δ13C Excursion Research Articles

Benthic response to the strong Silurian climatic fluctuations—implications from Gotland (Sweden)

Climatically the Silurian was a most unstable period, as it is shows several strong δ13C excursions, with the Lau excursion being the strongest short-lived positive δ13C excursion of the entire Phanerozoic. The causes of these excursions remain a topic of debate. In order to determine how benthic communities responded to these climatic perturbations, a microfacies analysis of carbonate rocks from the Silurian period (Late Telychian to the uppermost Ludfordian) of Gotland was conducted. The development of reefs was significantly enhanced during periods characterized by carbon isotope excursions, compared to time intervals without. In periods of elevated δ13C values (≥ 3 ‰) the abundance of bryozoans and microbes increases, and in some areas, they form reefs (bryostromatolites), while corals and stromatoporoids were the main reef builders in periods of lower isotope ratios (0–3 ‰). Furthermore, a significant increase of phosphatic components, gypsum pseudomorphs and the microproblematicum Palaeomicrocodium is observed in times of increased isotope ratios. A model is proposed that explains the changes in the benthic communities over time. It is based on previously published models and explains the geochemical processes from coastal areas to the open ocean. Changes are assumed from a humid climate in times without isotope excursions to an arid climate in times of isotope excursions. It emphasizes processes that may have led to the strong δ13C shifts during the Silurian period, and includes the phosphorus cycle in sea water as an important factor in the development of the benthic communities.

First multi-proxy chronostratigraphy of the lower Cambrian Byrd Group, Transantarctic Mountains and correlation within East Gondwana

Antarctica and Australia were sutured together at the equator during the major pulse of animal biodiversification associated with the Cambrian radiation. However, the lack of detailed systematic chemostratigraphic and biostratigraphic sampling of lower Cambrian sedimentary successions from Antarctica has significantly impeded precise age determination and correlation with Cambrian strata on other palaeocontinents. This study is the first to present integrated, simultaneously sampled biostratigraphic and chemostratigraphic (δ13C isotopes) data from the same measured stratigraphic sections through the lower Cambrian Byrd Group in the Transantarctic Mountains. Shelly fossil assemblages (brachiopods, tommotiids, molluscs, bradoriids, trilobites) from the Holyoake Range and Churchill Mountains facilitate direct correlation with the Dailyatia odyssei Zone of South Australia (Cambrian Stages 3–4), and trilobites provide strong correlation between the Starshot Formation and the Cymbric Vale Formation in western New South Wales. A new ID-TIMS radiometric date of 514.96 ± 0.16 Ma from a tuff in the lower Cymbric Vale Formation is similar to dates from tuff beds in the Third Plain Creek Member of the Mernmerna Formation in the Flinders Ranges, providing an important absolute-age tie point between these lower Cambrian successions. Chemostratigraphic data from the upper Shackleton Limestone in the Holyoake Range capture a negative δ13C excursion that can be correlated to negative values within the multi-peaked MICE (cycles V–VIII in Siberia). Integrated faunal and chemostratigraphic data indicate a Cambrian Stages 3–4 age, giving robust chronostratigraphic context for the upper Shackleton Limestone–Holyoake Formation–Starshot Formation succession for the first time, permitting reconstruction of the depositional history of the lower Cambrian of Antarctica and global correlation of Byrd Group strata.

Integrated record of the Late Lutetian Thermal Maximum at IODP site U1508, Tasman Sea: The deep-sea response

The Late Lutetian Thermal Maximum (LLTM) was a transient and brief global warming event recorded in the middle Eocene, at 41.52 Ma. The biotic response to the LLTM has been documented at only a few marine sites so far. Here, we present the first record of deep-sea benthic foraminiferal assemblage changes during the LLTM in the southwest Pacific at International Ocean Discovery Program Hole U1508C (1609 m water depth) in the Tasman Sea. The LLTM coincides with a negative excursion in bulk sediment δ13C (0.47‰) and benthic foraminifera δ13C (0.36‰), with changes in the relative abundance of benthic foraminiferal species and in the deep-water organic geochemistry. The decrease in diversity of the assemblages indicates environmental stress during the event, potentially linked to oxygen deficiency, as evidenced by the occurrence of dysoxic taxa (e.g. Lenticulina spp., Turrillina brevispira). Although calcareous taxa dominate, the presence of corrosion-resistant species and poorly preserved foraminiferal tests suggest slightly CaCO3-corrosive bottom waters, but no dissolution was evident. We suggest the shallowing of the thermocline and enhanced water column stratification at this site during the LLTM.

