From sprawl to compactness and back: population dynamics (1848–2011) and the economic structure of a Mediterranean city

In the recent decades southern European cities experienced morphological changes and a demographic transition towards zero (or negative) growth and aging. Population dynamics shifted from the impressive growth of the post-industrial period into recent de-concentration processes determining the spillover of commercial and residential settlements across the surrounding rural areas. Based on long-term demographic data, the present study hypothesizes that urban expansion did not follow a one-way linear path from compactness to dispersion while reflecting differentiated growth patterns based on the specific socioeconomic context at the local scale. Along a sufficiently long time period non-linear expansion waves alternating settlement densification and scattering are expected to be found especially in complex urban contexts such as those developed in the Mediterranean region. This hypothesis was tested for a paradigmatic case study in southern Europe (Athens, Greece) using demographic data covering 160 years (1848–2011). Urban growth in Athens was assessed through the analysis of long-term census data made available on a district level. These data provide information on the spatial distribution of resident population and characterize distinct expansion waves according to the dominant socioeconomic context. Results of the study were discussed in the light of the debate on future development of the Mediterranean cities and the (changing) economic relations with the surrounding region.     

Subsidence and uplift of Sidoarjo (East Java) due to the eruption of the Lusi mud volcano (2006–present)

Global positioning system (GPS) and satellite-based InSAR (Interferometric Synthetic Aperture Radar) measurements of the subsidence and uplift of a populated area of Sidoarjo, East Java are due to the eruption of the Lusi mud volcano (2006–present). These data are the first direct quantitative measurements of deformation due to the growth of a mud volcano edifice. The GPS data were recorded over periods of a few hours to several months and show that between June 2006 and September 2007, the earth’s surface has been subsiding at rates of 0.1–4 cm/day. Maximum rates of subsidence occurred in an area 300–400 m to the northwest of the main mud volcano vent. Horizontal displacements were 0.03–0.9 cm/day and were also towards this area. In general uplifts of up to 0.09 cm/day were recorded in areas outside of the edifice. Changes in elevation measured using satellite imagery (InSAR technique) provide regional datasets of subsidence and uplift. They confirm that during the first year a roughly circular area was undergoing sag-like subsidence centered to the northwest of the main vent and that uplift was occurring 3–4 months after the initiation of the eruption due to the movement Watukosek fault system. Subsidence occurred due to the weight of mud and man-made dams and the collapse of the overburden due to removal of mud from the subsurface. Assuming constant rates of subsidence of 4 cm/day, then in the centre of the edifice there would be up to 44 m of subsidence in 3 years, and up to 16 m in 10 years. The mud volcano is now in a self-organizing state with new fluid conduits forming as a result of the collapse. An erratum to this article can be found at     

CO<Subscript>2</Subscript> Air–Sea Exchange due to Calcium Carbonate and Organic Matter Storage,and its Implications for the Global Carbon Cycle

Release of CO₂ from surface ocean water owing to precipitation of CaCO₃ and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a quotient θ = (CO₂ flux to the atmosphere)/(CaCO₃ precipitated). θ depends not only on water temperature and atmospheric CO₂ concentration but also on the CaCO₃ and organic carbon masses formed. In CO₂ generation by CaCO₃ precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric CO₂ concentration (195–375 ppmv), and increasing CaCO₃ precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic carbon counteracts the CO₂ production by carbonate precipitation and it results in lower CO₂ emissions from the surface layer. When atmospheric CO₂ increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO₂ transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric CO₂. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon from land, the calculated CO₂ flux to the atmosphere is a function of the CaCO₃ and C_org net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO₂ flux near the Last Glacial Maximum is 17 to 7×10¹² mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time it is 38–42×10¹² mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic emissions of CO₂ to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total alkalinity owing to the CaCO₃ storage and consequent emission of CO₂. These results are in agreement with the conclusions of a number of other investigators. As the CO₂ uptake in mineral weathering is a major flux in the global carbon cycle, the CO₂ weathering pathway that originates in the CO₂ produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large imbalances of the atmosphere and land organic reservoir at 10²–10⁴-year time scales.     

Evidence for late Paleoproterozoic (ca 1690–1665 Ma) high- to ultrahigh-temperature metamorphism in southern Australia: Implications for Proterozoic supercontinent models

Oxidised metasediments in the western Gawler Craton southern Australia record late Paleoproterozoic high-temperature (HT) to ultrahigh-temperature (UHT) metamorphism. The HT-UHT rocks are magnetite-rich and come from drill core in an unexposed region of the Gawler Craton. Coarse-grained cordierite-bearing assemblages that potentially contained osumilite are overprinted by orthopyroxene-sillimanite-bearing assemblages, which in turn are overprinted by garnet. This microstructural record indicates a metamorphic evolution involving early high-T, low-P conditions that were overprinted by lower thermal gradient assemblages. In situ LA–ICP–MS monazite U-Pb age dating yields a range of ages between 1850 and 1530 Ma with large populations at ca 1690–1650 Ma and ca 1600 Ma. Elsewhere in the Gawler Craton HT and UHT metamorphism occurred in the earliest Mesoproterozoic (ca 1580 Ma). The timing of the Australian UHT events coincides with several other documented examples and occurred during the postulated existence of the Columbia supercontinent. If arguments that link the formation of UHT belts to supercontinental amalgamation are valid, then the existence of ca 1700 to 1600 Ma UHT metamorphism may place additional constraints on the timing of Columbian assembly.     

