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We present an updated time frame for the 30 m thick late Miocene sedimentary Trachilos section from the island of Crete that contains the potentially oldest hominin footprints. The section is characterized by normal magnetic polarity. New and published foraminifera biostratigraphy results suggest an age of the section within the Mediterranean biozone MMi13d, younger than ~ 6.4 Ma. Calcareous nannoplankton data from sediments exposed near Trachilos and belonging to the same sub-basin indicate deposition during calcareous nannofossil biozone CN9bB, between 6.023 and 6.727 Ma. By integrating the magneto- and biostratigraphic data we correlate the Trachilos section with normal polarity Chron C3An.1n, between 6.272 and 6.023 Ma. Using cyclostratigraphic data based on magnetic susceptibility, we constrain the Trachilos footprints age at ~ 6.05 Ma, roughly 0.35 Ma older than previously thought. Some uncertainty remains related to an inaccessible interval of ~ 8 m section and the possibility that the normal polarity might represent the slightly older Chron C3An.2n. Sediment accumulation rate and biostratigraphic arguments, however, stand against these points and favor a deposition during Chron C3An.1n. [ABSTRACT FROM AUTHOR]

Paleomagnetism is the most important source of information for determining the position of tectonic plates in the geological past. Over the last few decades significant advancements have been made in improving the reliability of paleomagnetic data, ranging from analytical methods to statistical assessments. Here I present the first version of PmagDiR, an R-based open-source package which displays, assesses the reliability, and, when possible, corrects given paleomagnetic directions distributions. The main functions of the package are to: plot paleomagnetic directions, their averages and confidence boundaries; apply different direction cut-offs to identify and filter outliers; perform a test for antipodality; compare the distribution shape with the one predicted by a widely accepted paleosecular variation model; correct flawed distributions for paleomagnetic inclination flattening; and correct for strain-derived paleomagnetic directions deviations when the strain fabric is known. Furthermore, directions can be converted in virtual geomagnetic poles and plotted on a spherical projection for comparison with reference apparent polar wander paths. All main functions of the PmagDiR package return results as comma separated value text files as well as vector graphic files (pdf), optimized for publication purpose with minimal manipulation. [ABSTRACT FROM AUTHOR]

Earth's past climate exhibits short-term (1500-year) pronounced fluctuations during the last glacial period, called Dansgaard-Oeschger (DO) glacial events, which have never been detected in pre-Quaternary times. The record of DO equivalent climate variability in Mesozoic strata can provide constraints on understanding these events. Here we highlight a prominent 1500-year cyclicity in a Jurassic (~ 155 Ma) ice-free sedimentary record from the Tethyan Basin. This Jurassic 1500-year cyclicity is encoded in high-resolution magnetic susceptibility (MS) proxy data reflecting detrital variations, and expressed as marl-limestone couplets. Additionally, MS data detect the modulation of these DO-scale couplets by supercouplet sets, reflecting the precession and its harmonics. We suggest that this Jurassic DO-like cyclicity may originate from paleo-monsoon-like system, analogous to the record of DO events in the Pleistocene East Asian monsoon archives. Paleogeographic reconstructions and atmosphere-ocean simulations further support the potential existence of strong, ancient monsoon circulations in the Tethyan Basin during the Jurassic., (© 2022. The Author(s).)

Middle Permian Qixia Formation in the southwestern region of Sichuan (SW China) has experienced multiphase fluidisation, resulting in an unclear understanding of the reservoir reconstruction effect. In this study, a systematic analysis of the Qi2 member in Wulong Town was carried out by combining field outcrop petrology and geochemistry. The results demonstrated that multiple sets of crystalline dolomite-bioclastic limestone cycles were stacked vertically in the Qi2 member, accompanied by the development of fractures and karst channels. The dolomite was mainly composed of silty-fine dolomite (D1) and recrystallised dolomite (D2). Furthermore, obvious multiphase dolomitic cements (Cd1-Cd2) were present in the fractures and pores. Early karst is known to have lithologic mutation surface development and karst channel development at the top of several secondary cycles. The vadose silt dolomites (Cd1) having karst channels developed dull luminescence under cathode luminescence (CL). Both the geochemical indicators of elements and rare earth element (REE) content indicated dysoxic-oxic environmental conditions. The hydrothermal solution displayed tectonic carniole characteristics in the strata burial stage. Fractures and pores were filled with hydrothermal minerals such as coarse dolomites-saddle dolomites (Cd2, with some caused by recrystallisation of the Cd1 hydrothermal solution) and fluorites. Coarse dolomites-saddle dolomites developed dull-red luminescence with a bright-red rim under CL and their δ18OVPDB values were more negative than those of middle Permian limestone samples. Both the geochemical indicators of elements and REE content indicated the suboxic-anoxic environmental conditions. Karstification had minor constructive impact on the reservoir of the Qi2 member in Baoxing in southwestern Sichuan. Most products of karstification were distributed as fillings in channels. Aside from creating certain networked fractures, the hydrothermal solution was mainly filled with hydrothermal minerals along the fractures, pores and early karst channels. Karst and the hydrothermal solution mainly damaged the middle and upper parts of the middle Permian Qixia Formation in Southwest Sichuan. The impact of episodic fluid on the restoration of the carbonate reservoir was mainly restricted by channels for fluid migration and thickness differences among the reservoir. However, certain thick-layered and massive crystalline dolomite may hold promise for exploration. [ABSTRACT FROM AUTHOR]

Ancient fluvial deposits typically display repetitive changes in their depositional architecture such as alternating intervals of coarse-grained highly amalgamated (HA), laterally-stacked, channel bodies, and finer-grained less amalgamated (LA), vertically-stacked, channels encased in floodplain deposits. Such patterns are usually ascribed to slower, respectively higher, rates of base level rise (accommodation). However, "upstream" factors such as water discharge and sediment flux also play a potential role in determining stratigraphic architecture, yet this possibility has never been tested despite the recent advances in the field of palaeohydraulic reconstructions from fluvial accumulations. Here, we chronicle riverbed gradient evolution within three Middle Eocene (~ 40 Ma) fluvial HA-LA sequences in the Escanilla Formation in the south-Pyrenean foreland basin. This work documents, for the first time in a fossil fluvial system, how the ancient riverbed systematically evolved from lower slopes in coarser-grained HA intervals, and higher slopes in finer-grained LA intervals, suggesting that bed slope changes were determined primarily by climate-controlled water discharge variations rather than base level changes as often hypothesized. This highlights the important connection between climate and landscape evolution and has fundamental implications for our ability to reconstruct ancient hydroclimates from the interpretation of fluvial sedimentary sequences. [ABSTRACT FROM AUTHOR]

The spectacular fossil fauna and flora preserved in the Upper Cretaceous terrestrial strata of North America's Western Interior Basin record an exceptional peak in the diversification of fossil vertebrates in the Campanian, which has been termed the 'zenith of dinosaur diversity'. The wide latitudinal distribution of rocks and fossils that represent this episode, spanning from northern Mexico to the northern slopes of Alaska, provides a unique opportunity to gain insights into dinosaur paleoecology and to address outstanding questions regarding faunal provinciality in connection to paleogeography and climate. Whereas reliable basin-wide correlations are fundamental to investigations of this sort, three decades of radioisotope geochronology of various vintages and limited compatibility has complicated correlation of distant fossil-bearing successions and given rise to contradictory paleobiogeographic and evolutionary hypotheses. Here we present new U–Pb geochronology by the CA-ID-TIMS method for 16 stratigraphically well constrained bentonite beds, ranging in age from 82.419 ± 0.074 Ma to 73.496 ± 0.039 Ma (2σ internal uncertainties), and the resulting Bayesian age models for six key fossil-bearing formations over a 1600 km latitudinal distance from northwest New Mexico, USA to southern Alberta, Canada. Our high-resolution chronostratigraphic framework for the upper Campanian of the Western Interior Basin reveals that despite their contrasting depositional settings and basin evolution histories, significant age overlap exists between the main fossil-bearing intervals of the Kaiparowits Formation (southern Utah), Judith River Formation (central Montana), Two Medicine Formation (western Montana) and Dinosaur Park Formation (southern Alberta). Pending more extensive paleontologic collecting that would allow more rigorous faunal analyses, our results support a first-order connection between paleoecologic and fossil diversities and help overcome the chronostratigraphic ambiguities that have impeded the testing of proposed models of latitudinal provinciality of dinosaur taxa during the Campanian. [ABSTRACT FROM AUTHOR]

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene. [ABSTRACT FROM AUTHOR]

The Late Miocene global cooling (LMGC; approximately 7.9–5.8 Ma) was associated with remarkable changes in monsoon dynamics, biogenic bloom in the global oceans, and the rise of modern ecosystems at the expense of old biota. However, the possible linkage between the environmental changes and ecosystem shifts during the LMGC is still debated. In this paper, we show the high-resolution changes in the fluxes of selected radiolarian species, suggesting a drastic reorganization in the paleoceanography and ecosystem in the Japan Sea during the LMGC. The endemic radiolarian Cycladophora nakasekoi dominated the Japan Sea until 7.4 Ma when the Japan Sea sediment changed from dark radiolarian-rich sediment to organic-poor diatom ooze. Changes in the fluxes of C. nakasekoi and Tricolocapsa papillosa, the latter related to changes in the Pacific central water (PCW), show 100, 200, and ~ 500 ka cycles with their high flux mostly within the darker sediment intervals during the low-eccentricity period until 7.4 Ma, suggesting that orbitally paced PCW inflow might have been the major nutrient source into the Japan Sea. At about 7.4 Ma, these species decreased at the expense of increased Larcopyle weddellium, a radiolarian related to the North Pacific intermediate water (NPIW), and Cycladophora sphaeris, a subarctic radiolarian species, implying a decrease in PCW inflow and an increase in the inflow of NPIW and subarctic shallow water. Such a change would have been related to the LMGC-induced weakening in the Pacific Meridional overturning circulation and the southward shift of the subarctic front due to intensified East Asian winter monsoon. Such a drastic reorganization in the hydrography in the Japan Sea probably caused changes in nutrient provenance from the PCW to the NPIW and resulted in faunal turnover, marked by the disappearance of the old regional and endemic faunal components, such as C. nakasekoi. [ABSTRACT FROM AUTHOR]

The Pliensbachian–Toarcian boundary interval is characterized by a ~ 3‰ negative carbon-isotope excursion (CIE) in organic and inorganic marine and terrestrial archives from sections in Europe, such as Peniche (Portugal) and Hawsker Bottoms, Yorkshire (UK). A new high-resolution organic-carbon isotope record, illustrating the same chemostratigraphic feature, is presented from the Southern Hemisphere Arroyo Chacay Melehue section, Chos Malal, Argentina, corroborating the global significance of this disturbance to the carbon cycle. The negative carbon-isotope excursion, mercury and organic-matter enrichment are accompanied by high-resolution ammonite and nannofossil biostratigraphy together with U–Pb CA-ID-TIMS geochronology derived from intercalated volcanic ash beds. A new age of ~ 183.73 + 0.35/− 0.50 Ma for the Pliensbachian–Toarcian boundary, and 182.77 + 0.11/− 0.15 for the tenuicostatum–serpentinum zonal boundary, is assigned based on high-precision U–Pb zircon geochronology and a Bayesian Markov chain Monte Carlo (MCMC) stratigraphic age model. [ABSTRACT FROM AUTHOR]

Global marine archives from the early Pleistocene indicate that glacial-interglacial cycles, and their corresponding sea-level cycles, have predominantly a periodicity of ~ 41 kyrs driven by Earth's obliquity. Here, we present a clastic shallow-marine record from the early Pleistocene in Southeast Asia (Cholan Formation, Taiwan). The studied strata comprise stacked cyclic successions deposited in offshore to nearshore environments in the paleo-Taiwan Strait. The stratigraphy was compared to both a δ18O isotope record of benthic foraminifera and orbital parameters driving insolation at the time of deposition. Analyses indicate a strong correlation between depositional cycles and Northern Hemisphere summer insolation, which is precession-dominated with an obliquity component. Our results represent geological evidence of precession-dominated sea-level fluctuations during the early Pleistocene, independent of a global ice-volume proxy. Preservation of this signal is possible due to the high-accommodation creation and high-sedimentation rate in the basin enhancing the completeness of the stratigraphic record. [ABSTRACT FROM AUTHOR]

