Long‐term Holocene groundwater fluctuations in a chalk catchment: evidence from Rock‐Eval pyrolysis of riparian peats

The depositional history of peat‐dominated wetlands can be used to understand palaeoclimate and palaeohydrology and also constrain the impacts of future climate change. However, in chalkland valleys, seasonal water table fluctuations and a high alkalinity have diminished key environment indicators such as pollen, and there is a need for alternative investigative techniques. The method of Rock‐Eval pyrolysis can track changes in organic matter source and degradation, potentially relating to historic changes in vegetation cover. This is the first Rock‐Eval on cores from a groundwater‐dependent riparian chalk valley wetland combined with radiocarbon dating. The dating showed that the cores represented approximately 4000 years of depositional history. Changes in hydrocarbon chemistry including normal alkane composition of the peat indicated shifts of around 500 to 1000 years between terrestrial and more aquatic species, relating to periods of climate wetness. These climatic shifts are broadly consistent with other evidence from ombrotrophic peatland and lacustrine sediments across northwest Europe. However, the connection between climate wetness and groundwater dependent chalkland wetlands is complicated by external anthropogenically driven factors relating to land use and vegetation cover changes in the catchment. Nonetheless, this study suggests that Rock‐Eval pyrolysis is a useful and cost‐effective tool that can provide evidence for long‐term Holocene groundwater fluctuations. Copyright © 2016 British Geological Survey.     

Comparative Evaluation of Single‐Well LNAPL Tracer Testing at Five Sites

Light nonaqueous phase liquid (LNAPL) tracer testing is a technique used to directly measure LNAPL flow in situ and evaluate LNAPL mobility and recoverability. The test method consists of adding a fluorescent oil‐soluble tracer to LNAPL within a well, isolating small volumes of LNAPL with known tracer concentrations for use as in‐well calibration standards, and measuring the rate of tracer concentration decline in the well over time. The test measures LNAPL flux through the well, which is directly related to LNAPL mobility and recoverability in the surrounding formation. Test results for a total of 29 wells at five sites are presented. Results from LNAPL tracer testing were comparable to results obtained through other methods, and the method offers a time‐averaged result measured over a relatively long period, in ambient conditions, and reflects the influences of heterogeneity and hydraulic changes. In some cases, tracer concentration decline followed unexpected patterns, and these data have led to a better understanding of test assumptions, mechanisms influencing tracer distribution, and options to improve test execution and data interpretation. Method improvements developed over the course of the field studies included refinement of pre‐test screening of LNAPL fluorescence and improvements to measurement equipment. Overall, the field studies confirmed the technical validity and usefulness of the LNAPL tracing technique to support LNAPL mobility and recoverability assessments.     

Glacial and periglacial floodplain sediments regulate hydrologic transfer of reactive iron to a high arctic fjord

The transport of reactive iron (i.e. colloidal and dissolved) by a glacier‐fed stream system draining a high relief periglacial landscape in the high Arctic archipelago of Svalbard is described. A negative, non‐linear relationship between discharge and iron concentration is found, indicative of increased iron acquisition along baseflow pathways. Because the glaciers are cold‐based and there are no intra‐ or sub‐permafrost groundwater springs, baseflow is principally supplied by the active layer and the colluvial and alluvial sediments in the lower valley. Collectively, these environments increase the flux of iron in the stream by 40% over a floodplain length of just 8 km, resulting in 6 kg Fe km^?2a^?1 of reactive iron export for a 20% glacierized watershed. We show that pyrite oxidation in shallow‐groundwater flowpaths of the floodplain is the most important source of reactive iron, although it is far less influential in the upper parts of the catchment where other sources are significant (including ironstone and secondary oxide coatings). Microbial catalysis of the pyrite oxidation occurs in the floodplain, enabling rapid, hyporheic water exchange to enhance the iron fluxes at high discharge and cause the non‐linear relationship between discharge and reactive iron concentrations. Furthermore, because the pyrite oxidation is tightly coupled to carbonate and silicate mineral weathering, other nutrients such as base cations and silica are also released to the stream system. Our work therefore shows that high Arctic floodplains should be regarded as critically important regulators of terrestrial nutrient fluxes to coastal ecosystems from glacial and periglacial sources. Copyright © 2015 John Wiley & Sons, Ltd.     

