排序方式： 共有53条查询结果，搜索用时 15 毫秒
31.
The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 14C yr BP, after which lake levels began to rise again to the Nipissing highstand at approximately 4700 14C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, as 21st-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases. 相似文献
32.
The influence of Asian summer monsoon variability on the water balance of a Tibetan lake 总被引：2，自引：0，他引：2
Carrie Morrill 《Journal of Paleolimnology》2004,32(3):273-286
Past water-balance changes in Tibetan lakes are generally attributed to changes in the strength of the summer monsoon. However, the water balance of a lake reflects many different water fluxes, which are controlled by many climatic and hydrologic processes. In this research, weather data and evaporation models are used to determine the climatic cause of a recent water-balance change in Ahung Co, a lake in central Tibet. Between 1995 and 2001, lake level rose at least 20 cm and the lake began to overflow. Results indicate that an increase in summer monsoon precipitation over the lake and drainage basin is responsible for the rise in lake level. Stronger monsoon conditions between 1995 and 2001 also led to decreased lake evaporation and basin evapotranspiration due to increased clouds and humidity. This contributed to the rise in lake level, but to a much smaller extent than the increase in monsoon precipitation. Lake evaporation during the spring and fall was also reduced between 1995 and 2001 due to longer lasting ice cover. Variations in ice cover play a small role in the overall water balance of Ahung Co, however, because the lake area is small compared to the drainage basin area. If these results hold true for the past, water-balance fluctuations inferred from the geochemistry of sediments from Ahung Co provide a record of variations in monsoon precipitation during the Holocene. 相似文献
33.
Holocene variations in the Asian monsoon inferred from the geochemistry of lake sediments in central Tibet 总被引：4，自引：0，他引：4
Carrie Morrill Jonathan T. Overpeck Julia E. Cole Caiming Shen 《Quaternary Research》2006,65(2):232-243
We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of % carbonate, % organic carbon, C/N and δ13C of bulk organic matter, δ13C and δ18O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (∼7500 cal yr B.P.). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr B.P. and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ∼4000 yr. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period. 相似文献
34.
Tarrío-Saavedra Javier Sánchez-Carnero Noela Prieto Andrés 《Mathematical Geosciences》2020,52(5):669-692
Mathematical Geosciences - Seabed classification in coastal environments is usually accomplished using multivariate methods applied to acoustic features from corrected or uncorrected echoes. This... 相似文献
35.
The morphometry of 85 gnammas (weathering pits) from Big Stone County in western Minnesota allows the assessment of the relative ages of the gnamma population. The ratio between maximum and minimum depths is independent of the initial size of the cavity and only depends on the weathering evolution. Therefore, the distribution of depth ratios can be used to assess the gnamma population age and the history of weathering. The asymmetrical distribution of depth ratios measured in Big Stone County forms three distinct populations. When these sets are analyzed independently, the correlation (r2) between maximum and minimum depths is greater than 0·95. Each single population has a normal distribution of depth ratios and the average depth ratios (δ‐value) for each population are δ1 = 1·60 ± 0·05, δ2 = 2·09 ± 0·04 and δ3 = 2·42 ± 0·08. The initiation of gnamma formation followed the exhumation of the granite in the region. This granite was till and saprolite covered upon retreat of the ice from the Last Glacial Maximum. Nearby outcrops are striated, but the study site remained buried until it was exhumed by paleofloods issuing from a proglacial lake. These Holocene‐aged gnammas in western Minnesota were compared with gnammas of other ages from around the world. Our new results are in accordance with the hypothesis that δ‐values represent the evolution of gnammas with time under temperate‐ to cold‐climate dynamics. Phases of the formation of new gnammas may result from changes in weathering processes related to climate changes. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
36.
Carrie J. Miller Daniel L. Roelke Stephen E. Davis Hsiu-Ping Li George Gable 《Estuarine, Coastal and Shelf Science》2008
Changes in magnitude and frequency of inflow results in subsequent alterations in the delivery of nutrients essential for phytoplankton growth and competition producing variations in community composition and nutritional value of phytoplankton. Zooplankton demographics are likely directly influenced by pulsed inflows due to flushing losses, whereas they are also indirectly affected by changes in prey quality. In this study, we report the potential effect of pulsed inflows on the plankton community of the Guadalupe Estuary. Microcosms were used that allowed control of light intensity and photoperiod, turbulence, temperature, nutrient loading, and flushing magnitude and periodicity. Our microcosm experiments were novel as they utilized natural plankton communities, thereby allowing the simultaneous interaction between hydrology, resource availability and grazing. Results show differences in microcosms according to magnitude and frequency of flushing. For example, copepod population density was greatest at the annual mean inflow magnitude. At half the annual mean inflow a decline in prey quality likely resulted in poorer grazer performance, and at double the annual mean inflow magnitude increased flushing losses prevented the incidence of higher copepod densities. Similarly, pulsed inflows resulted in greater copepod population densities, higher overall phytoplankton biomass, and dominance of centric diatoms (known to be faster growing and more edible). While reduced freshwater inflow associated with increasing anthropogenic demands often strains the needs of estuarine systems, the effects of reduced river input may be alleviated with several management options including manipulation of the timing, frequency, and magnitude of freshwater inflows. Before implications for management can be discerned from these findings, however, larger scale experiments are needed focus on the roles of inflow magnitude and frequency. 相似文献
37.
