Fluid ascent through the solid lithosphere and its relation to earthquakes

The Earth is continuously expelling gases and liquids from great depths—juvenile volatiles from the mantle and recycled metamorphic products. Some of these fluids ascend through liquid rock in volcanic processes, but others utilize fractures and faults as conduits through the solid lithosphere. The latter process may have a major influence on earthquakes, since fluids at near lithostatic pressures appear to be required to activate deep faults that would otherwise remain locked.Fluids can be driven upward through solid rock by buoyancy, but only if present in sufficient concentration to form large-scale domains occupying interconnected fracture porosity. A growing fluid domain becomes so mobilized only when it attains the critical vertical dimension required for hydrostatic instability. This dimension, depending on the ultimate compressive yield strength of the rock, may be as much as several kilometers.Any column of fluid ascending through fractures in the solid lithosphere from a prolific deep source must become organized into a vertical sequence of discrete domains, separated by fluid-pressure discontinuities. This is required because a continuous hydrostatic-fluid-pressure profile extending from an arbitrarily deep source to the surface cannot be permitted by the finite strength of rock. A vertically stacked sequence of domains allows the internal fluid-pressure profile to approximate the external rock-stress profile in a stepwise fashion. The pressure discontinuity below the base of the uppermost hydrostatic domain may be responsible for some occurrences of so-called anomalous geopressures. An ascending stream of fluid that percolates upward from a deep source through a column of domains must encounter a sequence of abrupt pressure decreases at the transitions between successive domains. If supercritical gases act as solvents, the dissolved substances may drop out of solution at such pressure discontinuities, resulting in a local concentration of minerals and other substances.At great depths, brittle fracture would normally be prevented by high pressure and temperature, with all excessive stress discharged by ductile flow. Rock strata invaded by an ascending fluid domain are weakened, however, because cracks generated or reactivated by the high-pressure fluid can support the overburden, greatly reducing internal friction. This reduction of strength may cause a previously stressed rock to fail, resulting in hydraulic shear fracture. Thus, earthquakes may be triggered by the buoyant migration of deep-source fluids.The actual timing of the failure that leads to such an earthquake may be determined by the relatively rapid inflation of a fluid domain and not by any significant increase in the probably much slower rate of regional tectonic strain. Many earthquake precursory phenomena may be secondary symptoms of an increase in pore-fluid pressure, and certain coseismic phenomena may result from the venting of high-pressure fluids when faults break the surface. Instabilities in the migration of such fluid domains may also contribute to or cause the eruption of mud volcanoes, magma volcanoes, and kimberlite pipes.     

The mid-ocean ridge axial valley as a steady-state neck

In this paper the mid-ocean ridge axial valley is modelled as a steady-state lithospheric neck in which lithospheric stretching balances lithospheric accretion. Conversely, the axial high is a steady-state lithospheric bulge. The lithosphere is modelled as a thin plate with a Newtonian rheology. It is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases. An axial high will occur if the opposite condition holds. This is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates. By fitting our model to profiles across the Mid-Atlantic Ridge and the East Pacific Rise and assuming the lithospheric thickness at the ridge axis to be 5 km, we find accretion widths of 6–8 km. We find the width over which there is a significant increase in lithospheric viscosity to be also 6–8 km.     

Sensitivity analysis of CDOM spectral slope in artificial and natural samples: an application in the central eastern Mediterranean Basin

In the past two decades, optical properties of chromophoric dissolved organic matter (CDOM) in marine environments have been extensively studied. Many of these studies report CDOM properties for the offshore environment where this complex mixture of optically active compounds is strongly diluted. Nevertheless, autochthonous and allochthonous sources have been identified and sinks related to photodegradation and bacterial activity have been demonstrated. The calculation of the spectral slope of the CDOM absorption curve has been proven to be useful and is often reported. However, a rigorous uncertainty analysis of the slope calculation is rarely reported. In this paper, we propose a method to evaluate the uncertainty of CDOM spectral slope calculated between 270 and 400 nm, using both naturally sampled and artificial solutions. We use these results to study the ultra-oligotrophic waters of the Mediterranean Sea (central eastern basin), where little is known about CDOM spatial distribution. We show that dilutions of both artificial and natural samples produce a Gaussian distribution of spectral slopes, indicating that consistent values may be determined, with a typical uncertainty of ±0.0004 nm⁻¹ when absorption at 300 nm was greater then 0.1 m⁻¹ (0.1 m pathlength). Comparing the distribution of spectral slopes from central eastern basin samples to a Gaussian distribution, we show differences between measurements that were significantly different. These values allow us to distinguish possible sources (algal derived CDOM), sinks (e.g. photo-bleaching) at different depths. We propose a subdivision of CDOM compounds into refractory and semilabile/refractory pools and evaluate the CDOM spectral slope of algal derived CDOM released at or near deep chlorophyll maximum.     

