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1.
Brack A 《Planetary and Space Science》1996,44(11):1435-1440
Processing of organic molecules by liquid water was probably an essential requirement towards the emergence of terrestrial primitive life. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Geochemists favour now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere, very few building blocks are formed. Import of extraterrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, meteorites and micrometeorites. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars attesting the presence of an atmosphere capable of decelerating C-rich micro-meteorites. Therefore, primitive life may have developed on Mars, as well. Liquid water disappeared from the surface of Mars very early, about 3.8 Ga ago. The Viking missions did not find, at the surface of the Martian soil, any organic molecules or clear-cut evidence for microbial activities such as photosynthesis, respiration or nutrition. The results can be explained referring to an active photochemistry of Martian soil driven by the high influx of solar UV. These experiments do not exclude the existence of organic molecules and fossils of micro-organisms which developed on early Mars until liquid water disappeared. Mars may store below its surface some well preserved clues of a still hypothetical primitive life. 相似文献
2.
Historical pollution trends in a disturbed, estuarine sedimentary environment, SW Sweden 总被引：3，自引：0，他引：3
The objective of this article is to determine whether pollution trends can be reconstructed from sediments in the outer Göta älv estuary. Detailed interpretation of heavy metal profiles in the recent sediment, in combination with profiles of 210Pb- and 137Cs-activity, organic carbon content and grain size distribution, allows estimation of historical trends. This is based on the assumption that only certain intervals, which can be identified, are disturbed. The sediment data are compared with archive data on historical trends in emissions of heavy metals. Special attention is given to mercury, which shows high enrichment over long periods of time. It is found that the most polluted sediments were deposited between 1930 and 1980, but that episodic discharges have also occurred more recently. Nevertheless, the effects of remediation since the 1970s are apparent. 相似文献
3.
4.
Peter Brack Hans Rieber Roland Mundil Wolfgang Blendinger Florian Maurer 《Swiss Journal of Geoscience》2007,100(3):327-348
The depositional architecture and the geometric relationships between platform-slope deposits and basinal sediments along
with paleontological evidence indicate the time interval of the younger Anisian Reitziites reitzi ammonoid zone to largely represent the main stage of platform aggradation at the Cernera and Bivera/Clapsavon carbonate platforms.
Published and new U-Pb age data of zircons from volcaniclastic layers bracketing the stratigraphic interval of platform growth
constrain the duration of platform evolution to a time span shorter than 1.8±0.7m.y., probably in the order of 0.5-1m.y.,
reflecting fast rates of vertical platform aggradation exceeding 500 m/m.y. In the range of growth potentials for shallow-water
carbonate systems estimated in relation to the time span of observation, this high rate is in agreement with values for short
intervals of 105-106yrs (e.g., Schlager 1999).
After drowning, the platforms at Cernera and Bivera/Clapsavon were blanketed by thin pelagic carbonates. On the former platform
flanks the draping sediments in places comprise red nodular pelagic limestones (Clapsavon Limestone) similar in facies to
the Han Bulog Limestones occurring elsewhere in Middle Triassic successions of the Mediterranean Tethys. The drowning of vast
areas of former carbonate platforms possibly triggered the onset of bottom-water circulation in adjacent basins as suggested
by the abrupt transition from laminated to bioturbated pelagic nodular limestones in the Buchenstein Formation which occurred
close to the time of initial platform submergence. During the Late Ladinian the topographic features of the drowned platforms
were onlapped by rapidly deposited, predominantly clastic successions including coarse breccias and volcanic rocks sealing
and preserving the peculiar stratigraphic setting. 相似文献
5.
Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps) 总被引：1，自引：0，他引：1
Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at 250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar–40Ar ages of 30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition. 相似文献
6.
Holocene sedimentological change was investigated in 15 sediment piston cores (250 cm long) from the mouth of the Göta älv River, outside of Göteborg, which is sheltered by an archipelago. The main objective was to interpret recent, natural and human-induced influences upon the accumulation–erosion balance and the sedimentary environment from Holocene sediment units. The five main units are: consolidated glaciomarine clay (Facies gC) lowermost, consolidated estuarine clay (Facies dC), soft sub-recent sediment (Facies omC and Facies C) uppermost and dumped sediment (Facies D) in some parts of the study area. The sediment facies are divided by two major hiatuses: (a) the early Holocene hiatus between Facies gC and dC involves a time gap of at least 7000 years, ending at ca. 4000 BP and (b) the late Holocene hiatus between the consolidated clays (Facies gC and dC) and Facies omC spans 1000 years and ends ca. 50 BP (i.e. 100 years ago). Both hiatuses probably relate to the effects of isostatic land uplift. Surface sediments consist mainly of an organic-matter-rich clay facies, suggesting changes in the trophic state of the estuary during the last 100 years. Together with human-induced increased river erosion and loss of accommodation space in the estuary, this caused the onset of sedimentation in the estuary. The recent sediments are contaminated with heavy metals (Cu, Zn, Pb, Hg), with highest concentrations in the lower part of Facies omC. 相似文献
7.
