ALBERT

Description: The Ediacara biota has been long championed as a snapshot of the marine ecosystem on the eve of the Cambrian explosion, providing important insights into the early evolution of animals. Fossiliferous beds in the eponymous Ediacara Member of South Australia have been recently reinterpreted as paleosols and Ediacara fossils as lichens or microbial colonies that lived on terrestrial soils. This reinterpretation, here dubbed the terrestrial Ediacara hypothesis, would fundamentally change our views of biological evolution just prior to the Cambrian explosion. We take a comparative paleobiology approach to test this hypothesis. The Ediacara Member shares a number of forms with assemblages in Ediacaran marine black shales in South China, shales that show no evidence of pedogenesis. Thus, the shared Ediacara fossils, and by extension other co-occurring fossils, are unlikely to have been terrestrial organisms. A terrestrial interpretation is also inconsistent with functional morphological evidence; some of the shared forms are not morphologically adapted to address the most critical challenges for terrestrial life (e.g., mechanical support and desiccation). Thus, the terrestrial Ediacara hypothesis can be falsified on comparative paleobiological and functional morphological grounds, and we urge paleopedologists to critically reevaluate evidence for pedogenesis in the Ediacara Member and other Ediacaran successions.

Description: Two great earthquakes have occurred recently along the Sunda Trench, the 2004 M9.2 Sumatra-Andaman earthquake and the 2005 M8.7 Nias earthquake. These earthquakes ruptured over 1600 km of adjacent crust within 3 mo of each other. We quantitatively present poroelastic deformation analyses suggesting that postseismic fluid flow and recovery induced by the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake. Simple back-slip simulations indicate that the megapascal (MPa)-scale pore-pressure recovery is equivalent to 7 yr of interseismic Coulomb stress accumulation near the Nias earthquake hypocenter, implying that pore-pressure recovery of the Sumatra-Andaman earthquake advanced the timing of the Nias earthquake by [~]7 yr. That is, in the absence of postseismic pore-pressure recovery, we predict that the Nias earthquake would have occurred in 2011 instead of 2005.

Description: Recent changes in speed, thinning, and retreat rates of marine-terminating outlet glaciers have raised concerns about the future stability of the Greenland Ice Sheet. Establishing a longer term record of outlet glacier retreat rates is essential to provide a context for present-day observations and to improve and constrain numerical models of outlet glacier behavior. New exposure dating (10Be) of streamlined bedrock surfaces and glacial erratic boulders of Sermilik Fjord, southeast Greenland, the present-day drainage route of Helheim Glacier, documents rapid retreat (∼80 m a−1) of this major marine-terminating outlet glacier at the close of the last glaciation. The glacier front retreated ∼80 km to within 20 km of the present-day (2010) position of Helheim Glacier in

Description: Silicic calderas represent the surface expressions of large silicic magma bodies. Although development of sizable volumes of granite, sensu lato, is considered key in stabilizing continental crust, the factors that contribute to its origin are debated. Based on a comprehensive literature review, we analyze the characteristics, tectonic settings, and spatial distributions of 108 large ([≥]5 km diameter), Quaternary, silicic (〉63 wt% SiO2) calderas in arc settings. Generally, arcs associated with trench-normal convergence rates between 70 and 95 mm/yr are more likely to host silicic calderas, indicating that a larger flux of basalt favors formation of evolved magma, although New Zealand is a notable exception. The nature of the crust controls the types of calderas that develop; large and rhyolitic calderas tend to occur on old, thick, continental crust near preexisting structures and/or areas under local extension. In continental margins, silicic calderas are distributed more widely behind the volcanic front than are typical arc volcanoes. In addition, arcs with greater caldera densities tend to be young or have migrated, suggesting that the overlying crustal column can be depleted of a low-melting-temperature component over time. These results allow geologists studying batholiths to use silica content, size, and distribution of plutons to infer the tectonic properties of ancient arcs.

Description: Localities bearing abundant enrolled Cambrian trilobites are extremely rare, but here we report a new and exceptional such assemblage with a high diversity of enrolled specimens from the middle Cambrian of Spain. This novel find demonstrates that encapsulated enrollment was a behavioral response available to many of the early benthic trilobite morphotypes. This result provides a new perspective for reviewing the evolution of trilobite enrollment through the Paleozoic. We find a significant and consistent increase in the proportion of enrolled trilobites within assemblages bearing abundant articulated trilobites. Similarly, the mean number of coaptative devices also increased progressively during the Paleozoic. Such devices likely provided later enrolled trilobites with greater structural integrity, and may support an escalatory model of trilobite defense evolution. Likewise, environmental circumstances favoring the preservation of enrolled trilobites may have become more common during post-Cambrian time.

Description: A paucity of integrative data can lead to disparate pre-Pangean paleogeographic reconstructions, such as those for the Neoproterozoic-Cambrian paleogeography of the blocks of modern-day China. Reconstructions for the north China block, in particular, have relied on sparse paleomagnetic and biogeographic data and, as a result, have yielded discordant paleogeographic models. Here we present new detrital zircon grain age distributions from siliciclastic rocks, coupled with species-level polymerid trilobite biogeography, that suggest close ties between north China and the northeastern Indian margin during the Cambrian. In combination, these data require north China to have been in paleogeographic continuity with northern India as a part of core Gondwanaland, contrasting with the traditional view that north China was an isolated outboard terrane. The shared record of Cambrian-Ordovician tectonism in both northern India and north China likely represents the same event, which affected this region of Gondwanaland.

Description: Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km 3 of ejecta as dense-rock equivalent; recurrence interval of ~10 5 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the "AD860B" ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ~100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.

Description: We use LiDAR imagery to identify two fault scarps on latest Pleistocene glacial outwash deposits along the North Fork Nooksack River in Whatcom County, Washington (United States). Mapping and paleoseismic investigation of these previously unknown scarps provide constraints on the earthquake history and seismic hazard in the northern Puget Lowland. The Kendall scarp lies along the mapped trace of the Boulder Creek fault, a south-dipping Tertiary normal fault, and the Canyon Creek scarp lies in close proximity to the south-dipping Canyon Creek fault and the south-dipping Glacier Extensional fault. Both scarps are south-side-up, opposite the sense of displacement observed on the nearby bedrock faults. Trenches excavated across these scarps exposed folded and faulted late Quaternary glacial outwash, locally dated between ca. 12 and 13 ka, and Holocene buried soils and scarp colluvium. Reverse and oblique faulting of the soils and colluvial deposits indicates at least two late Holocene earthquakes, while folding of the glacial outwash prior to formation of the post-glacial soil suggests an earlier Holocene earthquake. Abrupt changes in bed thickness across faults in the Canyon Creek excavation suggest a lateral component of slip. Sediments in a wetland adjacent to the Kendall scarp record three pond-forming episodes during the Holocene—we infer that surface ruptures on the Boulder Creek fault during past earthquakes temporarily blocked the stream channel and created an ephemeral lake. The Boulder Creek and Canyon Creek faults formed in the early to mid-Tertiary as normal faults and likely lay dormant until reactivated as reverse faults in a new stress regime. The most recent earthquakes—each likely M w 〉 6.3 and dating to ca. 8050–7250 calendar years B.P. (cal yr B.P.), 3190–2980 cal. yr B.P., and 910–740 cal. yr B.P.—demonstrate that reverse faulting in the northern Puget Lowland poses a hazard to urban areas between Seattle (Washington) and Vancouver, British Columbia (Canada).

Description: Surface modification processes leading to large debris accumulations in high-relief mountain areas are important for understanding landscape evolution, especially in some of Earth’s most active orogens. The Arroumd rock avalanche at the foot of the NW face of Mount Aksoual (3912 m above sea level [masl]) in the Jebel Toubkal area of the High Atlas, Morocco, represents one of the largest mass movement landforms in North Africa. The age and origin of this extensive feature have been contested for over a century. Late Pleistocene moraines are also present in the same valley, adjacent to the avalanche debris. The mean of six 10 Be cosmogenic exposure ages shows that a series of catastrophic rock slope failures occurred at 4.5 ± 0.5 ka, while a set of eight exposure ages from two of the three mapped moraines has a far larger spread from 1.5 to 7.5 ka. This suggests that the avalanche events were effective agents in modifying the true surface exposure age of the Pleistocene moraines in the Arroumd valley. This has resulted in similar mean 10 Be apparent exposure ages for the preexisting Late Pleistocene moraine surfaces and Holocene catastrophic rock slope failures. Similar rock avalanche deposits are present in other glaciated valleys in the High Atlas. We conclude that the trigger for collapse was seismic activity related to proximity of the major Tizi n’Test fault. These findings have important implications for interpreting and dating glacial landforms in tectonically active settings.