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91.
Jessica L. Gary Steven M. Colman Nigel J. Wattrus C. F. M. Lewis 《Journal of Paleolimnology》2012,47(3):299-311
Glacial Lake Agassiz, the largest of the North American glacial lakes, discharged through several different outlets during
its history, although the timing and location of discharge remain controversial. However, one discharge event is well established
based on extensive onshore observations: drainage through the Nipigon Lake area into the Superior basin about 10,700 years
ago, following retreat of ice of the Marquette advance from the basin. High-resolution, single-channel seismic-reflection
data collected with a small airgun were acquired to test the hypothesis that the Post-Marquette drainage event left diagnostic
stratigraphic and geomorphic signatures beneath Lake Superior. The unique bathymetry of northwestern Lake Superior, where
water depth plunges off Nipigon and Black Bays, makes this location ideal for the characterization of the post-Marquette depositional
and erosional features. According to our hypothesis, the initial, sudden discharge of high-velocity water would have eroded
channels through the bays. The steep and sudden drop-off into the Superior basin would have caused the flow to slow and drop
much of the sediment it was carrying. Our results confirm the existence of both erosional features and depositional sediment
packages related to Lake Agassiz discharge at this time. The erosional features include deeply incised bedrock channels in
the bays. The depositional features comprise subaqueous fans that are thickest in the deep water areas adjacent to the outlets
and thin lakeward and laterally away from the channels. The seismic character of the basal units of the fans, proximal to
the channels, is chaotic and only very weakly stratified, suggesting that these deposits represent coarse sediment laid down
during the initial, high-energy stages of the flood. These sediments are overlain by a stratified package which is interpreted
as the fine grained sediment associated with the later, low-energy stages of the flood. The combination of channels and subaqueous
fans is inferred to be diagnostic of high-energy Lake Agassiz discharge into the Superior basin, and they serve as analogs
for hypothesized discharge at other times. 相似文献
92.
The ‘era of interdisciplinarity’ heralds collaborative inquiry as effective for addressing complex issues at the nexus of disciplinary interests. Geographers have long argued that they are particularly well-suited to contribute to interdisciplinary endeavors because of the breadth and depth that the discipline enfolds. However, within the literature about geography and interdisciplinarity, we find only two rather limiting conversations. The first conversation is concerned with the role(s) and position of geography within academia and focuses on what geographers can do to distinguish themselves while also improving their interactions with scholars from other disciplines. The second conversation largely revolves around how best to conduct interdisciplinarity ‘in the lab and classroom’ and focuses on practical issues associated with making collaborative research operate smoothly for multiple, disciplinary participants. We propose opening up intellectual space for a third conversation about the benefits, challenges and contributions of individual disciplines in interdisciplinary environments. Using survey data, we analyze how geography was perceived by collaborators from various disciplines in an interdisciplinary Urban Ecology program at the University of Washington. We offer this pilot study as a heuristic for others wishing to perform similar small-scale reflexive exercises and advance this “third conversation”. 相似文献
93.
Michael J.S. Belton Peter Thomas Peter Schultz Lori Feaga Olivier Groussin Casey Lisse Jessica Sunshine W. Alan Delamere 《Icarus》2007,187(1):332-344
We consider the hypothesis that the layering observed on the surface of Comet 9P/Tempel 1 from the Deep Impact spacecraft and identified on other comet nuclei imaged by spacecraft (i.e., 19P/Borrelly and 81P/Wild 2) is ubiquitous on Jupiter family cometary nuclei and is an essential element of their internal structure. The observational characteristics of the layers on 9P/Tempel 1 are detailed and considered in the context of current theories of the accumulation and dynamical evolution of cometary nuclei. The works of Donn [Donn, B.D., 1990. Astron. Astrophys. 235, 441-446], Sirono and Greenberg [Sirono, S.-I., Greenberg, J.M., 2000. Icarus 145, 230-238] and the experiments of Wurm et al. [Wurm, G., Paraskov, G., Krauss, O., 2005. Icarus 178, 253-263] on the collision physics of porous aggregate bodies are used as basis for a conceptual model of the formation of layers. Our hypothesis is found to have implications for the place of origin of the JFCs and their subsequent dynamical history. Models of fragmentation and rubble pile building in the Kuiper belt in a period of collisional activity (e.g., [Kenyon, S.J., Luu, J.X., 1998. Astron. J. 115, 2136-2160; 1999a. Astron. J. 118, 1101-1119; 1999b. Astrophys. J. 526, 465-470; Farinella, P., Davis, D.R., Stern, S.A., 2000. In: Mannings, V., Boss, A.P., Russell, S.S. (Eds.), Protostars and Planets IV. Univ. of Arizona Press, Tucson, pp. 1255-1282; Durda, D.D., Stern, S.J., 2000. Icarus 145, 220-229]) following the formation of Neptune appear to be in conflict with the observed properties of the layers and irreconcilable with the hypothesis. Long-term residence in the scattered disk [Duncan, M.J., Levison, H.F., 1997. Science 276, 1670-1672; Duncan, M., Levison, H., Dones, L., 2004. In: Festou, M., Keller, H.U., Weaver, H.A. (Eds.), Comets II. Univ. of Arizona Press, Tucson, pp. 193-204] and/or a change in fragmentation outcome modeling may explain the long-term persistence of primordial layers. In any event, the existence of layers places constraints on the environment seen by the population of objects from which the Jupiter family comets originated. If correct, our hypothesis implies that the nuclei of Jupiter family comets are primordial remnants of the early agglomeration phase and that the physical structure of their interiors, except for the possible effects of compositional phase changes, is largely as it was when they were formed. We propose a new model for the interiors of Jupiter family cometary nuclei, called the talps or “layered pile” model, in which the interior consists of a core overlain by a pile of randomly stacked layers. We discuss how several cometary characteristics—layers, surface texture, indications of flow, compositional inhomogeneity, low bulk density low strength, propensity to split, etc., might be explained in terms of this model. Finally, we make some observational predictions and suggest goals for future space observations of these objects. 相似文献
94.
Climate sensitivity of the NCAR Community Climate Model (CCM2) to horizontal resolution 总被引:3,自引:1,他引:3
The dependence on horizontal resolution of the climate simulated by the National Center for Atmospheric Research Community Climate Model (CCM2) is explored. Simulations employing R15, T21, T31, T42, T63, and T106 horizontal spectral truncations are compared. Parameters associated with the diagnostic cloud scheme are modified for each resolution to provide similar global average cloud radiative forcing at each resolution. Overall, as with earlier studies, there are large differences between the low resolution R15 and T21 simulations and the medium resolution T42 simulation. Many climate statistics show a monotonic signal with increasing resolution, with the largest variation occurring from low to medium resolution. Although the monotonic signal is often from the low resolution simulations toward atmospheric analyses, in some cases it continues beyond the analyses at the highest resolution. Where convergence occurs, it is not always to the atmospheric analyses, and the highest resolution simulations are not the best by all measures. Although many climate statistics converge, the processes that maintain the climate do not, especially when considered on a regional basis. The implication is that the finer scales are required to capture the nonlinear processes that force the medium scales. Overall, it appears that, at a minimum, T42 resolution is required, but higher resolution would be better. Applications at T42 should take into consideration how model errors indicated by these resolution signals might affect any findings.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
95.
LIU Cheng LIU Ruiliang ZHOU Pengcheng LU Chun YANG Zengxin A. Mark POLLARD Peter HOMMEL MA Jian CUI Jianfeng Peter BRAY TONG Jianyi Jessica RAWSON 《《地质学报》英文版》2020,94(3):594-602
Tianshanbeilu is the largest Bronze Age site in eastern Xinjiang, China. Stretching across the entire second millennium BC, it performed a prominent role in connecting the Hexi corridor, Central China and the steppe. A further insight into the metallurgical tradition and the metal supply network is of vital importance to improve our understanding of its multi-connected nature. This paper offers a new set of chemical and isotopic data on the copper-based objects at Tianshanbeilu, including alloying elements, trace elements (impurities) and lead isotopes. Combining the concentrations of arsenic and antimony reveals that arsenic was introduced to copper partially due to the use of specific minerals tethrahedrite-tennantite. Lead isotopes demonstrate that multiple sources of copper were employed at Tianshanbeilu and a majority of them are characterized by common lead, which appears rather different from those of the Central Plains and the Hexi corridor, but highly consistent with local ores. Surprisingly, one object at Tianshanbeilu contains the well-known highly radiogenic lead. This object undoubtedly marks the westernmost boundary of the distribution of the highly radiogenic lead. We also anticipate that more lead isotopic analyses in NW China will further contribute to the study of the highly radiogenic lead in Central China. 相似文献
96.
Partitioning of Ca, Mn, Mg, and Fe2+ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400°C with melts in equilibrium with forsterite-rich olivine (Fo >95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q4, Q3, and Q2 type with nonbridging oxygen, therefore, in the Q3 and Q2 units.For melts with high Q3/Q2-abundance ratio (corresponding to NBO/T near 1), increasing Na/(Na+Ca) [and Na/(Na+Ca+Mn+Mg+Fe2+)] results in a systematic decrease of the partition coefficients, KCaol/melt, KMnol/melt, KMgol/melt, and KFe2+ol/melt, because of ordering of the network-modifying Ca, Mn, Mg, and Fe2+ among nonbridging oxygen in Q3 and Q2 structural units. This decrease is more pronounced the smaller the ionic radius of the cation. With decreasing Q3/Q2 abundance ratio (less-polymerized melts) this effect becomes less pronounced.Activity-composition relations among network-modifying cations in silicate melts are, therefore, governed by availability of energetically nonequivalent nonbridging oxygen in individual Qn-species in the melt. As a result, any composition change that enhances abundance of highly depolymerized Qn-species will cause partition coefficients to decrease. 相似文献
97.
James N. Jensen J. Donald Johnson Jessica St Aubin Russell F. Christman 《Organic Geochemistry》1985,8(1):71-76
Monochloramine is interesting both as a selective oxidant of fulvic acid and as a drinking water disinfectant. In this study, the controlled reaction of aquatic fulvic acid with monochloramine did not result in products detectable by ether extraction-gas chromatography-flame ionization detection techniques. Evidence of a reaction was shown by bleaching of the fulvic acid solution (decreased absorbance at 465 nm) and chlorine substitution. Chlorine-containing products were quantified by the total carbon adsorbable organic halide (TOX) parameter. Bleaching and organic chlorine formation were much less extensive than in the reaction of free chlorine with fulvic acid. Monochloramine was shown to produce an organic chlorine fraction more hydrophilic and with higher molecular size than that produced by chlorine and fulvic acid. Results suggest that monochloramine may be useful tool for the investigation of certain humic functionalities because it reacts rather selectively and to a small extent with aquatic fulvic acid. 相似文献
98.
99.
100.
Andrew Swales R. Bruce Williamson Laurence F. Van Dam Morag J. Stroud Matt S. McGlone 《Estuaries and Coasts》2002,25(1):43-56
Cores were collected from the length of Pakuranga estuary, a small urban estuary in Auckland, New Zealand, to determine sedimentation
and contaminant history, and in particular the impact of urbanization. Catchment sediment loads for the most recent history
(1953–1995), including urbanization since 1960, were reconstructed using the landcover history and soil erosion modeling.
Pollen and14C dating and pre-urban landcover history were used to reconstruct early estuary sedimentation (i.e., post-3000 yr BP to 1960).
Heavy metal concentrations, particle size,137Cs, pollen, and catchment sediment loads were all needed to disentangle the complex estuarine response to urbanization.137Cs profiles did not reflect the historical fallout pattern, but deposition of137Cs-labelled eroded catchment soil, coinciding with peaks in urban construction. Temporal variations in stormwater137Cs concentrations are likely due to varying contributions from137Cs-rich topsoil and137Cs-poor subsoils. A similar pattern was observed in heavy-metal concentrations and attributed to street runoff rather than
topsoil being diluted by metal-poor subsoils. Dating of the sediment profiles showed that during urbanization sedimentation
rates in the tidal creek and estuary were higher than sedimentation rates associated with past agricultural landuse and the
original forest landcover. Urbanization has brought about substantial environmental changes in the upper estuary through continued
infilling of shallow, intertidal areas, contamination by heavy metals to levels of ecological concern, sediment textural changes,
and rapid mangrove colonization of formerly bare intertidal sediments. 相似文献