The Southern Alps host volcano-sedimentary basins that formed during post-Variscan extension and strike-slip in the Early
Permian. We present U–Pb ages and initial Hf isotopic compositions of magmatic zircons from silicic tuffs and pyroclastic
flows within these basins, from caldera fillings and from shallow intrusions from a 250 km long E–W transect (Bozen–Lugano–Lago
Maggiore) and compare these with previously published data. Basin formation and magmatism are closely related to each other
and occurred during a short time span between 285 and 275 Ma. The silicic magmatism is coeval with mafic intrusions of the
Ivrea-Verbano Zone and within Austroalpine units. We conclude that deep magma generation, hybridisation and upper crustal
emplacement occurred contemporaneously along the entire transect of the Southern Alps. The heat advection in the lower crust
by injected mantle melts was sufficient to produce crustal partial melts in lower crustal levels. The resulting granitoid
melts intruded into the upper crust or rose to the surface forming large caldera complexes. The compilation of Sr and Nd isotopic
data of these rocks demonstrates that the mantle mixing endmember in the melts may not be geochemically enriched but has a
depleted composition, comparable to the Adriatic subcontinental mantle exhumed to form the Tethyan sea floor during Mesozoic
continental breakup and seafloor spreading. Magmatism and clastic sedimentation in the intracontinental basins was interrupted
at 275 Ma for some 10–15 million years, forming a Middle Permian unconformity. This unconformity may have originated during
large-scale strike-slip tectonics and erosion that was associated with crustal thinning, upwelling and partial melting of
mantle, and advection of melts and heat into the crust. The unconformity indeed corresponds in time to the transition from
a Pangea-B plate reconstruction for the Early Permian to the Late Permian Pangea-A plate assembly (Muttoni et al. in Earth
Planet Sci Lett 215:379–394, 2003). The magmatic activity would therefore indicate the onset of >2,000 km of strike-slip movement along a continental-scale
mega-shear, as their model suggests. 相似文献
8.
Ray DuVarney Charlie Bleau Garry Motter Richard Dekany Mitchell Troy Gary Brack 《Experimental Astronomy》2001,11(3):237-249
SciMeasure, in collaboration with Emory University and the Jet Propulsion Laboratory, has developed a very versatile CCD controller for use in adaptive optics, optical interferometry, and other applications requiring high-speed readout rates and/or low read noise. The overall architecture of this controller system will be discussed and its performance using both EEV CCD39 and MIT/LL CCID-19 detectors will be presented. This controller is used in the adaptive optics system, developed by JPL, for the 200′′ Hale telescope at Palomar Mountain. Early diffraction-limited science results, recently achieved by the AO system, are presented. We gratefully acknowledge the financial support of NASA through SBIR contracts NAS8–97195 and NAS8–98081. 相似文献
9.
On reliable data-driven partial GNSS ambiguity resolution 总被引：1，自引：1，他引：0
Andreas Brack 《GPS Solutions》2015,19(3):411-422
10.
Landsat Thematic Mapper (TM) digital data acquired in the fall of 1988 and digital elevation model (DEM) data were evaluated for use in characterizing the spatial distribution of defoliation conditions within the boreal montane spruce‐fir ecosystem in the Black Mountains (Mtns.) of North Carolina. Correlation coefficients between the TM waveband data and field estimates of defoliation taken from 22 one hectare field plots were low (0.10 to ‐0.54). TM band four (near infrared) was the only waveband found to be significantly correlated with needle loss. Defoliation was also shown to be significantly correlated with digital elevation and aspect data. A multivariate linear regression model predicting percent defoliation was developed from the TM, elevation, and aspect data for the 21 field plots. In addition to having a high R2(0.85) the model was shown to reliably predict defoliation conditions throughout the ecosystem. A color‐coded classified image depicting the spatial distribution of defoliation conditions within the study site was generated by applying the model to the TM and DEM data. 相似文献