Major Carbon Cycle Perturbations Research Articles

Climatic controls on water-mass chemistry in a Paleocene lacustrine setting, Subei Basin, eastern China

Abstract The Paleocene epoch was characterized by global climate fluctuations and major carbon-cycle perturbations. During the greenhouse climate that characterized the early Cenozoic, a short-lived late Paleocene global cooling event has been recognized from marine records. However, the response of the terrestrial system to this climate cooling event is poorly understood. Here, we present major- and trace-element analyses, iron speciation systematics, carbonate carbon isotope data, and mineralogical observations from lacustrine sediments in Member II of the Paleocene Funing Formation (E1f2), utilizing well-preserved drill core from the Subei Basin, eastern China. Both chemical (chemical index of alteration [CIA], Al/K ratios) and mineralogical (mineralogical index of alteration, clay/feldspar ratios) proxies yield consistent weathering and paleoclimatic interpretations, suggesting a transition from cool and arid climatic conditions to warmer and more humid climatic conditions. Correlation of carbon isotopes between the Subei Basin and deep-sea records implies that this terrestrial setting records the short-lived Paleocene climate cooling event. The combination of climate records and paleosalinity proxies (B/Ga and S/total organic carbon [TOC]) indicates a relatively high-salinity water column (brackish to saline) under cool and arid climatic conditions in the Subei Basin, suggesting that elevated salinity was likely produced via net-evaporative conditions, rather than marine incursions. A shift toward less saline brackish conditions up section reflects an increase in precipitation and freshwater runoff under warmer and more humid climatic conditions. Iron speciation and redox-sensitive trace-metal systematics reveal fluctuating redox conditions, from oxic through to anoxic ferruginous, but with the distinct development of better ventilated conditions as freshwater inputs increased under more humid conditions. Our findings demonstrate the sensitivity of terrestrial climate to the late Paleocene climate cooling event and further reveal the chemical response of a lacustrine setting to a cooling episode in a greenhouse world.

Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ13C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ13C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (εp) increased by 5.6‰ between the surface and the LEZ-variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ13C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in εp result in significant, but smaller, differences in the δ13C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ13C values of both phytol and total organic carbon and affect interpretations of εp over geologic time.

Warming, acidification, and calcification feedback during the first hyperthermal of the Cenozoic—The Latest Danian Event

Abstract The Latest Danian Event (LDE; ca. 62.15 Ma) is a major double-spiked eccentricity-driven transient warming event and carbon cycle perturbation (hyperthermal) in the early Paleocene, which has received significantly less attention compared to the larger events of the late Paleocene–early Eocene. A better understanding of the nature of the LDE may broaden our understanding of hyperthermals more generally and improve our knowledge of Earth system responses to extreme climate states. We present planktic and benthic foraminiferal Mg/Ca and B/Ca records that shed new light on changes in South Atlantic temperature and carbonate chemistry during the LDE. Our planktic Mg/Ca record reveals a pulsed increase in sea-surface temperature of at least ~1.5 °C during the older carbon isotope excursion, and ~0.5 °C during the younger isotope excursion. We observe drops in planktic and benthic B/Ca, synchronous with pronounced negative excursions in benthic δ13C, which suggest a shift in the carbonate system toward more acidic, dissolved inorganic carbon–rich conditions, in both the surface and deep ocean. Conditions remained more acidic following the LDE, which we suggest may be linked to an enhanced ocean alkalinity sink due to changes in the makeup of planktic calcifiers, hinting at a novel feedback between calcifier ecology and ocean-atmosphere CO2.

Benthic Foraminiferal Response to the Aptian−Albian Carbon Cycle Perturbation in the Atlantic Ocean

