Surface atoms have fewer interatomic bonds than those in the bulk that they often relax and reconstruct on extended two-dimensional surfaces. Far less is known about the surface structures of nanocrystals. Here, we show that coherent diffraction patterns recorded from individual nanocrystals are very sensitive to the atomic structure of nanocrystal surfaces. Nanocrystals of Au of 3-5 nm in diameter were studied by examining diffraction intensity oscillations around the Bragg peaks. Both results obtained from modelling the experimental data and molecular dynamics simulations strongly suggest inhomogeneous relaxations, involving large out-of-plane bond length contractions for the edge atoms (approximately 0.2 A); a significant contraction (approximately 0.13 A) for {100} surface atoms; and a much smaller contraction (approximately 0.05 A) for atoms in the middle of the {111} facets. These results denote a coordination/facet dependence that markedly differentiates the structural dynamics of nanocrystals from bulk crystalline surfaces.

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International audienceThe Tarim Basin in western China formed the easternmost margin of a shallow epicontinental seathat extended across Eurasia and was well connected to the western Tethys during the Paleogene.Climate modelling studies suggest that the westward retreat of this sea from Central Asia may havebeen as important as the Tibetan Plateau uplift in forcing aridification and monsoon intensificationin the Asian continental interior due to the redistribution of the land-sea thermal contrast. However,testing of this hypothesis is hindered by poor constraints on the timing and precise palaeogeographicdynamics of the retreat. Here, we present an improved integrated bio- and magnetostratigraphicchronological framework of the previously studied marine to continental transition in the southwestTarim Basin along the Pamir and West Kunlun Shan, allowing us to better constrain its timing,cause and palaeoenvironmental impact. The sea retreat is assigned a latest Lutetian–earliest Bartonianage (ca. 41 Ma; correlation of the last marine sediments to calcareous nannofossil Zone CP14and correlation of the first continental red beds to the base of magnetochron C18r). Higher up in thecontinental deposits, a major hiatus includes the Eocene–Oligocene transition (ca. 34 Ma). This suggeststhe Tarim Basin was hydrologically connected to the Tethyan marine Realm until at least theearliest Oligocene and had not yet been closed by uplift of the Pamir–Kunlun orogenic system. Thewestward sea retreat at ca. 41 Ma and the disconformity at the Eocene–Oligocene transition are bothtime-equivalent with reported Asian aridification steps, suggesting that, consistent with climatemodelling results, the sea acted as an important moisture source for the Asian continental interior

International audienceA vast shallow epicontinental sea extended across Eurasia and was well-connected to the Western Tethys before it retreated westward and became isolated as the Paratethys Sea. However, the palaeogeography and the timing of this westward retreat are too poorly constrained to determine potential wider environmental impacts, let alone understanding underlying mechanisms of the retreat such as global eustasy and tectonism associated with the Indo-Asia collision. Here, an improved chronostratigraphic and palaeogeographic framework is provided for the onset of the proto-Paratethys Sea retreat at its easternmost extent in the Tarim Basin in western China is provided. Five different third-order sea-level cycles can be recognised from the Cretaceous-Palaeogene sedimentary record in the Tarim Basin, of which the last two stepped successively westwards as the sea retreated after the maximum third incursion. New biostratigraphic data from the fourth and fifth incursions at the westernmost margin of the Tarim Basin are compared to our recent integrated bio-magneto-stratigraphic results on the fourth incursion near the palaeodepocentre in the south-western part of the basin. While the fourth incursion extended throughout the basin and retreated at ~ 41 Ma (base C18r), the last and fifth incursion is restricted to the westernmost margin and its marine deposits are assigned a latest Bartonian-early Priabonian age from ~ 38.0 to ~ 36.7 Ma (near top C17n.2n to base C16n.2n). Similar to the fourth, the fossil assemblages of the fifth incursion are indicative of shallow marine, near-shore conditions and their widespread distribution across Eurasia suggests that the marine connection to the Western Tethys was maintained. The lack of diachronicity of the fourth incursion between the studied sections across the southwest Tarim Basin suggests that the sea entered and withdrew relatively rapidly, as can be expected in the case of eustatic control on a shallow epicontinental basin. However, the westward palaeogeographic step between the fourth and fifth incursions separated by several millions of years rather suggests the combined long-term effect of tectonism, possibly associated with early uplift of the Pamir-Kunlun Shan thrust belt. The fourth and fifth regressions are time-equivalent with significant aridification steps recorded in the Asian interior, thus supporting climate modelling results showing that the stepwise sea retreat from Central Asia amplified the aridification of the Asian interior