Claire L. A. Dell Guilherme O. Longo Carrie Manfrino Deron E. Burkepile 《Marine Ecology》2020,41(5):e12613
Many Caribbean reefs have shifted from coral dominance to macroalgal dominance, often by brown algae such as Dictyota and Lobophora. However, the north side of Cayman Brac in the Cayman Islands is dominated seasonally by the green macroalga Microdictyon (percent cover of Microdictyon is 4% in the winter and ~40% of the reef in the summer), although it is absent from the south side of the island and the remainder of the country. Indeed, Microdictyon is rare in much of the Caribbean, so this situation on Cayman Brac provides an opportunity to investigate the conditions that facilitate its distribution and dominance. The impact of herbivory, competition, nutrient input and other abiotic conditions were examined as factors that could influence the distribution and dominance of Microdictyon. While herbivory or nutrient input are frequently found to be key drivers of benthic community composition on coral reefs, here consumption of Microdictyon by herbivores was low, and thus, the alga was not subject to strong top-down control by herbivory. So, in this case, neither herbivore abundance nor feeding preference appeared to influence the distribution of Microdictyon. Nutrient input was also similar to both sides of the island suggesting nutrients played little role in differential distribution. But, in a controlled transplant experiment where Microdictyon was protected from herbivory and competition, it experienced almost complete mortality (93.3%) when transplanted to the south side, compared to only 11.8% mortality on the north. The south side was exposed to the strongest wave action 92% of the days in our study and was on average a slight, but significant 0.2°C warmer. Thus, these data suggest physical forcing (i.e. wave exposure) is the most likely factor dictating Microdictyon distribution. Conversely, a combination of reduced herbivory and increased competitive strength may explain the seasonal dominance of Microdictyon on the north side of Cayman Brac. Microdictyon was a competitive inferior to the other common algae in winter but increased in competitive strength to equal other species in summer. These results add to the literature on Microdictyon and on the forces impacting benthic community structure of coral reefs. 相似文献
38.
Large wood frequency and volume were examined as a function of landscape characteristics at different spatial and temporal scales in 50 reaches of the Upper Little Tennessee River basin with drainage areas ranging from 0.3 to 30.1 km2. Riparian forest cover was described laterally at the reach scale and longitudinally 1 km upstream in all tributaries. Riparian cover was analyzed with geomorphic and additional landscape variables to isolate factors that most influence wood in streams. Forested area immediately surrounding the reach was the strongest predictor of wood frequency and volume, although upstream riparian cover can explain additional variation in wood distributions. An optimal forested buffer width around the stream for large wood was not evident. The relationship between the riparian forest and wood weakens in bigger channels, as fluvial transport of pieces increases. Resurveys demonstrate that large wood is most dynamic in wide, forested reaches and changes function during floods to store sediment and organic matter. 相似文献
39.
David Morrill Anderson Bruce A. Bauer Charles R. Buckner Edward Gille Wendy S. Gross Michael Hartman Carrie Morrill Anju M. Shah Eugene R. Wahl 《Earth Science Informatics》2011,4(4):161-167
Reducing uncertainty in global temperature reconstructions of the past millennium remains the key issue in applying this record
to society’s pressing climate change problem. Reconstructions are collaborative, built on the research of hundreds of scientists
who apply their diverse scientific expertise and field and laboratory skill to create the individual proxy reconstructions
that underlie the multi-proxy, global average temperature time series. Web 2.0 features have enabled collaborative efforts
that improve the characterization of uncertainty. Raw data shared via a repository (the World Data Center for Paleoclimatology)
enable new reconstructions from the collection of user-generated data. Standards propagated by expert communities facilitate
quality control and interoperability. Open access to data and computer code promote transparency and make the science accessible
to a broader audience. Blogs, wikis, and listservs share background information and highlight contentious as well as unique
aspects of paleo science. A novel approach now underway, titled the Paleoclimate Reconstruction Challenge, and based on the
sharing of simulated data (pseudo-proxies) and reconstruction results, seeks to facilitate method development, further reducing
uncertainty. Broadly-useful aspects of the Challenge may find application in other fields. 相似文献
40.
Predator‐prey interactions can shape biological communities and drive evolutionary change. The assessment of predation rates in modern and ancient ecosystems presents many challenges. We examined the utility of one common predation metric, shell repair frequency, as a proxy for crushing predation on marine gastropod populations. Our six localities near Bamfield, British Columbia, represented two contrasting predation regimes: three high‐wave‐energy (exposed) environments have low predation rates, while three low‐wave‐energy (protected) environments exhibit high predation rates. We measured the occurrence of repair scars for local populations of the gastropod Chlorostoma funebrale, and recorded multiple independent variables (shell measurements, water velocity, predatory crab abundance) at each locality. Fisher's exact test established that the repair frequencies observed in the protected localities were significantly greater than those of the exposed environments. Factors important in predicting the likelihood of an individual having a repair scar were examined using binomial logistic regression. A model containing the main effects of crab abundance, shell height, shell thickness and water velocity provided the best fit in predicting the presence of repair scars, with crab abundance having the largest contribution. The strong relationship between the presence of repair scars and predator abundance indicates that repair frequency is a valid tool for assessing predation intensity in gastropod populations. 相似文献