Abstract A planktic foraminiferal mass extinction, coeval with the major carbon cycle perturbation of Oceanic Anoxic Event (OAE) 1b, occurred at the Aptian−Albian boundary interval (AABI). However, the scarcity of high-resolution records across the AABI hampers an assessment of the impacts of OAE 1b on deep-water benthic foraminiferal assemblages. Here we present high-resolution benthic foraminiferal census counts at Deep Sea Drilling Project (DSDP) Site 511 (southern South Atlantic Ocean) and Ocean Drilling Program (ODP) Site 1049 (western subtropical North Atlantic Ocean) over the AABI. Our records at these bathyal sites provide conclusive evidence that there was no benthic foraminiferal extinction at the Aptian−Albian boundary, although marked reorganizations of relative abundances occurred. During the latest Aptian, cyclic increases in the abundance of infaunal species at both sites point to repeated pulses of reduced bottom water oxygenation and increased organic carbon flux to the ocean floor. Additionally, agglutinated and weakly calcified benthic foraminiferal species were relatively abundant during the latest Aptian, suggesting deep-water carbonate ion depletion in the Atlantic Ocean, although we did not identify signs of carbonate dissolution at these relatively shallow sites. At Site 511, abundances of infaunal foraminifera increased in tandem with the negative carbonate carbon isotope (δ13Ccarb) excursion of the Kilian sub-event within OAE 1b, suggesting decreased bottom water ventilation and increased organic carbon flux to the ocean floor during the sub-event. Bottom water ventilation and carbonate ion saturation improved during the earliest Albian in the Atlantic Ocean, followed by high-amplitude oscillations, as suggested by abundance trends of heavily calcified epifaunal foraminifera at Sites 511 and 1049.

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.

Tracking magmatism and oceanic change through the early Aptian Anoxic Event (OAE 1a) to the late Aptian: Insights from osmium isotopes from the westernmost Tethys (SE Spain) Cau Core

Some of the major Carbon cycle perturbations of the Phanerozoic occurred during the Aptian, in relation to magmatism. The highest temperatures reconstructed for the Cretaceous Period correspond to the Oceanic Anoxic Event of the early Aptian (OAE 1a), an episode of accelerated global change. Here we present a chemostratigraphic study based on osmium isotopes integrated with high-resolution Carbon-Oxygen stable isotope data from the Cau Core (Western Tethys, SE Spain), including a 6.4 Ma record from the early to the late Aptian. This high-resolution study of the continuous and expanded Cau section permits a thorough understanding of the duration of the Aptian events, as well as an evaluation of the mechanisms triggering the abrupt changes of the global carbon and osmium cycles and their interdependence. Here we show that the Large Igneous Province (LIP) Aptian magmatism initiated 550–750 kyr prior to the OAE 1a, and persisted for 1.4 Myr after the event, influencing the composition of seawater for 2.8 Myr. We show a continuous Os isotope record encompassing the OAE 1a and the late Aptian for the first time, and demonstrate that the recovery from the exceptionally unradiogenic composition of seawater Os produced by the dominance of the Ontong Java Plateau volcanism, was slow. Our results demonstrate the different time duration of some events, and the asynchronous relationship between the carbon and osmium cycles

Assessing the importance of thermogenic degassing from the Karoo Large Igneous Province (LIP) in driving Toarcian carbon cycle perturbations

The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO2 (pCO2) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a 12C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric pCO2 and δ13C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian pCO2 and δ13C proxy data, and that thermogenic carbon (δ13C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.

Processes and forcing mechanisms of the carbon cycle perturbation during Cretaceous Oceanic Anoxic Event 2

