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Yurena Yanes Christopher S. Romanek Heather A. Brant María R. Alonso 《Geochimica et cosmochimica acta》2009,73(14):4077-6869
Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a low-latitude oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10-2160 m a.s.l.) in Tenerife Island (∼28°N) of the Canary Archipelago.Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ18O value of the shell (δ18Oshell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the δ18O value of rain (δ18Orain). Calculated δ18O values of the snail body water (δ18Obody) derived from observed temperatures and δ18Oshell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.)59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ18O values for local rain water. The calculated δ18Obody values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the “altitude effect” observed for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ18Oshell values indicate lower temperatures, lower δ18Orain values and possibly, higher rainfall totals. A positive correlation between the mean δ13C values of shells (δ13Cshell) and plants by site suggests that shells potentially record information about the surrounding vegetation. The δ13Cshell values varied between −15.7 and −0.6‰ (VPDB), indicating that snails consumed C3 and C4/CAM plants, where more negative δ13Cshell values probably reflects the preferential consumption of C3 plants which are favored under wetter conditions. Individuals with more positive δ13Cshell values consumed a larger percentage of C4 plants (other potential factors such as carbonate ingestion or atmospheric CO2 contribution were unlikely) that were more common at lower elevations of the hotter and drier south facing transect. The relatively wide range of shell isotope values within a single site requires the analysis of numerous shells for meaningful paleoclimatic studies. Although small differences were observed in isotope composition among snail species collected at a single sampling site, they were not significant, suggesting that isotope signatures extracted from multi-taxa snail data sets may be used to infer environmental conditions over a broad range of habitats. 相似文献
13.
Brant W. Touchette Kirsten L. Rhodes Gracen A. Smith Mariana Poole 《Estuaries and Coasts》2009,32(5):917-925
Salt spray is one of many abiotic factors that can influence plant productivity and species composition in coastal ecosystems.
However, little is known about how marsh plants respond physiologically to the accumulation of sea aerosols on foliar tissues.
In this study, experimental microcosms maintained in controlled greenhouse conditions were used to evaluate how low- (1.7 mg
dm−2 day−1, weekly averages) and high- (8.6 mg dm−2 day−1) salt-spray loads would influence plant–water relations in Spartina alterniflora (Loisel.). While no differences in plant performance (e.g., changes in biomass and leaf area) were observed between the treatments
and control plants, a number of physiological modifications attributed to salt spray were observed. In general, salt-treated
plants underwent significant decreases in water potential (Ψ) and osmotic potential (Ψ
π) and increases in leaf conductance (g) and bulk modulus of elasticity (ε). It is likely that these physiological responses were used to generate lower Ψ while maintaining osmotic and water homeostasis. That is, by decreasing Ψ
π and increasing g and ε, more efficient water flow through the soil–plant–atmosphere continuum can be achieved, thus generating lower Ψ without promoting loss of turgor. 相似文献
14.
Richard W. Battarbee Gavin L. Simpson Helen Bennion Christopher Curtis 《Journal of Paleolimnology》2011,45(4):489-505
This paper has two aims: (1) to show for the first time how a natural typology can be established using palaeoecological methods;
and (2) to show how it can be used in lake restoration studies with respect to the definition of recovery targets for acidified
lakes. By defining the characteristic reference assemblages for low alkalinity site types rather than for a specific site
it allows success to be measured more broadly, unconstrained by the specific composition of the pre-acidification flora. We
analyse statistically the pre-acidification diatom assemblages of sediment cores from 121 low alkalinity lakes in the UK in
order to assess whether a reference typology for such lakes can be defined on the basis of their diatom floras. We use samples
dating to ~1850 AD to represent pre-acidification conditions. The results show that three main clusters can be identified,
two dominated by benthic taxa (Clusters 1 and 3) and one dominated by planktonic taxa (Cluster 2). Cluster 1 is characterised
by taxa such as Brachysira vitrea, Cymbella microcephala and Fragilaria spp., Cluster 2 by Cyclotella comensis, C. radiosa, Asterionella formosa, Aulacoseira subarctica and Achnanthes minutissima and Cluster 3 by Eunotia incisa, Frustulia rhomboides var. saxonica, Fragilaria virescens var. exigua, and Cymbella perpusilla. Although environmental data for 1850 AD are not available it is apparent from the contemporary distribution of the taxa
in the different clusters that Cluster 2 represents the most alkaline pre-acidification conditions. Some sites in this cluster
have been acidified, but some, especially the larger, deeper lakes have been enriched. Cluster 1 includes sites that contain
diatoms with relatively high pH optima (pH 6.4–7.4) whereas Cluster 3 sites contain diatoms with the lowest pre-acidification
pH conditions in the data-set. Sites in this cluster also have the lowest base cation concentrations at the present day and
include the sites in the UK that have been most affected by acid deposition. 相似文献
15.
