The Influence of Bioturbation on Iron and Sulphur Cycling in Marine Sediments: A Model Analysis

The geochemical cycles of iron and sulphur in marine sediments are strongly intertwined and give rise to a complex network of redox and precipitation reactions. Bioturbation refers to all modes of transport of particles and solutes induced by larger organisms, and in the present-day seafloor, bioturbation is one of the most important factors controlling the biogeochemical cycling of iron and sulphur. To better understand how bioturbation controls Fe and S cycling, we developed reactive transport model of a coastal sediment impacted by faunal activity. Subsequently, we performed a model sensitivity analysis, separately investigating the two different transport modes of bioturbation, i.e. bio-mixing (solid particle transport) and bio-irrigation (enhanced solute transport). This analysis reveals that bio-mixing and bio-irrigation have distinct—and largely opposing effects on both the iron and sulphur cycles. Bio-mixing enhances transport between the oxic and suboxic zones, thus promoting the reduction of oxidised species (e.g. iron oxyhydroxides) and the oxidation of reduced species (e.g. iron sulphides). Through the re-oxidation of iron sulphides, bio-mixing strongly enhances the recycling of Fe and S between their reduced and oxidised states. Bio-irrigation on the other hand removes reduced solutes, i.e. ferrous iron and free sulphide, from the sediment pore water. These reduced species are then reoxidised in the overlying water and not recycled within the sediment column, which leads to a decrease in Fe and S recycling. Overall, our results demonstrate that the ecology of the macrofauna (inducing bio-mixing or bio-irrigation, or both) matters when assessing their impact on sediment geochemistry. This finding seems particularly relevant for sedimentary cycling across Cambrian transition, when benthic fauna started colonizing and reworking the seafloor.     

Investigation of the change in soil fabric during cone penetration in silt using 2D measurements

Interpretation of CPTU testing in silt is non-trivial because of the partially drained conditions that are likely to occur during penetration. A better understanding of the pore pressure generation/dissipation is needed in order to obtain reliable design parameters. Following a previous study using X-ray computed tomography (micro-CT) with volumetric digital image correlation (3D-DIC) that clearly showed the formation of distinct dilation and compression areas around the cone; the present work takes a closer look at those areas in order to link volumetric behavior to changes in soil fabric. High-resolution 2D backscattered electron images of polished thin sections prepared from frozen samples at the end of penetration are used. The images have a spatial resolution of 0.4 µm/pixel that allow a clear identification of grains and pore spaces. Image processing techniques are developed to quantify local porosity and obtain the statistical distribution of the particle orientation for the zones around the cone tip and shaft. It is shown that the formation of compaction regions is related to the ability of the grains to rearrange and align along a well-defined preferred orientation forming a more closed-fabric characterized by high anisotropy values, while zones of dilation are associated with a more open packing with grains randomly oriented and with large voids within. These observations suggested that for a saturated soil, water will move from a compressive zone to a neighboring dilative zone, creating a short drainage path. By shedding light on the link between soil fabric and drainage patterns, this study contributes toward a better understanding of the measured macro-response during CPTU tests on silt.     

A TEM and EELS study of carbon in a melt fragment from the Gardnos impact structure

A carbon‐rich melt fragment from the Gardnos impact structure has been studied for a better understanding of the preservation and structural form(s) of carbon that have been processed by impact melting. The carbon was analyzed in situ in its original petrographic context within the melt fragment, using high‐resolution techniques including focused ion beam‐transmission electron microscopy and electron energy loss spectroscopy. Results show that the carbon is largely uniform and has a nanocrystalline grain size. The Gardnos carbon has a graphitic structure but with a large c/a ratio indicating disorder. The disorder could be a result of rapid heating to high temperatures during impact, followed by rapid cooling, with not enough time to crystallize into highly ordered graphite. However, temperature distribution during impact is extremely heterogenous, and the disordered Gardnos carbon could also represent material that avoided extreme temperatures, and thus, it was preserved. Understanding the structure of carbon during terrestrial impacts is important to help determine if the history of carbon within extraterrestrial samples is impact related. Furthermore, the degree of preservation of carbon during impact is key for locating and detecting organic compounds in extraterrestrial samples. This example from Gardnos, together with previous studies, shows that not all carbon is lost to oxidation during impact but that impact melting can encapsulate and preserve carbon where it is available.     

