Natural Hazards - Archaeological sites are increasingly threatened by primary impacts of climate change, including sea-level rise, flooding, and erosion. These important sites represent cultural... 相似文献
Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h−1, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Météo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.
West Hawk Lake (WHL) is located within the glacial Lake Agassiz basin, 140 km east of Winnipeg, Manitoba. The small lake lies
in a deep, steep-sided, meteorite impact crater, which has been partly filled by 60 m of sediment that today forms a flat
floor in the central part of the basin below 111 m of water. Four cores, 5–11 m in length, were collected using a Kullenberg
piston gravity corer. All sediment is clay, contains no unconformities, and has low organic content in all but the upper meter.
Sample analyses include bulk and clay mineralogy, major and minor elements, TOC, stable isotopes of C, N, and O, pollen, charcoal,
diatoms, and floral and faunal macrofossils. The sequence is divided into four units based mainly on thickness and style of
lamination, diatoms, and pollen. AMS radiocarbon dates do not provide a clear indication of age in the postglacial sequence;
possible explanations include contamination by older organic inwash and downward movement of younger organic acids. A chronological
framework was established using only selected AMS dates on plant macrofossils, combined with correlations to dated events
outside the basin and paleotopographic reconstructions of Lake Agassiz. The 822 1-cm-thick varves in the lower 8 m of the
cored WHL sequence were deposited just prior to 10,000 cal years BP (∼8,900 14C years BP), during the glacial Lake Agassiz phase of the lake. The disappearance of dolomite near the top of the varved sequence
reflects the reduced influence of Lake Agassiz and the carbonate bedrock and glacial sediment in its catchment. The lowermost
varves are barren of organisms, indicating cold and turbid glacial lake waters, but the presence of benthic and planktonic
algae in the upper 520 varves indicates warming; this lake phase coincides with a change in clay mineralogy, δ18O and δ13C in cellulose, and in some other parameters. This change may have resulted from a major drawdown in Lake Agassiz when its
overflow switched from northwest to east after formation of the Upper Campbell beach of that lake 9,300–9,400 14C years ago. The end of thick varve deposition at ∼10,000 cal years BP is related to the opening of a lower eastern outlet
of Lake Agassiz and an accompanying drop in West Hawk Lake level. WHL became independent from Lake Agassiz at this time, sedimentation
rates dropped, and only ∼2.5 m of sediment was deposited in the next 10,000 years. During the first two centuries of post-Lake
Agassiz history, there were anomalies in the diatom assemblage, stable O and C isotopes, magnetic susceptibility, and other
parameters, reflecting an unstable watershed. Modern oligotrophic conditions were soon established; charcoal abundance increased
in response to the reduced distance to the shoreline and to warmer conditions. Regional warming after ∼9,500 cal years BP
is indicated by pollen and diatoms as well as C and O isotope values. Relatively dry conditions are suggested by a rise in
pine and decrease in spruce and other vegetation types between 9,500 and 5,000 cal years BP (∼8,500–4,400 14C years BP), plus a decrease in δ13Ccell values. After this, there was a shift to slightly cooler and wetter conditions. A large increase in organic content and change
in elemental concentration in the past several thousand years probably reflects a decline in supply of mineral detritus to
the basin and possibly an increase in productivity. 相似文献
The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) mission is to return to Earth a pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single‐component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7–3 μm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/OH‐bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk‐integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3 μm. Our study prioritizes spectral indices to be used for OSIRIS‐REx spectral analysis and mapping and informs the reliability of all index‐derived data products, including a science value map for sample site selection. 相似文献
The chemical characteristics of aggregating material in the marine environment are largely unknown. We investigated neutral aldose (NA) abundance and composition in aggregation of marine snow and other organic matter (OM) size fractions in the field. Four sample sets were fractionated using membrane filtration and ultrafiltration into the following size fractions: particulate material, high-molecular-weight (HMW) material, and low-molecular-weight (LMW) material. We also collected three sample sets of marine-snow aggregates. Each sample set contained small, medium, and large aggregate size fractions and each size fraction consisted of 25–50 aggregates. For 7 marine-snow samples and for each water-sample size fraction, we determined monomeric and polymeric NA concentration, NA yield (amount of NA-C normalized to organic carbon), and composition; total organic carbon (TOC) concentration; transparent exopolymer particles (TEP) concentration, and TEP propensity (TEP concentration after inducing TEP formation in filtered samples). This is the first study to include compound-specific NA determinations on these four marine OM size fractions.The mass balances of organic carbon and NA indicated that there were no serious contamination or loss problems. Concentrations, yields, and NA mol fractions in water samples were similar to results from other studies. Glucose and galactose had the highest relative abundance in all size fractions. The NA yield increased with increasing molecular weight or particle size for all fractions except marine snow. The NA yield increased in the order: LMW< marine snow< HMW< particles. Marine snow had a higher average NA yield than the LMW fraction, but lower than particle and HMW-fractions. This indicates that OM in marine snow could have been diagenetically derived from particulate and HMW-fractions, that is, marine snow may include material from the particulate and the colloidal phase.TEP concentration or TEP propensity was positively correlated with concentrations of all individual NAs as well as the sum NA concentrations, indicating that TEP contains neutral sugars in addition to the acidic polysaccharides stained in the determination of TEP concentrations.Despite the relatively low NA yield in marine snow, marine snow was enriched in NA when compared with seawater, with enrichment factors of 34–225 (average 125). By combining data from this study with data from other studies, we estimate that < 10% of carbohydrates in marine snow comprise NAs.There was no clear correlation between marine-snow aggregate size and NA yield, that is, there appears to be no general age difference between small and large marine-snow aggregates. NA composition was similar among different marine-snow size fracions collected during the same day, indicating that aggregation/disaggregation reactions resulted in homogenizing NA composition in marine-snow aggregates of all sizes. The NA composition of marine snow was different from that of other OM size fractions, indicating either that bacterial degradation has modified the composition of marine snow to a larger extent than other OM size fractions or that marine snow is formed through the aggregation of selected subcomponents of OM. 相似文献
Sewage effluent input and population were chosen as predictors of microplastic presence in sediments at four sites in the River Thames basin (UK). Large microplastic particles (1 mm–4 mm) were extracted using a stepwise approach to include visual extraction, flotation and identification using Raman spectroscopy. Microplastics were found at all four sites. One site had significantly higher numbers of microplastics than other sites, average 66 particles 100 g? 1, 91% of which were fragments. This site was downstream of a storm drain outfall receiving urban runoff; many of the fragments at this site were determined to be derived of thermoplastic road-surface marking paints. At the remaining three sites, fibres were the dominant particle type. The most common polymers identified included polypropylene, polyester and polyarylsulphone. This study describes two major new findings: presence of microplastic particles in a UK freshwater system and identification of road marking paints as a source of microplastics.
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This study is the first to quantify microplastics of any size in river sediments in the UK and links their presence to terrestrial sources including sewage and road marking paints. 相似文献
Interannual variability is an important modulator of synoptic and intraseasonal variability in South America. This paper seeks
to characterize the main modes of interannual variability of seasonal precipitation and some associated mechanisms. The impact
of this variability on the frequency of extreme rainfall events and the possible effect of anthropogenic climate change on
this variability are reviewed. The interannual oscillations of the annual total precipitation are mainly due to the variability
in austral autumn and summer. While autumn is the dominant rainy season in the northern part of the continent, where the variability
is highest (especially in the northeastern part), summer is the rainy season over most of the continent, thanks to a summer
monsoon regime. In the monsoon season, the strongest variability occurs near the South Atlantic Convergence Zone (SACZ), which
is one of the most important features of the South American monsoon system. In all seasons but summer, the most important
source of variability is ENSO (El Ni?o Southern Oscillation), although ENSO shows a great contribution also in summer. The
ENSO impact on the frequency of extreme precipitation events is also important in all seasons, being generally even more significant
than the influence on seasonal rainfall totals. Climate change associated with increasing emission of greenhouse gases shows
potential to impact seasonal amounts of precipitation in South America, but there is still great uncertainty associated with
the projected changes, since there is not much agreement among the models’ outputs for most regions in the continent, with
the exception of southeastern South America and southern Andes. Climate change can also impact the natural variability modes
of seasonal precipitation associated with ENSO. 相似文献