Earth Sciences

Abstract The ϵHf and δ 18 O values in detrital zircons from the Slave craton, Canada, indicate three episodes of crust formation between ca. 4.5 and 2.8 Ga, namely at ca. 4.4–4.5 Ga, ca. 3.8 Ga, and ca. 3.4 Ga. Most of the juvenile crust appears to have been mafic in ...

ABSTRACT Granites sensu lato in the Sudetes in truded in sev eral ep i sodes dur ing the Variscan orogeny recording different stages of crust and mantle evolution. Correlating precise ages with geochemistry of the Variscan granites provides infor mation on the evolution of these sources within the Variscan orogen. The Variscan in trusive rocks from the Niemcza Zone (Bohemian Massif, Sudetes, SW Po land) include undeformed dioritic to syenitic rocks and magmatically foliated granodiorites. In this study we analysed low SiO2 (48–53 wt.%) monzodioritic rocks from Przedborowa and Koźmice. The monzodiorites contain late-magmatic zircons with ages of 341.8 ± 1.9 Ma for Przedborowa and 335.6 ± 2.3 Ma for Koźmice, interpreted as emplacement ages of the dioritic magmas. Older Przedborowa rocks are lower in K, Mg, Rb and Ni than the Koźmice rocks and similar compositional trend is also observed in the Central Bohemian Plutonic Complex. The implication is that the mantle underlying the Niemcza Zone became more enriched from ca. 342 to ca. 336 Ma, probably following the collision of the Saxothuringian and Moldanubian/Lugian domains. The magmatism related to the collision occurred ca. 12 Ma later than that in the Central Bohemian Plutonic Complex, but was accompanied by a similar change in magma chemistry from high-K (Przedborowa) to shoshonitic (Koźmice, Kośmin enclaves) and probably to ultrapotassic (Wilków Wielki).

Tong-Dzuy Thanh Department of Geology, Vietnam National University, 90, Nguyen Trai, Dong Da, Hanoi, Vietnam Philippe Janvier URA 12 du CNRS, Laboratoire de Pale´ ontologie, Muse´ um National d'Histoire Naturelle, 8, rue Buffon, 75005 Paris, France Ta Hoa ...

... between fish and tetrapods was further completed by a number of new early tetrapod discoveries in the ... are diverse, their polarity is now firmly established: from fishes to tetrapods within a ... are currently trying to find the factors that regulate such macro-evolutionary patterns that is ...

ABSTRACT Periphyton can play an important role for biogeochemical processes in large regulated rivers. The development of benthic algae during low flow periods are especially suitable in braided sections, near bank zones and shallow areas. Light availability, fine sediment accumulation and flow velocity are assumed to be the main factors controlling periphyton growth, while the effect of water level fluctuations in large rivers is often neglected. However, the latter could also have severe impacts on the development of benthic algae. The present study aims to analyse how short term (hourly and daily) water level fluctuations influence the periphyton development and species richness and how desiccation stress affects the specific primary production. An in-situ experiment was conducted for one month on a Danube River bank in the free-flowing stretch of the river east of Vienna. Plates with artificial substrates were exposed at several defined depths along a fluctuating water level gradient. A short term laboratory experiment was also conducted to assess the effect of drying and rewetting on benthic algal production. Our results showed the benthic chlorophyll-a content was 40 to 60 times lower and diatom species were 5 times fewer at a desiccation-affected zone compared with the permanently-immersed zone. However, after a short term flow pulse lasting 3 days, the differences between the two zones were alleviated. In the laboratory experiment, the benthic algal community from two different colonisation stages with different chlorophyll-a content and species composition (days 12 and 21 of development) were capable of efficient photosynthesis for 2 and 5 hours of desiccation, respectively, followed by no measurable photosynthetic activity for an additional hour. Complete recovery of photosynthesis took 3 and 5 hours after rehydration. The results of this study showed that the periphyton on a riparian zone of a large regulated river is a very dynamic community with a high capacity for resilience with regard to the highly fluctuating environmental conditions. Furthermore, short-term desiccation stress should be considered as an important form of disturbance that affects algal communities in altered river systems.

Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings