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1.
2.
Jack Wisdom 《Celestial Mechanics and Dynamical Astronomy》1986,38(2):175-180
The solution by Sessin and Ferraz-Mello (Celes. Mech.
32, 307–332) of the Hori auxiliary system for the motion of two planets with periods nearly commensurate in the ratio 21 is considerably simplified by the introduction of canonical variables. An analogous canonical transformation simplifies the elliptic restricted problem. 相似文献
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4.
We describe the tidal circulation and salinity regime of a coastal plain estuary that connects to the ocean through a flood
tide delta. The delta acts as a sill, and we examine the mechanisms through which the sill affects exchange of estuarine water
with the ocean. Given enough buoyancy, the dynamics of tidal intrusion fronts across the sill and selective withdrawal (aspiration)
in the deeper channel landward appear to control the exchange of seawater with estuarine water. Comparison of currents on
the sill and stratification in the channel reveals aspiration depths smaller than channel depth during neap tide. During neap
tide and strong vertical stratification, seawater plunges beneath the less dense estuarine water somewhere on the sill. Turbulence
in the intruding bottom layer on the sill promotes entrainment of fluid from the surface layer, and the seawater along the
sill bottom is diluted with estuarine water. During ebb flow, salt is effectively trapped landward of the sill in a stagnant
zone between the aspiration depth and the bottom where it can be advected farther upstream by flood currents. During spring
tide, the plunge point moves landward and off the sill, stratification is weakened in the deep channel, and aspiration during
ebb extends to the bottom. This prevents the formation of stagnant water near the bottom, and the estuary is flooded with
high salinity water far inland. The neapspring cycle of tidal intrusion fronts on flood coupled with aspiration during ebb
interacts with the sill to play an important role in the transport and retention of salt within the estuary. 相似文献
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7.
Jack B. Epstein 《Geological Journal》1986,21(3):283-306
Many contributions that have led to a better understanding of Appalachian geology have resulted directly from work in the folded Appalachian Mountain and Great Valley sections of the Valley and Ridge physiographic province of eastern Pennsylvania. Disagreements have been common since H.D. Rogers first described the geology of the area in 1858. Many differing opinions still exist regarding the stratigraphy, structural geology, geomorphology, and glacial geology. The rocks in the area, which range from Middle Ordovician to Late Devonian in age, are more than 25000 feet (7620 m) thick. This diversified group of sedimentary rocks was deposited in many different environments, ranging from deep sea, through neritic and tidal, to alluvial. In general, the Middle Ordovician through Lower Devonian strata are a sedimentary cycle related to the waxing and waning of Taconic tectonism. The sequence began with a greywacke-argillite suite (Martinsburg Formation) representing synorogenic basin deepening. This was followed by basin filling and pro-gradation of a sandstone-shale clastic wedge (Shawangunk Formation and Bloomsburg Red Beds) derived from the erosion of the mountains that were uplifted during the Taconic orogeny. The sequence ended with deposition of many thin units of carbonate, sandstone, and shale on a shelf marginal to a land area of low relief. Another tectonic-sedimentary cycle, related to the Acadian orogeny, began with deposition of Middle Devonian rocks. Deep-water shales (Marcellus Shale) preceded shoaling (Mahantango Formation) and turbidite sedimentation (Trimmers Rock Formation) followed by another molasse (Catskill Formation). 相似文献
8.
A late Quaternary ichthyofauna from Homestead Cave, Utah, provides a new source of information on lake history in the Bonneville basin. The fish, represented by 11 freshwater species, were accumulated between 11,200 and 1000 14C yr B.P. by scavenging owls. The 87Sr/86Sr ratio of Lake Bonneville varied with its elevation; 87Sr/86Sr values of fish from the lowest stratum of the cave suggest they grew in a lake near the terminal Pleistocene Gilbert shoreline. In the lowest deposits, a decrease in fish size and an increase in species tolerant of higher salinities or temperatures suggest multiple die-offs associated with declining lake levels. An initial, catastrophic, post-Provo die-off occurred at 11,300–11,200 14C yr B.P. and was followed by at least one rebound or recolonization of fish populations, but fish were gone from Lake Bonneville sometime before 10,400 14C yr B.P. This evidence is inconsistent with previous inferences of a near desiccation of Lake Bonneville between 13,000 and 12,000 14C yr B.P. Peaks in Gila atraria frequencies in the upper strata suggest the Great Salt Lake had highstands at 3400 and 1000 14C yr B.P. 相似文献
9.
J. O. Blanton A. J. Garrett J. S. Bollinger D. W. Hayes L. D. Koffman J. Amft 《Estuaries and Coasts》2009,32(3):573-592
In late December 1991, an accidental release of 5,700 CI of tritiated water (HTO) from the Savannah River Site was transported
via site streams into the Savannah River where it was carried downstream to the coastal zone. HTO released into a semitropical
Georgia estuary was forced into the tidal marshes surrounding the estuary as well as discharged directly into the Atlantic
Ocean. The spreading of HTO was studied with a 3D hydrodynamic model (ALGE) that includes flooding and draining of intertidal
areas. Comparisons of model simulations to measured HTO concentration showed that ALGE simulated well the general increase
and decrease of HTO as its plume passed a given area. The “sheet flow” approximation for marsh and small tidal creek flow
largely compensated for lack of model resolution and accurate bathymetry in areas with numerous small to medium-sized tidal
creeks. The water volume of the unresolved tidal creeks had to be accounted for in the simulations by increasing the initial
water depth over the marshes. ALGE and a simple box model both reproduced the trapping of HTO in intertidal areas. The time
scale over which intertidal areas import and export HTO back to the tidal channels varies between 10 and 30 days. 相似文献
10.
Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation? 下载免费PDF全文
Dominik C. Hezel Graeme M. Poole Jack Hoyes Barry J. Coles Catherine Unsworth Nina Albrecht Caroline Smith Mark Rehkämper Andreas Pack Matthew Genge Sara S. Russell 《Meteoritics & planetary science》2015,50(2):229-242
Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ56Fe values up to +0.35‰ relative to the solar system mean. On average, the δ56Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ56Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ56Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ18O (0.9–1.5‰) than their host chondrites, and the same is true for Δ17O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an 16O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts. 相似文献