Consistency of the activity–composition models of Holland,Green, and Powell (2018) with experiments on natural and synthetic compositions: A comparative study

The recent publication of new activity–composition models by Holland, Green and Powell (2018; Journal of Petrology 59 : 881–900), with a melt model calibrated for source compositions ranging from peridotitic to granitic, opens the door to the modelling of multiple petrogenetic processes at supersolidus conditions in which the composition of the melt phase changes considerably, without having to change the melt model. This melt model is also the first one using the internally consistent thermodynamic databases published by T. Holland and R. Powell that contains TiO₂ and Fe₂O₃, further expanding the application of this model to more realistic geological scenarios. The accompanying mineral models are also the first in containing some minor elements, like TiO₂ in garnet and K₂O in clinopyroxene. Consequently, it is relevant to test the applicability of these new models to a large P–T–X range of conditions before they can be used in full. Thermodynamic calculations made with the software Perple_X using these models were compared to experimental results, namely the modal proportions and the composition of the melt and several mineral solution phases. The experiments chosen for the comparative study covered a wide range of source compositions (from mafic to felsic), pressure (from 0.3 to 2.1 GPa), temperature (from 700 to 1,150°C) and total and added water content (structural water: 0.15–1.48 wt%; added water: 0–8 wt%; total water: 0.15%–8.15%). The results indicate that the extended melt model reproduces well the composition of the experimental melts, with an inverse correlation between component amount and fit: the best match is found for SiO₂ (−0.8% on average) and the worst match is found for those elements with the lowest amounts, TiO₂ and MgO (+241% and +235% on average, respectively; values indicate calculated minus experimental, times 100 and divided by experimental). The TiO₂ content in the melt model increases dramatically with increasing pressure, from +90% for P < 1.5 GPa to +593% for P > 1.5 GPa. No comparison was made on the Fe₂O₃ content, as the published iron contents of the experimental melts were always reported as FeO_t. In some cases, there is a substantial mismatch in the modal proportions between experiments and calculations, with the reactant phases less abundant and product phases more abundant in the calculations, an effect that is attributed to kinetic effects in the experiments and to the selected clinoamphibole model. Finally, the extended melt model was compared to the tonalitic melt model of Green et al. (2016; Journal of Metamorphic Geology, 34 : 845–869). Both melt models produce very similar results for SiO₂, Al₂O₃, Na₂O and K₂O, with slightly better results for the tonalitic melt model in FeO_t and MgO and for the extended melt model in CaO. No comparison is made on TiO₂ because the tonalitic melt model does not include this component. In summary, the new activity–composition models represent a significant contribution to thermodynamic calculations on the evolution of siliceous magmas where their composition, temperature and pressure changes substantially.     

U–Pb dating of granodiorite and granite units of the Los Pedroches batholith. Implications for geodynamic models of the southern Central Iberian Zone (Iberian Massif)

The first U–Pb geochronological results on the magmatic alignment of the Los Pedroches batholith are presented. The batholith is composed of a main granodioritic unit, several granite plutons and an important acid to basic dyke complex, all of them intrusive after the main Variscan regional deformation phase, D1, along the boundary between the Ossa-Morena and Central Iberian zones (SW Iberian Massif). Zircons from samples on both extremes of the granodiorite massif record nearly simultaneous magmatic crystallization at ca. 308 Ma, while the emplacement of granite plutons was diachronic between 314 and 304 Ma. The U–Pb results combined with new field and textural observations allow to better constrain the age of Variscan deformations D2 and D3 across the region, while the age of D1 remains imprecise. Transcurrent D2 shearing-tightening of D1 folds occurred around 314 Ma (lower Westphalian) in relation to the emplacement of the first granitic magmas. D3 faults and shear bands bearing a strong extensional component developed at ca. 308 Ma (upper Westphalian), associated to the intrusion of the main granodiorite pluton (granodiorite) of the batholith. Together with available geochemical and geophysical information, these results point to the Variscan reactivation of lithospheric fractures at the origin and subsequent emplacement of hybrid magmas within this sector of the Massif.     

