The end of the Ediacara biota: Extinction,biotic replacement,or Cheshire Cat?

The Ediacaran–Cambrian transition signals a drastic change in both diversity and ecosystem construction. The Ediacara biota (consisting of various metazoan stem lineages in addition to extinct eukaryotic clades) disappears, and is replaced by more familiar Cambrian and Paleozoic metazoan groups. Although metazoans are present in the Ediacaran, their ecological contribution is dwarfed by Ediacaran-type clades of uncertain phylogenetic affinities, while Ediacaran-type morphologies are virtually non-existent in younger assemblages. Three alternative hypotheses have been advanced to explain this dramatic change at, or near, the Ediacaran–Cambrian boundary: 1) mass extinction of most Ediacaran forms; 2) biotic replacement, with early Cambrian organisms eliminating Ediacaran forms; and 3) a Cheshire Cat model, with Ediacaran forms gradually disappearing from the fossil record (but not necessarily going extinct) as a result of the elimination of unique preservational settings, primarily microbial matgrounds, that dominated the Ediacaran. To evaluate these proposed explanations for the biotic changes observed at the Ediacaran–Cambrian transition, environmental drivers leading to global mass extinction are compared to biological factors such as predation and ecosystem engineering. We explore temporal and biogeographic distributions of Ediacaran taxa combined with evaluations of functional guild ranges throughout the Ediacaran. The paucity of temporally-resolved localities with diverse Ediacaran assemblages, combined with difficulties associated with differences in taphonomic regimes before, during, and after the transition hinders this evaluation. Nonetheless, the demonstration of geographic and niche range changes offers a novel means of assessing the downfall of Ediacara-type taxa at the hands of emerging metazoans, which we hypothesize to be most likely due to the indirect ecological impact metazoans had upon the Ediacarans. Ultimately, the combination of studies on ecosystem construction, biostratigraphy, and biogeography showcases the magnitude of the transition at the Ediacaran–Cambrian boundary.     

Middle-fan deposits from the late precambrian kongsfjord formation submarine fan,Northeast Finnmark,Northern Norway

The 3.2 km-thick late Precambrian Kongsfjord Formation Submarine Fan shows well-developed middle-fan facies-associations. Channel deposits are characterised by discrete packets of coarse-grained, medium to thick-bedded, amalgamated sandstone turbidites and other mass-flow deposits, generally 10 to 30 m thick. Individual beds, or packets of beds, wedge out and channel bases cut down by up to 11 m over a lateral distance of 150 m. Channel deposits often comprise a thinning-and-fining-upward sequence although they vary greatly in clarity. Interchannel deposits occur as packets, tens of centimetres to 25 m thick, of thin and very thin bedded Bouma T_cde siltstones and mudstones. Palaeocurrents within interchannel deposits commonly diverge from those of adjacent channel sandstones. Within the interchannel deposits, isolated beds or packets of beds occur that are both thicker bedded and coarser grained than the surrounding beds; these unusual deposits are sheet-like or fill small channels, and are interpreted as crevasse splays, lobes and channels. Packets, up to a few metres thick, of laterally discontinuous siltstone turbidites occur immediately above some of the channel sandstones, rarely below, and in some cases within interchannel deposits. These siltstones are thin to medium-bedded, show Bouma T_cd, with T_c often as climbing-ripple lamination, and commonly show soft-sediment deformation as slides, slumps, liquefaction and fluidisation structures. Palaeoflow within these packets, compared to adjacent channel sandstones, diverges by up to 90°, and in some cases channel sandstones are seen to pass laterally into these deposits with a swing in palaeocurrents from parallel to the inferred channel axis, to perpendicular to it. These deposits are thought to be levees. Channel-margin deposits are most distinctive, and they are recognised by extreme lateral wedging of channel sandstones, with concomitant thinning and fining of individual beds and their amalgamation towards the channel axis. Sliding and slumping of channel margin deposits is common. Throughout the Kongsfjord Formation Submarine Fan, channel sandstone palaeocurrents suggest a sediment-transport direction to the NE quadrant, although some channels funnelled sediment towards the southeast.     

