Life's origin: the cosmic, planetary and biological processes     

Scattergood T  Des Marais D  Jahnke L 《The Planetary report》1987,7(6):4-5

From elements formed in interstellar furnaces to humans peering back at the stars, the evolution of life has been a long, intricate and perhaps inevitable process. Life as we know it requires a planet orbiting a star at just the right distance so that water can exist in liquid form. It needs a rich supply of chemicals and energy sources. On Earth, the combination of chemistry and energy generated molecules that evolved ways of replicating themselves and of passing information from one generation to the next. Thus, the thread of life began. This chart traces the thread, maintained by DNA molecules for much of its history, as it weaves its way through the primitive oceans, gaining strength and diversity along the way. Organisms eventually moved onto the land, where advanced forms, including humans, ultimately arose. Finally, assisted by a technology of its own making, life has reached back out into space to understand its own origins, to expand into new realms, and to seek other living threads in the cosmos.  相似文献   

Thermal structures in the Kuroshio extension measured with a towed thermistor chain     

Yoshihiko Sekine  Fukuji Yamada  Yoshiaki Toba  Eugene C. Lafond 《Journal of Oceanography》1994,50(4):391-401

Two-dimensional temperature data observed by use of a 275 meter towed thermistor chain deployed from an oceanographic research vessel USS MARYSVILLE, which cruised with a speed of 6.2 knots in July 1966 across the Kuroshio Extension in the North Pacific, are investigated. Two-dimensional variations of the distribution of the isotherms along the ship's track are analyzed with special reference to their slope, wavelength and wave height. The results show that the slope and wave height of isotherms have a tendency to increase as the temperature decreases. Even if the contribution of wave heights smaller than 1.5 m is neglected, i.e., contribution of large scale slope with a horizontal scale of 5–30 km is subtracted, this tendency is still detected. In contrast to this, the wavelength evaluated by the crest to crest method has no dependency on the temperature. Power spectrum of the isotherm depth is proportional tok ^–1.87 for 13°C andk ^–2.13 for 27°C, wherek is the wave number. It is shown that the spectra of warmer isotherms are relatively well approximated by –2 power law (Garrett and Munk spectrum) for internal waves rather than the –5/3 power law (Kolmogorov spectrum) for three dimensional isotropic turbulence.  相似文献   

Formation of ‘Beach Rock’ at Siesta Key, Florida and its influence on barrier island development     

Darren Spurgeon  Richard A. Davis  Jr.  Eugene A. Shinnu 《Marine Geology》2003,200(1-4):19-29

Seaward-dipping strata of carbonate-cemented shell debris located along the coast of Siesta Key on the Gulf Coast of the Florida peninsula have long been interpreted to be beachrock equivalent in age to the Pleistocene Anastasia Formation (Stage 5e) of the east coast of Florida. Detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a Holocene deposit that is not beachrock but was lithified in a meteoric environment. Whole rock dates, dates from shells only, and from cement only demonstrate that these beach deposits were in place by at least 1800 yr BP and might have been there as long ago as 4300 yr BP. This means that some type of barrier island was in place at that time. Previous investigations have depicted Siesta Key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits. This suggests that the beach deposits might have been the site of the original position of Siesta Key. These data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited; sometime between 1800 and 4300 yr ago. This scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago.  相似文献   

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia     

Peter Ward  Mick Whitehouse  Mike Meredith  Eugene Murphy  Rachael Shreeve  Rebecca Korb  Jon Watkins  Sally Thorpe  Rachel Woodd-Walker  Andrew Brierley  Nathan Cunningham  Sharon Grant  Doug Bone 《Deep Sea Research Part I: Oceanographic Research Papers》2002,49(12):139

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.  相似文献   

Below- and Aboveground Spartina alterniflora Production in a Louisiana Salt Marsh     

