Reputed to be the driest desert in the world, the Atacama Desert in the Central Andes of Northern Chile is an extreme environment with high UV radiation, wide temperature variation, and minimum ...precipitation. Scarce lagoons associated with salt flats (salars) in this desert are the surface expression of shallow groundwater; these ponds serve as refugia for life and often host microbial communities associated with evaporitic mineral deposition. Results based on multidisciplinary field campaigns and associated laboratory examination of samples collected from the Puquios of the Salar de Llamara in the Atacama Desert during austral summer provide unprecedented detail regarding the spatial heterogeneity of physical, chemical, and biological characteristics of these salar environments. Four main lagoons ('Puquios') and more than 400 smaller ponds occur within an area less than 5 km
, and are characterized by high variability in electrical conductivity, benthic and planktonic biota, microbiota, lagoon bottom type, and style of mineral deposition. Results suggest that electrical conductivity is a driving force of system heterogeneity. Such spatial heterogeneity within the Puquios is likely to be expanded with temporal observations incorporating expected seasonal changes in electrical conductivity. The complexity of these Andean ecosystems may be key to their ability to persist in extreme environments at the edge of habitability.
Core MD04‐2822 from the Rockall Trough has apparent sedimentation rates of ∼ 1 m/kyr during the last deglaciation (Termination I). Component magnetization directions indicate a magnetic excursion at ...16.3 m depth in the core, corresponding to an age of 26.5 ka, implying an excursion duration of ∼350 years. Across Termination I, the mean grain size of sortable silt implies reduced bottom‐current velocity in the Younger Dryas and Heinrich Stadial (HS)−1A, and increased velocities during the Bølling‐Allerød warm period. Standard bulk magnetic parameters imply fining of magnetic grain size from the mid‐Younger Dryas (∼12 ka) until ∼ 8 ka. First‐order reversal curves (FORCs) were analyzed using ridge extraction to differentiate single domain (SD) from background (detrital) components. Principal component analysis (FORC‐PCA) was then used to discriminate three end members corresponding to SD, pseudo‐single domain (PSD), and multidomain (MD) magnetite. The fining of bulk magnetic grain size from 12 to 8 ka is due to reduction in concentration of detrital (PSD + MD) magnetite, superimposed on a relatively uniform concentration of SD magnetite produced by magnetotactic bacteria. The decrease in PSD+MD magnetite concentration from 12 to 8 ka is synchronized with increase in benthic δ13C, and with major (∼70 m) regional sea‐level rise, and may therefore be related to detrital sources on the shelf that had reduced influence as sea level rose, and to bottom‐water reorganization as Northern Source Water (NSW) replaced Southern Source Water (SSW).
Key Points
FORC‐PCA as a new magnetic grain‐size discriminator
Relationship of magnetic and sortable‐silt grain size proxies over Termination I
New putative magnetic excursion at 26.5 ka
To learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole ...U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly <1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly “pseudo‐single‐domain” behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank.
Plain Language Summary
Critical evidence documenting Earth's evolution has been provided by measuring the magnetic field generated by a magnetized ocean crust. Most of this field comes from the uppermost crust produced by seafloor volcanic eruptions. However, sometimes important contributions come from the lower crust that is not commonly exposed on the seafloor. To better understand these lower crustal contributions, rock samples were collected via ocean drilling at Atlantis Bank in the Indian Ocean. Previous magnetic work on this type of rock has mostly been limited to measurements that provide an average of all magnetic minerals present. In this study, we perform more detailed analyses and are able to link certain magnetic behaviors with specific populations of the magnetic mineral magnetite. In one form, magnetite is present in relatively large (tens of microns) oxide minerals with fine‐scale compositional variations. Magnetite is also present as tiny particles enclosed within other minerals (plagioclase and pyroxene and/or olivine). Because rock layers containing the large oxides are relatively close to the seafloor at Atlantis Bank, and because they contain a large quantity of magnetite, they will contribute most strongly to any magnetic field measurements made close to the seafloor.
Key Points
Magnetite in Atlantis Bank gabbros is present as single‐domain inclusions in silicate minerals and as intergrowths in coarse oxides
Fine‐scale lamellar intergrowths in the coarse oxides give rise to pseudo‐single‐domain‐like behavior and likely dominates near‐bottom anomalies
Thermomagnetic data are consistent with high‐temperature hydrothermal alteration in the uppermost 200–500 m
•We image paleoflooding events in stalagmites using scanning magnetic microscopy.•This approach compiles recurrence histories of extreme paleo-precipitation events.•Case study: Mean recurrence rates ...are ≤5 events per century for the last 500 years in SE Minnesota.•Flooding recurrence rates increase to 7 events per century during the last 100 years.•Land use changes increased the cave's hydrogeologic response rate, carrying more fine sediment.
Flooding events are major natural hazards that present significant risk to communities worldwide. Calculations of flood recurrence rate through time are important tools for regulating land use, determining insurance rates, and for the design and construction of levees and dams. Typically, flood recurrence rates are based on limited historical data or on evidence preserved in the geologic record as overbank deposits, tree ring scars, or high water scour marks. However, these approaches are either limited in their ability to produce continuous time series of flooding events or do not consider the effects of regional land use change. Here we use scanning superconducting quantum interference device (SQUID) microscopy to rapidly image the magnetization associated with flood layers in a polished surface of an annually laminated stalagmite from Spring Valley Caverns (SVC) in southeastern Minnesota. A time series of magnetization peaks, each of which corresponds to a flooding event, yields an average flood recurrence rate of ≤5 events per century for the last 500 years. This rate increases to ∼7 events per century since 1900, coincident with historical timber and agricultural land-use changes in Minnesota. This approach produces a continuous record of well-dated, extreme-precipitation events that can be examined within the context of land use change.
Speleothems, especially stalagmites, hold great potential as recorders of the Earth's ancient magnetic field. However, our understanding of the magnetic mineral assemblages within speleothems is not ...well developed. We present tandem rock magnetic characterization of bulk samples and electron microscopy of magnetic extracts from five North American stalagmites. Magnetite and goethite are present in all samples, albeit in varying abundances and morphologies. Magnetite grains are likely detrital, showing evidence of transport and weathering, including plumose markings and etch pits. These grains are unlikely to have precipitated abiotically in the cave environment. Titanomagnetite and exsolved intergrowths of Fe‐Ti oxides indicate that a significant portion of the magnetic mineral assemblage is allochthonous and was transported into the caves. These Ti‐rich grains display a range of morphologies from euhedral to well rounded, indicating that earlier morphological models for speleothem magnetization do not apply in all geologic settings. Goethite was observed in three morphologies: isolated needles, needle aggregates, and polycrystalline aggregates of randomly oriented nanoscale grains. The magnetite and titanomagnetite, as well as their partially oxidized equivalents, likely hold a depositional remanent magnetization (DRM), whereas if goethite grains were precipitated in situ on the stalagmite drip surface, they are more likely to hold a chemical remanent magnetization (CRM) because of their small size and low saturation magnetization. Until the remanence of goethite can be shown to be paleomagnetically meaningful, we propose that paleomagnetic studies of speleothems include a 150°C thermal demagnetization step to remove any goethite remanence.
Key Points
Magnetic minerals suggest that speleothem remanence falls on a CRM‐DRM continuum
Observed goethite suggests CRM; magnetite and titanomagnetite indicate DRM
Paleomagnetic studies on speleothems should analyze only DRM components
Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe0.5Nb0.5)O3 (PFN). Elastic softening begins at ~550 K, ...which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~425 K coincides with the value of T* reported for polar nanoregions starting to acquire a static or quasi-static component. Softening of the shear modulus by ~30-35% through ~395-320 K, together with a peak in acoustic loss, is due to classical strain/order parameter coupling through the cubic → tetragonal → monoclinic transition sequence of ferroelectric/ferroelastic transitions. A plateau of high acoustic loss below ~320 K is due to the mobility under stress of a ferroelastic microstructure but, instead of the typical effects of freezing of twin wall motion at some low temperature, there is a steady decrease in loss and increase in elastic stiffness below ~85 K. This is attributed to freezing of a succession of strain-coupled defects with a range of relaxation times and is consistent with a report in the literature that PFN develops a tweed microstructure over a wide temperature interval. No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is actually smeared out or suppressed. Instead, the sample is weakly ferromagnetic up to ~560 K, though it has not been possible to exclude definitively the possibility that this could be due to some magnetic impurity. Overall, evidence from the RUS data is of a permeating influence of static and dynamic strain relaxation effects which are attributed to local strain heterogeneity on a mesoscopic length scale. These, in turn, must have a role in determining the magnetic properties and multiferroic character of PFN.
Perovskites in the ternary system PbTiO
3
(PT)–PbZrO
3
(PZ)–Pb(Fe
0.5
Nb
0.5
)O
3
(PFN) have attracted close interest because they can display simultaneous ferroelectric, magnetic and ferroelastic ...properties. Those with the most sensitive response to external fields are likely to have compositions near the morphotropic phase boundary (MPB) which lies close to the binary join Pb(Zr
0.53
Ti
0.47
)O
3
(PZT)–PFN. In the present study, the strength and dynamics of strain coupling behaviour which accompanies the development of ferroelectricity and (anti)ferromagnetism in ceramic PZT–PFN samples have been investigated by resonant ultrasound spectroscopy. Elastic softening ahead of the cubic–tetragonal transition does not fit with models based on dispersion of the soft mode or relaxor characteristics but is attributed, instead, to coupling between acoustic modes and a central peak mode from correlated relaxations and/or microstructure dynamics. Softening of the shear modulus through the transition by up to ~50 % fits with the expected pattern for linear/quadratic strain/order parameter coupling at an improper ferroelastic transition and close to tricritical evolution for the order parameter. Superattenuation of acoustic resonances in a temperature interval of ~100 K below the transition point is indicative of mobile ferroelastic twin walls. By way of contrast, the first-order tetragonal–monoclinic transition involves only a small change in the shear modulus and is not accompanied by significant changes in acoustic dissipation. The dominant feature of the elastic and anelastic properties at low temperatures is a concave-up variation of the shear modulus and relatively high loss down to the lowest temperature, which appears to be the signature of materials with substantial local strain heterogeneity and a spectrum of strain relaxation times. No evidence of magnetoelastic coupling has been found, in spite of the samples displaying ferromagnetism below ~550 K and possible spin glass ordering below ~50 K. For the important multiferroic perovskite ceramics with compositions close to the MPB of ternary PT-PZ-PFN, there must be some focus in future on the role of strain heterogeneity.
Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr
0.53
Ti
0.47
O
3
–PbFe
0.5
Ta
0.5
O
3
(PZT–PFT) have been ...assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr
0.53
Ti
0.47
O
3
–PbFe
0.5
Nb
0.5
O
3
(PZT–PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe
0.5
Ta
0.5
O
3
. As for PZT–PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic–tetragonal–monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel–Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT–PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.
The point mutation S120G in human nucleoside diphosphate kinase A, identified in patients with neuroblastoma, causes a protein folding defect. The urea-unfolded protein cannot refold in vitro, and ...accumulates as a molten globule folding intermediate. We show here that the trimethylamine-
N-oxide (TMAO) corrects the folding defect and stimulated subunit association. TMAO also substantially increased the stability to denaturation by urea of both wild-type and S120G mutant. A non-native folding intermediate accumulated in the presence of 4.5–7
M urea and of 2
M TMAO. It was inactive, monomeric, contained some secondary structure but no tertiary structure and displayed a remarkable stability to denaturation.
Human erythrocyte nucleoside-diphosphate kinase (NDP kinase) is a hexameric enzyme consisting of two kinds of polypeptide
chains, A and B. By random association (A6, A5B...AB5, B6) these polypeptides ...form isoenzymes differing in their isoelectric
point. Chains A and B of NDP kinase were purified by ion-exchange chromatography under denaturing conditions. Upon mixing
and renaturation, the isozymic pattern of NDP kinase obtained by conventional methods was restored. Antibodies raised against
purified chains showed significant cross-reactivity, both in immunoblot experiments and activity inhibition studies. Sequence
determination showed that both chains consisted of 152 amino acid residues corresponding to Mr or 17,143 (chain A) and 17,294
(chain B), respectively. There was high homology between the two sequences (88% identity). The phosphorylation site on the
enzyme is located at His-118. Chain A was identical with human Nm23 protein, which has been reported as a potential suppressor
protein in tumor metastasis and chain B was identical with Nm23-H2 protein.
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