Impacts of ocean gateway and basin width on Tertiary tropical climate variability in a prototype model

Using a simple tropical climate model, we investigated possible impacts of changes in oceanic seaways (Panama and Tethys) and ocean basin sizes (great Pacific and narrow Atlantic) on tropical climate variability during Tertiary. Our model showed that the opening of seaways had little influence on climate variability in the tropical Pacific because the climate variability in the Pacific Ocean’s large basins were internally generated, regardless of the variation in the tropical Atlantic Ocean. Conversely, the climate variability in the tropical Atlantic Ocean was highly dependent on the tropical Pacific Ocean; thus, an opening seaway, particularly the Panama seaway, was crucial in generating the interannual variability in the tropical Atlantic Ocean. We also found that in the Pacific Ocean, basin size strongly modified the period and amplitude of the interannual variability of both the Pacific and Atlantic Oceans due to ocean wave dynamics.     

Quantification of groundwater discharge into lakes using radon-222 as naturally occurring tracer

A case study was carried out with the aim to practically test whether estimates of groundwater discharge rates into dredging lakes can be made via an uncomplicated and straightforward technique using radon-222 as naturally-occurring groundwater tracer. Lake Ammelshainer See, a dredging, seepage lake, approximately 25 km east of Leipzig, Germany, was chosen as the investigation site. In order to evaluate changes in the spatial and temporal radon-222 patterns in the lake during different stages of stratification, sampling campaigns were conducted in April 2007 (well mixed stage) and in May 2007 (thermal stratification stage). Groundwater flow estimates were made using a radon mass balance approach accounting for all radon fluxes into and out of the lake and assuming steady-state conditions with respect to these radon fluxes. Once all positive and negative radon fluxes related to the lake water volume were determined, groundwater discharge was estimated by using the advective radon input and the radon activity concentration of the pore water as key parameters. The results showed that in case of a lake with a size and shape of Lake Ammelshainer See (530,000 m²) reasonable groundwater discharge estimates can be made by collection and analyzing just a few water samples and a few samples from the sediment layer.     

Tracing anthropogenically driven groundwater discharge into a coastal lagoon from southern Brazil

We investigated the distribution of naturally occurring geochemical tracers (²²²Rn, ²²³Ra, ²²⁴Ra, ²²⁶Ra, CH₄, δ¹⁸O, and δ²H) in the water column and adjacent groundwater of Mangueira Lagoon as proxies of groundwater discharge. Mangueira Lagoon is a large (90 km long), shallow (4–5 m deep), fresh, and non-tidal coastal lagoon in southern Brazil surrounded by extensively irrigated rice plantations and numerous irrigation canals. We hypothesized that the annual, intense irrigation for rice agriculture creates extreme conditions that seasonally change groundwater discharge patterns in the adjacent lagoon. We further supposed that dredging of irrigation canals alters groundwater fluxes.

While the activities of ²²²Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH₄ and radium isotopes were only 1 order of magnitude higher. Therefore, ²²²Rn appears to be the preferred groundwater tracer in this system. Radon concentrations and conductivities were dramatically higher near the pump house of rice irrigation canals, consistent with a groundwater source. Modeling of radon inventories accounting for total inputs (groundwater advection, diffusion from sediments, and decay of ²²⁶Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicated that groundwater advection rates in the irrigation canals (25 cm/d) are over 2 orders of magnitude higher than along the shoreline (0.1 cm/d). Nearly 75% of the total area of the canals is found in the southern half of the lagoon, where groundwater inputs seem to be higher as also indicated by methane and stable isotope trends. In spite of the relatively small area of the canals, we estimate that they contribute nearly 70% of the total (57,000 m³/d) groundwater input into the entire Mangueira Lagoon. We suggest that the dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into the lagoon.     

New devices for sediment and water sampling

Four new devices for sediment and water sampling are described, viz. (1) a large-volume water sampler; (2) a water sampler for thin water layers, (3) a bottom-water sampler, and (4) a gravity corer with exchangeable plastic tubes for use in a small boat.     

Micro-scale S isotope studies of the Kharaelakh intrusion, Noril’sk region, Siberia: Constraints on the genesis of coexisting anhydrite and sulfide minerals

The coexistence of magmatic anhydrite and sulfide minerals in non-arc-related mafic magmas has only rarely been documented. Likewise the S isotope fractionation between sulfate and sulfide in mafic rocks has infrequently been measured. In the Kharaelakh intrusion associated with the world-famous Noril’sk ore district in Siberia coexisting magmatic anhydrite and sulfide minerals have been identified. Sulfur isotope compositions of the anhydrite-sulfide assemblages have been measured via both ion microprobe and conventional analyses to help elucidate the origin of the anhydrite-sulfide pairs. Magmatic anhydrite and chalcopyrite are characterized by δ³⁴S values between 18.8‰ and 22.8‰, and 9.3‰ and 13.2‰, respectfully. Coexisting anhydrite and chalcopyrite show Δ values that fall between 8.5‰ and 11.9‰. Anhydrite in the Kharaelakh intrusion is most readily explained by the assimilation of sulfate from country rocks; partial reduction to sulfide led to mixing between sulfate-derived sulfide and sulfide of mantle origin. The variable anhydrite and sulfide δ³⁴S values are a function of differing degrees of sulfate reduction, variable mixing of sulfate-derived and mantle sulfide, incomplete isotopic homogenization of the magma, and a lack of uniform attainment of isotopic equilibrium during subsolidus cooling. The δ³⁴S values of sulfide minerals have changed much less with cooling than have anhydrite values due in large part to the high sulfide/sulfate ratio. Variations in both sulfide and anhydrite δ³⁴S values indicate that isotopically distinct domains existed on a centimeter scale. Late stage hydrothermal anhydrite and pyrite also occur associated with Ca-rich hydrous alteration assemblages (e.g., thomsonite, prehnite, pectolite, epidote, xonotlite). δ³⁴S values of secondary hydrothermal anhydrite and pyrite determined by conventional analyses are in the same range as those of the magmatic minerals. Anhydrite-pyrite Δ values are in the 9.1-10.1‰ range, and are smaller than anticipated for the low temperatures indicated by the silicate alteration assemblages. The small Δ values are suggestive of either sulfate-sulfide isotopic disequilibrium or closure of the system to further exchange between ∼550 and 600 °C. Our results confirm the importance of the assimilation of externally derived sulfur in the generation of the elevated δ³⁴S values in the Kharaelakh intrusion, but highlight the sulfur isotopic variability that may occur in magmatic systems. In addition, our results confirm the need for more precise experimental determination of sulfate-sulfide sulfur isotope fractionation factors in high-T systems.     

A review of the stratigraphy and geological setting of the Palaeoproterozoic Magondi Supergroup,Zimbabwe – Type locality for the Lomagundi carbon isotope excursion

The Palaeoproterozoic Magondi Supergroup lies unconformably on the Archaean granitoid-greenstone terrain of the Zimbabwe Craton and experienced deformation and metamorphism at 2.06–1.96 Ga to form the Magondi Mobile Belt. The Magondi Supergroup comprises three lithostratigraphic units. Volcano-sedimentary rift deposits (Deweras Group) are unconformably overlain by passive margin, back-arc, and foreland basin sedimentary successions, including shallow-marine sedimentary rocks (Lomagundi Group) in the east, and deeper-water shelf to continental slope deposits in the west (Piriwiri Group). Based on the upward-coarsening trend and presence of volcanic rocks at the top of the Piriwiri and Lomagundi groups, the Piriwiri Group is considered to be a distal, deeper-water time-equivalent of the Lomagundi Group. The Magondi Supergroup experienced low-grade metamorphism in the southeastern zone, but the grade increases to upper greenschist and amphibolite facies grade to the north along strike and, more dramatically, across strike to the west, reaching upper amphibolite to granulite facies in the Piriwiri Group.     

The dark side of zircon: textural,age, oxygen isotopic and trace element evidence of fluid saturation in the subvolcanic reservoir of the Island Park-Mount Jackson Rhyolite,Yellowstone (USA)

The Island Park-Mount Jackson series in the Yellowstone volcanic field, Wyoming (USA), is a suite of rhyolitic domes and lavas that erupted between the caldera-forming eruptions of the Mesa Fall Tuff (1.3 Ma) and the Lava Creek Tuff (0.6 Ma). Combined zircon U/Pb geochronology, Raman spectroscopy, oxygen isotopic and trace elemental compositions document storage conditions of these magmas between consecutive supereruptions. Based on comparison with co-erupted melt compositions and textural criteria, four zircon compositional groups are identified that record different stages along a continuous magmatic evolution from trace element-poor rhyolite at high temperatures to extremely fractionated rhyolite where zircon trace elements are highly enriched (e.g., >?1000 ppm U). These latter zircon domains are dark in cathodoluminescence images and show broadened Raman peaks relative to near-endmember zircon, indicating that substitution of non-stoichiometric trace elements into zircon leads to distortion of the crystal lattice. Some of these zircon domains contain inclusions of U-Th-REE-phases, likely originating from coupled dissolution–reprecipitation of metastable trace element-rich zircon in the presence of a fluid phase. Rhyolite-MELTS simulations indicate that at the conditions required to produce the observed enrichment in trace elements, a fluid phase is likely present. These findings illustrate that zircons can be assembled from a variety of co-existing magmatic environments in the same magma reservoir, including near-solidus volatile-rich melts close to the magmatic–hydrothermal transition.     

Particle acceleration by ultrarelativistic shocks: theory and simulations

We consider the acceleration of charged particles near ultrarelativistic shocks, with Lorentz factor     . We present simulations of the acceleration process and compare these with results from semi-analytical calculations. We show that the spectrum that results from acceleration near ultrarelativistic shocks is a power law,     , with a nearly universal value     for the slope of this power law.
We confirm that the ultrarelativistic equivalent of the Fermi acceleration at a shock differs from its non-relativistic counterpart by the occurrence of large anisotropies in the distribution of the accelerated particles near the shock. In the rest frame of the upstream fluid, particles can only outrun the shock when their direction of motion lies within a small loss cone of opening angle     around the shock normal.
We also show that all physically plausible deflection or scattering mechanisms can change the upstream flight direction of relativistic particles originating from downstream by only a small amount:     . This limits the energy change per shock crossing cycle to     , except for the first cycle where particles originate upstream. In that case the upstream energy is boosted by a factor     for those particles that are scattered back across the shock into the upstream region.     

Crust and upper mantle structure of the Bohemian Massif from the dispersion of seismic surface waves

Summary The phase velocity dispersion of Rayleigh waves for the Moxa-Vienna (MOX-VIE) and Moxa-Kaperské Hory (MOX-KHC) profiles, and of both Rayleigh and Love waves for the Kaperské Hory-Ksi (KHC-KSP) profile have been measured and inverted into models of shearwave velocity vs. depth. The three paths cross, respectively, the central part of the Bohemian Massif, its western margin, and the Bohemian Pluton and Cretaceous. For the MOX-VIE profile mean and lower crustal shear wave velocities of 3.7 and 3.9 km/s, respectively, a mean Moho depth of 34 km, and no existence of a low-velocity layer in the lower crust were found. The model obtained for the MOX-KHC profile is characterized by a slightly lower velocity in the lower crust (3.8 km/s), by a slightly lower Moho depth (32 km), and by the appearance of a weak low-velocity channel between 55 and 140 km. The crustal section of the final model for the KHC-KSP profile agrees well with the KHKS82 model derived by Novotný from results of DSS along international profile VII. Our final Rayleigh-wave model has significantly lower shear-wave velocities down to 215 km in the mantle. A systematic difference of 0.18 km/s between the average velocities of Rayleigh and Love waves has been revealed for the depth range from 30 to 215 km. Since almost no contamination of the fundamental Love mode with higher modes has been observed, and since the investigated structure hardly contains an unresolved system of thin, alternately low- and high-velocity layers, the cause of the difference is evidently polarization anisotropy of the upper mantle beneath the Bohemian Massif. It is recommended that the discussed investigations should be supplemented with data from the fan of KSP-GRF (Gräfenberg Array, Germany) paths and from the KHC-BRG (Berggiesshübel, Germany) profile.