[1] We present an observational study of stratospheric gravity wave spectra and seasonal variations of potential energy density at the South Pole (90°S) and Rothera (67.5°S, 68.0°W), Antarctica. The gravity wave spectra are derived from the atmospheric relative density perturbation in the altitude range of 30-45 km measured by an iron Boltzmann lidar. The ground-relative wave characteristics obtained at each location are comparable, with an annual mean vertical wavelength of $4.1 km, vertical phase speed of $0.7 m s À1 , and period of $104 min. Approximately 44% of the observed waves show an upward phase progression while the rest display a downward phase progression in ground-based reference for both locations. Gravity wave potential energy density (GW-E P ) at Rothera is $4 times higher than the South Pole in winter but is comparable in summer. Clear seasonal variations of GW-E P are observed at Rothera with the winter average being 6 times larger than that of summer. The seasonal variations of GW-E P at the South Pole are significantly smaller than those at Rothera. The absence of seasonal variations in wave sources and critical level filtering at the South Pole is likely to be responsible for the nearly constant GW-E P . The minimum critical level filtering in winter at Rothera is likely to be a main cause for the winter enhanced GW-E P , as this would allow more orography-generated waves to reach the 30 to 45 km range. The stratospheric jet streams may also contribute to the winter enhancement at Rothera.

The characteristic remanent magnetization (ChRM) isolated from Paleogene carbonate rocks of the Zongpu Formation in Gamba (28.3°N, 88.5°E) of southern Tibet has previously been interpreted to be primary. These data are pertinent for estimating the width of Greater India and dating the initiation of India‐Asia collision. We have reanalyzed the published ChRM directions and completed thorough rock magnetic tests and petrographic observations on specimens collected throughout the previously investigated sections. Negative nonparametric fold tests demonstrate that the ChRM has a synfolding or postfolding origin. Rock magnetic analyses reveal that the dominant magnetic carrier is magnetite. “Wasp‐waisted” hysteresis loops, suppressed Verwey transitions, high frequency‐dependent in‐phase magnetic susceptibility, and evidence that >70% of the ferrimagnetic material is superparamagnetic at room temperature are consistent with the rock‐magnetic fingerprint of remagnetized carbonate rocks. Scanning electron microscopy observations and energy‐dispersive X‐ray spectrometry analysis confirm that magnetite grains are authigenic. In summary, the carbonate rocks of the Zongpu Formation in Gamba have been chemically remagnetized. Thus, the early Paleogene latitude of the Tibetan Himalaya and size of Greater India have yet to be determined and the initiation of collision cannot yet be precisely dated by paleomagnetism. If collision began at 59 ± 1 Ma at ~19°N, as suggested by sedimentary records and paleomagnetic data from the Lhasa terrane, then a huge Greater India, as large as ~3500–3800 km, is required in the early Paleogene. This size, in sharp contrast to the few hundred kilometers estimated for the Early Cretaceous, implies an ever greater need for extension within Greater India during the Cretaceous.

A systematic bias towards low palaeomagnetic inclination recorded in clastic sediments, that is, inclination shallowing, has been recognized and studied for decades. Identification, understanding and correction of this inclination shallowing are critical for palaeogeographic reconstructions, particularly those used in climate models and to date collisional events in convergent orogenic systems, such as those surrounding the Neotethys. Here we report palaeomagnetic inclinations from the sedimentary Eocene upper Linzizong Group of Southern Tibet that are ∼20 • lower than conformable underlying volcanic units. At face value, the palaeomagnetic results from these sedimentary rocks suggest the southern margin of Asia was located ∼10 • N, which is inconsistent with recent reviews of the palaeolatitude of Southern Tibet. We apply two different correction methods to estimate the magnitude of inclination shallowing independently from the volcanics. The mean inclination is corrected from 20.5 • to 40.0 • within 95 per cent confidence limits between 33.1 • and 49.5 • by the elongation/inclination (E/I) correction method; an anisotropy-based inclination correction method steepens the mean inclination to 41.3 ± 3.3 • after a curve fitting-determined particle anisotropy of 1.39 is applied. These corrected inclinations are statistically indistinguishable from the well-determined 40.3 ± 4.5 o mean inclination of the underlying volcanic rocks that provides an independent check on the validity of these correction methods. Our results show that inclination shallowing in sedimentary rocks can be corrected. Careful inspection of stratigraphic variations of rock magnetic properties and remanence anisotropy suggests shallowing was caused mainly by a combination of syn-and post-depositional processes such as particle imbrication and sedimentary compaction that vary in importance throughout the section. Palaeolatitudes calculated from palaeomagnetic directions from Eocene sedimentary rocks of the upper Linzizong Group that have corrected for inclination shallowing are consistent with palaeolatitude history of the Lhasa terrane, and suggest that the India-Asia collision began at ∼20 • N by 45-55 Ma.