<p>One of the most dramatic warming episodes of the Mesozoic occurred near the Pliensbachian-Toarcian transition (Early Jurassic). The occurrence of abundant exotic clasts and glendonites in marine strata of Siberia suggests cold conditions during the late Pliensbachian, which may have led to the episodic growth of high latitude ice-sheets. These conditions ended abruptly during the early Toarcian when temperature rose rapidly across an episode of global biogeochemical perturbation known as the Toarcian Oceanic Anoxic Event (T-OAE). The rapid marine transgression coinciding with the T-OAE onset has been tentatively attributed to the rapid demise of these polar ice-sheets, which possibly released large amounts of methane in the atmosphere through permafrost thawing. Nevertheless, the scarce quantitative estimates of Pliensbachian-Toarcian temperatures have exclusively been obtained from low paleolatitude sites. Plus, existing temperature records are mostly based on oxygen isotope thermometry and hence remain equivocal in the absence of constraints on the ocean oxygen composition of Pliensbachian-Toarcian oceans and its temporal variability. Clumped isotope (&#916;<sub>47</sub>) data from aragonite bivalve shells from one NE Siberian site have recently provided the first quantitative evidence for extreme Toarcian polar warmth, with marine temperature estimates exceeding ~15&#176;C north of the Anabar shield. In this study, we present new &#916;<sub>47</sub> data from bivalve samples from Tyung River, south of the Anabar shield that allow to substantially expand this record both spatially and temporally. Clumped isotope data from aragonite shells confirm elevated marine temperatures (~13&#176;C) at the end of the T-OAE in polar areas some 850 km away from the previous record. Upper Pliensbachian calcite shells of <em>Harpax</em> collected from coastal to deltaic, boulder-bearing deposits of a nearby site record much lower temperature (~3&#176;C) and extreme <sup>18</sup>O-depletion of environmental waters (&#948;<sup>18</sup>O = -6.5&#8240;VSMOW). These results provide the first quantitative evidence for near-freezing polar temperatures during the Late Pliensbachian, which is a key prerequisite for the hypothesis of episodic ice-sheet growth prior to the T-OAE. Beyond glacio-eustasy, our new data offer a rare glimpse of extreme changes in polar temperatures across a transition from coldhouse to greenhouse climate and will certainly prove useful for future earth system simulations of Mesozoic climates.&#160;</p>
Elastic constants of lizardite [Mg 3 Si 2 O 5 (OH) 4 ] were computed using first‐principles quantum mechanical calculations within the density functional theory. The predicted c ‐axis compressibility is much larger than measured. Modeling of the weak O‐H⋯O interactions between layers must be improved in order to better predict layered hydrated mineral elastic properties. The large computed bulk modulus range is consistent with equation of state and seismic velocities in chrysotile‐lizardite serpentinites, but shear wave velocities are lower than the lowest theoretical estimate. The low seismic velocities measured for chrysotile serpentinites can be attributed to specific contribution of the nanotube‐textured chrysotile. For antigorite, the data from acoustic and EoS measurements are consistent. If used for interpreting seismic velocities, the inferred degrees of serpentinization of the mantle wedge are higher than with the commonly used calibrations using chrysotile‐serpentinite properties. Serpentine is likely a dominant phase in low velocity areas of the mantle wedge at 30–50 km depths.
Abstract Tektites are terrestrial impact-generated glasses that are ejected long distance (up to 11,000 km), share unique characteristics and have a poorly understood formation process. Only four tektite strewn-fields are known, and three of them are sourced from known impact craters. Here we show that the recently discovered Pantasma impact crater (14 km diameter) in Nicaragua is the source of an impact glass strewn-field documented in Belize 530 km away. Their cogenesis is documented by coincidental ages, at 804 ± 9 ka, as well as consistent elemental compositions and isotopic ratios. The Belize impact glass share many characteristics with known tektites but also present several peculiar features. We propose that these glasses represent a previously unrecognized tektite strewn-field. These discoveries shed new light on the tektite formation process, which may be more common than previously claimed, as most known Pleistocene >10 km diameter cratering events have generated tektites.
