Li isotope fractionation in peridotites and mafic melts

We have measured the Li isotope ratios of a range of co-existing phases from peridotites and mafic magmas to investigate high-temperature fractionations of ⁷Li/⁶Li. The Li isotopic compositions of seven mantle peridotites, reconstructed from analyses of mineral separates, show little variation (δ⁷Li 3.2-4.9‰) despite a wide range in fertility and radiogenic isotopic compositions. The most fertile samples yield a best estimate of δ⁷Li ∼ 3.5‰ for the upper mantle. Bulk analyses of olivine separates from the xenoliths are typically ∼1.5‰ isotopically lighter than co-existing orthopyroxenes, suggestive of a small, high-temperature equilibrium isotope fractionation. On the other hand, bulk analyses of olivine phenocrysts and their host melts are isotopically indistinguishable. Given these observations, equilibrium mantle melting should generate melts with δ⁷Li little different from their sources (<0.5‰ lighter). In contrast to olivine and orthopyroxene, that dominate peridotite Li budgets, bulk clinopyroxene analyses are highly variable (δ⁷Li = 6.6‰ to −8.1‰). Phlogopite separated from a modally metasomatised xenolith yielded an extreme δ⁷Li of −18.9‰. Such large Li isotope variability is indicative of isotopic disequilibrium. This inference is strongly reinforced by in situ, secondary ion mass-spectrometry analyses which show Li isotope zonation in peridotite minerals. The simplest zoning patterns show isotopically light rims. This style of zoning is also observed in the phenocrysts of holocrystalline Hawaiian lavas. More dramatically, a single orthopyroxene crystal from a San Carlos xenolith shows a W-shaped Li isotope profile with a 40‰ range in δ⁷Li, close to the isotope variability seen in all terrestrial whole rock analyses. We attribute Li isotope zonation in mineral phases to diffusive fractionation of Li isotopes, within mineral phases and along melt pathways that pervade xenoliths. Given the high diffusivity of Li, the Li isotope profiles we observe can persist, at most, only a few years at magmatic temperatures. Our results thus highlight the potential of Li isotopes as a high-resolution geospeedometer of the final phases of magmatic activity and cooling.     

A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel,Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin

The coastline of County Down includes sites that are pivotal to understanding the history of the last glaciation of the northern Irish Sea Basin in relation to relative sea level and regional glacial readvances. The cliff sections display evidence that has been used to underpin controversial models of glaciomarine sedimentation in isostatically-depressed basins followed by emergent marine and littoral environments. They also provide crucial evidence claimed to constrain millennial-scale ice sheet oscillations associated with uniquely large and rapid sea-level fluctuations. This paper reviews previous work and reports new findings that generally supports the ‘terrestrial’ model of glaciation, involving subglacial accretion and deformation of sediment beneath grounded ice. Deep troughs were incised into the till sheet during a post Late Glacial Maximum draw-down of ice into the Irish Sea Basin. Ice retreat was accompanied by glaciomarine accretion of mud in the troughs during a period of high relative sea level. The trough-fills were over-ridden, compacted, deformed and truncated during a glacial re-advance that is correlated with the Clogher Head Readvance. Grounding-line retreat accompanied by rapid subaqueous ice-proximal sedimentation preserved a widespread subglacial stone pavement. Raised beach gravels cap the sequence. The evidence supports an uninterrupted fall in relative sea level from c. 30?m that is consistent with sea level curves predicted by current glacio-isostatic adjustment modelling. Critical evidence previously cited in support of subaerial dissection of the troughs, and hence rapid fall and rise in relative sea level prior to the deposition of the glaciomarine muds, is not justified.     

Edgar Hynes Thompson: pragmatism, ingenuity and mechanisms

In this contribution the author looks for an explanation of an uncharacteristically scathing review, by E. H. Thompson towards the end of his life, of a book, Theory of Errors and Generalized Matrix Inverses, by Arne Bjerhammar, a leading European academic.     

Terracing potential field data

Terracing is an operator that is applied to potential field data to produce regions of constant field amplitude that are separated by sharp boundaries, as an aid to interpretation. When applied to map data, the boundaries are defined by the zero contour of the 2D Laplacian derivative operator. An improved method is described here that defines the boundaries by the zero contour of the profile curvature. This approach gives superior results because the 2D Laplacian operator is composed only of derivatives in the EW and NS directions, while the profile curvature uses the curvature in the 'uphill' direction at each point, whatever that direction may be. The method is demonstrated on gravity data from South Africa. Source code in Matlab format is available from the authors on request.     

