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671.
The 230Th content of subsurface Bahamas seawater has been estimated from incrementally deposited aragonite skeletons of sclerosponges using U-series age models which imply syndepositional incorporation of 230Th. First order age models, assuming constant growth rate and zero initial 230Th activity, display an average surface age of 63 ± 18 y despite the fact that the surfaces of the organisms were known to be alive at the time collection in the 1990s. A knowledge of the initial 230Th activity is necessary to incorporate the surface age constraint into an age model. In the Bahamas, this approach estimates an initial 230Th/232Th activity ratio of 4.3 ± 1.1 when applied to several individuals of the Caribbean sclerosponge Ceratoporella nicholsoni. These results differ from the same approach applied to previous work on Jamaican sclerosponges accreting aragonite in an environment closer to crustal sources and with less carbonate substrate. We report new measurements of Jamaican sclerosponge suggesting initial activity ratios of 230Th/232Th of ∼1, albeit with a high amount of uncertainty, indicating a regional difference in initial thorium activities. The estimates of initial 230Th/232Th activity ratios in the Bahamas are similar to the dissolved ratio rather than the particulate ratio and can be explained by mixing of open ocean water and bank-top surface water, however no information regarding temporal changes in the amount of initial 230Th/232Th activity available for incorporation into biogenic aragonite can be constrained using this approach. Growth rates do not change after correction for initial thorium and they display an inverse relationship with depth. Although this paper deals uniquely with sclerosponge aragonite, it is likely that the approach can be used on any incrementally deposited biogenic aragonite, provided information is known a priori about the first order growth model and there is evidence that initial thorium incorporation is representative of ambient environment rather than individual physiology. 相似文献
672.
T. J. Griffis J. Zhang J. M. Baker N. Kljun K. Billmark 《Boundary-Layer Meteorology》2007,123(2):295-316
In recent years considerable effort has been focused on combining micrometeorological and stable isotope techniques to partition
net fluxes and to study biosphere–atmosphere exchange processes. While much progress has been achieved over the last decade,
some new issues are beginning to emerge as technological advances, such as laser spectroscopy, permit isotopic fluxes to be
measured more easily and continuously in the field. Traditional investigations have quantified the isotopic composition of
biosphere-atmosphere exchange by using the Keeling two-member mixing model (the classic Keeling plot). An alternative method,
based on a new capacity to measure isotopic mixing ratios, is to determine the isotope composition of biosphere–atmosphere
exchange from the ratio of flux measurements. The objective of this study was to critically evaluate these methods for quantifying
the isotopic composition of ecosystem respiration (δR) over a period of three growing seasons (2003–2005) within a heterogeneous landscape consisting of C3 and C4 species. For C4 canopies, the mixing model approach produced δR values that were 4–6‰ lower (isotopically lighter) than the flux-gradient method. The analyses presented here strongly suggest
that differences between flux and concentration footprint functions are the main factor influencing the inequality between
the mixing model and flux-gradient approaches. A mixing model approach, which is based on the concentration footprint, can
have a source area influence more than 20-fold greater than the flux footprint. These results highlight the fact that isotopic
flux partitioning is susceptible to problems arising from combining signals (concentration and fluxes) that represent very
different spatial scales (footprint). This problem is likely to be most pronounced within heterogeneous terrain. However,
even under ideal conditions, the mismatch between concentration and flux footprints could have a detrimental impact on isotopic
flux partitioning where very small differences in isotopic signals must be resolved. 相似文献
673.
Development and Implementation of a Bayesian-based Aquifer Vulnerability Assessment in Florida 总被引:4,自引:1,他引:4
Jonathan D. Arthur H. Alex R. Wood Alan E. Baker James R. Cichon Gary L. Raines 《Natural Resources Research》2007,16(2):93-107
The Florida Aquifer Vulnerability Assessment (FAVA) was designed to provide a tool for environmental, regulatory, resource
management, and planning professionals to facilitate protection of groundwater resources from surface sources of contamination.
The FAVA project implements weights-of-evidence (WofE), a data-driven, Bayesian-probabilistic model to generate a series of
maps reflecting relative aquifer vulnerability of Florida’s principal aquifer systems. The vulnerability assessment process,
from project design to map implementation is described herein in reference to the Floridan aquifer system (FAS). The WofE
model calculates weighted relationships between hydrogeologic data layers that influence aquifer vulnerability and ambient
groundwater parameters in wells that reflect relative degrees of vulnerability. Statewide model input data layers (evidential
themes) include soil hydraulic conductivity, density of karst features, thickness of aquifer confinement, and hydraulic head
difference between the FAS and the watertable. Wells with median dissolved nitrogen concentrations exceeding statistically
established thresholds serve as training points in the WofE model. The resulting vulnerability map (response theme) reflects
classified posterior probabilities based on spatial relationships between the evidential themes and training points. The response
theme is subjected to extensive sensitivity and validation testing. Among the model validation techniques is calculation of
a response theme based on a different water-quality indicator of relative recharge or vulnerability: dissolved oxygen. Successful
implementation of the FAVA maps was facilitated by the overall project design, which included a needs assessment and iterative
technical advisory committee input and review. Ongoing programs to protect Florida’s springsheds have led to development of
larger-scale WofE-based vulnerability assessments. Additional applications of the maps include land-use planning amendments
and prioritization of land purchases to protect groundwater resources. 相似文献
674.
