Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny

This study assesses the significance, geometry, and kinematics of greenschist-facies deformation along the Dent Blanche Basal Thrust (DBBT), a major tectonic contact in the Internal Western Alps of Switzerland and Italy. The DBBT separates continental units of the Dent Blanche nappe, the structurally highest unit in the Western Alps, from underlying Piemont-Ligurian ophiolites. Mylonites and deformation structures along the contact provide a record of its retrograde greenschist-facies evolution after earlier high-pressure metamorphism. A first phase of foreland-directed, reverse-sense, top-(N)W shearing (D1) occurred between ca. 43 and 39 Ma, related to exhumation of the Dent Blanche nappe from high-pressure conditions. It led to the formation of mylonitic fabrics under high- to medium-grade greenschist-facies conditions along the entire DBBT. A phase of ductile normal-sense top-SE shearing (D2) at ca. 38–37 Ma was mainly localized within underlying ophiolitic units and only partly affected the DBBT. Another phase of ductile deformation (D3) under medium- to low-grade greenschist-facies conditions at ca. 36–35 Ma occurred in response to underthrusting of European continental margin units and resulted in the updoming of the nappe stack. Especially the southeastern DBBT was characterized by bulk top-NW shearing, partly conjugate top-NW/top-SE shearing, and resulting orogen-perpendicular crustal extension. Subsequently, the DBBT was affected by a phase of orogen-perpendicular shortening (D4) and formation of folds and crenulations at ca. 34–33 Ma due to increasing compressional tectonics. Finally, a phase of semi-ductile to brittle normal-sense top-NW and conjugate shearing (D5) from ca. 32 Ma onwards particularly affected the southeastern segment and indicates exhumation of the DBBT through the ductile–brittle transition. This was followed by brittle NW–SE extensional deformation. This study suggests that the DBBT experienced a polyphase deformation and reactivation history under decreasing greenschist-facies metamorphic conditions during which different segments of this major shear zone were variably affected.     

Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments

Gravity driven mass-flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western Argentine Basin. In this study we present geochemical data from a total of 23 gravity cores. Pore-water SO₄ is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane (AOM). The different shapes of SO₄ profiles (concave, kink- and s-type) can be consistently explained by sedimentary slides possibly in combination with changes in the CH₄ flux from below, thus, mostly representing transient pore-water conditions. Since slides may keep their original sedimentary signature, a combined analysis and numerical modeling of geochemical, physical properties, and hydro acoustic data could be applied in order to reconstruct the sedimentary history. We present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day. The results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep-sea, depositing high amounts of reactive compounds. The high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by AOM. For the total region we estimate a SO₄ (or CH₄) flux of 6.6 × 10¹⁰ moles per year into the zone of AOM. Projected to the global continental slope and rise area, this may sum up to about 2.6 × 10¹² moles per year. Provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1.2 × 10¹² moles per year as previously estimated. Thus, AOM obviously contributes very significantly to the regulation of global sulfur reservoirs, which is hitherto not sufficiently recognized. This finding may have implications for global geochemical models, as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time.     

The high-supply,current-dominated continental margin of southeastern South America during the late Quaternary

The continental margin off the La Plata Estuary (SE South America) is characterized by high fluvial sediment supply and strong ocean currents. High-resolution sediment-acoustic data combined with sedimentary facies analysis, AMS-¹⁴C ages, and neodymium isotopic data allowed us to reconstruct late Quaternary sedimentary dynamics in relation to the two major sediment sources, the La Plata Estuary and the Argentine margin. Sediments from these two provinces show completely different dispersal patterns. We show that the northward-trending La Plata paleo-valley was the sole transit path for the huge volumes of fluvial material during lower sea levels. In contrast, material from the Argentine margin sector was transported northwards by the strong current system. Despite the large sediment volumes supplied by both sources, wide parts of the shelf were characterized by either persistent non-deposition or local short-term depocenter formation. The location and formation history of these depocenters were primarily controlled by the interplay of sea level with current strength and local morphology. The high sediment supply was of secondary importance to the stratigraphic construction, though locally resulting in high sedimentation rates. Thus, the shelf system off the La Plata Estuary can be considered as a hydrodynamic-controlled end-member.     