Faith A. Darby  R. Eugene Turner 《Estuaries and Coasts》2008,31(1):223-231

The monthly variations of below- and aboveground biomass of Spartina alterniflora were documented for a south Louisiana salt marsh from March 2004 to March 2005, and in March 2006 and 2007. The annual production rate above- and belowground was 1821 and 11,676 g m^?2, respectively (Smalley method), and the annual production rate per biomass belowground was 10.7 g dry weight^?1, which are highs along the latitudinal distributions of the plant’s range. The average root + rhizome/shoot ratio (R&R/S) was 2.6:1, which is lower than the R&R/S ratios of 4 to 5.1 reported for Spartina sp. marshes in the northeastern US. The belowground biomass increased from July to September and fluctuated between October and November, after which it declined until February when the growing season began. The belowground biomass was dominated by rhizomes, which declined precipitously in spring and then rose to a seasonal high in the month before declining again as the late summer rise in inflorescence began. Over half of the root biomass in a 30-cm soil profile was in the upper 10 cm, and in the 10- to 20-cm profile for rhizomes. The maximum March biomass above- and belowground was four to five times that of the minimum biomass over the four sampling years. The net standing stock (NSS) of N and P in live biomass aboveground compared to that in the belowground biomass was about 1.7 times higher and equal, respectively, but the NSS of N and P for the live + dead biomass was about six times higher belowground. The average nitrogen/phosphorous molar ratios of 16:1 aboveground is in agreement with the often tested N limitation of biomass accumulation aboveground, whereas the 37:1 belowground ratio suggests that there is an influence of P on R&R foraging for P belowground. Some implications for management and restoration are, in part, that salt marshes should be evaluated and examined using information on the plant’s physiology and production both below- and aboveground.  相似文献   

Biogeochemical cycles of carbon, sulfur, and free oxygen in a microbial mat   总被引：11，自引：0，他引：11  

Canfield DE  Des Marais DJ 《Geochimica et cosmochimica acta》1993,57(16):3971-3984

Complete budgets for carbon and oxygen have been constructed for cyanobacterial mats dominated by Microcoleus chthonoplastes from the evaporating ponds of a salt works located in Guerrero Negro, Baja California Sur, Mexico. Included in the budget are measured rates of O2 production, sulfate reduction, and elemental exchange across the mat/brine interface, day and night, at various temperatures and times of the year. We infer from this data the various sinks for O2, as well as the sources of carbon for primary production. To summarize, although seasonal variability exists, a major percentage of the O2 produced during the day did not diffuse out of the mat but was used within the mat to oxidize both organic carbon and the sulfide produced by sulfate reduction. At night, most of the O2 that diffused into the mat was used to oxidize sulfide, with O2 respiration of minor importance. During the day, the internal mat processes of sulfate reduction and O2 respiration generated as much or more inorganic carbon (DIC) for primary production as diffusion into the mat. Also, oxygenic photosynthesis was the most important process of carbon fixation, although anoxygenic photosynthesis may have been important at low light levels during some times of the year. At night, the DIC lost from the mat was mostly from sulfate reduction. Elemental fluxes across the mat/brine interface indicated that carbon with an oxidation state of greater than zero was taken up by the mat during the day and liberated from the mat at night. Overall, carbon with an average oxidation state of near zero accumulated in the mat. Both carbon fixation and carbon oxidation rates varied with temperature by a similar amount. These mats are thus closely coupled systems where rapid rates of photosynthesis both require and fuel rapid rates of heterotrophic carbon oxidation.  相似文献   

The concentration and isotopic composition of carbon in basaltic glasses from the Juan de Fuca Ridge, Pacific Ocean     

Blank JG  Delaney JR  Des Marais DJ 《Geochimica et cosmochimica acta》1993,57(4):875-887

The abundance and 13C/12C ratios of carbon were analyzed in basaltic glass from twenty locations along the Juan de Fuca Ridge using a 3-step combustion/extraction technique. Carbon released during the first two combustion steps at 400-500 degrees C and 600-650 degrees C is interpreted to be secondary, and only the carbon recovered during a final combustion step at approximately 1200 degrees C is thought to be indigenous to the samples. For carbon released at approximately 1200 degrees C, glasses analyzed as 1-2 mm chips contained 23-146 ppm C with delta 13C values of -4.8 to -9.3%, whereas samples crushed to 38-63 microns or 63-90 microns yielded 56-103 ppm C with delta 13C values of -6.1 to -9.2%. The concentrations and isotopic compositions of the primary carbon dissolved in the glasses and present in the vesicles are similar to those previously reported for other ocean-ridge basalts. The Juan de Fuca basaltic magmas were not in equilibrium with respect to carbon when they erupted and quenched on the sea floor. Evidence of disequilibrium includes (1) a large range of carbon contents among glasses collected at similar depths, (2) a highly variable calculated carbon isotopic fractionation between melt and vapor determined by comparing crushed and uncrushed splits of the same sample, and (3) a lack of correlation between vesicle abundance, carbon concentration, and depth of eruption. Variations in carbon concentration and delta 13C ratios along the ridge do not correlate with major element chemistry. The observed relationship between carbon concentrations and delta 13C values may be explained by late-stage, variable degrees of open-system (Rayleigh-like) degassing.  相似文献   

Preservation of biological information in thermal spring deposits: developing a strategy for the search for fossil life on Mars   总被引：1，自引：0，他引：1  

Walter MR  Des Marais DJ 《Icarus》1993,101(1):129-143

Current interpretations of the early history of Mars suggest many similarities with the early Earth and therefore raise the possibility that the Archean and Proterozoic history of life on Earth could have a counterpart on Mars. Terrestrial experience suggests that, with techniques that can be employed remotely, ancient springs, including thermal springs, could well yield important information. By delivering water and various dissolved species to the sunlit surface of Mars, springs very likely created an environment suitable for life, which could have been difficult, if not impossible, to attain elsewhere. The chemical and temperature gradients associated with thermal springs sort organisms into sharply delineated, distinctive and different communities, and so diverse organisms are concentrated into relatively small areas in a predictable and informative fashion. A wide range of metabolic strategies are concentrated into small areas, thus furnishing a useful and representative sampling of the existing biota. Mineral-charged springwaters frequently deposit chemical precipitates of silica and/or carbonate which incorporate microorganisms and preserve them as fossils. The juxtaposition of stream valley headwaters with volcanoes and impact craters on Mars strongly implies that subsurface heating of groundwater created thermal springs. On Earth, thermal springs create distinctive geomorphic features and chemical signatures which can be detected by remote sensing. Spring deposits can be quite different chemically from adjacent rocks. Individual springs can be hundreds of meters wide, and complexes of springs occupy areas up to several kilometers wide. Benthic microbial mats and the resultant stromatolites occupy a large fraction of the available area. The relatively high densities of fossils and microbial mat fabrics within these deposits make them highly prospective in any search for morphological evidence of life, and there are examples of microbial fossils in spring deposits as old as 300 Myr.  相似文献   

A high resolution relative paleointensity record from the Gerlache-Boyd paleo-ice stream region, northern Antarctic Peninsula     

Verónica Willmott  Eugene W. Domack  Stefanie Brachfeld 《Quaternary Research》2006,66(1):1-11

Herein we document and interpret an absolute chronological dating attempt using geomagnetic paleointensity data from a post-glacial sediment drape on the western Antarctic Peninsula continental shelf. Our results demonstrate that absolute dating can be established in Holocene Antarctic shelf sediments that lack suitable material for radiocarbon dating. Two jumbo piston cores of 10-m length were collected in the Western Bransfield Basin. The cores preserve a strong, stable remanent magnetization and meet the magnetic mineral assemblage criteria recommended for reliable paleointensity analyses. The relative paleomagnetic intensity records were tuned to published absolute and relative paleomagnetic stacks, which yielded a record of the last ∼8500 years for the post-glacial drape. Four tephra layers associated with documented eruptions of nearby Deception Island have been dated at 3.31, 3.73, 4.44, and 6.86 ± 0.07 ka using the geomagnetic paleointensity method. This study establishes the dual role of geomagnetic paleointensity and tephrochronology in marine sediments across both sides of the northern Antarctic Peninsula.  相似文献   

Accessibility Measures in Spatial Mismatch Models     

Eugene D. Perle  Harald Bauder  Nancy Beckett 《The Professional geographer》2002,54(1):106-110

Implicit in all spatial mismatch models is the concept of accessibility. However, there is an immense variety of measures utilized in the literature to index the concept, and it is not clear how much of the disparity in findings between studies is attributable to alternative measures. This research note attempts to provide some evidence on the impact of alternative measures of accessibility in employment probability models.  相似文献