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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

River Geomorphology Affects Biogeochemical Responses to Hydrologic Events in a Large River Ecosystem

Shifts in the frequency and intensity of high discharge events due to climate change may have important consequences for the hydrology and biogeochemistry of rivers. However, our understanding of event-scale biogeochemical dynamics in large rivers lags that of small streams. To fill this gap, we used high-frequency sensor data collected during four consecutive summers from a main channel and backwater site of the Upper Mississippi River. We identified high discharge events and calculated event concentration-discharge responses for both physical-chemical (nitrate, turbidity, and fluorescent dissolved organic matter) and biological (chlorophyll-a and cyanobacteria) constituents using metrics of hysteresis and slope. We found a range of responses across events, particularly for nitrate. Although fluorescent dissolved organic matter (FDOM) and turbidity exhibited more consistent responses across events, contrasting hysteresis metrics indicated that FDOM was flushed to the river from more distant sources than turbidity. Biological responses (chlorophyll a and cyanobacteria) differed more between sites than physical and chemical constituents. Lastly, we found that the event characteristics best explaining concentration responses differed between sites, with event magnitude more frequently related to responses in the main channel, and antecedent wetness conditions associated with response variation in the backwater. Furthermore, our results indicate that event responses in large rivers are distinct across the diverse habitats and biogeochemical components of a large floodplain river, which has implications for local and downstream ecosystems as the climate shifts.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2014

Structure–property relations of orthorhombic [(CH{sub 3}){sub 3}NCH{sub 2}COO]{sub 2}(CuCl{sub 2}){sub 3}·2H{sub 2}O

Large single crystals of orthorhombic [(CH{sub 3}){sub 3}NCH{sub 2}COO]{sub 2}(CuCl{sub 2}){sub 3}·2H{sub 2}O with dimensions up to 40×40×30 mm{sup 3} were grown from aqueous solutions. The elastic and piezoelastic coefficients were derived from ultrasonic resonance frequencies and their shifts upon variation of pressure, respectively, using the plate-resonance technique. Additionally, the coefficients of thermal expansion were determined between 95 K and 305 K by dilatometry. The elastic behaviour at ambient conditions is dominated by the 2-dimensional network of strong hydrogen bonds within the (001) plane leading to a corresponding pseudo-tetragonal anisotropy of the longitudinal elastic stiffness. The variation of elastic properties with pressure, however, as well as the thermal expansion shows strong deviations from the pseudo-tetragonal symmetry. These deviations are probably correlated with tilts of the elongated tri-nuclear betaine–CuCl{sub 2}–water complexes. Neither the thermal expansion nor the specific heat capacity gives any hint on a phase transition in the investigated temperature range. - Graphical abstract: Single crystal of orthorhombic [(CH{sub 3}){sub 3}NCH{sub 2}COO]{sub 2}(CuCl{sub 2}){sub 3}·2H{sub 2}O. - Highlights: • Large single crystals (40 ×40 ×30 mm{sup 3}) of [(CH{sub 3}){sub 3}NCH{sub 2}COO]{sub 2}(CuCl{sub 2}){sub 3}·2H{sub 2}O were grown. • The elastic and piezoelastic coefficients were derived from ultrasonic resonance frequencies. • Thermal expansion (95 K–305 K) and heat capacity (113 K–323 K) were determined. • The orthorhombic structure shows pseudo-tetragonal elastic anisotropy at ambient conditions. • The crystal structure is stable in the investigated range (1–1600 bar, 95–303 K)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Theory and Modeling of Petawatt Laser Pulse Propagation in Low Density Plasmas

Report describing accomplishments in all-optical control of self-injection in laser-plasma accelerators and in developing advanced numerical models of laser-plasma interactions. All-optical approaches to controlling electron self-injection and beam formation in laser-plasma accelerators (LPAs) were explored. It was demonstrated that control over the laser pulse evolution is the key ingredient in the generation of low-background, low-phase-space-volume electron beams. To this end, preserving a smooth laser pulse envelope throughout the acceleration process can be achieved through tuning the phase and amplitude of the incident pulse. A negative frequency chirp compensates the frequency red-shift accumulated due to wake excitation, preventing evolution of the pulse into a relativistic optical shock. This reduces the ponderomotive force exerted on quiescent plasma electrons, suppressing expansion of the bubble and continuous injection of background electrons, thereby reducing the charge in the low-energy tail by an order of magnitude. Slowly raising the density in the pulse propagation direction locks electrons in the accelerating phase, boosting their energy, keeping continuous injection at a low level, tripling the brightness of the quasi-monoenergetic component. Additionally, propagating the negatively chirped pulse in a plasma channel suppresses diffraction of the pulse leading edge, further reducing continuous injection. As a side effect, oscillations of the pulse tail may be enhanced, leading to production of low-background, polychromatic electron beams. Such beams, consisting of quasi-monoenergetic components with controllable energy and energy separation, may be useful as drivers of polychromatic x-rays based on Thomson backscattering. These all-optical methods of electron beam quality control are critically important for the development of future compact, high-repetition-rate, GeV-scale LPA using 10 TW-class, ultra-high bandwidth pulses and mm-scale, dense plasmas. These results emphasize that investment into new pulse amplification techniques allowing for ultrahigh frequency bandwidth is as important for the design of future LPA as are the current efforts directed to increasing the pulse energy.

43 PARTICLE ACCELERATORS↗
DOE OSTI2014

CMB aberration and Doppler effects as a source of hemispherical asymmetries

Our peculiar motion with respect to the CMB rest frame represents a preferred direction in the observed CMB sky since it induces an apparent deflection of the observed CMB photons (aberration) and a shift in their frequency (Doppler). Both effects distort the multipoles a{sub ℓm}'s at all ℓ's. Such effects are real as it has been recently measured for the first time by Planck according to what was forecast in some recent papers. However, the common lore when estimating a power spectrum from CMB is to consider that Doppler affects only the ℓ = 1 multipole, neglecting any other corrections. In this work we use simulations of the CMB sky in a boosted frame with a peculiar velocity β≡v/c = 1.23 × 10{sup −3} in order to assess the impact of such effect on power spectrum estimations in different regions of the sky. We show that the boost induces a north-south asymmetry in the power spectrum which is highly significant and non-negligible, of about (0.58±0.10)% for half-sky cuts when going up to ℓ ≈ 2500. We suggest that these effects are relevant and may account for some of the north-south asymmetries seen in the Planck data, being especially important at small scales. Finally we analyze the particular case of the ACT experiment, which observed only a small fraction of the sky and show that it suffers a bias of about 1% on the power spectrum and of similar size on some cosmological parameters: for example the position of the peaks shifts by 0.5% and the overall amplitude of the spectrum is about 0.4% lower than a full-sky case.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Preparation and characterization of microwave absorbing composite materials with GSs or FeCo/GS composites

Highlights: • It is found that adding GSs or FeCo/GSs composite into the sample SiC/NR can greatly increase RL and the absorbing peak shifts to low-frequency with increasing the adding amount. • The optimum adding amount of GSs is 2%. The sample GS-200-2% possesses the lowest RL value of −31.27 dB at 7.6 GHz. • The optimum adding amount of FeCo/GSs composite is 3%. The sample AGS-450-3% possesses the lowest RL value of −20.67 dB at 7.8 GHz. - Abstract: This study mainly explored the synthesis method of graphene sheets (GSs) and FeCo/graphene sheet (FeCo/GS) composites and evaluated the microwave absorbing characteristics of composite materials containing GSs or FeCo/GS composites. First, graphite oxide (GO) was synthesized by the modified Hummers method. The synthesized GO was further used to synthesize GSs and FeCo/GS composites. Afterward, GSs or FeCo/GS composites were mixed with silicon carbide (SiC) and natural rubber (NR) to prepare microwave absorbing materials with thicknesses of 2 mm. Finally, identification and characterization of these samples were performed by XRD, FE-SEM, TEM, TGA, ESCA, RS, AFM and VNA. GSs were synthesized successfully at a low annealing temperature of 200 °C, and their thicknesses were between 0.848 nm and 2.115 nm, which corresponds to 2–6 stacked layers of graphene. FeCo/GS composites were also synthesized successfully at three different annealing temperatures. The nanoparticles of metals and metal oxides were well-dispersed on the surfaces of and the spaces between GSs. It was found that adding GSs or FeCo/GS composites into the SiC/NR sample can greatly increase reflection loss (RL) and that the absorbing peak shifts to lower frequencies as the concentration of added particles increases. The optimum concentration of added GSs was 2 wt.%. The GS-200-2% sample possessed the lowest RL value of −31.27 dB at 7.6 GHz. The optimum concentration of added FeCo/GS composites was 3 wt.%. The AGS-450-3% sample possessed the lowest RL value of −20.67 dB at 7.8 GHz.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Water-stress-induced breakdown of carbon–water relations: indicators from diurnal FLUXNET patterns

Understanding of terrestrial carbon and water cycles is currently hampered by an uncertainty in how to capture the large variety of plant responses to drought. In FLUXNET, the global network of CO 2 and H 2 O flux observations, many sites do not uniformly report the ancillary variables needed to study drought response physiology. To this end, we outline two data-driven indicators based on diurnal energy, water, and carbon flux patterns derived directly from the eddy covariance data and based on theorized physiological responses to hydraulic and non-stomatal limitations.Hydraulic limitations (i.e. intra-plant limitations on water movement) are proxied using the relative diurnal centroid ( C ET * ), whichmeasures the degree to which the flux of evapotranspiration (ET) is shiftedtoward the morning. Non-stomatal limitations (e.g. inhibitions of biochemical reactions, RuBisCO activity, and/or mesophyll conductance) are characterized by the Diurnal Water–Carbon Index (DWCI), which measures the degree of coupling between ET and gross primary productivity (GPP) within each day.As a proof of concept we show the response of the metrics at six European sites during the 2003 heat wave event, showing a varied response of morning shifts and decoupling. Globally, we found indications of hydraulic limitations in the form of significantly high frequencies of morning-shifted days in dry/Mediterranean climates and savanna/evergreen plant functional types (PFTs), whereas high frequencies of decoupling were dominated by dry climates and grassland/savanna PFTs indicating a prevalence of non-stomatal limitations in these ecosystems. Wholly, both the diurnal centroid and DWCI were associated with high net radiation and low latent energy typical of drought. Using three water use efficiency (WUE) models, we found the mean differences between expected and observed WUE to be - 0.09 to0.44 µ mol mmol -1 and - 0.29 to - 0.40 µ mol mmol -1 for decoupled and morning-shifted days, respectively, compared to mean differences - 1.41 to - 1.42 µ mol mmol -1 in dry conditions, suggesting that morning shifts/hydraulic responses are associated with an increase in WUE,whereas decoupling/non-stomatal limitations are not.

58 GEOSCIENCES↗
DOE OSTI2018

Plasmon-Polaritons in Island Metal Films: A Solution in the Framework of the Coherent Potential Approximation

The photon propagator describing the interaction of light with a monolayer of metal particles (island film) is calculated in the coherent potential approximation. It is shown that the shift in the frequency peak of a dipole surface plasmon for a monolayer particle relative to the analogous frequency of the plasma resonance for an isolated particle is not the only manifestation of the influence of neighboring particles. Neighboring particles also produce a bimodal structure in the spectrum line of the plasmon resonance. The possibility of fine structure in the plasmon resonance spectrum lines is predicted.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Raman Spectrum of Gaseous Propane in Methane

Pressure effects on the Raman spectrum of gaseous propane in methane are investigated. It is found that propane Raman bands located in the range 2700–3100 cm{sup –1} shift to lower frequencies although Raman bands in the range 300–1600 cm{sup –1} change negligibly as the density of the medium containing the analyzed propane increases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Syntheses and Crystal Structures of Rare-Earth Oxyapatites Ca 2 RE 8 (SiO 4 ) 6 O 2 (RE = Pr, Tb, Ho, Tm)

