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1.
The minimum sieve mesh size for fossil chironomid analysis is usually set at 100 μm, to ensure adequate recovery of small
species. Yet taking into account the labor intensity of sorting and identifying fossil chironomid remains, the large numbers
of samples requiring processing in paleoclimate studies with high temporal resolution, and the increasing need to engage non-specialist
analysts in this work, it seems appealing to sieve samples through a larger mesh size that would mainly retain easier-to-see,
easier-to-extract and easier-to-identify fossil specimens. In this study we evaluated the influence of sieve mesh size on
chironomid-based quantitative paleoenvironmental reconstructions in African lakes. We developed two chironomid-salinity calibration
data sets based on either the >100 μm or >150 μm size fractions of surface-sediment fossil assemblages, and compared the performance
of salinity-inference models derived from them. We find that, despite ∼35% additional fossil loss, restriction to >150 μm
data did not appreciably affect individual taxon optima or tolerances, and resulted in only a modest reduction of mean taxon
richness per lake. Parameters of statistical model performance were as good, or better than those of models based on the >100 μm
data, albeit only after excluding two atypically dilute (<100 μS/cm) lakes from the calibration. Application of inference
models based on >150 μm or >100 μm data to a 200-year fossil record from Lake Abiyata, Ethiopia, produced very similar trends,
amplitudes and uncertainty ranges of inferred past salinity change. Restriction to >150 μm data reduced the mean fossil yield
of core samples by ∼20% on average, i.e. fossil loss was markedly lower than in surface-sediment samples. Using the larger
mesh size reduced sample processing time by up to 50%, partly by removing a significant proportion of visually obstructive
organic debris, and partly by allowing a greater fraction of chironomid fossils to be identified directly in the sorting tray.
The fraction of 1st instar group taxa in surface-sediment samples was reduced from 13% to 3%, increasing the mean taxonomic
resolution of fossil assemblages, and thus their ecological specificity. 相似文献
2.
All the chironomid head capsules in a sediment sample are normally extracted to determine the concentration of chironomids.
This method may be tedious since a sample may contain hundreds of head capsules. We have tested a method to estimate the concentration
of chironomids in sub-samples and assessed the potential taxonomic bias generated when only a part of each sediment sample
is sorted. To allow calculation of concentrations, a known quantity of exotic markers (microspheres with diameter 149–350 μm)
was added to the sample. The number of head capsules in the total sample was estimated according to the ratio between the
retrieved numbers of microspheres to head capsules in the sub-sample. According to our results, the chironomid concentrations
estimated from sub-samples were statistically similar to the concentrations obtained by processing the full sample. Also,
a sub-sample containing at least 75 head capsules was likely to have similar taxa abundances and taxa richness as the full
sediment sample. These results indicate that exotic markers may be added to chironomid samples for representative concentration
estimation. The method may potentially be used for other biological groups than chironomids. 相似文献
3.
Maarten van Hardenbroek Oliver Heiri Jonathan Grey Paul L. E. Bodelier Frederike Verbruggen André F. Lotter 《Journal of Paleolimnology》2010,43(2):235-245
We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules
of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally
13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We
first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from
other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments
that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect
on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰)
but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into
chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains.
We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links
and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of
MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface
in lakes. 相似文献
4.
Katherine Griffiths Neal Michelutti Jules M. Blais Lynda E. Kimpe John P. Smol 《Journal of Paleolimnology》2010,44(2):405-412
The mass transport of nutrients by migratory animals can markedly alter the biogeochemistry and ecology of recipient ecosystems,
particularly in nutrient-poor regions such as the Arctic. However, the role of biovectors in the global cycling of nutrients
is often overlooked. Here we investigate nitrogen dynamics in two seabird-affected ponds in the Canadian High Arctic. The
ponds lie at the base of a large seabird colony and have been greatly enriched in nutrients due to the input of guano and
other wastes. Using sediment cores that span the last ~200 years, we measured stable isotopes of nitrogen (δ15N) in bulk sediments as well from the subfossil remains of chironomid (Diptera) head capsules and Daphnia ephippia. The bulk-sediment samples from our seabird-affected ponds had elevated δ15N values relative to seabird-free sites elsewhere in the Arctic. In general, the chironomid δ15N profiles roughly paralleled those of bulk sediments in both study ponds, while the Daphnia profile remained relatively stable in contrast to the considerable variation recorded in the bulk sediments and chironomids.
Interestingly, no apparent pattern emerged among δ15N values recorded in the bulk sediments, chironomids, and Daphnia between the two study ponds. The stability recorded in the δ15N profiles from bulk sediments relative to the more variable invertebrate profiles point towards the complexity of nitrogen
uptake by chironomids and Daphnia at these sites. These data suggest that the bulk sediments are integrating the different fractions of the overall δ15N pool and thus may be most appropriate for reconstructing overall trends in lake trophic status. 相似文献
5.
Spatial variability of chironomid death assemblages in the surface sediments of a fluctuating tropical lake (Lake Naivasha,Kenya) 总被引:1,自引:0,他引:1
Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically
stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated
with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability
of surface-sediment chironomid assemblages in the fairly large (∼100–170 km2 since 1983) and shallow (Z
max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller
adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island
Crater, 1.9 km2, 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects,
and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms
of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally
higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake
variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes,
but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed
a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food
quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented
the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas
the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics
in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the
near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow
fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages
can be exploited for within-lake calibration of environmental gradients. 相似文献
6.
F. Verbruggen O. Heiri G.-J. Reichart J. W. De Leeuw K. G. J. Nierop A. F. Lotter 《Journal of Paleolimnology》2010,43(4):857-872
Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains
can be used to reconstruct past lake water δ18O and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable
oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical
composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms
of pyrolyzates show that NaClO2 causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated
with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially
after KOH treatment at high concentration (28%) and temperature (100°C). Scanning electron microscopy confirmed that a proteinaceous
matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in
head capsules pretreated with NaClO2. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid δ18O values. Our results suggest that deproteination results in a relative increase of chironomid δ18O, whereas removal of chitin leads to decreased δ18O values. We therefore discourage the use of acids or prolonged (≥1 h) exposure to hot alkali (70°C) prior to chironomid δ18O analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid
δ18O measurements. Caution and standardization are required when pretreating samples for chironomid δ18O analysis to ensure reliable, comparable, and reproducible results. 相似文献
7.
Within-lake variability of subfossil chironomid assemblages in shallow Norwegian lakes 总被引:3,自引:5,他引:3
Oliver Heiri 《Journal of Paleolimnology》2004,32(1):67-84
Subfossil chironomids in the surface sediments of five small and shallow Norwegian lakes were studied to determine the within-lake variability of fossil assemblages, changes in chironomid assemblages with respect to water depth, and the representativeness of single samples for the entire chironomid fauna of a lake. In each of the lakes studied, six short sediment cores in the deepest part of the lake basin and two littoral to deep-water transects of seven cores each were obtained using a gravity corer, and chironomid assemblages in the uppermost centimetre of sediment were analysed. In three of the five lakes, chironomid concentrations were highest in the deepest parts of the lake basins. In the remaining two lakes, concentrations were either very variable or, in a lake with clear indications of anoxia in the bottom waters, highest at intermediate water depth. Chironomid assemblages tended to be dominated by the same taxa within a lake basin. However, in each of the lakes studied there was a clear and statistically significant shift in chironomid assemblages with respect to water depth. The organic content of the sediments was statistically significant in explaining the variance in the chironomid assemblages only in lakes where organic matter content was closely related to water depth. Only a few chironomid taxa were restricted to the shallowest parts of the lake basins, whereas a number of chironomids were found exclusively in deep-water sediments. Chironomid head capsules of running water taxa and simuliid remains were generally found in sediments close to lake tributaries and in the deepest parts of the lake basins. Although any individual sample contained only a part of the total subfossil chironomid fauna (21–63% of the total taxa per lake), chironomids dominant in any section of the study lakes were found in most of the transect and mid-lake samples. 相似文献
8.