<p indent="0mm">Oceanic Anoxic Event 2 (OAE2, ~94 Ma) was one of two global oceanic anoxic events in the Cretaceous and has been one of the most studied paleoceanographic events of the Mesozoic. Owing to the hyperthermal climate state during OAE2, it serves as a key event in the study of Earth system evolution during extreme greenhouse warmth. Recently, high temporal and spatial resolution studies and the application of globally representative paleoenvironmental proxies have permitted major advances in our understanding of the triggering mechanisms and feedbacks of the Earth system during OAE2. By reviewing these findings, this paper analyzes the detailed processes involved in the OAE2 carbon cycle perturbation and associated environmental changes to elucidate the mechanisms that forced the evolution of carbon isotopes (<italic>δ</italic>¹³C) in seawater through the event. The most remarkable feature of OAE2 is a sharp positive <italic>δ</italic>¹³C excursion recorded globally, indicating a major perturbation of carbon cycle during the event. Outgassing from massive magmatism associated with large igneous province (LIP) emplacement was the major trigger of this carbon cycle perturbation. Increased primary productivity and extensive anoxia were responsible for the long-term positive <italic>δ</italic>¹³C excursion during OAE2. Interactions between paleoenvironmental parameters, such as weathering intensity, paleotemperature, CO₂ concentration, primary productivity and seawater redox condition controlled the short-term <italic>δ</italic>¹³C changes, which are superimposed on the long-term positive excursion of the entire OAE2 event. In this paper, we present a comprehensive model for the interactions between paleoenvironment changes and <italic>δ</italic>¹³C fluctuations through OAE2. The <italic>δ</italic>¹³C curve across the OAE2 has been previously divided into six segments by Li et al. (2017), labeled as C1 to C6, among which the major carbon cycle perturbation interval spans segments C2 to C5. During segment C2, a minor negative excursion in <italic>δ</italic>¹³C with a magnitude of ~1‰ is observed in open marine settings and relates to outgassing from LIP magmatic activity. However, enhanced productivity and deoxygenation of seawater during C2, especially in restricted marine settings, promoted the preservation of organic matter and this in turn led to an increase of <italic>δ</italic>¹³C. Based on simple mass balance calculation, we suggest that a 25% increase in the burial rate of organic carbon could diminish the effects of LIP outgassing on <italic>δ</italic>¹³C values in seawater and lead to relatively stable <italic>δ</italic>¹³C values during C2 in restricted seas. Segment C3 is characterized by a rapid increase in <italic>δ</italic>¹³C values that lasted for <sc>~280 ka.</sc> Significantly enhanced biological fixation of carbon is suggested to be responsible for the positive excursion in <italic>δ</italic>¹³C. A globally recorded negative excursion in <italic>δ</italic>¹³C has been found in the middle of this segment, which is consistent with an interval of maximum weathering intensity and global cooling. We suggest that reoxygenation of bottom water owing to cooling and consequent respiration of organic carbon from enhanced continental input and organic rich sediments could be the major cause of this decrease in <italic>δ</italic>¹³C in the middle of segment C3. Within the lower part of segment C4, LIP magmatism remained active and weathering intensity decreased, which led to a return to anoxic conditions and a highly fertilized state within the oceans. Consequent high burial rate of organic matter resulted in the positive trend in <italic>δ</italic>¹³C. Meanwhile, the sediment retention potential for nutrients (e.g., P) decreased under anoxic conditions, which led to sustained high primary productivity and high <italic>δ</italic>¹³C (<italic>δ</italic>¹³C plateau) that lasted for <sc>~370 ka.</sc> After this peak in <italic>δ</italic>¹³C, nutrient supply from weathering and LIP magmatism reached a minimum threshold to maintain a productivity-anoxia feedback, which in turn led to the inevitable recovery of <italic>δ</italic>¹³C (segment C5) to pre-OAE values and the termination of OAE2.

Determining the style and provenance of magmatic activity during the Early Aptian Oceanic Anoxic Event (OAE 1a)

Large igneous province (LIP) volcanism has been proposed as a key trigger of several major climate and environmental perturbations during the Phanerozoic Aeon. Large-scale carbon emissions associated with one or both of magmatic degassing from the Greater Ontong-Java Plateau (G-OJP) and intrusion of organic-rich sediments by High Arctic LIP (HALIP) sills have been widely suggested as the trigger of the Early Aptian Oceanic Anoxic Event (OAE 1a: ~120 Ma). However, the respective roles of the two LIPs and associated carbon sources in causing this crisis remain debated. Here, six records of OAE 1a from the Pacific, Tethyan, Arctic, and South Atlantic realms are investigated, combining mercury (Hg) concentrations and osmium- (Os-) isotope ratios as proxies of LIP activity. Together with previously published datasets, the results indicate globally consistent Os-isotope evidence for LIP activity during OAE 1a, but geographically variable stratigraphic Hg trends. Clear mercury enrichments that match Os-isotope evidence of LIP activity, and suggest a Hg-cycle perturbation during the onset of OAE 1a, are documented at one Pacific site extremely proximal to the G-OJP, but not in Arctic, Tethyan or Atlantic records. This pattern highlights significant G-OJP volcanism during the onset of OAE 1a, and re-emphasises the limited potential for submarine LIP eruptions to cause Hg-cycle perturbations except in areas very proximal to source. The absence of clear Hg peaks in basal OAE 1a strata from the Arctic (or anywhere outside of the Pacific) does not support intense HALIP activity at that time, suggesting that the G-OJP was the more volcanically active LIP when OAE 1a commenced. Thus, G-OJP emissions of mantle carbon were more likely to have played a major role in initiating OAE 1a than thermogenic volatiles associated with the HALIP. A transient pulse of HALIP-related subaerial eruptions and/or thermogenic volatile emissions during the early–middle part of OAE 1a, potentially evidenced by more widespread Hg enrichments in strata from that time (including in the Arctic), might have prolonged the event. However, a non-volcanic cause of these later Hg influxes cannot be excluded. These findings challenge previous suggestions that magmatic CO 2 emissions from LIPs were incapable of causing major carbon-cycle perturbations alone, and highlight the need for further investigations to establish whether the high volume/emplacement rate of the G-OJP (potentially an order of magnitude greater than other LIPs) made it a unique case that stands in contrast to other provinces where the role of thermogenic volatiles was likely more crucial. • Hg/TOC and Os-isotope records of OAE 1a combined to determine style of LIP activity • Hg and Os evidence of volcanism only correlates near to Greater Ontong-Java Plateau • Supports submarine volcanism as the trigger of OAE 1a, rather than HALIP activity

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes

The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global extent and temporal progression of de-oxygenation is poorly constrained. Here we present carbonate associated uranium isotope data for the PETM. A lack of resolvable perturbation to the U-cycle during the event suggests a limited expansion of seafloor anoxia on a global scale. We use this result, in conjunction with a biogeochemical model, to set an upper limit on the extent of global seafloor de-oxygenation. The model suggests that the new U isotope data, whilst also being consistent with plausible carbon emission scenarios and observations of carbon cycle recovery, permit a maximum ~10-fold expansion of anoxia, covering <2% of seafloor area.

Recycled carbon degassed from the Emeishan plume as the potential driver for the major end-Guadalupian carbon cycle perturbations

Massive gas emissions (e.g., CO2, CH4 and SO2) during the formation of large igneous provinces (LIPs) have been suggested as the primary cause of dramatic climatic change and the consequent ecological collapses and biotic crises. Thermogenic carbon of crustal sediments induced by intrusive magmatism throughout the LIPs is considered as the primary trigger for environmental catastrophe including mass extinction, as illustrated in the case of the Emeishan LIP in Southwest China. Here we evaluate the Emeishan LIP to address the causal link between carbon degassing and environmental crises during the end-Guadalupian of Middle Permian. An assessment of the carbon flux degassed from recycled oceanic crust in the Emeishan plume shows that recycled oceanic crust contributed significantly to the carbon flux. Using evidence from carbonate carbon isotopic records at the Gualupian-Lopingian (G-L) boundary stratotype at Penglaitan of South China, our study suggests that carbon degassed from massive recycled components in the Emeishan plume served as a major end-Guadalupian (Middle Permian) carbon isotope excursion. The model based on the Emeishan LIP also offers new insights into the important role of recycled carbon released from other LIPs in climatic change and mass extinctions, as in the cases of the end-Permian Siberian and end-Cretaceous Deccan Traps. Our work highlights that carbon released from subducted slabs is returned to the atmosphere via upwelling mantle plumes, which could drive global climatic change and mass extinction.

Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations

The Permian/Triassic boundary approximately 251.9 million years ago marked the most severe environmental crisis identified in the geological record, which dictated the onwards course for the evolution of life. Magmatism from Siberian Traps is thought to have played an important role, but the causational trigger and its feedbacks are yet to be fully understood. Here we present a new boron-isotope-derived seawater pH record from fossil brachiopod shells deposited on the Tethys shelf that demonstrates a substantial decline in seawater pH coeval with the onset of the mass extinction in the latest Permian. Combined with carbon isotope data, our results are integrated in a geochemical model that resolves the carbon cycle dynamics as well as the ocean redox conditions and nitrogen isotope turnover. We find that the initial ocean acidification was intimately linked to a large pulse of carbon degassing from the Siberian sill intrusions. We unravel the consequences of the greenhouse effect on the marine environment, and show how elevated sea surface temperatures, export production and nutrient input driven by increased rates of chemical weathering gave rise to widespread deoxygenation and sporadic sulfide poisoning of the oceans in the earliest Triassic. Our findings enable us to assemble a consistent biogeochemical reconstruction of the mechanisms that resulted in the largest Phanerozoic mass extinction. The end-Permian mass extinction was linked with ocean acidification due to carbon degassing associated with Siberian Trap emplacement, according to boron isotopes from fossil shells and reconstruction of the carbon cycle.

Thermogenic carbon release from the Central Atlantic magmatic province caused major end-Triassic carbon cycle perturbations

The Central Atlantic magmatic province (CAMP), the end-Triassic mass extinction (ETE), and associated major carbon cycle perturbations occurred synchronously around the Triassic-Jurassic (T-J) boundary (201 Ma). Negative carbon isotope excursions (CIEs) recorded in marine and terrestrial sediments attest to the input of isotopically light carbon, although the carbon sources remain debated. Here, we explore the effects of mantle-derived and thermogenic carbon released from the emplacement of CAMP using the long-term ocean-atmosphere-sediment carbon cycle reservoir (LOSCAR) model. We have tested a detailed emission scenario grounded by numerous complementary boundary conditions, aiming to model the full extent of the carbon cycle perturbations around the T-J boundary. These include three negative CIEs (i.e., Marshi/Precursor, Spelae/Initial, Tilmanni/Main) with sharp positive CIEs in between. We show that a total of ∼24,000 Gt C (including ∼12,000 Gt thermogenic C) replicates the proxy data. These results indicate that thermogenic carbon generated from the contact aureoles around CAMP sills represents a credible source for the negative CIEs. An extremely isotopically depleted carbon source, such as marine methane clathrates, is therefore not required. Furthermore, we also find that significant organic carbon burial, in addition to silicate weathering, is necessary to account for the positive δ13C intervals following the negative CIEs.

Formation of microbial organic carbonates during the Late Jurassic from the Northern Tethys (Amu Darya Basin, Uzbekistan): Implications for Jurassic anoxic events

Abstract The Late Jurassic was a period of major global carbon cycle perturbations with episodes of anoxia leading to regional accumulation of organic matter in sediments worldwide. The Tubiegatan section (SW Gissar Mountains, Uzbekistan) located in the Northern Tethys, shows atypical organic-rich limestone and marl deposits (up to 6% of total organic carbon) marked by pronounced negative excursions of δ13Ccarb (amplitude of ca. 12‰) and δ13Corg (amplitude of ca. 4‰) recorded during the Middle Oxfordian (Transversarium Zone). A transdisciplinary approach including sedimentology, palynofacies characterization, mineralogy, organic and inorganic geochemistry was carried out to elucidate the origin of these organic-rich deposits. Highest TOC are measured in nodular limestones, and lowest δ13Ccarb values in thinly laminated facies consisting in alternances of infra-millimeter-thick organic and carbonate laminae. In the latter, the presence of organic-carbonate peloids and of possible remnants of exopolymeric substances associated with clay indicate that these structures are probably mineralized laminated benthic microbial mats (i.e., stromatolites). Rock-Eval pyrolysis coupled to palynofacies analyses point to a dominant altered marine organic matter of probable algal/microbial origin, with subordinate continental phytoclasts inputs in the upper part of the organic-rich interval. Trace elements (U/Th, V/Cr and Mo/Al ratios) indicate two anoxic episodes coinciding with the highest TOC, punctuated by dysoxic periods. Such O2-depleted conditions have allowed the preservation and probably the development of anaerobic microbial communities in the microbial mats. In these latter, sulfate reduction probably had a significant contribution to the production of carbonates, which would explain the precipitation of pyrite and the relatively low δ13Ccarb values. The progressive decrease then disappearance of kaolinite from the base of the organic-rich interval, is interpreted as a progressive aridification of the Amu Darya Basin during the Transversarium Zone, culminating with the progradation of a large-scale gypsum sabkha overlying the organic deposits. Overall, the organic-rich deposits could record the onset of the disconnection of the Amu Darya Basin from the open sea to the south, induced by compression and subsequent uplifts in the Afghan and Central Iranian blocks. The elevated evaporation, coupled with the presence of hydrological barriers (such as coral reefs) could have led to the formation of local to regional anoxic conditions in the Amu Darya Basin. Similar microbial organic accumulations are recently known throughout the Tethys (e.g., Arabian Plate, Western Europe) and from other oceans (e.g., Central Atlantic, Pacific) during the Late Jurassic, suggesting common controlling factors. The increase of organic matter storage worldwide coupled with potential methane release could have in turn induced major perturbations of the carbon cycle during the Oxfordian-Kimmeridgian interval. The relatively shallow anoxia model proposed in this study contrasts with the well-known organic carbon-rich pelagic models proposed for the Jurassic anoxia (e.g., Toarcian, Kimmeridgian) and Cretaceous OAEs.