Siderophile element constraints on the formation of metal in the metal-rich chondrites Bencubbin, Weatherford, and Gujba

Laser ablation inductively coupled plasma mass spectrometry was used to measure abundances of P, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au in metal grains in the Bencubbin-like chondrites Bencubbin, Weatherford, and Gujba to determine the origin of large metal aggregates in bencubbinites. A strong volatility-controlled signature is observed among the metal grains. The refractory siderophiles Ru, Rh, Re, Os, Ir, and Pt are unfractionated from one another, and are present in approximately chondritic relative abundances. The less refractory elements Fe, Co, Ni, Pd, and Au are fractionated from the refractory siderophiles, with a chondritic Ni/Co ratio and a higher than chondritic Pd/Fe ratio. The moderately volatile siderophile elements Ga, Ge, As, Sn, and Sb are depleted in the metal, relative to chondritic abundances, by up to 3 orders of magnitude. The trace siderophile element data are inconsistent with the following proposed origins of Bencubbin-Weatherford-Gujba metal: (1) condensation from the canonical solar nebula, (2) oxidation of an initially chondritic metal composition, and (3) equilibration with a S-rich partial melt. A condensation model for metal-enriched (×10⁷ CI) gas is developed. Formation by condensation or evaporation in such a high-density, metal-enriched gas is consistent with the trace element measurements. The proposed model for generating such a gas is protoplanetary impact involving a metal-rich body.     

Detecting inter-aquifer leakage in areas with limited data using hydraulics and multiple environmental tracers,including <Superscript>4</Superscript>He, <Superscript>36</Superscript>Cl/Cl, <Superscript>14</Superscript>C and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr

The investigation of regionally extensive groundwater systems in remote areas is hindered by a shortage of data due to a sparse observation network, which limits our understanding of the hydrogeological processes in arid regions. The study used a multidisciplinary approach to determine hydraulic connectivity between the Great Artesian Basin (GAB) and the underlying Arckaringa Basin in the desert region of Central Australia. In order to manage the impacts of groundwater abstraction from the Arckaringa Basin, it is vital to understand its connectivity with the GAB (upper aquifer), as the latter supports local pastoral stations and groundwater-dependent springs with unique endemic flora and fauna. The study is based on the collation of available geological information, a detailed analysis of hydraulic data, and data on environmental tracers. Enhanced inter-aquifer leakage in the centre of the study area was identified, as well as recharge to the GAB from ephemeral rivers and waterholes. Throughout the rest of the study area, inter-aquifer leakage is likely controlled by diffuse inter-aquifer leakage, but the coarse spatial resolution means that the presence of additional enhanced inter-aquifer leakage sites cannot be excluded. This study makes the case that a multi-tracer approach along with groundwater hydraulics and geology provides a tool-set to investigate enhanced inter-aquifer leakage even in a groundwater basin with a paucity of data. A particular problem encountered in this study was the ambiguous interpretation of different age tracers, which is attributed to diffusive transport across flow paths caused by low recharge rates.     

Tertiary geodynamics of Sakhalin (NW Pacific) from anisotropy of magnetic susceptibility fabrics and paleomagnetic data

Sakhalin has been affected by several phases of Cretaceous and Tertiary deformation due to the complex interaction of plates in the northwest Pacific region. A detailed understanding of the strain is important because it will provide constraints on plate-scale processes that control the formation and deformation of marginal sedimentary basins. Anisotropy of magnetic susceptibility (AMS) data were obtained from fine-grained mudstones and siltstones from 22 localities in Sakhalin in order to provide information concerning tectonic strain. AMS data reliably record ancient strain tensor orientations before significant deformation of the sediments occurred. Paleomagnetically determined vertical-axis rotations of crustal rocks allow rotation of the fabrics back to their original orientation. Results from southwest Sakhalin indicate a N035°E-directed net tectonic transport from the mid-Paleocene to the early Miocene, which is consistent with the present-day relative motion between the Okhotsk Sea and Eurasian plates. Reconstruction of early–late Miocene AMS fabrics in east Sakhalin indicates a tectonic transport direction of N040°E. In west Sakhalin, the transport direction appears to have remained relatively consistent from the Oligocene to the late Miocene, but it has a different attitude of N080°E. This suggests local deflection of the stress and strain fields, which was probably associated with opening of the northern Tatar Strait. A northward-directed tectonic transport is observed in Miocene sediments in southeast Sakhalin, mid-Eocene sediments in east Sakhalin, and in Late Cretaceous rocks of west and northern Sakhalin, which may be associated with northwestward motion and subduction of the Pacific Plate in the Tertiary period. The boundaries of the separate regions defined by the AMS data are consistent with present-day plate models and, therefore, provide meaningful constraints on the tectonic evolution of Sakhalin.     

Dynamics of an outburst flood originating from a small and high-altitude glacier in the Arid Andes of Chile

The rise of total water levels at the coast is caused primarily by three factors that encompass storm surges, tides and wind waves. The accuracy of total water elevation (TWE) forecast depends not only on the cyclonic track and its intensity, but also on the spatial distribution of winds which include its speed and direction. In the present study, the cyclonic winds are validated using buoy winds for the recent cyclones formed in the Bay of Bengal since 2010 using Jelesnianski wind scheme. It is found that the cyclonic winds computed from the scheme show an underestimate in the magnitude and also a mismatch in its direction. Hence, the wind scheme is suitably modified based on the buoy observations available at different locations using a power law which reduces the exponential decay of winds by about 30%. Moreover, the cyclonic wind direction is also corrected by suitably modifying its inflow angle. The significance of modified exponential factor and inflow angle in the computation cyclonic winds is highlighted using statistical analysis. A hydrodynamic finite element-based Advanced Circulation 2D depth integrated (ADCIRC-2DDI) model is used here to compute TWE as a response to combined effect of cyclonic winds and astronomical tides. As contribution of wave setup plays an important role near the coast, a coupled ADCIRC + SWAN is used to perceive the contribution of wind waves on the TWE. The experiments are performed to validate computed surge residuals with available tide gauge data. On comparison of observed surge residuals with the simulations using modified winds from the uncoupled and coupled models, it is found that the simulated surge residuals are better compared, especially with the inclusion of wave effect through the coupled model.     