Transports of Nordic Seas water masses and excess SF6 through Fram Strait to the Arctic Ocean

To determine the exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait is one of the most important aspects, and one of the major challenges, in describing the circulation in the Arctic Mediterranean Sea. Especially the northward transport of Arctic Intermediate Water (AIW) from the Nordic Seas into the Arctic Ocean is little known. In the two-ship study of the circulation in the Nordic Seas, Arctic Ocean - 2002, the Swedish icebreaker Oden operated in the ice-covered areas in and north of Fram Strait and in the western margins of Greenland and Iceland seas, while RV Knorr of Woods Hole worked in the ice free part of the Nordic Seas. Here two hydrographic sections obtained by Oden, augmented by tracer and velocity measurements with Lowered Acoustic Doppler Current Profiler (LADCP), are examined. The first section, reaching from the Svalbard shelf across the Yermak Plateau, covers the region north of Svalbard where inflow to the Arctic Ocean takes place. The second, western, section spans the outflow area extending from west of the Yermak Plateau onto the Greenland shelf. Geostrophic and LADCP derived velocities are both used to estimate the exchanges of water masses between the Nordic Seas and the Arctic Ocean. The geostrophic computations indicate a total flow of 3.6 Sv entering the Arctic on the eastern section. The southward flow on the western section is found to be 5.1 Sv. The total inflow to the Arctic Ocean obtained using the LADCP derived velocities is much larger, 13.6 Sv, and the southward transport on the western section is 13.7 Sv, equal to the northward transport north of Svalbard. Sulphur hexafluoride (SF₆) originating from a tracer release experiment in the Greenland Sea in 1996 has become a marker for the circulation of AIW. From the geostrophic velocities we obtain 0.5 Sv and from the LADCP derived velocities 2.8 Sv of AIW flowing into the Arctic. The annual transport of SF₆ into the Arctic Ocean derived from geostrophy is 5 kg/year, which is of the same magnitude as the observed total annual transport into the North Atlantic, while the LADCP measurements (19 kg/year) imply that it is substantially larger. Little SF₆ was found on the western section, confirming the dominance of the Arctic Ocean water masses and indicating that the major recirculation in Fram Strait takes place farther to the south.     

Geochemical comparison between minettes and kersantites from the Western European Hercynian orogen: trace element and PbSrNd isotope constraints on their origin

Trace element and isotopic characteristics of late Carboniferous to early Permian minettes and kersantites have been determined. These lamprophyres have been sampled throughout the Western European Hercynian orogen, from Brittany to the west to Schwarzwald to the east. In spite of sharp petrological differences reflected by mineralogy and major element geochemistry, minettes and kersantites exhibit close identity with respect to trace element and isotopic features. These features comprise enrichment in incompatible elements, highCs/Rb and lowCe/Pb ratios, Ta and Ti relative depletion, high abundance in transition elements and highNi/Mg ratios. Pb isotope ratios are undistinguishable from those measured on Hercynian continental crust. Initial¹⁴³Nd/¹⁴⁴Nd ratios are between0.5120 (ε_i −5) and0.5122 (ε_i −1) for minettes and kersantites whereas initial⁸⁷Sr/⁸⁶Sr ratios vary between 0.7055–0.710 for minettes and 0.707–0.708 for kersantites. No simple mixing relations are visible on RbSr and SmNd isochron diagrams. The exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material.     

The Barents Sea Ice Sheet — A model of its growth and decay during the last ice maximum

On the basis of geomorphological and sedimentological data, we believe that the entire Barents Sea was covered by grounded ice during the last glacial maximum. ¹⁴C dates on shells embedded in tills suggest marine conditions in the Barents Sea as late as 22 ka BP; and models of the deglaciation history based on uplift data from the northern Norwegian coast suggest that significant parts of the Barents Sea Ice Sheet calved off as early as 15 ka BP. The growth of the ice sheet is related to glacioeustatic fall and the exposure of shallow banks in the central Barents Sea, where ice caps may develop and expand to finally coalesce with the expanding ice masses from Svalbard and Fennoscandia.The outlined model for growth and decay of the Barents Sea Ice Sheet suggests a system which developed and existed under periods of maximum climatic deterioration, and where its growth and decay were strongly related to the fall and rise of sea level.     

