Terrigenous sedimentation processes along the continental margin off NW Africa: implications from grain-size analysis of seabed sediments

The terrigenous fraction of seabed sediments recovered along the north‐west African continental margin illustrates spatial variability in grain size attributed to different transport mechanisms. Three subpopulations are determined from the grain‐size analyses (n = 78) of the carbonate‐free silt fraction applying an end‐member modelling algorithm (G. J. Weltje, 1997). The two coarsest end‐members are interpreted as representing aeolian dust, and the fine‐grained end‐member is related to fluvial supply. The end‐member model thus allows aeolian fallout to be distinguished from fluvial‐sourced mud in this area. The relative contributions of the end‐members show distinct regional variations that can be related to different transport processes and pathways. Understanding present‐day sediment dispersal and mixing is important for a better understanding of older sedimentary records and palaeoclimate reconstructions in the region.     

Reorganization of a deeply incised drainage: role of deformation,sedimentation and groundwater flow

Deeply incised drainage networks are thought to be robust and not easily modified, and are commonly used as passive markers of horizontal strain. Yet, reorganizations (rearrangements) appear in the geologic record. We provide field evidence of the reorganization of a Miocene drainage network in response to strike–slip and vertical displacements in Guatemala. The drainage was deeply incised into a 50‐km‐wide orogen located along the North America–Caribbean plate boundary. It rearranged twice, first during the Late Miocene in response to transpressional uplift along the Polochic fault, and again in the Quaternary in response to transtensional uplift along secondary faults. The pattern of reorganization resembles that produced by the tectonic defeat of rivers that cross growing tectonic structures. Compilation of remote sensing data, field mapping, sediment provenance study, grain‐size analysis and Ar⁴⁰/Ar³⁹ dating from paleovalleys and their fill reveals that the classic mechanisms of river diversion, such as river avulsion over bedrock, or capture driven by surface runoff, are not sufficient to produce the observed diversions. The sites of diversion coincide spatially with limestone belts and reactivated fault zones, suggesting that solution‐triggered or deformation‐triggered permeability have helped breaching of interfluves. The diversions are also related temporally and spatially to the accumulation of sediment fills in the valleys, upstream of the rising structures. We infer that the breaching of the interfluves was achieved by headward erosion along tributaries fed by groundwater flow tracking from the valleys soon to be captured. Fault zones and limestone belts provided the pathways, and the aquifers occupying the valley fills provided the head pressure that enhanced groundwater circulation. The defeat of rivers crossing the rising structures results essentially from the tectonically enhanced activation of groundwater flow between catchments.     

Monitoring playa lake inundation in the western United States: Modern analogues to late-Holocene lake level change

Closed basin playas are among the most sensitive hydrologic systems globally and are excellent indicators of current and past climatic variability. This variability can significantly impact hydrologic regimes and biotic communities, and is often expressed in lake-bed deposits and shoreline features. We analyzed two playa basins in western North America that lie to either side of the current divide between monsoon and westerly precipitation regimes. Using a 23-year sequence of Landsat images at a 16-day time step, we determined the playa inundation response to varying precipitation inputs. Our results show that a strongly contrasting lake-inundation response occurs in lake basins separated by only 200 km. The Animas/Lordsburg Basin shows a marked lake-area increase in response to winter precipitation events, while the more southerly Palomas Basin shows a stronger response to monsoonal and El Niño-type events. This sensitivity to different input sources over short distances may explain some of the apparent asynchronous behavior of playa response found in lake records. Comprehensive regional-scale inundation records could be used to understand the dynamics of playa inundation events and how these events are linked to atmospheric circulation, and possibly to understand the observed asynchronous behavior of lake basins during the late Pleistocene and Holocene.     

Morphotectonics of a high plateau on the northwestern flank of the Continental Rift of southeastern Brazil

Integration of landform and structural analysis allowed the identification of Late Pleistocene–Holocene pulses of tectonic activity in the Campos do Jordão Plateau with ages and regimes similar to the ones from the continental rift. Fault reactivation along Precambrian shear zones give rise to a series of conspicuous morphotectonic features, determine the formation of stream piracy phenomena, and divide the plateau into smaller blocks. Recognition of these tectonic pulses as well as of their effects in landform development—particularly clear on the Campos de São Francisco at the highest area of the SE edge of the plateau—show that besides the climate-related Quaternary environmental changes significant neotectonic instability should be considered in the geomorphic evolution of the Campos do Jordão Plateau.     

