Geological Time Intervals Research Articles

Carbonate δ13C chemostratigraphy of the Hunjiang Group in North China and a tentative stratigraphic framework for the Meso-Neoproterozoic transition

A robust stratigraphic framework for the Meso-Neoproterozoic transition is necessary for a better understanding of evolutionary and environmental changes during this critical geological time interval. Given the limited availability of radiometric and biostratigraphic data from late Mesoproterozoic to early Neoproterozoic strata, chemostratigraphic correlations have been used to build this framework. To contribute to Proterozoic chemostratigraphy, here we report new carbonate carbon isotope data from the Hunjiang Group in the Baishan area of Jilin Province in North China. The carbon isotope data (average 3.00 ‰ ± 1.15 ‰, 1 SD), together with existing detrital zircon constraints and microfossils, confirm a late Mesoproterozoic to early Neoproterozoic age of the Hunjiang Group. Using the dynamic time warping algorithm, we then constructed a global chemostratigraphic framework for the Meso-Neoproterozoic transition based on δ13Ccarb data from the North China, São Francisco, and Congo cratons. This exercise confirms the generally narrow range of δ13Ccarb fluctuations in the late Mesoproterozoic but also substantiates a negative δ13Ccarb excursion of notable magnitude (∼9 ‰) and of potential global extent at ca. 1 Ga. This negative excursion, known as the Majiatun excursion, likely marks the Meso-Neoproterozoic transition and the onset of the dynamic carbon cycle in the Neoproterozoic Era.

Mesoproterozoic siliciclastic stromatolites of Chapada Diamantina (Brazil): Morphological types, genesis and environmental context

Laminations of organosedimentary structures can be formed by three distinct processes: (i) bio-induced mineral precipitation, (ii) trapping and binding, and (iii) abiotic mineral precipitation and each of these processes predominates in a certain geological time interval. Agglutinated stromatolites are rare and seem to form in specific environmental conditions. Siliciclastic stromatolites are an even rarer class of agglutinated stromatolites, which are formed in response to the process of trapping and binding of siliciclastic grains by upright filaments or by extracellular polymeric substances (EPS). This study describes and interprets one of the oldest siliciclastic stromatolites of the geological record and discusses the microbial evidence and mechanisms that controlled their accumulation and preservation. Two compound sections were analyzed and ten thin sections were prepared to characterize the siliciclastic stromatolites from the Mesoproterozoic Caboclo Formation (Brazil). Stromatolites are present in two different lithofacies associations: (i) shoreface and (ii) offshore transition. The shoreface stromatolites are characterized by silty to sandy texture and poor lamination, and the distal ones by clay, silt grains, less sand, and distinct lamination. The volume of siliciclastic grains composing the stromatolite fabric is >85% in the shoreface and >30% in offshore transition. These stromatolites were deposited in a favorable environment for trapping and binding of siliciclastic sediments: a wave-dominated siliciclastic ramp with the availability of siliciclastic grains and frequent water column agitation. The marine environment is favorable for forming EPS with good adhesive properties. Besides, the Caboclo Formation age (1.3–1.0 Ga) coincides with an abrupt fall in the amount of atmospheric CO2 reflected in CaCO3 saturation in the oceans. This change in ocean chemistry may have led to a change in stromatolite fabric, from the sparry crust and fine-grained precipitated to agglutinated.

The Role of Continental Alkaline Magmatism in Mantle Carbon Outflux Constrained by a Machine Learning Analysis of Zircon

AbstractContinental alkaline magmatism has been suggested to play a significant role in releasing deep mantle carbon into the atmosphere, which can greatly impact the global climate. However, the temporal variations of alkaline magmatism and their potential to modulate climate over geologic time remain poorly constrained. The detrital zircon record is a frequently used proxy for tracking secular variations in particular magmatism. Here, we use a novel machine‐learning technique to discriminate zircon from carbonatites, kimberlites, and other alkaline rocks. A global compilation of detrital zircon yields continental alkaline magmatic flare‐ups between 1,050−850, 650−500, 250−200, and 50−0 Ma. Our estimates indicate relatively elevated contributions of total magmatic carbon outgassing from alkaline magmatism during the aforementioned magmatic flare‐ups. We infer that anomalous alkaline magmatism may influence global warming during specific intervals of geologic time, but when they are not that voluminous or persistent extensive arc magmatism may drive warming conditions.