Attenuation models inferred from intraplate earthquake recordings

Strong-motion recordings at 87 sites from 56 different intraplate earthquakes from North America, Europe, China and Australia have been used through a two-step regression analysis to develop new attenuation models for peak ground acceleration, and for pseudo-relative velocity for frequencies of 0.25, 0.5, 1.0, 2.0, 5.0 and 10.0 Hz, all for 5 per cent of critical damping. The estimates are obtained along with an analysis of residuals and scatter. A similar regression analysis has been performed also for Fourier spectra of acceleration, in which case the coefficient for the anelastic term has been interpreted in terms of a frequency dependent quality factor Q. The resulting Q-model shows a strong frequency sensitivity with values around 600–700 at 1 Hz, around 2000 at 10 Hz and around 5200 at 25 Hz. These PGA, PSV and Q results depend, however, on the underlying assumption for geometrical spreading, in particular for low frequencies.     

H-chondrites: Trace element clues to their origin

We have analyzed 10 H-chondrites for 20 trace elements, using RNAA. The meteorites included 4 of petrologic type 4 and 2 each of types 3, 5 and 6.The data show that H-chondrites are not isochemical. H3's are depleted by some 10% not only in Fe (Dodd, 1976), but also in the siderophiles Os, Re, Ir, Ni, Pd, Au, and Ge. Moreover, the abundance pattern of siderophiles varies systematically with petrologic type. As similar fractionations of REE have been observed by Nakamura (1974), it appears that both the proportions and compositions of the main nebular condensates varied slightly during accretion of the H-chondrites. Thus the higher petrologic types are independent nebular products, not metamorphosed descendants of lower petrologic types.Abundances of highly volatile elements (Cs, Br, Bi, Tl, In, Cd, Ar³⁶) correlate with petrologic type, declining by ≤ 10^?3 from Type 3 to Type 6. The trends differ from those for artificially heated Type 3's (Ikramuddinet al., 1977b; Herzoget al., 1979), but agree passably with theoretical curves for nebular condensation. Apparently the low volatile contents of higher petrologic types are a primary feature, not the result of metamorphic loss.The mineralogy of chondrites suggests that they accreted between 405 K (absence of Fe₃O₄) and 560 K (presence of FeS), and the abundances of Tl, Bi, and In further restrict this interval to 420–500 K. Accretion at 1070 ± 100 K, as proposed by Hutchisonet al. (1979, 1980), leads to some extraordinary problems. Volatiles must be injected into the parent body after cooling, which requires permeation of the body by 10¹¹ times its volume of nebular gas. This process must also achieve a uniform distribution of the less volatile elements (Rb, Cu, Ag, Zn, Ga, Ge, Sn, Sb, Se, F), without freezeout in the colder outer layers.Factor analysis of our data shows 3 groupings: siderophiles (Os, Re, Ir, Ni, Pd, Au, and Ge), volatiles (Ag, Br, In, Cd, Bi, and Tl) and alkalis (Rb and Cs). The remaining 5 elements (U, Zn, Te, Se, and Sb) remain unassociated.     

Introduction to soil degradation processes in drylands

Two efforts by UN organizations to diagnose and map the distribution and trends of soil degradation in drylands are briefly described and compared.Soil degradation by water is greatly accelerated under poor vegetation cover in the Tropics and Subtropics, probably for two major reasons: high rates of decomposition of organic matter and high intensities of rainfall. The consequences of soil losses and high water runoff can reach far downstream in the river basin, causing damage by sedimentation on floodplains and in reservoirs.Damages by wind erosion and deposition are also much accelerated by overexploitation of the meagre vegetation cover of drylands. Overgrazing, overcultivation, firewood collection are the consequences of human and animal pressure, leading to more or less long-lasting desertification. The finer material, clay, silt, and organic matter, is blown by dust storms over long distances and can be deposited as dry or wet fallout in oceans or on land under the dust trajectories.Salinization often affects highly productive soils under poor irrigation practices. It is therefore a form of dryland degradation which causes particularly high losses of potential crops.