Early Mars volcanic sulfur storage in the upper cryosphere and formation of transient SO2‐rich atmospheres during the Hesperian

In a previous paper (Chassefière et al. 2013 ), we have shown that most volcanic sulfur released to the early Mars atmosphere could have been trapped in the upper cryosphere under the form of CO₂‐SO₂ clathrates. Huge amounts of sulfur, up to the equivalent of an ~1 bar atmosphere of SO₂, would have been stored in the Noachian upper cryosphere, then massively released to the atmosphere during the Hesperian due to rapidly decreasing CO₂ pressure. It could have resulted in the formation of the large sulfate deposits observed mainly in Hesperian terrains, whereas no or little sulfates are found at the Noachian. In the present paper, we first clarify some aspects of our previous work. We discuss the possibility of a smaller cooling effect of sulfur particles, or even of a net warming effect. We point out the fact that CO₂‐SO₂ clathrates formed through a progressive enrichment of a pre‐existing reservoir of CO₂ clathrates and discuss processes potentially involved in the slow formation of a SO₂‐rich upper cryosphere. We show that episodes of sudden destabilization at the Hesperian may generate 1000 ppmv of SO₂ in the atmosphere and contribute to maintaining the surface temperature above the water freezing point.     

Setting nutrient thresholds to support an ecological assessment based on nutrient enrichment, potential primary production and undesirable disturbance

The EU Water Framework Directive recognises that ecological status is supported by the prevailing physico-chemical conditions in each water body. This paper describes an approach to providing guidance on setting thresholds for nutrients taking account of the biological response to nutrient enrichment evident in different types of water. Indices of pressure, state and impact are used to achieve a robust nutrient (nitrogen) threshold by considering each individual index relative to a defined standard, scale or threshold. These indices include winter nitrogen concentrations relative to a predetermined reference value; the potential of the waterbody to support phytoplankton growth (estimated as primary production); and detection of an undesirable disturbance (measured as dissolved oxygen). Proposed reference values are based on a combination of historical records, offshore (limited human influence) nutrient concentrations, literature values and modelled data. Statistical confidence is based on a number of attributes, including distance of confidence limits away from a reference threshold and how well the model is populated with real data. This evidence based approach ensures that nutrient thresholds are based on knowledge of real and measurable biological responses in transitional and coastal waters.     

Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.     

The upper cenozoic evolution of the Duero and Ebro fluvial systems (N-Spain): part I. paleogeography; part II. geomorphology

Lack of age dates in the terrigenous Cenozoic sediments of the Duero and the Ebro sedimentary basins has complicated tecto-stratic correlation across the two basins. We tentatively synthesize a range of existing studies and new data to construct a rough general paleogeography throughout Upper Cenozoic times. The more extensive erosion of the Ebro has been previously attributed to the earlier moment of opening. We tentatively analyse lithostratic data to conclude that the lower knick-point and different lithologies have also contributed to the deeper erosion in the Ebro Basin. We conclude from lithostratic data and field evidence that the W half of the Rioja was part of the Duero in earlier times and that the escarpment retreated westward through the Rioja in four subsequent episodes of erosion. The tilt of the NW Duero is a consequence of isostatic rebound to this erosion.     

A new pathway for Deep water exchange between the Natal Valley and Mozambique Basin?

Although global thermohaline circulation pathways are fairly well known, the same cannot be said for local circulation pathways. Within the southwest Indian Ocean specifically there is little consensus regarding the finer point of thermohaline circulation. We present recently collected multibeam bathymetry and PARASOUND data from the northern Natal Valley and Mozambique Ridge, southwest Indian Ocean. These data show the Ariel Graben, a prominent feature in this region, creates a deep saddle across the Mozambique Ridge at ca. 28°S connecting the northern Natal Valley with the Mozambique Basin. Results show a west to east change in bathymetric and echo character across the northern flank of the Ariel Graben. Whereby eroded plastered sediment drifts in the west give way to aggrading plastered sediment drift in the midgraben, terminating in a field of seafloor undulations in the east. In contrast, the southern flank of the Ariel Graben exhibits an overall rugged character with sediments ponding in bathymetric depressions in between rugged sub/outcrop. It is postulated that this change in sea-floor character is the manifestation of deep water flow through the Ariel Graben. Current flow stripping, due to increased curvature of the graben axis, results in preferential deposition of suspended load in an area of limited accommodation space consequently developing an over-steepened plastered drift. These deposited sediments overcome the necessary shear stresses, resulting in soft sediment deformation in the form of down-slope growth faulting (creep) and generation of undulating sea-floor morphology. Contrary to previous views, our works suggests that water flows from west to east across the Mozambique Ridge via the Ariel Graben.     