Effect of Secondary Circulations on the Surface–Atmosphere Exchange of Energy at an Isolated Semi-arid Forest

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.     

Evidence for current-controlled sedimentation along the southern Mozambique continental margin since Early Miocene times

Major plastered drift sequences were imaged using high-resolution multichannel seismics during R/V Meteor cruises M63/1 and M75/3 south of the Mozambique Channel along the continental margin of Mozambique off the Limpopo River. Detailed seismic-stratigraphic analyses enabled the reconstruction of the onset and development of the modern, discontinuous, eddy-dominated Mozambique Current. Major drift sequences can first be identified during the Early Miocene. Consistent with earlier findings, a progressive northward shift of the depocenter indicates that, on a geological timescale, a steady but variable Mozambique Current existed from this time onward. It can furthermore be shown that, during the Early/Middle Miocene, a coast-parallel current was established off the Limpopo River as part of a lee eddy system driven by the Mozambique Current. Modern sedimentation is controlled by the interplay between slope morphology and the lee eddy system, resulting in upwelling of Antarctic Intermediate Water. Drift accumulations at larger depths are related to the reworking of sediment by deep-reaching eddies that migrate southward, forming the Mozambique Current and eventually merging with the Agulhas Current.     

Radioisotopic dating of the Tortonian Global Stratotype Section and Point: implications for intercalibration of 40Ar/39Ar and astronomical dating methods

In this paper we present new ⁴⁰Ar/³⁹Ar data of volcanic ash layers intercalated in the astronomically dated sections of Monte dei Corvi and Monte Gibliscemi (Italy) to obtain better radioisotopic time constraints on the Serravallian/Tortonian boundary and to confirm the intercalibration of radioisotopic and astronomical time proposed by Kuiper et al. [2004 ; Fish Canyon Tuff (FCT)-sanidine at 28.21 ± 0.03 Ma]. The latter intercalibration is supported by astronomically calibrated FCT sanidine ages for two ash layers at Monte Gibliscemi (GiF-1: 28.28 ± 0.04; GiD-3: 28.16 ± 0.04 Ma; ±1 SE). As a consequence, our results support the astronomically calibrated age of 11.608 Ma for the Tortonian Global Stratotype Section and Point and, hence, the tuning of the Serravallian/Tortonian boundary interval. The Ancona and Respighi levels at Monte dei Corvi give a more diffuse picture, possibly because of contamination with detrital or xenocrystic material and the inferior quality of biotite for intercalibration purposes.     

Topographic modeling of Phoebe using Cassini images

High-resolution Cassini stereo images of Saturn's moon Phoebe have been used to derive a regional digital terrain model (DTM) and an orthoimage mosaic of the surface. For DTM-control a network of 130 points measured in 14 images (70-390 m/pixel resolution) was established which was simultaneously used to determine the orientation of the spin-axis. The J2000 spin-axis was found at Dec=78.0°±0.1° and RA=356.6°±0.3°, substantially different from the former Voyager solution. The control points yield a mean figure radius of 107.2 km with RMS residuals of 6.2 km demonstrating the irregular shape of this body. The DTM was computed from densely spaced conjugate image points determined by methods of digital image correlation. It has a horizontal resolution of 1-2 km and vertical accuracies in the range 50-100 m. It is limited in coverage, but higher in resolution than the previously derived global shape model of Phoebe [Porco et al., 2005. Cassini imaging science: initial results on Phoebe and Iapetus. Science 307, 1237-1242] and allows us to study the morphology of the surface in more detail. There is evidence for unconsolidated material from a steep and smooth slope at the rim of a 100 km impact feature. There are several conically shaped craters on Phoebe, which may hint at highly porous and low compaction material on the surface.