Loess record of the Pleistocene–Holocene transition on the northern and central Great Plains, USA

Various lines of evidence support conflicting interpretations of the timing, abruptness, and nature of climate change in the Great Plains during the Pleistocene–Holocene transition. Loess deposits and paleosols on both the central and northern Great Plains provide a valuable record that can help address these issues. A synthesis of new and previously reported optical and radiocarbon ages indicates that the Brady Soil, which marks the boundary between late Pleistocene Peoria Loess and Holocene Bignell Loess, began forming after a reduction in the rate of Peoria Loess accumulation that most likely occurred between 13.5 and 15 cal ka. Brady Soil formation spanned all or part of the Bølling-Allerød episode (approximately 14.7–12.9 cal ka) and all of the Younger Dryas episode (12.9–11.5 cal ka) and extended at least 1000 years beyond the end of the Younger Dryas. The Brady Soil was buried by Bignell Loess sedimentation beginning around 10.5–9 cal ka, and continuing episodically through the Holocene. Evidence for a brief increase in loess influx during the Younger Dryas is noteworthy but very limited. Most late Quaternary loess accumulation in the central Great Plains was nonglacigenic and was under relatively direct climatic control. Thus, Brady Soil formation records climatic conditions that minimized eolian activity and allowed effective pedogenesis, probably through relatively high effective moisture.Optical dating of loess in North Dakota supports correlation of the Leonard Paleosol on the northern Great Plains with the Brady Soil. Thick loess in North Dakota was primarily derived from the Missouri River floodplain; thus, its stratigraphy may in part reflect glacial influence on the Missouri River. Nonetheless, the persistence of minimal loess accumulation and soil formation until 10 cal ka at our North Dakota study site is best explained by a prolonged interval of high effective moisture correlative with the conditions that favored Brady Soil formation. Burial of both the Brady Soil and the Leonard Paleosol by renewed loess influx probably represents eolian system response that occurred when gradual change toward a drier climate eventually crossed the threshold for eolian activity. Overall, the loess–paleosol sequences of the central and northern Great Plains record a broad peak of high effective moisture across the late Pleistocene to Holocene boundary, rather than well-defined climatic episodes corresponding to the Bølling-Allerød and Younger Dryas episodes in the North Atlantic region.     

Upscaling of Hooke''s Law for Imperfectly Layered Rocks

In this paper, we consider the upscaling of Hooke's law and its parameters on the fine scale, to a similar law with upscaled parameters on a larger scale. It is assumed that the fine scale material properties of the rock are imperfectly layered. In the governing equations, the deviations from perfect layering introduce a small parameter that can be used in perturbation series expansions for the stress, the strain, and the displacement. In the approximation of order zero the upscaled compliance matrix contains the well-known Backus parameters; this approximation holds exactly for a perfect layering. However, many natural rock types are imperfectly layered and in that case the approximation of order zero may not be sufficiently accurate. Therefore, we consider also the first order corrections. The derivation and results are presented both for the most general case and for the much simpler case in which the fine scale Poisson ratio may be assumed constant. From thermodynamic principles, it follows that the compliance tensor is symmetric on the fine scale. However, it is shown that the argument for symmetry cannot be extended to upscaled rigidities. One of the most important conclusions is that upscaled compliance tensors are nonsymmetric when there are trends in the deviations from perfect layering.     

Alkalic magma modified by incorporation of diverse tholeiitic components: ‘Complex’ hybridization on Kahoolawe Island, Hawaii

Summary Kahoolawe Island is a 1.4 to 1 Ma shield volcano composed of shield, caldera-fill, and postshield tholeiitic lavas, and postshield alkalic basalt and hawaiite lavas. One postshield vent erupted alkalic lavas (K₂O 1.1–1.6 wt.%) with resorbed olivines of wide compositional range (cores, Fo_86–71), resorbed Na-plagioclase (An_57–30; largely andesine) and clinopyroxene (evolved; Mg#s 76–71), and groundmass orthopyroxene (Mg# 62). They also contain tholeiitic gabbro xenoliths, which, as a suite, have a continuum of mineral compositions — clinopyroxene Mg#s 83–74, orthopyroxene Mg#s 83–76, and plagioclase An_69–35 (e.g., includes andesine gabbro). Lava compositions do not fall on expected Hawaiian fractionation trends due to MgO enrichment (e.g., CaO 7 wt.% @ MgO 6 wt.%). This assortment of mineral and rock components within alkalic lavas with apparent Mg enrichment is owed to a complex history that began with protracted mixing among primitive and differentiated tholeiitic magmas, probably near the end of shield building. These hybrid magmas crystallized a compositional variety of olivines that were resorbed during reservoir replenishments, and also crystallizedin situ to form orthopyroxene-bearing gabbro on reservoir walls. When magma production rates declined during the shield to postshield transition of tholeiitic to alkalic magmatism, the tholeiitic hybrids in reservoirs fractionated to yield highly evolved phases such as andesine and clinopyroxene with Mg# < 75. When postshield hawaiite magmas subsequently entered reservoirs, alkalic-tholeiitic hybridization occurred; the resulting `complex' mixture of hawaiite+tholeiitic hybrids resorbed andesine and clinopyroxene crystals and, upon eruption, entrained xenoliths of gabbro. Mass balancing suggests that the alkalic-tholeiitic hybridization involved 44% hawaiite mixed with a nearly equal amount of tholeiitic hybrid (MgO 9.5 wt.%) plus olivine and andesine. This type of complex hybridization is a logical process for magmatism associated with tholeiitic to alkalic transitions and waning magma production, and this Kahoolawe example is the first to document such mixing in Hawaiian reservoirs.

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Die Modifikation alkalischer Magmen durch die Inkorporation tholeiitischer Komponenten: Komplexe Hybridisierung auf der Insel Kahoolawe, Hawaii

Zusammenfassung Die Insel Kahoolawe ist ein 1.4 bis 1 Ma alter Schildvulkan, der sich aus tholeiitischen Schild-, Kaldera- und Post-Schildlaven, sowie aus alkalibasaltischen und hawaiitischen Post-Schildlaven zusammensetzt. Ein Post-Schildschlot förderte alkalische Laven (K₂O 1.1–1.6 Gew.%) mit resorbierten Olivinen, die eine breit gestreute Zusammensetzung zeigen (Kerne, Fo_86–71), resorbiertem Na-Plagioklas (An_57–30; großteils Andesin) und entwickeltem Klinopyroxen (Mg# 76–71), sowie Orthopyroxen in der Grundmasse (Mg# 62). Sie führen auch tholeiitische Gabbro-Xenolithe, die als Suite kontinuierliche Mineralzusammensetzungen zeigen — Klinopyroxen Mg# 83–74, Orthopyroxen Mg# 83–76 und Plagioklas An_69–35 (z.B. inklusive Andesin-Gabbro). Die Lavenzusammensetzungen folgen wegen einer Mg-Anreicherung (z.B. CaO7 Gew.% @ MgO6 Gew.%) nicht dem für Hawaii erwarteten Fraktionierungstrend. Diese Ansammlung von Mineral- und Gesteinskomponenten in alkalischen Laven mit scheinbarer Mg-Anreicherung geht auf eine komplexe Entwicklungsgeschichte zurück, die mit einer länger andauern Mischung von primitiven und differentierten tholeiitischen Magmen, wahrscheinlich im Endstadium der Schildbildung, begann. Diese hybriden Magmen kristallisierten Olivin variabler Zusammensetzung, der im Zuge der Reservoir-Auffüllung resorbiert wurde und kristallisierte an den Reservoir-Wändenin situ als Orthopyroxen-führende Gabbros. Als die Magmenproduktionsraten im Übergang vom tholeiitischen Schild- zum alkalischen Post-Schild-Magmatismus geringer wurden, fraktionierten die tholeiitischen Hybride in den Reservoiren und führten zur Bildung von kochentwickelten Phasen, wie Andesin und Klinopyroxen mit Mg# < 75. Die Zufuhr von Post-Schildmagmen in die Reservoire verursachte eine alkalisch-tholeiitische Hybridisierung. Die resultierende Mischung von Hawaiit-Tholeiit-Hybriden resorbierte Andesin- und Klinopyroxen-Kristalle und verfrachtete die Gabbro-Xenolithe bei der Eruption. Massenbilanzen weisen darauf hin, daß die alkalisch-tholeiitische Hybridisierung 44% Hawait, gemischt mit etwa dem gleichen Anteil an tholeiitischem Hybrid (MgO29.5 Gew.%) plus Olivin und Andesin, beinhaltete. Diese Art komplexer Hybridisierung ist ein logischer Prozeß von Magmatismus mit tholeiitisch-alkalischem Übergangschemismus und ausklingender Magmenproduktion und dieses Beispiel von Kahoolawe ist das erste, das derartige Mischungsprozesse für Reservoire auf Hawaii dokumentiert.

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With 8 Figures     

The Rwenzori Mountains,a landslide-prone region?

With its exceptionally steep topography, wet climate, and active faulting, landslides can be expected to occur in the Rwenzori Mountains. Whether or not this region is prone to landsliding and more generally whether global landslide inventories and hazard assessments are accurate in data-poor regions such as the East African highlands are thus far unclear. In order to address these questions, a first landslide inventory based on archive information is built for the Rwenzori Mountains. In total, 48 landslide and flash flood events, or combinations of these, are found. They caused 56 fatalities and considerable damage to road infrastructure, buildings, and cropland, and rendered over 14,000 persons homeless. These numbers indicate that the Rwenzori Mountains are landslide-prone and that the impact of these events is significant. Although not based on field investigations but on archive data from media reports and laymen accounts, our approach provides a useful complement to global inventories overlooking this region and increases our understanding of the phenomenon in the Rwenzori Mountains. Considering the severe impacts of landslides, the population growth and related anthropogenic interventions, and the likelihood of more intense rainfall conditions, there is an urgent need to invest in research on disaster risk reduction strategies in this region and other similar highland areas of Africa.     

Engaging communities in climate adaptation: the potential of social networks

There has been a growing recognition regarding the use of social networks to engage communities in government actions. However, despite increasing awareness of social networks, there is very limited evidence for their application in relation to climate policy. This study fills this gap by assessing the potential of social networks for engaging local communities in climate adaptation policy, drawing on a case study of the Shoalhaven region in Australia. Participants from key representative groups were recruited using a purposive snowball sampling technique (N?=?24). By mapping knowledge acquisition and diffusion networks in relation to climate adaption at the local scale, this study identified key nodes within the networks. Findings demonstrate that although climate adaptation information was acquired from a diverse range of sources, the sharing knowledge networks were far more dispersed. Furthermore, although 165 knowledge sources were identified, three nodes had coverage cross the entire network, and as such acted as boundary spanners within the sharing network. This research demonstrates the utility of social network analysis to reveal the underlying knowledge networks and structures that influence community engagement pathways and in doing so outlines key implications in relation to engaging local communities in climate policy and action.

Policy relevance

The rapid development of adaptation as a mainstream strategy for managing the risks of climate change has resulted in the emergence of a broad range of adaptation policies and management strategies globally. However, the success of these initiatives is largely dependent on their acceptance and uptake by local communities, which to date remains a significant challenge. Accordingly, policy makers require novel approaches to overcome barriers to community engagement so as to enhance the likely success of community engagement pathways. This article demonstrates the value of using social network analysis to reveal the underlying knowledge network structures. This approach makes it possible to identify key individuals within a community who can disseminate adaptation information quickly across broad geographic ranges. By utilizing this approach, policy makers globally will be able to increase the extent to which adaption initiatives are accepted and adhered to by local communities, thus increasing their success.     

Formation of submarine canyons on the flanks of the Canary Islands

Detailed morphological data collected from the submarine flanks of the Canary Islands have revealed numerous submarine canyons down to water depths of >3,000 m. These canyons are interpreted to have formed by submarine erosion. We postulate formation of proto-canyons by downslope-eroding mass flows which originate on land, enter the sea, and continue below sea level for several tens of kilometers. Once proto-canyons have been formed, they become deepened by further erosion and failures of the canyon walls and/or floor. Large amounts of sediments, funnelled through the canyons from the islands into the adjacent deep-ocean sedimentary basins, play an important role in the evolution of volcanic aprons surrounding ocean islands. Some major canyon systems appear to have persisted for at least 14 million years.     

Winds and Waves in the Yellow and East China Seas: A Comparison of Spaceborne Altimeter Measurements and Model Results

Wind speed and wave height measured by satellite altimeters represent a good data source to the study of global and regional wind and wave conditions. In this paper, the TOPEX altimeter wind and wave measurements in the Yellow and East China Seas are analyzed. The results provide a glimpse on the statistical properties and the spatial distributions of the regional wind and wave conditions. These data are excellent for use in the validation and verification of numerical simulations on global and regional scales. The altimeter measurements are compared with model output of temporal statistics and spatial distributions. The results show that the model simulations are in good agreement with TOPEX measurements in terms of the local mean and standard deviation of the variables (wave height and wind speed). For the comparison of spatial distributions, the quality of agreement between numerical simulations and altimeter measurements varies significantly from cycle to cycle of altimeter passes. In many cases, trends in the spatial distributions of wave heights and wind speeds between simulations and measurements are opposite. The statistics of biases, rms differences, linear regression coefficients and correlation coefficients are presented. A rather large percentage (∼50%) of cases show poor agreement based on a combination of low correlation, large rms difference or bias, and poor regression coefficient. There are indications that wave age is a factor affecting the performance of wave modeling skills. Generally speaking, the error statistics in the wave field is correlated to the corresponding error statistics in the wind field under the condition of active wind-wave generation. The error statistics between the wave field and the wind field become less correlated for large wave ages. This revised version was published online in August 2006 with corrections to the Cover Date.