Late Quaternary glacial and climate history of the Pamir Mountains derived from cosmogenic Be exposure ages

Moraines southwest of Lake Yashilkul, Pamir, Tajikistan, were dated using ¹⁰Be exposure ages of boulder surfaces. We found evidence for (1) an extensive glaciation ∼60,000 yr ago; (2) a less extensive glacial advance, which deposited a characteristic hummocky moraine lobe with exposure ages ranging from ∼11,000 to 47,000 yr, probably deposited at or before 47,000 yr ago; and (3) lateral moraines with exposure ages of ∼40,000 yr, 27,000 yr and 19,000 yr, respectively. Increasing aridity in the Pamir is most likely responsible for the progressively limited extent of the glaciers during the Late Pleistocene.     

Discrimination between different water bodies from a multilayered aquifer (Kaluvelly watershed,India): Trace element signature

In the multilayered aquifer of Kaluvelly (India), comprising various sedimentary layers overlying a charnockitic basement, concentrations of trace elements were measured in aquifer formations and in groundwaters to identify geochemical tracers for water bodies. The two main sandstones (Cuddalore and Vanur) originate from the charnockites and the Cuddalore sandstone has experienced lateritization. In the studied area, two charnockite end-members were identified: a dioritic and a granitic one. Mineralogical composition and whole-rock Ti concentrations confirmed the charnockite which displayed the granitic composition as the parent rock of the two sandstones. Titanium distribution indicates that the Cuddalore sandstone originates from a more intense weathering of the parent material than the Vanur sandstone. Despite extensive differences in trace element contents recorded in aquifer formations, only a few trace elements were suitable to distinguish the water bodies. Among soluble elements, Li (in the Vanur aquifer) and Ba (in the charnockite and carbonaceous aquifers) can be used as tracers. As the input of these elements in solution is mainly regulated by the available stock, for a given mineralogical origin there is a direct link between the aquifer formation composition and water signature. With the exception of As, concentrations of redox-sensitive elements were not preserved during pumping because of oxidation, preventing their use as tracers. Low-mobility elements such as La, Ce, Th, Zr, Nb, Hf, or Ta were too insoluble to be detected in waters and/or to record the aquifer formation signature. Their input in solution was not regulated by the available stock but by the dissolution rate of rock-forming minerals. Only Ti can be used to distinguish between two out of the three aquifers (charnockite and Vanur). The specific behavior of Ti recorded in these waters may be linked to rutile inclusions within plagioclases and to the influence of climate on Ti solubility.     

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Major shoreline retreat and sediment starvation following Snowball Earth

While cap dolostones are integral to the Snowball Earth hypothesis, the current depositional model does not account for multiple geological observations. Here we propose a model that rationalises palaeomagnetic, sequence‐stratigraphic and sedimentological data and supports rapid deglaciation with protracted cap dolostone precipitation. The Snowball Earth hypothesis posits that a runaway ice‐albedo can explain the climate paradox of Neoproterozoic glacial deposits occurring at low palaeolatitudes. This scenario invokes volcanic degassing to increase atmospheric greenhouse gases to a critical threshold that overcomes the albedo effect and brings the planet back from the ice‐covered state. Once this occurs, Earth should shift rapidly from a snowball to an extreme greenhouse. However, cap dolostone units overlying glacial sediments, typically interpreted as transgressive deposits, exhibit multiple magnetic reversals indicating they accumulated in >10⁵ years. By reviewing modern post‐glacial systems, sequence stratigraphic concepts and principles of sedimentology, we suggest that cap dolostones are not restricted to the transgression but rather represent sediment starvation following a major landward shoreline migration associated with the demise of Snowball Earth. Thus, the duration in which cap dolostone accumulated is not directly coupled to the timescale of the Snowball Earth deglaciation.     

