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991.
The Tibesti massif, one of the most prominent features of the Sahara desert, covers an area of some 100,000 km2. Though largely absent from scientific inquiry for several decades, it is one of the world’s major volcanic provinces, and
a key example of continental hot spot volcanism. The intense activity of the TVP began as early as the Oligocene, though the
major products that mark its surface date from Lower Miocene to Quaternary (Furon (Geology of Africa. Oliver & Boyd, Edinburgh
(trans 1963, orig French 1960), pp 1–377, 1963)); Gourgaud and Vincent (J Volcanol Geotherm Res 129:261–290, 2004). We present here a new and consistent analysis of each of the main components of the Tibesti Volcanic Province (TVP), based
on examination of multispectral imagery and digital elevation data acquired from the Advanced Spaceborne Thermal Emission
and Reflection Radiometer (ASTER). Our synthesis of these individual surveys shows that the TVP is made up of several shield
volcanoes (up to 80 km diameter) with large-scale calderas, extensive lava plateaux and flow fields, widespread tephra deposits,
and a highly varied structural relief. We compare morphometric characteristics of the major TVP structures with other hot
spot volcanoes (the Hawaiian Islands, the Galápagos Islands, the Canary and Cape Verdes archipelagos, Jebel Marra (western
Sudan), and Martian volcanoes), and consider the implications of differing tectonic setting (continental versus oceanic),
the thickness and velocity of the lithosphere, the relative sizes of main volcanic features (e.g. summit calderas, steep slopes
at summit regions), and the extent and diversity of volcanic features. These comparisons reveal morphologic similarities between
volcanism in the Tibesti, the Galápagos, and Western Sudan but also some distinct features of the TVP. Additionally, we find
that a relatively haphazard spatial development of the TVP has occurred, with volcanism initially appearing in the Central
TVP and subsequently migrating to both the Eastern and Western TVP regions.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
992.
993.
The Gray Fossil Site (GFS) includes a small (<2 ha) paleosinkhole lake fill with an exceptionally well-preserved record of
sedimentation and fossils from the latest Miocene to earliest Pliocene. The uppermost lacustrine stratigraphy is characterized
by rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty clay (B)
laminae. Both the A and B components are almost exclusively comprised of exogenic sediment (including organic matter). Periodicities
of 24 and 4.4 are recorded within a continuous 96 interpreted year sequence of rhythmite sediment. In a small lake with a
poorly oxygenated bottom, the presence of laterally continuous laminated sediment that includes well-known periodicities in
rhythmite thickness is interpreted as representing annually generated varves that correspond to seasonal variations in sedimentation.
The distinctly larger fraction of medium sand-size quartz grains present within the A laminae, as well as the abrupt transitions
between A and B components suggest that the rhythmites represent deposition during alternating high-energy and lower-energy
seasons, which is consistent with a monsoonal precipitation pattern. The seasonal climate may relate to changes in the ocean
circulation pattern prior to 4.6 Ma that resulted in an increased temperature and atmospheric pressure gradient between the
east coast of North America and the Atlantic Ocean, but this climate phase seems to be only a temporary condition, as underlying
and overlying sediment are both consistent with drier conditions. The periodicity at 24 interpreted years is consistent with
the well-known Hale solar cycle. The 4.4 interpreted-year periodicity occurs within the ENSO frequency band, and if this documentation
of ENSO-like interannual climate change is correct, then it suggests that ENSO operated at times during the warm Earth conditions
characterizing the late Tertiary. 相似文献
994.
Jessica D. Tomkins Scott F. Lamoureux Dermot Antoniades Warwick F. Vincent 《Journal of Paleolimnology》2009,41(1):225-242
Sediment aggregates (“sedimentary pellets”) within the sedimentary record of Lake A (83°00′ N, 75°30′ W), Ellesmere Island,
Canada, are used to construct a 1000 year proxy record of ice-cover extent and dynamics on this perennially ice-covered, High
Arctic lake. These pellets are interpreted to form during fall or early winter when littoral sediment adheres to ice forming
around the lake’s periphery or during summer through the development of anchor ice. The sediment likely collects in ice interstices
and is concentrated in the upper ice layers through summer surface ice melt and winter basal ice growth. The pellets remain
frozen in the ice until a summer or series of summers with reduced ice cover allows for their deposition across the lake basin.
Sedimentary pellet frequency within multiple sediment cores is used to develop a chronology of ice-cover fluctuations. This
proxy ice-cover record is largely corroborated by a record of unusual sedimentation in Lake A involving iron-rich, dark-orange
to red laminae overlying more diffuse laminae with a lighter hue. This sediment sequence is hypothesized to represent years
with reduced ice cover through increased chemocline ventilation and iron deposition. During the past millennium, the most
notable period of inferred reduced ice cover is ca. 1891 AD to present. Another period of ice cover mobility is suggested
ca. 1582–1774 AD, while persistent ice cover is inferred during the 1800s and prior to 1582 AD. The proxy ice-cover record
corresponds well with most regional melt-season proxy temperature and paleoecological records, especially during the 1800s
and 1900s.
相似文献
Jessica D. TomkinsEmail: |
995.