Sustained increases in atmospheric oxygen and marine productivity in the Neoproterozoic and Palaeozoic eras

A geologically rapid Neoproterozoic oxygenation event is commonly linked to the appearance of marine animal groups in the fossil record. However, there is still debate about what evidence from the sedimentary geochemical record—if any—provides strong support for a persistent shift in surface oxygen immediately preceding the rise of animals. We present statistical learning analyses of a large dataset of geochemical data and associated geological context from the Neoproterozoic and Palaeozoic sedimentary record and then use Earth system modelling to link trends in redox-sensitive trace metal and organic carbon concentrations to the oxygenation of Earth’s oceans and atmosphere. We do not find evidence for the wholesale oxygenation of Earth’s oceans in the late Neoproterozoic era. We do, however, reconstruct a moderate long-term increase in atmospheric oxygen and marine productivity. These changes to the Earth system would have increased dissolved oxygen and food supply in shallow-water habitats during the broad interval of geologic time in which the major animal groups first radiated. This approach provides some of the most direct evidence for potential physiological drivers of the Cambrian radiation, while highlighting the importance of later Palaeozoic oxygenation in the evolution of the modern Earth system.

On the Application Limits of the Parameter in Studying Variations of the Ancient Geomagnetic Field

To describe secular geomagnetic variation on geological timescales, statistical models have been widely used in recent decades. Currently, the most popular among these is the TK03 model (Tauxe and Kent, 2004). As other statistical models, TK03 numerically characterizes the amplitude of secular geomagnetic variation and the shape of the distribution of paleomagnetic directions which are considered as directly reflecting the directions of the geomagnetic field on the considered interval of geological time. For this purpose, three main parameters are used: the scatter Sb (or S) of the virtual geomagnetic poles, the elongation E of the distribution of paleomagnetic directions, and the direction of elongation of the distribution of paleomagnetic directions. The correct application of these parameters to describe ancient secular variation requires the satisfaction of certain, sometimes rather strict conditions. These conditions for the Sb and E parameters were considered in a number of previous publications, while the limits and conditions of application of parameter have not been studied in detail so far. This paper presents the results of mathematical modeling that allow us to evaluate the stability of the calculated values of this parameter as a function of the latitude of sampling, the number of samples used for its determination, the length of the time series on which this parameter is determined, as well as inclination shallowing and the degree of averaging when is estimated in sedimentary rocks. We also consider the extent to which the parameter can be sensitive to the presence and characteristics of the equatorial dipole component in the total geomagnetic field.

Summer-Wet Hydrologic Cycle during the Middle Miocene of the United States: New Evidence from Fossil Fungi.

Hydrologic reconstructions from North America are largely unknown for the Middle Miocene. Examination of fungal palynomorph assemblages coupled with traditional plant-based palynology permits delineation of local, as opposed to regional, climate signals and provides a baseline for study of ancient fungas. Here, the Fungi in a Warmer World project presents paleoecology and paleoclimatology of 351 fungal morphotypes from 3 sites in the United States: the Clarkia Konservat-Lagerstätte site (Idaho), the Alum Bluff site (Florida), and the Bouie River site (Mississippi). Of these, 83 fungi are identified as extant taxa and 41 are newly reported from the Miocene. Combining new plant-based paleoclimatic reconstructions with funga-based paleoclimate reconstructions, we demonstrate cooling and hydrologic changes from the Miocene climate optimum to the Serravallian. In the southeastern United States, this is comparable to that reconstructed with pollen and paleobotany alone. In the northwestern United States, cooling is greater than indicated by other reconstructions and hydrology shifts seasonally, from no dry season to a dry summer season. Our results demonstrate the utility of fossil fungi as paleoecologic and paleoclimatic proxies and that warmer than modern geological time intervals do not match the "wet gets wetter, dry gets drier" paradigm. Instead, both plants and fungi show an invigorated hydrological cycle across mid-latitude North America.

The Anthropocene is best understood as an ongoing, intensifying, diachronous event

Current debate on the status and character of the Anthropocene is focussed on whether this interval of geological time should be designated as a formal unit of epoch/series rank in the International Chronostratigraphic Chart/Geological Time Scale, or whether it is more appropriate for it to be considered as an informal ‘event’ comparable in significance with other major transformative events in deeper geological time. The case for formalizing the Anthropocene as a chronostratigraphical unit with a base at approximately 1950 CE is being developed by the Anthropocene Working Group of the Subcommission on Quaternary Stratigraphy. Here we outline the alternative position and explain why the time‐transgressive nature of human impact on global environmental systems that is reflected in the recent stratigraphical record means that the Anthropocene is better seen not as a series/epoch with a fixed lower boundary, but rather as an unfolding, transforming and intensifying geological event.

Semantic Mapping of the Geologic Time Scale: A temporal reference

The Geologic Time Scale is an ordered hierarchical set of terms representing specific time intervals in Earth's history. The hierarchical structure is correlated to the geologic record and major geologic events in Earth’s history (Gradstein et al. 2020). In the absence of quantitative numeric age values from absolute dating methods, the relative time intervals in the geologic time scale provide us with the vocabulary needed for deciphering Earth’s history and chronological reconstruction. This temporal frame of reference is critical to establishing correlations between specimens and how they fit within the Earth’s 4.567 Ga (giga annum) history. Due to spatial and temporal variations in the stratigraphic record, the terminology used in conjunction with geologic time scales is largely inconsistent. For a detailed discussion regarding term use in geologic timescales, see Cohen et al. (2013). As a result, published values for geologic timescale terms are often ambiguous and highly variable, limiting interoperability and hindering temporal correlations among specimens. A solution is to map verbatim geologic timescale values to a controlled vocabulary, constructing a single temporal frame of reference. The harmonization process is governed by an established set of business rules that can ultimately become fully or partially automated. In this study, we examined the Global Biodiversity Information Facility’s (GBIF) published distinct verbatim values for Darwin Core terms in the GeologicalContext Class of Darwin Core to assess the the use of chronostratiphic terms, a process highlighted in Sahdev et al. (2017). Preservation of these verbatim values, the initial unmapped set of published values, is important. Many are derived directly from primary source material and possess special historical and regional significance. These include land mammal ages (e.g., Lindsay (2003)), biostratigraphic zones, regional terms, and terms with higher granularity than the International Commission of Stratigraphy’s (ICS) timescale allows (e.g., subages/substages). For the purposes of this study, we selected the 2023/6 version of the ICS chronostratigraphic timescale as the controlled vocabulary (Cohen et al. 2023). The ICS is the most widely adopted timescale, comprising the most generalized and universally applicable intervals of geologic time. After semantic analysis of the verbatim values (see Table 1 for comparative statistics), we established a comprehensive set of business rules to map to the ICS timescale controlled vocabulary. This process yielded a collection of documented procedures to transform the heterogeneous collection of published terms into a semantically consistent dataset. The end result is a single temporal frame of reference for published geologic and paleontological specimens through semantic mapping to improve the temporal correlations between geologic specimens globally through data interoperability. This talk will highlight the process of harmonizing a heterogeneous collection of published verbatim Geologic Time Scale values with an established controlled vocabulary through semantic mapping.