Sulfur isotope biogeochemistry of the Proterozoic McArthur Basin

Previous geochemical and biomarker studies of the late Paleo-Mesoproterozoic propose a stratified world, with strongly reducing (possibly sulfidic) deep-ocean conditions overlain by an oxygenated surface-ocean and atmosphere. To investigate such a scenario, we look to the structure of the biogeochemical sulfur cycle. We present sulfur (³²S, ³³S, ³⁴S, and ³⁶S in sulfides) isotope data from the McArthur Basin (Barney Creek, Reward, Velkerri, and McMinn formations) that allows for a direct evaluation of the surface biosphere. We are interested in investigating the types of information that can be gained by including ³³S and ³⁶S. When the ³⁴S/³²S fractionations are small, the inclusion of ³³S and ³⁶S provides little additional information, but does provide ancillary evidence for relative isotopic homogeneity (with the internal consistency of ³³S/³²S and ³⁶S/³²S). When the ³⁴S/³²S fractionations are large, direct information about the fractionation mechanisms can be obtained, with the potential to distinguish the biological from abiological processes. For example, the reconstruction of the Roper Group suggests that seawater sulfate concentrations were high enough to buffer against spatial heterogeneities. Overall, our findings agree with previously proposed redox structure of the Proterozoic ocean, highlight contributions from the oxidative sulfur cycle, and outline a new tool for interpreting the state of the surface sulfur cycle.     

High-pressure crystal chemistry of phenakite (Be2SiO4) and bertrandite (Be4Si2O7(OH)2)

Compressibilities and high-pressure crystal structures have been determined by X-ray methods at several pressures for phenakite and bertrandite. Phenakite (hexagonal, space group R \(\bar 3\) ) has nearly isotropic compressibility with β=1.60±0.03×10^?4 kbar^?1 and β=1.45±0.07×10^?4 kbar^?1. The bulk modulus and its pressure derivative, based on a second-order Birch-Murnaghan equation of state, are 2.01±0.08 Mbar and 2±4, respectively. Bertrandite (orthorhombic, space group Cmc2₁) has anisotropic compression, with β _a =3.61±0.08, β _b =5.78±0.13 and β _c =3.19±0.01 (all ×10^?4 kbar^?1). The bulk modulus and its pressure derivative are calculated to be 0.70±0.03 Mbar and 5.3±1.5, respectively. Both minerals are composed of frameworks of beryllium and silicon tetrahedra, all of which have tetrahedral bulk moduli of approximately 2 Mbar. The significant differences in linear compressibilities of the two structures are a consequence of different degrees of T-O-T bending.     

Epifaunal worm tubes on Lower Jurassic (Lower Lias) ammonites from Dorset

Over 40 ammonites, mostly Promicroceras, with epifaunal worm tubes are described from the Lower Jurassic, Charmouth Mudstone Formation (Lias Group) of Dorset. Serpulids that were overgrown by the ammonites or responded to the ammonites’ growth attached to juvenile, living ammonites. Some epifaunal serpulids attached post-mortem, indicating oxygenated bottom water, which was rare in the lower Charmouth Mudstone Formation. Other serpulids do not conform to either pre- or post-mortem growth predictions and require individual assessment. The commonest pattern of growth for serpulids on live juvenile ammonites was attachment in the umbilical seam, with later growth onto, and finally around, the venter. Reconstructing this pattern shows that serpulids kept their aperture at 6 o’clock with respect to the orientation of the living ammonite (105-115° behind the ammonite aperture) throughout life. Reorientation of growth lines in serpulid worms just before the aperture suggests some worm tubes were fully grown. The 6 o’clock position of the aperture enabled feeding currents generated by the worms to parallel currents generated by swimming ammonites, thus maximizing food gathering and confirming that ammonites swam backwards. The mid-ventral position enabled the worm to deploy its branchia on both sides of the ammonite. Growth on ammonites was beneficial to the worms, but parasitic to the ammonites. Promicroceras with epifaunal worm tubes died at smaller sizes than unencumbered examples and size at death correlates inversely to extra weight of worm tubes. Uniformitarian comparisons suggest fossil serpulid worms grew in one season and that Promicroceras reached full size in two or three years.