Structure,transport and potential vorticity of the Gulf Stream at 68°W: Revisiting older data sets with new techniques

The stream-coordinates mean structure of the Gulf Stream at 68°W is derived using new methods for both defining stream coordinates and interpreting bottom pressure and inverted echo sounder travel times collected during the extensive Synoptic Ocean Prediction experiment. These new analyses provide pictures of the vertical structure of Gulf Stream flows that are demonstrably dynamically consistent with the density field at all depths, in contrast to previous work that relies on simple vertical interpolations to fill gaps between sparse current meter measurements. This new view of the Gulf Stream suggests a slightly higher total mean transport, with the increases coming from both baroclinic and barotropic components, and slightly stronger recirculation cells, particularly on the southern side. The recirculation of the Gulf Stream appears to have a weak baroclinic component, perhaps 10% of the total. A significant advantage of the methodology is the ability to obtain sensible vertical and horizontal gradients of currents and density so that the vertical and cross-stream structures of the components of the mean potential vorticity can be clearly imaged. One new feature from this calculation is that the along-stream gradient of the cross-stream velocity, a term that is often ignored in potential vorticity analyses, is non-negligible (though small) and is asymmetric about the current axis. Both the derived structure and implied dynamics of the circulation can be significantly altered by small changes to the method of calculating daily stream coordinates, e.g., by carefully filtering out observations in rings or not. Arrays of pressure-equipped inverted echo sounders provide the opportunity (at reasonable cost) for properly defining the stream coordinates of energetic jets such as the Gulf Stream.     

Ice-marginal environment and ecosystem prior to initial advance of the late Palaeozoic ice sheet in the Mount Butters area of the central Transantarctic Mountains, Antarctica

Basal rocks of the Upper Carboniferous to Lower Permian Pagoda Formation at Mount Butters provide an unusual view of periglacial conditions in the central Transantarctic Mountains region prior to the initial advance of the Gondwanide ice sheet. These rocks were deposited on a high relief unconformity that developed on granite. Deposition within relief on the unconformity, possibly in the lee of a granite buttress, protected the rocks from erosion during subsequent overriding by the ice sheet. The succession reflects deposition in a glacial‐fed to ice‐contact lake that contained a freshwater crustacean fauna. Centimetre‐ to decimetre‐scale basal layers include breccia and coarse‐grained sandstone. The occurrence of breccia resting on weathered granite suggests sedimentation as scree and as mass flow deposits. Overlying decimetre‐to metre‐scale stratified diamictites interbedded with metre‐scale, coarsening‐upward successions of siltstone to cross‐laminated sandstone suggest lacustrine deposition by suspension settling, rain out of ice‐rafted debris, and deltaic progradation. Thin zones with abundant conchostracans and/or with prolific trace fossils, in addition to less common remains of other crustaceans, attest to the presence of a low diversity benthic fauna. Conchostracans are concentrated in a series of thin beds that reflect moderately lengthy, perhaps seasonal, periods of free‐flowing water. Patchy vertical and lateral distribution of intense bioturbation and profuse trace fossils probably reflect repeated colonization events during times of favourable environmental conditions. Massive diamictite overlies the basal rocks and indicates that the ice‐marginal lake was subsequently overridden by the late Palaeozoic ice sheet. Occurrences of lodgement till, glacitectonite and deformation till suggest deposition from temperate or warm‐based ice, whereas underlying lacustrine and deltaic deposits, along with a crustacean and trace fossil fauna, suggest temperate periglacial conditions. Previous studies have stressed that upper Palaeozoic glacigenic deposits in Antarctica, and in Gondwanaland, record deglaciation events. In contrast, rocks at Mt. Butters provide an unusual glimpse into an ice‐margin lake and its fauna just prior to ice sheet advance.     

An experimental study of rare earth element partitioning between a shergottite melt and pigeonite: implications for the oxygen fugacity of the mart ian interior

A series of experiments on a synthetic, pigeonite-saturated, basaltic shergottite were carried out to constrain the variation of D(Eu/Gd)_{pigeonite/melt} and D(Eu/Sm)_{pigeonite/melt} with oxygen fugacity (f_O2). The experiments have been run under both dry and hydrous conditions. The shergottite was doped with 0.1, 0.5, and 1.0 wt.% Eu, Gd, and Sm oxides in different experiments and was equilibrated at liquidus conditions for 24 hours. D(Eu/Gd)_{pigeonite/melt} in dry melts ranges from 0.156 ± 0.014 (f_O2 = IW − 1) to 0.630 ± 0.102 (IW + 3.5). D(Eu/Sm)_{pigeonite/melt} in dry melts ranges from 0.279 ± 0.021 (IW − 1) to 1.114 ± 0.072 (IW + 3.5). Due to difficulties with low-f_O2 experiments, hydrous distribution coefficients were measured, but were not used in the calibration of the Eu-oxybarometers. These two Eu-oxybarometers provide an accurate way to measure f_O2 recorded during pigeonite crystallization, thereby yielding a record of f_O2 during the earliest period of Martian meteorite parent magma crystallization history.Using this new calibration, Martian meteorite pigeonite cores record f_O2 values of IW − 0.6 (±0.3) (QUE94201) to IW + 1.9 (±0.6) (Shergotty). These new values differ in magnitude, but not trend, from previously published data. The pigeonite Eu-oxybarometer yields an f_O2 range in the basaltic shergottites of 2 to 3 orders of magnitude. Several processes have been proposed to explain the origin of this f_O2 range, the majority of which rely on assimilation of an oxidized source. A potential correlation between this new pigeonite data and recent Fe-Ti oxide data, however, is consistent with intrinsic f_O2 differences in the magma source region being responsible for the measured f_O2 variations. This implies that the Martian meteorite source region, the mantle or lithosphere, may be heterogeneous in nature. However, the process of assimilation cannot be completely ruled out in that an assimilation event that took place before crystallization commenced would result in the overprinting of the source region f_O2 signature.     

Ecohydrologic separation alters interpreted hydrologic stores and fluxes in a headwater mountain catchment

Recent studies have demonstrated that compartmentalized pools of water preferentially supply either plant transpiration (poorly mobile water) or streamflow and groundwater (highly mobile water) in some catchments, a phenomenon referred to as ecohydrologic separation. The omission of processes accounting for ecohydrologic separation in standard applications of hydrological models is expected to influence estimates of water residence times and plant water availability. However, few studies have tested this expectation or investigated how ecohydrologic separation alters interpretations of stores and fluxes of water within a catchment. In this study, we compare two rainfall‐runoff models that integrate catchment‐scale representations of transport, one that incorporates ecohydrologic separation and one that does not. The models were developed for a second‐order watershed at the H.J. Andrews Experimental Forest (Oregon, USA), the site where ecohydrologic separation was first observed, and calibrated against multiple years of stream discharge and chloride concentration. Model structural variations caused mixed results for differences in calibrated parameters and differences in storage between reservoirs. However, large differences in catchment storage volumes and fluxes arise when considering only mobile water. These changes influence interpreted residence times for streamflow‐generating water, demonstrating the importance of ecohydrologic separation in catchment‐scale water and solute transport.     

Estimating sand concentrations using ADCP-based acoustic inversion in a large fluvial system characterized by bi-modal suspended-sediment distributions

Quantifying sediment flux within rivers is a challenge for many disciplines due, mainly, to difficulties inherent to traditional sediment sampling methods. These methods are operationally complex, high cost, and high risk. Additionally, the resulting data provide a low spatial and temporal resolution estimate of the total sediment flux, which has impeded advances in the understanding of the hydro-geomorphic characteristics of rivers. Acoustic technologies have been recognized as a leading tool for increasing the resolution of sediment data by relating their echo intensity level measurements to suspended sediment. Further effort is required to robustly test and develop these techniques across a wide range of conditions found in natural river systems. This article aims to evaluate the application of acoustic inversion techniques using commercially available, down-looking acoustic Doppler current profilers (ADCPs) in quantifying suspended sediment in a large sand bed river with varying bi-modal particle size distributions, wash load and suspended-sand ratios, and water stages. To achieve this objective, suspended sediment was physically sampled along the Paraná River, Argentina, under various hydro-sedimentological regimes. Two ADCPs emitting different sound frequencies were used to simultaneously profile echo intensity level within the water column. Using the sonar equation, calibrations were determined between suspended-sand concentrations and acoustic backscatter to solve the inverse problem. The study also analyzed the roles played by each term of the sonar equation, such as ADCP frequency, power supply, instrument constants, and particle size distributions typically found in sand bed rivers, on sediment attenuation and backscatter. Calibrations were successfully developed between corrected backscatter and suspended-sand concentrations for all sites and ADCP frequencies, resulting in mean suspended-sand concentration estimates within about 40% of the mean sampled concentrations. Noise values, calculated using the sonar equation and sediment sample characteristics, were fairly constant across evaluations, suggesting that they could be applied to other sand bed rivers. © 2018 John Wiley & Sons, Ltd.     

Variability of African farming systems from phenological analysis of NDVI time series

Food security exists when people have access to sufficient, safe and nutritious food at all times to meet their dietary needs. The natural resource base is one of the many factors affecting food security. Its variability and decline creates problems for local food production. In this study we characterize for sub-Saharan Africa vegetation phenology and assess variability and trends of phenological indicators based on NDVI time series from 1982 to 2006. We focus on cumulated NDVI over the season (cumNDVI) which is a proxy for net primary productivity. Results are aggregated at the level of major farming systems, while determining also spatial variability within farming systems. High temporal variability of cumNDVI occurs in semiarid and subhumid regions. The results show a large area of positive cumNDVI trends between Senegal and South Sudan. These correspond to positive CRU rainfall trends found and relate to recovery after the 1980??s droughts. We find significant negative cumNDVI trends near the south-coast of West Africa (Guinea coast) and in Tanzania. For each farming system, causes of change and variability are discussed based on available literature (Appendix A). Although food security comprises more than the local natural resource base, our results can perform an input for food security analysis by identifying zones of high variability or downward trends. Farming systems are found to be a useful level of analysis. Diversity and trends found within farming system boundaries underline that farming systems are dynamic.     

Loggerhead turtle movements reconstructed from 18O and 13C profiles from commensal barnacle shells

Commensal barnacles, Chelonibia testudinaria, from logger-head turtles have ¹⁸O and ¹³C variations in their calcitic shells that record the environments in which the turtles live. Isotopic profiles from the barnacle shells can thus be interpreted to reconstruct movements of the host turtle between open ocean and brackish-water regimes.