Late Pleistocene paleohydrology near the boundary of the Sonoran and Chihuahuan Deserts,southeastern Arizona,USA

Ground-water discharge (GWD) deposits form in arid environments as water tables rise and approach or breach the ground surface during periods of enhanced effective precipitation. Where preserved, these deposits contain information on the timing and elevation of past ground-water fluctuations. Here we report on the investigation of a series of GWD deposits that are exposed in discontinuous outcrops along a ～150-km stretch of the San Pedro Valley in southeastern Arizona, near the boundary of the Sonoran and Chihuahuan Deserts. Chronologic, isotopic, geochemical, faunal assemblage (ostracodes and gastropods), and sedimentological evidence collectively suggest that the elevation of the regional water table in the valley rose in response to a change in climate ～50 ka ago and remained relatively high for the next ～35 ka before falling during the Bølling–Allerød warm period, rebounding briefly during the Younger Dryas cold event, and falling again at the onset of the Holocene. The timing of these hydrologic changes coincides closely with variations in δ¹⁸O values of calcite from a nearby speleothem to the west and changes in lake levels at pluvial Lake Cochise to the east. Thus, in southeastern Arizona, the assumption that changes in climate are reflected in all aspects of the hydrologic cycle of a region simultaneously is validated. The timing of these changes also broadly coincides with variations in the GISP2 δ¹⁸O record, which supports the hypothesis that atmospheric teleconnections existed between the North Atlantic and the deserts of the American Southwest during the late Pleistocene.     

Practicing community geography in times of crisis

Community geography emphasizes the centrality of community engagement to socially transformative research. This introduction to a special issue of GeoJournal on community geography outlines how this growing subfield provides a model for collaborative action with the crises of our time, from white supremacy through climate change. As the co-editors of this special issue, we summarize the contents of these 14 articles, grouping them by the shared themes of power, institutional partnerships, pedagogy, and methods.

     

Ionic conductivity in sodium magnesium silicates

Na₂MgSiO₄ crystals prepared hydrothermally at 700° C and 3,000 atm are related to carnegieite with SG Pmn2₁, a=7.015(2), b=10.968(2), and c=5.260(1). Na conductivity in Na₂MgSiO₄ is 3.0×10^?5 (ohm-cm)^?1 at 300° C but can be raised to 1.1×10^?3 (ohm-cm)^?1 by creating Na vacancies in the composition Na_1.9Mg_0.9Al_0.1O₄. Na₄Mg₂Si₃O₁₀ is also a cristobalite-related carnegieite with the orthorhombic cell a=10.584(7), b=14.328(7), and c=5.233(5). The Na conductivity of Na₄Mg₂Si₃O₁₀ is 4.8×10^?3 (ohm-cm)^?1 at 300° C.     

Doing community geography

Community Geography offers researchers, community groups, and students opportunities to engage in action oriented applied geographical research. Creating and sustaining these research programs can be challenging, programs can involve many partners from both academic and the community, have different goals and purposes, and utilize a variety of methods to perform research. In this paper we offer a framework of three primary overarching principles for implementing CG projects; (1) Who, (2) Why, and (3) How. (1) “Who” describes who is involved in CG, including researchers, community partners, academic institutions, (2) “Why” describes the justifications and benefits of taking this approach. (3) “How” explains how CG borrows methodologies from many disciplines within geography and beyond. Our examples are not exhaustive; rather, they serve as starting points to inspire researchers interested in CG.

     

Episodic Cenozoic tectonism and the development of the NW European ‘passive’ continental margin

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to reconcile with the various syn-to post-rift mechanisms of permanent and/or transient movements proposed in the hypothetical context of a plume beneath Iceland. The epeirogenic movements can be explained as dynamic topographic responses to changing forms of small-scale convective flow in the upper mantle: tilting as coeval upwelling and downwelling above an edge-driven convection cell, sagging as a loss of dynamic support above a former upwelling. The inferred Cenozoic succession of epeirogenic tilting, sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening, a process that may also be the underlying cause of plate reorganisations. The postulated episodes of flow reorganisation in the NE Atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin, as well as on other Atlantic-type ‘passive’ margins.     

Spatial variability in beach biogeomorphology in a tropical archipelago

Beaches of tropical island coasts exhibit high levels of diversity in composition and form in comparison with their continental counterparts. To investigate the nature and origin of this diversity, individual beach morphology and sedimentology was investigated in the British Virgin Islands (BVI), a Caribbean archipelago of > 60 high volcanic and low reef islands. The islands exhibit a diversity of orientations (some facing the Atlantic and some the Caribbean), elevation and gradient, rock type and wave energy. An examination of 100 beaches in the archipelago revealed a first-order division into sand (70 beaches) and coral rubble (30 beaches). These beaches occur in seven planform types (determined by the antecedent geological framework) and are further subdivided according to shoreface type (seagrass, sandy shoreface, or reef). Mainland-attached headland-embayment beaches are the most common form of sand beach while coral rubble beaches usually occur as barriers that enclose salt ponds and wetlands. Among sand beaches, carbonate content is greatest on Atlantic-facing beaches, and coral rubble beaches are more common on Caribbean-facing beaches. Grain size characteristics on sandy beaches are highly variable and range from fine to very coarse sands while coral rubble beaches range up to boulder-sized clasts. The local source material is a primary determinant of sediment composition. The local factors such as the underlying geology, source and availability of sediments are the primary determinants of beach form, composition and texture in the BVI. Oceanographic and climatic conditions such as the prevailing easterly trade winds and waves which seasonally range in direction from east-northeast to southeast as well as beach orientation to Atlantic- or Caribbean-facing waves also contribute to the variability, but in a secondary role. © 2019 John Wiley & Sons, Ltd.     

Time-series analysis of six whale-fall communities in Monterey Canyon,California, USA

Dead whale carcasses that sink to the deep seafloor introduce a massive pulse of energy capable of hosting dynamic communities of organisms in an otherwise food-limited environment. Through long-term observations of one natural and five implanted whale carcasses in Monterey Canyon, CA, this study suggests that: (1) depth and related physical conditions play a crucial role in species composition; (2) the majority of species in these communities are background deep-sea taxa; and (3) carcass degradation occurs sub-decadally. Remotely operated vehicles (ROVs) equipped with studio quality video cameras were used to survey whales during 0.8 to seven year periods, depending on the carcass. All organisms were identified to the lowest possible taxon. Community differences among whale-falls seemed to be most strongly related to depth and water temperature. The communities changed significantly from initial establishment shortly after a carcass’ arrival at the seafloor through multiple years of steady degradation. The majority of species found at the whale-falls were background taxa commonly seen in Monterey Bay. While populations of species characterized as bone specialists, seep restricted, and of unknown habitat affinities were also observed, sometimes in great abundance, they contributed minimally to overall species richness. All whale carcasses, shallow and deep, exhibited sub-decadal degradation and a time-series of mosaic images at the deepest whale site illustrates the rapidity at which the carcasses degrade.     

Seismological service of Canada: Instrumentation and data processing

The Earth Physics Branch of the Department of Energy, Mines and Resources operates a variety of seismological installations throughout Canada including nineteen Standard and fourteen Regional Stations. At Yellowknife there is an 18-element short-period medium-aperture array with on-site processing which produces a daily detection bulletin. At Ottawa and Victoria, regional telemetered networks have been established which employ wide dynamic range digital data transmission and on-line event detectors. A six-element transportable array employing FM telemetry in the UHF band is used for local seismicity studies. Microprocessor-controlled event-triggered casette recorders are being developed as a further aid to regional seismicity studies. On the West Coast the Branch operates a strong-motion network that includes 28 accelerometers. At Ottawa, the data management group controls the flow of data and arranges for the distribution of seismograms, tape recordings, phase data and other derived information to Branch seismologists and other outside agencies.     

Resource concerns for territorial conflict

The objective of this paper is to engage constructively with literature on armed conflict and resources in order to draw attention both to assumptions that hinder a more accurate or useful understanding of natural resources and their role in territorial conflict as well as to aspects of this field of research that merit further work. In much of the literature on resource conflict, territorial aspects are either implicitly or explicitly tied to resource conflict largely through the assumption that the value of territory is intrinsic. Little of the literature attempts to advance theory, specifically, either on the particular role of resources or on the meaning and objectives of conflict beyond a one-dimensional objective of maintaining or securing control of territory. The paper considers themes of resource scarcity and resource abundance in conflict. A third section focuses on an analysis both of natural resource databases and of conflict databases (both international and civil) which emphasizes problematic results of blending these databases to demonstrate correlations. UIUC territorial conflict management workshop