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We employed the modern analog technique to quantitatively reconstruct temperature and precipitation over the past 2500 yr based on fossil pollen records from six high-elevation lakes in northern Colorado. Reconstructed annual temperatures for the study area did not deviate significantly from modern over the past 2500 yr despite hemispheric expressions of Medieval Climate Anomaly warmth and Little Ice Age cooling. Annual precipitation, however, shifted from lower than modern rates from 2500 to 1000 cal yr BP to higher than modern rates after 1000 cal yr BP, a greater than 100 mm increase in precipitation. Winter precipitation accounts for the majority of the change in annual precipitation, while summer precipitation rates did not change significantly over the past 2500 yr. The large change in winter precipitation rates from the first to second millennium of the Common Era is inferred from a shift in fossil pollen assemblages dominated by subalpine conifers, which have southern sites as modern analogs, to assemblages representing open subalpine vegetation with abundant Artemisia spp. (sagebrush), which have more northern modern analogs. The change helps to explain regional increases in lake levels and shifts in some isotopic and tree-ring data sets, highlighting the risk of large reductions in snowpack and water supplies in the Intermountain West.
Fremont societies of the Uinta Basin incorporated domesticates into a foraging lifeway over a 1,000-year period from AD 300 to 1300. Fremont research provides a unique opportunity to critically examine the social and ecological processes behind the adoption and abandonment of domesticates by hunter-gatherers. We develop and integrate a 2,115-year precipitation reconstruction with a Bayesian chronological model for the growth of Fremont societies in the Cub Creek reach of Dinosaur National Monument. Comparison of the archaeological chronology with the precipitation record suggests that the florescence of Fremont societies was an adaptation to multidecadal precipitation variability with an approximately 30-plus-year periodicity over most, but not all, of the last 2,115 years. Fremont societies adopted domesticates to enhance their resilience to periodic droughts. We propose that reduced precipitation variability from AD 750 to AD 1050, superimposed over consistent mean precipitation availability, was the tipping point that increased maize production, initiated agricultural intensification, and resulted in increased population and development of pithouse communities. Our study develops a multidecadal/multigenerational model within which to evaluate the strategies underwriting the adoption of domesticates by foragers, the formation of Fremont communities, and the inherent vulnerabilities to resource intensification that implicate the eventual dissolution of those communities.
Differences in pipe wall microstructure at various positions throughout the wall thickness of high strength aluminum alloy thick-wall pipes produced by reverse hot extrusion were investigated. The microstructures of the inner wall (IW), outer wall (OW), and half wall (HW) were compared. Further, heterogeneity in the mechanical properties of the pipe throughout the wall thickness was also investigated. Results revealed that the volume fraction of precipitation was highest at the HW position because of the higher Zn and Mg contents. Further, approximately 26% of grains were recrystallized in the OW position due to the greater strain during extrusion, while the recrystallization fractions of the IW and HW positions were 13% and 21%, respectively. The effects of precipitation strengthening and deformation strengthening contribute to the highest ultimate tensile strength and Vickers hardness of the HW position, and to the higher elongation of the IW and OW positions.
Binary Fe–Cu alloys are effective prototypes for investigating radiation-induced formation and growth of nanometric Cu-rich precipitates (CRPs) in nuclear reactor pressure vessels. In this report, the temporal evolution of CRPs during thermal aging of Fe–Cu binary alloys has been investigated by using complementary techniques such as atom probe tomography (APT) and small-angle neutron scattering (SANS). We report a detailed quantitative evolution of a rarely observed morphological transformation of Cu precipitates from spherical to ellipsoid with a significant change (approximately two times) in aspect ratio, an effect known to be associated with the 9R-3R structural transition of the precipitates. It is demonstrated through APT that the precipitates remain spherical up to 8 h, however, they subsequently convert to oblate ellipsoid upon further aging. SANS analysis also detected signs of this morphological transition in reciprocal space. Furthermore, SANS quantifies evolution of the precipitates and corroborates well with the APT results. Interestingly, the power-law exponent of the temporal evolution for mean size and number density agree reasonably well with the Lifshitz–Slyozov–Wagner model, in spite of the complex morphological evolution of the precipitates.
