This study focuses on unraveling the microphysical origins of the nonlinear elastic effects, which are pervasive in the Earth’s crust. Here, we examine the influence of grain shape on the elastic nonlinearity of granular assemblies. We find that the elastic nonlinearity of angular sand particles is of the same order of magnitude as that previously measured in spherical glass beads. However, while the elastic nonlinearity of glass beads increases by an order of magnitude with RH, that of sand particles is rather RH independent. We attribute this difference to the angularity of sand particles: absorbed water on the spherical grains weakens the junctions making them more nonlinear, while no such effect occurs in sand due to grain interlocking. Additionally, for one of the nonlinear parameters that likely arises from shearing/partial slip of the grain junctions, we observe a sharp amplitude threshold in sand which is not observed in glass beads.

Anomalous mercury (Hg) contents recorded near the Permian-Triassic boundary (PTB) are often linked to Siberian Traps Large Igneous Province (STLIP) volcanism and the Permian-Triassic boundary mass extinction (PTBME). However, mounting evidence indicates that the relation between STLIP volcanism and Hg “anomalies” is not straightforward. This study focuses on the timing and provenance of volcanic fluxes around the PTBME in South China. We constrain carbon isotope (δ13C) and Hg concentration and isotope records by utilizing high-precision U-Pb zircon ages from two expanded deep-water marine sections spanning the Late Permian to Early Triassic in the Nanpanjiang Basin. Results reveal two episodes of Hg enrichment. The oldest episode predates the onset of a large negative δ13C excursion, which is documented to be older than 252.07 ± 0.130 Ma. The second episode occurred between 251.822 ± 0.060 Ma and 251.589 ± 0.062 Ma, coinciding with the nadir of the δ13C excursion. Volcanic ash geochemistry and Hg isotope compositions suggest that mercury was sourced from subduction-related volcanic arc magmatism in the Tethys region, which peaked between 251.668 ± 0.079 Ma and 251.589 ± 0.052 Ma. These results are compatible with suggestions that regional arc volcanism contributed to the causes of the PTBME in South China and provide evidence that Hg anomalies close to the PTB are not a reliable stratigraphic marker for the PTB extinction event. This study demonstrates that the relations between volcanism, environmental perturbations and mass extinction during the Permian-Triassic transition are better resolved with the aid of high-precision U-Pb zircon ages.

A document by Mike Turley. Click on the document to view its contents.

A document by Sebastián Carrasco. Click on the document to view its contents.

We present a first study showing that organization of trade cumulus (Tc) clouds can significantly enhance Tc response to climate change. Among four recently identified states of Tc organization, the “Flower” state has the highest and the “Sugar” state the lowest cloud fraction and cloud radiative effect. Using large-eddy simulations, we show that the organized “Flower” Tc state is strongly suppressed at the end of the 21st century, unlike the less organized “Sugar” Tc state and Tc studied previously. The primary cause of the suppression is down-welling long-wave radiation from increased greenhouse gas concentrations, which weakens the mesoscale circulation that organizes clouds into the “Flower” Tc state. The cumulus-valve mechanism, which is thought to limit Tc response to climate change, does not prevent this response. Our work unravels an unrecognized role of cloud organization in the cloud response to climate change.

Significant volumes of magma are intruded into the crust during continental break-up, which can influence rift evolution by altering thermo-mechanical structure of the crust and thereby its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine-hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper-lower crustal boundary (10-15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid-crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift.

Subglacial hydrology can exert an important control on ice flow by affecting drag at the ice-bedrock interface. Here, we report on a series of subglacial drainage events (outbursts) along the Northeast Greenland Ice Stream (NEGIS), initiating as far inland as ~500 km from the margin of Zachariae Isstrøm. The drainage events are associated with local transient uplift, followed by prolonged subsidence, measured by satellite synthetic aperture radar interferometry (DInSAR). In downstream regions, drainage events are associated with a local speed-up in ice flow. The high spatiotemporal resolution of the DInSAR measurements allows for a detailed mapping of the drainage propagation pathway. We show that multiple drainage cascades have occurred along the same identified pathway over the years 2020-2022. Finally, the propagation speed of subglacial water flow is found to vary greatly along NEGIS, suggesting that fundamental differences may exist in the subglacial environment.

Mass-extinction with instantaneous and short-term effects on extreme climate and deteriorated ocean environment across the Cretaceous/Paleogene boundary (K/PgB) has been verified by an array of geological records, however, a longer-term (~100−1000 Kyr) post-K/PgB variation remain poorly understood, particularly due to the scarcity of terrestrial records. This study presents carbon isotope analyses of pedogenic carbonates in the Nanxiong Basin, South China to reconstruct carbon cycles and atmospheric CO2 concentrations (pCO2) spanning 70.0–62.0 Ma. Combined with data from Songliao Basin (China) and Tornillo Basin (USA), δ13C displays a post-K/PgB (66.0-64.5 Ma) vibration that is correlative to the surface ocean but mirroring to the bottom ocean. The vibration shows a pattern of collapse and smooth towards rebound, constituting a process of ~400 Kyr (millennia) deterioration, ~300 Kyr stabilization and ~800 Kyr recovery for the longer-term ecosystem and environment. A similar pattern is observed for the reconstructed pCO2, correlating to changes of sea surface temperature (SST) but contrasting bottom water temperature (BWT). With the discrepancy of longer-term proxy variations, it is proposed that ecosystems and environments in terrestrial and surface ocean had experienced a more unstable, difficult and erratic recovery process and were much more sensitive to climatic changes than in deep ocean for ~1.5 million years in the aftermath of the end-Cretaceous mass extinction. In addition, the decoupling of proxy variations from expected effects implies Deccan volcanism and Chicxulub impact may not have played a key role in the longer-term CO2 perturbation and environmental change following the K/PgB.

A document by Euan G. Nisbet. Click on the document to view its contents.

A machine-learning classifier for radiation waveforms of negative return strokes (RSs) is built and tested based on the Random Forest classifier using a large dataset consisting of 14,898 negative RSs and 159,277 intracloud (IC) pulses with 3-D location information. Eleven simple parameters including three parameters related with pulse characteristics and eight parameters related with the relative strength of pulses are defined to build the classifier. Two parameters for the evaluation of the classifier performance are also defined, including the classification accuracy, which is the percentage of true RSs in all classified RSs, and the identification efficiency, which is the percentage of correctly classified RSs in all true RSs. The tradeoff between the accuracy and the efficiency is examined and simple methods to tune the tradeoff are developed. The classifier achieved the best overall performance with an accuracy of 98.84% and an efficiency of 98.81%. With the same technique, the classifier for positive RSs is also built and tested using a dataset consisting of 8,700 positive RSs. The classifier has an accuracy of 99.04% and an efficiency of 98.37%. We also demonstrate that our classifiers can be readily used in various lightning location systems. By examining misclassified waveforms, we show evidence that some RSs and IC discharges produce special radiation waveforms that are almost impossible to correctly classify without 3-D location information, resulting in a fundamental difficulty to achieve very high accuracy and efficiency in the classification of lightning radiation waveforms.

Supercooled fogs can have an important radiative impact at the surface of the Greenland Ice Sheet, but they are difficult to detect and our understanding of the factors that control their lifetime and radiative properties is limited by a lack of observations. This study demonstrates that spectrally resolved measurements of downwelling longwave radiation can be used to generate retrievals of fog microphysical properties (phase and particle effective radius) when the fog visible optical depth is greater than ~0.25. For twelve cases of fog under otherwise clear skies between June and September 2019 at Summit Station in central Greenland, nine cases were mixed-phase. The mean ice particle (optically-equivalent sphere) effective radius was 24.0±7.8 µm, and the mean liquid droplet effective radius was 14.0±2.7 µm. These results, combined with measurements of aerosol particle number concentrations, provide observational evidence supporting the hypotheses that (a) low surface aerosol particle number concentrations can limit fog liquid water path, (b) fog can act to increase near-surface aerosol particle number concentrations through enhanced mixing, and (c) multiple fog events in quiescent periods gradually deplete near-surface aerosol particle number concentrations.

Future changes in Sahel precipitation are uncertain because of large differences among projections from various models. In order to explore this uncertainty, we use a storyline approach which seeks to identify alternative plausible evolutions of Sahel precipitation and their driving factors. By analysing projections from the CMIP6 climate models, we show that changes in North Atlantic and in Euro-Mediterranean temperatures explain up to 50% of the central Sahel precipitation change uncertainty. We then construct several storylines of Sahel precipitation change based on future plausible changes in North Atlantic and Euro-Mediterranean temperatures. In one storyline, an amplified warming of both the North Atlantic and the Euro-Mediterranean areas promotes a northward shift of the West African monsoon, increasing precipitation over the central Sahel, while, in another storyline, a moderate warming in both regions is associated with a small change in precipitation over the central Sahel and a decrease in precipitation over the western Sahel, at the end of the 21st century. These results indicate that reducing uncertainty in future changes in Sahel precipitation could be achieved by reducing uncertainty in the future warming of the North Atlantic and the Euro-Mediterranean areas.

A document by Xiaohan Song. Click on the document to view its contents.

In Iran, both local and regional scale land-surface deformation has resulted from the decline in groundwater levels \cite{Motagh_2008}. Moreover, the gap between groundwater extraction and renewal is so large that the resulting short-term impacts are likely to be irreversible \cite{Olen_2021}.Here we use Earth Observation (EO) data to calculate vertical subsidence rates due to groundwater extraction in Tehran, Iran’s capital city. This data includes Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR); very high-resolution (VHR) Pléiades optical stereo imagery; and ICESAT-2 laser altimetry data.The Centre for Observing and Monitoring Earthquakes and Tectonics (COMET) Looking into Continents from Space (LiCSAR) automated processing system is used to process six years (2015-2021) of Sentinel-1 SAR acquisitions \cite{Lazeck__2020} for interferometric (InSAR) analysis. The system generates short baseline networks of interferograms. We also correct for atmospheric noise using the GACOS system \cite{Yu_2018a,Yu_2018} and perform time-series analysis using open-source LiCSBAS software \cite{Morishita_2020}. Vertical and horizontal (east-west) velocities are calculated and mapped to produce nationwide and regional velocity fields \cite{Watson_2022}. Preliminary results indicate maximum subsidence rates in Tehran exceed 100 mm/year.VHR Digital Elevation Models (DEMs) and laser altimetry ground returns are also used to calculate land subsidence rates in Tehran. By comparing rates calculated using all three EO techniques we aim to validate InSAR velocities whilst investigating and constraining the benefits, drawbacks, and biases associated with each technique. Phase bias \cite{De_Zan_2015}, for example, may be introduced to calculated InSAR velocities when using the LiCSBAS short baseline network strategy as subsiding regions in Iran are often vegetated cropland.Publishing the open-source COMET-LiCS Subsidence Portal was the focus of previous work (https://comet-subsidencedb.org/). The portal presents automatically processed LiCSAR Sentinel-1 interferograms and LiCSBAS velocity timeseries for 99 subsiding regions across Iran. Interactive tools allow stakeholders to make quick, critical assessments related to extents and rates of subsidence. Validating portal data using DEMs and laser altimetry is essential before expanding the portal to have a global focus.

A document by Mohammed Gharioua. Click on the document to view its contents.

Global warming due to carbon dioxide-based radiative forcing did not resist to a critical analysis largely based on fundamentals of chemistry, physics and thermodynamics. This finding led to giving water an essential role in an alternative mechanism in which heat and not CO2 is climate determinant. This mechanism is based on ice ↔ liquid water and liquid water ↔ vapor interphase equilibria combined with the physics of infrared waves when they pass through the atmosphere. Accordingly, future global average temperature and ocean level rises should be smaller than predicted in the case of radiative forcing. Climatic events depending on chaotic perturbations, increases in strength and frequency are expected if anthropogenic heat releases become significant relative to solar heat supplies. Applied to distant past climate fluctuations, the water-based heat-management mechanism showed that ice melting, evaporation and humidity also determined the ups and downs of temperature and ocean level during glaciation-deglaciation alternating periods. The present times are part of the last post-deglaciation pseudo plateau in which variations of global temperature are limited to ± 2°C, a range respected during the last 8,000 years, including the recent industrial era, and comparable to plateau periods in distant past. It is in this plateau period that anthropogenic heat releases are presently complementing historical heat supplies from the Sun. Heat being a physical phenomenon independent of the sources, global temperature and level of oceans should continue to vary within the rather narrow ranges typical of past plateau periods provided anthropogenic heat releases remain negligible relative to solar supplies. If it is not the case, ice melting and evaporation may become unable to compensate anthropogenic heat supplies. The + 2°C limit would then be exceeded with progressively more evaporation, more winds, more hurricanes, more tornadoes, more clouds, more rains, more floodings and droughts, and at the end shortening of the time to next glaciation. Trend may seem already manifest on the basis of local recent climatic events felt unusual. However, the stock of ices is such that the evolution should extend over several centuries, may be more if evaporation acts in complement.

A document by Fanny Lehmann. Click on the document to view its contents.

A document by Uddalak Biswas. Click on the document to view its contents.