6 research outputs found
Nariai, Bertotti-Robinson and anti-Nariai solutions in higher dimensions
We find all the higher dimensional solutions of the Einstein-Maxwell theory
that are the topological product of two manifolds of constant curvature. These
solutions include the higher dimensional Nariai, Bertotti-Robinson and
anti-Nariai solutions, and the anti-de Sitter Bertotti-Robinson solutions with
toroidal and hyperbolic topology (Plebanski-Hacyan solutions). We give explicit
results for any dimension D>3. These solutions are generated from the
appropriate extremal limits of the higher dimensional near-extreme black holes
in a de Sitter, and anti-de Sitter backgrounds. Thus, we also find the mass and
the charge parameters of the higher dimensional extreme black holes as a
function of the radius of the degenerate horizon.Comment: 10 pages, 11 figures, RevTeX4. References added. Published versio
Towards an understanding of late Quaternary variations in the continental biogeochemical cycle of silicon : multi-isotope and sediment-flux data for Lake Rutundu, Mt Kenya, East Africa, since 38ka BP
Silicon is an essential nutrient for marine diatoms, which dominate the export of organic carbon to the deep ocean. Despite the dominance of the oceanic Si budget by fluvial inputs and the role of the land biosphere in controlling Si losses from rocks and soils to rivers, few studies have considered how continental biogeochemical Si fluxes varied on an orbital timescale. We reconstruct changes in Si cycling by the catchment-lake ecosystem of Lake Rutundu, Mt Kenya (3078 m a.s.l.), over the last ca. 38 ka, using a novel combination of lake-sediment fluxes and stable-isotope (13C, 15N, 18O, 30Si) data. Under glacial conditions (38.3-14.3 ka BP), high diatom productivity was maintained by substantial losses of dissolved SiO2 and soil nutrients from a sparse, leaky, terrestrial ecosystem. During the following period of enhanced monsoon rainfall and seasonality (14.3-9.5 ka BP), rapid Si cycling by fire-prone, mesic grassland was associated with substantial aeolian transport of opal phytoliths by smoke plumes, but greatly reduced nutrient losses in runoff. Invasion of tall, subalpine shrubs after 9.5 ka BP further enhanced landscape stability, leading to very low sediment fluxes of both phytoliths and diatoms. This case study offers new insights into processes that may have operated at biome to continental scales during the late Quaternar