Climate and weather conditions may have substantial effects on the ecology of both parasites and hosts in natural populations. The strength and shape of the effects of weather on parasites and hosts are likely to change as global warming affects local climate. These changes may in turn alter fundamental elements of parasite–host dynamics. We explored the influence of temperature and precipitation on parasite prevalence in a metapopulation of avian hosts in northern Norway. We also investigated if annual change in parasite prevalence was related to winter climate, as described by the North Atlantic Oscillation (NAO). We found that parasite prevalence increased with temperature within-years and decreased slightly with increasing precipitation. We also found that a mild winter (positive winter NAO index) was associated with higher mean parasite prevalence the following year. Our results indicate that both local and large scale weather conditions may affect the proportion of hosts that become infected by parasites in natural populations. Understanding the effect of climate and weather on parasite–host relationships in natural populations is vital in order to predict the full consequence of global warming.
Polycrystalline Ni-based superalloys for aerospace and power generation applications are often precipitation hardened to achieve strengthening at elevated temperatures. Here, atom probe microscopy has become an essential tool to study the complex morphology of nanoscale precipitates. This study focuses on Alloy 718, which is hardened by semi-coherent, ordered γ′ (Ni3(Al, Ti)) and γ″ (Ni3(Nb)) particles. According to previous research, these particles often occur as duplets or triplets with a stacking sequence dependent on prior processing. This creates various interfaces with a strong impact on the mechanical properties, highlighting the importance of quantitative studies which are challenging with electron microscopy. We present atom probe data reconstruction and analysis approaches particularly suited for precipitation hardened superalloys. While voltage atom probe allows for an accurate reconstruction, the acquired data volume is often limited. Laser-assisted atom probe provides statistically significant data, but the loss of crystallographic information requires correlation with voltage-mode datasets. We further describe an advanced iso-surface method where initially arbitrarily chosen concentration thresholds of Al + Ti for γ′ and Nb for γ″ particles are optimized. Recognizing the importance of the precipitate stacking order, the different types of precipitate interfaces are quantified, and these methods may be applicable to other engineering alloys.
Microbial-induced calcite precipitation (MICP) has gained much attention in soil improvement studies, where it can enhance the physical properties of sandy soil. Small-scale sand cylinder tests were conducted to investigate the formation and failure of calcium carbonate precipitation bonding between individual sand particles. Bonding formation by precipitation was examined by scanning electron microscopy. Energy-dispersive X-ray spectroscopy analysis verified the existence of calcium, carbon, and oxygen, which could form CaCO3 after MICP-treatment. Focused-ion-beam milling was applied to study the interior structure of calcium carbonate precipitation during the MICP process.
The southern Cape of South Africa is important to understanding regional climate because it straddles the transition between the winter and summer rainfall zones. We examine late Quaternary changes in rainfall seasonality and aridity through analysis of micromammal assemblages from three sites: Boomplaas Cave and Nelson Bay Cave in the aseasonal rainfall zone and Byneskranskop 1 in the winter rainfall zone. Our interpretation is based on analysis of 123 modern micromammal assemblages accumulated by barn owls (Tyto alba), which empirically links species composition to climate. The Pleistocene record (∼65 to 12 ka) from Boomplaas Cave, together with the last glacial maximum (LGM) samples from Nelson Bay Cave, indicates enhanced winter rainfall, especially during the LGM. Boomplaas Cave documents progressive aridification from the LGM to the earliest Holocene, followed by a return to moderately humid conditions through the Holocene. Byneskranskop 1 indicates a dominance of winter rains over the last 17 ka and a shift from an arid middle Holocene to a humid later Holocene. Agreement between the micromammal record and other local and regional proxies reinforces the potential of southern African micromammal assemblages as paleoclimate indicators.