The role of biophysical interactions within the ijzermonding tidal flat sediment dynamics

This paper focuses on the importance of biophysical interactions on short-term and long-term sediment dynamics. Therefore, various biological (macrobenthos, photopigments, colloidal EPS) and physical parameters (grain size, water content, sediment stability, bed level) were determined (bi)monthly in nine sampling plots on the IJzermonding tidal flat (Belgium, 51°08′N, 2°44′E) during three consecutive years (July 2005–June 2008). Results showed that sediment stability varied on the short timescale and was directly influenced by biota, while bed level varied mainly on the long-term due to interannual variability. The short-term dynamic relationships between mud content, water content, fucoxanthin and macrobenthos density resulted in a seasonal mud deposition and erosion cycle, and directly influenced sediment stability. Moreover, macrobenthos was proven to be the most important parameter determining sediment stability. On the long-term, a shift was observed from high fucoxanthin/chla concentration, high mud content and zero to moderate densities of Corophium volutator towards low fucoxanthin/chl a and mud content and high Corophium densities, which resulted in a transition from net accretion to net erosion. However, most measured variables proved to be poor predictors for these long-term bed level changes, indicating that external physical forces, such as waves and storminess, probably were the most important factors triggering long-term sediment dynamics. Nevertheless, biota indirectly influenced bed level changes by mediating short-term changes in sediment stability, thereby influencing the erodability of the sediment. The macrobenthos, and especially the mud shrimp Corophium, was suggested as the (indirect) driving destabilising factor for the sampling plots in the IIzermonding when considering the long-term evolution.     

Strong slope-aspect control of regolith thickness by bedrock foliation

The porous near-surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long-term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope-aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north-facing slope, regolith on the south-facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.     

Investigating potential future changes in surface water flooding hazard and impact

Surface water flooding (SWF) is a recurrent hazard that affects lives and livelihoods. Climate change is projected to change the frequency of extreme rainfall events that can lead to SWF. Increasingly, data from Regional Climate Models (RCMs) are being used to investigate the potential water-related impacts of climate change; such assessments often focus on broad-scale fluvial flooding and the use of coarse resolution (>12 km) RCMs. However, high-resolution (<4 km) convection-permitting RCMs are now becoming available that allow impact assessments of more localised SWF to be made. At the same time, there has been an increasing demand for more robust and timely real-time forecast and alert information on SWF. In the UK, a real-time SWF Hazard Impact Model framework has been developed. The system uses 1-km gridded surface runoff estimates from a hydrological model to simulate the SWF hazard. These are linked to detailed inundation model outputs through an Impact Library to assess impacts on property, people, transport, and infrastructure for four severity levels. Here, a set of high-resolution (1.5 km and 12 km) RCM data has been used as input to a grid-based hydrological model over southern Britain to simulate Current (1996–2009) and Future (~2100s; RCP8.5) surface runoff. Counts of threshold-exceedance for surface runoff and precipitation (at 1-, 3- and 6-hr durations) are analysed. Results show that the percentage increases in surface runoff extremes, are less than those of precipitation extremes. The higher-resolution RCM simulates the largest percentage increases, which occur in winter, and the winter exceedance counts are greater than summer exceedance counts. For property impacts, the largest percentage increases are also in winter; however, it is the 12-km RCM output that leads to the largest percentage increase in impacts. The added-value of high-resolution climate model data for hydrological modelling is from capturing the more intense convective storms in surface runoff estimates.     

Occurrence and distribution of microplastics in marine sediments along the Belgian coast

Plastic debris is known to undergo fragmentation at sea, which leads to the formation of microscopic particles of plastic; the so called ‘microplastics’. Due to their buoyant and persistent properties, these microplastics have the potential to become widely dispersed in the marine environment through hydrodynamic processes and ocean currents. In this study, the occurrence and distribution of microplastics was investigated in Belgian marine sediments from different locations (coastal harbours, beaches and sublittoral areas).Particles were found in large numbers in all samples, showing the wide distribution of microplastics in Belgian coastal waters. The highest concentrations were found in the harbours where total microplastic concentrations of up to 390 particles kg⁻¹ dry sediment were observed, which is 15-50 times higher than reported maximum concentrations of other, similar study areas.The depth profile of sediment cores suggested that microplastic concentrations on the beaches reflect the global plastic production increase.     

Resistive wave breaking in the Earth's outer core

The equations for an electrically conducting fluid in cylindrical coordinates are linearized assuming that the inertial terms in the momentum equation can be ignored (small Rossby number), and that the ratio of the Elsasser number and magnetic Reynolds number is one. After these assumptions, the governing equations are linearized about an ambient solution which vanishes at the the equator. Upon assuming large Elsasser and magnetic Reynolds number, the solutions to the linearized equations are approximated by wave trains having very short wave length (relative to the core radius) but which vary slowly (on a scale of the core radius). The period of the waves is much longer than a day but much shorter than the period of the slow hydromagnetic oscillations. These waves are found to be trapped in a region about the equator and away from the axis of rotation. The waves break at a latitudinal wave region boundary, in the sense that the waves become exponentially large in a boundary layer, having as an exponent some positive power of the large azimuthal wave number. This behavior is amplified as the Elsasser number becomes smaller while still remaining relatively large. Waves in more Earth-like parameter regimes are discussed briefly.