Alexis Licht

Asian monsoons in a late Eocene greenhouse world

The strong present-day Asian monsoons are thought to have originated between 25 and 22 million years (Myr) ago, driven by Tibetan–Himalayan uplift. However, the existence of older Asian monsoons and their response to enhanced greenhouse conditions such as those in the Eocene period (55–34xa0Myr ago) are unknown because of the paucity of well-dated records. Here we show late Eocene climate records revealing marked monsoon-like patterns in rainfall and wind south and north of the Tibetan–Himalayan orogen. This is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from Myanmar, and by aeolian dust deposition in northwest China. Our climate simulations support modern-like Eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhanced greenhouse conditions counterbalanced the negative effect of lower Tibetan relief on precipitation. These strong monsoons later weakened with the global shift to icehouse conditions 34xa0Myr ago.

Eolian cannibalism: Reworked loess and fluvial sediment as the main sources of the Chinese Loess Plateau

The loess and paleosol sequences of the Chinese Loess Plateau are composed of Quaternary dust, the origin of which has been the subject of considerable debate. Some recent U-Pb geochronological studies of eolian zircons have proposed the existence of two major wind pathways: from the north and northwest, through the Badan Jaran, Tengger, and Mu Us Deserts during interglacials, and from the west, through the Qaidam Basin during glacials. Others have emphasized the importance of Yellow River sediment supply in the Chinese Loess Plateau sediment budget. However, tracking dust source regions through U-Pb dating in a statistically robust manner is particularly complex given the similar age peaks in the age probability distributions of potential source regions in Central Asia. This paper presents 2410 new U-Pb ages of detrital zircons from wind-eroded strata, Quaternary eolian deposits, and modern river sands in central China in order to increase the robustness and the spatial resolution of zircon age distributions in dust source regions. We then propose a new mixture modeling technique to statistically address the contribution of these different sources to the Chinese Loess Plateau sedimentary budget. Our contribution estimates indicate that eolian supply to the Chinese Loess Plateau is dominated (60%–70%) by reworking of Yellow River sediment. Moreover, evidence of Qaidam Basin–sourced zircons (15%–20%) in both loess (glacial) and paleosol (interglacial) layers corroborates the existence of an erosive wind pathway through the Qaidam Basin during glacials and implies that a substantial portion of the interglacial dust is recycled from older glacial loess. We propose that sediment reworking of Yellow River sediment and older loess deposits by wind on the Chinese Loess Plateau homogenized the eolian zircon populations toward a glacial provenance due to higher (2–20 times) dust accumulation rates during glacials. These findings suggest that the Chinese Loess Plateau has evolved as a more dynamic landform than previous thought, where wind deflation, fluvial input, lateral transport, and accumulation of sediment are equally important. These internal reworking effects would then significantly bias the paleoclimatic interpretations based on eolian dust properties of the Chinese Loess Plateau.

A palaeo Tibet–Myanmar connection? Reconstructing the Late Eocene drainage system of central Myanmar using a multi-proxy approach

Strain resulting from the collision of India with Asia has caused fundamental changes to Asian drainage patterns, but the timing and nature of these changes are poorly understood. One frequently proposed hypothesis involves the connection of the palaeo Tsangpo drainage to a precursor to the Irrawaddy River of central Myanmar in the Palaeogene. To test this hypothesis, we studied the provenance of Palaeogene fluvio-clastic sedimentary rocks that crop out in central Myanmar, namely the Late Middle Eocene–Early Oligocene Pondaung and Yaw Formations. Isotopic analysis on bulk-rock and petrographic data indicate a primary magmatic arc source, and a secondary source composed of recycled, metamorphosed basement material. Although the exact location of both sources is hardly distinguishable because Burmese and Tibetan provinces share common lithological features, the presence of low-grade metamorphic fragments, the heterogeneity in Sr–Nd isotopic values of bulk sediments and westward-directed palaeoflow orientations indicate a proximal source area located on the eastern Asian margin. Central Myanmar was the locus of westward-prograding deltas opening into the Indian Ocean, supplied by the unroofing of an Andean-type cordillera that extended along the Burmese margin. We found no evidence to support a palaeo Tsangpo–Irrawaddy River, at least during the Late Eocene. Supplementary material: Data locations, and isotopic and petrographic results are available at www.geolsoc.org.uk/SUP18655.

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.

Impact of the North American monsoon on isotope paleoaltimeters: Implications for the paleoaltimetry of the American southwest

Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (δ18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect δ18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional δ18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary δ18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. δ18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing δ18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. δ18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our δ18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ∼1 km) had already been attained by the Eocene, but are also compatible with < 1 km of uplift by post-Laramide mechanisms. Our results show the limitations of standard δ18Oc-elevation models in complex hydrological systems and suggest that similar mechanisms may have led to summer-biased paleoaltimetry estimates for the initial stages of other orogenies —in the American Southwest and elsewhere.

First Paleomagnetic Constraints on the Latitudinal Displacement of the West Burma Block

Cenozoic collision between India and Eurasia produced the Himalayan-Tibetan orogen, which is commonly considered as the archetypical orogen for continent-continent collision systems. However, there is still no consensus on the amount and mechanism of post-collisional convergence, as well as on the roles of the numerous tectonic terranes comprising the orogen (Jagoutz et al. 2015, 2016; Replumaz et al. 2013, Royden et al. 2008, van Hins-bergen et al. 2011). The West Burma block exhibits a unique geodynamic evolution within this system, influenced by oblique subduction of the Indian plate and significant strike-slip motions along the dextral Sagaing Fault. Furthermore, it is at a key location for paleoenvi-ronmental reconstructions (Cai et al. 2016, Licht et al. 2013). Despite this, robust paleo-magnetic data from the West Burma block is largely absent.Here we report new paleomagnetic, petrological and U-Pb age data to constrain the latitudinal displacement of West Burma. To this end, 45 sites were drilled in the intrusives, extrusives and sediments of the Wuntho arc, Myanmar. Paleomagnetic results were obtained at 30 sites. In addition, 135 paleomagnetic results were obtained from a Late-Eocene mono-clinic sedimentary section in the Chindwin basin, Myanmar.Wuntho arc U-Pb ages cluster in the range 110–90 Ma, indicating a Late-Cretaceous age. Paleomagnetic results from this area show declination values of around 50°–100°, im-plying clockwise rotation of the overall arc dispersed by local-block rotations related to faulting, and inclination values close to zero, corresponding to near-equatorial paleolatitude. Tilt corrections are not available for sites in intrusive rocks. However, the sampling is dis-tributed over a large area (1000 km2) and the results are found inconsistent with regional tilt-ing of the arc. The occurrence of remagnetization after tilting of the country rocks in several sites by the intrusive batholith also support the clockwise rotations and the low paleolati-tude. In the Late-Eocene sediments, normal and reverse polarity magnetizations, alongside the occurrence of numerous ~10 cm thick siderite-rich layers with stable magnetizations, in-dicate a primary detrital or a very early diagenetic origin for the acquisition of the magneti-zation. The sediments constrain a low inclination after tilt correction, which is coherent with the inferred near-equatorial position from the older Wuntho arc rocks. Based on these re-sults, we suggest that accretion of the West Burma block occurred at near-equatorial lati-tude, and that it subsequently underwent significant clockwise rotation and northward translation during the Cenozoic.

A stem bat (Chiroptera: Palaeochiropterygidae) from the late middle Eocene of northern Anatolia: implications for the dispersal and palaeobiology of early bats

Fragmentary remains of an Eocene bat are described from the middle Eocene Lülük Member of the Uzunçarşidere Formation on the Pontide terrane, in what is now north-central Anatolia. The new taxon most closely resembles the palaeochiropterygids Lapichiropteryx and Stehlinia in terms of its known dental morphology, and it is referred to the stem chiropteran family Palaeochiropterygidae on this basis. Geological and palaeontological data indicate that the Pontide terrane was an island situated along the northern margin of Neotethys during the middle Eocene. The presence of a late-surviving stem chiropteran in an island context potentially illuminates dispersal patterns and capabilities among the earliest bats, which already enjoyed a nearly global distribution by the early Eocene. Other palaeochiropterygids for which postcranial material is known share little in common with extant bats that are capable of long-range dispersal across open water. The new Turkish bat taxon is consistent with a hypothetical dispersal corridor between Western Europe and India via islands on the northern margin of Neotethys and suggests a larger range of skeletal and locomotor variation within Palaeochiropterygidae than is currently recognised.

Chronology of subduction and collision along the İzmir-Ankara suture in Western Anatolia: records from the Central Sakarya Basin

ABSTRACT Western Anatolia is a complex assemblage of terranes, including the Sakarya Terrane and the Tauride-Anatolide Platform that collided during the late Cretaceous and Palaeogene (80–25 Ma) after the closure of the Izmir-Ankara Ocean. Determining the precise timing at which this ocean closed is particularly important to test kinematic reconstructions and geodynamic models of the Mediterranean region, and the chronology of suturing and its mechanisms remain controversial. Here, we document the Cretaceous-Eocene sedimentary history of the Central Sakarya Basin, along the northern margin of the Neotethys Ocean, via various approaches including biostratigraphy, geochronology, and sedimentology. Two high-resolution sections from the Central Sakarya Basin show that pelagic carbonate sedimentation shifted to rapid siliciclastic deposition in the early Campanian (~ 79.6 Ma), interpreted to be a result of the build-up of the accretionary prism at the southern margin of the Sakarya Terrane. Rapid onset of deltaic progradation and an increase in accumulation rates in the late Danian (~ 61 Ma), as well as a local angular unconformity are attributed to the onset of collision between the Sakarya Terrane and the Tauride-Anatolide Platform. Thus, our results indicate that though deformation of the subduction margin in Western Anatolia started as early as the Campanian, the closure of the İzmir-Ankara Ocean was only achieved by the early Palaeocene.

FIRST PALEOMAGNETIC CONSTRAINTS ON THE LATITUDINAL DISPLACEMENT OF THE WEST BURMA BLOCK

Cenozoic collision between India and Eurasia produced the Himalayan-Tibetan orogen, which is commonly considered as the archetypical orogen for continent-continent collision systems. However, there is still no consensus on the amount and mechanism of post-collisional convergence, as well as on the roles of the numerous tectonic terranes comprising the orogen (Jagoutz et al. 2015, 2016; Replumaz et al. 2013, Royden et al. 2008, van Hins-bergen et al. 2011). The West Burma block exhibits a unique geodynamic evolution within this system, influenced by oblique subduction of the Indian plate and significant strike-slip motions along the dextral Sagaing Fault. Furthermore, it is at a key location for paleoenvi-ronmental reconstructions (Cai et al. 2016, Licht et al. 2013). Despite this, robust paleo-magnetic data from the West Burma block is largely absent.Here we report new paleomagnetic, petrological and U-Pb age data to constrain the latitudinal displacement of West Burma. To this end, 45 sites were drilled in the intrusives, extrusives and sediments of the Wuntho arc, Myanmar. Paleomagnetic results were obtained at 30 sites. In addition, 135 paleomagnetic results were obtained from a Late-Eocene mono-clinic sedimentary section in the Chindwin basin, Myanmar.Wuntho arc U-Pb ages cluster in the range 110–90 Ma, indicating a Late-Cretaceous age. Paleomagnetic results from this area show declination values of around 50°–100°, im-plying clockwise rotation of the overall arc dispersed by local-block rotations related to faulting, and inclination values close to zero, corresponding to near-equatorial paleolatitude. Tilt corrections are not available for sites in intrusive rocks. However, the sampling is dis-tributed over a large area (1000 km2) and the results are found inconsistent with regional tilt-ing of the arc. The occurrence of remagnetization after tilting of the country rocks in several sites by the intrusive batholith also support the clockwise rotations and the low paleolati-tude. In the Late-Eocene sediments, normal and reverse polarity magnetizations, alongside the occurrence of numerous ~10 cm thick siderite-rich layers with stable magnetizations, in-dicate a primary detrital or a very early diagenetic origin for the acquisition of the magneti-zation. The sediments constrain a low inclination after tilt correction, which is coherent with the inferred near-equatorial position from the older Wuntho arc rocks. Based on these re-sults, we suggest that accretion of the West Burma block occurred at near-equatorial lati-tude, and that it subsequently underwent significant clockwise rotation and northward translation during the Cenozoic.