Submarine slope failures along the convergent continental margin of the Middle America Trench

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ∼1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well-defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated

Characterization of microbial communities in carbonate sediments

Microbial communities in carbonate sediments from the alkaline Lake Neusiedl and the Aldabra Atoll were characterized. The aim was to determine the microbial community composition and function in the context of their contribution to biogeochemical cycles and carbonate precipitation. Total DNA and RNA were extracted from sediment and water samples. 16S ribosomal RNA genes and transcripts were amplified and sequenced to determine the bacterial community composition. Metagenomes were assembled from selected sampling sites to determine the functional potential encoded within the microbial community. Detailed insights into bacterial genomes and metabolism were gained through isolation and characterisation of two novel bacterial species derived from Aldabra. The first sampling campaign represents the proof-of-concept study at Lake Neusiedl (Chapter C.1 & C.2). In this study the sampling procedure for the push-cores and water column was established. Bacterial 16S rRNA genes were amplified from the total DNA, sequenced, and analysed. The results showed that freshwater picoplanktonic Alphaproteobacteria and Actinobacteriota were abundant in the water column (Chapter C.1). Together with Synechococcales sheaths they may provide nucleation sites for carbonate precipitation in the water column. The sediment followed the standard biogeochemical succession and showed signs of diatom dissolution (Chapter C.2). This was linked to high abundance of heterotrophic Gammaproteobacteria and fermenting Chloroflexota, which likely contributed to maintaining the neutral pH and supported the dissolution process. The main sampling campaign to the Aldabra Atoll took place at the end of the dry season in November 2017. Sediment cores and water samples were taken at three sampling sites in the lagoon and one pool at the island rim (Chapter C.3). The bacterial community composition was identified using both 16S rRNA genes and transcripts, covering both present and past members of the community. The sampling sites Cinq Cases and Westpool D were selected for direct metagenome sequencing and analysis, as these were landlocked pools with a history of stromatolites (Chapter C.5). The sand sediment was oxic with low bacterial diversities and dominant Pseudomonas. The surface was covered by a slightly lithified crust, potentially linked to tidally induced carbonate oversaturation and precipitation driven by the activity of Gloeocapsopsis (Chapter C.3). In the mud and silt sediments bioturbation and tidal mixing led to a mixed surface and sulphate reduction zone. These were followed by atypical low bacterial phylogenetic diversity zones with high proportions of Gammaproteobacteria. Their onset was linked to changes in redox conditions, sediment age and available organic material (Chapter C.3). This was supported by results from the analysis of abundant metagenome-assembled genomes (MAGs) of the low-diversity zones at Cinq Cases. The MAGs harboured key genes for aerobic metabolism and denitrification (Chapter C.5). MAGs and 16S rRNA genes from Westpool D suggested that a biofilm comprising Gloeocapsa, Salinivibrio and Francisella is responsible for biologically induced carbonate precipitation of the local stromatolites. The unlithified microbial mat at the bottom of the pond harboured Cyanobium and Arthrospira, indicating that only specific Cyanobacteria support carbonate precipitation (Chapter C.5). To identify novel bacteria and provide information on the vast majority of uncultured taxa, we enriched halophilic members of the bacterial community. Two isolates were selected and characterized both physiologically and genomically (Chapter C.4). Pontibacillus sp. ALD_SL1 was isolated form the mudflat of the South Lagoon and exhibited a high relative abundance (30%) in the active bacterial community of the water column at Cinq Cases. Psychroflexus sp. ALD_RP9 was isolated from the bacterial bloom at Westpool D. Its ability to form extensive EPS to protect itself from salt and solar radiation may result in binding Ca2+-ions. Upon EPS degradation, local increase of Ca2+ and rearrangement of the EPS residues support the nucleation of carbonates. This study encompasses the first characterization of microbial communities from the Aldabra Atoll using amplicon, metagenome, and genome analyses. The study highlights the different modes of carbonate precipitation, which can occur in the lacustrine and lagoonal environments. It also provides a basis for in-depth analysis of individual members of the community and their involvement in sediment biogeochemical cycling.2021-12-0

Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis

[1] Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip

Ichthyosaurier aus der kreide argentiniens

Im Museum zu La Plata befinden .sich einige wenige Ichthyosaurierreste, von denen einer eine gewisse Wiechtigkeit besitzt. Diese Reste stammen aus dem Tithon des Cerro Lotena, 50 km südöstlich von Zapala im Territorium Neuquen, aus dem Tithon von Minas de Plafadita bei Salas in der Provinz Mendoza und aus dem Neokom des Cerro Belgrano am Lago Belgrano im Territorium Santa Cruz. Der zuletzt genannte Fund ist interessant, weil ausser mehreren Wirbeln noch Teile der Vorderextremitäten erhalten sind, und man aus der Kreide noch fast nichts von zusammenhängenden Extremitäten kennt. Die Extremitätenstruktur aber ist zur Beurteilung von lchthyosauriern von besonderer Wichtigkeit. In dem Zeitraum von Trias bis Kreide, in dem überhaupt die Ichthyosaurier vorkommen. sind die zu Flossen umgebildeten Extremitáten (speciell Vorderextremitäten) nach zwei Prinzipien aufgebaut, sie sind entweder longipinnat oder latipinnat, d. h. in dem ersten Falle schliessen sich die Fingerstrahlen in direkter Linie an je ein proximales Carpale an, folglich sind dort weniger als 5 echte Finger vorhanden, in dem zweiten Falle aber gehen von dem mit zwei distalen Facetten versehenen Intermedium 2 Fingerstrahlen aus, folglich sind hier — wenigstens bei den triassischen und altjurassischen Gattungen—5 echteFinger vor handen; erst im oberen Jura tritt eine sekundäre Reduktion auf 4 echte Fingerstrahlen ein. Ausserdiesen echten Fingerstrahlen kommen sowohl auf der ulnaren wie auf der radialen Seite noch sesamoide Ketten von Polygonalknöchelchen vor, die Fingerstrahlen vortäuschen können. So sind eine Anzahl verschiedener Strukturmöglichkeiten gegeben. Es braucht hier nicht näher ausgeführt zu werden, dass das ganze Skelett der Ichthiyosaurier mit diesen Merkmalen in Korrelation steht.Facultad de Ciencias Naturales y Muse

Retrieval of cloud spherical albedo from top-of-atmosphere reflectance measurements performed at a single observation angle

International audienceThe paper is devoted to the derivation of the simple analytical relationship between the cloud spherical albedo and the cloud reflection function. The relationship obtained can be used for the derivation of the spherical albedo from backscattered solar light measurements performed by radiometers on geostationary and polar orbiting satellites. The example of the application of the technique to MODIS data is shown

Subduction erosion and basal friction along the sediment-starved convergent margin off Antofagasta, Chile

Subduction erosion is commonly associated with strong interplate coupling and a consequent abrasion of the upper plate. Northern Chile is an often cited example of a strongly coupled erosional margin. Its crystalline basement is inferred to form a strong upper plate, the trench axis contains little detectable sediment, and the subducting lower plate has a high‐relief horst‐graben topography. With little water‐rich sediment to reduce interplate friction, the high relief of an igneous ocean crust thrust beneath continental basement should generate high friction interplate abrasion. However, a prestack depth‐migrated seismic record images slope debris that collects in a frontal prism. This debris, including ∼30% pore fluid, fills subducting grabens and is subsequently incorporated into an ∼1.5‐km‐thick interplate reflective layer. The subduction zone thrust passes through the upper part of this layer. Interplate seismicity and taper analyses indicate basal friction at levels that are common in sedimented convergent margins. The continued growth of lower plate grabens after subduction probably accommodates upper plate material, a process that erodes the upper plate. Erosion is aided by weakening of the upper plate rock framework beneath the continental slope. This erosion undermines the upper plate and tips it seaward thereby steepening the continental slope which induces midslope gravity tectonics. Despite sediment starvation, a frontal prism constructed of remolded slope debris elevates pore pressure to reduce interplate friction. Coeval erosion and prism building control the size of the frontal prism. Processes other than high friction abrasion best explain subduction erosion along northern Chile

Initial Tectonic Deformation of Hemipelagic Sediment at the Leading Edge of the Japan Convergent Margin

Diatomaceous mudstones at depth under the deep sea terrace and the trench inner slope off Japan have been variably affected by tectonic stress. Veins, healed fractures, and microfaults occur at all sites except the shallow Site 435 on the upper trench inner slope and Site 436 on the Pacific Plate. Veins, fractures, and faults occur in cores from below 620 meters (lower to middle Miocene) in the landward sites (438 and 439) on the deep sea terrace, and are probably related to normal faulting seen in seismic records. The depth to "consolidated" sediment and to the first occurrence of veins and healed fractures shallows progressively toward the trench. The intensity of deformation also appears to increase seaward. However, no sediments younger than upper Pliocene are deformed. Open fractures may exist in situ at Sites 434 and 441 at levels between about 150 and 500 meters sub-bottom. The Japan Transect sediments—in contrast to deposits in the zone of initial deformation at other convergent margins though highly deformed, are not highly overconsolidated. However, sediment at depth in the trench inner slope sites is overconsolidated relative to that at the same depth in the landward reference site. None of the deformed Japan margin sediments recovered at Legs 56 and 57 sites originated by accretion of oceanic plate material—also in contrast to sediments at some of the margins previously studied. We suggest that tectonic stress related to convergence has been communicated to the slope sediments on the trench inner slope, either continuously or periodically, causing rapid tectonic dewatering and inducing fracturing and faulting. If episodic, the latest of these deformational periods may have occurred during the late Pliocene. The faults and fractures are either rehealed by continued overburden pressure (sediment loading) or may remain open at shallower levels. Fracturing and dewatering of semiconsolidated sediment beneath an unconsolidated but impermeable mud veneer may cause overpressured zones at depths of 200 to 500 meters. These overpressured zones possibly locally reduce shear strength and cause downslope mass movement of sediment, even on low-angle slopes on the trench inner slope

Interplate seismicity at the CRISP site: the 2002 Osa earthquake sequence

The Costa Rica Seismogenesis Project (CRISP) is designed to explore the processes involved in the nucleation of large interplate earthquakes in erosional subduction zones. On 16 June 2002 a magnitude Mw=6.4 earthquake and its aftershocks may have nucleated at the subduction thrust to be penetrated and sampled by CRISP, ~40 km west of Osa Peninsula. Global event locations present uncertainties too large to prove that the event actually occurred at a location and depth reachable by riser drilling. We have compiled a database including foreshocks, the main shock, and ~400 aftershocks, with phase arrival times from all the seismological networks that recorded the 2002 Osa sequence locally. This includes a temporal network of ocean-bottom hydrophones (OBH) that happened to be installed close to the area at the time of the earthquake. The coverage increase provided by the OBH network allow us to better constrain the event relocations, and to further analyze the seismicity in the vicinity of Osa for the six months during which they were deployed. Moreover, we undertook teleseismic waveform inversion to provide additional constraints for the centroid depth of the 2002 Osa earthquake, allowing further study of the focal mechanism. Along the Costa Rican seismogenic zone, the 2002 Osa sequence is the most recent. It nucleated in the SE region of the forearc where this erosional margin is underthrust by a seamount covered ocean plate. A Mw=6.9 earthquake sequence occurred in 1999, co-located with a subducted ridge and associated seamounts. The Osa mainshock and first hours of aftershocks began in the CRISP area, ~30 km seaward of the 1999 sequence. In the following two weeks, subsequent aftershocks migrated into the 1999 aftershock area and also clustered in an area updip from it. The Osa updip seismicity apparently occurred where interplate temperatures are ~100°C or less. In this study, we present the relocation of the 2002 Osa earthquake sequence and background seismicity using different techniques and a moment tensor inversion for the mainshock, and discuss the corresponding uncertainties, in an effort to provide further evidence that the planned Phase B of CRISP will be successful in drilling the seismogenic coupling zone

The Carnivorous Saurischia in the Jura and Cretaceous Formations, principally in Europe

The carnivorous Saurischia of the jurassic and cretaceous formations of North América are relatively well known by the many papers principally of Marsh, Osborn, Lambe, Gilmore, and others. All the more one misses the same for Europe, though many finds and descriptions are existing there even from an earlier time than in North América. But most of these specimens are much more fragmentary and incomplete, it being impossible to make such methodical excavations as the country is more cultivated.Facultad de Ciencias Naturales y Muse