Epiphytic diatoms as flood indicators 总被引:1,自引:0,他引:1
Johan A. Wiklund Natalie Bozinovski Roland I. Hall Brent B. Wolfe 《Journal of Paleolimnology》2010,44(1):25-42
The hydroecology of floodplain lakes is strongly regulated by flood events. The threat of climate warming and increasing human
activities requires development of scientific methods to quantify changes in the frequency of short-lived flood events, because
they remain difficult to identify using conventional paleolimnological and monitoring approaches. We developed an approach
to detect floods in sediment records by comparing the abundance and composition of epiphytic diatom communities in flooded
and non-flooded ponds of the Peace-Athabasca Delta (PAD), Canada, that grew on submerged macrophytes (Potamogeton zosteriformis, P.
perfoliatus) and an artificial substrate (polypropylene sheets) during the open-water season of 2005. Analysis of similarity tests showed
that epiphytic diatom community composition differs significantly between flooded and non-flooded ponds. After accounting
for the “pond effect,” paired comparisons of the three substrates determined that variation in community composition between
the artificial substrate and macrophytes was similar to that between the macrophyte taxa. Similarity percentage analysis identified
diatom taxa that discriminate between flooded and non-flooded ponds. The relative abundance of ‘strong flood indicator taxa’
was used to construct an event-scale flood record spanning the past 180 years using analyses of sedimentary diatom assemblages
from a closed-drainage pond (PAD 5). Results were verified by close agreement with an independent paleoflood record from a
nearby flood-prone oxbow pond (PAD 54) and historical records. Comparison of epiphytic diatoms in flooded and non-flooded
lakes in this study provides a promising approach to detect changes in flood frequency, and may have applications for reconstructing
other pulse-type disturbances such as hurricanes and pollutant spills. 相似文献
996.
The Geology and Petroleum Resources of Kansas: A Review From Alpha to Omega or From the Pleistocene to the Precambrian 总被引:1,自引:0,他引:1
Daniel F. Merriam 《Natural Resources Research》2010,19(4):293-316
The geological story of Kansas is told through the rocks that are present. It is a simple story in generalities but complex
in detail. Knowing the story, gives insight into understanding the occurrence and location of possible economic valuable minerals,
such as petroleum. This is a brief review of Kansas geology with respect to the known occurrence of oil and gas. Kansas is
part of the Midcontinent oil province with oil having been discovered 150 years ago and commercial production commencing in
1873. Although many prospects remain in Kansas, the state has gone from the number 1 producer in the U.S. in 1916 to 8th today.
Exploration for new oil and gas production therefore is going to have to be more imaginative and utilize new approaches and
techniques to find the elusive petroleum. There are possibilities however for the prospector who can search diligently. Although
the big fields probably have been discovered, the prospects today are deeper, in more undetectable traps, and in essentially
untested places. 相似文献
997.
Sofia U. Holmgren Christian Bigler Ólafur Ingólfsson Alexander P. Wolfe 《Journal of Paleolimnology》2010,43(2):393-412
Lake sediments from four small lakes on western Spitsbergen (Svalbard Archipelago, Norwegian High Arctic) preserve biostratigraphic and isotopic evidence for a complex suite of twentieth century environmental changes. At Lake Skardtjørna and Lake Tjørnskardet on Nordenskiöldkysten, there is a marked diatom floristic change coupled to increased diatom concentrations beginning around 1920. At Lake Istjørna and Lake Istjørnelva, 25 km southwest of Longyearbyen, both diatom total valve and chrysophyte stomatocyst concentrations have increased dramatically since the beginning of the 1900s. The early twentieth century changes are probably related to climate warming after the Little Ice Age. However, the most pronounced changes in diatom assemblages seem to have occurred in the last few decades. At the same time, nitrogen stable isotopes in sediment organic matter in two of the lakes became progressively depleted by ~2‰, which is consistent with diffuse atmospheric inputs from anthropogenic sources and attendant fertilization. These data suggest that climate change and nitrogen deposition may be acting together in driving these lakes towards new ecological states that are unique in the context of the Holocene. 相似文献
998.
Laurent Millet Charline Giguet-Covex Valérie Verneaux Jean-Claude Druart Thierry Adatte Fabien Arnaud 《Journal of Paleolimnology》2010,44(4):963-978
This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter
analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in
the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows
the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period,
a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state.
Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated
hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the
TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom
taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major
turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the
sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic
anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions,
a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either
the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological
approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological
state of the lake. 相似文献
999.
Anthropogenic global warming caused by CO2 emissions is strongly and fundamentally linked to future energy production. The Special Report on Emission Scenarios (SRES)
from 2000 contains 40 scenarios for future fossil fuel production and is used by the IPCC to assess future climate change.
Previous scenarios were withdrawn after exaggerating one or several trends. This study investigates underlying assumptions
on resource availability and future production expectations to determine whether exaggerations can be found in the present
set of emission scenarios as well. It is found that the SRES unnecessarily takes an overoptimistic stance and that future
production expectations are leaning toward spectacular increases from present output levels. In summary, we can only encourage
the IPCC to involve more resource experts and natural science in future emission scenarios. The current set, SRES, is biased
toward exaggerated resource availability and unrealistic expectations on future production outputs from fossil fuels. 相似文献
1000.
Yongqiang Xiong Fengchang Wu Jidun Fang Lifang Wang Yun Li Haiqing Liao 《Journal of Paleolimnology》2010,44(1):217-231
In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study
of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that
two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm
sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly
major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded.
Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth.
(2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes
in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that
primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost
10 cm, notable excursions to less negative δ13Corg and δ15Ntotal and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered
a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments. 相似文献