P. J. van Hengstum E. G. Reinhardt J. I. Boyce C. Clark 《Journal of Paleolimnology》2007,37(4):603-618
This study examines the anthropogenic alteration of sedimentation in Frenchman’s Bay in Lake Ontario, using high-resolution
particle size analysis in two 200 cm cores. Lithofacies were determined using the particle size data of both the terrigenous
sediment and terrigenous sediment + diatom fractions. Terrigenous particle size data from the centre of the lagoon provided
the most representative record of anthropogenic impacts. Three distinctive lithofacies were recognized: (1) a Natural Wetland
(NW) lithofacies (106–200 cm) had an average mean particle size of 49.4 μm, a mode of 29.4 μm and an average standard deviation
of 119.1 μm; (2) an Agricultural and Deforestation (AD) lithofacies (40–105 cm) had a statistically significant lower average
mean (30.8 μm), mode (13.5 μm), and standard deviation (48.5) μm; (3) an Urbanized (U) lithofacies (0–40 cm) showed a continued
trend towards smaller particle sizes with an average mean of 21.2 μm, a mode of 9.4 μm, and an average standard deviation
of 32.7 μm. The lithofacies correlated with previously identified trends in thecamoebian biofacies and magnetic susceptibility
data showing post-colonial lagoon eutrophication and increased overland soil erosion. The up-core trend towards finer and
less variable particle sizes is attributed to erosion of fine-grained watershed sediments (glacial Lake Iroquois silts and
clay) during land-clearance and modification of natural drainage patterns. The influx of silts and clays into the lagoon is
also recorded by increased sediment accumulation rates and a reduction in seasonal sediment variability in the wetland. Based
on the 210Pb dates, sedimentation rates increased at 1850 ±56 AD (AD lithofacies) and suggest an exponentially increasing trend in accumulation
rates. Increasing sedimentation rates can be attributed to the progressive loss of native vegetation and intensified erosion
of Lake Iroquois deposits via stream and hillslope erosion. Ecologically, the increased input of fine-grained sediments into
the wetland has resulted in reduced water clarity and has altered the wetland substrate contributing to wetland loss in Frenchman’s
Bay. 相似文献
9.
Sieving samples for chironomid analysis with a 150 μm mesh was shown to greatly reduce sample preparation time, and use of
only larger specimens did not affect chironomid-inferred salinities in African lakes. Here, we tested if this method is suitable
for temperature reconstruction in colder lakes at higher latitudes. Removal of specimens <150 μm in two training sets, one
from Canada and one from Sweden, had little impact on the performance statistics of the calibration models. Chironomid abundance,
however, decreased greatly because more than half of the head capsules in assemblages were <150 μm. This had major impacts
on the temperature reconstructions. Inferences were on average 2°C warmer with the modified models (all specimens >150 μm)
than those obtained with the full model (all specimens >100 μm). General patterns of temperature change were also altered.
For Lake 7 on Southampton Island, Canada, a cooling trend was reconstructed with the full Canadian model while the modified
Canadian model yielded a warming trend. When only specimens >150 μm were used, two to three times more wet sediment was needed
to obtain a sufficient number of head capsules. These results indicate that, in cold lakes (mean July/August air temperature
≤11°C), large proportions of head capsules are <150 μm, and sieving the samples in a 150 μm mesh leads to altered temperature
reconstructions. 相似文献
10.
Testate amoebae are informative about palaeoecological conditions, but the methods generally used for their analyses in lake
sediments differ from those used for their analyses in peats, making comparisons difficult. This study examines how filter
mesh size and total number of individuals counted affect species richness, Shannon diversity, equitability, density and assemblage
structure. We analysed the complete testate amoeba contents of six sediment samples from Lake Lautrey, France. The abundance
of testate amoebae was high (1,403–10,870 shells cm−3), and species smaller than 63 μm in both length and width represented up to 89% of total abundance and 43% of species richness.
A simulation showed that using 47- or 63-μm mesh-size filters reduced inter-sample differences and changed the patterns of
abundance, species richness and assemblage structure, causing loss of information and leading to potential erroneous palaeoecological
interpretation. Rarefaction analyses suggest that although 170 shells are sufficient to assess the general structure of assemblages,
such small sample sizes can underestimate species richness by overlooking taxa with relative abundances <4%. Total counts
of 400 shells yield better estimates of assemblage structure and recover at least 50% of total species richness, although
species with absolute frequencies below 2% may still be missed. Higher counts are required to obtain reliable estimates of
species richness and assemblage structure in samples that have high testate amoeba densities but are dominated by a few small
taxa. Further studies should determine the bioindicator value and functional roles of small and/or rare species in lakes and
thus to what extent overlooking them affects palaeoecological interpretations. 相似文献
11.
Kerosene, a grade mineral oil, is commonly used to extract beetles from sediment. Here, the use of kerosene to extract chironomid
head capsules was tested on 10 samples from sediment of different lakes, with different organic matter content as measured
by loss on ignition, and estimated ages. Our results revealed that this flotation tool is very effective in extracting either
full or half chironomid head capsules. The mean extraction efficiency was 89.3 ± 8.0% with an estimated relative abundance
error ranging from −1% to 1% for 46 of the 57 identified taxa. Larger chironomids (400–500 μm width), which are often full
of sediment particles, have the highest relative abundance error, with a maximum of 4.3% for Corynocera oliveri-type. A canonical correspondence analysis showed that, despite this small bias, samples retrieved with the kerosene flotation
do not differ from the whole sample assemblages. These results give us confidence in the use of this flotation technique for
chironomid sample preparation. 相似文献
12.
Several techniques are available to examine the isotopic composition of historic lake waters, providing data that can subsequently
be used to examine environmental changes. A recently-developed technique is the stable oxygen isotope analysis of subfossil
chironomid (Diptera: Chironomidae) head capsules (mostly chitin) preserved in lake sediments. This technique involves a high
Temperature Conversion Elemental Analyzer (TC/EA), which has been a relatively recent addition to the suite of online peripherals
for analyzing the stable isotopic composition of organic samples. The highly precise and accurate 18O/16O and D/H measurements obtainable using the TC/EA with samples in the microgram range make this instrumentation suitable for
studying geochemical and biological processes. Preparation of organic samples for isotopic analysis typically requires first
weighing each sample into silver/tin capsules. These capsules can introduce oxygen and hydrogen contamination (a “blank effect”),
which is especially problematic for analysis of small organic samples (e.g. less than 100 μg). Here we tested tin and silver
capsules from two manufacturers and a range of sizes to assess contamination to small organic samples on the TC/EA. We also
assessed how a method for cleaning silver capsules affected our analysis of commercial chitin. In general, capsules made of
silver have less detectible oxygen than those made of tin, and capsules from the two manufacturers varied in their detectible
oxygen. There was no detectable H contamination from silver capsules. In addition to our empirical findings, we present a
model demonstrating the influence that contaminant oxygen can have on the δ18O of small organic samples. Sample mass becomes an important issue for such analyses. In light of our findings, we recommend
a minimum sample mass ≥50 μg (approximately 120 whole chironomid head capsules) on a TC/EA-IRMS (Deltaplus XP system). Finally, we present a detailed protocol for preparing and transferring chironomid head capsules into silver capsules
that minimizes the influence of contaminant oxygen. This protocol provides the paleo-community with another potential method
for reconstructing paleoenvironments. 相似文献
13.
Floodplain lakes are rarely analysed for fossil chironomids and usually not incorporated in modern chironomid-climate calibration
datasets because of the potential complex hydrological processes that could result from flooding of the lakes. In order to
investigate this potential influence of river inundations on fossil chironomid assemblages, 13 regularly inundated lakes and
20 lakes isolated from riverine influence were sampled and their surface sediments analysed for subfossil chironomid assemblages.
The physical and chemical settings of all lakes were similar, although the variation in the environmental variables was higher
in the lakes isolated from riverine influence. Chironomid concentration and taxon richness show significant differences between
the two classes of lakes, and the variation in these variables is best explained by loss-on-ignition of the sediments (LOI).
Relative chironomid abundances show some differences between the two groups of lakes, with several chironomid taxa occurring
preferentially in one of the two lake-types. The variability in chironomid assemblages is also best explained by LOI. Application
of a chironomid-temperature inference model shows that both types of lakes reconstruct July air temperatures that are equal
to, or slightly underestimating, the measured temperature of the region. We conclude that, although there are some differences
between the chironomid assemblages of floodplain lakes and of isolated lakes, these differences do not have a major effect
on chironomid-based temperature reconstruction. 相似文献
14.
Alexandra Schmäh 《Journal of Paleolimnology》1993,9(2):99-108
In paleolimnology, subfossil head capsules of chironomids play an important role as ecological indicators of lake history. It is important to determine, therefore, whether fossil assemblages are representative of former biocoenoses. There is evidence that headcapsules washed in from other places can make up a significant percentage of the total. As interpretations are usually drawn from the examination of a single core, it is of special interest to know whether a fossil assemblage of a single site properly reflects limnological conditions of the whole lake. This study examined the taxonomic distribution of subfossil chironomids in the surficial sediments of the Bodensee-Untersee, with the aim of assessing the variability in chironomid assemblages. Apparently, most of the head capsules of the profundal fossil assemblages in the Untersee had been washed in from the littoral zone or from the slope. Although the Bodensee-Untersee is a rather large lake, variability is surprisingly low among all samples. Therefore a correct interpretation from a single core may be possible. 相似文献
15.
A. P. Fedotov S. S. Vorobyeva K. E. Vershinin D. K. Nurgaliev I. V. Enushchenko S. M. Krapivina K. V. Tarakanova G. A. Ziborova P. G. Yassonov A. S. Borissov 《Journal of Paleolimnology》2012,47(4):617-630
We analysed a 620-cm-long sediment record from Lake Kotokel located in East Siberia (Russia) for subfossil diatoms, chironomids
and pollen to provide a reconstruction of the climate history of the area for the last 12.2 kyr. The subfossil records show
differing time lags in their responses to climate change; diatoms and chironomids were more sensitive to climate change than
the pollen record. Changes in the biogenic proxies seem related with changes in insolation, the temperature of the North Atlantic
and solar activity. The chironomids Chironomus plumosus-type and Einfeldia carbonaria-type and the diatom Aulacoseira granulata were interpreted as markers of warm climate condition. The proxy records were divided into four periods (A, B, C and D) suggesting
differing climate in East Siberia during the Holocene. Period D (12.2–9.5 kyr BP) at the beginning of the Holocene, according
to chironomid and diatom records, was characterized by warm climate with summer temperatures close to modern. However, forest
vegetation had not become fully established yet. During Period C (9.5–5.8 kyr BP), the climate seemed to gradually become
colder and wetter from the beginning of Period C to 7 kyr BP. From 7 to 5.8 kyr BP, the climate seemed to remain cold, but
aridity increased. Period B (5.8–1.7 kyr BP) was characterised by frequent and sharp alternations between warm and cold conditions.
Unstable conditions during this time are also registered in records from Lakes Baikal, Khubsugul and various other shallow
lakes of the region. Optimal warm and wet conditions seemed to occur ca. 4 kyr BP. During Period A (the last 1.5 kyr) the
diatom and chironomid records show evidence of cold conditions at 1.5–1 kyr BP, but the forest vegetation did not change significantly. 相似文献
16.
David F. Porinchu Glen M. MacDonald Amy M. Bloom Katrina A. Moser 《Journal of Paleolimnology》2002,28(3):355-375
Surface lake sediment was recovered from 57 lakes along an elevation gradient in the central, eastern Sierra Nevada of California. The surface sediment was analysed for subfossil chironomid remains in order to assess the modern distribution of chironomids in the region. The lakes sampled for the calibration dataset were between 2.0 and 40.0 m in depth, spanned an altitudinal gradient of 1360 m and a surface water temperature gradient of approximately 14 °C. Redundancy analysis (RDA) identified that five of the measured environmental variables – surface water temperature, elevation, depth, strontium, particulate organic carbon – accounted for a statistically significant amount of the variance in chironomid community composition. Quantitative transfer functions, based on weighted-averaging (WA), partial least squares (PLS) and weighted-averaging partial least squares (WA-PLS), were developed to estimate surface water temperature from the chironomid assemblages. The best model was a WA model with classical deshrinking, which had a relatively high coefficient of determination (r2 = 0.73), low root mean square error of prediction (RMSEP = 1.2 °C) and a low maximum bias (0.90 °C). The results from this study suggest that robust quantitative estimates of past surface water temperature can be derived from the application of these models to fossil chironomid assemblages preserved in late-Quaternary lake sediment in this region. 相似文献
17.
The larval head capsules of Sciaridae (black fungus gnats) are transported into lakes from terrestrial habitats, in most cases
via streams or rivers, and preserve well in lake sediments. The abundance of sciarid remains can provide information on the
importance of terrestrial and running-water input into fossil insect assemblages in lake sediments if examined in relation
to the number of fossils of aquatic insects such as the Chironomidae, Thaumaleidae, Ceratopogonidae, or Simuliidae. Here we
describe fossil head capsules of sciarid larvae and provide an example of how these remains can be used to constrain past
changes in taphonomic processes that influence fossil chironomid records. 相似文献
18.
A multiple stable isotope record of Late Quaternary limnological changes and chironomid paleoecology from northeastern Iceland 总被引:1,自引:0,他引:1
We present results from multiple stable isotope analyses (δ18O of chironomid larval head capsules, chironomid adult thoraxes and other insect remains and δD, δ13C, δ15N of total organic matter—TOM) of a lake sediment core (04-SVID-03) taken from Stora Vidarvatn in northeastern Iceland to
reconstruct past environmental, limnological and δ18O of past lake water changes during the Holocene. Core 04-SVID-03 represents a ∼12,000 cal. yrs BP to present record. Large
magnitude changes in δ18O occurred during the Holocene at the site. Downcore shifts in δ18O of chironomids did not correlate with measurements of the δ13C and δ15N of chironomid head capsules, implying that the δ18O changes were not primarily driven by changes in chironomid diet during the Holocene. The δD of TOM provided a proxy of relative
lake-water δD changes at the site and also showed large magnitude changes during the record. This approach was supported by
analyses of a modern training set where δDTOM analyses were conducted using surface sediments from a suite of freshwater lakes over a large latitudinal gradient. The magnitude
of changes in both the δ18O and δD and the relatively negative δ18O values throughout much of the core suggest that the proxies represent more paleoenvironmental information than solely temperature.
Additional possible influences on lake-water isotopic composition are discussed, including changes in the seasonality of precipitation,
in the patterns of air masses supplying precipitation to Iceland and in the dominant mode of the North Atlantic Oscillation. 相似文献
19.
