Isotopic evidence (Sr/Sr, δLi) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and ⁸⁷Sr/⁸⁶Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ⁷Li values, partly close to those of hydrothermal vent fluids (δ⁷Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.     

Chondritic micrometeorites from the Transantarctic Mountains

Abstract– On the basis of morphological and petrographic characteristics, eight “giant” unmelted micrometeorites in the 300–1100 μm size range were selected from the Transantarctic Mountain micrometeorite collection, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of scanning electron microscopy, electron probe microanalyses, and synchrotron X‐ray diffraction allow their classification as chondritic micrometeorites. The large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions, therefore allowing a better definition of their parent material. A large variety of material is observed: five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite, one to CV chondrite, one to CM chondrite, and one to CI chondrite parent materials. Besides reporting the first occurrence of a CV‐like micrometeorite, our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin.     

Hydrological modelling for assessing spatio-temporal groundwater recharge variations in the water-stressed Amathole Water Supply System,Eastern Cape,South Africa

To increase the resilience of regional water supply systems in South Africa in the face of anticipated climatic changes and a constant increase in water demand, water supply sources require diversification. Many water-stressed metropolitan regions in South Africa depend largely on surface water to cover their water demand. While climatic and river discharge data is widely available in these regions, information on groundwater resources – which could support supply source diversification – is scarce. Groundwater recharge is a key parameter that is used to estimate groundwater amounts that can be sustainably exploited at a sub-watershed level. Therefore, the objective of this study was to develop a reliable hydrological modelling routine that enables the assessment of regional spatio-temporal variations of groundwater recharge to discern the most promising areas for groundwater development. Accordingly, we present a semi-distributed hydrological modelling approach that incorporates water balance routines coupled with baseflow modelling techniques to yield spatio-temporal variations of groundwater recharge on a regional level. The approach is demonstrated for the actively managed catchment areas of the Amathole Water Supply System situated in a semi-arid part of the Eastern Cape of South Africa. In the investigated study area, annual groundwater recharge exhibits a high spatio-temporal heterogeneity and is estimated to vary between ~0.5% and 8% of annual rainfall. Despite some uncertainties induced by limited data availability, calibration and validation of the model were found to be satisfactory and yielded model results similar to (point) data of annual groundwater recharge reported in earlier studies. Our approach is therefore found to derive crucial information for efficiently targeting more detailed groundwater exploration studies and could work as a blueprint for orientating groundwater potential exploration in similar environments.     

A multi‐storage groundwater concept for the SWAT model to emphasize nonlinear groundwater dynamics in lowland catchments

Hydrological models are useful tools to analyze present and future conditions of water quantity and quality. The integrated modelling of water and nutrients needs an adequate representation of the different discharge components. In common with many lowlands, groundwater contribution to the discharge in the North German lowlands is a key factor for a reasonable representation of the water balance, especially in low flow periods. Several studies revealed that the widely used Soil and Water Assessment Tool (SWAT) model performs poorly for low flow periods. This paper deals with the extension of the groundwater module of the SWAT model to enhance low flow representation. The current two‐storage concept of SWAT was further developed to a three‐storage concept. This was realized due to modification of the groundwater module by splitting the active groundwater storage into a fast and a slow contributing aquifer. The results of this study show that the groundwater module with three storages leads to a good prediction of the overall discharge especially for the recession limbs and the low flow periods. The improved performance is reflected in the signature measures for the mid‐segment (percent bias ?2.4% vs ?15.9%) and the low segment (percent bias 14.8% vs 46.8%) of the flow duration curve. The three‐storage groundwater module is more process oriented than the original version due to the introduction of a fast and a slow groundwater flow component. The three‐storage version includes a modular approach, because groundwater storages can be activated or deactivated independently for subbasin and hydrological response unit level. Copyright © 2013 John Wiley & Sons, Ltd.     