Late Cretaceous marine flooding and installation of a mangrove swamp environment in the Prebetic (Betic External Zones, SE Spain)

AbstractThe Cenomanian (Upper Cretaceous) sedimentary rocks of the External Prebetic (Betic Cordillera) in the Sierra de Montearagón-Carcelén (SE Spain) record the transgression and flooding of continental deposits and the subsequent development of an inner shallow carbonate platform. The transgressive surface is densely colonized by infaunal trace-makers of the Glossifungites ichnofacies (Gastrochaenolites and Glossifungites), which indicate sediment starvation and erosion as well as colonization of a firmground. The first deposits were a thick calcarenite bar (2 to 11 m) with large-scale cross-bedding pointing to a high-energy environment in a shallow carbonate platform. The record of the Rosselia ichnofacies (Rosselia and Ophiomorpha) confirms a loose sandy bottom under high-energy conditions. The overlaying stratigraphic succession is characterized by subtidal laminated marly limestones and intertidal limestones with rhizoliths interpreted as a mangrove swamp environment. These facies are organized in shallowing-upwards sedimentary sequences. An episodic high-energy event is represented by bivalve-rich limestones representing shell lags of disarticulated valves. This facies could be related to a climatic perturbation evidenced by a negative excursion of δ13C and δ18O. The successive shallowing-upwards sedimentary sequences persisted thanks to subsidence related to the tectonically controlled depocenters located in the south of the Sierra de Montearagón-Carcelén. The differential subsidence, evidenced by the increasing thickness towards the south of the sector under study, suggests the activity of listric faults that controlled the depocenters. This is the first report of the Cenomanian transgression in this sector of the External Prebetic and the first record of the very extensive mangrove swamp that developed close to the land that emerged.

The Early to Late Cretaceous (Albian–Cenomanian) transition in Putumayo, Colombia: a biostratigraphic and carbon isotope stratigraphic correlation for northwestern South America

Despite the rich geological and paleontological record of Colombia, the Putumayo region near Ecuador remains one of the least explored regions in the equatorial Neotropics, mainly due to its thick vegetation, deep rock weathering, soil formation, and ground cover, geographic remoteness, and inaccessibility of well-exposed outcrops. This has resulted in limited comparisons with neighboring basins and thus the generation of more comprehensive biostratigraphic correlations with western and northern South America, as well as other paleobiogeographic regions (e.g., Tethys, Western Interior Seaway). Here, we present 67 occurrences of uppermost Lower Cretaceous to lowermost Upper Cretaceous ammonoids and other macrofossils (e.g., bivalves, decapod crustaceans, osteichthyan fish remains, plant remains), as well as a δ13Corg chemostratigraphic curve derived from 105 samples from the middle Albian of the uppermost Caballos Formation and the upper Albian to lower Cenomanian lower Villeta Formation, collected in situ from a stratigraphic section exposed along the Mocoa–San Francisco Road, Putumayo, Colombia, here called the San Francisco section. The chemostratigraphic, carbon-isotope curve for the region shows a great similarity with the shape of the accepted curve for the late Albian Oceanic Anoxic Event 1d, which is characterized by a worldwide long positive δ13C excursion anomaly. The new biostratigraphic and isotopic data serve as a point of reference to compare the Putumayo Basin with neighboring basins (e.g., Marañon Basin in Perú, Oriente Basin in Ecuador, Upper Magdalena Valley Basin in Colombia), and thus the generation of more comprehensive upper Lower Cretaceous and lower Upper Cretaceous biostratigraphic and chemostratigraphic correlations for northwestern South America.

Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

Multiple abrupt warming events ("hyperthermals") punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ13C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation's sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ13C and δ18O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ13C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ13C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ13C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ13C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

Early Mississippian global δ13C excursion is not a diagenetic artifact

Shallow-water platform carbonate δ13C may provide a record of changes in ocean chemistry through time, but early marine diagenesis and local processes can decouple these records from the global carbon cycle. Recent studies of calcium isotopes (δ44/40Ca) in shallow-water carbonates indicate that δ44/40Ca can be altered during early marine diagenesis, implying that δ13C may also potentially be altered. Here, we tested the hypothesis that the platform carbonate δ13C record of the Kinderhookian-Osagean boundary excursion (KOBE), ∼353 m.y. ago, reflects a period of global diagenesis using paired isotopic (δ44/40Ca and clumped isotopes) and trace-element geochemistry from three sections in the United States. There is little evidence for covariation between δ44/40Ca and δ13C during the KOBE. Clumped isotopes from our shallowest section support primarily sediment-buffered diagenesis at relatively low temperatures. We conclude that the δ13C record of the KOBE as recorded in shallow-water carbonate is consistent with a shift in the dissolved inorganic carbon reservoir and that, more generally, ancient shallow-water carbonates can retain records of primary seawater chemistry.

