2,146 research outputs found
Amino acid incorporation into mitochondrial ribosomes of Neurospora crassa wild-type and Mi-1 mutant
Spin-axis relaxation in spin-exchange collisions of alkali atoms
We present calculations of spin-relaxation rates of alkali-metal atoms due to
the spin-axis interaction acting in binary collisions between the atoms. We
show that for the high-temperature conditions of interest here, the spin
relaxation rates calculated with classical-path trajectories are nearly the
same as those calculated with the distorted-wave Born approximation. We compare
these calculations to recent experiments that used magnetic decoupling to
isolate spin relaxation due to binary collisions from that due to the formation
of triplet van-der-Waals molecules. The values of the spin-axis coupling
coefficients deduced from measurements of binary collision rates are consistent
with those deduced from molecular decoupling experiments. All the experimental
data is consistent with a simple and physically plausible scaling law for the
spin-axis coupling coefficients.Comment: text+1 figur
Energy input is primary controller of methane bubbling in subarctic lakes
Emission of methane (CH4) from surface waters is often dominated by ebullition (bubbling), a transport mode with highâspatiotemporal variability. Based on new and extensive CH4 ebullition data, we demonstrate striking correlations (r2 between 0.92 and 0.997) when comparing seasonal bubble CH4 flux from three shallow subarctic lakes to four readily measurable proxies of incoming energy flux and daily flux magnitudes to surface sediment temperature (r2 between 0.86 and 0.94). Our results after continuous multiyear sampling suggest that CH4 ebullition is a predictable process, and that heat flux into the lakes is the dominant driver of gas production and release. Future changes in the energy received by lakes and ponds due to shorter iceâcovered seasons will predictably alter the ebullitive CH4 flux from freshwater systems across northern landscapes. This finding is critical for our understanding of the dynamics of radiatively important trace gas sources and associated climate feedback
Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes
The release of longâstored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CHâ) clumped isotope (Îââ) and ÂčâŽC measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Îââ values are negatively correlated with CHâ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CHâ Îââ and the fraction modern carbon. These correlations imply that CHâ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Îââ values, are not positively correlated with CHâ flux estimates, highlighting the likely importance of environmental variables other than CHâ production rates in controlling ebullition fluxes
Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes
The release of longâstored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CHâ) clumped isotope (Îââ) and ÂčâŽC measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Îââ values are negatively correlated with CHâ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CHâ Îââ and the fraction modern carbon. These correlations imply that CHâ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Îââ values, are not positively correlated with CHâ flux estimates, highlighting the likely importance of environmental variables other than CHâ production rates in controlling ebullition fluxes
Enhanced X-ray variability from V1647 Ori, the young star in outburst illuminating McNeil's Nebula
We report a ~38 ks X-ray observation of McNeil's Nebula obtained with XMM on
2004 April 4. V1647 Ori, the young star in outburst illuminating McNeil's
Nebula, is detected with XMM and appears variable in X-rays. We investigate the
hardness ratio variability and time variations of the event energy distribution
with quantile analysis, and show that the large increase of the count rate from
V1647 Ori observed during the second half of the observation is not associated
with any large plasma temperature variations as for typical X-ray flares from
young low-mass stars. X-ray spectral fitting shows that the bulk (~75%) of the
intrinsic X-ray emission in the 0.5-8 keV energy band comes from a soft plasma
component (0.9 keV) reminiscent of the X-ray spectrum of the classical T Tauri
star TW Hya, for which X-ray emission is believed to be generated by an
accretion shock onto the photosphere of a low-mass star. The hard plasma
component (4.2 keV) contributes ~25% of the total X-ray emission, and can be
understood only in the framework of plasma heating sustained by magnetic
reconnection events. We find a hydrogen column density of NH=4.1E22 cm-2, which
points out a significant excess of hydrogen column density compared to the
value derived from optical/IR observations, consistent with the picture of the
rise of a wind/jet unveiled from ground optical spectroscopy. The X-ray flux
observed with XMM ranges from roughly the flux observed by Chandra on 2004
March 22 (~10 times greater than the pre-outburst X-ray flux) to a value two
times greater than that caught by Chandra on 2004 March 7 (~200 times greater
than the pre-outburst X-ray flux). We have investigated the possibility that
V1647 Ori displays a periodic variation in X-ray brightness as suggested by the
combined Chandra+XMM data set (abridged).Comment: 11 pages and 8 Figures. Accepted for publication by Astronomy &
Astrophysic
Decadal-scale hotspot methane ebullition within lakes following abrupt permafrost thaw
Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO2) and methane (CH4) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH4 ebullition with (a) an assessment of lake area changes delineated from high-resolution (1â2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH4 ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release 14C-depleted CH4 from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH4 hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took
place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older 14C ages of CH4 released from hotspot seeps in older, expanding thermokarst lakes (14CCH4 20 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger 14CCH4 in new lakes (14CCH4 8526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger 14C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH4-hotspot seep emissions following formation of thermokarst lakes as climate warms
Race, Slavery, and the Expression of Sexual Violence in Louisa Picquet, The Octoroon
Historically, victims of sexual violence have rarely left written accounts of their abuse, so while sexual violence has long been associated with slavery in the United States, historians have few accounts from formerly enslaved people who experienced it first-hand. Through a close reading of the narrative of Louisa Picquet, a survivor of sexual violence in Georgia and Louisiana, this article reflects on the recovery of evidence of sexual violence under slavery through amanuensis-recorded testimony, the unintended evidence of survival within the violent archive of female slavery, and the expression of âraceâ as an authorial device through which to demonstrate the multigenerational nature of sexual victimhood
Search for astronomical neutrinos from blazar TXS 0506+056 in super-kamiokande
We report a search for astronomical neutrinos in the energy region from several GeV to TeV in the direction of the blazar TXS 0506+056 using the Super-Kamiokande detector following the detection of a 100 TeV neutrinos from the same location by the IceCube collaboration. Using Super-Kamiokande neutrino data across several data samples observed from 1996 April to 2018 February we have searched for both a total excess above known backgrounds across the entire period as well as localized excesses on smaller timescales in that interval. No significant excess nor significant variation in the observed event rate are found in the blazar direction. Upper limits are placed on the electron- and muon-neutrino fluxes at the 90% confidence level as 6.0 Ă 10â7 and 4.5 Ă 10â7â9.3 Ă 10â10 [erg cmâ2 sâ1], respectively
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