Distribution and sedimentary characteristics of tsunami deposits along the Cascadia margin of western North America

Tsunami deposits have been found at more than 60 sites along the Cascadia margin of Western North America, and here we review and synthesize their distribution and sedimentary characteristics based on the published record. Cascadia tsunami deposits are best preserved, and most easily identified, in low-energy coastal environments such as tidal marshes, back-barrier marshes and coastal lakes where they occur as anomalous layers of sand within peat and mud. They extend up to a kilometer inland in open coastal settings and several kilometers up river valleys. They are distinguished from other sediments by a combination of sedimentary character and stratigraphic context. Recurrence intervals range from 300–1000 years with an average of 500–600 years. The tsunami deposits have been used to help evaluate and mitigate tsunami hazards in Cascadia. They show that the Cascadia subduction zone is prone to great earthquakes that generate large tsunamis. The inclusion of tsunami deposits on inundation maps, used in conjunction with results from inundation models, allows a more accurate assessment of areas subject to tsunami inundation. The application of sediment transport models can help estimate tsunami flow velocity and wave height, parameters which are necessary to help establish evacuation routes and plan development in tsunami prone areas.     

Reconstructing 4000 years of mass movement and tsunami history in a deep peri‐Alpine lake (Lake Geneva,France‐Switzerland)

The study of mass movements in lake sediments provides insights into past natural hazards at historic and prehistoric timescales. Sediments from the deep basin of Lake Geneva reveal a succession of six large‐scale (volumes of 22 × 10⁶ to 250 × 10⁶ m³) mass‐transport deposits, associated with five mass‐movement events within 2600 years (4000 cal bp to 563 ad ). The mass‐transport deposits result from: (i) lateral slope failures (mass‐transport deposit B at 3895 ± 225 cal bp and mass‐transport deposits A and C at 3683 ± 128 cal bp ); and (ii) Rhône delta collapses (mass‐transport deposits D to G dated at 2650 ± 150 cal bp , 2185 ± 85 cal bp , 1920 ± 120 cal bp and 563 ad , respectively). Mass‐transport deposits A and C were most probably triggered by an earthquake, whereas the Rhône delta collapses were likely to be due to sediment overload with a rockfall as the external trigger (mass‐transport deposit G, the Tauredunum event in 563 ad known from historical records), an earthquake (mass‐transport deposit E) or unknown external triggers (mass‐transport deposits D and F). Independent of their origin and trigger mechanisms, numerical simulations show that all of these recorded mass‐transport deposits are large enough to have generated at least metre‐scale tsunamis during mass movement initiation. Since the Tauredunum event in 563 ad , two small‐scale (volumes of 1 to 2 × 10⁶ m³) mass‐transport deposits (H and I) are present in the seismic record, both of which are associated with small lateral slope failures. Mass‐transport deposits H and I might be related to earthquakes in Lausanne/Geneva (possibly) 1322 ad and Aigle 1584 ad , respectively. The sedimentary record of the deep basin of Lake Geneva, in combination with the historical record, show that during the past 3695 years, at least six tsunamis were generated by mass movements, indicating that the tsunami hazard in the Lake Geneva region should not be neglected, although such events are not frequent with a recurrence time of 0·0016 yr^?1.     

Influence of the mechanical behaviour of brittle–ductile fold–thrust belts on the development of foreland basins

A two‐dimensional mathematical model considering coupling between a deforming elasto‐visco‐plastic fold–thrust belt, flexural subsidence and diffusional surface processes is solved using the Finite Element Method to investigate how the mechanical behaviour of brittle–ductile wedges influences the development of foreland basins. Results show that, depending mainly on the strength of the basal décollement, two end‐member types of foreland basin are possible. When the basal detachment is relatively strong, the foreland basin system is characterised by: (1) Highly asymmetrical orogen formed by thrusts concentrated in the incoming pro‐wedge. (2) Sedimentation on retro‐side takes place in one major foredeep basin which grows throughout orogen evolution. (3) Deposition on the pro‐side occurs initially in the foredeep, and continues in the wedge‐top before isolated basins are advected towards the orogen core where they become uplifted and exhumed. (4) Most pro‐wedge basins show an upward progression from low altitude, foredeep deposits at the base to high altitude, wedge‐top deposits near the surface. In contrast, when the basal detachment behaves weakly due to the presence of low viscosity material such as salt, the foreland basin system is characterised by (1) Broad, low relief orogen showing little preferential vergence and predominance of folding relative to faulting. (2) Deposition mainly in wedge‐top basins showing growth strata. (3) Many basins are initiated contemporaneously but form discontinuously due to the locus of active deformation jumping back and forth between different structures. Model results successfully reproduce first order observations of deforming brittle–ductile wedges and foreland basins. Moreover, the results support and provide a framework for understanding the existence of two main end‐member foreland basin types, simple and complex, associated with fold–thrust belts whose detachments are relatively strong and weak, respectively.     

Microborer ichnocoenoses in Quaternary corals from New Caledonia: reconstructions of paleo-water depths and reef growth strategies in relation to environmental changes

Coral reef growth and development depend on several environmental factors, including tectonic and climatic parameters and local ecological drivers. Reef growth is especially sensitive to sea-level variations. Paleo-water depth reconstructions are essential tools used to determine reef growth patterns during different periods of reef growth. Assemblages of corals and/or coralline algae have been commonly used in such paleodepth reconstructions. This study shows that using microendolith ichnocoenoses can sometimes provide better accuracy than traditional coralgal analyses, particularly in the depth-range 0–10 m where coralgal assemblages usually show broad distribution ranges. Holocene and Pleistocene cores from two barrier reef sites on the west coast of Grande Terre in New Caledonia are examined here. Holocene reef development at these sites feature examples of microendolith ichnocoenoses that document rapid environmental changes and small sea-level variations of about 2–5 m in amplitude, and record these changes with more accuracy than coral and coralline algae assemblages which are highly dependant on the hydrodynamic energy of the setting. During the Pleistocene, which was less chronologically constrained, the microendolith ichnocoenoses also reflect paleo-water depths and reef-growth patterns at different periods of reef history.     

Denudation rates on cratonic landscapes: comparison between suspended and dissolved fluxes,and 10Be analysis in the Nyong and Sanaga River basins,south Cameroon

South Cameroon is located in a tropical and tectonically quiescent region, with landscapes characterized by thick highly weathered regolith, indicative of the long‐term predominance of chemical weathering over erosion. Currently this region undergoes huge changes due to accelerated mutations related to a growing population and economical developments with associated needs and increasing pressures on land and natural resources. We analysed two of the main south Cameroon rivers: the Nyong River and Sanaga River. The Sanaga catchment undergoes a contrasted tropical climate from sub‐humid mountainous and humid climate and is impacted by deforestation, agriculture, damming, mining and urbanization, especially in the Mbam sub‐basin, draining the highly populated volcanic highlands. By contrast, the Nyong catchment, only under humid tropical climate, is preserved from anthropogenic disturbance with low population except in the region of Yaoundé (Méfou sub‐basin). Moreover the Nyong basin is dam‐free and less impacted by agriculture and logging. We explore both denudation temporal variability and the ratio between chemical and physical denudation through two catchment‐averaged erosion and denudation datasets. The first one consists of an 11‐year long gauging dataset, while the second one comes from cosmogenic radionuclides [CRNs, here beryllium‐10 (¹⁰Be)] from sand sampled in the river mainstreams (timescale of tens to hundreds of thousands of years). Modern fluxes estimated from gauging data range from 5 to 100 m/Ma (10 to 200 t/km²/yr); our calculations indicate that the usual relative contribution of chemical versus physical denudation is 60% and 40%, respectively, of the total denudation. Beryllium‐10 denudation rates and sediment fluxes range from 4.8 to 40.3 m/Ma or 13 to 109 t/km²/yr, respectively, after correction for quartz enrichment. These fluxes are slightly less than the modern fluxes observed in Cameroon and other stable tropical areas. The highest ¹⁰Be‐derived fluxes and the highest physical versus chemical denudation ratios are attributed to anthropogenic impact. Copyright © 2016 John Wiley & Sons, Ltd.     

Global sea-level rise and its relation to the terrestrial reference frame

We examined the sensitivity of estimates of global sea-level rise obtained from GPS-corrected long term tide gauge records to uncertainties in the International Terrestrial Reference Frame (ITRF) realization. A useful transfer function was established, linking potential errors in the reference frame datum (origin and scale) to resulting errors in the estimate of global sea level rise. Contrary to scale errors that are propagated by a factor of 100%, the impact of errors in the origin depends on the network geometry. The geometry of the network analyzed here resulted in an error propagation factor of 50% for the Z component of the origin, mainly due to the asymmetry in the distribution of the stations between hemispheres. This factor decreased from 50% to less than 10% as the geometry of the network improved using realistic potential stations that did not yet meet the selection criteria (e.g., record length, data availability). Conversely, we explored new constraints on the reference frame by considering forward calculations involving tide gauge records. A reference frame could be found in which the scatter of the regional sea-level rates was limited. The resulting reference frame drifted by 1.36 ± 0.22? mm/year from the ITRF2000 origin in the Z component and by ?0.44 ± 0.22?mm/year from the ITRF2005 origin. A bound on the rate of global sea level rise of 1.2 to 1.6?mm/year was derived for the past century, depending on the origin of the adopted reference frame. The upper bound is slightly lower than previous estimates of 1.8?mm/year discussed in the IPCC fourth report.     

A New Interlaboratory Characterisation of Silicon,Rare Earth Elements and Twenty‐Two Other Trace Element Concentrations in the Natural River Water Certified Reference Material SLRS‐6 (NRC‐CNRC)

The natural river water reference material SLRS‐6 (NRC‐CNRC) is the newest batch of a quality control material routinely used in many international environmental laboratories. This work presents a nine‐laboratory compilation of measurements of major and trace element concentrations and their related uncertainties, unavailable in the NRC‐CNRC certificate (B, Cs, Li, Ga, Ge, Hf, Nb, P, Rb, Rh, Re, S, Sc, Se, Si, Sn, Th, Ti, Tl, W, Y, Y, Zr and REEs). Measurements were mostly made using inductively coupled plasma‐mass spectrometry. The results are compared with equivalent data for the last batch of the material, SLRS‐5, measured simultaneously with SLRS‐6 in this study. In general, very low concentrations, close to the quantification limits, were found in the new batch. The Sr isotopic ratio is also reported.