9 research outputs found
On the Necessity of Correction of the Rules of Rejection of “Gross Errors” in the Statistical Processing of the Results of Testing of Soils
Analysis of coexistence of the recent invader Diaphanosoma orghidani Negrea with the aboriginal species D. brachyurum (Lievin) (Crustacea, Cladocera) in the Rybinsk reservoir
Spin-conserving carrier recombination in conjugated polymers
The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4?300 K enables a direct quantification of spin conversion in coulombically bound electron?hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined
Invisible Structure of the Cathode Spot of a Welding Arc with a Tungsten Anode and a Film Cathode Burning in an Inert Gas
A shift of thermokarst lakes from carbon sources to sinks during the Holocene epoch
Thermokarst lakes formed across vast regions of Siberia and Alaska during the last deglaciation and are thought to be a net source of atmospheric methane and carbon dioxide during the Holocene epoch1, 2, 3, 4. However, the same thermokarst lakes can also sequester carbon5, and it remains uncertain whether carbon uptake by thermokarst lakes can offset their greenhouse gas emissions. Here we use field observations of Siberian permafrost exposures, radiocarbon dating and spatial analyses to quantify Holocene carbon stocks and fluxes in lake sediments overlying thawed Pleistocene-aged permafrost. We find that carbon accumulation in deep thermokarst-lake sediments since the last deglaciation is about 1.6 times larger than the mass of Pleistocene-aged permafrost carbon released as greenhouse gases when the lakes first formed. Although methane and carbon dioxide emissions following thaw lead to immediate radiative warming, carbon uptake in peat-rich sediments occurs over millennial timescales. We assess thermokarst-lake carbon feedbacks to climate with an atmospheric perturbation model and find that thermokarst basins switched from a net radiative warming to a net cooling climate effect about 5,000 years ago. High rates of Holocene carbon accumulation in 20 lake sediments (47 ± 10 grams of carbon per square metre per year; mean ± standard error) were driven by thermokarst erosion and deposition of terrestrial organic matter, by nutrient release from thawing permafrost that stimulated lake productivity and by slow decomposition in cold, anoxic lake bottoms. When lakes eventually drained, permafrost formation rapidly sequestered sediment carbon. Our estimate of about 160 petagrams of Holocene organic carbon in deep lake basins of Siberia and Alaska increases the circumpolar peat carbon pool estimate for permafrost regions by over 50 per cent (ref. 6). The carbon in perennially frozen drained lake sediments may become vulnerable to mineralization as permafrost disappears7, 8, 9, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene
A shift of thermokarst lakes from carbon sources to sinks during the Holocene epoch
Thermokarst lakes formed across vast regions of Siberia and Alaska during the last deglaciation and are thought to be a net source of atmospheric methane and carbon dioxide during the Holocene epoch1–4. However, the same thermokarstlakes can also sequester carbon5 , and it remains uncertain whether carbon uptake by thermokarst lakes can offset their greenhouse gas emissions. Here we use field observations of Siberian permafrost exposures, radiocarbon dating and spatial analyses to quantify Holocene carbon stocks and fluxes in lake sediments overlying thawed Pleistocene-aged permafrost.We find that carbon accumulationin deep thermokarst-lake sediments since thelast deglaciationis about 1.6 times larger than the mass of Pleistocene-aged permafrost carbon released as greenhouse gases when the lakes first formed. Although methane and carbon dioxide emissions following thaw lead toimmediate radiative warming, carbon uptakein peat-rich sediments occurs over millennial timescales. We assess thermokarstlake carbon feedbacks to climate with an atmospheric perturbation model and find that thermokarst basins switched from a net radiative warming to a net cooling climate effect about 5,000 years ago. High rates of Holocene carbon accumulation in 20 lake sediments (47 6 10 grams of carbon per square metre per year; mean 6 standard error) were driven by thermokarst erosion and deposition of terrestrial organic matter, by nutrient release from thawing permafrost that stimulatedlake productivity and by slow decompositionin cold, anoxic lake bottoms.When lakes eventually drained, permafrost formation rapidly sequestered sediment carbon. Our estimate of about 160 petagrams of Holocene organic carbon in deep lake basins of Siberia and Alaska increases the circumpolar peat carbon pool estimate for permafrost regions by over 50 per cent (ref. 6). The carbon in perennially frozen drained lake sediments may become vulnerable to mineralization as permafrost disappears7–9, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene