Hemmo A. Abels

Tibetan plateau aridification linked to global cooling at the Eocene-Oligocene transition

Continental aridification and the intensification of the monsoons in Asia are generally attributed to uplift of the Tibetan plateau and to the land–sea redistributions associated with the continental collision of India and Asia, whereas some studies suggest that past changes in Asian environments are mainly governed by global climate. The most dramatic climate event since the onset of the collision of India and Asia is the Eocene–Oligocene transition, an abrupt cooling step associated with the onset of glaciation in Antarctica 34 million years ago. However, the influence of this global event on Asian environments is poorly understood. Here we use magnetostratigraphy and cyclostratigraphy to show that aridification, which is indicated by the disappearance of playa lake deposits in the northeastern Tibetan plateau, occurred precisely at the time of the Eocene–Oligocene transition. Our findings suggest that this global transition is linked to significant aridification and cooling in continental Asia recorded by palaeontological and palaeoenvironmental changes, and thus support the idea that global cooling is associated with the Eocene–Oligocene transition. We show that, with sufficient age control on the sedimentary records, global climate can be distinguished from tectonism and recognized as a major contributor to continental Asian environments.

Asian aridification linked to the first step of the Eocene-Oligocene climate Transition (EOT) in obliquity-dominated terrestrial records (Xining Basin, China)

Asian terrestrial records of the Eocene-Oligocene Transition (EOT) are rare and, when available, often poorly constrained in time, even though they are crucial in understanding the atmospheric impact of this major step in Cenozoic climate deterioration. Here, we present a detailed cyclostratigraphic study of the continuous continental EOT succession deposited between ∼35 to 33 Ma in the Xining Basin at the northeastern edge of Tibetan Plateau. Lithology supplemented with high-resolution magnetic susceptibility (MS), median grain size (MGS) and color reflectance (a) records reveal a prominent ∼3.4 m thick basic cyclicity of alternating playa gypsum and dry mudflat red mudstones of latest Eocene age. The magnetostratigraphic age model indicates that this cyclicity was most likely forced by the 41kyr obliquity cycle driving oscillations of drier and wetter conditions in Asian interior climate from at least 1 million year before the EOT. In addition, our results suggest a duration of∼0.9 Myr for magnetochron C13r that is in accordance with radiometric dates from continental successions in Wyoming, USA, albeit somewhat shorter than in current time scales. Detailed comparison of the EOT interval in the Tashan section with marine records suggest that the most pronounced lithofacies change in the Xining Basin corresponds to the first of two widely recognized steps in oxygen isotopes across the EOT. This first step precedes the major and second step (i.e. the base of Oi-1) and has recently been reported to be mainly related to atmospheric cooling rather than ice volCorrespondence to: H. A. Abels (abels@geo.uu.nl) ume growth. Coincidence with lithofacies changes in our Chinese record would suggest that the atmospheric impact of the first step was of global significance, while the major ice volume increase of the second step did not significantly affect Asian interior climate.

Stratigraphic continuity and fragmentary sedimentation: the success of cyclostratigraphy as part of integrated stratigraphy

Abstract The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (104 years up to 106 years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record.

Resilience of the Asian atmospheric circulation shown by Paleogene dust provenance

The onset of modern central Asian atmospheric circulation is traditionally linked to the interplay of surface uplift of the Mongolian and Tibetan-Himalayan orogens, retreat of the Paratethys sea from central Asia and Cenozoic global cooling. Although the role of these players has not yet been unravelled, the vast dust deposits of central China support the presence of arid conditions and modern atmospheric pathways for the last 25 million years (Myr). Here, we present provenance data from older (42–33 Myr) dust deposits, at a time when the Tibetan Plateau was less developed, the Paratethys sea still present in central Asia and atmospheric pCO2 much higher. Our results show that dust sources and near-surface atmospheric circulation have changed little since at least 42 Myr. Our findings indicate that the locus of central Asian high pressures and concurrent aridity is a resilient feature only modulated by mountain building, global cooling and sea retreat.

Orbitally forced Azolla blooms And Middle Eocene Arctic hydrology: Clues from palynology

The high abundances and cyclic distribution of remains of the freshwater fern Azolla in early-Middle Eocene sediments from the Arctic Ocean have previously been related to episodic surface-water freshening, which was speculated to be orbitally modulated. Our integrated palynological and cyclostratigraphical analysis of the recovered Azolla interval in Integrated Ocean Drilling Program (IODP) core 302-M0004A-11X resulted in the recognition of two clear periodicities: a dominant ~1.2 m cyclicity, which we relate to changes in obliquity (~40 k.y.), and a weaker ~0.7 m cyclicity, which we link to precession (~21 k.y.). Cycles in the abundances of Azolla, cysts of freshwater-tolerant dinoflagellates, and swamp-vegetation pollen show covariability in the obliquity domain. This strong correlation suggests periods of enhanced rainfall and runoff during Azolla blooms, presumably linked to increased local summer temperatures during obliquity maxima. Larix and bisaccate conifer pollen covary at the precession frequency, with peak occurrences corresponding to precession minima, possibly as a result of enhanced continental runoff from a more remote source area and a stronger seasonal contrast. Following the sudden demise of Azolla ca. 48.1 Ma, runoff (cycles) continued to influence the central Arctic at decreased intensity. This and a concomitant decline in swamp-vegetation pollen suggest edaphically drier conditions on land and decreased runoff into the Arctic Ocean, causing salinity changes, which might have been fatal for Azolla. Moreover, a sea-level rise, inferred from overall decreasing total terrestrial palynomorph concentrations, possibly facilitated oceanic connections.

Repetitive mammalian dwarfing during ancient greenhouse warming events

Ancient mammals dwarf in size during multiple episodes of extreme global warming. Abrupt perturbations of the global carbon cycle during the early Eocene are associated with rapid global warming events, which are analogous in many ways to present greenhouse warming. Mammal dwarfing has been observed, along with other changes in community structure, during the largest of these ancient global warming events, known as the Paleocene-Eocene Thermal Maximum [PETM; ~56 million years ago (Ma)]. We show that mammalian dwarfing accompanied the subsequent, smaller-magnitude warming event known as Eocene Thermal Maximum 2 [ETM2 (~53 Ma)]. Statistically significant decrease in body size during ETM2 is observed in two of four taxonomic groups analyzed in this study and is most clearly observed in early equids (horses). During ETM2, the best-sampled lineage of equids decreased in size by ~14%, as opposed to ~30% during the PETM. Thus, dwarfing appears to be a common evolutionary response of some mammals during past global warming events, and the extent of dwarfing seems related to the magnitude of the event.

Steppe development on the Northern Tibetan Plateau inferred from Paleogene ephedroid pollen

Abstract Steppe vegetation represents a key marker of past Asian aridification and is associated with monsoonal intensification. Little is, however, known about the origin of this pre-Oligocene vegetation, its specific composition and how it changed over time and responded to climatic variations. Here, we describe the morphological characters of Ephedraceae pollen in Eocene strata of the Xining Basin and compare the pollen composition with the palynological composition of Late Cretaceous and Paleocene deposits of the Xining Basin and the Quaternary deposits of the Qaidam Basin. We find that the Late Cretaceous steppe was dominated by Gnetaceaepollenites; in the transition from the Cretaceous to the Paleocene, Gnetaceaepollenites became extinct and Ephedripites subgenus Ephedripites dominated the flora with rare occurrences of Ephedripites subgen. Distachyapites; the middle to late Eocene presents a strong increase of Ephedripites subgen. Distachyapites; and the Quaternary/Recent is marked by a significantly lower diversity of Ephedraceae (and Nitrariaceae) compared to the Eocene. In the modern landscape of China, only a fraction of the Paleogene species diversity of Ephedraceae remains and we propose that these alterations in Ephedreaceae composition occurred in response to the climatic changes at least since the Eocene. In particular, the strong Eocene monsoons that enhanced the continental aridification may have played an important role in the evolution of Ephedripites subgen. Distachyapites triggering an evolutionary shift to wind-pollination in this group. Conceivably, the Ephedraceae/Nitrariaceae dominated steppe ended during the Eocene/Oligocene climatic cooling and aridification, which favoured other plant taxa.

Clay mineralogical constraints on weathering in response to early Eocene hyperthermal events in the Bighorn Basin, Wyoming (Western Interior, USA)

