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61.
Shifts in the hydrologic regime of Florida’s Apalachicola River have been attributed to anthropogenic changes throughout its watershed, including local dam construction. To assess impacts of those shifts on floodplain forests, we reconstructed tree growth using dendrochronology and compared these trends with hydrological and climatic variables. Comparisons of stream-gage data before and after dam construction on the Apalachicola River revealed statistically significant mean declines in annual average stage. Mean minimum annual stages, rise rates, and fall rates also decreased, while hydrograph reversals increased. Growth in four tree species correlated strongly with site-specific inundation parameters. A wetter climate in the two decades following dam construction and fine-scale fluctuation of the hydrograph may have set the stage for positive growth releases. Logging and hurricane wind throw events may have also contributed. However, drier conditions in the last two decades are now exacerbated by stage-discharge declines that had been masked previously. Tree growth rates and recruitment have decreased and, in the absence of a major disturbance, the forest canopy is composed of an older cohort of individuals. Our findings highlight how hydrograph variability, climate change, and vegetation disturbance are all relevant for gaging and anticipating the range of impacts of river modification on floodplain forests. 相似文献
62.
Colette C.A. Starheim Christopher Gomez Justin Harrison Claire Kain Nicholas J. Brewer Kirsty Owen Danang Sri Hadmoko Heather Purdie Peyman Zawar‐Reza Ian Owens Patrick Wassmer Franck Lavigne 《New Zealand geographer》2013,69(1):26-38
Previous research on debris‐flow deposit structure typically reports little to no visually discernible stratigraphy. The preliminary findings presented here provide evidence for more complex internal deposit architecture with inverse grading and subunits thought to reflect individual flow surges. Ground‐penetrating radar surveys, geospatial data and field observations are used to describe 10 subunits traceable over the 14 lateral radargrams imaging the lower 38 m of the deposit. Additional subunits are depicted further upslope in a longitudinal transect. As well as demonstrating the need for continued investigation of deposit architecture using non‐traditional techniques, these results may help improve future interpretations of post‐event deposits. 相似文献
63.
64.
Caleb J. Schiff Darrell S. Kaufman Alexander P. Wolfe Justin Dodd Zachary Sharp 《Journal of Paleolimnology》2009,41(1):189-208
The oxygen isotope ratios of diatoms (δ18Odiatom), and the oxygen and hydrogen isotope ratios of lake water (δW) of lakes in south Alaska provide insight into past changes
in atmospheric circulation. Lake water was collected from 31 lakes along an elevation transect and diatoms were isolated from
lake sediment from one lake (Mica Lake) in south Alaska. In general, δW values from coastal lakes overlap the global meteoric
water line (GMWL). δW values from interior lakes do not lie on the GMWL; they fall on a local evaporation line trajectory
suggesting source isotopes are depleted with respect to maritime lakes. Sediment cores were recovered from 58 m depth in Mica
Lake (60.96° N, 148.15° W; 100 m asl), an evaporation-insensitive lake in the western Prince William Sound. Thirteen calibrated
14C ages on terrestrial macrofossil samples were used to construct an age-depth model for core MC-2, which spans 9910 cal years.
Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a
newly designed, stepwise fluorination technique, which uses a CO2 laser-ablation system, coupled to a mass spectrometer, and has an external reproducibility of ±0.2‰. δ18Odiatom values from Mica Lake sediment range between 25.2 and 29.8‰. δ18Odiatom values are relatively uniform between 9.6 and 2.6 ka, but exhibit a four-fold increase in variability since 2.6 ka. High-resolution
sampling and analyses of the top 100 cm of our lake cores suggest large climate variability during the last 2000 years. The
20th century shows a +4.0‰ increase of δ18Odiatom values. Shifts of δ18Odiatom values are likely not related to changes in diatom taxa or dissolution effects. Late Holocene excursions to lower δ18Odiatom values suggest a reduction of south-to-north storm trajectories delivered by meridional flow, which likely corresponds to
prolonged intervals when the Aleutian Low pressure system weakened. Comparisons with isotope records of precipitation (δP)
from the region support the storm-track hypothesis, and add to evidence for variability in North Pacific atmospheric circulation
during the Holocene.
相似文献
Zachary SharpEmail: |
65.
TerEx Toolbox for semi‐automated selection of fluvial terrace and floodplain features from lidar 下载免费PDF全文
Terraces and floodplains are important indicators of near‐channel sediment dynamics, serving as potential sediment sources and sinks. Increasing availability of high resolution topography data over large areas calls for development of semi‐automated techniques for identification and measurement of these features. In this study we introduce a novel tool that accommodates user‐defined parameters including, a local‐relief threshold selected by a variable‐size moving window, minimum area threshold, and maximum distance from the channel to identify and map discrete terrace and floodplain surfaces. Each of the parameters can easily be calibrated for a given watershed or reach. Subsequently, the tool automatically measures planform area, absolute elevation, and height relative to the local river channel for each terrace polygon. We validate the tool in two locations where terrace maps were previously developed via manual digitization from lidar and extensive field mapping campaigns. The tool is also tested on six different types of rivers to provide examples of starting selection parameters, and to test effectiveness of the tool across a wide range of landscapes. Generally, the tool provides a high quality draft map of terrace and floodplain surfaces across the wide range of environmental conditions for which it has been tested. We find that the tool functions best in catchments where the terraces are spatially extensive, with distinct differences between the terrace and floodplain. The most challenging environments for semi‐automated terrace and floodplain mapping include steep catchments with dense riparian vegetation, and very small terraces (~10 m2 in areal extent). We then apply the tool to map terraces and floodplains in the Root River watershed, southeastern Minnesota and generate exceedance plots for terrace heights. These plots provide a first pass analysis to indicate the tributaries and reaches of the river where terraces constitute a significant source of sediment. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
66.
Laura J. Morrissey Karin M. Barovich Martin Hand Katherine E. Howard Justin L. Payne 《地学前缘(英文版)》2019,10(1):175-194
U-Pb monazite and zircon geochronology and calculated metamorphic phase diagrams from drill holes in the northern Gawler Craton, southern Australia, reveal the presence of ca. 1.45 Ga magmatism and metamorphism. Magmatism and granulite facies metamorphism of this age has not previously been recognised in the Gawler Craton. The magmatic rocks have steep LREE-enriched patterns and high Ga/Al values, suggesting they are A-type granites. Calculated metamorphic forward models suggest that this event was associated with high apparent thermal gradients and reached pressures of 3.2 -5.4 kbar and temperatures of 775-815℃. The high apparent thermal gradients may reflect pluton-enhanced metamorphism, consistent with the presence of A-type granites. The recognition of ca. 1.45 Ga tectonism in the northern Gawler Craton is added to a compilation of ca. 1.50 -1.40 Ga magmatism, shear zone reactivation, rift basin development and isotope resetting throughout the South and North Australian Cratons that shows that this event was widespread in eastern Proterozoic Australia. This event is stylistically similar to ca. 1.45 Ga A-type magmatism and high thermal gradient metamorphism in Laurentia in this interval and provides further support for a connection between Australia and Laurentia during the Mesoproterozoic. The tectonic setting of the 1.50-1.40 Ga event is unclear but may record rifting within the Nuna(or Columbia) supercontinent, or a period of intracontinental extension within a long-lived convergent setting. 相似文献
67.
Tom?RaimondoEmail authorView authors OrcID profile Justin?Payne Benjamin?Wade Pierre?Lanari Chris?Clark Martin?Hand 《Contributions to Mineralogy and Petrology》2017,172(4):17
A persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements, particularly the rare-earth elements (REEs). This is despite the fact that trace elements are generally less vulnerable to diffusive resetting, and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage, including the growth and/or dissolution of accessory minerals. We outline a technique for the routine acquisition of high-resolution 2D trace element maps by LA-ICP-MS, and introduce an extension of the software package XMapTools for rapid processing of LA-ICP-MS data to visualise and interpret compositional zoning patterns. These methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U–Pb monazite and Sm–Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to ‘see through’ multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism, metasomatism, and deformation. 相似文献
68.
