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The microstructure of permafrost ground contains clues to its formation and hence its preconditioning to future change.
We applied X-ray computed microtomography (CT) to obtain high-resolution data (Delta x = 50 mu m) of the composition of a 164 cm long permafrost core drilled in a Yedoma upland in north-eastern Siberia.
The CT analysis allowed the microstructures to be directly mapped and volumetric contents of excess ice, gas inclusions, and two distinct sediment types to be quantified. Using laboratory measurements of coarsely resolved core samples, we statistically estimated the composition of the sediment types and used it to indirectly quantify volumetric contents of pore ice, organic matter, and mineral material along the core.
We conclude that CT is a promising method for obtaining physical properties of permafrost cores which opens novel research potentials.
Impact of Late Pleistocene climate variability on paleo-erosion rates in the western Himalaya
(2022)
It has been proposed that at short timescales of 10(2)-10(5) yr, climatic variability can explain variations in sediment flux, but in orogens with pronounced climatic gradients rate changes caused by the oscillating efficiency in rainfall, runoff, and/or sediment transport and deposition are still not well-constrained.
To explore landscape responses under variable climatic forcing, we evaluate time windows of prevailing sediment aggradation and related paleo-erosion rates from the southern flanks of the Dhauladhar Range in the western Himalaya.
We compare past and present Be-10-derived erosion rates of well-dated Late Pleistocene fluvial landforms and modern river sediments and reconstruct the sediment aggradation and incision history based on new luminescence data.
Our results document significant variations in erosion rates ranging from 0.1 to 3.4 mm/yr over the Late Pleistocene.
We find that, during times of weak monsoon intensity, the moderately steep areas (hillslope angles of 27 +/- 13 degrees) erode at lower rates of 0.1-0.4 mm/yr compared to steeper (>40 degrees) crestal regions of the Dhauladhar Range that erode at 0.8-1.3 mm/yr.
In contrast, during several millennia of stronger monsoon intensity, both the moderately steep and high slope areas record higher erosion rates (>1-3.4 mm/yr). Lithological clast-count analysis shows that this increase of erosion is focused in the moderately steep areas, where Lesser Himalayan rocks are exposed.
Our data thus highlight the highly non-linear response of climatic forcing on landscape evolution and suggest complex depositional processes and sedimentary signals in downstream areas. (C) 2021 Elsevier B.V. All rights reserved.
Bastnsite [REE(CO3)F] is the main mineral of REE ore deposits in carbonatites. Synthetic bastnsite-like compounds were precipitated from aqueous solutions by many different methods, but previous attempts to model magmatic crystallization of bastnsite from hydrous calciocarbonatite melts were unsuccessful.
Here we present the first experimental evidence that bastnsite and two other REE carbonates, burbankite, and lukechangite, can crystallize from carbonatite melt in the synthetic system La(CO3)F-CaCO3-Na2CO3 at temperatures between 580 and 850 degrees C and a pressure 100 MPa.
The experiments on starting mixtures of reagent-grade CaCO3, Na2CO3, La-2(CO3)(3), and LaF3 were carried out in cold-seal rapid-quench pressure vessels.
The studied system is an isobaric pseudoternary join of a quinary system where CO2 and fluorides act as independent components. Liquidus phases in the run products are calcite, nyerereite, Na carbonate, bastnsite, burbankite solid solution (Na,Ca)(3)(Ca,La)(3)(CO3)(5), and lukechangite Na3La2(CO3)(4)F. Calcite and bastnite form a eutectic in the boundary join La(CO3)F-CaCO3 at 780 +/- 20 degrees C and 58 wt% La(CO3)F. Phase equilibria in the boundary join La(CO3)F-Na2CO3 are complicated by peritectic reaction between Ca-free end-member of burbankite solid solution petersenite (Pet) and lukechangite (Luk) with liquid (L): Na4La2(CO3)(5) (Pet) + NaF (L) = Na3La2(CO3)(4)F (Luk) + Na2CO3 (Nc).
The right-hand-side assemblage becomes stable below 600 +/- 20 degrees C. In ternary mixtures, bastnsite (Bst), burbankite (Bur), and calcite (Cc) are involved in another peritectic reaction: 2La(CO3)F (Bst) + CaCO3 (Cc) + 2Na(2)CO(3) (L) = Na2CaLa2(CO3)(5) (Bur) + 2NaF (L). Burbankite in equilibrium with calcite replaces bastnsite below 730 +/- 20 degrees C. Stable solidus assemblages in the pseudoternary system are: basnsite-burbankite-fluorite-calcite, basnasite-burbankite-fluorite-lukechangite, bastnsite-burbankite-lukechangite, burbankite-lukechangite-nyerereitecalcite, and burbankite-lukechangite-nyerereite-natrite. Addition of 10 wt% Ca-3(PO4)(2) to one of the ternary mixtures resulted in massive crystallization of La-bearing apatite and monazite and complete disappearance of bastnsite and burbankite.
Our results confirm that REE-bearing phosphates are much more stable than carbonates and fluorocarbonates.
Therefore, primary crystallization of the latter from common carbonatite magmas is unlikely. Possible exceptions are carbonatites at Mountain Pass that are characterized by very low P2O5 concentrations (usually at or below 0.5 wt%) and extremely high REE contents in the order of a few weight percent or more. In other carbonatites, bastnsite and burbankite probably crystallized from highly concentrated alkaline carbonate-chloride brines that were found in melt inclusions and are thought to be responsible for widespread fenitization around carbonatite bodies.
From June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high-temperature zones at high resolution.
The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlio geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short-period and broadband stations.
In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators.
During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant.
Here, we describe the network and the recorded datasets.
Furthermore, we showsome initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high-resolution event location to delineate faults and body- and surface-wave tomographies to image the subsurface velocity structure in great detail.
Martian atmospheric spectral end-members retrieval from ExoMars Thermal Infrared (TIRVIM) data
(2022)
Key knowledge about planetary composition can be recovered from the study of thermal infrared spectral range datasets.
This range has a huge diagnostic potential because it contains diagnostic absorptions from a planetary surface and atmosphere. The main goal of this study is to process and interpret the dataset from the Thermal Infrared channel (TIRVIM) which is part of the Atmospheric Chemistry Suite of the ExoMars2016 Trace Gas Orbiter mission to find and characterize dust and water ice clouds in the atmosphere.
The method employed here is based on the application of principal component analysis and target transformation techniques to extract the independent variable components present in the analyzed dataset. Spectral shapes of both atmospheric dust and water ice aerosols have been recovered from the analysis of TIRVIM data.
The comparison between our results with those previously obtained on Thermal Emission Spectrometer (TES) data and with previous analysis on TIRVIM data, validates the methodology here applied, showing that it allows to correctly recover the atmospheric spectral endmembers present in the TIRVIM data.
Moreover, comparison with atmospheric retrievals on PFS, TES and IRIS data, allowed us to assess the temporal stability and homogeneity of dust and water ice components in the Martian atmosphere over a time period of almost 50 years.
The wetland cover is defined as the spatially homogenous region of a wetland attributed to the underlying biophysical conditions such as vegetation, turbidity, hydric soil, and the amount of water.
Here, we present a novel method to derive the wetland-cover types (WCTs) combining three commonly used multispectral indices, NDVI, MNDWI, and NDTI, in three large Ramsar wetlands located in different geomorphic and climatic settings across India. These wetlands include the Kaabar Tal, a floodplain wetland in east Ganga Plains, Chilika Lagoon, a coastal wetland in eastern India, and Nal Sarovar in semi-arid western India.
The novelty of our approach is that the derived WCTs are stable in space and time, and therefore, a given WCT across different wetlands or within different zones of a large wetland will imply similar underlying biophysical attributes.
The WCTs can therefore provide a novel tool for monitoring and change detection of wetland cover types.
We have automated the proposed WCT algorithm using the Google Earth Engine (GEE) environment and by developing ArcGIS tools. The method can be implemented on any wetland and using any multispectral imagery dataset with visible and NIR bands.
The proposed methodology is simple yet robust and easy to implement and, therefore, holds significant importance in wetland monitoring and management.
The magnitude of earthquakes on continental normal faults rarely exceeds 7.0 Mw. However, because of their vicinity to large population centers they can be highly destructive.
Long recurrence time, relatively small deformations, and limited observations hinder our understanding of the deformation patterns and mechanisms controlling the magnitude of events.
Here, this problem is addressed with 2D thermomechanical modeling of normal fault seismic cycles.
The 2020 Samos, Greece Mw7.0 earthquake is used as an example as it is one of the largest and most studied continental normal fault earthquakes. The modeling approach employs visco-elasto-plastic rheology, compressibility, free surface, and a rate-and-state friction law for the fault.
Modeling of the Samos earthquake suggests the pore fluid pressure ratio on the fault ranges from 0 to 0.7. The model demonstrates that most of the deformation during interseismic and coseismic periods, besides on the fault, occurs in the hanging wall and footwall below the seismogenic part of the fault. The largest vertical surface displacement during the earthquake is the subsidence of the hanging wall in the vicinity of the fault, while the uplift of the footwall and remote part of the hanging wall is significantly smaller.
Modeling of the seismic cycles on normal faults with different setups shows the dependency of the magnitude on the thermal profile and dipping angle of the fault; low heat flow and low dipping angle are favorable conditions for the largest events, while steep normal faults in the areas of high heat flow tend to have the smallest magnitudes.
The formation of the Central Andes dates back to similar to 50 Ma, but its most pronounced episode, including the growth of the Altiplano-Puna Plateau and pulsatile tectonic shortening phases, occurred within the last 25 Ma.
The reason for this evolution remains unexplained. Using geodynamic numerical modeling we infer that the primary cause of the pulses of tectonic shortening and growth of the Central Andes is the changing geometry of the subducted Nazca plate, and particularly the steepening of the mid-mantle slab segment which results in a slowing down of the trench retreat and subsequent increase in shortening of the advancing South America plate.
This steepening first happens after the end of the flat slab episode at similar to 25 Ma, and later during the buckling and stagnation of the slab in the mantle transition zone. Processes that mechanically weaken the lithosphere of the South America plate, as suggested in previous studies, enhance the intensity of the shortening events.
These processes include delamination of the mantle lithosphere and weakening of foreland sediments.
Our new modeling results are consistent with the timing and amplitude of the deformation from geological data in the Central Andes at the Altiplano latitude.
Plain Language Summary
The Central Andes is a subduction-type orogeny that formed as a result of the interaction between the Nazca oceanic plate and the South American continental plate over the last 50 million years. Growth of the Andes is primarily the result of crustal shortening. Nevertheless, "geological" data compiled from previous studies have shown that phases of drastic pulsatile shortening occur at 15 and 5 Ma.
In this study, we used high-resolution 2D numerical geodynamic simulations to investigate the link between oceanic and continental plate dynamics and their interaction. We find that when the oceanic plate steepens in the mantle transition zone, the trench retreat is hindered. Coupled with the weakening of the continental plate through the slab flattening and subsequent delamination of the lithospheric mantle, this leads to pulsatile shortening phases of a magnitude equivalent to that suggested by the data.
Magma-filled dikes may feed erupting fissures that lead to alignments of craters developing at the surface, yet the details of activity and migrating eruptions at the crater row are difficult to monitor and are hardly understood.
The 2021 Tajogaite eruption at the Cumbre Vieja, La Palma (Spain), lasted 85 days and developed a pronounced alignment of craters that may be related to changes within the volcano edifice.
Here, we use COSMO-SkyMed satellite radar data and ground-based time-lapse photographs, offering a high-resolution dataset to explore the locations and characteristics of evolving craters.
Our results show that the craters evolve both gradually and suddenly and can be divided into three main phases. Phase 1, lasting the first 6 weeks of the eruption, was characterized by a NW-SE linear evolution of up to seven craters emerging on the growing cone.
Following two partial collapses of the cone to the northwest and a seismicity increase at depth, Phase 2 started and caused a propagation of the main activity toward the southeastern side, together with the presence of up to 11 craters along this main NW-SE trend. Associated with strong deep and shallow earthquakes, Phase 3 was initiated and continued for the final 2 weeks of the eruption, expressed by the development of up to 18 craters, which became dominant and clustered in the southeastern sector in early December 2021. In Phase 3, a second and oblique alignment and surface fracture was identified.
Our findings that crater and eruption changes coincide together with an increase in seismic activity at depth point to a deep driver leading to crater and morphology changes at the surface.
These also suggest that crater distributions might allow for improved monitoring of changes occurring at depth, and vice versa, such that strong seismicity changes at depth may herald the migration and new formation of craters, which have major implications for the assessment of tephra and lava flow hazards on volcanoes.
On 12 August 2021, a > 220 s lasting complex earthquake with Mw > 8.2 hit the South Sandwich Trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001 to 2 Hz, including point and finite fault inversions and the back-projection of high-frequency signals.
We also determined moment tensor solutions for 88 aftershocks. The rupture initiated simultaneously with a rupture equivalent to a M-w 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture, which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench.
The slow rupture covered nearly two-thirds of the entire subduction zone length, and with M-w 8.2 released the bulk of the total moment of the whole earthquake.
Tsunami modeling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to M-w 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.
Plain Language Summary
The earthquake of 12 August 2021 along the deep-sea trench of the South Sandwich Islands in the South Atlantic reached a magnitude of 8.2 and triggered a tsunami. The automatic earthquake parameter determination of different agencies showed very different results shortly after the earthquake and partially underestimated the tsunami potential of the earthquake.
A possible reason was the complex rupture process and that the tsunami was generated by a long and shallow slow slip rupture sandwiched between more conventional fast slip subevents at its northern and southern ends. In addition, the fault surface, which extended over 450 km, was highly curved striking 150 degrees-220 degrees.
We investigated the different components of the seismic wavefields in different frequency ranges and with different methods.
The analysis shows how even complex earthquakes can be deciphered by combining analyzing methods. The comparison with aftershocks and the triggered tsunami waves confirms our model that explains the South Sandwich rupture by four subevents in the plate boundary along the curved deep-sea trench. Here, the depth, rupture velocities, and slip on each segment of the rupture vary considerably.
The method can also be applied to other megathrust earthquakes and help to further improve tsunami warnings in the future.