ALBERT

Description: Extension rate is known to control key processes during rifted margin formation such as crust-mantle coupling, decompression melting, magmatism, and serpentinisation. Here we build on recent advances in plate tectonic reconstructions by quantifying the extension velocity history of Earth’s major rifted margins during the last 240 million years. We find that many successful rifts start with a slow phase of extension followed by rapid acceleration that introduces a fast phase. The transition from slow to fast rifting takes place long before crustal break-up: approximately half of the present day rifted margin area was created during the slow, and the other half during the fast rift phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. In these models, rift velocities are not imposed but instead evolve naturally in response to the changing strength of the rift. Our results demonstrate that abrupt plate acceleration during continental rifting is controlled by a rift-intrinsic strength-velocity feedback. The abruptness of rift acceleration is thereby governed by the nonlinearity of lithospheric localization. Realistic brittle and power-law rheologies lead to a speed-up duration between two and ten million years. For successful rifts that generate a new ocean basin, the duration of rift speed-up is notably almost independent of the applied extensional force. Instead, the force controls the duration of the slow phase: higher forces shorten the slow phase while lower forces prolong it. If the force is too low, however, delocalisation processes prevent the rift from reaching the point of speed-up and produce a failed rift, even if the extensional system was active for many million years.

Description: The deep carbon cycle connects CO2 concentrations within the atmosphere to the vast carbon reservoir in Earth’s mantle: subducted lithosphere carries carbon into the mantle, while extensional plate boundaries and arc volcanoes release it back to Earth’s surface. The length of plate boundaries thereby exerts first-order control on global CO2 fluxes on geological time scales. Here we provide a global census of rift length from the Triassic to present day, combining a new plate reconstruction analysis technique with data from the geological rift record.We find that the most extensive rift phase during the fragmentation of Pangea occurred in the Jurassic/Early Cretaceous with extension along the South Atlantic (9700 km) and North Atlantic rifts (9100 km), within East Gondwana (8500 km), the failed African rift systems (4900 km), and between Australia and Antarctica (3700 km). The combined extent of these and other rift systems amounts to more than 50.000 km of simultaneously active continental rifts. During the Late Cretaceous, in the aftermath of this massive rift episode, the global rift length dropped by 60% to 20.000 km. We further show that a second pronounced rift episode starts in the Eocene with global rift lengths of up to 30.000 km. It is well-accepted that volcanoes at plate boundaries release large amounts of CO2 from the Earth’s interior. Recent work, however, highlights the importance of deep-cutting faults and diffuse degassing on CO2 emissions in the East African Rift, which appear to be comparable to CO2 release rates at mid-ocean ridges worldwide. Upscaling measured CO2 fluxes from East Africa to all concurrently active global rift zones with due caution, we compute the first-order history of cumulative rift-related CO2 degassing rates for the last 250 Myr. We demonstrate that rift-related CO2 release in the Early Cretaceous may have reached 400% of present-day rates. In first-order agreement with paleo-atmospheric CO2 concentrations from proxy indicators, our degassing rates correlate with the two distinct periods of elevated atmospheric CO2 in the Mesozoic and Cenozoic. Compiling the length of other plate boundaries through time (mid-ocean ridges, subduction zones, continental arcs), we do not find such a correlation with the paleo-CO2 record, which leads us to suggest that rift-related degassing constitutes an important element of the deep carbon cycle.

Description: Plate tectonic processes and mantle convection form a self-organized system whose surface expression is characterized by repeated Wilson cycles. Conventional numerical models often capture only specific aspects of plate-mantle interaction, due to imposed lateral boundary conditions or simplified rheologies. Here we study continental rift evolution using a 2D spherical annulus geometry that does not require lateral boundary conditions. Instead, continental extension is driven self-consistently by slab pull, basal drag and trench suction forces. We use the numerical code StagYY to solve equations of conservation of mass, momentum and energy and transport of material properties. This code is capable of computing mantle convection with self-consistently generated Earth-like plate tectonics using a pseudo-plastic rheology. Our models involve an incompressible mantle under the Boussinesq approximation with internal heat sources and basal heating. Due to the 2D setup, our models allow for a comparably high resolution of 10 km at the mantle surface and 15 km at the core mantle boundary. Viscosity variations range over 7 orders of magnitude. We find that the causes for rift initiation are often related to subduction dynamics. Some rifts initiate due to increasing slab pull, others because of developing trench suction force, for instance by closure of an intraoceanic back-arc basin. In agreement with natural settings, our models reproduce rifts forming in both young and old collision zones. Our experiments show that rift dynamics follow a characteristic evolution, which is independent of the specific setting: (1) continental rifts initiate during tens of million of years at low extension rates (few millimetres per year) (2) the extension velocity increases during less than 10 million years up to several tens of millimetres per year. This speed-up takes place before lithospheric break-up and affects the structural architecture of rifted margins. (3) high divergence rates persist until break-up is achieved and often reduce several tens of millions of years after continental separation. By illustrating the geodynamic connection between subduction dynamics and rift evolution, our results allow new interpretations of plate tectonic reconstructions. Rift acceleration during the transition from phase 1 to phase 2 induces elevated convergence rates at the opposite side of the continents. This leads to enhanced subduction velocities, e.g. between North America and the Farallon plate 200 million years ago, or to the closure of potential back-arc basins such as in the proto-Andean ranges of South America. Post-rift deceleration occurs when the global plate system re-equilibrates after the phase of enhanced stress during continental rupture. This phenomenon of a plate slow-down after mechanical rupture occurred in the real-world aftermath of Australia-Antarctica separation, South Atlantic opening, and North Atlantic break-up.