Terrestrial records of two hyperthermal events in the Cretaceous-Paleogene boundary suggest different control mechanisms

Two hyperthermal events with different carbon cycle perturbations occurred across the Cretaceous-Palaeogene boundary, i.e., the late Maastrichtian Warming Event and the Early Danian Dan-C2 event. However, the roles played by Deccan volcanism and orbital forcing in these two hyperthermal events are still debated. Here we obtain a terrestrial δ13Ccarb record in the Nanxiong Basin (southeastern China) and compare it with marine records. The results show that both hyperthermal events can be well distinguished and that the Dan-C2 event is characterized by a typical hyperthermal event. In addition, the δ13C excursion during the late Maastrichtian Warming Event was more muted and prolonged than that during the Dan-C2 event, and the short-eccentricity cycle disappeared in the marine record during the late Maastrichtian Warming Event, indicating that Deccan volcanism perturbed the carbon cycle during the late Maastrichtian Warming Event, while the Dan-C2 event was less influenced by volcanic perturbation.

Stratigraphic evidence for a major unconformity within the Ediacaran System

The subdivision and correlation of the Ediacaran System within the geologic timescale is desirable, but challenging. The Ediacaran Doushantuo Formation (∼635–550 Ma) in South China is one of the most important sedimentary successions for studying the co-evolution of Earth's environment and life at that time. However, its stratigraphic continuity is confusing and deserves scrutiny. In this study we evaluate stratigraphic continuity of the Doushantuo Formation from a key section deposited in an alleged deep-water “basinal” setting. Using litho-, chemo-, and chrono-stratigraphic data anchored by two new CA-ID-TIMS U-Pb zircon dates of 629.7 Ma and 556.4 Ma from two tuff layers, we reveal that this section contains a cryptic subaerial unconformity with a >44 Myr gap in the sedimentary record. The unconformity occurs immediately prior to the EN3/Shuram δ13C excursion in the mid-Ediacaran, and can be traced regionally and may be correlative to other mid-Ediacaran unconformities globally. We suggest that its development is related to glacio-eustatic sea-level changes during the mid-Ediacaran. This finding provides a critical stratigraphic boundary for future subdivision and correlation of the Ediacaran System in South China, and we advocate for a reconsideration of the completeness of the Ediacaran stratigraphic record in this area.

Doubthouse climate influences on the carbon cycle and organic matter enrichment in lacustrine basins: Astrochronological and paleontological perspectives

The Doubthouse climate is a typical cooling climate between the Early Eocene Climate Optimum (EECO) and the Middle Eocene Climate Optimum (MECO). The precise timing of this climate’s occurrence and its effects on the carbon cycle and organic matter enrichment remain unknown. This work studied the upper 4th Mbr of Shahejie Fm in the Eocene Dongpu Depression (DD), Bohai Bay Basin, to reveal the controlling effect of the Doubthouse climate on organic matter enrichment. The astronomical time scale (ATS) of the upper 4th Mbr of Shahejie Fm in the DD is established as 41.38–43.31 Ma (±0.1 Myr) using a natural gamma logging curve. The simulated sedimentation rate is 14.6 cm/kyr. The organic matter is mostly from freshwater algae such as green algae, cyanobacteria, and dinoflagellates. The positive excursion (4.07 ‰) of δ13C corresponds to the increased weathering intensity and the humid climate, which benefit algae growth. The paleo-productivity and sedimentation rates during the Doubthouse climate period increased. Besides, the cooling climate inhibited the respiration of the algae. Therefore, CO2 intake > organic carbon consumption was recorded as δ13C excursion. Hence, we propose that the doubthouse climate is the controlling factor of organic matter enrichment.

