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1.
Prediction of Resource Volumes at Untested Locations Using Simple Local Prediction Models 总被引:1,自引:1,他引:0
This paper shows how local spatial nonparametric prediction models can be applied to estimate volumes of recoverable gas resources
at individual undrilled sites, at multiple sites on a regional scale, and to compute confidence bounds for regional volumes
based on the distribution of those estimates. An approach that combines cross-validation, the jackknife, and bootstrap procedures
is used to accomplish this task. Simulation experiments show that cross-validation can be applied beneficially to select an
appropriate prediction model. The cross-validation procedure worked well for a wide range of different states of nature and
levels of information. Jackknife procedures are used to compute individual prediction estimation errors at undrilled locations.
The jackknife replicates also are used with a bootstrap resampling procedure to compute confidence bounds for the total volume.
The method was applied to data (partitioned into a training set and target set) from the Devonian Antrim Shale continuous-type
gas play in the Michigan Basin in Otsego County, Michigan. The analysis showed that the model estimate of total recoverable
volumes at prediction sites is within 4 percent of the total observed volume. The model predictions also provide frequency
distributions of the cell volumes at the production unit scale. Such distributions are the basis for subsequent economic analyses.
相似文献
Emil D. AttanasiEmail: |
2.
The Arman field in western Kazakhstan is estimated to hold recoverable reserves of 3.65 million metric tons of oil and 74
million cubic meters of gas. The field began production in 1994 as a joint venture between Oryx Energy, MangistauMunaiGas,
and the State Holding Company Zharkyn, and currently is operated by Royal Dutch Shell in a 50:50 joint venture with Lukoil.
The geology, crude composition, and production history of Arman is outlined, followed by a review of the field development,
contract structure, and operating and capital expenditures. An assessment of field profitability concludes the discussion.
相似文献
Mark J. KaiserEmail: |
3.
American Association of Petroleum Geologists Energy Minerals Division 《Natural Resources Research》2007,16(3):243-261
This article contains a brief summary of some of the 2006 annual committee reports presented to the Energy Minerals Division
(EMD) of the American Association of Petroleum Geologists. The purpose of the reports is to advise EMD leadership and members
of the current status of research and developments of energy resources (other than conventional oil and natural gas that typically
occur in sandstone and carbonate rocks), energy economics, and geospatial information. This summary presented here by the
EMD is a service to the general geologic community. Included in this summary are reviews of the current research and activities
related to coal, coalbed methane, gas hydrates, gas shales, geospatial information technology related to energy resources,
geothermal resources, oil sands, and uranium resources.
相似文献
American Association of Petroleum Geologists, Energy Minerals DivisionEmail: |
4.
Timothy L. Cook Raymond S. Bradley Joseph S. Stoner Pierre Francus 《Journal of Paleolimnology》2009,41(1):77-94
Sediments in Lower Murray Lake, northern Ellesmere Island, Nunavut Canada (81°21′ N, 69°32′ W) contain annual laminations
(varves) that provide a record of sediment accumulation through the past 5000+ years. Annual mass accumulation was estimated
based on measurements of varve thickness and sediment bulk density. Comparison of Lower Murray Lake mass accumulation with
instrumental climate data, long-term records of climatic forcing mechanisms and other regional paleoclimate records suggests
that lake sedimentation is positively correlated with regional melt season temperatures driven by radiative forcing. The temperature
reconstruction suggests that recent temperatures are ~2.6°C higher than minimum temperatures observed during the Little Ice
Age, maximum temperatures during the past 5200 years exceeded modern values by ~0.6°C, and that minimum temperatures observed
approximately 2900 varve years BC were ~3.5°C colder than recent conditions. Recent temperatures were the warmest since the
fourteenth century, but similar conditions existed intermittently during the period spanning ~4000–1000 varve years ago. A
highly stable pattern of sedimentation throughout the period of record supports the use of annual mass accumulation in Lower
Murray Lake as a reliable proxy indicator of local climatic conditions in the past.
相似文献
Pierre FrancusEmail: |
5.
Tad W. Patzek 《Natural Resources Research》2006,15(4):255-270
This paper analyzes energy efficiency of the industrial corn-ethanol cycle. In particular, it critically evaluates earlier
publications by DOE, USDA, and UC Berkeley Energy Resources Group. It is demonstrated that most of the current First Law net-energy
models of the industrial corn-ethanol cycle are based on nonphysical assumptions and should be viewed with caution. In particular,
these models do not (i) define the system boundaries, (ii) conserve mass, and (iii) conserve energy. The energy cost of producing
and refining carbon fuels in real time, for example, corn and ethanol, is high relative to that of fossil fuels deposited
and concentrated over geological time. Proper mass and energy balances of corn fields and ethanol refineries that account
for the photosynthetic energy, part of the environment restoration work, and the coproduct energy have been formulated. These
balances show that energetically production of ethanol from corn is 2–4 times less favorable than production of gasoline from
petroleum. From thermodynamics it also follows that ecological damage wrought by industrial biofuel production must be severe.
With the DDGS coproduct energy credit, 3.9 gallons of ethanol displace on average the energy in 1 gallon of gasoline. Without
the DDGS energy credit, this average number is 6.2 gallons of ethanol. Equivalent CO2 emissions from corn ethanol are some 50% higher than those from gasoline, and become 100% higher if methane emissions from
cows fed with DDGS are accounted for. From the mass balance of soil it follows that ethanol coproducts should be returned
to the fields.
相似文献
Tad W. PatzekEmail: |
6.
Tad W. Patzek 《Natural Resources Research》2006,15(3):205-212
This paper analyzes the Illinois State Variety Test results for total and extractable starch content in 708 samples of 401
commercial varieties of corn. It is shown that the normally distributed extractable starch content has the mean of 66.2% and
the standard deviation of 1.13%. The corresponding maximum theoretical yield of ethanol is 0.364 kg EtOH/kg dry corn, and
the standard deviation is 0.007. In the ethanol industry units, this yield translates to 2.64 gal EtOH/nominal wet bushel,
and the standard deviation is 0.05 gal/bu.
The U.S. ethanol industry consistently has inflated its ethanol yields by counting 5 volume percent of # 14 gasoline denaturant
(8% of energy content) as ethanol. Also, imports from Brazil and higher alcohols seem to have been counted as U.S. ethanol.
The usually accepted USDA estimate of mean ethanol yield in the U.S., 2.682 gal EtOH/bu, is one standard deviation above the
rigorous statistical estimate in this paper.
相似文献
Tad W. PatzekEmail: |
7.
By applying the multi-Hubbert curve analysis to coal production in the United States, we demonstrate that anthracite production
can be modeled with a single Hubbert curve that extends to the practical end of commercial production of this highest-rank
coal. The production of bituminous coal from existing mines is about 80% complete and can be carried out at the current rate
for the next 20 years. The production of subbituminous coal from existing mines can be carried out at the current rate for
40–45 years. Significant new investment to extend the existing mines and build new ones would have to commence in 2009 to
sustain the current rate of coal production, 1 billion tons per year, in 2029. In view of the existing data, we conclude that
there is no spare coal production capacity of the size required for massive coal conversion to liquid transportation fuels.
Our analysis is independent of other factors that will prevent large-scale coal liquefaction projects: the inefficiency of
the process and either emissions of greenhouse gases or energy cost of sequestration.
相似文献
Tad W. PatzekEmail: |
8.
The United States has the world’s largest coal reserves and Montana the highest potential for mega-mine development. Consequently,
a large-scale effort to convert coal to liquids (CTL) has been proposed to create a major source of domestic transportation
fuels from coal, and some prominent Montanans want to be at the center of that effort. We calculate that the energy efficiency
of the best existing Fischer–Tropsch (FT) process applied to average coal in Montana is less than 1/2 of the corresponding
efficiency of an average crude oil refining process. The resulting CO2 emissions are 20 times (2000%) higher for CTL than for conventional petroleum products. One barrel of the FT fuel requires
roughly 800 kg of coal and 800 kg of water. The minimum energy cost of subsurface CO2 sequestration would be at least 40% of the FT fuel energy, essentially halving energy efficiency of the process. We argue
therefore that CTL conversion is not the most valuable use for the coal, nor will it ever be, as long as it is economical
to use natural gas for electric power generation. This finding results from the low efficiency inherent in FT synthesis, and
is independent of the monumental FT plant construction costs, mine construction costs, acute lack of water, and the associated
environmental impacts for Montana.
相似文献
Tad W. PatzekEmail: |
9.
C.-Y. Cynthia Lin 《Natural Resources Research》2009,18(1):19-28
This paper uses annual data on world oil price and consumption from 1965 to 2006 to calibrate a Hotelling model of optimal
nonrenewable resource extraction. Numerical solutions are generated for various specifications of the elasticity of demand
for both isoelastic demand and linear demand under each of two possible market structures: perfect competition and monopoly.
Prior to the 1973 oil crisis, the model that best fits actual data is one of perfect competition with linear demand and a
demand elasticity of −0.4. For the periods 1973–1981 and 1981–1990, the model that best fits actual data is one of monopoly
with linear demand and demand elasticities of −0.8 and −0.7, respectively, suggesting that the market was strongly influenced
by OPEC during this time. Under the model that best fits the most recent period (perfect competition with linear demand and
demand elasticity −0.5), the real oil price (in 1982–1984 U.S.$) should fall in the range $60.87–$66.31/barrel over the years
2010–2030.
相似文献
C.-Y. Cynthia LinEmail: |
10.
A lake sediment core recovered from Lake V57 on Victoria Island, Nunavut, Canada, spanning the last 2000 years, was analyzed
for sub-fossil midge remains and organic-matter content (estimated by loss-on-ignition (LOI)). Significant changes in midge
community composition occurred during the last 2000 years, with a distinct midge community appearing after 1600 AD. The chironomid
community between 0 and 1600 AD was dominated by Heterotrissocladius, Tanytarsus, Abiskomyia, and Paracladius. At approximately 1600 AD, Heterotrissocladius decreased in relative abundance and taxa such as Corynocera ambigua, C. oliveri, Psectrocladius sordidellus type, and Pentanneurini increased in relative abundance. Previously published midge-based inference models for average July
air temperature (AJAT) and summer surface–water temperature (SSWT) were applied to the subfossil midge stratigraphy. The AJAT
reconstruction indicates relatively cool conditions existed between 1100 and 1600 AD, with exceptional warming occuring after
1600 AD, as lake productivity inferred from organic-matter content increased concomitantly with midge-inferred AJAT and SSWT.
The cooler conditions between 1200 and 1600 AD, and the pattern of warming over recent centuries inferred from Lake V57 is
broadly consistent with temperature-sensitive biogenic silica records from other sites in the central Canadian Arctic and
the treeline zone to the south suggesting a regionally synchronous response to climate forcing.
相似文献
David F. PorinchuEmail: |
11.
Jeremy B. Platt 《Natural Resources Research》2007,16(3):263-275
Fuel prices in 2006 continued at record levels, with uranium continuing upward unabated and coal, SO2 emission allowances, and natural gas all softening. This softening did not continue for natural gas, however, whose prices
rose, fell and rose again, first following weather influences and, by the second quarter of 2007, continuing at high levels
without any support from fundamentals. This article reviews these trends and describes the remarkable increases in fuel expenses
for power generation. By the end of 2005, natural gas claimed 55% of annual power sector fuel expenses, even though it was
used for only 19% of electric generation. Although natural gas is enormously important to the power sector, the sector also
is an important driver of the natural gas market—growing to over 28% of the market even as total use has declined. The article
proceeds to discuss globalization, natural gas price risk, and technology developments. Forces of globalization are poised
to affect the energy markets in new ways—new in not being only about oil. Of particular interest in the growth of intermodal
traffic and its a little-understood impacts on rail traffic patterns and transportation costs, and expected rapidly expanding
LNG imports toward the end of the decade. Two aspects of natural gas price risk are discussed: how understanding the use of
gas in the power sector helps define price ceilings and floors for natural gas, and how the recent increase in the natural
gas production after years of record drilling could alter the supply–demand balance for the better. The article cautions,
however, that escalation in natural gas finding and development costs is countering the more positive developments that emerged
during 2006. Regarding technology, the exploitation of unconventional natural gas was one highlight. So too was the queuing
up of coal-fired power plants for the post-2010 period, a phenomenon that has come under great pressure with many consequences
including increased pressures in the natural gas market. The most significant illustration of these forces was the early 2007
suspension of development plans by a large power company, well before the Supreme Court’s ruling on CO2 as a tailpipe pollutant and President Bush’s call for global goals on CO2 emissions.
相似文献
Jeremy B. PlattEmail: |
12.
Climate of the past millennium inferred from varved proglacial lake sediments on northeast Baffin Island,Arctic Canada 总被引:1,自引:0,他引:1
This study uses 239+240Pu-dated varved sediments from Big Round Lake, a proglacial lake on northeast Baffin Island, Arctic Canada to generate a 1000-year-long,
annual-resolution record of past climate. Varve thickness is positively correlated with July–August–September temperature
measured at Clyde River, 70 km to the north of the lake (r = 0.46, p < 0.001). We therefore interpret the variability and trends in varve thickness to partially represent summer temperature.
The coolest Little Ice Age temperatures occurred in this record from 1575 to 1760 AD and were approximately 1.5°C cooler than
today (average from 1995 to 2005 AD) and 0.2°C cooler than the last millennium (average from 1000 to 2000 AD). Pre-twentieth-century
warmth occurred during two intervals, 970–1150 AD and 1375–1575 AD; temperatures were approximately 1.2°C cooler than today,
but 0.1°C warmer than the last millennium. The Big Round Lake varve-thickness record contains features similar to that reconstructed
elsewhere in the eastern Canadian Arctic. This high-resolution quantitative record expands our understanding of arctic climate
during the past millennium.
相似文献
Elizabeth K. ThomasEmail: |
13.
Holocene climate and glacier variability at Hallet and Greyling Lakes,Chugach Mountains,south-central Alaska 总被引:2,自引:0,他引:2
Evidence from lake sediments and glacier forefields from two hydrologically isolated lake basins is used to reconstruct Holocene
glacier and climate history at Hallet and Greyling Lakes in the central Chugach Mountains of south-central Alaska. Glacial
landform mapping, lichenometry, and equilibrium-line altitude reconstructions, along with changes in sedimentary biogenic-silica
content, bulk density, and grain-size distribution indicate a dynamic history of Holocene climate variability. The evidence
suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning
between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a
remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500 to 800 AD,
and the second during the Little Ice Age (LIA) from ~1400 to 1900 AD. During the LIA, the reconstructed equilibrium-line altitude
in the region was no more than 83 ± 44 m (n = 21) lower than the modern, which is based on the extent of glaciers during 1978. Differences between the summer temperature
inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased
winter precipitation from 1300 to 1500 AD, and reduced winter precipitation from 1800 to 1900 AD, likely associated with variability
in the strength of the Aleutian Low.
相似文献
Darrell S. KaufmanEmail: |
14.
The environmental controls on modern peatland testate amoebae (Protozoa: Rhizopoda) in the North of Ireland were investigated
to assess the potential for Holocene palaeoclimate research within this region. Canonical Correspondence Analysis (CCA) revealed
that hydrological factors (water table depth and moisture content) are the most important abiotic controls on organism distribution.
A series of partial CCAs showed that water table depth explains 15.8% and moisture content explains 5.5% of the total variance.
Monte-Carlo permutation tests showed that the results are highly significant (p < 0.002; p < 0.040 respectively). Transfer functions were generated for water table depth using weighted averaging tolerance downweighted
(WA-Tol) regression and for moisture content using weighted averaging partial least squares regression (WA-PLS). The performance
of the models was assessed using leave-one-out cross-validation (jacknifing). After removal of outlier samples, the improved
transfer functions were found to perform well with an r
jack2 and root mean square error of predictionjack of 0.83, 4.99 cm for water table depth and 0.76, 4.60% for moisture content respectively. The water table transfer function
was applied to a fossil peat sequence from this region and reconstruction errors were generated by 1,000 bootstrap cycles.
The water table reconstruction was also carried out using an established pan-European transfer function and was found to be
similar to that based on the North of Ireland dataset. This demonstrates the persistent and comparable control of hydrological
variables on the distribution of testate amoebae taxa across Europe and implies that regional training sets can suffice as
long as no-analogue situations are not encountered.
相似文献
G. T. SwindlesEmail: |
15.
Pollen-based reconstructions of late Holocene climate from the central and western Canadian Arctic 总被引:1,自引:0,他引:1
Two lake-sediment cores from the western and central Canadian Arctic were used to investigate late Holocene climate variability
in the region. Both cores were analyzed for pollen, organic matter, biogenic silica, and magnetic susceptibility, and were
dated using a combination of 210Pb and 14C techniques. Core MB01, from southwestern Victoria Island, provides a 2600-year-long record. Fossil pollen percentages, along
with other parameters, suggest the occurrence of a cold period around 2400 cal year BP (450 BC), followed by slightly warmer
conditions by 1800 cal year BP (150 AD), and a return to cooler conditions throughout much of the last millennium. Core SL06,
from southern Boothia Peninsula, shows more subtle changes in pollen percentages over its 2500-year duration, but an increase
in Cyperaceae and decrease in Oxyria pollen around 1400 cal year BP (550 AD) are indicative of warmer conditions at that time. Quantitative climate reconstructions
from these pollen sequences were compared to two other pollen-based climate records from the region and indicate the presence
of a widespread wet period ~1500 cal year BP (450 AD), and a cool and dry Little Ice Age. In the reconstructions based on
pollen percentage data, the twentieth century summer temperature and annual precipitation in the central and western Canadian
Arctic were comparable to that which occurred over the last 2500 years. However, pollen-influx values increase in the most
recent sediments, suggesting high plant productivity during the late twentieth century.
相似文献
Matthew C. PerosEmail: |
16.
A sedimentary record from lake Stora Viearvatn in northeast Iceland records environmental changes over the past 2000 years.
Downcore data include chironomid (Diptera: Chironomidae) assemblage data and total organic carbon, nitrogen, and biogenic
silica content. Sample scores from detrended correspondence analysis (DCA) of chironomid assemblage data are well correlated
with measured temperatures at Stykkishólmur over the 170 year instrumental record, indicating that chironomid assemblages
at Stora Viearvatn have responded sensitively to past temperature changes. DCA scores appear to be useful for quantitatively
inferring past temperatures at this site. In contrast, a quantitative chironomid-temperature transfer function developed for
northwestern Iceland does a relatively poor job of reconstructing temperature shifts, possibly due to the lake’s large size
and depth relative to the calibration sites or to the limited resolution of the subfossil taxonomy. The pre-instrumental climate
history inferred from chironomids and other paleolimnological proxies is supported by prior inferences from historical documents,
glacier reconstructions, and paleoceanographic studies. Much of the first millennium AD was relatively warm, with temperatures
comparable to warm decades of the twentieth century. Temperatures during parts of the tenth and eleventh centuries AD may
have been comparably warm. Biogenic silica concentrations declined, carbon:nitrogen ratios increased, and some chironomid
taxa disappeared from the lake between the thirteenth and nineteenth centuries, recording the decline of temperatures into
the Little Ice Age, increasing soil erosion, and declining lake productivity. All the proxy reconstructions indicate that
the most severe Little Ice Age conditions occurred during the eighteenth and nineteenth centuries, a period historically associated
with maximum sea-ice and glacier extent around Iceland.
相似文献
Peter G. LangdonEmail: |
17.
A computer methodology is presented that allows natural aggregate producers, local governmental, and nongovernmental planners
to define specific locations that may have sand and gravel deposits meeting user-specified minimum size, thickness, and geographic
and geologic criteria, in areas where the surficial geology has been mapped. As an example, the surficial geologic map of
the South Merrimack quadrangle was digitized and several digital geographic information system databases were downloaded from
the internet and used to estimate the sand and gravel resources in the quadrangle. More than 41 percent of the South Merrimack
quadrangle has been mapped as having sand and (or) gravel deposited by glacial meltwaters. These glaciofluvial areas are estimated
to contain a total of 10 million m3 of material mapped as gravel, 60 million m3 of material mapped as mixed sand and gravel, and another 50 million m3 of material mapped as sand with minor silt. The mean thickness of these areas is about 1.95 meters. Twenty tracts were selected,
each having individual areas of more than about 14 acres (5.67 hectares) of stratified glacial-meltwater sand and gravel deposits,
at least 10-feet (3.0 m) of material above the watertable, and not sterilized by the proximity of buildings, roads, streams
and other bodies of water, or railroads. The 20 tracts are estimated to contain between about 4 and 10 million short tons
(st) of gravel and 20 and 30 million st of sand. The five most gravel-rich tracts contain about 71 to 82 percent of the gravel
resources in all 20 tracts and about 54–56 percent of the sand. Using this methodology, and the above criteria, a group of
four tracts, divided by narrow areas sterilized by a small stream and secondary roads, may have the highest potential in the
quadrangle for sand and gravel resources.
相似文献
David M. SutphinEmail: |
18.
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: |
19.
Quantitative reconstructions of mean July temperatures (T
jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern
Fennoscandia and the Kola Peninsula are presented. T
jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error
of prediction (RMSEP) of 0.99°C. The most obvious trend in the inferred temperatures from all sites is the general decrease
in T
jul during the last 2000 years. Pollen-inferred T
jul values on average 0.18 ± 0.56°C (n = 91) higher than present (where “present” refers to the last three decades based on pollen-inferred T
jul in core-top samples) are indicated between 0 and 1100 AD (2000–850 cal year BP), and temperatures −0.2 ± 0.47°C (n = 78) below present are inferred between 1100 and 1900 AD (850–50 cal year BP). No consistent temperature peak is observed
during the ‘Medieval Warm Period’, ca. 900–1200 AD (1100–750 cal year BP), but the cooler period between 1100 and 1900 AD
(850–50 cal year BP) corresponds in general with the ‘Little Ice Age’ (LIA). Consistently with independent stable isotopic
data, the composite pollen-based record suggests that the coldest periods of the LIA date to 1500–1600 AD (450–350 cal year
BP) and 1800–1850 AD (150–100 cal year BP). An abrupt warming occurred at about 1900 AD and the twentieth century is the warmest
century since about 1000 AD (950 cal year BP).
相似文献
A. E. BjuneEmail: |
20.
Melissa L. Chipman Gina H. Clarke Benjamin F. Clegg Irene Gregory-Eaves Feng Sheng Hu 《Journal of Paleolimnology》2009,41(1):57-75
We analyzed sediments of the past 2000 years from Ongoke Lake, southwest Alaska, for organic carbon, organic nitrogen, biogenic
silica (BSi), and diatom assemblages at decadal to centennial resolution to infer limnological changes that may be related
to climatic variation in southwestern Alaska. The chronology is based on a 210Pb profile from bulk sediments and nine AMS 14C ages from terrestrial plant macrofossils. Four of the 14C ages span a core depth interval of 60.5 cm but are statistically indistinguishable from one another with a mean of ~1300
AD, which compromises the determination of temporal trends at Ongoke Lake and comparison with other paleoclimate records.
The diatom record suggests changes in the duration of ice cover and strength of thermal stratification that are probably related
to temperature variation. This variation includes a cold interval around the first millennium cooling (FMC) and a warm interval
spanning the medieval climate anomaly (MCA). However, the lake-sediment record shows no clear signals of temperature variation
for the period of the Little Ice Age (LIA) or the twentieth century. Climatic changes during these periods may have been manifested
through effective-moisture (precipitation minus evapotranspiration) variation in the Ongoke Lake area. We estimate water depths
and infer effective-moisture fluctuations by applying a regional transfer function to our diatom record. Together with inferences
from diatom autecologies, this water-depth reconstruction suggests that effective moisture increased steadily from 50 BC to
350 AD, which was followed by relatively dry conditions between 550 and 750 AD and relatively wet conditions between 750 and
1450 AD. Effective moisture was low from ~1450 to 1850 AD, coinciding with the LIA; an alternative age model places this interval
between ~1315 and 1850 AD. During the past 150 years, effective moisture increased, with estimated water depths reaching peak
values in the second half of the twentieth century. This study offers the first paleolimnological record for inferring centennial-scale
climatic variation over the past two millennia from southwestern Alaska.
相似文献
Feng Sheng HuEmail: |