La fréquence des orages dans les Alpes de la Suisse Occidentale

Résumé Des observations inédites permettent d'établir un nouveau profil des fréquences orageuses dans les Alpes occidentales de la Suisse. Le nombre de jours d'orage, notablement plus élevé qu'on ne l'admettait jusqu'ici, décroit de l'extérieur vers l'intérieur du massif alpin et jusque dans sa chaine la plus méridionale. Quelques données numériques documentent la variation diurne et annuelle de l'activité orageuse en Valais.

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Summary Unpublished observations allow the etablishment of a new cross section of the thunderstorm frequency in the western Alps of Switzerland. The number of thunderstorm days is much more frequent than has yet been admitted, diminishing from the outer towards the inner region of the alpine massive as far as the extreme southern chain. Some statistics record the diurnal and annual variations of the activity of thunderstorms in Valais.

     

Über die Indikatorgleichung eines Seismometers

Summary Based on the recordings of the free oscillations of a seismometer, an approximate method is developed for calculating the non-linear terms of the corresponding indicator equation. An equivalent linear indicator equation is calculated with the results.     

Cretaceous continental margin evolution revealed using quantitative seismic geomorphology,offshore northwest Africa

The application of high‐resolution seismic geomorphology, integrated with lithological data from the continental margin offshore The Gambia, northwest Africa, documents a complex tectono‐stratigraphic history through the Cretaceous. This reveals the spatial‐temporal evolution of submarine canyons by quantifying the related basin depositional elements and providing an estimate of intra‐ versus extra‐basinal sediment budget. The margin developed from the Jurassic to Aptian as a carbonate escarpment. Followed by, an Albian‐aged wave‐dominated delta system that prograded to the palaeo‐shelf edge. This is the first major delivery of siliciclastic sediment into the basin during the evolution of the continental margin, with increased sediment input linked to exhumation events of the hinterland. Subaqueous channel systems (up to 320 m wide) meandered through the pro‐delta region reaching the palaeo‐shelf edge, where it is postulated they initiated early submarine canyonisation of the margin. The canyonisation was long‐lived (ca. 28 Myr) dissecting the inherited seascape topography. Thirteen submarine canyons can be mapped, associated with a Late Cretaceous‐aged regional composite unconformity (RCU), classified as shelf incised or slope confined. Major knickpoints within the canyons and the sharp inflection point along the margin are controlled by the lithological contrast between carbonate and siliciclastic subcrop lithologies. Analysis of the base‐of‐slope deposits at the terminus of the canyons identifies two end‐member lobe styles, debris‐rich and debris‐poor, reflecting the amount of carbonate detritus eroded and redeposited from the escarpment margin (blocks up to ca. 1 km³). The vast majority of canyon‐derived sediment (97%) in the base‐of‐slope is interpreted as locally derived intra‐basinal material. The average volume of sediment bypassed through shelf‐incised canyons is an order of magnitude higher than the slope‐confined systems. These results document a complex mixed‐margin evolution, with seascape evolution, sedimentation style and volume controlled by shelf‐margin collapse, far‐field tectonic activity and the effects of hinterland rejuvenation of the siliciclastic source.     

High-resolution IP<Subscript>25</Subscript>-based reconstruction of sea-ice variability in the western North Pacific and Bering Sea during the past 18,000 years

Due to its strong influence on heat and moisture exchange between the ocean and the atmosphere, sea ice is an essential component of the global climate system. In the context of its alarming decrease in terms of concentration, thickness and duration, understanding the processes controlling sea-ice variability and reconstructing paleo-sea-ice extent in polar regions have become of great interest for the scientific community. In this study, for the first time, IP₂₅, a recently developed biomarker sea-ice proxy, was used for a high-resolution reconstruction of the sea-ice extent and its variability in the western North Pacific and western Bering Sea during the past 18,000 years. To identify mechanisms controlling the sea-ice variability, IP₂₅ data were associated with published sea-surface temperature as well as diatom and biogenic opal data. The results indicate that a seasonal sea-ice cover existed during cold periods (Heinrich Stadial 1 and Younger Dryas), whereas during warmer intervals (Bølling-Allerød and Holocene) reduced sea ice or ice-free conditions prevailed in the study area. The variability in sea-ice extent seems to be linked to climate anomalies and sea-level changes controlling the oceanographic circulation between the subarctic Pacific and the Bering Sea, especially the Alaskan Stream injection though the Aleutian passes.     

Die U.S. Geological Survey in Zahlen

Ohne Zusammenfassung     

A combined EOF/variational approach for mapping radar-derived sea surface currents

A technique for reconstruction of the 2d surface velocity field from radar observations is proposed. The method consecutively employs two processing techniques: At the first stage raw radial velocity data are subject to EOF analysis, which enables to fill gaps in observations and provides estimates of the noise level and integral parameters characterizing small-scale variability of the sea surface circulation. These parameters are utilized at the second stage, when the cost function for variational interpolation is constructed, and the updated radial velocities are interpolated on the regular grid.Experiments with simulated and real data are used to assess the method's skill and compare it with the conventional 2d variational (2dVar) approach. It is shown that the proposed technique consistently improves performance of the 2dVar algorithm and becomes particularly effective when a radar stops operating for 1–2 days and/or a persistent gap emerges in spatial coverage of a basin by the HFR network.     

Seasonal and Annual Fluxes of Nutrients and Organic Matter from Large Rivers to the Arctic Ocean and Surrounding Seas

River inputs of nutrients and organic matter impact the biogeochemistry of arctic estuaries and the Arctic Ocean as a whole, yet there is considerable uncertainty about the magnitude of fluvial fluxes at the pan-Arctic scale. Samples from the six largest arctic rivers, with a combined watershed area of 11.3?×?10⁶?km², have revealed strong seasonal variations in constituent concentrations and fluxes within rivers as well as large differences among the rivers. Specifically, we investigate fluxes of dissolved organic carbon, dissolved organic nitrogen, total dissolved phosphorus, dissolved inorganic nitrogen, nitrate, and silica. This is the first time that seasonal and annual constituent fluxes have been determined using consistent sampling and analytical methods at the pan-Arctic scale and consequently provide the best available estimates for constituent flux from land to the Arctic Ocean and surrounding seas. Given the large inputs of river water to the relatively small Arctic Ocean and the dramatic impacts that climate change is having in the Arctic, it is particularly urgent that we establish the contemporary river fluxes so that we will be able to detect future changes and evaluate the impact of the changes on the biogeochemistry of the receiving coastal and ocean systems.