Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid. This solute Ca and Mg gets subsequently buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction has provided the climatic feedback that has maintained Earth's climate equable and inhabitable over the entire history of the Earth. To understand how Earth's climate functions, it is therefore vital to understand silicate weathering and to estimate the flux carbon dioxide associated with modern chemical weathering. Modern day silicate weathering fluxes are estimated from the chemistry of rivers or natural waters. Natural waters contain positively charged elements or cations such as Ca, Mg, Na and K, and it has been understood for decades that the relative and absolute concentrations of these elements depend of the type of rocks that are drained. For example, Ca is mainly derived from the weathering of limestones, whereas Na and K are mainly derived from the weathering of silicate minerals such as feldspar. This distinction is important because only the Ca derived from silicate weathering is important for carbon dioxide consumption. Therefore, the Ca flux from silicate weathering is usually estimated based on Na, which has been thought to a more reliable estimate of silicate weathering. However, chemical weathering is more complex than simple mineral dissolution and a series of other chemical reactions also occur such as cation exchange. This is a process whereby the positively charged cations in solution are attracted to negatively charged mineral surfaces on clays, a process known to buffer groundwaters. One of the key chemical exchanges is Ca for Na, meaning that Na may not provide a true estimate of silicate weathering at all. Recent isotopic data suggests that cation exchange might be more significant that previously thought, which until now has been very hard to fingerprint. One method is to use naturally occurring tracers or isotopes, that allow chemical reactions to be tracked. In this work, it is proposed to examine the naturally occurring isotopes of the elements Li and Mg to examine the role of cation exchange in global budgets. However, to be able to do this successfully, a series of experimental work is proposed to examine the behaviour of the isotopes of Mg and Li during cation exchange. Once we understand how our tracers work we can use them to re-evaluate our understanding of natural waters, and better estimate fluxes of carbon dioxide associated with chemical weathering, with the ultimate aim of better understanding Earth's climate.

Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid. This solute Ca and Mg gets subsequently buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction has provided the climatic feedback that has maintained Earth's climate equable and inhabitable over the entire history of the Earth. To understand how Earth's climate functions, it is therefore vital to understand silicate weathering and to estimate the flux carbon dioxide associated with modern chemical weathering. Modern day silicate weathering fluxes are estimated from the chemistry of rivers or natural waters. Natural waters contain positively charged elements or cations such as Ca, Mg, Na and K, and it has been understood for decades that the relative and absolute concentrations of these elements depend of the type of rocks that are drained. For example, Ca is mainly derived from the weathering of limestones, whereas Na and K are mainly derived from the weathering of silicate minerals such as feldspar. This distinction is important because only the Ca derived from silicate weathering is important for carbon dioxide consumption. Therefore, the Ca flux from silicate weathering is usually estimated based on Na, which has been thought to a more reliable estimate of silicate weathering. However, chemical weathering is more complex than simple mineral dissolution and a series of other chemical reactions also occur such as cation exchange. This is a process whereby the positively charged cations in solution are attracted to negatively charged mineral surfaces on clays, a process known to buffer groundwaters. One of the key chemical exchanges is Ca for Na, meaning that Na may not provide a true estimate of silicate weathering at all. Recent isotopic data suggests that cation exchange might be more significant that previously thought, which until now has been very hard to fingerprint. One method is to use naturally occurring tracers or isotopes, that allow chemical reactions to be tracked. In this work, it is proposed to examine the naturally occurring isotopes of the elements Li and Mg to examine the role of cation exchange in global budgets. However, to be able to do this successfully, a series of experimental work is proposed to examine the behaviour of the isotopes of Mg and Li during cation exchange. Once we understand how our tracers work we can use them to re-evaluate our understanding of natural waters, and better estimate fluxes of carbon dioxide associated with chemical weathering, with the ultimate aim of better understanding Earth's climate.

Two-thirds of the Earth's surface is paved by oceanic crust formed by seafloor spreading at the 60,000 km-long global mid-ocean ridge (MOR) system. As the rigid ocean plates are pulled apart, at rates varying from <10 to 160 mm/year, the Earth's mantle is drawn up from beneath, partly melting as it does so. The melt separates from the mantle and rises to the surface to form a continuous layer of 'magmatic' crust, typically about 6 km thick, made of basalt at the surface and gabbro, its slowly cooled equivalent, beneath. However, over the past 15 years we have come to realise that, at spreading rates below about 40 mm/yr, this simple model cannot be correct. Instead, large tracts of mantle rocks may be exposed on the seafloor, with no magmatic crust being present. Plate separation on slow-spreading MORs such as the Mid-Atlantic Ridge (MAR) may instead be taken up in part on great dislocations - unusually large geological faults known as 'detachments' - on which tens of km of extension may be accommodated. Where exposed on the seafloor these faults typically form flat or gently domed surfaces on which mantle rocks and/or gabbro are exposed. These structures are known as 'oceanic core complexes' (OCCs). We think OCCs form when the magma supply dwindles and seawater is able to penetrate down a fault and access mantle rocks beneath. These rocks, called 'peridotites', are made mostly of the mineral olivine, which reacts easily with water to produce the weak minerals serpentine and talc, lubricating the fault and allowing it to continue slipping and develop into a long-lived detachment. Very recently, several workers (including PI Reston) have proposed that detachment faulting is far more common than previously supposed, to the extent that up to half of all Atlantic seafloor may be generated by such 'tectonic' spreading. They view detachments as regionally continuous features that underlie all the seafloor on one side of the ridge axis, but only emerge at the surface in a few places, the OCCs. But is detachment faulting really so widespread? From a detailed study of the 13N region of the MAR, Co-Is MacLeod and Searle came to the quite different, and much less extreme, view that detachments are discontinuous and restricted to individual OCCs. They are interspersed between volcanically active, magma-rich ridge segments, and triggered by localised waning of magma supply. In this model detachments are episodically 'killed' by renewed magmatism, often delivered laterally from adjoining segments. How can we distinguish these very different hypotheses about the mechanism of seafloor spreading? The key data needed are: (1) the sub-surface geometry and extent of the detachments beneath the ridge axis, (2) the amount and detailed distribution of magmatic crust, and (3) the asymmetry of spreading rates associated with OCCs and volcanic seafloor (they should be similar in the regional and differ in the local detachment models). We propose to obtain these data in a comprehensive seismic and seabed magnetic survey of the MAR in the 13N region, where detachment faults are active at the ridge axis today. We will use a large array of ocean-bottom seismographs (OBSs) to image 3D velocity variations related to different rock types using 'seismic tomography' - akin to medical CT scanning - and conduct a multi-channel reflection survey, which will image sub-surface discontinuities - like a simple X-ray. We will then leave the OBSs (to be recovered on a later cruise) to record the locations of natural micro-earthquakes in the region. These will show directly the 3D geometry and linkage of active faults. Finally, we will deploy the autonomous robot vehicle Autosub 6000, which will be programmed to make very detailed maps of magnetic field reversals (yielding seafloor age and spreading rate) and seafloor topography (helping structural interpretations) while we perform the seismic experiments.

Plate tectonics has been a fundamental tenet of Earth Science for nearly 50 years, but fundamental questions remain, such as where is the base of the plate and what makes a plate, "plate-like?" A better understanding of the transition from the rigid lithospheric plate to the weaker mantle beneath has important implications for the driving forces of plate tectonics, natural hazards, and climate change. There are many proxies used to estimate the depth and nature of the base of tectonic plates, but to date no consensus has been reached. For example, temperature is known to have a strong effect on the mechanical behaviour of rocks, and if this were the sole process governing the definition of the plate, then we would expect to see a thin plate near a mid ocean ridge and a very thick plate beneath old seafloor. However numerous geophysical studies observe what are interpreted as nearly constant thickness plate at all seafloor ages. This has led scientists to propose other mechanisms, such as dehydration of the mantle to strengthen the mantle to form a rigid plate. Similarly, observations of very strong anomalies have led others to suggest that melt might exist to weaken the mantle beneath the plates. However many of these observations come from only one ocean, the Pacific, from indirect, remote observations, at different areas and scales, and with different sensitivities to earth properties. Although results have been promising, comparisons among studies are challenging, hindering a complete understanding of the tectonic plate. We will systematically image the entire length of an oceanic plate, from its birth at the Mid Atlantic Ridge to its oldest formation on the African margin. This is a large-scale focused effort with multiple scales of resolution and sensitivity, from a metre to kilometre scale using seismic and electromagnetic methods. This scale, focus, and interdisciplinary approach will finally determine the processes and properties that make a plate strong and define it. The project will be accomplished through a large, focused international collaboration that involves EU partners (3.5 M euro) and industry (6.4M euro), both already funded.

Earthquakes are a very destructive and yet unpredictable manifestations of the Earth internal dynamics. They correspond to a rapid motion along geological faults, generating seismic waves as they propagate along the fault strands. The propagation of ruptures along faults induces dramatic stresses and deformation of the rocks hosting the fault, which become increasingly damaged (i.e, degraded) as multiple earthquakes occur along a fault over geological timescales. In turn, this damage of the off-fault rocks has an impact on the dynamic rupture processes: damage generation and earthquake rupture are coupled phenomena. A better knowledge of the dynamic damage processes can thus truly improve our understanding of the physics of earthquakes, and hence help to better predict strong motion and earthquake hazard. It is the goal of this proposal to investigate how dynamic ruptures can induce damage in the surrounding rocks, the specific characteristics of this damage, how it affects the rocks properties, and finally to build an earthquake rupture model which includes the couplings between rupture propagation and off-fault damage. The proposed approach is multidisciplinary, and includes: (1) field characterisation of naturally damaged samples around the San Jacinto fault in South California; (2) laboratory rock deformation experiments at very high deformation rates; and (3) the development of a numerical modeling approach, tested against experimental data, which will allow simulations of fully coupled earthquake rupture processes to be performed. By far the most challenging aspect of the study of dynamic damage is to perform rock deformation experiments at deformation rates and pressure conditions relevant to earthquake ruptures. To achieve this, our proposal includes the design and construction of a novel deformation apparatus which will allow high speed compression and decompression tests to be performed on rock samples. This apparatus will be unique in Europe and will cover an unprecedented range of deformation conditions.