This Special Issue was produced as an output from the EU Integrated Project Euro-limpacs which aimed to evaluate the impacts of global change on European freshwater ecosystems using a combination of approaches,
including monitoring, experiments, modelling and palaeolimnology. The papers focus on the last of these approaches. They examine
the role of lake sediment records in determining reference conditions for a range of environmental pressures including acidification,
eutrophication, metal pollution, organic carbon and sediment accumulation rates. The findings are especially relevant to the
European Union’s Water Framework Directive which requires an assessment of lake ecological status based on deviation from
reference conditions. The contributions consider a range of issues relating to the use of palaeolimnological data in defining
reference conditions and lake status including human versus natural variability, concepts of pristine and reference conditions,
shifting baselines, and quantification of degree of change. This introductory paper sets the context for the volume by briefly
describing how palaeolimnology has evolved as a science, able now to contribute uniquely to the understanding of lake ecosystem
change, especially with respect to the role of human activity over recent decades and centuries. 相似文献
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Ileana Bladé Brant Liebmann Didac Fortuny Geert Jan van Oldenborgh 《Climate Dynamics》2012,39(3-4):709-727
Climate models predict substantial summer precipitation reductions in Europe and the Mediterranean region in the twenty-first century, but the extent to which these models correctly represent the mechanisms of summertime precipitation in this region is uncertain. Here an analysis is conducted to compare the observed and simulated impacts of the dominant large-scale driver of summer rainfall variability in Europe and the Mediterranean, the summer North Atlantic Oscillation (SNAO). The SNAO is defined as the leading mode of July–August sea level pressure variability in the North Atlantic sector. Although the SNAO is weaker and confined to northern latitudes compared to its winter counterpart, with a southern lobe located over the UK, it significantly affects precipitation in the Mediterranean, particularly Italy and the Balkans (correlations of up to 0.6). During high SNAO summers, when strong anticyclonic conditions and suppressed precipitation prevail over the UK, the Mediterranean region instead is anomalously wet. This enhanced precipitation is related to the presence of a strong upper-level trough over the Balkans—part of a hemispheric pattern of anomalies that develops in association with the SNAO—that leads to mid-level cooling and increased potential instability. Neither this downstream extension nor the surface influence of the SNAO is captured in the two CMIP3 models examined (HadCM3 and GFDL-CM2.1), with weak or non-existent correlations between the SNAO and Mediterranean precipitation. Because these models also predict a strong upward SNAO trend in the future, the error in their representation of the SNAO surface signature impacts the projected precipitation trends. In particular, the attendant increase in precipitation that, based on observations, should occur in the Mediterranean and offset some of the non-SNAO related drying does not occur. Furthermore, the fact that neither the observed SNAO nor summer precipitation in Europe/Mediterranean region exhibits any significant trend so far (for either the full century or the recent half of the record) does not increase our confidence in these model projections. 相似文献
17.
We report accurate timing of second and third contacts made from videotape of the total solar eclipse of 23 November, 1984, observed in Papua New Guinea. The magnitude of the discrepancies between predicted and observed times indicates that the secular change in the size of the Sun reported by some observers is within the uncertainty. 相似文献
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Isabel J. Bishop Helen Bennion Ian R. Patmore Carl D. Sayer 《Journal of Paleolimnology》2018,60(2):153-165
Preventing biodiversity loss is a key aim of modern conservation, and paleolimnology can inform conservation strategies for target species and habitats where other data are unavailable. Care must be taken to fully understand the possibilities and limits of such techniques, particularly where they concern single species. This study uses plant and seed distribution data to inform macrofossil reconstructions of the rare macrophyte Najas flexilis (Slender Naiad) in Scotland, UK. It answers three questions: (a) How does the location of N. flexilis seeds in the surface sediments relate to the distribution of N. flexilis plants? (b) How do the numbers of seeds in surface sediments correlate with % cover of N. flexilis plants across lakes with differing N. flexilis abundances? (c) What are the implications of these findings for paleolimnology? Percentage N. flexilis cover and number of N. flexilis seeds in surface sediments were recorded at ~100 sample points at each of three sites; one where the species was abundant, one where it was occasional and one where it was extinct. At all sites, N. flexilis seeds were present in surface sediments across the entire lake. No correlation between % cover N. flexilis and the number of seeds in surface sediments was found within individual sites. The distribution of seeds in these lakes appeared to be related to multiple environmental and ecological variables including latitude and longitude (proxies for water currents). This is attributed to the ability of seed-bearing N. flexilis plants to fragment and float large distances on water. Between sites, there was a significant difference in the mean seed counts, with higher mean seed counts corresponding to higher abundances of N. flexilis plants. It is concluded that N. flexilis is likely to be well represented in sediment cores taken from any location within a basin, but that care should be taken when inferring changes in N. flexilis abundance from changes in the numbers of seeds in sediment samples. This work demonstrates that the reproductive ecology (number of seeds produced and dispersal mechanisms) is an important factor to consider when attempting reconstructions of single aquatic plant populations from macrofossil records. 相似文献
19.