Paleoenvironmental and Paleoecological Dynamics of the U.S. Atlantic Coastal Plain Prior to and During the Paleocene-Eocene Thermal Maximum

Abstract We studied the rapid paleo-environmental changes and the corresponding biotic responses of benthic foraminifera of a shallow shelf site during the late Paleocene and the Paleocene-Eocene Thermal Maximum (PETM). The PETM is globally characterized by a negative δ13C excursion in marine and terrestrial sediments. Isotope data from the Atlantic Coastal Plain from the South Dover Bridge core, Maryland, show an additional small δ13C excursion just below the base of the PETM: the “pre-onset excursion” (POE). The benthic foraminiferal and coupled grain-size record of the late Paleocene indicates a well-oxygenated, current-dominated environment with a stable, high food supply. During the POE, bottom currents become subdued and finer-grained sediment accumulation increased. These changes are partially reversed after the end of the POE. Before the PETM the river influence increases again, food supply becomes more pulsed and the benthic taxa, typically connected to the PETM, start to appear in those gradually warming conditions. During the PETM, the environment shifts to a river-dominated one, with strongly reduced currents. The low-diversity PETM fauna thrives under episodic low-oxygen conditions, caused by river-induced stratification, while the Paleocene assemblage nearly vanishes from the record. Gradually the environment begins to recover, the grain size shows an uptick in bottom currents and pre-PETM foraminifera become more abundant again, indicating increased oxygen levels and a more stable food supply. While the overall environmental shifts at South Dover Bridge fit within the observations across the shelf, the POE related insights are so far unique. Our bathymetric reconstructions show an outer neritic paleodepth (∼100 m) during the Paleocene, with a modest sea level rise in the core phase of the PETM, which is subsequently reversed during the recovery phase.

Apparent Diachroneity of Calcareous Nannofossil Datums During the Early Eocene in the High‐Latitude South Pacific Ocean

AbstractThe late Paleocene to early Eocene interval is characterized by a series of carbon perturbations that caused transient warming (hyperthermal) events, of which the Paleocene‐Eocene Thermal Maximum (PETM) was the largest. These hyperthermals can be recognized in the pelagic sedimentary record as paired negative δ13C and δ18O excursions, in addition to decreased calcium carbonate and increased iron content caused by carbonate dissolution. However, current data are predominantly sourced from the equatorial‐to subequatorial regions. Here we present a new high‐latitude late Paleocene—early Eocene record, recovered during International Ocean Discovery Program (IODP) Expedition 378 on the Campbell Plateau off New Zealand, in the southwest Pacific Ocean. To construct an age model, we correlated our chemostratigraphic and biostratigraphic data to existing astronomically‐tuned age models from Walvis Ridge (South Atlantic Ocean) and Demerara Rise (equatorial Atlantic Ocean). Our results indicate that the Site U1553 composite section spans ∼7 million years of the latest Paleocene to early Eocene (50.5–57.5 Ma), and preserves many of the early Eocene hyperthermals; including a PETM interval that is more expanded than elsewhere in this region. However, construction of the age model also revealed discrepancies between the chemostratigraphic and biostratigraphic tie points used for correlation. This is likely due to latitudinal diachroneity in the calcareous nannofossil biostratigraphic datums, which are primarily based on low‐to mid‐latitude assemblages. Therefore, our study highlights the need to establish a revised calcareous nannofossil biozonation that is more appropriate for high‐latitude age models.

Paleoproterozoic positive δ13C excursion recorded in the carbonatite of southeast Tarim Craton, northwest China

The Paleoproterozoic Aketashitage carbonatite of northwest China is one of few high-δ13C carbonatites worldwide. Its absence of associated alkaline silicate rocks positions it as an intriguing case for probing the complex formation process of carbonatites. To provide further constraints on the genesis of these carbonatites, we have executed an exhaustive study encompassing petrography, geochemistry, geochronology, and radiogenic-stable isotope analysis. The Aketashitage carbonatites exhibit complex intrusive relationships with surrounding wall-rocks and house a diverse array of xenoliths, unmistakably signifying their igneous origin. Their rare earth elements (REE) diagrams are characterized by a nearly flat pattern and pronounced positive Eu anomalies, indicating a significant impact from hydrothermal fluids. These carbonatites are notably deficient in REEs, Ba, Sr, U, Th, Nb, and Ta. Their 143Nd/144Nd ratios and εNd (t) values range from 0.5109 to 0.5111 and −8.13 to −6.11, respectively. Remarkably, their δ13CV-PDB values (V-PDB, Vienna Pee Dee Belemnite) are exceptionally elevated, ranging from +13.31 to +15.48 ‰, thereby setting them apart from typical mantle-derived carbonatites, which exhibit δ13C values below 0 ‰. These comprehensive geochemical traits diverge significantly from those of mantle-derived carbonatites but share striking resemblances with crustal-origin carbonate rocks. Moreover, petrographic and mineralogical analyses reveal indicators of regional metamorphism within these carbonatites, proposing their possible for formation through the anatexis of high-grade marbles during the peak metamorphic event of the Paleoproterozoic era. Coupled with determined formation ages, these results imply a potential connection of Aketashitage carbonatites formation to the collision and amalgamation of the supercontinent Columbia. Moreover, the notable δ13C enrichment observed in these carbonatites presents substantial evidence for the impactful reach of the global Lomagundi-Jatuli event on the Tarim Craton.