A novel quantitative assessment of late Holocene precipitation in the Levant is presented, including mean and variance of annual precipitation and their trends. A stochastic framework was utilized and allowed, possibly for the first time, linking high-quality, reconstructed rises/declines in Dead Sea levels with precipitation trends in its watershed. We determined the change in mean annual precipitation for 12 specific intervals over the past 4500 yr, concluding that: (1) the twentieth century was substantially wetter than most of the late Holocene; (2) a representative reference value of mean annual precipitation is 75% of the present-day parameter; (3) during the late Holocene, mean annual precipitation ranged between −17 and +66% of the reference value (−37 to +25% of present-day conditions); (4) the driest intervals were 1500–1200 BC and AD 755–890, and the wettest intervals were 2500–2460 BC, 130–40 BC, AD 350–490, and AD 1770–1940; (5) lake-level rises and declines probably occurred in response to trends in precipitation means and are less likely to occur when precipitation mean is constant; (6) average trends in mean annual precipitation during intervals of ≥200 yr did not exceed 15 mm per decade. The precipitation trends probably reflect shifts in eastern Mediterranean cyclone tracks.
The composition of nano-sized alloy carbides formed by interphase precipitation in V–Nb and V–Ti multiple microalloyed low-carbon steels is analyzed by using three-dimensional atom probe. Carbide-forming alloying elements including V, Nb, and Ti, are simultaneously precipitated from the early stage of isothermal treatment, whose atoms are uniformly distributed in the carbide particles, even after prolonged holding. Cluster analysis by the maximum separation method, with parameters optimized using different methods, is carried out to extract alloy carbides from matrix. The composition of alloy carbides evaluated by site fraction of substitutional carbide-forming alloying elements indicates that at the early stage of their formation, Nb and Ti are more strongly enriched than V.
Understanding the consequences of environmental fluctuations for parasite dynamics requires a long-term view stretching over many transmission cycles. Here we studied the dynamics of three malaria parasites (Plasmodium azurophilum, P. leucocytica and P. floridense) infecting the lizard Anolis gundlachi, in the rainforest of Puerto Rico. In this malaria–anole system we evaluated temporal fluctuations in individual probability of infection, the environmental drivers of observed variation and consequences for host body condition and Plasmodium parasites assemblage. We conducted a total of 15 surveys including 10 from 1990 to 2002 and five from 2015 to 2017. During the early years, a lizard's probability of infection by all Plasmodium species appeared stable despite disturbances ranging from two hurricanes to short droughts. Over a longer timescale, probability of infection and overall prevalence varied significantly, following non-linear relationships with temperature and rainfall such that highest prevalence is expected at intermediate climate measures. A perplexing result was that host body condition was maximized at intermediate levels of rainfall and/or temperature (when risk of infection was highest), yet we found no significant decreases in body condition due to infection. Plasmodium parasite species composition varied through time with a reduction and near local extinction of P. floridense. Our results emphasize the need for long-term studies to reveal host–parasite dynamics, their drivers and consequences.
Large changes in landscape, vegetation, and culture in northwestern (NW) Europe during the first millennium AD seem concurrent with climatic shifts. Understanding of this relation requires high-resolution palaeoclimate reconstructions. Therefore, we compiled available climate reconstructions from sites across NW Europe (extent research area: 10°W–20°E, 45°–60°N) through review of literature and the underlying data, to identify supraregional climatic changes in this region. All reconstructions cover the period from AD 1 to 1000 and have a temporal resolution of ≤50 yr. This resulted in 22 climate reconstructions/proxy records based on different palaeoclimate archives: chironomids (1), pollen (6), Sphagnum mosses (1), stalagmites (8), testate amoebae (4), and tree rings (2). Comparing all temperature reconstructions, we conclude that summer temperatures between AD 1 and 250 were relatively high, and the period between AD 250 and 700 was characterised by colder summer conditions. The period from AD 700 to 1000 was again characterised by warmer summers. These temperature shifts occurred in the whole of NW Europe. In contrast, the compilation of precipitation reconstructions does not show a common pattern across NW Europe either as a result of a heterogeneous precipitation pattern or the lack of suitable and consistent precipitation proxies.
Continental paleoclimate proxies with near-global coverage are rare. Land snail δ18O is one of the few proxies abundant in Quaternary sediments ranging from the tropics to the high Arctic tundra. However, its application in paleoclimatology remains difficult, attributable in part to limitations in published calibration studies. Here we present shell δ18O of modern small (<10 mm) snails across North America, from Florida (30°N) to Manitoba (58°N), to examine the main climatic controls on shell δ18O at a coarse scale. This transect is augmented by published δ18O values, which expand our coverage from Jamaica (18°N) to Alaska (64°N). Results indicate that shell δ18O primarily tracks the average annual precipitation δ18O. Shell δ18O increases 0.5–0.7‰ for every 1‰ increase in precipitation δ18O, and 0.3–0.7‰ for every 1°C increase in temperature. These relationships hold true when all taxa are included regardless of body size (ranging from ~1.6 to ~58 mm), ecology (herbivores, omnivores, and carnivores), or behavior (variable seasonal active periods and mobility habits). Future isotopic investigations should include calibration studies in tropical and high-latitude settings, arid environments, and along altitudinal gradients to test if the near linear relationship between shell and meteoric precipitation δ18O observed on a continental scale remains significant.
The Inner Mongolian Plateau lies along the northern limit reached by the East Asian summer monsoon. This geographic setting makes it especially sensitive to environmental change and an excellent site for understanding Quaternary East Asian monsoon variability. In this study we present new results of hydrogen isotopic compositions of fatty acids extracted from sediments, which were used to construct Holocene paleoprecipitation (or moisture) changes in Northern China. The hydrogen isotopic composition (D/H ratio) of n-acids in the sedimentary sequence of the Duoerji peat, Inner Mongolia, was determined with gas chromatography and mass spectrometry. Changes in the precipitation from middle Inner Mongolia are recorded by the D/H ratio of n-C20, n-C22, n-C24, n-C26, n-C28 acids (δD). From 10–9 ka, the relatively high δD values indicate reduced precipitation in the Early Holocene. Subsequently, increased precipitation is reflected by reduced δD values from 9–5.5 ka. After 5.5 ka, gradually increasing δD values record an overall decrease in precipitation. The precipitation trends established for the Duoerji sequence are consistent with other major paleoclimate proxies in the East Asian monsoon region, especially with a distinct Holocene optimum of increased monsoonal activity from 9–5.5 ka. The δD resulting paleo-precipitation record clearly shows that the Holocene climate in Northern China is basically controlled by the insolation changes.
Coherent cuboidal B2 nanoprecipitation in body-centered cubic (BCC)-based high-entropy alloys (HEAs) is important for the improvement of mechanical strength. The present work primarily investigated the effect of Ti substitution for Al on the cuboidal B2 nanoprecipitates in BCC Al0.7NiCoFeCr2 HEAs. A series of (Al,Ti)0.7NiCoFeCr2 HEAs with different Al/Ti ratios were prepared by suction-cast processing, and their microstructures and mechanical properties were then characterized comprehensively. It was found that the substitution of Ti for Al can change the phase structures of ordered precipitation, from the B2-AlNi to a highly ordered L21-Ni2AlTi phase. Especially, a small amount addition of Ti (≤4.2 at.%, Al/Ti ratio ≥2/1) renders the HEAs with cuboidal L21 nanoparticles coherently precipitated into the BCC matrix, which is attributed to the moderate lattice misfit (ε = 0.5–0.6%) between BCC and L21 phases. HEAs with such coherent microstructures exhibit high compressive yield strength of about 1700–1800 MPa. When the Ti content reaches up to 6.25 at.%, the matrix of the alloy will be turned into the σ phase, rather than BCC, leading to a heavy brittleness.
In this work, differential scanning calorimetry (DSC) was used to characterize and analyze the precipitation/dissolution kinetics of second phase particles during the cooling/reheating process in a vanadium microalloyed steel. The results indicated that three obvious exothermic peaks were detected on the cooling DSC curve. Furthermore, three corresponding endothermic peaks were also detected on the heating DSC curve. Combined with thermodynamic calculation and transmission electron microscopy analysis, these three exothermic peaks along cooling DSC curve were defined as the precipitation reaction of V(CN), the reaction of austenite transformation into ferrite and the precipitation reaction of VC, respectively. Meanwhile, three corresponding reverse reactions for cooling were also defined along the reheating DSC curve. The linear regression result revealed that the precipitation activation energies for V(CN) and VC were identified as 311.2 kJ/mol and 167.6 kJ/mol, respectively. The dissolution activation energies for VC and V(CN) were identified as 255.4 kJ/mol and 592.6 kJ/mol, respectively.
The solute equilibrium partition coefficients (ki) of C, Si, Mn, P, and S in high sulfur steel during the solidification process were investigated by the thermodynamic calculation. The effect of MnS precipitation on ki was explored. The results showed that the precipitation of MnS inclusion would influence the concentrations of solutes Mn and S, leading to the changing of ki. Due to the precipitation of MnS, the kC and kS decreased first and then increased with temperature decreasing, while kSi, kMn, and kP changed monotonously. The impacts of solidification temperature on kSi and kMn were greater than that on kC, kS, and kP. With the increase of S content, kC, kSi, and kP increased while kMn and kS decreased. Whereas, an opposite effect was found with the increase of Mn content. The order of influence extent by S and Mn contents was kSi > kS > kMn > kC > kP.
The hydrated carbonate mineral ikaite (CaCO3·6H2O) is thermodynamically unstable at all known conditions on Earth. Regardless, ikaite has been found in marine sediments, as tufa columns and in sea ice. The reason for these occurrences remains unknown. However, cold temperatures (<6°C), high pH and the presence of Mg2+ and SO42– in these settings have been suggested as factors that promote ikaite formation. Here we show that Mg concentration and pH are primary controls of ikaite precipitation at 5°C. In our experiments a sodium carbonate solution was mixed with seawater at a temperature of 5°C and at a constant rate. To test the effect of Mg2+ and SO42– we used synthetic seawater which allowed us to remove these elements from the seawater. The pH was controlled by different ratios of Na2CO3 and NaHCO3 in the carbonate solution. We found that ikaite precipitated when both seawater and synthetic seawater from which SO4 had been removed were used in the experiments. However, ikaite did not precipitate in experiments conducted with synthetic seawater from which Mg had been removed. In these experiments, calcite precipitated instead of ikaite. By varying the Mg concentration of the synthetic seawater and the pH of the sodium carbonate solution, we constructed a kinetic stability diagram for ikaite and calcite as a function of Mg concentration and pH. One possible explanation of our finding is that Mg2+ inhibits calcite nucleation and thereby allows metastable ikaite to form instead.
The effect of equal-channel angular pressing (ECAP) at various temperatures (310, 330, and 350 °C) on precipitations and strengthening mechanisms of Mg–9Al–1Si alloys was investigated. The results indicated that the average grain size decreased gradually with decreasing of ECAP temperature. The distribution of the Mg2Si phase changed a little when the ECAP temperature increased. However, the different morphologies of β-Mg17Al12 phase were observed, including continuous and uncontinuous precipitation of particles at 310 and 350 °C. The continuous β-Mg17Al12 phase was hardly found and the refined β-Mg17Al12 phase was distributed dispersedly in the matrix at 330 °C. Thus, the mechanical properties of the Mg–9Al–1Si alloy was optimum: ultimate tensile strength and elongation were ∼350.8 MPa and ∼14.77%, respectively. It can be deduced that both grain refinement strengthening and precipitation strengthening play significant roles in strength increment of the alloy during the ECAP process. However, precipitation strengthening is the predominant mechanism.
We developed a 1108 yr chronology of tree-ring widths, based on 64 Himalayan pencil juniper (Juniperus semiglobosa Regel) trees, for the Pamir-Alay Mountains, central Asia. Dendroclimatological analysis demonstrates that precipitation has significant effects on tree growth in the semiarid mountainous area of northwestern Tajikistan located on the edge of the great midlatitude Karakum and Kyzylkum deserts. The highest level of linear correlation (r=0.67) is observed between tree growth and seasonalised winter (previous December–February) precipitation. Our studies also show that moisture (precipitation/Palmer Drought Severity Index) from the previous June to the current September was the dominant climatic factor accounting for interannual variations in tree-ring width, suggesting that this should be considered in climate reconstruction. Using the transfer function method, we reconstructed the region’s drought history over the period AD 908–2015. The results of this moisture reconstruction showed that the most recent millennium was characterised by series of dry and wet stages. The driest periods occurred before 1000, 1200–1250, and at the end of the eighteenth century and beginning of the nineteenth century. The wettest conditions existed in 1650–1700 and after 1990.