عنوان
پدید آورنده
موضوع
رده
کتابخانه
محل استقرار
TLpq303891441
انگلیسی
Stable isotopic composition of soil carbonate and fossil teeth in paleoecologic reconstruction
[Thesis]
J. Quade
The University of Utah
1990
137
Ph.D.
The University of Utah
1990
The relationship of the usd\delta\spusdC and usd\delta\spusdO content of Holocene soil carbonates to variations in climate, plant cover, parent material, and soil depth was examined along several elevation transects in the southern Great Basin. usd\delta\spusdC of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth, indicating a decrease in the ratio of atmosphere to plant-derived CO2 downprofile. This pattern is consistent with an open system controlled by diffusion and isotopic equilibrium between carbon species, suggesting that kinetic or Rayleigh fractionation effects are not important. The higher proportion of C4 and CAM relative to C3 plants at low elevations, in combination with lower soil respiration rates, explain the progressive enrichment in soil carbonate with elevation decrease. It is likely that there has been complete isotopic exchange of C/C between HCO3\sp- in solution and soil CO2 prior to carbonate precipitation, irrespective of parent material. Layered carbonate and silica encrust fault fractures exposed in Trench 14 near Yucca Mountain, site of the proposed high-level nuclear waste repository in southern Nevada. The isotopic content of the fracture carbonates shows that they are pedogenic and likely formed in the presence of vegetation and rainfall typical of glacial climate. The fracture carbonates differ isotopically from carbonates deposited by deep circulation springs in the region. The regional water table therefore remained below the level of the Trench 14 during the Quaternary as the carbonates formed. Sediments comprising the Siwalik Group of northwestern Pakistan contain a long succession of buried paleosols, usually accompanied by pedogenic carbonate, spanning 16 to 0.4 Myr. The usd\delta\spusdC contents of carbonates show that prior to 7.3 million years ago, C3 plants, probably composed of mostly trees and shrubs, covered floodplains. By the early Pliocene and thereafter, C4 grassland had replaced the earlier forests. The usd\delta\spusdC of fossil tooth enamel from large floodplain herbivores reflect a dietary shift consistent with this biomass turnover. The appearance of grasslands likely marks the onset or a strong intensification of the Asian monsoon.
Earth sciences
Geochemistry
Nevada
Pakistan
Paleoecology
teeth
J. Quade
مطالعه متن کتاب p
[Thesis]
276903
a
Y