Beta diversity dynamics in East Asian angiosperm woody plants: taxonomic turnover in relation to temperature gradients during the Cenozoic

Information on the paleo-beta diversity of fossil assemblages and its patterning in different geological time intervals helps us to better understand the community-level response of biodiversity patterns to current global warming. We focused on the impact of paleoclimate changes on large-scale taxonomic sorting related to geography; specifically, how cooler and warmer climatic conditions affect the distance-dependency of beta diversity. Using a dataset of Cenozoic fossil assemblages of angiosperm woody plants (7,468 data points; 310 genera in 95 families) in the East Asian archipelago, we modeled the distance-dependency of genus turnover (Sørensen’s pairwise dissimilarity) through the Oligocene, Miocene, Pliocene, Pleistocene, Last Glacial Period, Holocene, and the present day. The genus turnover of angiosperm woody plants during the Oligocene to Holocene interval was positively correlated with geographical and climatic distance, but was significant only after the Last Glacial Period to the present day. During the warm Tertiary period, genus turnover was mostly independent of geographical distance. Spatial/climatic distance-dependent turnover under colder environments involved a climate-induced sorting process to spatially diversify woody-plant assemblages across the archipelago. Moreover, the predominance of distant-independent turnover suggested the effect of dispersal release under warmer, stable climates. Our findings suggest that future tropicalization in temperate habitats could promote geographical homogenization of biodiversity patterns.

The influence of oxygen and electronegativity on iron mineral chemistry throughout Earth’s history

The mineral record contains chemical signatures that reflect the evolving redox conditions of Earth’s crust, and reveal trends in the bioavailability of life's critical elements. In particular, shifting redox states of transition metal elements that are used as co-factors by enzymes across all domains of life can be tracked in preserved mineral chemistry. The transition metal iron (Fe) is one of the most abundant elements in Earth’s core, mantle, and crust, and is commonly a major constituent of rock-forming minerals. Additionally, Fe is the most widely used metal in biology, and Earth’s redox history has profoundly impacted the chemical speciation and availability of Fe in aqueous systems. The diverse mineral chemistry of Fe can therefore reveal new insights into the redox evolution of Earth’s crust and subsequent biological impacts. Here, we apply a new mineral chemistry network analysis platform, dragon, to investigate the mineral chemistry of iron (Fe) over geologic-time. We present bipartite network graphs of iron minerals linked to their constituent elements or ions for several geological time intervals. These graphs illustrate the increasing importance of oxygen (O) abundance in the atmosphere from the Paleoarchean to the Mesoproterozoic in regard to the the emergence of new Fe minerals, as well as the influence of free O on trends in element electronegativity in mineral formation and electron transfer processes. The proportion of oxygen containing Fe minerals had the sharpest increase during the time periods of the Kenorland and Columbia supercontinent assembly events. Indeed, the rise of O is associated with O becoming more centralized in the network as well as with an increase through time in the number of Fe minerals containing combinations of high and low electronegativity elements. The importance of oxygen in the expansion of Fe mineral chemical diversity, and its influence on the bioavailability of Fe in the environment, is illustrated clearly in the Fe mineral chemistry network.

Relative species abundance and population densities of the past: developing multispecies occupancy models for fossil data

AbstractThe number of individuals of species varies, but estimating abundance, given incomplete and biased sampling in both contemporary and fossilized communities, is challenging. Here, we describe a new occupancy model in a hierarchical Bayesian framework with random effects, in which multispecies occupancy and detection are modeled as a means to estimate relative species abundance and relative population densities. The modeling framework is suited for temporal samples of fossil communities with repeated sampling including multiple species with similar preservation potential. We demonstrate our modeling framework using a fossil community of benthic organisms to estimate relative species abundance dynamics and changing relative population densities of focal species in nine (geological) time intervals over 2.3 Myr. We also explore potential explanatory factors (paleoenvironmental proxies) and temporal autocorrelation that could provide extra information on unsampled time intervals. The modeling framework is applicable across a wide range of questions on species-level dynamics in paleoecological community settings.

Evaluation of Boulder Deposits Linked to Late Neogene Hurricane Events

The Neogene is a globally recognized interval of geologic time that lasted from 23 until 1 [...]

New Constraints on Global Geochemical Cycling During Oceanic Anoxic Event 2 (Late Cretaceous) From a 6‐Million‐year Long Molybdenum‐Isotope Record

AbstractIntervals of extreme warmth are predicted to drive a decrease in the oxygen content of the oceans. This prediction has been tested for the acme of short (<1 million years) episodes of significant marine anoxia in the Phanerozoic geological record known as Oceanic Anoxic Events (OAEs). However, there is a paucity of data spanning prolonged multimillion‐year intervals of geological time before and after OAEs. We present a Mo‐isotope record from limestones and marlstones of the Eagle Ford Group, South Texas, which was deposited in the southern Cretaceous Western Interior Seaway of North America during a 6‐million‐year period encompassing OAE 2 (Late Cenomanian–early Turonian: ∼94 Ma). Mo‐isotope compositions from deposits that formed in euxinic (sulfidic) conditions before OAE 2 allow the paleo‐seawater composition to be constrained to 1.1%–1.9%. This range of values overlaps previous estimates of up to ∼1.5% for the peak of OAE 2 determined from similarly sulfidic sediments deposited in the restricted proto‐North Atlantic Ocean. Mo‐isotopes thus varied by less than a few tenths of per mil across one of the most extreme intervals of global deoxygenation in the Late Phanerozoic. Rather than a limited change in oceanic deoxygenation, we suggest that the new data reflect changes to global iron cycling linked to basalt‐seawater interaction, terrestrial weathering and expanded partially oxygenated shallow shelf‐seas that played a key role in the burial of isotopically light molybdenum, thus acting as a counterbalance to its removal into sulfidic sediments.

Visualization of evolutionary relationships through geologic time in Timescale Creator

Evolutionary trees showing interpretations of the divergences, lineages, extinctions and relative abundances of life forms through geologic time provide a very useful visualization for both public audiences and research communities. The Java-based TimeScale Creator platform was originally designed to present graphics of Earth history with a streamlined interface. We have added an evolutionary tree display column in addition to the array of other geo-history columns for chart making and on-screen exploration through any interval of geologic time. Features in the evolutionary tree display include on-screen images of life forms, single-click on/off control of color-coded family branches, pop-up windows giving additional details, embedded evolution charts of morphotypes that developed within genotypes, an option to make more-compact charts by allowing new evolutionary branches to reposition over earlier extinct branches, and export/import of some common evolution-data sharing formats. A suite of current public datapacks using this evolutionary-tree visualization include major vertebrate groups (including dinosaurs), marine plankton through the past ca. 70 million years, and the Tree of Life of extant organisms. Students and researchers can upload their own datasets using a simple tabular-data format from Excel-type spreadsheets. The package and external datapacks are freely available at www.timescalecreator.org. The intended audiences range from Earth-science classes in schools to paleontologists and evolutionary biologists who wish to compare their phylogenies to other geologic events and trends of Earth history.

ВОДОСБОРНЫЙ БАССЕЙН КАК КОМПЛЕКСНАЯ СОЦИОПРИРОДНАЯ ИЕРАРХИЧЕСКАЯ СИСТЕМА

The theoretical and methodological basis of the systems hierarchical spatial and temporal analysis of a drainage basin, which addresses the problems of effective management in socio-natural systems of different ranks, is considered. It is proposed to distinguish 9 orders of forms that are relevant to the analysis of drainage basins, where the first level is represented by individual aggregates and particles, and the last - by basins of large and the largest rivers. As part of the allocation of geological, historical and modern time intervals, the specificity of the implementation of processes in basins of different scales from changing states, through functioning to evolution is demonstrated. The interrelation of conditions and factors that determine the processes occurring within the drainage basins is revealed. It is shown that a specific combination of conditions and factors that determine processes in the drainage basin is associated with the hierarchy of the objects under consideration, i.e. the choice of a spatial-temporal hierarchical level is crucial for the organization of study within drainage basins. At one hierarchical level, some phenomenon can be considered as a factor, and at another - as a condition. For example, tectonic processes can be considered as an active factor in the evolution of large river basins in the geological perspective, but for small drainage basin, this is already a conservative background condition. It is shown that at the historical time the anthropogenic factor often comes to the fore, with the appearance of which in the functioning of the drainage basin, there is a need to take into account the entire complex of socio-environmental problems that can affect the sustainable state of various territories, especially in the field of water and land use. Hierarchical levels of managing subjects are identified, which are primarily responsible for effective management at the appropriate hierarchical level of the organization of the socio-natural system within the catchment area, starting from an individual to humankind as a whole.

Giant Gaussian process models of geomagnetic palaeosecular variation: a directional outlook

SUMMARY Assessment of long-term palaeosecular variation (PSV) of the geomagnetic field is frequently based on simplified versions of a class of statistical models known as giant Gaussian processes (GGP) used to represent temporal variations in spherical harmonic descriptions of the field. Here we propose a new type of analysis to assess the shape and dispersion of the directional distributions caused by PSV. The quantities analysed in this study are equal-area coordinates of rotated distributions of palaeomagnetic directions, ${x_E}$ (east−west) and ${x_N}\ $(north−south) and their standard deviations (${\sigma _E}$ and ${\sigma _N}$). These are easy to determine, and can readily be numerically predicted for any GGP model, avoiding the need for the numerous simulations generally used to determine the scatter and/or elongation of directional distributions. Mean predictions of $\overline {{x_N}} $ for a simplified GGP model are different from the expected geocentric axial dipole (GAD) directions, in agreement with inclination differences noted in previous studies. The best estimates for palaeomagnetic inclination are the expected directions from the mean of virtual geomagnetic poles (VGPs) calculated using an iterative angular cut-off process. Predictions of ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$ vary with latitude and are symmetric about the Equator. The N–S direction (${\sigma _{\rm N}}$) is always larger than E–W (${\sigma _{\rm E}}$), but the difference decreases from a maximum at the Equator to the poles, where ${\sigma _{\rm N}} = \ {\sigma _{\rm E}}$. A simplified GGP model is used to show that the parameter α (affecting variances in all Gauss coefficients) is positively correlated with ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}\ $ while the β parameter, the ratio of dipole to quadrupole family standard deviations, modifies the latitudinal dependence of ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$. Experimental error in ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$ can be accommodated using the common statistical parameters found in palaeomagnetic data sets, as ${\alpha _{95}}$ from site-mean directions. Predictions of simplified GGP models are compared with both numerical simulations and real data spanning the last 10 Ma. The latitudinal dependence of the proposed measures of PSV (${\sigma _{\rm E}}$ and ${\sigma _N}$) provide useful diagnostics for testing the validity of a GGP model. For the past 10 Ma the best-fitting GGP model with a mean GAD field set to $g_1^0 = \ - 18\ \mu T$ has α = 6.7 µT and β = 4.2. These new directional diagnostics will be used to investigate changes in overall geomagnetic field behaviour over other geological time intervals.

Biogenic carbonate mercury and marine temperature records reveal global influence of Late Cretaceous Deccan Traps

The climate and environmental significance of the Deccan Traps large igneous province of west-central India has been the subject of debate in paleontological communities. Nearly one million years of semi-continuous Deccan eruptive activity spanned the Cretaceous-Paleogene boundary, which is renowned for the extinction of most dinosaur groups. Whereas the Chicxulub impactor is acknowledged as the principal cause of these extinctions, the Deccan Traps eruptions are believed to have contributed to extinction patterns and/or enhanced ecological pressures on biota during this interval of geologic time. We present the first coupled records of biogenic carbonate clumped isotope paleothermometry and mercury concentrations as measured from a broad geographic distribution of marine mollusk fossils. These fossils preserve evidence of simultaneous increases in coastal marine temperatures and mercury concentrations at a global scale, which appear attributable to volcanic CO2 and mercury emissions. These early findings warrant further investigation with additional records of combined Late Cretaceous temperatures and mercury concentrations of biogenic carbonate.

