Refining the Early Devonian time scale using Milankovitch cyclicity in Lochkovian–Pragian sediments (Prague Synform, Czech Republic)

The Early Devonian geological time scale (base of the Devonian at , Becker et al., 2012) suffers from poor age control, with associated large uncertainties between 2.5 and 4.2 Myr on the stage boundaries. Identifying orbital cycles from sedimentary successions can serve as a very powerful chronometer to test and, where appropriate, improve age models. Here, we focus on the Lochkovian and Pragian, the two lowermost Devonian stages. High-resolution magnetic susceptibility ( – 5 to 10 cm sampling interval) and gamma ray spectrometry (GRS – 25 to 50 cm sampling interval) records were gathered from two main limestone sections, Požár-CS (118 m, spanning the Lochkov and Praha Formations) and Pod Barrandovem (174 m; Praha Formation), both in the Czech Republic. An additional section (Branžovy, 65 m, Praha Formation) was sampled for GRS (every 50 cm). The and GRS records are very similar, so variations are driven by variations in the samples' paramagnetic clay mineral content, reflecting changes in detrital input. Therefore, climatic variations are very likely captured in our records. Multiple spectral analysis and statistical techniques such as: Continuous Wavelet Transform, Evolutive Harmonic Analysis, Multi-taper method and Average Spectral Misfit, were used in concert to reach an optimal astronomical interpretation. The Požár-CS section shows distinctly varying sediment accumulation rates. The Lochkovian (essentially equivalent to the Lochkov Formation (Fm.)) is interpreted to include a total of nineteen 405 kyr eccentricity cycles, constraining its duration to . The Praha Fm. includes fourteen 405 kyr eccentricity cycles in the three sampled sections, while the Pragian Stage only includes about four 405 kyr eccentricity cycles, thus exhibiting durations of and respectively. Because the Lochkov Fm. contains an interval with very low sediment accumulation rate and because the Praha Fm. was cross-validated in three different sections, the uncertainty in the duration of the Lochkov Fm. and the Lochkovian is larger than that of the Praha Fm. and Pragian. The new floating time scales for the Lochkovian and Pragian stages have an unprecedented precision, with reduction in the uncertainty by a factor of 1.7 for the Lochkovian and of ∼6 for the Pragian. Furthermore, longer orbital modulation cycles are also identified with periodicities of ∼1000 kyr and 2000–2500 kyr

Trace metal distribution in the bed, bank and suspended sediment of the Ravensbourne River and its implication for sediment monitoring in an urban river

Purpose This study aims to identify a suitable sediment compartment for sediment quality monitoring by: (a) studying the concentration of trace metals (Cd, Cu, Ni, Pb and Zn) in the bed, bank and suspended sediment compartments of the Ravensbourne River to establish any differences in trace metal concentrations with compartment; (b) determining the influence of sediment particle size fractions ( 0.05) in the concentrations of metals between the suspended sediment and the < 63 μm bed sediment fraction, but there was a significant difference (p < 0.05) between the suspended sediment and the < 63 μm bank sediment fraction. There were also significant differences between the concentrations of metals in the < 63 μm and the 63 μm–2 mm fractions. Generally, the Ravensbourne River did not comply with the draft UK sediment quality guidelines for the metals analysed. Conclusions This study shows the importance of identifying a suitable sediment compartment to sample for compliance with sediment quality standards. The bed and suspended sediments are the most widely used sediment compartments for sediment monitoring, but collecting sufficient mass of the < 63 μm sediment fraction for monitoring presents a challenge for urban gravel bed rivers like the Ravensbourne River. It seems appropriate to establish individual monitoring regimes for different rivers

The geochemistry and origin of fluids in the carbonate structure of the Hranice Karst with the world\u27s deepest flooded cave of the Hranicka Abyss, Czech Republic