Series of transient greenhouse warming intervals in the early Eocene provide an opportunity to study the response of rock weathering and erosion to changes in temperature and precipitation. During greenhouse warming, chemical weathering is thought to increase the uptake of carbon from the atmosphere, while physical weathering and erosion control sediment supply. A large ancient greenhouse warming event is the Paleocene-Eocene Thermal Maximum at 56 Ma. In many coastal sites, an increase in the abundance of kaolinite clay during the Paleocene-Eocene Thermal Maximum is interpreted as the result of reworking from terrestrial strata due to enhanced runoff caused by increased seasonal precipitation and storminess during a time of decreased vegetation cover. In the continental interior of North America, Paleocene-Eocene Thermal Maximum paleosols show more intense pedogenesis and drying, which are indicated by deeply weathered and strongly oxidized soil profiles. The weathering and oxidation could be related to temperature and precipitation changes, but also to increased time available for weathering and increased soil permeability in coarser sediment. Here, we provide evidence for enhanced climate seasonality, increased erosion of proximal laterites and intrabasinal floodplain soils, and a potential slight increase in chemical weathering during the smaller early Eocene hyperthermals (Eocene Thermal Maximum 2, including H1 and H2) postdating the Paleocene-Eocene Thermal Maximum, for which no previous clay mineral data were available. Hyperthermal soil formation at the site of floodplain deposition causes a similar, insignificant clay mineralogical change as occurred during the background climates of the early Eocene by showing small increases in smectite and decreases in illite-smectite and illite. Remarkably, the detrital sediments during the hyperthermals show a similar pedogenic-like increase of smectite and decreases of mixed-layer illite-smectite and illite, while the kaolinite and chlorite proportions remained low and unchanged. Since sedimentation rates and provenance were similar during the events, enhanced smectite neoformation during soil formation in more proximal settings, and associated reworking, is the likely process causing this clay mineralogical change. The hundreds to thousands of year time scales at which individual paleosols were formed were probably too short for significant alteration of the rocks by in situ chemical weathering despite changing climates during the two post−Paleocene-Eocene Thermal Maximum greenhouse warming episodes. The relatively small signal, however, raises the question of whether increased chemical weathering can indeed be a strong negative feedback mechanism to enhanced greenhouse gas warming over the time scales at which these processes act.

Sedimentary geology of the middle Carboniferous of the Donbas region (Dniepr-Donets basin, Ukraine)

The Paleozoic Dniepr-Donets Basin in Belarus, Ukraine, and Russia forms a major hydrocarbon province. Although well- and seismic data have established a 20 km thick stratigraphy, field-studies of its sediments are scarce. The inverted Donbas segment (Ukraine) exposes the middle Carboniferous part of the basins stratigraphy. Here, we provide detailed sedimentological data from 13 sections that cover 1.5 of the total of 5 km of the Bashkirian and Moscovian stages and assess the paleoenvironment and paleo-current directions. Middle Carboniferous deposition occurred in a shelf environment, with coal deposition, subordinate fluvial facies, and abundant lower and middle shoreface facies, comprising an intercalated package of potential source and reservoir rocks. Sedimentary facies indicate a paleodepth range from below storm wave base to near-coastal swamp environments. Sedimentation and subsidence were hence in pace, with subtle facies changes likely representing relative sea-level changes. Paleocurrent directions are remarkably consistently southeastward in time and space in the different sedimentary facies across the Donbas Fold Belt, illustrating a dominant sedimentary infill along the basin axis, with little basin margin influence. This suggests that the middle Carboniferous stratigraphy of the Dniepr-Donets basin to the northwest probably contains significant amounts of fluvial sandstones, important for assessing hydrocarbon reservoir potential.

An expanded lower Eocene shelf sequence from the eastern Aquitaine Basin, SW France: biostratigraphy, biofacies, and stable carbon and oxygen isotopes

The early Eocene is characterized by a succession of orbitally-controlled global stable carbon isotope excursions, with some being linked to climatic and related biotic perturbations. The impact of these isotopic excursions has been primarily studied in deep-sea sections under comparably stable conditions. In order to investigate the impact of global post-PETM isotopic signals on shallow marine settings, the Ypresian neritic ʻBlue Marls’ of the Corbieres (SW France) were investigated. High-resolution records of microfossil biota and stable carbon and oxygen isotopes pinpoint biostratigraphic, paleoecologic and geochemic constraints. Calcareous nannofossil biostratigraphy positions the sequence in the upper part of zone NP11, possibly ranging into basal NP12, which is conformable with larger benthic foraminifera data indicating shallow benthic zone SBZ8. This implies a time span of about 0.4 Myr and high overall sedimentation rates of about ~ 32 cm/kyr for the section. A shallowing upward trend from outer neritic to coastal settings is observed in the development of the lithostratigraphy and the microfossil assemblage. The assemblages can be subdivided in seven larger biofacies and four ostracod assemblage zones. The lower third of the section is characterized by strongly fluctuating and partly high plankton/benthos-ratios for neritic settings. A final pronounced peak in plankton occurrence is associated with strong decrease of benthic biota, suggesting anoxic conditions in the outer neritic environment. Several local negative δ13C- and δ18O-excursions can be identified in the section. The upper, most pronounced and consistent negative δ13C excursion is tentatively linked to global carbon isotope excursion K (ETM3) based on the biostratigraphic constraints.