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events

Concentrations of suspended particulate matter (SPM), NO₃-N and P fractions: PO₄-P, dissolved organic P (DOP), particulate P (PP) and bioavailable exchangeable P were examined over 5 storm events in two nested agricultural catchments in NE Scotland: a (51 km²) catchment and its headwater (4 km²). NO₃-N showed anticlockwise hysteresis for all storms in both catchments. In contrast, the headwater showed strong clockwise hysteresis of SPM, dissolved and particulate P concentrations, but which weakened through summer to spring. Less pronounced hysteresis of P forms in the larger catchment was attributed to a combination of factors: a less energetic system, nutrient leaching from the floodplain, a point source of a small sewage treatment works and the occurrence of coarser soil and sediment parent materials with less P adsorption and transport capacity. The headwater exhibited a strong ‘first flush’ effect of sediment and dissolved P, particularly following dry conditions, received a significant transfer of readily-solubilized organic P from the surrounding soils in late summer and after manure applications in winter, and was the likely cause of large sediment associated P signals observed in the 51 km² catchment. Our results suggest that steeper gradient headwaters should be targeted for riparian improvements to mitigate soil erosion from headwater fields. The efficiency of riparian erosion controls is also dependant on the size of the store of fine sediment material within the stream channel and this may be large.     

Virtual velocity of sand transport in water

Using the 160‐m‐long flume at Tsukuba University we undertook an experiment to provide a first estimate of the virtual velocity of sand in the size range 0.5–2.0 mm. For the flow velocity used in our experiment this sediment‐size range would conventionally be regarded as suspended sediment. The virtual velocity was found to be 37–41% of the flow velocity. Paradoxically, virtual velocity decreases as particle size decreases. Such a lower virtual velocity of finer sediment is not inconceivable. First, trapping of the sediment appears to be a function of bed roughness, and there is a probable relationship between bed roughness and trapping efficiency for particles of different sizes. Second, finer particles are more likely to find sheltered positions on a rough bed and thus experience lower mobility, relative to the more exposed coarser grains, as observed for bedload transport. Third, the virtual velocity of particles undergoing bedload transport has been found, in some instances, to be lower for finer clasts. We combine our data with previous studies of virtual velocity of bedload to develop, for the first time, a hypothesis for a holistic analysis of sediment movement in rivers. Copyright © 2017 John Wiley & Sons, Ltd.     

Variability of the useful life of reservoirs in tropical locations: A case study from the Burdekin Falls Dam,Australia

Large dam construction for irrigation, hydropower, water supply, and flood control in tropical to sub-tropical areas increased markedly after 1950. Many of the dams built during this period have filled with sediment and no longer perform to their original design specifications; in many cases, forecast dam life was greatly overestimated. This study investigates the useful life of Burdekin Falls Dam (BFD), northeastern Australia and compares the findings with other tropical reservoirs. Using two independent methods it is estimated that between 61 and 65 million m³ of sediment has been deposited in the reservoir over the 24 years of operation through 2011. This sediment volume equates to an average of 0.15% of capacity lost per year since construction was completed. If current sediment loads/climate regimes persist, reservoir capacity will be reduced by 50% after 345 years. However, the useful life of the BFD reduces to just 276 years when drawdown data are considered; these data show reservoir use would be affected once 40% of storage was filled with sediment, with a 60% drawdown return period of 1 in 15 years. When compared to similar large tropical to sub-tropical reservoirs, the BFD has a slightly longer reservoir useful life than dams in India and a much longer half-life than for both similar-sized and larger dams in China, Brazil, and Iran. Properties of the BFD that promote a longer useful life include a lower trap efficiency, relatively low annual sediment load delivered to the reservoir, limited sediment deposition behind the dam wall (and uniform distribution of deposited sediment), and the export of highly turbid annual floodwaters before settling and deposition of any remaining sediment within the reservoir.     

Monitoring coastal morphology: the potential of low-cost fixed array action cameras for 3D reconstruction

The combination of structure-from-motion with multi-view stereo (SfM-MVS) photogrammetry has become an increasingly popular method for the monitoring and three-dimensional (3D) reconstruction of coastal environments. Climate change is driving the potential for increased coastal landward retreat meaning geomorphological monitoring using methods such as SfM-MVS has become essential for detecting and tracking impacts. SfM-MVS has been well-researched with a variety of platforms and spatial and temporal resolutions using mainly rectilinear digital cameras in coastal settings. However, there has been no assessment of the potential of fixed multi-camera arrays to monitor landward retreat or on the significance of camera placement in relation to the scene. This study presents an innovative method of image acquisition using a purpose-built camera grid and GoPro© action camera to evaluate the combined effects of camera height, obliqueness and overlap at a site of known landward retreat. This approach examines the effect of camera placement on scene reconstruction to aid the design of a multi-camera array. SfM-MVS dense point clouds display millimetre accuracy when compared to equivalent terrestrial laser scans and strong image network geometry with internal precision estimates of < 3 mm. Comparable point cloud reconstruction can be achieved with a small number of images stationed in appropriate positions. Initial results show as few as five images positioned at a cliff to camera ratio of 3:4.18 and camera obliqueness of 40° can provide reconstruction in the range of millimetres (mean error of 4.79 mm). These findings illustrate the importance of camera placement when using multiple cameras and aid the design of a low-cost, fixed multi-camera array for use at sites of small-scale landward retreat. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd