Preliminary investigation of a late Wisconsinan fauna from K1 cave, Queen Charlotte Islands (Haida Gwaii), Canada

Recent investigations of a limestone solution cave on the Queen Charlotte Islands (Haida Gwaii) have yielded skeletal remains of fauna including late Pleistocene and early Holocene bears, one specimen of which dates to ca. 14,400 ¹⁴C yr B.P. This new fossil evidence sheds light on early postglacial environmental conditions in this archipelago, with implications for the timing of early human migration into the Americas.     

Detrital thermochronology – a new perspective on hinterland tectonics, an example from the Andean Amazon Basin, Ecuador

In order to understand the significance of detrital grain ages in sedimentary basins, a new approach is presented. Five characteristic paths, identified by the change in age of detrital grain populations combined with the change in lagtime over time, can be related to different geodynamic settings in the source regions. When lagtime and grain age increase over time, a change in source must be invoked – this is usually a direct response to a geological event. A constant cooling age, a vertical path, associated implicitly with increasing lagtime, implies erosion of materials that had passed through the closure temperature rapidly – exhuming sufficient rock to supply detritus over the time of the path. Constant lagtimes, regardless of the lagtime itself, are indicative of thermochronological stability in the source region. This can involve fast or slow cooling. Finally, decreasing lagtimes support the notion of increasing cooling rates in the source regions over time. A test study is presented from sediments of the northern Ecuadorian Sub‐Andean Zone where geological events had previously been identified using alternative methods. The addition of heavy‐mineral studies increased the precision in the interpretation. At 90 Ma, rapidly decreasing lagtimes point to a phase of tectonic activity. From about 85 Ma until about 60 Ma the lagtimes were approximately zero. This represents a phase of rapid exhumation of the source regions correlating with the previously identified Pallatanga event. An associated increase of metamorphic minerals occurs over this time span, pointing to increased erosion from deeper horizons. At about 70 Ma, the oldest source region, the shield to the east, was switched off. This timing correlates with a change from marine to continental conditions in the basin, a change in palaeocurrent directions from the east to the west, as well as an associated influx of material from the growing Cordillera Real. At about 55 Ma, a change in source is identified by a change in slope of the lagtime curve together with a change in heavy minerals. From 50 to 35 Ma a renewed period of tectonism in the source region is correlated with the docking of the Macuchi terrane which clearly had an effect of increased erosion in the Cordillera Real bringing in higher grade metamorphic minerals. From about 32 Ma onwards the lagtime has been somewhat constant at about 30 Myr. This does not imply, however, a steady‐state environment as it is well known from other geological evidence that there have been other events within this time frame. One must be cautious about over‐interpreting the lagtime as a method to determine steady state in any region. It is a matter of scale.     

Experimental impact cratering: A summary of the major results of the MEMIN research unit

This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network (MEMIN). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two-stage light-gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane-driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high-temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters.     

A Long Way to Go: Collective Paths to Racial Justice in Geography

Despite decades of recognition and worry about diversity, our discipline remains persistently white. That is, it is dominated by white bodies and it continues to conform to norms, practices, and ideologies of whiteness. This is a loss. At best, it limits the possibilities and impact of our work as geographers. At worst, it perpetuates harmful exclusions in our discipline: its working environments, its institutions, and its knowledge production. This remains deeply concerning for many geographers, and there has been important research, commentary, and institutional activity over the years. Yet, research shows us that little meaningful progress has been made. We know that mentoring is one vital part of the journey toward change. As such, we reflect here on our experience developing a research collective built on a transformative mentoring practice. We outline the key challenges, strategies, and tentative successes of the collective in supporting women of color undergraduate, graduate, and faculty geographers, arguing that such feminist formations are a vital part of the path to intellectual racial justice in our field. Key Words: diversity, feminist geography, higher education, mentoring, race.     

Closely related thyasirid bivalves associate with multiple symbiont phylotypes

Species of thyasirid bivalves are considered to be representative of early stages of chemosymbiosis, given that bacterial symbionts occur outside gill epithelial cells, vary among species in their abundance and nutritional importance, and are environmentally acquired. For these reasons, assessing the extent of host–symbiont specificity in thyasirids could provide valuable information on the evolution of chemosymbiosis. We show that individuals of two closely related and sympatric Thyasira cf. gouldi operational taxonomic units collected from three sites in a fjord in Newfoundland, Canada, associate with one of three distinct, closely related symbiont phylotypes. While associations show some site‐specificity, there is flexibility in host–symbiont pairings within the fjord, further supporting an early and relatively unspecific stage of chemosymbiosis in this family. Morphologic differences observed both within and among symbiont phylotypes suggest physiologic variation, possibly induced by small‐scale differences in sedimentary conditions.     

Submergence and Herbivory as Divergent Causes of Marsh Loss in Long Island Sound

Tidal marsh degradation has been attributed to a number of different causes, but few studies have examined multiple potential factors at the same sites. Differentiating the diverse drivers of marsh loss is critical to prescribing successful interventions for conservation and restoration of this important habitat. We evaluated two hypotheses for vegetation loss at two marshes in Long Island Sound (LIS): (1) marsh submergence, caused by an imbalance between sea-level rise and marsh accretion, and (2) defoliation associated with herbivory by the purple marsh crab, Sesarma reticulatum. At our western LIS site, we found no evidence of herbivory: crabs were scarce, and crab-exclusion cages provided no benefit. We attribute degradation at that site to submergence, a conclusion supported by topographic and hydrologic data showing that loss of vegetation occurred only in wetter parts of the marsh. In contrast, at our central LIS site, our observations were consistent with herbivory as a driving force: There were substantial populations of Sesarma, crab-exclusion cages allowed plants to thrive, and vegetation loss took place across a variety of elevations. We also analyzed soil conditions at both sites, in order to determine the signatures of different degradation processes and assess the potential for restoration. At the submergence site, unvegetated soils exhibited high bulk density, low organic content, and low soil strength, posing significant biogeochemical challenges to re-colonization by vegetation. At the herbivory site, unvegetated soils had a characteristic “riddled-peat” appearance, resulting from expansion and erosion of Sesarma burrow networks. The high redox potential and organic content of those soils suggested that revegetation at the herbivory site would be likely if Sesarma populations could be controlled before erosion leads to elevation loss.