Four different rare-earth oxyapatites of Ca 2 RE 8 (SiO 4 ) 6 O 2 (RE = Pr, Tb, Ho, Tm) were synthesized using a solution-based method followed by drying, calcination, and high-temperature sintering in air. X-ray powder diffraction and Raman spectroscopy were performed on the synthesized oxyapatites. Here, the RE oxyapatites crystallize in the hexagonal space group P6 3 /m with similar unit cell parameters, increasing linearly with larger RE cations. The unit cell volumes increase linearly whereas the densities decrease nonlinearly with larger RE cations. Raman spectra showed intense bands of the symmetric bending and stretching modes of SiO 4 at ~ 400 and 860 cm -1 regions, respectively. The bands generally shifted to higher frequencies with smaller RE cations in the structures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Structural Analysis of ZnO Nanoparticles Reinforced P(3HB-co-15 mol% 3HHx) Bioplastic Composite

Bioplastics are gaining interest due to their biodegradable and biocompatible nature which can be used as a replacement for petroleum based plastics. Poly (3-hydroxybutyrate-co-15 mol% 3-hydroxyhexanoate) [P(3HB-co-15 mol% 3HHx)]/ZnO nanoparticles (NPs) blended bioplastic films were fabricated by solution casting method using chloroform as solvent. The structural characteristics such as peak intensity analysis, crystallite size, dislocation density, and texture coefficient of ZnO NPs mixed P(3HB-co-15 mol% 3HHx) samples were studied using X-Ray Diffraction (XRD) and Fourier Transform Infra-Red (FTIR) analyses. It is clear from the XRD analyses that the crystallinity of P(3HB-co-15 mol% 3HHx) was decreased considerably as ZnO NPs concentration increased. The crystallite size of P(3HB-co-15 mol% 3HHx) was decreased with an increase in ZnO NPs concentration and observed within 150 nm and the dislocation density was decreased with respect to the orientation of P(3HB-co-15 mol% 3HHx) crystals. Simultaneously, the structural properties of ZnO NPs in P(3HB-co-15 mol% 3HHx) matrix were affected noticeably with respect to the ZnO NPs and copolymer concentrations. The characteristic peak positions from FTIR spectra of P(3HB-co-15 mol% 3HHx) copolymer shifted towards higher frequency and evidenced the existence of structural defects. Overall, it was found from both XRD and FTIR analyses that the presence of ZnO NPs affected the crystallinity of P(3HB-co-15 mol% 3HHx) copolymer due to the formation of intermolecular bonds, which restricted the preferential orientation of P(3HB-co-15 mol% 3HHx) molecules which was observed from the texture coefficient analyses. Based on these observations, ZnO NPs at low concentrations can be used with P(3HB-co-15 mol% 3HHx) copolymer effectively and the resulting composite may be used for packaging application.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Effects of Co Substitution on the Microstructural, Infrared, and Electrical Properties of Mg{sub 0.6 − x}Co{sub x}Zn{sub 0.4}Fe{sub 2}O{sub 4} Ferrites

Spinel ferrites having Mg{sub 0.6 − x}Co{sub x}Zn{sub 0.4}Fe{sub 2}O{sub 4} (0 ≤ x ≤ 0.6) compositions were prepared using the sol–gel method. XRD patterns show that samples have the cubic phase of the spinel ferrite with the Fd3̄m space group. The obtained values of the lattice constant and porosity increase with increasing cobalt concentration. FTIR spectra show only the tetrahedral stretching peaks that are shifting towards higher frequencies with increasing Co content. The substitution of Mg by Co leads to the decrease of conductivity of the samples. The behaviors of the real part of permittivity and loss tangent have been investigated using Maxwell theory. Electrical modulus and impedance results show the presence of the electrical relaxation phenomenon for the samples with non-Debye type. The Nyquist representations have been analyzed using a proposed electrical circuit, and the results show that the resistances of the prepared samples resulted mostly from the grain boundary effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol for 3 minutes per day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.56 , p = 0.02) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.83, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07 , p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

3D human airway epithelium↗
DOE OSTI2022

Performance of multifunctional piezoelectric energy harvesting microgyroscopes with material degradation

Most microelectromechanical systems (MEMS) are made from brittle materials that are prone to cracking. Damage from the formation of cracks can occur anytime during the manufacturing process or due to mechanical loading of MEMS devices. Cracks can increase the structures flexibility and lead to changes in performance or total failure. Understanding the potential influences cracks in MEMS is necessary for the longevity of these systems. The energy harvesting microgyroscope studied in this work is susceptible to crack propagation in its silicon substrate and lead zirconate titanate piezoelectric patch. To this end, an electromechanical model is developed to capture the impacts of cracks on the performance of a damaged energy harvesting microgyroscope. Numerical methods are used to approximate the damage associated with distributed crack networks. Then, the static and dynamic responses of the damaged microgyroscope are analyzed for two possible multifunctional setups. Here, the current study revealed that cracks can significantly affect the static and dynamic behaviors of the microgyroscope energy harvester. Combinations of cracking and piezoelectric property degradation cause a decrease in the static pull-in voltage and shift the resonance frequencies of the system. Additionally, it is shown that the systems sensitivity to changes in the base rotation and energy harvesting capabilities vary widely depending on the severity of material degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ballistic-diffusive phonon transport in cellulose nanocrystals by ReaxFF molecular dynamics simulations

Cellulose nanocrystals (CNCs), a ubiquitous nano-sized natural biopolymer, have drawn considerable attention in recent years due to their excellent mechanical and chemical properties. However, thermal properties of CNCs, which are critical for their potential applications as structural or functional materials, are less investigated. In this work, their thermal properties were systematically studied by molecular dynamics (MD) simulations in terms of the effects of polymorphs, size, temperature, and strain. It was found that the thermal conductivities depend on the polymorphs of CNCs, but the thermal conductance of unit chain of each polymorph is similar. Moreover, the strong dependence of the thermal conductivities of CNCs on their length and cross-sectional area indicates the existence of remarkable ballistic-diffusive phonon transport. Single chain CNCs can achieve a thermal conductivity of ~6 W/(m∙K), implying their potential as high thermal conductive polymer. The mechanical strain can enhance the alignment of cellulose skeleton, which reduces phonon scattering and thus increases thermal conductivity. Possible underlying mechanism of the varying thermal conductivity was further discussed and attributed to the shift of phonon frequencies and varied degree of orientation. This study reveals the sophisticated ballistic-diffusive phonon transport in CNCs, and pave path to future design of CNC-based materials with desirable thermal properties.

42 ENGINEERING↗
DOE OSTI2011

Synthesis, structural and magnetic studies of the CuCr{sub 1-x}Rh{sub x}O{sub 2} delafossite solid solution with 0 {<=} x {<=} 0.2

Highlights: {yields} Rh non-magnetic replacement of the Cr in the magnetic frustrated delafossite CuCrO{sub 2}. {yields} Delafossite solid solution is maintained up to 0.2 in this system. {yields} Strain generated by Rh substitution is strongly anisotropic. {yields} M{sup 3+}-O bond strength is affected. {yields} High temperature paramagnetic behavior and weak ferromagnetism at low temperature. -- Abstract: The CuCr{sub 1-x}Rh{sub x}O{sub 2} series is investigated by X-ray diffraction, magnetization measurements and Raman spectroscopy on ceramic samples. It is found that a delafossite solid solution is maintained up to x = 0.2 in CuCr{sub 1-x}Rh{sub x}O{sub 2}. The small observed variation in cell parameters is consistent with the small difference between the ionic radii of Cr{sup 3+} and Rh{sup 3+}. A significant broadening of X-ray reflections is observed and when analyzed using the Williamson-Hall relationship showed that the strain generated by Rh substitution is strongly anisotropic, affecting mainly (Cr,Rh)-O bonds in the ab plane. Room temperature Raman spectra displayed three main Raman active modes. All modes shift to lower frequency and undergo significant changes in intensity with increasing Rh content, showing the effect of Rh atoms on the M{sup 3+}-O bond strength. The magnetic behavior of CuCr{sub 1-x}Rh{sub x}O{sub 2} samples was investigated as a function of temperature and applied field. At high temperature paramagnetic behavior, and at low temperature, evidence for weak ferromagnetism, reinforced by a hysteresis loop at 4 K is observed. The magnetic behavior of CuCr{sub 1-x}Rh{sub x}O{sub 2} is attributed to the disorder of Cr and Rh in octahedral sites resulting in short-range Cr-O-Cr and Cr-O-Rh interactions, which give rise to short-range weak ferromagnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Confinement and surface effects on the physical properties of rhombohedral-shape hematite (α-Fe{sub 2}O{sub 3}) nanocrystals

