oxygenation events.

Drowning of a carbonate platform as a precursor stage of the Early Toarcian global anoxic event (Southern Provence sub‐Basin, South‐east France)

AbstractThe Toarcian Oceanic Anoxic Event is well‐known as coinciding with a carbonate crisis, coupled with organic matter accumulation and perturbation of the carbon cycle expressed by carbon‐isotope excursions. In this palaeoenvironmental setting, the present research attempts to better constrain the palaeoenvironmental conditions leading to the drowning of a carbonate platform during Late Pliensbachian to Early Toarcian times. This study is based on the integrated sedimentological, diagenetic and geochemical (stable isotopes and Rock‐Eval pyrolysis) analysis of several stratigraphic successions located in the Southern Provence sub‐Basin (South‐east France). Eodiagenetic ferroan calcite cements below two hardground surfaces at the Pliensbachian–Toarcian transition record episodic eutrophication events. Such events were precursor stages of a global palaeoenvironmental perturbation (i.e. a global carbon cycle perturbation) that led to the demise of the carbonate platform and occurred prior to the Toarcian Oceanic Anoxic Event. Subsequently, the acme of the palaeoenvironmental deterioration that coincided with the Toarcian Oceanic Anoxic Event is recorded by a pronounced negative carbon‐isotope excursion (−3‰) followed by a positive one (2·87‰), organic matter preservation and inferred high temperature of surface sea water (ca 25°C) as given by oxygen‐isotope values from chondrichthyan tooth enamel. Sedimentological comparisons, as well as chemostratigraphic and biostratigraphical correlations between the Early Toarcian sedimentary series of the Southern Provence and Eastern Dauphinois sub‐basins, indicate that tectonic tilting occurred earlier in the Southern Provence sub‐Basin; this resulted in the preservation of organic‐rich deposits in the first sub‐basin and depositional hiatus in the second sub‐basin. Thus, local conditions, such as contrasting structural setting, could markedly modify the sedimentary signature of a global climatic event.

Geochemical evidence for widespread euxinia in the Later Cambrian ocean

Widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long-term inhibitor of evolutionary radiation on early Earth. In recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later Cambrian oceans, possibly influencing evolutionary events during this time. Physical evidence of widespread anoxia in Cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored. Here we present sulphur isotope records from six globally distributed stratigraphic sections of later Cambrian marine rocks (about 499 million years old). We find a positive sulphur isotope excursion in phase with the Steptoean Positive Carbon Isotope Excursion (SPICE), a large and rapid excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation. Numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large-scale anoxic and sulphidic (euxinic) conditions. Independently, molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia. These results identify the SPICE interval as the best characterized ocean anoxic event in the pre-Mesozoic ocean and an extreme example of oxygen deficiency in the later Cambrian ocean. Thus, a redox structure similar to those in Proterozoic oceans may have persisted or returned in the oceans of the early Phanerozoic eon. Indeed, the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in Cambrian oceans.

Toxic Cambrian oceans

Isotopic evidence from carbon and sulphur points to the spread of anoxia and toxic sulphide as the chief culprits in at least one of a series of crises for marine ecosystems during the nascent stages of early animal evolution. See Letter p.80 It has been suggested that the Cambrian ocean was oxygen deficient, but physical evidence for widespread anoxia has been lacking. Gill et al. present sulphur isotope data from Cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation at the time. A prolonged period of anoxia during the Cambrian may explain the previously enigmatic peculiarities seen in the fossil record.

Campanian Maastrichtian carbon isotope stratigraphy: shelf-ocean correlation between the European shelf sea and the tropical Pacific Ocean

The long-term climate cooling during Campanian - Maastrichtian times is not well understood to date, especially because of the uncertainty introduced by low temporal resolution of biostratigraphy and the pronounced provincialism between tropical and temperate taxa. Two new high-resolution carbon isotope records derived from the boreal shelf-sea section at Lagerdorf-Kronsmoor-Hemmoor, northern Germany and the tropical Pacific at Deep Sea Drilling Project Site 305, Shatsky Rise, reduce these uncertainties. The records can be correlated with an accuracy not achieved by biostratigraphic methods so far. Distinct carbon isotope events in the late Campanian and the early Maastrichtian can be identified at both localities suggesting to represent global carbon cycle perturbations. Especially, the negative carbon isotope excursion in the early Maastrichtian, a pronounced feature of open-ocean records from the Pacific and Southern oceans, is recognized for the first time at a shelf-sea locality related to the North Atlantic Ocean. Furthermore, two short-term positive excursions are identified as superimposed signals to this event. The improved stratigraphy provides the unique opportunity to recognize leads and lags between the carbon cycle and ocean circulation of different marine settings and ecosystems, leading to a better understanding of their causes and effects.

Early Hettangian benthic–planktonic coupling at Doniford (SW England): Palaeoenvironmental implications for the aftermath of the end-Triassic crisis

This paper presents a high-resolution micropalaeontological study, integrated with geochemical and sedimentological analyses of the Triassic–Jurassic (Tr–J) transition at the Doniford section (SW England). Our analyses indicate the renewal of surface carbonate production in the Blue Lias Formation (early Hettangian) after the end-Triassic mass extinction. Based on the high abundance of surface carbonate producers (schizosphaerellids), which occurred in phase with development at sea-floor of calcareous hyaline benthic foraminifers, we deduce that a benthic–pelagic coupling resulted in both the initial restoration of the biological carbonate pump, and also a more efficient export production. The presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end-Triassic maximal perturbation. Complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black-shale sediments. The hypothesis of a methane-release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early Jurassic is discounted, based on evidence from the decoupled carbon isotope records of carbonate and organic matter. Apart from local control of a high amplitude sea-level change across the Tr–J boundary, the succession and nature of events at Doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the Central Atlantic Magmatic Province (CAMP) volcanism. A super greenhouse climate in the aftermath of the end-Triassic mass extinction appears to be the results of the combination of the long-term CO 2 accumulation in the atmosphere system due to the CAMP volcanic activity, as well as the inefficiency of the biological pump due to reduced export production.

What happens when the ocean is overheated? The foraminiferal response across the Paleocene-Eocene Thermal Maximum at the Alamedilla section (Spain)

The global warming and major perturbation of the global carbon cycle that occurred during the Paleocene–Eocene Thermal Maximum (PETM) have been investigated in the lower bathyal–upper abyssal Alamedilla section (Spain). Geochemical anomalies and dramatic faunal changes (including the globally recognized extinction event of deep-sea benthic foraminifera and the rapid evolutionary turnover of planktic foraminifera and calcareous nannofossils) are associated with the PETM at Alamedilla. Biotic changes in the plankton and benthos indicate environmental instability ∼11–14 k.y. before the onset of carbon isotope excursion that marks the Paleocene/Eocene boundary. The reorganization of the planktic ecosystem points to warm and oligotrophic conditions in surface waters during the earliest Eocene, whereas faunal and geochemical data indicate that the extinctions of benthic foraminifera occurred over an interval with a high CaCO3 content and oxic conditions at the seafloor. The proliferation of disaster taxa ( Glomospira spp.) after the extinctions has been related to a potential source of isotopically light carbon in the western Tethys and North Atlantic. Significant changes in foraminiferal test size are documented across the PETM. We suggest that increased temperatures played an important role in benthic foraminiferal test size, increasing their metabolic rates and, consequently, their food requirements. Decreased planktic foraminiferal test size may be related to decreased nutrient availability or surface-water density. However, the differences in test size evolution among different species of both benthic and planktic foraminifera may be related to interspecific competition and ecological adaptations to direct or indirect consequences of the carbon addition during the PETM.

A new chronology for the end-Triassic mass extinction

The transition from the Triassic to Jurassic Period, initiating the ‘Age of the dinosaurs’, approximately 200 Ma, is marked by a profound mass extinction with more than 50% genus loss in both marine and continental realms. This event closely coincides with a period of extensive volcanism in the Central Atlantic Magmatic Province (CAMP) associated with the initial break-up of Pangaea but a causal relationship is still debated. The Triassic–Jurassic (T–J) boundary is recently proposed in the marine record at the first occurrence datum of Jurassic ammonites, post-dating the extinction interval that concurs with two distinct perturbations in the carbon isotope record. The continental record shows a major palynological turnover together with a prominent change in tetrapod taxa, but a direct link to the marine events is still equivocal. Here we develop an accurate chronostratigraphic framework for the T–J boundary interval and establish detailed trans-Atlantic and marine–continental correlations by integrating astrochronology, paleomagnetism, basalt geochemistry and geobiology. We show that the oldest CAMP basalts are diachronous by 20 kyr across the Atlantic Ocean, and that these two volcanic pulses coincide with the end-Triassic extinction interval in the marine realm. Our results support the hypotheses of Phanerozoic mass extinctions resulting from emplacement of Large Igneous Provinces (LIPs) and provide crucial time constraints for numerical modelling of Triassic–Jurassic climate change and global carbon-cycle perturbations.