Consistent land- and atmosphere-based U.S. carbon sink estimates.
(41/768)
For the period 1980-89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 10(15) grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980-89 are similar to those for 1985-89 and 1990-94, indicating a relatively stable U.S. sink throughout the period. (+info)
Stomatal oscillations at small apertures: indications for a fundamental insufficiency of stomatal feedback-control inherent in the stomatal turgor mechanism.
(42/768)
Continuous measurements of stomatal aperture simultaneously with gas exchange during periods of stomatal oscillations are reported for the first time. Measurements were performed in the field on attached leaves of undisturbed Sambucus nigra L. plants which were subjected to step-wise increases of PPFD. Oscillations only occurred when stomatal apertures were small under high water vapour mole fraction difference between leaf and atmosphere (DeltaW). They consisted of periodically repeated opening movements transiently leading to very small apertures. Measurements of the area of the stomatal complex in parallel to the determination of aperture were used to record volume changes of guard cells even if stomata were closed. Stomatal opening upon a light stimulus required an antecedent guard cell swelling before a slit occurred. After opening of the slit the guard cells again began to shrink which, with some delay, led to complete closure. Opening and closing were rhythmically repeated. The time-lag until initial opening was different for each individual stoma. This led to counteracting movements of closely adjacent stomata. The tendency to oscillate at small apertures is interpreted as being a failure of smoothly damped feedback regulation at the point of stomatal opening: Volume changes are ineffective for transpiration if stomata are still closed; however, at the point of initial opening transpiration rate rises steeply. This discontinuity together with the rather long time constants inherent in the stomatal turgor mechanism makes oscillatory overshooting responses likely if at high DeltaW the 'nominal value' of gas exchange demands a small aperture. (+info)
Remotely sensed biological production in the equatorial Pacific.
(43/768)
A combination of ship, buoy, and satellite observations in the tropical Pacific during the period from 1992 to 2000 provides a basin-scale perspective on the net effects of El Nino and La Nina on biogeochemical cycles. New biological production during the 1997-99 El Nino/La Nina period varied by more than a factor of 2. The resulting interannual changes in global carbon sequestration associated with the El Nino/La Nina cycle contributed to the largest known natural perturbation of the global carbon cycle over these time scales. (+info)
Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth.
(44/768)
The low O2 content of the Archean atmosphere implies that methane should have been present at levels approximately 10(2) to 10(3) parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space (upward arrow space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --> CH4 + 2O2 --> CO2 + O2 + 4H(upward arrow space)]. Expected irreversible oxidation (approximately 10(12) to 10(13) moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized. (+info)
Climate change as the dominant control on glacial-interglacial variations in C3 and C4 plant abundance.
(45/768)
Although C4 plant expansions have been recognized in the late Miocene, identification of the underlying causes is complicated by the uncertainties associated with estimates of ancient precipitation, temperature, and partial pressure of atmospheric carbon dioxide (PCO2). Here we report the carbon isotopic compositions of leaf wax n-alkanes in lake sediment cores from two sites in Mesoamerica that have experienced contrasting moisture variations since the last glacial maximum. Opposite isotopic trends obtained from these two sites indicate that regional climate exerts a strong control on the relative abundance of C3 and C4 plants and that in the absence of favorable moisture and temperature conditions, low PCO2 alone is insufficient to drive an expansion of C4 plants. (+info)
Impact melting of frozen oceans on the early Earth: implications for the origin of life.
(46/768)
Without sufficient greenhouse gases in the atmosphere, the early Earth would have become a permanently frozen planet because the young Sun was less luminous than it is today. Several resolutions to this faint young Sun-frozen Earth paradox have been proposed, with an atmosphere rich in CO2 being the one generally favored. However, these models assume that there were no mechanisms for melting a once frozen ocean. Here we show that bolide impacts between about 3.6 and 4.0 billion years ago could have episodically melted an ice-covered early ocean. Thaw-freeze cycles associated with bolide impacts could have been important for the initiation of abiotic reactions that gave rise to the first living organisms. (+info)
Gas chromatographic separation of nitrogen, oxygen, argon, and carbon monoxide using custom-made porous polymers from high purity divinylbenzene.
(47/768)
Existing porous polymers were surveyed for their ability to separate the subject gases. Certain products that showed more promise than others were synthesized and the existing synthetic procedures studied and modified to produce new polymers with enhanced ability to separate the subject gases. Evaluation of the porous polymers was carried out practically by gas chromatography at ambient temperature. The modified synthetic procedures were somewhat simpler than the originals. The new porous polymers made with high purity divinylbenzene enabled use of shorter columns to obtain the separations desired. (+info)
From exobiology to cosmobiology at LISA and elsewhere.
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Since the emergence of Exobiology, back to the l960ties, this field drastically increased and, although differently named, is today a largely recognized scientific domain of wild interdisciplinarity. It includes not only the search for extraterrestrial living Systems, in particular by direct exploration of planetary bodies and studies of extraterrestrial materials, but also the study on the origins of life on Earth and, in connection to this field, the study of extraterrestrial organic chemistry. The exobiology programmes currently developed at LISA are related to this last aspect. They include the study of prebiotic-like chemistry in the gas and solid phases, based on laboratory simulation experiments, theoretical modeling and future in situ measurements in Titan's atmosphere and in cometary nuclei. A national program of exobiology, coordinated by LISA is under development in France, it covers many of the various aspects of Exobiology, including the study of life in extreme environments, as a reference tool for extraterrestrial life, the study of the primitive environment of the Earth, of the organic chemistry in comets and on Titan, of Mars and Europa and even of extrasolar planets as potential niches for extraterrestrial living systems, associated to the determination of the electromagnetic signatures of life. In parallel to this general program, a proposal for a large simulation chamber to be used as a national facility in particular to simulate the organic chemistry in various planetary environments, and in the interstellar medium, is under preparation. International cooperations linked to these programmes, in particular in the frame of the development of an exobiology facility on the International Space Station, would be of crucial interest. (+info)