Fischer plots are widely used in paleoenvironmental research as graphic representations of sea- and lake-level changes through mapping linearly corrected variation of accumulative cycle thickness over cycle number or stratum depth. Some kinds of paleoenvironmental proxy data (especially subsurface data, such as natural gamma-ray logging data), which preserve continuous cyclic signals and have been largely collected, are potential materials for constructing Fischer Plots. However, it is laborious to count the cycles preserved in these proxy data manually and map Fischer plots with these cycles. In this paper, we introduce an original open-source Python code "PyFISCHERPLOT" for constructing Fischer Plots in batches utilizing paleoenvironmental proxy data series. The principle of constructing Fischer plots based on proxy data, the data processing and usage of the PyFISCHERPLOT code and the application cases of the code are presented. The code is compared with existing methods for constructing Fischer plots. [ABSTRACT FROM AUTHOR]

Cereals are a central resource for the human diet and are traditionally assumed to have evolved from wild grasses at the onset of the Neolithic under the pressure of agriculture. Here we demonstrate that cereals may have a significantly longer and more diverse lineage, based on the study of a 0–2.3 Ma, 601 m long sedimentary core from Lake Acıgöl (South-West Anatolia). Pollen characteristic of cereals is abundant throughout the sedimentary sequence. The presence of large lakes within this arid bioclimatic zone led to the concentration of large herbivore herds, as indicated by the continuous occurrence of coprophilous fungi spores in the record. Our hypothesis is that the effects of overgrazing on soils and herbaceous stratum, during this long period, led to genetic modifications of the Poaceae taxa and to the appearance of proto-cereals. The simultaneous presence of hominins is attested as early as about 1.4 Ma in the lake vicinity, and 1.8 Ma in Georgia and Levant. These ancient hominins probably benefited from the availability of these proto-cereals, rich in nutrients, as well as various other edible plants, opening the way, in this region of the Middle East, to a process of domestication, which reached its full development during the Neolithic. [ABSTRACT FROM AUTHOR]

The wavelet local multiple correlation (WLMC) is introduced for the first time in the study of climate dynamics inferred from multivariate climate time series. To exemplify the use of WLMC with real climate data, we analyse Last Millennium (LM) relationships among several large-scale reconstructed climate variables characterizing North Atlantic: i.e. sea surface temperatures (SST) from the tropical cyclone main developmental region (MDR), the El Niño-Southern Oscillation (ENSO), the North Atlantic Multidecadal Oscillation (AMO), and tropical cyclone counts (TC). We examine the former three large-scale variables because they are known to influence North Atlantic tropical cyclone activity and because their underlying drivers are still under investigation. WLMC results obtained for these multivariate climate time series suggest that: (1) MDRSST and AMO show the highest correlation with each other and with respect to the TC record over the last millennium, and: (2) MDRSST is the dominant climate variable that explains TC temporal variability. WLMC results confirm that this method is able to capture the most fundamental information contained in multivariate climate time series and is suitable to investigate correlation among climate time series in a multivariate context. [ABSTRACT FROM AUTHOR]

Although the responses of North Atlantic Deep Water (NADW) is deeply connected to orbital rhythms, those under different tectonic and atmospheric boundary conditions remain unknown. Here, we report suborbitally resolved benthic foraminiferal stable isotope data from J-anomaly Ridge in the North Atlantic from ca. 26.4–26.0 Ma. Our results indicate that the formation of NADW during that time interval was increased during the obliquity-paced interglacial periods, similar to in the Plio-Pleistocene. During the late Oligocene, the interglacial poleward shifts of the stronger westerlies in the southern hemisphere, which occurred due to the higher thermal contrasts near the upper limit of the troposphere, reinforced the Antarctic Circumpolar Current (ACC) and, in turn, the Atlantic meridional overturning circulation (AMOC). However, such a response mode in deep ocean circulation did not occur during the middle Eocene because of different tectonic boundary conditions and the immature states of the ACC. Instead, the middle Eocene interglacial conditions weakened the formation of the proto-type NADW due to less heat loss rate in high-latitude regions of the North Atlantic during high obliquity periods. Our findings highlight the different responses of deep ocean circulation to orbital forcing and show that climate feedbacks can be largely sensitive to boundary conditions. [ABSTRACT FROM AUTHOR]

Earth's orbital variations on timescales of 104–105 years, known as Milankovitch cycles, have played a critical role in pacing climate change and ecosystem dynamics, through glacial and/or monsoon dynamics. However, the climatic and biotic consequences of these cycles on much longer (~ 107 years) timescales remain unclear, due to a lack of long proxy records with precise age constraints. Here, we show ~ 10-Myr scale variations in early Mesozoic (250–180 Ma) records of lake-level, desert distribution, biogenic-silica burial flux, atmospheric CO2 levels (pCO2), and sea-surface-temperature (SST). Their phase relationships, coupled with carbon cycle modeling results, suggest that orbitally-paced summer monsoon dynamics modulates changes in terrestrial weatherability by ~ 20%, affecting changes in pCO2 of up to 500–1,000 ppmv and 3–7 °C SST. We also infer that these ~ 10-Myr scale climatic variations could have been causally linked to biotic turnover, size variations in dinosaur footprints, and tetrapod dispersal, potentially through spatio-temporal variations in resource availability and arid-hot climatic barriers at low-middle latitudes. [ABSTRACT FROM AUTHOR]

Millennial-scale palaeoclimate variability has been documented in various terrestrial and marine palaeoclimate proxy records throughout the Northern Hemisphere for the last glacial cycle. Its clear expression and rapid shifts between different states of climate (Greenland Interstadials and Stadials) represents a correlation tool beyond the resolution of e.g. luminescence dating, especially relevant for terrestrial deposits. Usually, comparison of terrestrial proxy datasets and the Greenland ice cores indicates a complex expression of millennial-scale climate variability as recorded in terrestrial geoarchives including loess. Loess is the most widespread terrestrial geoarchive of the Quaternary and especially widespread over Eurasia. However, loess often records a smoothed representation of millennial-scale variability without all fidelity when compared to the Greenland data, this being a relevant limiting feature in integrating loess with other palaeoclimate records. To better understand the loess proxy-response to millennial-scale climate variability, we simulate a proxy signal smoothing by natural processes through application of low-pass filters of δ18O data from Greenland, a high-resolution palaeoclimate reference record, alongside speleothem isotope records from the Black Sea-Mediterranean region. We show that low-pass filters represent rather simple models for better constraining the expression of millennial-scale climate variability in low sedimentation environments, and in sediments where proxy-response signals are most likely affected by natural smoothing (by e.g. bioturbation). Interestingly, smoothed datasets from Greenland and the Black Sea-Mediterranean region are most similar in the last ~15 ka and between ~50–30 ka. Between ~30–15 ka, roughly corresponding to the Last Glacial Maximum and the deglaciation, the records show dissimilarities, challenging the construction of robust correlative time-scales in this age range. From our analysis it becomes apparent that patterns of palaeoclimate signals in loess-palaeosol sequences often might be better explained by smoothed Greenland reference data than the original high-resolution Greenland dataset, or other reference data. This opens the possibility to better assess the temporal resolution and palaeoclimate potential of loess-palaeosol sequences in recording supra-regional climate patterns, as well as to securely integrate loess with other chronologically better-resolved palaeoclimate records. [ABSTRACT FROM AUTHOR]

Recent studies have proved that high elevation environments, especially remote wetlands, are exceptional ecological sensors of global change. For example, European glaciers have retreated during the 20th century while the Sierra Nevada National Park in southern Spain witnessed the first complete disappearance of modern glaciers in Europe. Given that the effects of climatic fluctuations on local ecosystems are complex in these sensitive alpine areas, it is crucial to identify their long-term natural trends, ecological thresholds, and responses to human impact. In this study, the geochemical records from two adjacent alpine bogs in the protected Sierra Nevada National Park reveal different sensitivities and long-term environmental responses, despite similar natural forcings, such as solar radiation and the North Atlantic Oscillation, during the late Holocene. After the Industrial Revolution both bogs registered an independent, abrupt and enhanced response to the anthropogenic forcing, at the same time that the last glaciers disappeared. The different response recorded at each site suggests that the National Park and land managers of similar regions need to consider landscape and environmental evolution in addition to changing climate to fully understand implications of climate and human influence. [ABSTRACT FROM AUTHOR]

