The balance of forces in modern plate tectonics is broadly understood and subduction initiation appears to be a robust manifestation of the system. However, when and how it began are not. The principal hurdle to attaining mobile lid tectonics is overcoming the strength of the lithosphere, but this raises a chicken-or-egg paradox: what caused this strong layer to generate weakened planar features on which subduction could initiate? Answering this question will require knowledge of the interrelationships among the full range of relevant lithospheric rheologies but at present few firm conclusions can be drawn from theory. What is clear is that such models will never achieve ab initio reconstruction of early Earth behavior. Mantle convection, a highly non-linear, dispersive, chaotic system, is uninvertible and the recent recognition that global tectonic mode may be pathway dependent only underscores this fact. If we’re ever to understand when plate tectonics got underway, we’re going to have to acquire that knowledge from the geologic record.
University of Sheffield
Optical dating, based on OSL (Optically Stimulated Luminescence) or IRSL (Infra-Red Stimulated Luminescence) signals of quartz or feldspar, allows us to determine how long sediment grains have been buried since they last exposed to light, or heated above around 300°C. The luminescence signal measured in the laboratory results from the recombination and optical emission from electrons that became trapped at metastable sites in the crystal lattice as a result of environmental ionizing radiation. The dating technique works well on timescales of years to hundreds of thousands of years; an upper time limit is reached when all available electron trapping sites are filled. Technical improvements allow us to measure age estimates for individual sand-sized grains from sediment bodies. A new measurement approach termed MET (Multiple Elevated Temperature) IRSL, developed over the last five years or so, allows us to determine the duration of light exposure before burial commenced. This is clearly useful as a way to quantify whether sufficient daylight exposure was received by each grain to reset the IRSL signal fully (one of the remaining limitations of conventional IRSL and OSL approaches), but also has the potential for tracking the pathways for grains through the environment indifferent sediment transport systems. Combined with techniques to assess light exposure durations for rock surfaces using IRSL developed by Sohbati and colleagues at the Risø laboratory, Denmark, these methods have great potential for understanding many different aspects of sediment transport, including human modification of processes and source to sink storage.