Origin of Aitken particles in remote regions of the Southern Hemisphere

Seasonal variations in total particle concentrations at five remote Southern Hemisphere sites in Samoa, Tasmania and Antarctica are compared. The dominant particle production mechanism is deduced to be photochemical, with concentrations dictated by the availability of radiation in the eight months centred on the winter solstice. There is little variation with latitude in the mean summer concentrations. The middle troposphere is shown to act as an important reservoir of photochemically-produced particles in Tasmania and probably in the Antarctic. It is suggested that primary-particle production is mainly through the oxidation of dimethyl sulphide and that these particles then act as centres of condensation for sulphuric acid formed by the oxidation of sulphur dioxide.     

Late Quaternary colluvial deposits in the Bocaina Plateau,southeastern Brazil highlands: age and palaeoenvironmental consequences

Despite the generalized occurrence of colluvial deposits in the humid tropical areas of southeastern Brazil, regional correlation is difficult because the deposits are discontinuous, and chronological data are very scarce and scattered. For the first time, colluvial deposits in the Bocaina Plateau are described, including 18 radiocarbon ages from 12 profiles. The plateau is located on the eastern flank of the Continental Rift of Southeastern Brazil and is the highest part of the Serra do Mar, with elevations up to 2000 m above sea level. Because the Bocaina Plateau is part of the summit surfaces of southeastern Brazil, it was subjected to specific climatic conditions during the Quaternary. Colluvial deposits on the lower hillslopes and edges of amphitheatres can show complex sequences with up to three intercalated dark humic horizons, corresponding to Late Pleistocene and Holocene phases of morphodynamic activity and pedogenesis. Ages of palaeosols vary from 650±50 a BP to 36 880±980 a BP. Landforms and colluvia on the Bocaina Plateau are very similar to those found on the opposite flank of the continental rift, on the Campos do Jordão Plateau. Ages of buried soils point to similar soil formation and colluviation episodes in the two plateaus. As in Campos do Jordão, the succession of erosive, depositional and pedogenetic processes on the altos campos hillslopes is probably related to late Quaternary climate changes.     

Unraveling the roles of asymmetric uplift,normal faulting and groundwater flow to drainage rearrangement in an emerging karstic landscape

Landscape adjustment to tectonic, lithologic and climatic forcing leads to drainage reorganization and migration of divides. The respective contribution of these forcings, especially on carbonate landscapes is not well defined. Here, we have addressed this issue by combining field observations, satellite image interpretation and digital elevation model (DEM) quantitative analysis to assess drainage response to spatially heterogeneous rainfall, asymmetric uplift, and normal faulting on an emerging carbonated platform (Sumba Island, Indonesia). We map geomorphic markers of fluvial dynamics and drainage rearrangement and compute a χ parameter that incorporates the contributions of unevenly distributed precipitation and asymmetric uplift to estimate erosional disequilibrium across drainage divides. We find that asymmetric emergence of Sumba Island created an initial parallel drainage, asymmetric across a divide that propagates landwards. Soon after establishing itself on the emerging slopes this drainage was disturbed by normal faulting, which has become the main force driving drainage rearrangement. Vertical offsets across normal fault scarps first triggered aggradation within valleys over the hanging walls, and then disconnected upstream reaches from downstream reaches, leading to the formation of wind gaps atop the fault scarps and upstream perched sedimentary basins. The defeat of rivers by growing fault scarps was catalysed by the possibility for surface water to be rerouted near the fault scarps into underground water networks inside the underlying carbonates. At the end of the process, the opposite drainage across the main water divide captured the struggling drainage. Capture mechanisms include initial groundwater capture of the perched alluvial aquifers, followed by ground sapping at the head of the opposite drainage and surface stream diversion by avulsion. Finally, normal faulting is the main driving force of drainage rearrangement allowing avulsion and karstic rerouting whereas asymmetric uplift and climate forcings have shown a low efficiency. The role of karstification is more ambiguous, catalysing or inhibiting drainage rearrangement. Copyright © 2018 John Wiley & Sons, Ltd.     

Social mix revisited: within- and across-neighborhood ties between ethnic minorities of differing socioeconomic backgrounds

Creating social mix in neighborhoods has been an important urban policy during the past years. The aim is the development of category-crossing ties between the middle and lower classes. However, studies found little evidence of such ties. This paper identifies two reasons why this might be the case. First, studies usually analyze the resource flow from native middle classes to disadvantaged residents. Second, the resource flow is analyzed within one neighborhood only. Based on interviews and network analysis with second generation, upwardly mobile Turkish-Germans in Berlin, I show that they act as cultural and language brokers, benefiting lower class co-ethnics. They provide exactly the information and resources to people with a lower class background that are envisioned by social mixing programs. Moreover, the resource flow occurs within but also across neighborhoods, whereby ‘second neighborhoods’ such as the previous neighborhood or the neighborhood where the workplace is located are particularly important.