Using DORIS measurements for modeling the vertical total electron content of the Earth’s ionosphere

The Doppler orbitography and radiopositioning integrated by satellite (DORIS) system was originally developed for precise orbit determination of low Earth orbiting (LEO) satellites. Beyond that, it is highly qualified for modeling the distribution of electrons within the Earth’s ionosphere. It measures with two frequencies in L-band with a relative frequency ratio close to 5. Since the terrestrial ground beacons are distributed quite homogeneously and several LEOs are equipped with modern receivers, a good applicability for global vertical total electron content (VTEC) modeling can be expected. This paper investigates the capability of DORIS dual-frequency phase observations for deriving VTEC and the contribution of these data to global VTEC modeling. The DORIS preprocessing is performed similar to commonly used global navigation satellite systems (GNSS) preprocessing. However, the absolute DORIS VTEC level is taken from global ionospheric maps (GIM) provided by the International GNSS Service (IGS) as the DORIS data contain no absolute information. DORIS-derived VTEC values show good consistency with IGS GIMs with a RMS between 2 and 3 total electron content units (TECU) depending on solar activity which can be reduced to less than 2 TECU when using only observations with elevation angles higher than \(50^\circ \) . The combination of DORIS VTEC with data from other space-geodetic measurement techniques improves the accuracy of global VTEC models significantly. If DORIS VTEC data is used to update IGS GIMs, an improvement of up to 12  % can be achieved. The accuracy directly beneath the DORIS satellites’ ground-tracks ranges between 1.5 and 3.5 TECU assuming a precision of 2.5 TECU for altimeter-derived VTEC values which have been used for validation purposes.     

Settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions

The time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature (1,280–1,500 °C) centrifuge-assisted experiments on a chromite-basalt melt system at 100–1,500g (0.5 GPa). Subsequently, the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions (LMIs) is assessed. Centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule. The experimentally observed mechanical settling velocity of a suspension of ~24 vol% chromite is calculated to be about half (~0.53) of the Stokes settling velocity, with a sedimentation exponent n of 2.35 (3). Gravitational settling leads to an orthocumulate layer with a porosity of 0.52 (all porosities as fraction). Formation times for such a layer from a magma with initial chromite contents of 0.1–1 vol% are 140–3.5 days, equal to a growth rate of 0.007–0.3 m/day for grain sizes of 1–2 mm. More compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction: An increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts, reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt. The relation between the porosity in the cumulate pile and effective pressure integrated over time (Δρ · h · a · t) is best fit with a logarithmic function, in fact confirming that a (pressure) dissolution–reprecipitation process is the dominant mechanism of compaction. The experimentally derived equation allows calculating compaction times: 70–80 % chromite at the bottom of a 1-m-thick chromite layer are reached after 9–250 years, whereas equivalent compaction times are 0.2–0.9 years for olivine (both for 2 mm grain size). The experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 10⁹ Pa s, much lower than previously reported. As long as melt escape from the compacting cumulate remains homogeneous, fluidization does not play any role; however, channelized melt flow may lead to suspension and upward movement of cumulate crystals. In LMIs, chromitite layers are typically part of a sequence with layers of mafic minerals, compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades.     

Is rainfall intensity significant in the rainfall–runoff process within tropical rainforests of northeast Queensland? The Hewlett regression analyses revisited

Following the statistical analyses of long‐term rainfall‐runoff records from research basins in humid temperate latitudes, Hewlett and co‐workers extended the global challenge to disprove their findings that rainfall intensity was non‐significant. This paper responds to Hewlett's challenge as no preceding analyses have involved forested basins in a tropical cyclone‐prone area. Based on a 7 year rainfall‐runoff record, quickflow (QF), peak flow (QP) and quickflow response ratios (QRR) were regressed as dependent variables against rainfall parameters (intensity, P_i, amount, P), storm duration, D and antecedent flow, I. These data sets were categorised into total streamflow (Q) classes and stratified into three seasons, (monsoon, post‐monsoon and dry) for forested and cleared catchments. Where rainfall variable collinearity met acceptable levels, the addition of P_i to regression models including P, D, I contributed up to 9% and 66% of the respective variations in quickflow and peak flow. For the highest Q storm classes (monsoon), P_i alone accounted for up to 67% and 91% of the variation in Q_F and Q_P respectively and was the dominant influence on Q_P for all seasons. The very high rainfall intensities experienced in the monsoon season is a causal factor why these results differ from those of other research drainage basins. Surprisingly, P_i continued to have a significant influence on Q_F for dry season classes when less‐intense rainfall occurs. Further the results were similar for both catchments across all seasons. P was the dominant independent variable affecting Q_F above a threshold Q of 50 mm (monsoon), as rainfall contributes directly to saturation overland flow and return flow under saturated conditions. Further although QRR increased with increasing Q for each season, the regression results for that parameter were poor possibly due to the non‐linearity of the rainfall‐runoff relationship. Copyright © 2010 John Wiley & Sons, Ltd.