Exploring Geological and Topographical Controls on Low Flows with Hydrogeological Models     

Claire Carlier  Stefanie B. Wirth  Fabien Cochand  Daniel Hunkeler  Philip Brunner 《Ground water》2019,57(1):48-62

This study investigates how catchment properties influence low-flow dynamics. With 496 synthetic models composed of a bedrock and an alluvial aquifer, we systematically assess the impact of the hydraulic conductivity of both lithologies, of the hillslope and of the river slope on catchment dynamics. The physically based hydrogeological simulator HydroGeoSphere is employed, which allows obtaining a range of low-flow indicators. The hydraulic conductivity of the bedrock K_bedrock, a proxy for transmissivity, is the only catchment property exerting an overall control on low flows and explains 60% of the variance of Q95/Q50. The difference in dynamics of catchments with same K_bedrock depends on hillslope gradients and the alluvial aquifer properties. The buffering capacity of the bedrock is mainly related to K_bedrock and the hillslope gradient. We thus propose the dimensionless bedrock productivity index (BPI) that combines these characteristics with the mean net precipitation. For bedrock only models, the BPI explains 82% of the variance of the ratio of Q95 to mean net precipitation. The alluvial aquifer can significantly influence low flows when the bedrock productivity is limited. Although our synthetic catchment setup is simple, it is far more complex than the available analytical approaches or conceptual hydrological models. The direct application of the results to existing catchments requires nevertheless careful consideration of the local geological topographic and climatic conditions. This study provides quantitative insight into the complex interrelations between geology, topography and low-flow dynamics and challenges previous studies which neglect or oversimplify geological characteristics in the assessment of low flows.  相似文献   

Origin and evolution of the atmospheres of early Venus,Earth and Mars     

Helmut Lammer  Aubrey L. Zerkle  Stefanie Gebauer  Nicola Tosi  Lena Noack  Manuel Scherf  Elke Pilat-Lohinger  Manuel Güdel  John Lee Grenfell  Mareike Godolt  Athanasia Nikolaou 《Astronomy and Astrophysics Review》2018,26(1):2

We review the origin and evolution of the atmospheres of Earth, Venus and Mars from the time when their accreting bodies were released from the protoplanetary disk a few million years after the origin of the Sun. If the accreting planetary cores reached masses \(\ge 0.5 M_\mathrm{Earth}\) before the gas in the disk disappeared, primordial atmospheres consisting mainly of H\(_2\) form around the young planetary body, contrary to late-stage planet formation, where terrestrial planets accrete material after the nebula phase of the disk. The differences between these two scenarios are explored by investigating non-radiogenic atmospheric noble gas isotope anomalies observed on the three terrestrial planets. The role of the young Sun’s more efficient EUV radiation and of the plasma environment into the escape of early atmospheres is also addressed. We discuss the catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans and we describe the geochemical evidence for additional delivery of volatile-rich chondritic materials during the main stages of terrestrial planet formation. The evolution scenario of early Earth is then compared with the atmospheric evolution of planets where no active plate tectonics emerged like on Venus and Mars. We look at the diversity between early Earth, Venus and Mars, which is found to be related to their differing geochemical, geodynamical and geophysical conditions, including plate tectonics, crust and mantle oxidation processes and their involvement in degassing processes of secondary \(\hbox {N}_2\) atmospheres. The buildup of atmospheric \(\hbox {N}_2\), \(\hbox {O}_2\), and the role of greenhouse gases such as \(\hbox {CO}_2\) and \(\hbox {CH}_4\) to counter the Faint Young Sun Paradox (FYSP), when the earliest life forms on Earth originated until the Great Oxidation Event \(\approx \) 2.3 Gyr ago, are addressed. This review concludes with a discussion on the implications of understanding Earth’s geophysical and related atmospheric evolution in relation to the discovery of potential habitable terrestrial exoplanets.  相似文献   

Resurrecting beach ridge archaeology: Parallel depositional records from St. Lawrence Island and Cape Krusenstern,Western Alaska     

Owen K. Mason  Stefanie L. Ludwig 《Geoarchaeology》1990,5(4):349-373

Archaeological sites on gravel beach ridge plains offer a treacherously facile method of reconstructing cultural chronology based on the assumption that settlements were preferentially situated nearest the sea. the initial phase of beach ridge methodology in Alaska dates from its 1930s use by Henry Collins at St. Lawrence Island and its 1950s use by Louis Giddings in Kotzebue Sound. Numerous questions of cultural and depositional chronology remain unresolved. At Gambell, on St. Lawrence Island, three sets of ridges span the period since about 2000 B.P., with a prominent disconformity after Punuk culture times at ca. 1100 B.P. Reviewing the ¹⁴C dates (n = 83) we find that the Gambell sequence (n = 50) broadly parallels that of Kotzebue Sound, especially in the similar erosional disconformity at 1200-1000 B.P., related to increased storminess in the North Pacific. the Cape Krusenstern sequence is only loosely constrained by ¹⁴C dates (n = 33) disproportionately concentrated on 7 of the 114 ridge fragments. the dating of early Choris culture is especially problematic, which seems to occur both before and after the Old Whaling culture, well-dated at 2900-2800 B.P. on the 53rd ridge. However, reanalyzing the depositional sequence, we find that some of the more easterly Choris ridges probably represent erosional events after the Old Whaling occupation.  相似文献