Oxygen and Hydrogen Isotopic Signatures of Large Atmospheric Ice Conglomerations

Specific studies about the stable isotope composition (¹⁸O/¹⁶O and D/H) of atmospheric icy conglomerations are still scarce. The present work offers, for the first time, a very detailed analysis of oxygen and hydrogen isotopic signatures of unusually large ice conglomerations, or “megacryometeors”, that fell to the ground in Spain during January 2000. The hydrochemical analysis is based on the bulk isotopic composition and systematic selective sampling (deuterium isotopic mapping) of eleven selected specimens. δ¹⁸O and δD (V-SMOW) of all samples fall into the Meteoric Water Line matching well with typical tropospheric values. The distribution of the samples on Craig's line suggests either a variation in condensation temperature and/or different residual fractions of water vapour (Rayleigh processes). Three of the largest megacryometeors exhibited unequivocally distinctive negative values (δ¹⁸O = −17.2%₀ and δD = −127 %₀ V-SMOW), (δ¹⁸O = −15.6%₀ and δD = −112%₀ V-SMOW) and (δ¹⁸O = −14.4%₀ and δD = −100%₀ V-SMOW), suggesting an atmospheric origin typical of the upper troposphere. Theoretical calculations indicate that the vertical trajectory of growth was lower than 3.2 km. During the period in which the fall of megacryometeors occurred, anomalous atmospheric conditions were observed to exist: a substantial lowering of the tropopause with a deep layer of saturated air below, ozone depression and strong wind shear. Moreover, these large ice conglomerations occurred during non-thunderstorm conditions, suggesting an alternative process of ice growth was responsible for their formation.     

Red Mangrove Seedling Survival, Growth, and Reproduction: Effects of Environment and Maternal Genotype

An adequate understanding of the nature and extent of response to stressors and resources by marine foundation plant species requires study of local adaptation and plasticity in traits. Analyses of variation among genotypes in growth and morphology and genotype × environment interactions are necessary for restoration in, for example, different combinations of tide, soil, and salinity regimes, and for assessing how foundation plant species will respond to global climate change. We conducted a field experiment to assess differences in responses among 86 half-sibling (same maternal tree) seedling families of the red mangrove (Rhizophora mangle) to different environmental conditions of hydrology imposed at low- and high-intertidal settings. At 3 years, Rhizophora survival and growth varied with maternal tree genotype, elevation, and genotype × elevation. This effect was independent of size of propagules at planting. Growth also differed among the five islands planted, a main effect that is a composite of a number of microenvironmental differences stemming from island size and shape, proximity to passes (and thus fetch), and island slope. Significant genotype × island interactions for some response variables further support the hypothesis that seedlings of different maternal genotypes can perform very differently under various suites of environmental conditions. Planted seedlings reproduced at an early age, and at 3 years there were differences in reproductive output among genotypes but not an overall mean difference between plants at low or high elevation. Whether our results show adaptation to local conditions or differences in plasticity among genotypes will require further study as the plants mature further to demonstrate fitness differences. However, either adaptation or plasticity provides a basis for maintenance of Rhizophora dominance over a wider range of environmental conditions and may be important for adaptation to conditions that will vary with global climate change.     

Genetic effects of a large-scaleSpartina alterniflora (smooth cordgrass) dieback and recovery in the northern Gulf of Mexico

A large-scale dieback event struck marshes along the northwestern Gulf of Mexico coast during summer 2000, in apparent response to a prolonged and severe drought. Along the Louisiana coast, large areas of the dominant marsh species,Spartina alterniflora, turned brown, followed by death of at least the aboveground structures of entire plant mortality. Key ecological and genetic measures were studied in a dieback-affected marsh in southwest Louisiana (C83 marsh, Sabine National Wildlife Refuge), for which existed predieback ecologic and genetic datasets. Effects on genetic diversity only were studied in a second set of sites in southeastern Louisiana (near Bay Junop), where the dieback was more widespread. We hypothesized that stem density, live aboveground biomass, and genetic diversity would be significantly reduced compared to predieback conditions and to nearby unaffected marshes. Stem densities and biomass levels approached predieback conditions 14 months after first observance of the dieback in the Sabine marsh and were similar to or exceeded the same measures for a nearby unaffected marsh. DNA extracted from leaf samples in the Sabine and Bay Junop sites was used to construct genotype profiles using AFLPs and analyzed using the complement of Simpson’s Index (1-D), the richness measure G/N, average heterozygosity <H>, and the estimated proportion of polymorphic genes <P>. Genetic diversity was relatively unaffected by the dieback at either the Sabine or Bay Junop sites. Evidence from field observations and the results of the genetic analyses suggest that seedling recruitment is an important factor in the recovery of both the Bay Junop and C83 sites, although re-growth from surviving below-ground rhizomes appeared to dominate recovery at the latter site. Survival of below-ground structures, leading to the rapid recovery observed, indicates a high level of resilience of the Sabine marsh to drought-induced stress. Still, the genetic diversity ofS. alterniflora-dominated marshes may be promoted by occasional disturbance events, which produce open areas in which seedling recruitment can occur.     

Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K<Subscript>2</Subscript>CO<Subscript>3</Subscript> to 3.2 GPa

Brackets on the melting temperature of K₂CO₃ were experimentally determined at 1.86 ± 0.02 GPa (1,163–1,167°C), 2.79 ± 0.03 GPa (1,187–1,195°C), and 3.16 ± 0.04 GPa (1,183–1,189°C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K₂CO₃ fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K₂CO₃, the high-pressure density and compressibility of K₂CO₃ liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K′₀ = dK ₀/dP, where K ₀ = 1/β₀) is between 16.2 and 11.6, with a best estimate of 13.7, in a third-order Birch–Murnaghan equation of state (EOS). This liquid K′₀ leads to a density of 2,175 ± 36 kg/m³ at 4 GPa and 1,500°C, which is ∼30% lower than that reported in the literature on the basis of the falling-sphere method at the same conditions. The uncertainty in the liquid K′₀ leads to an error in melt density of ± 2% at 4 GPa; the error decreases with decreasing pressure. With a K′₀ of 13.7, the compressibility of K₂CO₃ at 1,500°C and 1 bar (K ₀ = 3.8 GPa) drops rapidly with increasing pressure ( ), which prevents a density crossover with silicate melts, such as CaAlSi₂O₈ and CaMgSi₂O₆, at upper mantle depths.     

Determination of Cl Production Rates Derived from the Well-Dated Deglaciation Surfaces of Whidbey and Fidalgo Islands, Washington

The ³⁶Cl dating method is increasingly being used to determine the surface-exposure history of Quaternary landforms. Production rates for the ³⁶Cl isotopic system, a critical component of the dating method, have now been refined using the well-constrained radiocarbon-based deglaciation history of Whidbey and Fidalgo Islands, Washington. The calculated total production rates due to calcium and potassium are 91±5 atoms ³⁶Cl (g Ca)⁻¹ yr⁻¹ and are 228±18 atoms ³⁶Cl (g K)⁻¹ yr⁻¹, respectively. The calculated ground-level secondary neutron production rate in air, P_f(0), inferred from thermal neutron absorption by ³⁵Cl is 762±28 neutrons (g air)⁻¹ yr⁻¹ for samples with low water content (1–2 wt.%). Neutron absorption by serpentinized harzburgite samples of the same exposure age, having higher water content (8–12 wt.%), is 40% greater relative to that for dry samples. These data suggest that existing models do not adequately describe thermalization and capture of neutrons for hydrous rock samples. Calculated ³⁶Cl ages of samples collected from the surfaces of a well-dated dacite flow (10,600–12,800 cal yr B.P.) and three disparate deglaciated localities are consistent with close limiting calibrated ¹⁴C ages, thereby supporting the validity of our ³⁶Cl production rates integrated over the last 15,500 cal yr between latitudes of 46.5° and 51°N. Although our production rates are internally consistent and yield reasonable exposure ages for other localities, there nevertheless are significant differences between these production rates and those of other investigators.     

Change in Extent of Meadows and Shrub Fields in the Central Western Cascade Range,Oregon∗

We examined change in areal extent of mountain meadows and fields of deciduous shrubs and conifer saplings in the central western Cascade Range of Oregon, based on analysis of aerial photographs taken in 1946 and 2000. These nonforest vegetation patches are distinctive habitats in dominantly forested landscapes, such as the Cascades, and change in extent of these habitats is of interest to scientists and land managers. We mapped and dated even-aged forest stands of probable postfire origin around the nonforest patches, using tree-ring analysis and interpretation of aerial photographs. We used these and archival data to interpret possible influences of past wildfire and sheep grazing on the extent of nonforest patches. The total area of nonforest vegetation patches decreased from 5.5 percent of the study area in 1946 to 2.5 percent in 2000. Significantly more cases of contracted patches were observed for mesic and xeric meadows which have adjacent forest established after wildfire in the approximately 150 years preceding 1946. A higher proportion of mesic (47 percent, n = 47) than xeric (17 percent, n= 115) meadows contracted between 1946 and 2000. Broad-leaved shrub fields were unchanged, probably because of topo-edaphic controls, dense cover of shrubs, and snow effects; but all fields of conifer saplings underwent succession to forest. We observed no strong influences of sheep grazing on the extent of meadows.     

2-D seismic trenching of colluvial wedges and faults

Images of the depth and shape of colluvial wedges by 3-D travel time tomography can be valuable in estimating the past history of ancient earthquakes and assessing the earthquake hazard of a fault. Unfortunately, 3-D seismic surveys can be both costly and time consuming. In this paper, we report our first successful results of detecting the shapes and depths of colluvial wedges with 2-D travel time tomography. The locations of the colluvial wedges are along the Oquirrh fault and the Salt Lake City segment of the Wasatch fault. We also report that the tomogram from one of our 2-D surveys suggests the possibility of detecting multiple colluvial wedges at depth. Using both reflection and tomographic images at another site clearly reveals the presence of a known shallow fault and the contact between native soil and recent fill. Our results suggest the synergistic use of both seismic tomography and reflection imaging, we denote as seismic trenching, as a means of detecting shallow colluvial wedges and faults. Seismic trenching has the potential to significantly expand the lateral extent and depth of investigation of paleoseismology.     

The anatomy of exhumed river-channel belts: Bedform to belt-scale river kinematics of the Ruby Ranch Member,Cretaceous Cedar Mountain Formation,Utah, USA

Many published interpretations of ancient fluvial systems have relied on observations of extensive outcrops of thick successions. This paper, in contrast, demonstrates that a regional understanding of palaeoriver kinematics, depositional setting and sedimentation rates can be interpreted from local sedimentological measurements of bedform and barform strata. Dune and bar strata, channel planform geometry and bed topography are measured within exhumed fluvial strata exposed as ridges in the Ruby Ranch Member of the Cretaceous Cedar Mountain Formation, Utah, USA. The ridges are composed of lithified stacked channel belts, representing at least five or six re-occupations of a single-strand channel. Lateral sections reveal well-preserved barforms constructed of subaqueous dune cross-sets. The topography of palaeobarforms is preserved along the top surface of the outcrops. Comparisons of the channel-belt centreline to local palaeotransport directions indicate that channel planform geometry was preserved through the re-occupations, rather than being obscured by lateral migration. Rapid avulsions preserved the state of the active channel bed and its individual bars at the time of abandonment. Inferred minimum sedimentation durations for the preserved elements, inferred from cross-set thickness distributions and assumed bedform migration rates, vary within a belt from one to ten days. Using only these local sedimentological measurements, the depositional setting is interpreted as a fluvial megafan, given the similarity in river kinematics. This paper provides a systematic methodology for the future synthesis of vertical and planview data, including the drone-equipped 2020 Mars Rover mission, to exhumed fluvial and deltaic strata.     

Channel adjustments to reservoir construction and gravel extraction along Stony Creek, California

Stony Creek (drainage area 1920 km²), a tributary to the Sacramento River 200 km north of San Francisco, has experienced channel adjustments to both dam construction and instream gravel extraction. Black Butte Dam, constructed in 1963, reduced flood peaks and trapped the annual 100,000 m³ of bedload sediment formerly delivered from the upper watershed. The reach below the dam changed from an active braided channel to an incised, single-thread channel shortly after dam construction. About 25 km downstream of the dam, large instream gravel mines annually extract 230,000–580,000 m{su3} and have induced channel incision of over 5 m, necessitating bridge repairs costing US$1.4 million.