Late Holocene linkages between decade–century scale climate variability and productivity at Lake Tanganyika, Africa     

Andrew S. Cohen  Kiram E. Lezzar  Julia Cole  David Dettman  Geoffrey S. Ellis  Meagan Eagle Gonneea  Pierre-Denis Plisnier  Victor Langenberg  Maarten Blaauw  Derrick Zilifi 《Journal of Paleolimnology》2006,36(2):189-209

Microlaminated sediment cores from the Kalya slope region of Lake Tanganyika provide a near-annually resolved paleoclimate record between ∼∼2,840 and 1,420 cal. yr B.P. demonstrating strong linkages between climate variability and lacustrine productivity. Laminae couplets comprise dark, terrigenous-dominated half couplets, interpreted as low density underflows deposited from riverine sources during the rainy season, alternating with light, planktonic diatomaceous ooze, with little terrigenous component, interpreted as windy/dry season deposits. Laminated portions of the studied cores consist of conspicuous dark and light colored bundles of laminae couplets. Light and dark bundles alternate at decadal time scales. Within dark bundles, both light and dark half couplets are significantly thinner than within light bundles, implying slower sediment accumulation rates during both seasons over those intervals.Time series analyses of laminae thickness patterns demonstrate significant periodicities at interannual–centennial time scales. Longer time scale periodicities (multidecadal to centennial scale) of light and dark half couplet thicknesses are coherent and in some cases are similar to solar cycle periods on these time scales. Although laminae thickness cycles do not strongly covary with the actual Δ¹⁴C record for this same time period, two large Δ¹⁴C anomalies are associated with substantial decreases in both light and dark laminae thickness. In contrast to the multidecadal– centennial time scale, significant annual to decadal periodicities, which are broadly consistent with ENSO/PDO forcing and their impact on East African climate, are not coherent between light and dark half couplets. The coherency of light–dark couplets at decadal–centennial time scales, but not at shorter time scales, is consistent with a model of a long-term relationship between precipitation (recorded in wet season dark laminae thickness) and productivity (light laminae thickness), which is not manifest at shorter time scales. We hypothesize that this coupling results from long-term recharging of internal nutrient loading during wet periods (higher erosion of soil P) and reduced loading during drought intervals. The relationship is not expressed on short time scales during which the dominant control on productivity is wind-driven, dry season upwelling, which is uncorrelated with wet-season precipitation. Our record greatly extends the temporal record of this quasi-periodic behavior throughout the late Holocene and provides the first evidence linking decade- to century-scale episodes of enhanced productivity to enhanced precipitation levels and nutrient recharge in a productive tropical lake.  相似文献   

On the 70 birthday of Wilfried Schönborn     

Günther Friedrich  Heike Zimmermann-Timm  Anita Lange  Rainer Koschel 《Limnologica》2004,34(3):173-175

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Potential impacts of geothermal energy use and storage of heat on groundwater quality,biodiversity, and ecosystem processes     

Christian Griebler  Heike Brielmann  Christina M. Haberer  Sigrid Kaschuba  Claudia Kellermann  Christine Stumpp  Florian Hegler  David Kuntz  Simone Walker-Hertkorn  Tillmann Lueders 《Environmental Earth Sciences》2016,75(20):1391

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Reflectance anisotropy and syn-deformational coalification of the Jewel seam in the Cadomin area, Alberta, Canada     

Willem Langenberg  Wolfgang Kalkreuth 《International Journal of Coal Geology》1991,19(1-4)

The Jewel seam of the Cadomin area is complexly folded and cut by several thrust faults. Vitrinite reflectance anisotropy was determined from twenty oriented coal blocks, collected from various structural positions. Five of the blocks have uniaxial anisotropies, five have biaxial positive anisotropies, the remaining ten are biaxial negative. The biaxial coals display maximum reflectance axes parallel to nearby fold axes, which suggests a relationship between vitrinite anisotropy and deformation. The biaxial vitrinite reflectance ellipsoids result from superposition of tectonic strains on a primary, sedimentary burial related, uniaxial anisotropy. A tectonic stress field was probably present during the later stages of sedimentary burial and subsequent deformation.Coalification resulted largely from pre-deformational, sedimentary burial, but the final coalification stage was syn-deformational, as illustrated by the intersection of isorank surfaces and the Jewel seam and the biaxial vitrinite reflectance anisotropy.  相似文献   

The effect of a front on the general circulation—A model study of the Rhine plume area     

Heike Langenberg  Dr. Thomas Pohlmann 《Ocean Dynamics》1994,46(4):341-353

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Direct effects of Daphnia-grazing, not infochemicals, mediate a shift towards large inedible colonies of the gelatinous green alga Sphaerocystis schroeteri     

Heike Kampe  Marie König-Rinke  Jürgen Benndorf 《Limnologica》2007,37(2):137-145

The influence of Daphnia galeata×hyalina grazing and of infochemicals released by the daphnids on the colony size and growth rate of the colonial gelatinous green alga Sphaerocystis schroeteri (Chlorococcales) was investigated in laboratory batch experiments run for 96 h. High zooplankton grazing pressure was exerted by a final concentration of 100 daphnids L⁻¹ in the Daphnia treatments. Infochemicals were obtained by filtration (0.2 μm) of water from D. galeata×hyalina cultures (200 ind. L⁻¹ exposed for 24 h). This filtrate was added to the S. schroeteri cultures in two concentrations corresponding to 7 and 50 daphnids L⁻¹, respectively. The growth rate of S. schroeteri was neither affected significantly by direct Daphnia grazing nor by the presence of Daphnia infochemicals, in comparison to the control. However, the portion of inedible S. schroeteri colonies (diameter>50 μm) increased under direct grazing pressure, whereas the Daphnia infochemicals did not influence the colony size significantly. We conclude that the shift in colony size by direct zooplankton grazing denotes an effective defence mechanism against size selective feeding for colonial gelatinous green algae. This effective defence in combination with unchanged growth rates of the larger colonies (under non-limiting nutrient and light conditions) falsifies the assumption of a trade-off between minimising grazing losses and maximising growth by optimising the colony size.  相似文献   