Philip F. Hopkins Rachel S. Somerville Thomas J. Cox † Lars Hernquist Shardha Jogee Dusan Kere&#; Chung-Pei Ma Brant Robertson ‡ Kyle Stewart 《Monthly notices of the Royal Astronomical Society》2009,397(2):802-814
Transformation of discs into spheroids via mergers is a well-accepted element of galaxy formation models. However, recent simulations have shown that the bulge formation is suppressed in increasingly gas-rich mergers. We investigate the global implications of these results in a cosmological framework, using independent approaches: empirical halo-occupation models (where galaxies are populated in haloes according to observations) and semi-analytic models. In both, ignoring the effects of gas in mergers leads to the overproduction of spheroids: low- and intermediate-mass galaxies are predicted to be bulge-dominated ( B / T ∼ 0.5 at <1010 M⊙ , with almost no 'bulgeless' systems), even if they have avoided major mergers. Including the different physical behaviour of gas in mergers immediately leads to a dramatic change: bulge formation is suppressed in low-mass galaxies, observed to be gas-rich (giving B / T ∼ 0.1 at <1010 M⊙ , with a number of bulgeless galaxies in good agreement with observations). Simulations and analytic models which neglect the similarity-breaking behaviour of gas have difficulty reproducing the strong observed morphology–mass relation. However, the observed dependence of gas fractions on mass, combined with suppression of bulge formation in gas-rich mergers, naturally leads to the observed trends. Discrepancies between observations and models that ignore the role of gas increase with redshift; in models that treat gas properly, galaxies are predicted to be less bulge-dominated at high redshifts, in agreement with the observations. We discuss implications for the global bulge mass density and future observational tests. 相似文献
20.
Sandra Costa-B?ddeker Helen Bennion Tatiane Araújo de Jesus Ana Luiza S. Albuquerque Rubens C. L. Figueira Denise de C. Bicudo 《Journal of Paleolimnology》2012,48(4):751-766
We studied the eutrophication history of a tropical shallow reservoir in the S?o Paulo metropolitan region, southeast Brazil. We analyzed grain size, geochemistry, diatom assemblages, and land-use records in a sediment core from the reservoir to infer its trophic state history during the last ~110?years (1894?C2005). Eighty diatom species were observed in the core and shifts in the relative abundances of planktonic and benthic taxa indicate major limnological changes associated with complex interactions between hydrologic factors and eutrophication. Discostella stelligera was associated with deforestation and water physical changes whereas Aulacoseira granulata, a species abundant throughout the core, was mostly associated with high flux conditions and erosion events, regardless of trophic state. Eutrophication was triggered by construction of the city zoo (1958) and installation of the S?o Paulo State Department of Agriculture (1975) within the Gar?as watershed, and increasing loads of untreated sewage from these institutions. The data suggest that deterioration in water quality began after ~1975 and markedly accelerated after ~1990. The reservoir has been hypereutrophic since 1999. Steady increases in geochemical proxies for trophic state, along with a decrease in C/N ratios, indicated higher nutrient concentrations and the prevalence of autochthonous production towards the core top. Appearance of Achnanthidium catenatum ~1993 highlighted the onset of a marked eutrophication phase. The subsequent dominance of Planothidium rostratum and Cyclotella meneghiniana suggested a sharp shift to a hypereutrophic state since 1999. Land-use history proved valuable for validating the chronology and interpreting anthropogenic impacts. Multi-proxy analysis of the sediment record provided an effective tool for tracking ecological shifts in the reservoir ecosystem. This study provides the first reconstruction of lake eutrophication history in Brazil and highlights the importance of hydrological/physical changes as drivers of diatom assemblage shifts in reservoirs, which may confound trophic state inferences based on shifts in the planktonic/benthic diatom ratio. 相似文献