Paired organic matter and pyrite δ34S records reveal mechanisms of carbon, sulfur, and iron cycle disruption during Ocean Anoxic Event 2

The sulfur (S) isotope composition of pyrite in the sedimentary record has played an important part in our understanding of the evolution of biogeochemical cycles throughout Earth history. However, the kinetics of pyritization are complex and depend strongly on the reactivity and mineralogy of available iron. As a second major sink for sulfide in anoxic sediments, organic matter (OM) provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide. To first order, roughly parallel pyrite and OM δ34S profiles reflect changes in sulfide, while independent patterns require alternative explanations, including changes in iron availability or OM characteristics. We apply this framework to Ocean Anoxic Event 2 (OAE-2, ∼94 Mya), a period of enhanced burial of reduced C and S (in OM and pyrite) that has been associated with an expansion of reducing marine conditions. We present paired S-isotope records for pyrite and OM along with profiles of OM S:C ratio and S redox speciation from four well-characterized lithologic sections with a range of depositional environments (Pont d'Issole, Cismon, Tarfaya Basin, and Demerara Rise) to reconstruct both local redox structure and global mechanisms impacting the C, S and Fe cycles around OAE-2.OM sulfurization appears to be a major control on OM preservation at all four sites. Similar to modern anoxic environments, there is a positive correlation between OM S:C ratios and TOC concentrations for sites with more reducing conditions, implying a link between OM sulfurization and burial. At consistently anoxic sites like Tarfaya Basin and Demerara Rise, strongly sulfurized OM with a consistent S redox speciation and S-isotope composition most likely formed rapidly in sinking particles before, during, and after OAE-2. Particle-hosted OM sulfurization may therefore have been a central mechanism facilitating the massive burial of OM in anoxic environments during this and other periods of enhanced global carbon burial. At the same time, a nearly 25‰ negative shift in the δ34S values of pyrite – but not OM – occurs at multiple, globally distributed sites prior to the onset of OAE-2, indicating slower pyritization reactions that likely reflect changes in iron delivery due to expanding regional or global anoxia. The combination of pyrite and organic S isotopes thus provides novel constraints on the interwoven cycles of carbon, iron, and sulfur across a major carbon cycle perturbation.

Changes in CO2 during Ocean Anoxic Event 1d indicate similarities to other carbon cycle perturbations

Past greenhouse intervals of the Mesozoic were repeatedly punctuated by Ocean Anoxic Events (OAEs), major perturbations to the global carbon cycle and abrupt climate changes that may serve as relevant analogs for Earth's greenhouse gas-forced climate future. The key to better understanding these transient climate disruptions and possible CO2-forced tipping-points resides in high-resolution, precise, and accurate estimates of atmospheric CO2 for individual OAEs. Here we present a high-temporal resolution, multi-proxy pCO2 reconstruction for the onset of mid-Cretaceous (Albian–Cenomanian Boundary) OAE1d. Coupling of pCO2 estimates with carbon isotopic compositions (δ13C) of charcoal, vitrain, and cuticle from the Rose Creek Pit (RCP), Nebraska, reveals complex phasing, including a lag between the well-documented negative δ13C excursion defining the onset of OAE1d and the CO2 increase. This lag indicates that increased CO2 or other C-based greenhouse gases may not have been the primary cause of the negative excursion. Our study reveals a pCO2 increase within the interval of the negative δ13C excursion, reaching a maximum of up to ∼840 ppm (95% confidence interval −307 ppm/+167 ppm) toward its end. The reconstructed magnitude of CO2 increase (∼357 ppm) is similar to that of Late Cretaceous OAE2 but of smaller magnitude than that of other major carbon cycle perturbations of the Mesozoic assessed via stomatal methods (e.g., the Toarcian OAE [TOAE], Triassic–Jurassic boundary event, Cretaceous–Paleogene Boundary event). Furthermore, our results indicate a possible shared causal or developmental mechanism with OAE1a and the TOAE.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