The origin of fluids in the Hranice Karst containing the deepest flooded abyss in the world has been investigated using hydrogeological, hydrogeochemical, and isotopic data. At least a part of the CO2 gas originates in the mantle as indicated by very enriched δ13C(DIC) values and from existing He isotope analyses. The origin of groundwater in the karstic aquifer which is exploited at the Teplice nad Bečvou Spa is meteoric with a recharge area about 200 m above the Bečva River valley as indicated by depleted values of δ2H and δ18O compared to the river water. Based on detectable tritium, the groundwater is from 20 to 50 years old. Water in the Hranicka Abyss and in the Zbrašov Aragonite Caves is a mixture of carbonate aquifer groundwater with the river and/or shallow groundwater comprising variable proportions of both end-members. Water in Death Cave Lake seems to be affected by agriculture contaminated shallow groundwater as indicated by increased nitrate concentration. Inverse geochemical modeling of aquifer geochemistry suggested two scenarios: (1) reaction of Mg-rich calcite with deep hypogenic CO2 (about 30 mmol/l) plus dissolution of trace amounts of halite and sylvite and cation exchange; (2) reaction of Mg-depleted calcite and Mg-silicate (talc) as a source of Mg together with deep CO2. Both scenarios were calibrated using δ13C(DIC) values and gave satisfactory results. A conceptual model of the site has been developed which includes a gravity-driven flow system where meteoric water which has recharged in the surrounding uplands is heated at depth and acquires large amounts of hypogenic CO2, which preferentially dissolves Mg-carbonates along the pre-existing tectonic features. The Miocene transgression followed by the later incision of the Bečva Valley played an important role in groundwater circulation and the origin of fluids

New palaeomagnetic data from the Palaeozoic carbonates of the Moravo-Silesian Zone (Czech Republic): evidence for a timing and origin of the late Variscan remagnetization

Palaeomagnetic studies were carried out in the Devonian–Early Carboniferous carbonates of the Moravo-Silesian Zone — MSZ (Czech Republic) in order to evaluate the timing and origin of late Variscan magnetic over printing. Sampling localities were spread out along the strike of the MSZ from the SW to NE. Previously published thermal maturity data have demonstrated a significant gradient from SW (burial temperatures 150–200gradeC) to NE of the region (250–300gradeC). A late Variscan remagnetization direction (component A), carried by magnetite, was identified in 6 localities. Three phases of the remagnetization in the MSZ might be distinguished which might be assigned to Early to Late Carboniferous, Late Carboniferous and Early Permian. They are coeval with remagnetization events distinguished in Ardennes. A correlation exists between thermal indices and un blocking temperature spectra of component A. Thermal activation nomograms show that component A might be either a thermoviscous or thermochemical remanent magnetization acquired due to a thermal event (deep burial) of 1–10 My duration and stabilized during subsequent uplift. A more ancient component B, identified in the SW part, previously interpreted as primary, is shown to be a synfolding remagnetisation. Itindicates 70grade clockwise rotations before the acquis tion of the component A

Oxygen and carbon stable isotope records of the Lochkovian-Pragian boundary interval from the Prague Basin (Lower Devonian, Czech Republic)

Climate changes close to the Lochkovian-Pragian Event are still widely discussed. Carbonate δ18O and δ13C and conodont apatite δ18O from medial to distal carbonate ramp sediments were analysed to provide further stable isotope data from the stratotype area in the Prague Basin. The uppermost Silurian to lower Emsian δ18O trends are put into an updated conodont biostratigraphy framework. Carbonate δ18O and δ13C are discussed in the context of facies-dependent diagenesis. The δ13C pattern measured from different sections enables the correlation of intervals with development of different facies inside the Prague Basin. Positive δ13C shifts are often coupled with trends of decreasing computed gamma-ray (CGR) values interpreted as regressions, whereas negative δ13C shifts commonly coincide with opposite trends in CGR. The Lochkovian-Pragian boundary interval coincided with an increase in carbonate δ13C and an increase in the δ18O of conodont apatite and carbonate, which we interpret as consequence of climate cooling. Generally, the Lochkovian was warmer than the Pragian and earliest Emsian