Description: The South American continent as we know it formed during the break-up of West Gondwana between 150 and 110 million years ago, when the South Atlantic Rift system evolved into the South Atlantic ocean. Using state-of-the-art global tectonic reconstructions in conjunction with numerical and analytical modelling, we investigate the geodynamics of rift systems as they evolve into an ocean basin. We find that rifts initially stretch very slowly along the future splitting zone, but then move apart very quickly before the onset of rupture. In case of the split between South America and Africa, the divergence rate increased from initially 5 to 7 millimetres per year to over 40 millimetres per year within few million years. Intriguingly, abrupt rift acceleration did not only occur during the splitting of West Gondwana, but also during the separation of Australia and Antarctica, North America and Greenland, Africa and South America, in the North Atlantic or the South China Sea. We elucidate the underlying process by reproducing the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The mechanical models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength–velocity feedback similar to a rope that snaps when pulled apart. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

Description: Probabilistic forecasting of volcanic ash dispersion typically involves simulating an ensemble of realistic event scenarios to estimate the probability of a particular hazard threshold being exceeded. While the ensemble size, the sampling procedure used, and the desired threshold all influence the uncertainty in the probability estimate, current practice does not usually quantify and communicate this uncertainty. We present the application of standard statistical methods to estimate the variance in probabilistic ensembles and communicate confidence intervals, using the example of volcanic ash transport from a representative explosive eruption in Iceland. For stochastic (random) sampling of the wind data, we show how the variance of an exceedance probability depends on the threshold of interest and the ensemble size, and illustrate how we can use the relative variance to compare the uncertainty between estimates of probabilities of different magnitudes. Further, we demonstrate how the variance can be reduced using a stratified sampling approach to ensemble design; in the chosen example we consider a set of 29 Northern European weather regimes known as Grosswetterlagen (GWL). We show that sampling wind fields from within the GWL regimes allows the uncertainty to be quantified just as easily, and reduces the number of samples required to achieve the same variance, compared to stochastic sampling. Our results show that uncertainty in ash dispersion forecasts can be straightforwardly calculated and communicated, and highlight the need for the volcanic ash forecasting community and operational end-users to jointly choose acceptable levels of variance for ash forecasts in the future.

Description: Tidal information and knowledge of longer-term sea level change are vital for safety of coastal communities and infrastructure. Madagascar has limited tidal prediction and no national sea-level monitoring capability. Working with Malagasy partners, the Direction Générale de la Météorologie (DGM), PASS-SWIO aims to establish a sea level monitoring system for Madagascar based on deployment of a low-cost relocatable tide gauge (Portagauge). Longer-term sea level variability can be derived from satellite altimetry data, however, for ‘absolute’ sea level measurements to be applied at the coast they require adjustment to local relative sea level, via ‘ground-truthing’ to some known fixed point on land. The GNSS-IR technique allows sea level to be inferred relative to the same geodetic reference frame as satellite altimetry. Co-location of GNSS with conventional tide gauge sensors allows these measurements to be connected to satellite altimetry, which can substitute for long-term observations from tide gauges.Portagauge, which uses GNSS-IR alongside a conventional radar gauge, will be deployed at Toamasina (NE Madagascar). DGM will be trained to operate the Portagauge and to process and analyse tide gauge and satellite altimeter data (Jason-2, Jason-3, Sentinel-3A and 3B). By cross validating portagauge data against satellite data, they will generate analysis of tidal and non-tidal sea-level characteristics for the Madagascar coastal region. The project will define a road map to establish a long-term, sustainable, national sea-level monitoring system for Madagascar. This will provide a model sea level monitoring system for developing island states and coastal nations, based on low-cost tide gauges and satellite data.

Description: Tide gauges can capture sea level variability on multiple timescales, from high frequency events like waves, tides and tsunamis, to seasonal and interannual changes and the longer-term trends associated with Climate Change. However, tide gauges are costly to maintain to the stringent accuracy requirements demanded by the IOC-UNESCO’s Global Sea Level Observing System (GLOSS) for monitoring sea level rise. As a result, they are often maintained to lower accuracy standards, for example, where their primary purpose is for short-term operational forecasting. This diminishes the supply of suitable sea level data for scientific studies. In addition, a sparsity of Global Navigation Satellite System (GNSS) receivers at the coast leads to large uncertainties in rates of land motion at tide gauges, which also hampers the estimation of long-term sea level trends. Through the Horizon Europe EuroSea project and the National Oceanography Centre’s (NOC) UK Tide Gauge project, we have devised prototype low-cost and largely maintenance-free tide gauge systems, which can be powered by renewable energy and which monitor both land motion and sea level using novel techniques such as ground-based GNSS Interferometric Reflectometry (GNSS-IR). These systems eliminate the need for costly ongoing levelling exercises and also incorporate customisations to local monitoring needs, such as sensors for lightning detection and wave height. The systems are being tested in a variety of coastal environments, including the UK, the Mediterranean Sea and South America. It is hoped that there is potential to advance these technological solutions as a global standard, via the GLOSS community.