A Compensated-Redlich-Kwong (CORK) equation for volumes and fugacities of CO2 and H2O in the range 1 bar to 50 kbar and 100–1600°C

We present a simple virial-type extension to the modified Redlich-Kwong (MRK) equation for calculation of the volumes and fugacities of H₂O and CO₂ over the pressure range 0.001–50 kbar and 100 to 1400°C (H₂O) and 100 to 1600°C (CO₂). This extension has been designed to: (a) compensate for the tendency of the MRK equation to overestimate volumes at high pressures, and (b) accommodate the volume behaviour of coexisting gas and liquid phases along the saturation curve. The equation developed for CO₂ may be used to derive volumes and fugacities of CO, H₂, CH₄, N₂, O₂ and other gases which conform to the corresponding states principle. For H₂O the measured volumes of Burnham et al. are significantly higher in the range 4–10 kbar than those presented by other workers. For CO₂ the volume behaviour at high pressures derived from published MRK equations are very different (larger volumes, steeper (P/T)_V, and hence larger fugacities) from the virial-type equations of Saxena and Fei. Our CORK equation for CO₂ yields fugacities which are in closer agreement with the available high pressure experimental decarbonation reactions.     

Occurrence Apparent Velocities for Identification and Quantification of Space–Time Clustering Precursory to a Large Earthquake. Application to Large (M > 7.0) Earthquakes in Southern California and Northern Baja California

Space–time seismic clusters, localized bursts of seismic activity, are a feature of background seismicity before the occurrence of large earthquakes, a feature that agrees with observations of diminishing Gutenberg–Richter b-value, fractal dimension, and entropy, and is therefore suggestive of high stress. However, identification and quantification of these space–time clusters, particularly when they are small, is not an easy task and requires a priori assumptions. A novel method for space–time cluster identification, based on an extension of the concept of apparent velocities, is proposed because space–time clusters in the background seismicity have a particular signature in the apparent velocity domain. The contents of histogram peaks due to clusters in the apparent velocity histogram can be used to quantify the cluster activity compared with null hypothesis levels. Identification of the earthquakes corresponding to the apparent velocities in the peaks allows identification of cluster activity in time and space. Apparent velocity peaks do appear in real catalog data for southern California and northern Baja California before the Landers 1992 M = 7.3, Hector Mine 1999 M = 7.1, El Mayor-Cucapah 2010 M = 7.2, and Ridgecrest 2019 M = 7.1 earthquakes, and they appear only within 15 to 25 years before the occurrence of large earthquakes. They are not observed either long before the large earthquakes or after them, and hence could be related to high local states of stress and be of value as a possible precursory observable.

     

Cation exchanged Fe(II) and Sr compared to other divalent cations (Ca, Mg) in the bure Callovian–Oxfordian formation: Implications for porewater composition modelling

Iron and Sr bearing phases were thoroughly investigated by means of spectrometric and microscopic techniques in Callovian–Oxfordian (COX) samples originating from the ANDRA Underground Research Laboratory (URL) in Bure (France). Strontium was found to be essentially associated with celestite, whereas Fe was found to be distributed over a wide range of mineral phases. Iron was mainly present as Fe(II) in the studied samples (∼93% from Mössbauer results). Most of the Fe(II) was found to be in pyrite, sideroplesite/ankerite and clay minerals. Iron(III), if present, was associated with clay minerals (probably illite, illite-smectite mixed layer minerals and chlorite). No Fe(III) oxy(hydro)xide could be detected in the samples. Strontianite was not observed either. Based on these observations, it is likely that the COX porewater is in equilibrium with the following carbonate minerals, calcite, dolomite and ankerite/sideroplesite, but not with strontianite. It is shown that this equilibrium information can be combined with clay cation exchange composition information in order to give direct estimates or constraints on the solubility products of the carbonate minerals dolomite, siderite and strontianite. As a consequence, an experimental method was developed to retrieve the cation exchanged Fe(II) in very well preserved COX samples.     

Evidence for Late Miocene to Recent contamination of arc andesites by crustal melts in the Chilean Andes (25–26°S) and its geodynamic implications

Chemical and isotopic data from 12 volcanic centers of the southern Central Volcanic Zone (CVZ) in Chile, whose ages of 20, 16, 11, 8, 5, 2 and <1 Ma bracket the peak of shortening and crustal thickening in the mid-Miocene, show systematic differences with age. The composition of andesites erupted before and after crustal thickening are similar in terms of most major and trace elements, but the post-Miocene andesites show enrichments in Th, U, Cs and Rb, as well as high ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios coupled with low εNd values which indicate greater crustal contamination compared with the older equivalents. Comparison of contamination indicators with age shows that contamination was low from 20 Ma to 8 Ma, increased sharply between 8 and 5 Ma, and remained at a high level into the Quaternary. Constant ratios of fluid-mobile vs immobile elements (Cs/Th or Ba/Zr) in even the most contaminated rocks indicate that fluid interaction was negligible. The contaminated andesites display disequilibrium textures and contain phenocrysts with a mixed population of melt inclusions. We suggest that the main process of crustal contamination was mixing with crustal melts. This is supported by geophysical evidence for a zone of partial melting in the mid and lower crust under the magmatic arc and by the presence of late Miocene to Pliocene crustal-derived felsic ignimbrites in the CVZ.