Highlights: • Uniform rhombohedral hematite nanocrystals (RHNCs) have been obtained. • A detailed formation mechanism of these HNCS has been proposed. • Phonon confinement effects were revealed in the RHNCS vibrational bands. • Quantum confinement effects on the optical and electronic properties were found. - Abstract: Morphological, microstructural and vibrational properties of hematite (α-Fe{sub 2}O{sub 3}) nanocrystals with a rhombohedral shape and rounded edges, obtained by forced hydrolysis of iron(III) solutions under a fast nucleation, have been investigated in detail as a function of aging time. These studies allowed us to propose a detailed formation mechanism and revealed that these nanocrystals are composed of four {104} side facets, two {110} faces at the edges of the long diagonal of the nanocrystals and two {−441} facets as the top and bottom faces. Also, the presence of nanoscopic pores and fissures was evidenced. The vibrational bands of such nanocrystals were shifted to lower frequencies in comparison with bulk hematite ones as the nanocrystal size was reduced due to phonon confinement effects. Also, the indirect and direct transition band gaps displayed interesting dependences on the aging time arising from quantum confinement and surface effects.

36 MATERIALS SCIENCE↗
DOE OSTI2013

New insight into UO 2 F 2 particulate structure by micro-Raman spectroscopy

Uranyl fluoride particles produced via hydrolysis of uranium hexafluoride have been deposited on different substrates: polished graphite disks, silver foil, stainless steel and gold-coated silicon wafer, and measured with micro-Raman spectroscopy (MRS). All three metallic substrates enhanced the Raman signal delivered by UO 2 F 2 in comparison to graphite. The fundamental stretching of the U–O band appeared at 867 cm –1 in case of the graphite substrate, while in case of the others it was shifted to lower frequencies (down to 839 cm –1 ). All applied metallic substrates showed the expected effect of Raman signal enhancement; however the gold layer appeared to be most effective. Lastly, application of new substrates provides more information on the molecular structure of uranyl fluoride precipitation, which is interesting for nuclear safeguards and nuclear environmental analysis.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

A mechanistic understanding of oxygen isotopic changes in the Western United States at the Last Glacial Maximum

At the Last Glacial Maximum (LGM), records suggest drier conditions in the northwest United States and wetter conditions in the southwest United States relative to present-day as well as widespread changes in the isotopic composition of water. However, the mechanisms responsible for these changes remain ambiguous. Here, we explore differences in western United States hydroclimate between the LGM and preindustrial with a water isotope tracer enabled Earth System Model. We then use proxy forward models to compare simulated and recorded δ 18 O in speleothems. We find that the pattern of hydroclimate response in the western United States at the LGM relates to a combination of 1) increased frequency and southward shifted wintertime extratropical cyclones in the North Pacific, 2) greater rainout of moisture as it moves over the continent, and 3) reduced evaporation in the cooler LGM climate. The simulated lower δ 18 O of precipitation at the LGM relates predominantly to an increase in cool season moisture removal efficiency, with a secondary contribution from relatively more cool season precipitation. Both surface temperatures and North American ice sheets contribute to these hydroclimate changes at the LGM. Comparisons between δ 18 O from proxy forward models and speleothem records in the western United States show general agreement at the LGM, with increasing depletion moving towards the continental interior. Finally, this study highlights the similarities and differences between hydrologic and δ 18 O changes at the LGM and emphasizes the utility of model-proxy comparison for interpretation.

54 ENVIRONMENTAL SCIENCES↗