Continental margin off Sergipe and Alagoas, northeastern Brazil: A reconnaissance geophysical study of morphology and structure

The stable continental margin of northeastern Brazil is unusually narrow, probably because of the small size and tropical character of the drainage basins of the hinterland, and correspondingly low rates of land erosion and marine sedimentation. The continental shelf, which is mainly a marine erosion surface, is also remarkably shallow, either because of upwarping or, more probably, because of the ineffectiveness of Pleistocene marine erosional processes on steeply sloping continental margins. Sediment accumulation is confined to the Sāo Francisco delta, seaward of which are fossil (?) lagoonal deposits, and to a poorly developed nearshore sand prism.The margin formed by seaward progradation of sediment on a subsiding basement, but the present morphology of the continental slope reflects chiefly Pleistocene canyon cutting and mass gravitational movements of sediment, which have exposed older strata in the upper slope. Beneath the continental slope is a magnetic anomaly (like the slope anomaly off the eastern U.S.A.), probably caused by a deeply buried dike of oceanic basalt, and apparently associated with a buried ridge which may have formed the seaward margin of the Sergipe—Alagoas Basin during the early history of the South Atlantic. Similar structures may be typical of the narrow easternmost part of the Brazilian margin.     

A laser-based fluorometry system for investigations of seawater and porewater fluorescence

A highly sensitive laser-induced fluorescence (LIF) system has been developed to study the fluorescence of dissolved organic carbon (DOC) in the marine environment. The LIF detector has a detection limit of 10 attomoles (10 × 10⁻¹⁸ moles) of pterin and eliminates internal quenching in highly fluorescent samples such as anoxic porewaters encountered when using conventional fluorometry. LIF analysis is rapid, reproducible, and uses only 100 μl of a sample. This small size requirement permits fluorescence analyses of samples often available only in limited amounts, such as porewaters, hydrothermal vent waters, and rainwaters. In addition, the LIF detection system may greatly simplify extraction and separation procedures required to characterize the fluorescent components of DOC.     

Geologic setting of the Snake Pit hydrothermal site: An active vent field on the Mid-Atlantic Ridge

TheSnake Pit Hydrothermal Site lies on the axis of the Mid-Atlantic Ridge at 23°22′ N latitude, about 30 km south of the Kane Transform Intersection. Active ‘black smoker’ vents and a surrounding field of hydrothermal sediment occur at the crest of a laterally extensive neovolcanic ridge. It is one of the first active hydrothermal vent fields to be found on a slow-spreading ridge axis and despite significant differences in its geologic setting from those of the East Pacific Rise, has many similarities to its fast-spreading counterparts. Although preliminary reports have documented many interesting aspects of these vents and their surroundings, new data collected from the manned submersible ALVIN and the deep-towed ANGUS camera system define the regional tectonic setting as well as the local geologic environment of this fascinating area. The Snake Pit vents are located on a local peak of a volcanic constructional ridge at a depth of 3450 m, 700–800 m deeper than vents known from the East Pacific Rise, Galapagos, or Juan de Fuca spreading centers. The vent field is at least 600 m long and up to 200 m wide and is covered by a thick blanket of greenish to yellow-orange hydrothermal sediment. Both active and extinct vents are perched along the crests of steep-sided sulfide mounds that reach heights of over 40 m. High-temperature (350° C) fluids are vented from black smoker chimneys and low-temperature (226° C) fluids seep from sulphide domes and subordinate anhydrite constructions. Water temperatures, flow rates, fluid chemistries, and mineralization are strikingly similar to vents of faster spreading ridge crests; however, a somewhat distinct fauna inhabit the area.     

A numerical study of the effects of wind forcing on the Chile Current System

A high-resolution, multi-level, primitive equation ocean model is used to examine the response of the coastal region from 22.5°S to 35°S of the Chile Current System to both equatorward and climatological wind forcing. The results from both types of forcing show that an equatorward surface current, a poleward undercurrent, upwelling, meanders, filaments and eddies develop in response to the predominant equatorward wind forcing. When climatological wind forcing is used, an offshore branch of the equatorward surface current is also generated. These features are consistent with available observations of the Chile Current System. The model results support the hypothesis that wind forcing is an important mechanism for generating currents, eddies and filaments in the Chile eastern boundary current system and in other eastern boundary current regions which have predominantly equatorward wind forcing.     

Modeling ferrous-ferric iron chemistry with application to martian surface geochemistry

The Mars Global Surveyor, Mars Exploration Rover, and Mars Express missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major recent mission findings are the presence of jarosite (a ferric sulfate salt), which requires formation from an acid-sulfate brine, and the occurrence of hematite and goethite on Mars. Recent ferric iron models have largely focused on 25 °C, which is a major limitation for models exploring the geochemical history of cold bodies such as Mars. Until recently, our work on low-temperature iron-bearing brines involved ferrous but not ferric iron, also obviously a limitation. The objectives of this work were to (1) add ferric iron chemistry to an existing ferrous iron model (FREZCHEM), (2) extend this ferrous/ferric iron geochemical model to lower temperatures (<0 °C), and (3) use the reformulated model to explore ferrous/ferric iron chemistries on Mars.The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Ferric chloride and sulfate mineral parameterizations were based, in part, on experimental data. Ferric oxide/hydroxide mineral parameterizations were based exclusively on Gibbs free energy and enthalpy data. New iron parameterizations added 23 new ferrous/ferric minerals to the model for this Na-K-Mg-Ca-Fe(II)-Fe(III)-H-Cl-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-O₂-CH₄-H₂O system.The model was used to develop paragenetic sequences for Rio Tinto waters on Earth and a hypothetical Martian brine derived from acid weathering of basaltic minerals. In general, model simulations were in agreement with field evidence on Earth and Mars in predicting precipitation of stable iron minerals such as jarosites, goethite, and hematite. In addition, paragenetic simulations for Mars suggest that other iron minerals such as lepidocrocite, schwertmannite, ferricopiapite, copiapite, and bilinite may also be present on the surface of Mars. Evaporation or freezing of the Martian brine led to similar mineral precipitates. However, in freezing, compared to evaporation, the following key differences were found: (1) magnesium sulfates had higher hydration states; (2) there was greater total aqueous sulfate (SO_4T = SO₄ + HSO₄) removal; and (3) there was a significantly higher aqueous Cl/SO_4T ratio in the residual Na-Mg-Cl brine. Given the similarities of model results to observations, alternating dry/wet and freeze/thaw cycles and brine migration could have played major roles in vug formation, Cl stratification, and hematite concretion formation on Mars.     

Simultaneous inner- and outer-sphere arsenate adsorption on corundum and hematite

The ability to predict the fate and transport of arsenic in aquatic environments, its impact on water quality and human health, and the performance and cost-effectiveness of water treatment systems relies on understanding how it interacts with solid surfaces. In situ resonant surface X-ray scattering measurements of arsenate adsorption at pH 5 in 0.01 M NaCl on corundum and hematite (012) surfaces demonstrate that arsenate surface complexation is unexpectedly bimodal, adsorbing simultaneously as inner- and outer-sphere species. In addition, this bimodal behavior is found to be independent of the total arsenate solution concentration, and thus surface coverage, over the range of 10⁻⁶ to 10⁻³ M. Alternative mechanisms to produce the observed As distributions, such as arsenate dimerization or surface precipitation of an aluminum or ferric arsenate, are inconsistent with the experimentally-determined total and As-specific density profiles. Based on the location of the outer-sphere arsenate in relation to the surfaces studied, possible binding mechanisms include electrostatic attraction, hydrogen bonding to surface oxygen functional group, and configurational stabilization by interfacial water. Although the observation of outer-sphere arsenate surface complexes on a metal oxide surface is unprecedented, it is unclear if such species were absent in previous molecular-scale studies, as it is difficult for methods commonly used to investigate the mechanisms of arsenate adsorption to conclusively identify or rule out the presence of outer-sphere species when inner-sphere species are also present.     

Assembling “Japantown”? A critical toponymy of urban dispossession in Vancouver,Canada

Geographic scholarship in critical toponymy has highlighted the importance of place naming as a form of discursive power within processes of urbanization. This paper builds on such literature and advances a novel theory of toponymic assemblage to interpret findings from a participatory research project in the Downtown Eastside of Vancouver, Canada. We foreground neighborhood history in the form of a Japanese Canadian enclave and its wartime uprooting and dispossession, and trace the historical antecedents of a resurrected toponymy of “Japantown” that has appropriated and renarrated Japanese Canadian history to facilitate further rounds of dispossession. Using a genealogical method, we highlight three “moments” of Japanese Canadian uprooting, return, presence, and activism, demonstrating how toponymies are assembled in place in heterogeneous and historically contiguous ways. This approach expands on current research in critical toponymy, offering a novel methodology for exploring the enrolment of toponymy, discourse, and materiality in the formation of place.