Geological dykes - sheets of rock that are often oriented vertically or steeply inclined to the bedding of preexisting rocks - typically intrude because stresses either 1) overcome rock strength or 2) exploit existing fractures created by preceding tectonic activity. Normally, it is impossible to tell these two possibilities apart because intrusion occurs along the rift zone - i.e., in the same direction as the faults within the rift. More generally, it is also poorly known how many types of fractures increase in size to form larger faults for similar reasons. Some existing mechanical models can explain how the displacements of faults scales with their length. However, they leave open questions of how fractures not showing such scaling develop. The role of pre-existing fractures in creating pathways for dyke propagation could be important for guiding the propagation. This potential "irrationality" of dyke intrusion is crucial for interpreting the nature (and source) of intense earthquake crises in volcanic systems, and ultimately for managing volcanic crises when knowledge of potential eruption sites would otherwise be an asset. For instance, if dykes are shown to preferentially follow pre-existing structural weaknesses, then detailed mapping of faults could provide important constraints for volcano eruption hazard maps and scenario-planning. An exciting opportunity to tackle this outstanding scientific problem is now presented by a rare, intense earthquake crisis in one of the most geometrically extreme, fissure-fed volcanoes on Earth, the volcanic ridge of São Jorge Island (Azores), which contains faults oblique to the rift zone. Starting on 19 March 2022, the region's seismicity levels raised extraordinarily from only 5 earthquakes recorded in 01/01-18/03, to over 27,000 M 2-3.3 events recorded from March 19th until now. Unfortunately, current earthquake locations are substantially uncertain because of geometric limitations of the existing seismic network, which includes only seismic stations in the islands. These uncertainties prevent us from relating the earthquakes to known faults and volcanic centres. Further, the limited data coverage and quality of existing networks have hindered the construction of detailed 3-D seismic tomography images of the region, with only 1-D velocity models being available based on land data. In order to address these issues, we propose to deploy a temporary seismic network of five ocean bottom seismometers (OBSs) around São Jorge and ten land broadband (BB) stations on São Jorge and surrounding islands. This will substantially enhance the region's seismic data coverage, leading to an unprecedented dataset: (1) showing how seismicity associated with a dyke intrusion relates to known faults; and (2) enabling the construction of the first detailed 3-D subsurface images of the crust and of the volcanic edifice in this rare example of a dyke in an environment with faults oblique to the rift zone. More generally, this project will bring key new insights into the structure and plumbing network of tall and narrow fissure-fed volcanic systems such as São Jorge. It will also shed new light on the mechanics of dyke intrusions and their kinematic evolution in general.

The Atlantic Ocean's conveyor belt circulation is a fundamental component of the global climate system, transporting heat from low to high latitudes, and thus warming Northern Europe. The strength of this circulation is thought to have varied abruptly in the past, giving rise to rapid climate changes of more than 10 degrees C in a decade during the last glacial period. Changes of this nature today would have a severe impact on society, so we want to know more about the sensitivity of this circulation. In order to do this, we will study intervals of rapid climate and circulation change in the past. To better understand these past circulation changes we will reconstruct the concentration of radiocarbon in surface and deep waters in the North Atlantic Ocean. This is known as a radiocarbon reservoir age, and it is highly sensitive to the rate of ocean circulation. Therefore, by reconstructing reservoir ages, we can tell how quickly the ocean was circulating during intervals of rapid climate change. We also need to know what the reservoir age was in the past if we want to use radiocarbon as a dating tool, to tell the age of geological and archeological objects and events. Radiocarbon can be thought of as a stopwatch for a geological sample. For a marine sample, however, there is already some time on the clock when we press go. This extra time before starting the clock is the reservoir age, and we must know what it is in order to accurately tell geological time. By reconstructing reservoir ages, we will therefore improve understanding of rapid circulation and climate change, and also improve the most important dating tool used in earth and archeological sciences. To reconstruct radiocarbon reservoir ages we need to measure the radiocarbon content of a sample, and also to know its age independently, so we can work out what was already on the clock when the sample formed. To do this we will make radiocarbon measurements on shells taken from sediment cores from the North Atlantic, and pair them with a range of exciting new techniques that can tell their age. Firstly we will look for layers of volcanic ash in the sediment cores, which we can date using their argon content, and match to precisely dated ash layers in ice cores and on Iceland. Secondly we can look at changes in sea surface temperature records, and match these to the same events that are precisely dated in ice cores. Thirdly we will use the concentration of thorium in sediments to tell how much sediment accumulated between these ash and temperature tie points. Fourthly, we will combine all this information using statistical modelling, which will also provide a good measure of the uncertainty in our results. This work will create maps of reservoir ages and how they changed in the North Atlantic over the last 10 to 50 thousand years, with a special focus on times of rapid climate change. To help us link the reservoir ages to different circulation regimes, we will use a climate model that can simulate radiocarbon. We will make this model's ocean circulation operate in different ways, and see which circulations best match our data. This will allow us to better understand how ocean circulation changed in the past to cause rapid climate change, and improve confidence in how ocean circulation may operate in the future. Finally, we will package our reservoir age maps into a tool that can be used by earth scientists and archeologists to improve their radiocarbon dating.