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1.
Testate amoebae as paleoindicators of surface-moisture changes on Michigan peatlands: modern ecology and hydrological calibration 总被引:6,自引:0,他引:6
Robert K. Booth 《Journal of Paleolimnology》2002,28(3):329-348
Peatland testate amoebae are sensitive indicators of local hydrology and have been used as proxies for surface moisture conditions in fossil studies. However, few regional calibration datasets exist in North America, and knowledge of testate amoeba ecology and distribution patterns are limited. The objectives of this study were to (1) investigate the relationship between testate amoebae, environment, and Sphagnum species in Michigan peatlands; (2) generate transfer functions from this dataset that can be applied to fossil data; and (3) describe vertical variation of testate amoebae inhabiting Sphagnum moss. Testate amoeba assemblages from 139 microsites within 11 peatlands in Michigan were compared to assess variability between and within peatlands. Most peatlands contained similar testate amoeba assemblages, although within individual peatlands the amount of assemblage variability is correlated to the amount of environmental heterogeneity. Of the measured environmental variables, depth to water table showed the strongest relationship with testate amoebae. Depth to water table can be reconstructed from fossil data with a mean error of ±7.5 cm, although predictive ability deteriorates in extremely dry environments (>30 cm water table depth). Vertical variation in testate amoeba assemblages was investigated at 89 Sphagnum-dominated microsites by directly comparing amoeba assemblages and the abundance and frequency of common taxa in upper and lower portions of the Sphagnum stem. Except for extremely dry microsites, considerable vertical variation in assemblage composition exists. Many agglutinate taxa are more abundant on lower portions of the Sphagnum stem, and taxa containing symbiotic zoochlorellae are more abundant on upper portions. Refinements in sampling procedures and analysis may improve the predictive ability of transfer functions. 相似文献
2.
C. Schnitchen D. J. Charman E. Magyari M. Braun I. Grigorszky B. Tóthmérész M. Molnár Zs. Szántó 《Journal of Paleolimnology》2006,36(1):1-17
Peatlands offer the potential for high resolution records of water balance over Holocene timescales, yet this potential is
under-exploited in many areas of the world. Within Europe, peatlands are mostly confined to areas north of 55° N, but several
areas of southern and eastern Europe contain small peatlands which may be suitable for palaeoclimatic reconstruction. In this
paper we test the potential of peatlands in the Carpathian region for deriving quantified estimates of water table changes
using testate amoebae analysis. A training set for palaeohydrological reconstruction from testate amoebae assemblages was
obtained by collecting surface samples from 13 peatlands, including 9 from Hungary and 4 from Transylvania (Romania). Using
a simple measure of mean annual water tables estimated from staining of PVC tape, we found that some peatlands were heavily
influenced by runoff and groundwater, and were therefore not suitable as modern analogues of ombrotrophic climatically sensitive
sites. The relationship between the testate amoebae assemblages in the modern samples and the environmental variables was
explored using CCA. The CCA biplot showed that the most important variables are depth to water table and moisture content,
confirming that hydrology is a key control on taxon distribution. pH was a secondary gradient. A transfer function for % moisture
and depth to water table was established and applied to fossil assemblages from a sequence from Fenyves-tető, Transylvania,
Romania. The reconstructed water table shows a number of variations which have parallels with other palaeoclimatic records
from Europe and the North Atlantic prominent phases of higher water tables are associated with the periods 8000–8300 cal BP,
3000–2500 cal BP and after 600 cal BP. We suggest that these were periods of particular intensification of westerly airflow
which affected eastern Europe as well as western and central Europe. 相似文献
3.
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: |
4.
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: |
5.
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: |
6.
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: |
7.
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: |
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.