Glendonites track methane seepage in Mesozoic polar seas

During the Phanerozoic, Earth has experienced a number of transient global warming events associated with major carbon cycle perturbations. Paradoxically, many of these extreme greenhouse episodes are preceded or followed by cold climate, perhaps even glacial conditions, as inferred from the occurrence of glendonites in high latitudes. Glendonites are pseudomorphs of ikaite (CaCO3·6H2O), a hydrated carbonate mineral increasingly stable at low temperatures. Here, we show that methane seepage and oxidation provide an overriding control on Mesozoic glendonite formation (i.e., ikaite fossilization). Geochemical and petrological analyses of 33 Early Jurassic to Early Cretaceous glendonites from five sections in Siberia (Russia) reveal that most of their infilling carbonate phases are reminiscent of methane-derived authigenic carbonates. Bulk glendonites and surrounding sediments exhibit exceptionally high and low carbon isotope values (+20‰ to –45‰ VPDB [Vienna Peedee belemnite]), typical for carbon sources linked to methane generation and oxidation. Gas inclusion data confirm the presence of methane and longer-chain hydrocarbon gases, suggesting a thermogenic source for the methane. Glendonite-bearing layers can be traced for hundreds of kilometers, suggesting widespread trapping of methane in the sub-seafloor during the Jurassic. As such, glendonites constitute an unexplored archive for detecting past episodes of methane release and oxidation in polar settings.

The Monterey Event within the Central Mediterranean area: The shallow‐water record

AbstractThe middle Miocene is an important time to understand modern global climate evolution and its consequences on marine systems. The Mid‐Miocene Climatic Optimum (between 17·0 Ma and 13·5 Ma) was the warmest time interval of the past 35 million years during which atmospheric CO2 concentrations were lower than today. In the Mid‐Miocene Climatic Optimum, a significant carbon cycle perturbation occurred, expressed as a last long‐term positive carbon isotope shift, known in literature as the Monterey Carbon Isotope Excursion and recorded in open‐ocean settings. In this work, the lower to middle Miocene carbon isotope records from three different domains of the Central Mediterranean are analysed with the aim of identifying the local carbonate platform response to the major global carbon cycle perturbation of the Monterey Event. Carbon and oxygen isotope ratios have been measured on samples belonging to three different stratigraphic sections, two of them are representative of shallow‐water settings (Latium–Abruzzi and Apula platforms), and the latter of a hemipelagic setting (Umbria–Marche Basin). A well‐defined Monterey Carbon Isotope Excursion is recorded also in these shallow‐water sections. Despite their expected problematic stratigraphic constraints, a reliable age model is provided by calcareous nannofossil biostratigraphy and strontium isotope stratigraphy. In both of the carbonate platform successions examined, the Monterey Carbon Isotope Excursion coincides with a spread of bryozoans over other carbonate‐producing biota. The high productivity of the bryozoan‐dominated factory in the aphotic zone had an important control on the platform depositional profile. The high rates of sediment production in the deeper aphotic and oligophotic zones produced a depositional profile of a low‐angle ramp.

Orbital control on the timing of oceanic anoxia in the Late Cretaceous

Abstract. The oceans at the time of the Cenomanian–Turonian transition were abruptly perturbed by a period of bottom-water anoxia. This led to the brief but widespread deposition of black organic-rich shales, such as the Livello Bonarelli in the Umbria–Marche Basin (Italy). Despite intensive studies, the origin and exact timing of this event are still debated. In this study, we assess leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world by providing a 6 Myr long astronomically tuned timescale across the Cenomanian–Turonian boundary. We procure insights into the relationship between orbital forcing and the Late Cretaceous carbon cycle by deciphering the imprint of astronomical cycles on lithologic, physical properties, and stable isotope records, obtained from the Bottaccione, Contessa and Furlo sections in the Umbria–Marche Basin. The deposition of black shales and cherts, as well as the onset of oceanic anoxia, is related to maxima in the 405 kyr cycle of eccentricity-modulated precession. Correlation to radioisotopic ages from the Western Interior (USA) provides unprecedented age control for the studied Italian successions. The most likely tuned age for the base of the Livello Bonarelli is 94.17 ± 0.15 Ma (tuning 1); however, a 405 kyr older age cannot be excluded (tuning 2) due to uncertainties in stratigraphic correlation, radioisotopic dating, and orbital configuration. Our cyclostratigraphic framework suggests that the exact timing of major carbon cycle perturbations during the Cretaceous may be linked to increased variability in seasonality (i.e. a 405 kyr eccentricity maximum) after the prolonged avoidance of seasonal extremes (i.e. a 2.4 Myr eccentricity minimum). Volcanism is probably the ultimate driver of oceanic anoxia, but orbital periodicities determine the exact timing of carbon cycle perturbations in the Late Cretaceous. This unites two leading hypotheses about the inception of oceanic anoxia in the Late Cretaceous greenhouse world.