Oceanic organic carbon as a possible first-order control on the carbon cycle during the Bathonian–Callovian

Abstract Oceans are the largest, readily exchangeable, superficial carbon reservoir; a current challenge in investigating past and present environments and predict future evolution relates to the role of oceanic carbon in regulating Earths' carbon cycle and climate. At least one paired δ13Ccarb-TOC decoupling event is noted in the Late Bathonian–Early Callovian. Provokingly, we suggest that this decoupling event and other carbon isotopic events in the Bathonian–Callovian resulted from the addition and removal of carbon from the oceanic organic carbon (OOC) reservoir, likely dominated by dissolved and fine particulate oceanic organic carbon. The decoupling event is characterised by a mainly invariant δ13CTOC record and increasingly more positive δ13Ccarb values. The δ13Ccarb-TOC decoupling is tentatively explained by the expansion of the OOC reservoir, which increased the residence time of carbon in the oceans and buffered (slow equilibrium) δ13COOC (approximated by δ13CTOC) to changes in δ13C of oceanic inorganic carbon, comprised mainly of dissolved inorganic carbon (approximated by δ13Ccarb). Reconversion of OOC to CO2 may have resulted in negative δ13C excursions and increased atmospheric pCO2, whereas the change from accumulation of OOC to export of organic carbon to sediments may have resulted in positive δ13C excursions and widespread accumulation of organic matter. It is speculated that other small-scale (~ 1‰) δ13C excursions in the Mesozoic record may have resulted from similar episodes of OOC accumulation and oxidation. Our study calls attention to the potential of the OOC reservoir to act as a critical driver for planetary-scale changes over short geological time intervals.

Late Cretaceous Transtension in the Eastern Tibetan Plateau: Evidence From Postcollisional A‐Type Granite and Syenite in the Changdu Area, China

AbstractThe Late Cretaceous is an important geological time interval for the Tibetan Plateau because it corresponds to the period when the tectonic regime changed from Lhasa‐Qiangtang collision to Indo‐Asian assembly. However, the nature of and controls on the change in tectonic regime are poorly constrained. In this paper, we report results of a study of two intrusions in the Changdu area of the Eastern Tibetan Plateau. Zircon U‐Pb dating shows that both intrusions formed at ca. 77.6–74.3 Ma. The Bangda intrusion has A‐type granite affinity and a peraluminous character, whereas the Ruduo intrusion is a metaluminous syenite. Both intrusions have very similar trace element compositions, slightly enriched zircon εHf(t) values (−9.3 and −1.7), and EM‐2‐like Sr‐Nd‐Pb isotope ratios. These features of the two intrusions indicate that their magmas were derived from partial melting of an alkali‐rich basaltic lower crust and a small proportion of mantle melt. The occurrence of alkaline intrusions is consistent with Late Cretaceous extension in the Eastern Tibetan Plateau. Based on the results of this study and previous data, we propose an intraplate extensional tectonic model, in which there was NS‐NNW‐directed Late Cretaceous transtension in the Eastern Tibetan Plateau following the Lhasa‐Qiangtang collision. This extension is interpreted to have been triggered by the Bangong‐Nujiang slab break‐off at around 110 Ma and driven by the far‐field subduction of the Neo‐Tethys oceanic crust.

Text-mined fossil biodiversity dynamics using machine learning.

Documented occurrences of fossil taxa are the empirical foundation for understanding large-scale biodiversity changes and evolutionary dynamics in deep time. The fossil record contains vast amounts of understudied taxa. Yet the compilation of huge volumes of data remains a labour-intensive impediment to a more complete understanding of Earth's biodiversity history. Even so, many occurrence records of species and genera in these taxa can be uncovered in the palaeontological literature. Here, we extract observations of fossils and their inferred ages from unstructured text in books and scientific articles using machine-learning approaches. We use Bryozoa, a group of marine invertebrates with a rich fossil record, as a case study. Building on recent advances in computational linguistics, we develop a pipeline to recognize taxonomic names and geologic time intervals in published literature and use supervised learning to machine-read whether the species in question occurred in a given age interval. Intermediate machine error rates appear comparable to human error rates in a simple trial, and resulting genus richness curves capture the main features of published fossil diversity studies of bryozoans. We believe our automated pipeline, that greatly reduced the time required to compile our dataset, can help others compile similar data for other taxa.