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61.
Kurt A. Refsnider Gifford H. Miller Marilyn L. Fogel Bianca Fréchette Roxane Bowden John T. Andrews G. Lang Farmer 《Quaternary Research》2014
The mineralogy and isotopic compositions of subglacially precipitated carbonate crusts (SPCCs) provide information on conditions and processes beneath former glaciers and ice sheets. Here we describe SPCCs formed on gneissic bedrock at the bed of the Laurentide Ice Sheet (LIS) during the last glacial maximum on central Baffin Island. Geochemical data indicate that the Ca in the crusts was likely derived from the subglacial chemical weathering Ca-bearing minerals in the local bedrock. C and Sr isotopic analyses reveal that the C in the calcite was derived predominantly from older plant debris. The δ18O values of the SPCCs suggest that these crusts formed in isotopic equilibrium with basal ice LIS preserved in the Barnes Ice Cap (BIC). Columnar crystal fabric and the predominance of sparite over micrite in the SPCCs are indicative of carbonate precipitation under open-system conditions. However, the mean δ18O value of the calcite crusts is ~ 10‰ higher than those of primary LIS ice preserved in the BIC, demonstrating that SPCCs record the isotopic composition of only basal ice. Palynomorph assemblages preserved within the calcite and basal BIC ice include species last endemic to the Arctic in the early Tertiary. The source of these palynomorphs remains enigmatic. 相似文献
62.
Maya García-Comas Manuel López-Puertas Bianca M. Dinelli Alberto Adriani Angioletta Coradini 《Icarus》2011,214(2):571-583
After molecular nitrogen, methane is the most abundant species in Titan’s atmosphere and plays a major role in its energy budget and its chemistry. Methane has strong bands at 3.3 μm emitting mainly at daytime after absorption of solar radiation. This emission is strongly affected by non-local thermodynamic equilibrium (non-LTE) in Titan’s upper atmosphere and, hence, an accurate modeling of the non-LTE populations of the emitting vibrational levels is necessary for its analysis. We present a sophisticated and extensive non-LTE model which considers 22 CH4 levels and takes into account all known excitation mechanisms in which they take part. Solar absorption is the major excitation process controlling the population of the v3-quanta levels above 1000 km whereas the distribution of the vibrational energy within levels of similar energy through collisions with N2 is also of importance below that altitude. CH4-CH4 vibrational exchange of v4-quanta affects their population below 500 km. We found that the ν3 → ground band dominates Titan’s 3.3 μm daytime limb radiance above 750 km whereas the ν3 + ν4 → ν4 band does below that altitude and down to 300 km. The ν3 + ν2 → ν2, the 2ν3 → ν3, and the 13CH4ν3 → ground bands each contribute from 5% to 8% at regions below 800 km. The ν3 + 2ν4 → 2ν4and ν2 + ν3 + ν4 → ν2 + ν4 bands each contribute from 2% to 5% below 650 km. Contributions from other CH4 bands are negligible. We have used the non-LTE model to retrieve the CH4 abundance from 500 to 1100 km in the southern hemisphere from Cassini-VIMS daytime measurements near 3.3 μm. Our retrievals show good agreement with previous measurements and model results, supporting a weak deviation from well mixed values from the lower atmosphere up to 1000 km. 相似文献
63.
Alexander P. Wolfe Bianca Frchette Pierre J. H. Richard Gifford H. Miller Steven L. Forman 《Quaternary Science Reviews》2000,19(17-18)
A 137 cm sediment core from Fog Lake, a small upland basin that was not glaciated during the Late Wisconsinan, reveals the following stratigraphic succession: (I) basal diamicton, (II) laminated silts, (III) compacted dewatered gyttja, (IV) stratified minerogenic sediments, and (V) overlying gyttja. Units I–III, and most of unit IV, were deposited prior to the Holocene; unit V represents approximately the last 8000 14C years. Alternate luminescence dating techniques (TL and IRSL) place unit III in latter oxygen isotope stage (OIS) 5, >74,000 years BP. A suite of 20 AMS 14C dates suggests that much of the post-OIS 5 paleoenvironmental record is characterized by either extremely low continuous sediment accumulation, or by one or more episodes of more rapid sedimentation. Diverse pollen and spore assemblages have concentrations that are unexpectedly high throughout the pre-Holocene sediments (>10,000 grains cm−3), by comparison to the Holocene (5000 grains cm−3). Exceedingly low sediment accumulation rates appear to have resulted in the concentration of atmospherically derived pollen and spores. Diatom assemblages record the evolution of the lake's chemistry and past algal production. Several abrupt changes occurred synchronously in both the terrestrial and aquatic components of the system, and these appear to represent ecological responses to pre-Holocene climatic variability. The sediments of Fog Lake provide the first integrated paleoecological record older than Holocene for a vast region of the Canadian Arctic, thereby refining our understanding of long-term arctic ecosystem ontogeny. 相似文献
64.
Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake 总被引:5,自引:0,他引:5
Marcello Bianca Carmelo Monaco Luigi Tortorici Licio Cernobori 《Geophysical Journal International》1999,139(2):370-394
We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400 kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3 mm yr −1 and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45 km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7 mm yr −1 is calculated, and a recurrence interval of about 550 ± 50 yr is proposed for large events similar to that of January 1693. 相似文献