Liam Reinhardt Douglas Jerolmack Brad J. Cardinale Veerle Vanacker Justin Wright 《地球表面变化过程与地形》2010,35(1):78-101
There appears to be no single axis of causality between life and its landscape, but rather, each exerts a simultaneous influence on the other over a wide range of temporal and spatial scales. These influences occur through feedbacks of differing strength and importance with co‐evolution representing the tightest coupling between biological and geomorphological systems. The ongoing failure to incorporate these dynamic bio‐physical interactions with human activity in landscape studies limits our ability to predict the response of landscapes to human disturbance and climate change. This limitation is a direct result of the poor communication between the ecological and geomorphological communities and consequent paucity of interdisciplinary research. Recognition of this failure led to the organization of the Meeting of Young Researchers in Earth Science (MYRES) III, titled ‘Dynamic Interactions of Life and its Landscape’. This paper synthesizes and expands upon key issues and findings from that meeting, to help chart a course for future collaboration among Earth surface scientists and ecologists: it represents the consensus view of a competitively selected group of 77 early‐career researchers. Two broad themes that serve to focus and motivate future research are identified: (1) co‐evolution of landforms and biological communities; and (2) humans as modifiers of the landscape (through direct and indirect actions). Also outlined are the state of the art in analytical, experimental and modelling techniques in ecological and geomorphological research, and novel new research avenues that combine these techniques are suggested. It is hoped that this paper will serve as an interdisciplinary reference for geomorphologists and ecologists looking to learn more about the other field. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
69.
David P. Hamilton Katherine R. O’Brien Michele A. Burford Justin D. Brookes Chris G. McBride 《Aquatic Sciences - Research Across Boundaries》2010,72(3):295-307
The factors affecting vertical distributions of chlorophyll fluorescence were examined in four temperate, warm monomictic
lakes. Each of the lakes (maximum depth >80 m) was sampled over 2 years at intervals from monthly to seasonal. Profiles were
taken of chlorophyll fluorescence (as a proxy for algal biomass), temperature and irradiance, as well as integrated samples
from the surface mixed layer for chlorophyll a (chl a) and nutrient concentrations in each lake. Depth profiles of chlorophyll fluorescence were also made along transects of the
longest axis of each lake. Chlorophyll fluorescence maxima occurred at depths closely correlated with euphotic depth (r
2 = 0.67, P < 0.01), which varied with nutrient status of the lakes. While seasonal thermal density stratification is a prerequisite
for the existence of a deep chlorophyll maximum (DCM), our study provides evidence that the depth of light penetration largely
dictates the DCM depth during stratification. Reduction in water clarity through eutrophication can cause a shift in phytoplankton
distributions from a DCM in spring or summer to a surface chlorophyll maximum within the surface mixed layer when the depth
of the euphotic zone (z
eu) is consistently shallower than the depth of the surface mixed layer (z
SML). Trophic status has a key role in determining vertical distributions of chlorophyll in the four lakes, but does not appear
to disrupt the annual cycle of maximum chlorophyll in winter. 相似文献
70.
Jonathan H. Sharp Kohei Yoshiyama Alexander E. Parker Matthew C. Schwartz Susan E. Curless Allison Y. Beauregard Justin E. Ossolinski Allen R. Davis 《Estuaries and Coasts》2009,32(6):1023-1043
The Delaware River and Bay Estuary is one of the major urbanized estuaries of the world. The 100-km long tidal river portion
of the estuary suffered from major summer hypoxia in the past due to municipal and industrial inputs in the urban region;
the estuary has seen remarkable water quality improvements from recent municipal sewage treatment upgrades. However, the estuary
still has extremely high nutrient loading, which appears to not have much adverse impact. Since the biogeochemistry of the
estuary has been relatively similar for the past two decades, our multiple year research database is used in this review paper
to address broad spatial and seasonal patterns of conditions in the tidal river and 120 km long saline bay. Dissolved oxygen
concentrations show impact from allochthonous urban inputs and meteorological forcing as well as biological influences. Nutrient
concentrations, although high, do not stimulate excessive algal biomass due to light and multiple nutrient element limitations.
Since the bay does not have strong persistent summer stratification, there is little potential for bottom water hypoxia. Elevated
chlorophyll concentrations do not exert much influence on light attenuation since resuspended bottom inorganic sediments dominate
the turbidity. Dissolved inorganic carbon and dissolved and particulate organic carbon distributions show significant variability
from watershed inputs and lesser impact from urban inputs and biological processes. Ratios of dissolved and particulate carbon,
nitrogen, and phosphorus help to understand watershed and urban inputs as well as autochthonous biological influences. Owing
to the relatively simple geometry of the system and localized anthropogenic inputs as well as a broad spatial and seasonal
database, it is possible to develop these biogeochemical trends and correlations for the Delaware Estuary. We suggest that
this biogeochemical perspective allows a revised evaluation of estuarine eutrophication that should have generic value for
understanding other estuarine and coastal waters. 相似文献