Wavelet analysis of turbulence in cirrus clouds

Two flights of the UK Meteorological Offices Hercules aircraft through daytime frontal cirrus around Scotland have been analysed using wavelet analysis on the vertical velocity time-series from the horizontal runs. It is shown that wavelet analysis is a useful tool for analysing the turbulence data in cirrus clouds. It finds the largest scales involved in producing turbulence, as does Fourier analysis, such as the 2-km spectral peaks corresponding to convective activity during flight A283. Wavelet spectra have the added advantage that the position is shown, and so they identify smaller-scale, highly localised processes such as the production of turbulent kinetic energy by the breaking of Kelvin-Helmholtz waves due to the vertical shear in the horizontal wind. These may be lost in Fourier spectra obtained for long time-series, though they contribute something to the average spectral density at the appropriate scale. The main disadvantage of this technique is that only octave frequency bands are resolved.     

Theory of decametric radio emissions from Jupiter

It is suggested that the Jovian decametric emissions (DAM) originate in a cyclotron instability of weakly relativistic electrons trapped in the Jovian magnetic field. The resulting radiation has a group velocity in the magnetosphheric plasma which may be of order 10²km/sec, and thus takes much more time to escape the magnetosphere than if the group velocity were at or near the speed of light. Therefore, the asymmetry of the Io phase with respect to sources east and west of the Earth-Jupiter line does not imply an asymmetric beaming of DAM; it is caused by the delay the waves experience in traversing the magnetosphere. The frequency drifts of milli- and decasecond bursts are also explained. It is found that the rotation of the magnetosphere can play an important role, since the observer views the propagation velocity of the waves as the sum of their group velocity and the velocity of the medium itself. The rotation velocity is in opposite directions, relative to the observer, for sources east and west of the Earth-Jupiter line; the resultant vector addition gives positive frequency drifts for decasecond bursts from the early and fourth sources, and negative drifts for bursts from the main and third sources. The negative drifts of millisecond bursts may be the result of large density gradients of plasma in a temporarily compressed magnetosphere.     

The nation-state and the city: introduction to a debate

In recent years, the urban political landscape has become infused with a potent and incendiary mixture of popular-democratic, anti-globalisation, and separatist rhetoric. One central process driving this has been the problematic relations between the nation-state and the city. This Debates and Interventions forum aims to prompt a debate about where the city should be situated intellectually in respect of the nation-state. Four questions are addressed: (1) How do contemporary processes of urban development reflect changing national geopolitical priorities? (2) To what extent are struggles around immigration, citizenship and recognition conducted within cities rather than around the nation-state? (3) To what degree does the city (as e.g. a physical form, social environment or discourse) help to mobilise, propel and/or contest this new urban politics? And (4) Is the concept of a “post-national” global order appropriate for understanding the city and urban politics today?     

Trends of the EU’s territorial and consumption-based emissions from 1990 to 2016

In this paper, a model for the distribution of the Global Carbon Budget between the countries of the world is presented. The model is based on the criteria of equity while also taking into account the different historical responsibilities. The Global Carbon Budget corresponds to the quantity of carbon dioxide emissions that can still be released into the atmosphere while maintaining the increase in the average earth surface temperature below 2 °C, and it is therefore compatible with the long-term objective defined in the Paris Agreement. The results of applying the model are shown both for the 15 emitters that currently top the ranking for world emissions as well as for the other countries, which are grouped together in three main groups: Other African, Other Latin American and Caribbean, and the Rest of the World. Mitigation curves compatible with the carbon budget allocated to the different countries are presented. When comparing each emitter’s historical emissions for the period 1971–2010 with the proposed distribution for the period 2011–2050 obtained using the model, it can be seen that developed countries must face the future with a greatly reduced carbon budget, whereas developing countries can make use of a carbon budget that is higher than their cumulative historical emissions. Finally, there is a discussion about how a model with these characteristics could be useful when implementing the Paris Agreement.