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 epoch. However, the same thermokarst lakes can also sequester carbon, 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 disappears, potentially negating the climate stabilization provided by thermokarst lakes during the late Holocene.

Competitive versus cooperative attitudes in crossed categorization effects: testing the category dominance and equivalence models.

Participants' (N = 256) competitive or cooperative attitudes toward national groups were activated by their reading 1 of 2 selected newspaper articles. Participants then judged the competence and attractiveness of a stranger who was categorized, separately for race and nationality, as belonging to an out-group or in-group. As predicted, the activated competitive attitude produced responses prescribed by the model of category dominance by nationality. In the condition of cooperative attitudes, however, competence responses were consistent with the model of equivalence (i.e., no effect of category) and attraction responses were consistent with the model of category dominance by race. The authors discuss reasons for the discrepant models in the condition of cooperative attitudes and the implications of these findings.

Clinical evaluation of a microporous membrane oxygenator.

1. The microporous membrane oxygenator has a maximum priming volume of 400 ml. and transfers up to 250 ml. of O2. 2. CO2 transfer is excessive when ventilating the oxygenator with 100% O2. CO2 should be added to the ventilating gas. 3. The oxygenator has performed satisfactorily in 90 cases of cardiac surgery with perfusion time up to nine hours. 4. Precautions are described to prevent possible accumulation of water vapor in the oxygenator.