ALBERT

Description: Strike-slip faults are classically associated with pull-apart basins where continental crust is thinned between two laterally offset fault segments. We propose a subsidence mechanism to explain the formation of a new type of basin where no substantial segment offset or syn-strike-slip thinning is observed. Such “flexural strike-slip basins” form due to a sediment load creating accommodation space by bending the lithosphere. We use a two-way coupling between the geodynamic code ASPECT and surface-processes code FastScape to show that flexural strike-slip basins emerge if sediment is deposited on thin lithosphere close to a strike-slip fault. These conditions were met at the Andaman Basin Central fault (Andaman Sea, Indian Ocean), where seismic reflection data provide evidence of a laterally extensive flexural basin with a depocenter located parallel to the strike-slip fault trace.

Description: Carbonatites host some unique ore deposits, especially REE, and fractional crystallization might be a potentially powerful mechanism for control enrichment of carbonatitic magmas by these metals to economically significant levels. At present, data on distribution coefficients of REE during fractional crystallization of carbonatitic melts at volcanic conditions are extremely scarce. Here we present an experimental study of REE partitioning between carbonatitic melts and calcite in the system CaCO3-Na2CO3 with varying amounts of P2O5, F, Cl, SiO2, SO3 at 650–900 °C and 100 MPa using cold-seal pressure vessels and LA-ICP-MS. The presence of phosphorus in the system generally increases the distribution coefficients but its effect decreases with increasing concentration. The temperature factor is high: at 770–900 °C DREE ≥ 1, while at lower temperatures DREE become below unity. Silicon also promotes the fractionation of REE into calcite, while sulfur contributes to retention of REE in the melt. Our results imply that calcite may impose significant control upon REE fractionation at the early stages of crystallization of carbonatitic magmas and might be a closest proxy for monitoring the REE content in initial melt.

Description: Deep hydrothermal Mo, W and base metal mineralization found in the Detroit City portal of the Sweet Home mine in the Alma district (Colorado Mineral Belt) was deposited in response to magmatic activity and the formation of Climax-type Mo deposits during the Oligocene. This study presents extensive geochemical analyses of fluid inclusions inminerals from early greisen-like vein mineralization to better understand the fluid system responsible for ore formation. Quartz and fluorite, which are associated with molybdenite, huebnerite and/or pyrite mineralization, precipitated from low- to medium-salinity (1.5-11.5 wt.% equiv. NaCl), CO2-bearing fluids at temperatures between 360 and 415°C and probably under a fluctuating pressure regime at depths of at least 3.5 km. The formation of greisen-like and base metal mineralization at the Detroit City portal of the Sweet Home mine is related to fluids of different origin. Early magmatic fluids were the principal source for mantle-derived volatiles (CO2, H2S/SO2, noble gases) and mixed with significant amounts of heated meteoric water. Fluid mixing of magmatic fluids with meteoric water is constrained by δ 2H-δ 18O relationships of fluid inclusion water in different minerals (Fig. 1). Whether molybdenum was derived from magmatic fluids remains unclear. Fluid inclusions in huebnerite suggest that W originated from source rocks that are enriched in organic matter rather than from magmatic fluids. The deep hydrothermal mineralization at the Detroit City portal of the Sweet Home mine shows features similar to deep hydrothermal vein mineralization found in Climax-type Mo deposits and their periphery, suggesting that fluid migration and the deposition of ore and gangue minerals in the Sweet Home mine was triggered by a deep-seated magmatic intrusion. The findings of this study are in good agreement with the results of previous fluid inclusion studies of the mineralization of the Sweet Home mine (Lüders et al., 2009) and from Climaxtype Mo porphyry deposits in the Colorado Mineral Belt (Hall et al., 1974).

Description: The birth and expansion of continental plateaus exert a strong control on our planet's climate and the distribution and evolution of its biodiversity. It has been proposed that the Tibetan Plateau has been steadily growing by southward expansion. Here we demonstrate that the shape of the southeastern margin of the plateau has remained unchanged for the last 10 Myr despite vast amounts of exhumation. Our finding is based on a new, high-resolution thermochronological dataset from the deep gorges of the Salween and Mekong rivers, which we interpret using a physics-based model combined with an optimization method. We show that our scenario also agrees with a wide range of other, independent geological and geophysical data. This finding demonstrates that plateau margins can reach large-scale topographic steady-state between outward growth and surface erosion, which has important implications for our understanding of the evolution of Earth's climate and biodiversity in the recent geological past.

Description: By means of 4 years of Challenging Minisatellite Payload (CHAMP) zonal wind observations and a Thermosphere-Ionosphere Electrodynamics General Circulation Model simulation, longitudinal and seasonal variations of thermospheric superrotation at magnetic equator are investigated first. The superrotation shows four- and three-peaked longitudinal patterns in March and September equinoxes, and a two-peaked variation during solstices. The superrotation is stronger in December than in June solstice, and stronger in March than in September equinox. The mean correlation between the zonal variation of superrotation and nighttime eastward wind is about 0.8, while it is 0.6 with daytime westward zonal wind. The interaction between the neutral wind and geomagnetic field plays a more important role in the superrotation, rather than the F-region electron density. The lower atmospheric tides tend to suppress the superrotation, but contribute to the longitudinal patterns of the superrotation. The viscous force is also important for the longitudinal modulation of the superrotation.

Description: We report the concurrent observations of F-region plasma changes and field-aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all-sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F-layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave-induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere.

Description: The LArge-scale Reservoir Simulator (LARS) has been previously developed to study hydrate dissociation in hydrate-bearing systems under in-situ conditions. In the present study, a numerical framework of equations of state describing hydrate formation at equilibrium conditions has been elaborated and integrated with a numerical flow and transport simulator to investigate a multi-stage hydrate formation experiment undertaken in LARS. A verification of the implemented modeling framework has been carried out by benchmarking it against another established numerical code. Three-dimensional (3D) model calibration has been performed based on laboratory data available from temperature sensors, fluid sampling, and electrical resistivity tomography. The simulation results demonstrate that temperature profiles, spatial hydrate distribution, and bulk hydrate saturation are consistent with the observations. Furthermore, our numerical framework can be applied to calibrate geophysical measurements, optimize post-processing workflows for monitoring data, improve the design of hydrate formation experiments, and investigate the temporal evolution of sub-permafrost methane hydrate reservoirs.

Description: Tropical forests store 40–50 per cent of terrestrial vegetation carbon1. However, spatial variations in aboveground live tree biomass carbon (AGC) stocks remain poorly understood, in particular in tropical montane forests2. Owing to climatic and soil changes with increasing elevation3, AGC stocks are lower in tropical montane forests compared with lowland forests2. Here we assemble and analyse a dataset of structurally intact old-growth forests (AfriMont) spanning 44 montane sites in 12 African countries. We find that montane sites in the AfriMont plot network have a mean AGC stock of 149.4 megagrams of carbon per hectare (95% confidence interval 137.1–164.2), which is comparable to lowland forests in the African Tropical Rainforest Observation Network4 and about 70 per cent and 32 per cent higher than averages from plot networks in montane2,5,6 and lowland7 forests in the Neotropics, respectively. Notably, our results are two-thirds higher than the Intergovernmental Panel on Climate Change default values for these forests in Africa8. We find that the low stem density and high abundance of large trees of African lowland forests4 is mirrored in the montane forests sampled. This carbon store is endangered: we estimate that 0.8 million hectares of old-growth African montane forest have been lost since 2000. We provide country-specific montane forest AGC stock estimates modelled from our plot network to help to guide forest conservation and reforestation interventions. Our findings highlight the need for conserving these biodiverse9,10 and carbon-rich ecosystems.

Description: In this study, the effect of disturbance dynamo electric field (DDEF) induced by subauroral polarization streams (SAPS) on the variations of the equatorial electrojet (EEJ) and its counter electrojet (CEJ) during the geomagnetic storm on June 1, 2013 is analyzed in detail for the first time. Observations from ground-based magnetometers showed that the SAPS-induced EEJ flows westward and eastward in the daytime and dawn/dusk sectors, respectively. The effects of SAPS on EEJ are mainly associated with the changes of zonal ionospheric electric field, while the changes in the ionospheric conductivity play a secondary role. By using Thermosphere Ionosphere Electrodynamic General Circulation Model simulations, the zonal electric field induced by SAPS associated with the DDEF is examined. The results of the simulations show that the DDEF has a significant impact on the EEJ variability. The daytime westward EEJ at the dip equator is mainly driven by disturbance zonal wind, with secondary contributions from disturbance meridional wind. A similar mechanism can be observed in the dawn/dusk sector when the eastward EEJ is produced; however, it has a much weaker intensity than that during the daytime.

Description: CO2 release from particulate organic carbon (POC) oxidation during fluvial transit can influence climate over a range of timescales. Identifying the mechanistic controls on such carbon fluxes requires determining where POC oxidation occurs in river systems. While field data show POC oxidation and replacement moving downstream in lowland rivers, flume studies show that oxidation during active fluvial transport is limited. This suggests that most fluvial POC oxidation occurs during transient floodplain storage, but this idea has yet to be tested. Here, we isolate the influence of floodplain storage time on POC oxidation by exploiting a chronosequence of floodplain deposits above the modern groundwater table in the Rio Bermejo, Argentina. Measurements from 15 floodplain cores with depositional ages from 1 y to 20 ky show a progressive POC concentration decrease and 13C-enrichment with increasing time spent in floodplain storage. These results from the Rio Bermejo indicate that over 80% of fluvially-deposited POC can be oxidized over millennial timescales in aerated floodplains. Furthermore, POC in the oldest floodplain cores is more 14C-enriched than expected based on the independently-dated floodplain ages, indicating that a portion of this oxidized POC is replaced by autochthonous POC produced primarily by floodplain vegetation. We suggest floodplain storage timescales control the extent of oxidation of fluvially-deposited POC, and may play a prominent role in determining if rivers are significant atmospheric CO2 sources.