The Earth's biota originated and developed to its current complex state through interacting with multilevel physical forcing of our planet's climate and near and outer space phenomena. In the present study, we focus on the time scale of hundreds to thousands of years in the most recent time interval – the Holocene. Using a pollen paleocommunity dataset from southern Lithuania (Čepkeliai bog) and applying spectral analysis techniques, we tested this record for the presence of statistically significant cyclicities, which can be observed in past solar activity. The time series of non-metric multidimensional scaling (NMDS) scores, which in our case are assumed to reflect temperature variations, and Tsallis entropy-related community compositional diversity estimates q* revealed the presence of cycles on several time scales. The most consistent periodicities are characterized by periods lasting between 201 and 240 years, which is very close to the DeVries solar cycles (208 years). A shorter-term periodicity of 176 years was detected in the NMDS scores that can be putatively linked to the subharmonics of the Gleissberg solar cycle. In addition, periodicities of ≈3,760 and ≈1,880 years were found in both parameters. These periodic patterns could be explained either as originating as a harmonic nonlinear response to precession forcing, or as resulting from the long-term solar activity quasicycles that were reported in previous studies of solar activity proxies. [ABSTRACT FROM AUTHOR]

Geological sediment archives document a rich periodic series of astronomically driven climate, but record also abrupt, severe climatic changes called events, the multi-Myr boundary conditions of which have generally been ascribed to acyclic processes from Earth's interior dynamics. These events have rarely been considered together within extended time series for potential correlation with long-term (multi-million year, Myr) cycling. Here I show a coupling between events and multi-Myr cycles in a temperature and ice-volume climatic proxy of the geological past 115 Myr. I use Cenozoic through middle Cretaceous climatic variations, as recorded in benthic foraminifera δ18O, to highlight prominent ~9 and ~36 Myr cyclicities. These cyclicities were previously attributed either to astronomical or tectonic variations. In particular, I point out that most of the well-known events during the past 115 Myr geological interval occur during extremes in the ~9 and ~36 Myr cycling. One exception is the early Cenozoic hyperthermal events including the salient Paleocene-Eocene Thermal Maximum (~56 Ma), which do not match extremes in long-period cyclicities, but to inflection point of these cycles. Specific focus on climatic events, as inferred from δ18O proxy, suggest that some "events", marked by gradual trends within the ~9 and ~36 Myr cycle extremes, would principally be paced by long-term cycling, while "events", recorded as abrupt δ18O changes nearby cycle extremes, would be rather induced by acyclic processes. The connection between cyclic and acyclic processes, as triggers or feedbacks, is very likely. Such link between cycling and events in Earth's past climate provides insight into celestial dynamics governing perturbations in Earth's surface systems, but also the potential connection between external and Earth's interior processes. [ABSTRACT FROM AUTHOR]

The organic carbon isotope (δ13Corg) curve for ~1.7-km-thick mid-Cretaceous strata of the Chaqiela section in Gamba area, southern Tibet is presented in this study. C-isotopic chemostratigraphic correlation combined with biostratigraphic constraints show that the Chaqiela section spans early Aptian through early Campanian period, and that almost all of the carbon cycle perturbations and Oceanic Anoxic Events during the mid-Cretaceous period are well recorded in the continental margin area of the southeastern Tethys Ocean. Significantly, two levels of methane-derived authigenic carbonates were identified at the onset of OAE1b near the Aptian-Albian boundary. We suggest that an increase in methane release from gas hydrates, potentially driven by sea-level fall and bottom water temperature increase, may have contributed to the large negative δ13Corg excursions and global warming during OAE1b. [ABSTRACT FROM AUTHOR]