Weathering of granite in Antarctica: II. Thermal stress at the grain scale

Granular disintegration has long been recognized and referred to in weathering texts from all environments, including the Antarctic. Despite this universal identification and referral, few to no data exist regarding thermal conditions at this scale and causative mechanisms remain little more than conjecture. Here, as part of a larger weathering study, thermal data of individual grains (using infrared thermometry and ultra‐fine thermocouples) composing a coarse granite, as well as the thermal gradients in the outer 10 cm (using thermistors), were collected from a north‐facing exposure. Measurements were also made regarding the surface roughness of the rock. Based on recorded temperatures, the nature of the rock surface and the properties of the minerals, an argument is made for complex stress fields that lead to granular disintegration. Mineral to mineral temperature differences found to occur were, in part, due to the changing exposure to solar radiation through the day (and through seasons). Because the thermal conductivity and the coefficient of thermal expansion of quartz are not equal in all directions, coupled with the vagaries of heating, this leads to inter‐granular stresses. Although fracture toughness increases with a decrease in temperature, it is suggested that the tensile forces resulting from falling temperatures are able to exceed this and produce granular disassociation. The lack of equality with respect to crystal axis of both thermal conductivity and expansion in quartz further exacerbates the propensity to failure. Grain size and porosity also influence the thermal stresses and may help explain why some grains are held in place despite disassociation near the surface. While the data presented here appear to beg more questions than providing answers, they do provide a basis for better, more detailed studies of this important weathering scale. Copyright © 2007 John Wiley & Sons, Ltd.     

Should Geography Educators Adopt Electronic Textbooks?

Significant changes in textbook publishing are at college instructors' door. Textbook representatives are directed to mesmerize potential clients with eye-catching displays and statements about digital literacy of the next generation of college students. Based on a one-year pilot study, in 2014–2015, of an electronic textbook, this commentary shares real experiences with the digital tools, contextualizes the shift in textbook publishing and research on student outcomes, and provides insight in navigating through the Brave New World of digital texts and adaptive learning tools.     

Men and women in Boston learn about terrorism differently: gendering the urban values–vulnerability nexus

This paper expands the values-vulnerability nexus for studying urban vulnerability to terrorism. Gender is among the most salient social structures affecting a person’s interpretation of risk, and urban geographers have demonstrated that gender also deeply affects the experience of urban life. However, the values that shape urban vulnerability to terrorism have been conceptualized more broadly, usually focusing on the underpinning economic organization of cities that terrorists exploit rather than on concepts such as gender. As a result, the role of gender in understanding societal responses to terrorism—specifically risk perception and preparedness—is underdeveloped, including in the 2008 US National Survey of Disaster and Preparedness, which sought to understand Americans’ responses to terrorism. This paper outlines a gender values–vulnerability nexus for studying terrorism. Using interview data from Boston, Massachusetts conducted before and after the 15 April 2013 Boston Marathon attacks, I demonstrate that gender deeply affected how Bostonians learned about and responded to terrorism. The gender values–vulnerability nexus explores the role of gendered political, social, economic, and geographical dimensions in urban vulnerability to terrorism. The role of gender has the potential to substantially increase vulnerability for urban populations, particularly given gender-similar social networks and urban contexts that continue to yield different experiences for men and women. The implications of these findings may signal challenges for urban emergency managers, public safety officials, and ordinary people in American cities.     

Changes in Climate Extremes Over the Australian Region and New Zealand During the Twentieth Century

Analyses of high quality data show that there have been some interesting recent changes in the incidence of some climate extremes in the Australian region and New Zealand.     

Mapping the Globalization of the Temporary Staffing Industry*

Very little is currently known about the globalization of the temporary staffing industry, a strategically significant sector given its role in promulgating wider labor market flexibility. This article starts to rectify this research lacuna in four ways: by conceptualizing the international expansion of temporary staffing and comparing it to other business service sectors, by identifying and mapping the top twenty transnational staffing agencies, by offering a typology of the leading transnational agencies based on their functional and geographic characteristics, and by charting a research agenda for future work on this sector.     

The unique,widely distributed,estuarine sea anemone,Nematostella vectensis Stephenson: A review,new facts,and questions

The small, burrowing, edwardsiid sea anemone Nematostella vectensis is widely distributed in estuaries and bays. Most typically it occurs in pools in marshes though it may occur subtidally as well. We have compiled records of its occurrence in North America from Nova Scotia to Georgia along the shores of the Atlantic Ocean, from Florida to Louisiana in the Gulf of Mexico and from California to Washington on the Pacific coast. To date we have found no records of its presence in Alabama or Texas, though it is present in all other of the contiguous coastal states of the United States. The species also occurs in England. We have obtained living specimens from many locations and have crossed females from England, Maryland, Georgia, California, Oregon, and Washington with males from Nova Scotia, Maryland, Georgia, and Oregon. These 24 crosses all yielded viable first-generation anemones that in turn produced second-generation animals. We accept this as proof that this widely distributed anemone is a single species. We have obtained living N. vectensis from 11 areas. Of these, only samples from Maine, Maryland, Georgia, and Oregon contained both sexes. The sample from Nova Scotia was all male and our samples from England, New Hampshire, California, and Washington were all female. We hypothesize that the unisexual samples were from clones resulting from asexual reproduction in this species. *** DIRECT SUPPORT *** A01BY066 00015     

Isotopic and elemental fractionation of solar wind implanted in the Genesis concentrator target characterized and quantified by noble gases

Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The ²⁰Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass‐dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions.