Sediment budgeting of short-term backfilling processes: The erosional collapse of a Carolingian canal construction     

Johannes Schmidt  Lukas Werther  Johannes Rabiger-Völlmer  Franz Herzig  Birgit Schneider  Ulrike Werban  Peter Dietrich  Stefanie Berg  Sven Linzen  Peter Ettel  Christoph Zielhofer 《地球表面变化过程与地形》2020,45(14):3449-3462

Sediment budgeting concepts serve as quantification tools to decipher the erosion and accumulation processes within a catchment and help to understand these relocation processes through time. While sediment budgets are widely used in geomorphological catchment-based studies, such quantification approaches are rarely applied in geoarchaeological studies. The case of Charlemagne's summit canal (also known as Fossa Carolina) and its erosional collapse provides an example for which we can use this geomorphological concept and understand the abandonment of the Carolingian construction site. The Fossa Carolina is one of the largest hydro-engineering projects in Medieval Europe. It is situated in Southern Franconia (48.9876°N, 10.9267°E; Bavaria, southern Germany) between the Altmühl and Swabian Rezat rivers. It should have bridged the Central European watershed and connected the Rhine–Main and Danube river systems. According to our dendrochronological analyses and historical sources, the excavation and construction of the Carolingian canal took place in AD 792 and 793. Contemporary written sources describe an intense backfill of excavated sediment in autumn AD 793. This short-term erosion event has been proposed as the principal reason for the collapse and abandonment of the hydro-engineering project. We use subsurface data (drillings, archaeological excavations, and direct-push sensing) and geospatial data (a LiDAR digital terrain model (DTM), a pre-modern DTM, and a 3D model of the Fossa Carolina] for the identification and sediment budgeting of the backfills. Dendrochronological findings and radiocarbon ages of macro remains within the backfills give clear evidence for the erosional collapse of the canal project during or directly after the construction period. Moreover, our quantification approach allows the detection of the major sedimentary collapse zone. The exceedance of the manpower tipping point may have caused the abandonment of the entire construction site. The spatial distribution of the dendrochronological results indicates a north–south direction of the early medieval construction progress. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

Debris cover on thaw slumps and its insulative role in a warming climate     

S. Zwieback  J. Boike  P. Marsh  A. Berg 《地球表面变化过程与地形》2020,45(11):2631-2646

Thaw slumps in ice-rich permafrost can retreat tens of metres per summer, driven by the melt of subaerially exposed ground ice. However, some slumps retain an ice-veneering debris cover as they retreat. A quantitative understanding of the thermal regime and geomorphic evolution of debris-covered slumps in a warming climate is largely lacking. To characterize the thermal regime, we instrumented four debris-covered slumps in the Canadian Low Arctic and developed a numerical conduction-based model. The observed surface temperatures >20° C and steep thermal gradients indicate that debris insulates the ice by shifting the energy balance towards radiative and turbulent losses. After the model was calibrated and validated with field observations, it predicted sub-debris ice melt to decrease four-fold from 1.9 to 0.5 mas the thickness of the fine-grained debris quadruples from 0.1 to 0.4 m. With warming temperatures, melt is predicted to increase most rapidly, in relative terms, for thick (∼0.5–1.0 m) debris covers. The morphology and evolution of the debris-covered slumps were characterized using field and remote sensing observations, which revealed differences in association with morphology and debris composition. Two low-angle slumps retreated continually despite their persistent fine-grained debris covers. The observed elevation losses decreased from ∼1.0 m/yr where debris thickness ∼0.2 mto 0.1 m/yr where thickness ∼1.0 m. Conversely, a steep slump with a coarse-grained debris veneer underwent short-lived bursts of retreat, hinting at a complex interplay of positive and negative feedback processes. The insulative protection and behaviour of debris vary significantly with factors such as thickness, grain size and climate: debris thus exerts a fundamental, spatially variable influence on slump trajectories in a warming climate. © 2020 John Wiley & Sons, Ltd.  相似文献   

Forward: Modeling for Sustainability and Adaptation          下载免费PDF全文

Reed M. Maxwell  Alexis Navarre‐Sitchler  Matt Tonkin 《Ground water》2018,56(4):515-516

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Aragonite depositional facies in a Late Ordovician calcite sea,Eastern Laurentia     

Noel P. James  Guy M. Narbonne  Alexis K. R. Armstrong 《Sedimentology》2020,67(7):3513-3532

The Black River (Upper Ordovician – Sandbian) and Trenton (Upper Ordovician – Katian) groups are traditionally interpreted as a deepening-upward succession deposited in a progressively subsiding Appalachian Basin margin that contained warm-water, marine, photozoan deposits that pass upward into cool-water, marine, heterozoan carbonates. This succession is customarily interpreted to reflect an incursion of cold, high-latitude ocean waters into the area. This view is herein confirmed for coeval carbonates in the northern part of the basin, particularly the St. Lawrence Platform. They are now well explained in this study on the basis of recent studies of cool-water carbonates and calcite–aragonite seas. Overall the succession is one of Sandbian photozoan ramp deposits succeeded by Katian heterozoan ramp carbonates that changed back to photozoan ramp deposits prior to the Hirnantian glaciation. The current interpretation, that deposition took place throughout a calcite sea time, seems at odds with this series of strata. Instead it is herein proposed that deposition took place during an aragonite sea time wherein calcite sea-like sediments accumulated under cold ocean-water temperatures. Such an interpretation is supported by recent experimental data that supports the importance of seawater temperature on CaCO₃ polymorph precipitation. If correct, this means that some of the evidence for calcite sea deposition through time brought about by global tectonics, should be re-evaluated to make sure it was not simply cool-water carbonate production.  相似文献   

Metop-GRAS in-orbit instrument performance   总被引：2，自引：1，他引：1  

Magnus Bonnedal  Jacob Christensen  Anders Carlström  Anders Berg 《GPS Solutions》2010,14(1):109-120

The GRAS instrument on the Metop-A satellite provides more than 600 radio occultation measurement profiles per day. The instrument is characterized by its wide antenna coverage, high signal-to-noise ratio and an ultra-stable clock reference. The conventional dual-frequency tracking of GPS signals is under dynamic atmosphere conditions complemented by open loop tracking with sampling of the signal at a 1 kHz rate, providing an unprecedented view of the signal spectral environment. This paper presents the instrument performance as derived from analysis of in-orbit measurement data. We show that the noise figure is low enough to enable mapping of external radio noise variations over the earth’s surface. An error propagation model is presented to relate instrument characteristics to bending angle performance. This model is also used to illustrate the relation between filter bandwidth, resolution and measurement noise. The Doppler model, guiding open loop measurements, is found to be accurate to better than 20 Hz with a possibility for improvement to 10 Hz. The high performance at low altitudes enables the presence of surface reflections at the −20-dB level to be identified in more than 50% of the occultations. The potential performance improvements for next generation receivers are discussed.  相似文献