France Oris Mariusz Lamentowicz Aurélie Genries Brice Mourier Olivier Blarquez Adam A. Ali Laurent Bremond Christopher Carcaillet 《Journal of Paleolimnology》2013,49(4):633-646
Testate amoebae that inhabit peat are sensitive indicators of water table position. In this study, we used testate amoebae in sediments from a mire in the western Alps (Lac du Thyl) to: (1) reconstruct the hydrology of the site over the last 7,000 years, (2) determine how hydrological changes affected testate amoebae diversity and (3) infer past trophic state shifts. The study site is located in one of the driest valleys of the Alps and is thus very sensitive to hydrological changes. Our study revealed that the water table depth increased (dry conditions) between 5,800 and 4,000 cal year BP. This triggered establishment of a Sphagnum-type peat and acidic conditions from 5,700 to 4,000 cal year BP. These processes were independent of ongoing transformations of the terrestrial vegetation and soil in the catchment area. After 1,690 cal year BP, the depth to the water table decreased (wetter conditions) and a minerotrophic fen developed. At the same time, the diversity of testate amoebae increased, probably as a result of deforestation that supported the expansion of grassland. Climate and land use were apparently more important factors controlling the lake hydrology than were changes in vegetation and soil in the catchment. Testate amoebae diversity was linked to land cover. Changes in pH were controlled indirectly by external forcing (climate), but more directly by fluctuations in the level of the water table (internal forcing) and autogenous expansion of Sphagnum. 相似文献
10.
Yangmin Qin Edward A. D. Mitchell Mariusz Lamentowicz Richard J. Payne Enrique Lara Yansheng Gu Xianyu Huang Hongmei Wang 《Journal of Paleolimnology》2013,50(3):319-330
Testate amoebae are a diverse and abundant group of protozoa that constitute a large proportion of biomass in many ecosystems and probably fill important roles in ecosystem function. These microorganisms have attracted the interest of paleoecologists because the preserved shells of testate amoebae and their known hydrological preferences enable reconstruction of past hydrological change. In ombrotrophic peatlands, surface wetness reflects hydroclimate, so testate amoebae can play an important role in reconstruction of Holocene climate change. Previous studies, however, have been geographically restricted, mostly to North America and Europe. We studied the ecology of testate amoebae in peatlands from central China in relation to hydrology, pH and metal concentrations. We found that testate amoeba community structure was correlated with depth to water table (DWT) and that the hydrological preferences of species generally matched those of previous studies. We developed a weighted average DWT transfer function that enables prediction of water table depth with a cross-validated mean error of <5 cm. Our results demonstrate the potential for using testate amoebae to reconstruct paleohydrology in China. Such studies could contribute to our understanding of Holocene climate changes in China, particularly regarding past Asian monsoon activity. 相似文献
11.
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: |
12.
Glen M. MacDonald David F. Porinchu Nicolas Rolland Konstantine V. Kremenetsky Darrell S. Kaufman 《Journal of Paleolimnology》2009,41(1):129-141
Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern
Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record,
we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W.
A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition
(LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer
water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses
suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing,
namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented
in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century.
As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA.
Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other
period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during
the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they
present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might
have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen
and lower temperatures in the hypolimnion of the more stratification-prone lake.
相似文献
Glen M. MacDonaldEmail: |
13.
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: |
14.
Mariusz Lamentowicz 《Journal of Paleolimnology》2010,43(3):499-511
Our palaeoecological study covers 73 years of history (1929–2002) of a kettle hole peatland inundated by water from a nearby,
dammed lake. Testate amoebae, pollen and non-pollen palynomorphs (NPPs) were used to track the shift to wetter conditions
in the peatland. Lead-210 was used to try and construct the chronology. We investigated how peatland testate amoebae communities
changed since the damming of a nearby river. Furthermore, we evaluated how rapidly local vegetation responded to the increase
in wetness, and how vegetation changes correlated with shifts in testate amoebae and NPPs. The Mukrza kettle hole provided
palaeoecological evidence of trophic state and hydrological changes since the lake filled with water in 1929. Three stages
of development were revealed. The first two were associated with initial inundation, and the third was related to Sphagnum expansion and acidification. Quantitative reconstruction of groundwater level and pH, inferred using testate amoebae, confirmed
our hypotheses about changes in hydrology and trophic state. Subfossil desmid remains lend qualitative support to the reconstruction.
The ecology of several testate amoeba taxa is discussed in the context of succession and population establishment. There was
complete species replacement since the time of inundation. Our investigation has two important applied aspects: (1) it enables
prediction of the response of peatlands to a rise in water table on restored sites; and (2) it provides analogues for palaeoclimatological
studies. The history of the Mukrza mire is an example of how palaeoecological studies can be used to assess the degree of
change in peatlands transformed by human activities. 相似文献
15.
Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands 总被引:1,自引:0,他引:1
Edward A. D. Mitchell Richard J. Payne Mariusz Lamentowicz 《Journal of Paleolimnology》2008,40(2):603-618
Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some
experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what,
if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence
for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative
palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative
abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications
of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations
using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent
the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance.
However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell
type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous
experimental work did not lead to significant difference in inferred water table. Our results show that differential shell
preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation
of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant,
and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic
peatlands. 相似文献
16.
The analysis of chironomid taxa and environmental datasets from 46 New Zealand lakes identified temperature (February mean air temperature) and lake production (chlorophyll a (Chl a)) as the main drivers of chironomid distribution. Temperature was the strongest driver of chironomid distribution and consequently produced the most robust inference models. We present two possible temperature transfer functions from this dataset. The most robust model (weighted averaging-partial least squares (WA-PLS), n = 36) was based on a dataset with the most productive (Chl a > 10 μg l−1) lakes removed. This model produced a coefficient of determination () of 0.77, and a root mean squared error of prediction (RMSEPjack) of 1.31°C. The Chl a transfer function (partial least squares (PLS), n = 37) was far less reliable, with an of 0.49 and an RMSEPjack of 0.46 Log10μg l−1. Both of these transfer functions could be improved by a revision of the taxonomy for the New Zealand chironomid taxa, particularly the genus Chironomus. The Chironomus morphotype was common in high altitude, cool, oligotrophic lakes and lowland, warm, eutrophic lakes. This could reflect the widespread distribution of one eurythermic species, or the collective distribution of a number of different Chironomus species with more limited tolerances. The Chl a transfer function could also be improved by inputting mean Chl a values into the inference model rather than the spot measurements that were available for this study. 相似文献
17.
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: |
18.
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: |
19.
Diatom-based models for reconstructing past water quality and productivity in New Zealand lakes 总被引:17,自引:12,他引:5
The trophic status of lakes in New Zealand is, on average, low compared to more densely populated areas of the globe. Despite this, trends of eutrophication are currently widespread due to recent intensification in agriculture. In order to better identify baseline productivity and establish long-term trends in lake trophic status, diatom-based transfer functions for productivity-related parameters were developed. Water quality data and surface sediment diatom assemblages from 53 lakes across the North and South Islands of New Zealand were analysed to determine species responses to the principal environmental gradients in the data set. Repeat sampling of water chemistry over a 12-month period enabled examination of species responses to annual means as well as means calculated for stratified and mixed periods. Variables found to be most strongly correlated with diatom species distributions were chlorophyll a (Chl a), total phosphorus (TP), dissolved reactive phosphorus (DRP), ionic concentration (measured as electrical conductivity (EC)) and pH. These variables were used to develop diatom-based transfer functions using weighted averaging regression and calibration (simple, tolerance down-weighted and with partial least squares algorithm applied). Overall, models derived for stratified means were weaker than those using annual or isothermal means. For specific variables, the models derived for the isothermal mean of EC (WA-tol r2jack = 0.79; RMSEP = 0.15 log10 S cm–1),the annual mean of pH (WA r2jack = 0.72; RMSEP = 0.25 pH units) and the isothermal mean of Chl a (WA r2jack = 0.71; RMSEP = 0.18 log10 mg m–3 Chl a) performed best. The models derived for TP were weak in comparison (for the annual mean of TP: WA r2jack = 0.50; RMSEP = 0.24 log10 mg m–3 TP) and residuals on estimates for this model were correlated with several other water quality variables, suggesting confounding of species responses to TP concentrations. The model derived for the isothermal mean of DRP was relatively strong (WA-tol r2jack = 0.78; RMSEP = 0.17 log10 mg m–3 DRP); however, residual values for this model were also found to be strongly correlated with several other water quality variables. It is concluded that the poor performance of the TP and DRP transfer functions relative to that of the Chl a model reflects the coexistence of nitrogen and phosphorus limitation within the lakes in the data set. In spite of this, the suite of transfer functions developed from the training set is regarded as a valuable addition to palaeolimnological studies in NewZealand. 相似文献
20.
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.
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Tad W. PatzekEmail: |