675.
Roger F. Reinking A. Shelby Frisch Brad W. Orr David L. Korn Luc R. Bissonnette Gilles Roy 《Boundary-Layer Meteorology》2003,109(3):255-284
Trapped Kelvin–Helmholtz (K–H) waves and vortices were monitored as they were generated immediately upwind of a mountain and driven into the barrier by a low-level jet. A stratus cloud visually revealed the embedded, propagating, gravity-shear waves. Interactions of the waves with the mountain were deciphered using remote sensing measurements of the structure, motions, and microphysics within the cloud and conceptual models based on existing theories. The observations show that the mountain acted as a dam to the flow that was primed for, but did not spontaneously induce, the waves. In response to the blocking, the waves spatially developed a pattern of formation, amplification, and breakdown between the upstream flow and the barrier, and altered the associated clouds in the process. Notably, radar signatures of velocity variance depicted organized, intertwined ribbons of relatively large vorticity within the wave layer. These provided measured evidence of the vortex sheet and streamwise vortex tubes predicted by advanced K–Hinstability theory, the three-dimensional version of Scorer's `stripe', the layer of rotational fluid between opposed flows that led to the wave generation. A theory of resonant interaction of wave trains, but with blocking imposed, appears to explain the internal structure of the pile-up of the flow and wave amplification approaching the barrier. Evolution of the supporting atmospheric thermal structure and introduction of a boundary-layer flow reversal follow a current model of blocking, although some features may have developed more directly from wave-driven mixing. The remote sensors also measured the influence of the waves on the cloud liquid water, including a cumulative enlargement of droplets as they were carried through a series of waves. 相似文献
676.
J. K. Vry J. Baker R. Maas T. A. Little R. Grapes M. Dixon 《Journal of Metamorphic Geology》2004,22(3):137-157
New (garnet Sm–Nd and Lu–Hf) and existing (Rb–Sr, 40Ar/39Ar, U–Pb and Sm–Nd) ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the Alpine Schist belt and Southern Alps orogen, New Zealand are diachronous and partly Cenozoic in age. The dominant metamorphic isograds in the Alpine Schist formed during crustal thickening at a previously unsuspected time, at c. 86 Ma, immediately prior to the opening of the Tasman Sea at c. 84–82 Ma. Obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism, and fabric elements can be highly diachronous. A detailed timing history for the growth of a single garnet is recorded by a Sm–Nd garnet–whole rock age of 97.8 ± 8.1 Ma for the inmost garnet core (zone 1), Lu–Hf ages of 86.2 ± 0.2 Ma and 86.3 ± 0.2 Ma for overgrowth zones 2 and 3, a step‐leach Sm–Nd age of 12 ± 37 Ma for zone 4, and growth of the garnet rim (zone 5) over the Alpine Fault mylonite foliation during the modern phase of oblique collision that began at c. 5–6 Ma. Plate convergence along the New Zealand portion of the Gondwana margin continued after c. 105 Ma, almost certainly culminating in the oblique collision of a large oceanic plateau (Hikurangi Plateau). The metamorphism of the Alpine Schist at c. 86 Ma is evidence of that hit. The mid‐ to late‐Cretaceous extension that is widespread elsewhere in the New Zealand region is attributed to upper plate extension and slab roll‐back. The effects of the collision with the Hikurangi Plateau may have contributed to the changing plate motions in the region leading up to the opening of the Tasman Sea at c. 82 Ma. 相似文献
677.
678.
679.
Peter T. Harris Michael G. Hughes Elaine K. Baker Robert W. Dalrymple Jock B. Keene 《Continental Shelf Research》2004,24(19):2431-2454
Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l−1 from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s−1 within the distributary channels to <50 cm s−1 on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10−3 degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates. 相似文献
680.
186Os enrichments in volcanic rocks and peridotite-derived iridosmine grains have been attributed to contributions from Earth’s outer core to the mantle, and apparently constrain the scale of mantle convection and an early timing for inner–outer core segregation more than 3.5 Gyr ago. Here, we highlight that marine ferromanganese crusts and nodules are characterised by high Pt/Os ratios and Pt–Os contents that develop much larger 186Os excesses over geological time (≥0.2%/Gyr) than those hypothesised for Earth’s outer core (<0.005–0.01%/Gyr). 187Os/188Os ratios in ferromanganese crusts are radiogenic due to sequestering of continental Os from seawater. Similarly, ancient ferromanganese materials may have had 186Os excesses (>0.1%) as a result of high Pt/Os ratios in continental crust, even prior to in-growth of 186Os after formation due to their high Pt/Os ratios. Past recycling of small amounts of these materials into the Earth’s mantle will produce coupled 187Os–186Os excesses and little change in Re and platinum-group-element concentrations, as observed in Hawaiian picrites, and in contrast to the predicted result of outer core addition to the mantle. 187Os and 186Os enrichments in the Hawaiian mantle source are potentially consistent with it comprising recycled oceanic lithosphere, pelagic sediments and ferromanganese materials, and questions the notion that Os isotopes can be used to uniquely identify core–mantle interactions and the depth at which mantle sources for volcanism originate. 相似文献