Faunal and paleoenvironmental changes at a Cambrian (Jiangshanian; Steptoean–Sunwaptan boundary interval) trilobite extinction event, in contrasting deep- and shallow-subtidal settings, Nevada and Oklahoma

Abstract Successions in Oklahoma and Nevada record trilobite extinction and replacement near the Steptoean–Sunwaptan boundary in inner-shelf and outer-shelf settings, respectively. Prior to the extinctions, different trilobite biofacies occupied these environments, but faunas became similar in composition across the environmental gradient in the overlying I. “major” and Taenicephalus zones. Faunal changes in the outer shelf at the I. “major” Zone begin at a drowning unconformity that brought dark, laminated calcisiltite and silty lime mudstone above a subtidal carbonate succession. In contrast, Oklahoma shows facies continuity in a succession of tidally influenced bioclastic carbonates. Loss of genera and a dramatic abundance “spike” of Irvingella are features of the I. “major” Zone in both regions. Turnover of biofacies occurred in the succeeding Taenicephalus Zone, with both the inner and outer shelf dominated by Orygmaspis (Parabolinoides). Blooms of orthid brachiopods in shallow water settings are underappreciated signals of faunal change in the extinction interval. Although absent from the outer shelf in Nevada, orthids became abundant enough in Oklahoma to form shell beds in the lower Taenicephalus Zone, but became rare in overlying strata. Carbon isotope stratigraphy includes a modest positive δ13C excursion that peaks in the extinction interval at 1.4‰ (Oklahoma) and 2.2‰ (Nevada), which is congruent with previous reports from Utah and Wyoming. Although consistent with regional upwelling of dysoxic waters, the absence of sedimentary evidence for significant environmental change over much of the shelf is problematic. This suggests that physical environmental change acted primarily as a catalyst for cascading ecological and biogeographic effects.

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

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

Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution

High resolution stable isotopic (C and O) and geochemical studies of Paleoproterozoic carbonates were carried out in the Umra sub-basin of the Aravalli Supergroup, Rajasthan, India. All the sections, Umra-Dedkiya (UM), Umra-Jampa (BD), and Matoon (MU and GU), have yielded characteristic Lomagundi type positive values, with δ13Ccarb > 4 ‰ VPDB (maximum up to 6.86 ‰). Samples of two sections (UM and BD) have also yielded several distinct negative δ13Ccarb values (up to −6.14 ‰ VPDB). Geochemical validation of trace element and REE data indicate both these types of excursions to be of pristine character, with no signature of organic-remineralisation or hypersaline depositional condition. Mineralogical assemblages lack hypersalinity indicator minerals, such as Na-tourmaline, Na-Albite or scapolite, in the positive δ13C bearing carbonate samples. This supports the inference regarding the observed positive δ13C excursions mimicking the global Paleoproterozoic Lomagundi event. Chemostratigraphic correlation within the basin and with one of the global strato-type Paleoproterozoic carbonate unit from Paso Severino Formation, Uruguay, indicate interruptions in the initial part of the Lomagundi episode marked by several negative δ13C excursion events before achieving a stable positive δ13C character. This phenomenon is also evident from variation in paleoproductivity proxies, such as TOC, TN, Ba and P values, consistent with δ13C fluctuations. Mathematical model calculations have corroborated such a conclusion. All these observations motivated us to present a modified version of the global Paleoproterozoic δ13Ccarb curve and a six stage basin evolution model to explain the evolution of δ13C vis-à-vis variation in atmospheric oxygen.