Insights about the formation of a complex impact structure formed in basalt from numerical modeling: The Vista Alegre structure,southern Brazil

We present the outcomes of simulations of the formation of the Vista Alegre impact structure, Paraná Basin, Brazil. The target comprised a thick sequence of volcanic rocks of predominantly basaltic composition of the Serra Geral Formation that had been deposited on top of sedimentary rocks (sandstones) of the Pirambóia/Botucatu formations. The cratering process was modeled using the iSALE shock physics code. Our best‐fit model suggests that (1) the crater was originally ~10 km in size; (2) it was formed in ~115 s by a stony projectile of 1000 m in diameter, for an assumed impact velocity of 12 km s^?1; (3) target rocks underwent a peak pressure of ~20 GPa, in agreement with previous petrographic investigations of shock deformation. Furthermore, the model points out that the sedimentary strata below the layer of volcanic rocks were raised by ~650 meters at the central part of the crater, which resulted in the current partial exposure of the sandstones at the surface. The outcomes of our modeling suggest that parameters like cohesion and strength of the target rocks, after shock compression, determined the final morphology of the crater, especially the absence of a topographically prominent central peak. Finally, the results of the numerical modeling are roughly in agreement with gravity data over the structure, in particular with respect to the presence of the uplifted sedimentary strata, which are responsible for a low gravity signature at the center of the structure.     

Tellurium isotopic composition of the early solar system—A search for effects resulting from stellar nucleosynthesis, Sn decay, and mass-independent fractionation

New precise Te isotope data acquired by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) are presented for selected extraterrestrial and terrestrial materials. Bulk samples of carbonaceous, ordinary and enstatite chondrites as well as the metal and sulfide phases of iron meteorites were analyzed to search for nucleosynthetic isotope anomalies and to find evidence of formerly live ¹²⁶Sn, which decays to ¹²⁶Te with a half-life of 234,500 yr. None of the meteorites show evidence of mass dependent Te isotope fractionations larger than 2‰ for δ^126/128Te. Following internal normalization of the data to ¹²⁵Te/¹²⁸Te, the Te isotope ratios of all analyzed meteorites were found to be identical to a terrestrial standard, within uncertainties. This provides evidence that the regions of the solar disk that were sampled during accretion of the meteorite parent bodies were well mixed and homogeneous on a large scale, with respect to Te isotopes. The data acquired for bulk carbonaceous chondrites indicate that the initial ¹²⁶Sn/¹¹⁸Sn ratio of the solar system was <4 × 10⁻⁵, but this is dependent on the assumption that no redistribution of Sn and Te occurred since the start of the solar system. Five Archean sedimentary sulfides that display both mass dependent and mass-independent isotope effects for S yield internally normalized Te isotope data, which indicate that mass-independent Te isotope effects are absent. The mass dependent fractionations in these samples are constrained to be less than ∼1‰ for δ^126/128Te.     

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.     

Stratigraphy of nutrients and metals in sediment profiles of two dimictic lakes in North-Eastern Germany

A sedimentological study of two dimitic lakes in North-Eastern Germany provided a record of anthropogenic impacts and historical changes of water quality. The upper 50 cm sediment profiles were compared for major nutrients and selected major trace elements. The sediments were dated by ²¹⁰Pb and ¹³⁷Cs measurement. The upper 50 cm sediment profiles represent approximately the last 100 years of history in both lakes. Element analyses show different characteristic stratigraphic patterns in both lakes. Based on the nutrient and metal stratification, three characteristic time periods can be documented for both lakes. In addition to agricultural use of the catchment area, atmospheric pollution greatly influenced the metal concentration in the sediment layers. Variation in the external loading and redox conditions in the hypolimnion explain the variation in the composition and accumulation of metals in the sediment stratigraphy. No increases or changes in the trophic level of either lake could be documented based on the accumulation of the nutrients C, N and P. The ratio of Fe/Mn and Fe/Ca characterized the changing redox conditions. The stratigraphy of Pb and Zn agrees with the historical variation in atmospheric pollution and confirms literature values for Central and North Europe. The drop in Pb and Zn over the last 10–15 years is a regional effect in North-Eastern Germany.