Stratigraphy of the Cenomanian–Turonian Oceanic Anoxic Event OAE2 in shallow shelf sequences of NE Egypt     

Ahmed El-Sabbagh  Abdel Aziz Tantawy  Gerta Keller  Hassan Khozyem  Jorge Spangenberg  Thierry Adatte  Brian Gertsch 《Cretaceous Research》2011,32(6):705-722

Two shallow water late Cenomanian to early Turonian sequences of NE Egypt have been investigated to evaluate the response to OAE2. Age control based on calcareous nannoplankton, planktic foraminifera and ammonite biostratigraphies integrated with δ¹³C stratigraphy is relatively good despite low diversity and sporadic occurrences. Planktic and benthic foraminiferal faunas are characterized by dysoxic, brackish and mesotrophic conditions, as indicated by low species diversity, low oxygen and low salinity tolerant planktic and benthic species, along with oyster-rich limestone layers. In these subtidal to inner neritic environments the OAE2 δ¹³C excursion appears comparable and coeval to that of open marine environments. However, in contrast to open marine environments where anoxic conditions begin after the first δ¹³C peak and end at or near the Cenomanian–Turonian boundary, in shallow coastal environments anoxic conditions do not appear until the early Turonian. This delay in anoxia appears to be related to the sea-level transgression that reached its maximum in the early Turonian, as observed in shallow water sections from Egypt to Morocco.  相似文献   

Anaerobic Metabolism in Tidal Freshwater Wetlands: I. Plant Removal Effects on Iron Reduction and Methanogenesis     

Jason K. Keller  Ariana E. Sutton-Grier  Allyson L. Bullock  J. Patrick Megonigal 《Estuaries and Coasts》2013,36(3):457-470

For energetic reasons, iron reduction suppresses methanogenesis in tidal freshwater wetlands; however, when iron reduction is limited by iron oxide availability, methanogenesis dominates anaerobic carbon mineralization. Plants can mediate this microbial competition by releasing oxygen into the rhizosphere and supplying oxidized iron for iron reducers. We utilized a plant removal experiment in two wetland sites to test the hypothesis that, in the absence of plants, rates of iron reduction would be diminished, allowing methanogenesis to dominate anaerobic metabolism. In both sites, methanogenesis was the primary anaerobic mineralization pathway, with iron reduction dominating only early and late in the growing season in the site with a less organic soil. These patterns were not influenced by the presence of plants, demonstrating that plants were not a key control of microbial metabolism. Instead, we suggest that site conditions, including soil chemistry, and temperature are important controls of the pathways of anaerobic metabolism.  相似文献   

A User Interface for the Kansas Geological Survey Slug Test Model     

by Steven P. Esling  John E. Keller 《Ground water》2009,47(4):587-590

The Kansas Geological Survey (KGS) developed a semianalytical solution for slug tests that incorporates the effects of partial penetration, anisotropy, and the presence of variable conductivity well skins. The solution can simulate either confined or unconfined conditions. The original model, written in FORTRAN, has a text-based interface with rigid input requirements and limited output options. We re-created the main routine for the KGS model as a Visual Basic macro that runs in most versions of Microsoft Excel and built a simple-to-use Excel spreadsheet interface that automatically displays the graphical results of the test. A comparison of the output from the original FORTRAN code to that of the new Excel spreadsheet version for three cases produced identical results.  相似文献   

Towards a detailed distal tephrostratigraphy in the Central Mediterranean: The last 20,000 yrs record of Lago Grande di Monticchio     

Sabine Wulf  Michael Kraml  Jörg Keller 《Journal of Volcanology and Geothermal Research》2008

A detailed compilation of distal tephrostratigraphy comprising the last 20,000 yrs is given for the Central Mediterranean region. A total of 47 distinct ash layers identified in the maar lake sediments of Lago Grande di Monticchio (Basilicata, Southern Italy) are compared with proximal and distal terrestrial-marine tephra deposits in the circum-central Mediterranean region. The results of these studies provide valuable information for reconstructing the Late Pleistocene and the Holocene dispersal of pyroclastic deposits from south Italian explosive volcanoes, in particular Somma-Vesuvius, the Campi Flegrei caldera, Ischia Island and Mount Etna. Prominent tephras are discussed with respect to their reliability as dating and correlation tools in sedimentary records. Ashes from Plinian eruptions of Somma-Vesuvius (i.e. Avellino, Mercato, Greenish, Pomici di Base), for instance, are well-defined by their distribution patterns and their unique composition. The widespread Y-1 tephra from Mount Etna, on the other hand, derived most likely from two distinct Plinian events with changing wind conditions, and therefore becomes a less reliable stratigraphic marker. Statistical–numerical calculations are presented in order to discriminate between Holocene tephras from the Campi Flegrei caldera (i.e. Astroni 1–3, Agnano Monte Spina, Averno 1, Lagno Amendolare), since these ashes are characterized by an almost indistinguishable chemical fingerprint. As a highlight, numerous Campanian eruptions of proposed low-intensity have been identified in the distal site of Monticchio suggesting a revision of existing tephra dispersal maps and re-calculation of eruptive conditions. In summary, the tephra record of Monticchio is one of the key successions for linking both, terrestrial records from Central-southern Italy and marine sequences from the Tyrrhenian, Adriatic and Ionian Seas.  相似文献   

Preserving the Ocean Circulation: Implications for Climate Policy   总被引：3，自引：2，他引：1  

Klaus Keller  Kelvin Tan  François M. M. Morel  David F. Bradford 《Climatic change》2000,47(1-2):17-43

Climate modelers have recognized the possibility of abrupt climate changes caused by a reorganization of the North Atlantic's current pattern (technically known as a thermohaline circulation collapse). This circulation system now warms north-western Europe and transports carbon dioxide to the deep oceans. The posited collapse of this system could produce severe cooling in northwestern Europe, even when general global warming is in progress. In this paper we use a simple integrated assessment model to investigate the optimal policy response to this risk. Adding the constraint of avoiding a thermohaline circulation collapse would significantly reduce the allowable greenhouse gas emissions in the long run along an optimal path. Our analysis implies that relatively small damages associated with a collapse (less than 1% of gross world product) would justify a considerable reduction of future carbon dioxide emissions.  相似文献   

Iridium and the K/T boundary at El Caribe, Guatemala     

G. Keller  W. Stinnesbeck 《International Journal of Earth Sciences》2000,88(4):840-843

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About the correlation between the distribution of the azooxantellate scleractinian corals and the abundance of the zooplankton in the surface waters     

N. B. Keller  N. S. Oskina 《Oceanology》2009,49(6):811-823

The correlation between the azooxantellate scleractinian coral distribution and the abundance of the zooplankton in the Atlantic Ocean is discussed. The analysis is based on a dataset of 943 specimens (82 species) sampled at 87 stations that were performed during the scientific cruises of Russian research vessels in the Atlantic. The hypothesis concerning if the coral biodiversity and population density correlate with the zooplankton abundance was tested. The relationships between these parameters are contradictory. High coral biodiversity and population density were registered for the areas of low and average zooplankton abundance in the surface waters.  相似文献   

Comparison of scenario‐neutral approaches for estimation of climate change impacts on flood characteristics     

Luise Keller  Ole Rssler  Olivia Martius  Rolf Weingartner 《水文研究》2019,33(4):535-550

Scenario‐neutral assessments of climate change impact on floods analyse the sensitivity of a catchment to a range of changes in selected meteorological variables such as temperature and precipitation. The key challenges of the approach are the choice of the meteorological variables and statistics thereof and how to generate time series representing altered climatologies of the selected variables. Different methods have been proposed to achieve this, and it remains unclear if and to which extent they result in comparable flood change projections. Here, we compare projections of annual maximum floods (AMFs) derived from three different scenario‐neutral methods for a prealpine study catchment. The methods chosen use different types of meteorological data, namely, observations, regional climate model output, and weather generator data. The different time series account for projected changes in the seasonality of temperature and precipitation, in the occurrence statistics of precipitation, and of daily precipitation extremes. Resulting change in mean AMF peak magnitudes and volumes differs in sign between the methods (range of ?6% to +7% for flood peak magnitudes and ?11% to +14% for flood volumes). Moreover, variability of projected peak magnitudes and flood volumes depends on method with one approach leading to a generally larger spread. The differences between the methods vary depending on whether peak magnitudes or flood volumes are considered and different relationships between peak magnitude and volume change result. These findings can be linked to differing flood regime changes among the three approaches. The study highlights that considering selected aspects of climate change only when performing scenario‐neutral studies may lead to differing representations of flood generating processes by the approaches and thus different quantifications of flood change. As each method comes with its own strengths and weaknesses, it is recommended to combine several scenario‐neutral approaches to obtain more robust results.  相似文献   

IGU Commission International Hydrological Programme     

Prof. Dr. R. Keller  Prof. Dr. H.-E. Müller  K.-R. Nippes Dr. 《GeoJournal》1984,8(2):192-192

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Long-term (10⁵) or short-term (10³) δ¹³C excursion near the Palaeocene-Eocene transition: evidence from the Tethys     

Gangyi Lu  Gerta Keller  Thierry Adatte  Nieves Ortiz  Eustoquio Molina 《地学学报》1996,8(4):347-355

Expanded sedimentary records from the Tethys reveal unique faunal and isotopic changes across the Palaeocene-Eocene (P-E) transition. Unlike in the open oceans, the Tethys exhibits a gradual decrease of 1.5% in δ¹³C values prior to the rapid δ¹³C excursion. Associated with the 6¹³C excursion is a decrease in calcite burial, increase in detrital content and appearance of a unique opportunistic planktic foraminifera1 assemblage (e.g. compressed acarininids). The existence of a prelude decrease in δ¹³C values in the Tethys suggests that the P-E δ¹³C excursion may have occurred in two steps and over a few hundred thousand years, rather than as one step over a few thousand years as previously suggested. This slower excursion rate is readily explained by changing organic carbon weathering or burial rates and avoids the need of invoking ad hoc scenarios.  相似文献