Aptian–Albian sequence stratigraphy, biostratigraphy, chemostratigraphy, and chronostratigraphy: Sonoran shelf and Tamaulipas Basin, Mexico

The Late Jurassic extensional Bisbee Basin in northwestern Mexico and southwestern United States lay between the Pacific magmatic arc and subduction zone and the cratonic Comanche shelf. Aptian to late Albian fluvial, deltaic, and shallow marine sediments and patch reefs of the Bisbee Group filled the basin. The upper Barremian(?)–lower Aptian Cerro de Oro Formation and its equivalent Rancho La Colgada Formation are overlain by the Morita, Mural, Cintura, and La Juana formations. The paralic Morita Formation and the marine Mural Limestone grade southeast into the marine Agua Salada and Lampazos formations, which correlate with deep-water facies in the Chihuahua trough and Tamaulipas Basin. Three late Aptian to middle Albian long-term depositional cycles were approximately 2.5 myr in duration.Foraminifers, calcareous algae, bivalves, rudists, corals, dinoflagellates and scarce ammonites compose the Mural Limestone biostratigraphy. First-occurrence datums of Aptian–Albian benthic Foraminifera are integrated with planktonic Foraminifera and rudists and define biostratigraphic zones that correlate with the Comanche shelf. In the Chihuahua trough and Tamaulipas Basin planktonic Foraminifera and nannofossils identify global zones. Stage boundaries on the Sonora shelf are correlated by a comprehensive biostratigraphic database.Carbon isotope chemozones correlate with Tethyan chemostratigraphy. The lower Aptian OAE 1a and uppermost Aptian–lowest Albian OAE1b chemozone are in the Lampazos section. The negative δ13C excursions and fossil evidence correlate with global Jacob, Kilian, Paquier, and Leenhardt levels. Low-oxygen water masses flooded the shallow shelf and even affected coral-rudist buildups.

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.

Meteoric diagenesis influenced by East Asian Summer Monsoon: A case study from the Pleistocene carbonate succession, Xisha Islands, South China Sea

The East Asian Monsoon (EAM) has significantly influenced various depositional and diagenetic processes in carbonate sediments in much of East and South Asia during the Cenozoic. The relationship between the EAM and marine carbonate platforms, which are widely developed in this region, however, remains largely unknown. Based on petrographic, mineralogical, and geochemical analyses of the carbonate succession (∼237 m thick) on the Xisha Islands, South China Sea, meteoric diagenesis and its relationship with the East Asian Summer Monsoon (EASM) during the Pleistocene were studied. Various petrographic features, stable isotopes (δ13C, δ18O), and trace elements (Sr, Fe, Mn, Rare Earth Elements and Y) indicate that this succession was extensively altered by meteoric diagenesis, producing four exposed surfaces embedded in the shallow coral reef sediments. Given this data, we suggest that the meteoric diagenesis that affected this succession occurred during periods of low sea-level, and was potentially related to the relative intensity of the EASM. During strong EASM periods, particularly during the Mid-Pleistocene Transition ∼0.9 Ma, meteoric diagenesis was intense, as suggested by the complete alteration of unstable minerals, extreme dissolution, the presence of reddish-brown iron oxides, and a large negative excursion of δ13C (−6.2‰). The influence of the EASM on meteoric diagenesis of Pleistocene carbonate platforms can be identified throughout the South China Sea. This study sheds light on the relationship between meteoric diagenesis and paleo-climate conditions in carbonate platform regions that are influenced by monsoons.

Using chemostratigraphy and biostratigraphy methods to define the unconformity underlying the Upper Ordovician Pagoda Formation in the Upper Yangtze Platform

The discovery of hydrocarbon in the Pagoda Formation in South China has received attention from the petroleum industry. This research paper identifies the discontinuity surface underlying the Pagoda Formation. The Upper Ordovician strata were correlated in different regions using the conodont biostratigraphy, and δ13C and δ18O isotope stratigraphy. The biostratigraphic result illustrates that three conodont zones can be established in each study section, and one to three conodont zones are missing in the lower Pagoda Formation and the strata underlying the Pagoda Formation of the Upper Yangtze Platform. Moreover, the chemostratigraphic result highlighted that the development of the lower δ13C isotope positive excursion events in the Xiaogangwan and Liangcun areas was less complete than that in the Jiaodingshan area. Combined with the cyclostratigraphic results, the development of the δ13C isotope positive excursion is not complete. The development of the δ13C excursion event is more complete in the south and east than in the center and north. This demonstrates that the Pagoda Formation began its deposition at different times and the transgression process may have advanced from the southeast to the north and that the contact between the Pagoda Formation and the underlying formation is not conformable. It is proposed that the Late Ordovician in the South China Plate was affected by the tectonic movement and the cooling paleoclimate at that time, which resulted in the discontinuity between the Pagoda Formation and the underlying strata. The unconformity could be the channel for hydrocarbon migration or for the high-quality oil and gas reservoirs being developed and preserved under the unconformity surface.