Karel Žák

Palaeoenvironments and palaeoceanography changes across the Jurassic/Cretaceous boundary in the Arctic realm: case study of the Nordvik section (north Siberia, Russia)

The Jurassic/Cretaceous transition was accompanied by significant changes in palaeoceanography and palaeoenvironments in the Tethyan Realm, but outside the Tethys such data are very scarce. Here we present results of a study of the most complete section in the Panboreal Superrealm, the Nordvik section. Belemnite δ18O data show an irregular decrease from values reaching up to +1.6‰ in the Middle Oxfordian and from +0.8 to −1.7‰ in the basal Ryazanian, indicating a prolonged warming. The biodiversity changes were strongly related to sea-level oscillations, showing a relatively low belemnite and high ammonite diversity during sea-level rise, accompanied by a decrease of the macrobenthos taxonomical richness. The most prominent sea-level rise is marked by the occurrence of open sea ammonites with Pacific affinities. Peak abundances of spores and prasinophytes correlate with a negative excursion in organic carbon δ13C near the J/K boundary and could reflect blooms of green algae caused by disturbance of the marine ecosystem.

Cryogenic cave carbonates from the Cold Wind Cave, Nízke Tatry Mountains, Slovakia: Extending the age range of cryogenic cave carbonate formation to the Saalian

. Hovorka D. & Spišiak J., 1988 – Vulkanizmus mezozoika Západných Karpát. Veda, Bratislava, 263 p. (In

Cryogenic Mineral Formation in Caves

Abstract Freezing of karst water in caves forces the segregation of solutes, a process of rejection of dissolved ions by the advancing ice-water front during the growth of ice crystals. This process causes supersaturation of the unfrozen residual part of the solution and precipitation of some of dissolved compounds as minerals. Water evaporation and solution degassing additionally enhance the mineral formation. The cryogenic cave minerals constitute a variety of speleothems, which differ in practically all aspects from their counterparts formed in caves unaffected by freezing. The morphology and mineralogy of cryogenic cave minerals largely depend on the initial chemical composition of the karst water, the thickness of the water layer that freezes, and the freezing rate. The most common cryogenic minerals in the ice caves of limestone karst are fine-grained (powdery) carbonates produced by rapid water freezing in thin water layers. In contrast, slower freezing of large water volumes at cave temperature near 0°C produces coarse-grained cryogenic cave carbonates, which are typically associated with present or past permafrost conditions. Overall, the cryogenic cave carbonates are characterized by C and O isotope signatures different from that of speleothems in temperate environments. Apart from the cryogenic carbonates, several other freeze-related minerals have been identified in caves. By far, the richest diversity of cryogenic minerals occurs in gypsum-hosted ice caves.

Zhamanshin astrobleme provides evidence for carbonaceous chondrite and post-impact exchange between ejecta and Earth’s atmosphere

Chemical fingerprints of impacts are usually compromised by extreme conditions in the impact plume, and the contribution of projectile matter to impactites does not often exceed a fraction of per cent. Here we use chromium and oxygen isotopes to identify the impactor and impact-plume processes for Zhamanshin astrobleme, Kazakhstan. ε54Cr values up to 1.54 in irghizites, part of the fallback ejecta, represent the 54Cr-rich extremity of the Solar System range and suggest a CI-like chondrite impactor. Δ17O values as low as −0.22‰ in irghizites, however, are incompatible with a CI-like impactor. We suggest that the observed 17O depletion in irghizites relative to the terrestrial range is caused by partial isotope exchange with atmospheric oxygen (Δ17O = −0.47‰) following material ejection. In contrast, combined Δ17O–ε54Cr data for central European tektites (distal ejecta) fall into the terrestrial range and neither impactor fingerprint nor oxygen isotope exchange with the atmosphere are indicated.Identifying the original impactor from craters remains challenging. Here, the authors use chromium and oxygen isotopes to indicate that the Zhamanshin astrobleme impactor was a carbonaceous chrondrite by demonstrating that depleted 17O values are due to exchange with atmospheric oxygen.