Seasonal dynamics of aggregates and their typical biocoenosis in the Elbe Estuary     

Heike Zimmermann-Timm  Henry Holst  Stefan Müller 《Estuaries and Coasts》1998,21(4):613-621

In 1995, an extensive investigation was carried out in the Elbe Estuary in Germany. Special attention was paid to organisms, including bacteria, amoebae, ciliates, flagellates, rotifers, larvae ofDreissena polymorpha, and nematodes, dispersed in the water column and associated firmly or loosely with different types of aggregates. The abundance, size, and colonization by microorganisms of the aggregates varied during the seasons. The largest aggregates were found during spring and summer, when diatoms, rotifers, and crustaceans were present. The most aggregates were encountered in spring and from summer to autumn. Most of the species observed during the study were more common in pelagic habitats than in benthic ones. However, the presence of ciliates in the groups Hypotrichida and Sessilida as well as as flagellates in the groups Biosoecida, Cercomonadida, Choanoflagellida, Chrysomonadida, Kinetoplastida, and amoebae and some nematodes in the open water depends, upon the availability of surfaces, because they are sessile or poor swimnters, and some flagellates and amoebae need to attach themselves to an object to feed. Most organisms were much more abundant in or on aggregates than in the surrounding water during spring and summer, which indicates that aggregates enhance the habitat and promote the development of the organisms. From spring through autumn, the structure of the community associated with the aggregates is influenced by the pelagic environment. The presence of the species in the benthic community was detected only during summer.  相似文献   

A mathematical model for orientation data from macroscopic conical folds     

D. Kelker  C. W. Langenberg 《Mathematical Geology》1982,14(4):289-307

Statistical techniques are developed to classify folds into one of three classes: cylindrical, conical, or neither. A translated version of Bingham's distribution on the sphere is applied to orientation data fron conical folds. Iterative least-squares techniques are used to determine the best-fitting small circle (or cone), and confidence intervals for the cone axis and half apical angle are developed. Examples of a cylindrical and a conical fold are given. Another fold is neither cylindrical nor conical and is classified as pseudoconical. Relationships between the Bingham and Fisher distributions are presented.  相似文献   

Current state and trends in Canadian Arctic marine ecosystems: II. Heterotrophic food web, pelagic-benthic coupling, and biodiversity     

Gérald Darnis  Dominique Robert  Corinne Pomerleau  Heike Link  Philippe Archambault  R. John Nelson  Maxime Geoffroy  Jean-éric Tremblay  Connie Lovejoy  Steve H. Ferguson  Brian P. V. Hunt  Louis Fortier 《Climatic change》2012,115(1):179-205

As part of the Canadian contribution to the International Polar Year (IPY), several major international research programs have focused on offshore arctic marine ecosystems. The general goal of these projects was to improve our understanding of how the response of arctic marine ecosystems to climate warming will alter food web structure and ecosystem services provided to Northerners. At least four key findings from these projects relating to arctic heterotrophic food web, pelagic-benthic coupling and biodiversity have emerged: (1) Contrary to a long-standing paradigm of dormant ecosystems during the long arctic winter, major food web components showed relatively high level of winter activity, well before the spring release of ice algae and subsequent phytoplankton bloom. Such phenological plasticity among key secondary producers like zooplankton may thus narrow the risks of extreme mismatch between primary production and secondary production in an increasingly variable arctic environment. (2) Tight pelagic-benthic coupling and consequent recycling of nutrients at the seafloor characterize specific regions of the Canadian Arctic, such as the North Water polynya and Lancaster Sound. The latter constitute hot spots of benthic ecosystem functioning compared to regions where zooplankton-mediated processes weaken the pelagic-benthic coupling. (3) In contrast with another widely shared assumption of lower biodiversity, arctic marine biodiversity is comparable to that reported off Atlantic and Pacific coasts of Canada, albeit threatened by the potential colonization of subarctic species. (4) The rapid decrease of summer sea-ice cover allows increasing numbers of killer whales to use the Canadian High Arctic as a hunting ground. The stronger presence of this species, bound to become a new apex predator of arctic seas, will likely affect populations of endemic arctic marine mammals such as the narwhal, bowhead, and beluga whales.  相似文献   

Warming the early earth—CO₂ reconsidered     

Philip von Paris  Heike Rauer  J. Lee Grenfell  Beate Patzer  Barbara Stracke  Franz Schreier 《Planetary and Space Science》2008,56(9):1244-1259

Despite a fainter Sun, the surface of the early Earth was mostly ice-free. Proposed solutions to this so-called “faint young Sun problem” have usually involved higher amounts of greenhouse gases than present in the modern-day atmosphere. However, geological evidence seemed to indicate that the atmospheric CO₂ concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO₂ necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies. For the late Archaean and early Proterozoic period of the Earth, we calculate that CO₂ partial pressures of only about 2.9 mb are required to keep its surface from freezing which is compatible with the amount inferred from sediment studies. This conclusion was not significantly changed when we varied model parameters such as relative humidity or surface albedo, obtaining CO₂ partial pressures for the late Archaean between 1.5 and 5.5 mb. Thus, the contradiction between sediment data and model results disappears for the late Archaean and early proterozoic.  相似文献