Hilde Eggermont Oliver Heiri James Russell Mathias Vuille Leen Audenaert Dirk Verschuren 《Journal of Paleolimnology》2010,43(3):413-435
Fossil assemblages of chironomid larvae (non-biting midges) preserved in lake sediments are well-established paleothermometers
in north-temperate and boreal regions, but their potential for temperature reconstruction in tropical regions has never before
been assessed. In this study, we surveyed sub-fossil chironomid assemblages in the surface sediments of 65 lakes and permanent
pools in southwestern Uganda (including the Rwenzori Mountains) and central and southern Kenya (including Mount Kenya) to
document the modern distribution of African chironomid communities along the regional temperature gradient covered by lakes
situated between 489 and 4,575 m above sea level (a.s.l). We then combined these faunal data with linked Surface-Water Temperature
(SWTemp: range 2.1–28.1°C) and Mean Annual Air Temperature (MATemp: range 1.1–24.9°C) data to develop inference models for
quantitative paleotemperature reconstruction. Here we compare and discuss the performance of models based on different numerical
techniques [weighted-averaging (WA), weighted-averaging partial-least-squares (WA-PLS) and a weighted modern analogue technique
(WMAT)], and on subsets of lakes with varying gradient lengths of temperature and other environmental variables. All inference
models calibrated against MATemp have a high coefficient of determination (
r\textjack2 r_{\text{jack}}^{2} = 0.81–0.97), low maximum bias (0.84–2.59°C), and low root-mean-squared error of prediction (RMSEP = 0.61–1.50°C). The statistical
power of SWTemp models is generally weaker (
r\textjack2 r_{\text{jack}}^{2} = 0.77–0.95; maximum bias 1.55–3.73°C; RMSEP = 1.39–1.98°C), likely because the surface-water temperature data are spot measurements
failing to catch significant daily and seasonal variation. Models based on calibration over the full temperature gradient
suffer slightly from the limited number of study sites at intermediate elevation (2,000–3,000 m), and from the presence of
morphologically indistinguishable but ecologically distinct taxa. Calibration confined to high-elevation sites (>3,000 m)
has poorer error statistics, but is less susceptible to biogeographical and taxonomic complexities. Our results compare favourably
with chironomid-based temperature inferences in temperate regions, indicating that chironomid-based temperature reconstruction
in tropical Africa can be achieved. 相似文献
20.
Tomi P. Luoto 《Journal of Paleolimnology》2012,47(1):141-157
Surface sediment samples from Lake Moaralmsee in the Austrian Alps were examined for fossil remains of aquatic insects and
mites. This study investigated the influence of water depth on the fauna, to explore the possibility of using such fossil
remains in sediment cores to reconstruct past water level changes. In addition, instar-specific patterns of chironomid (Diptera:
Chironomidae) head capsule accumulation were examined to evaluate whether the smaller, lighter-weight early instars are more
easily transported within the lake basin, creating a potential source of error for paleolimnological inferences. Results showed
that intra-lake distribution of these zoological remains is closely related to water depth and suggested that the fossils
accumulate near each species’ habitat. In addition, the ratio between exoskeletons of oribatid mites (Acari: Oribatida) and
chironomid head capsules was strongly related to water depth. Examination of instar-specific accumulation patterns of all
chironomid remains showed no significant relationship between specific instars and water depth, though littoral samples consisted
only of the 3rd and 4th instars. A taxon-specific examination revealed that the early instars of Paracladius are significantly focused to the deeper parts of the basin. Because most taxa displayed significant relations with water
depth, a transfer function was developed, relating fossil chironomids to water depth. This model has a high coefficient of
determination and a low estimate of prediction error. In this study, Paracladius was found to prefer shallow and intermediate water depths, hence enhanced offshore transport of early instar head capsules
may weaken model performance statistics. Results indicate that intra-lake calibration sets of invertebrate remains have great
potential in paleolimnological research, though there is a possible risk of spatial autocorrelation. Such datasets also contribute
to the understanding of the modern ecology of the fauna because fossil assemblages in surface deposits provide habitat-specific
autecological information. More effort should be directed at evaluating how remains of different instars are transported within
other lake basins, because selective offshore transport of head capsules of different larval stages can potentially cause
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