From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreadingdirection-parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

[1] It is demonstrated for the first time how model parameter, structural and data uncertainties can be accounted for explicitly and simultaneously within the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. As an example application, 72 variants of a single soil moisture accounting store are tested as simplified hypotheses of runoff generation at six experimental grassland field-scale lysimeters through model rejection and a novel diagnostic scheme. The fields, designed as replicates, exhibit different hydrological behaviors which yield different model performances. For fields with low initial discharge levels at the beginning of events, the conceptual stores considered reach their limit of applicability. Conversely, one of the fields yielding more discharge than the others, but having larger data gaps, allows for greater flexibility in the choice of model structures. As a model learning exercise, the study points to a "leaking" of the fields not evident from previous field experiments. It is discussed how understanding observational uncertainties and incorporating these into model diagnostics can help appreciate the scale of model structural error.

Abstract. High-resolution, deep-towed side-scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading Mid-Atlantic Ridge near 29øN. Sonar data allow us to identify individual fault scarps, to measure fault widths and spacing, and to calculate horizontal fault displacements (heave) and tectonic strain. We find that over long periods of time (> 1 Myr on average), tectonic strain is -10% on average and does not vary significantly along axis. There is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment, indicated by -50% lower tectonic strain on the east flank than on the west flank. These variations in tectonic strain do not correlate directly with changes in fault spacing and heave. Fault spacing and heave increase from the center of the segment toward the end (inside comer) on the west flank and from the outside to the inside comer across the axis. These parameters remain relatively constant along the segment on the east flank and across the axis at the segment center. Tectonic strain appears to be decoupled from magmatic accretion at timescales > 1 Myr, as the decrease in magma supply from the segment center toward the end (inferred from variations in crustal thickness along the axis) is not correlated with a complementary increase in tectonic strain. Instead, tectonic strain remains relatively constant along the axis at-7% on the east flank and at-15% on the west flank. These results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis.

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Phosphorus losses from land to water will be impacted by climate change and land management for food production, with detrimental impacts on aquatic ecosystems. Here we use a unique combination of methods to evaluate the impact of projected climate change on future phosphorus transfers, and to assess what scale of agricultural change would be needed to mitigate these transfers. We combine novel high-frequency phosphorus flux data from three representative catchments across the UK, a new high-spatial resolution climate model, uncertainty estimates from an ensemble of future climate simulations, two phosphorus transfer models of contrasting complexity and a simplified representation of the potential intensification of agriculture based on expert elicitation from land managers. We show that the effect of climate change on average winter phosphorus loads (predicted increase up to 30% by 2050s) will be limited only by large-scale agricultural changes (e.g., 20–80% reduction in phosphorus inputs).

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Abstract. Benchmarking model performance across large samples of catchments is useful to guide model selection and future model development. Given uncertainties in the observational data we use to drive and evaluate hydrological models, and uncertainties in the structure and parameterisation of models we use to produce hydrological simulations and predictions, it is essential that model evaluation is undertaken within an uncertainty analysis framework. Here, we benchmark the capability of several lumped hydrological models across Great Britain by focusing on daily flow and peak flow simulation. Four hydrological model structures from the Framework for Understanding Structural Errors (FUSE) were applied to over 1000 catchments in England, Wales and Scotland. Model performance was then evaluated using standard performance metrics for daily flows and novel performance metrics for peak flows considering parameter uncertainty. Our results show that lumped hydrological models were able to produce adequate simulations across most of Great Britain, with each model producing simulations exceeding a 0.5 Nash–Sutcliffe efficiency for at least 80 % of catchments. All four models showed a similar spatial pattern of performance, producing better simulations in the wetter catchments to the west and poor model performance in central Scotland and south-eastern England. Poor model performance was often linked to the catchment water balance, with models unable to capture the catchment hydrology where the water balance did not close. Overall, performance was similar between model structures, but different models performed better for different catchment characteristics and metrics, as well as for assessing daily or peak flows, leading to the ensemble of model structures outperforming any single structure, thus demonstrating the value of using multi-model structures across a large sample of different catchment behaviours. This research evaluates what conceptual lumped models can achieve as a performance benchmark and provides interesting insights into where and why these simple models may fail. The large number of river catchments included in this study makes it an appropriate benchmark for any future developments of a national model of Great Britain.

In this study, temporal changes in extractability of [ 14 C]pyrene were followed in two soils with differing organic matter contents under sterile and nonsterile conditions over 24 weeks. The nonsterile pasture soil was the only incubation to show significant loss of [ 14 C]pyrene-associated activity over the 24-week incubation. Sequential extraction using methanol:water (1:1), followed by 1-butanol and finally dichloromethane-Soxhlet showed changes in the relative proportions of extractability with increased soil-PAH contact time. Significant decreases in methanol:water and 1-butanol extractability were recorded over the 24week incubation. The nonsterile pasture soil exhibited the greatest decrease in 1-butanol extraction. Significant nonextractable residues were formed with increased soilpyrene contact time in all soils, with the largest increase found in the nonsterile pasture soil. These residues were investigated by the alkaline extraction of the humic material and saponification of the resultant humin. After 24week soil-pyrene contact time, the bioavailability of the added [ 14 C]pyrene was assessed by bacterial mineralization. A comparison was made between bioavailability and the amount of 14 C activity extracted by the sequential scheme of solvents. Methanol:water significantly underestimated the bioavailable fraction, whereas 1-butanol overestimated the bioavailability of the [ 14 C]pyrene-associated activity.