Response of arid northeast Mexico to global climate changes during the late Pleistocene to the middle Holocene

Anthropogenic global warming might cause expansion of the drylands and trigger socio-economic challenges in the water-deficit subtropical regions. Changes in hydroclimate during the intervals of variable global temperature over the recent geological past, however, could provide useful information about the possible responses of these arid ecosystems to the near future warmer conditions. We evaluated hydroclimates of two different parts of subtropical North America by generating new records of surface processes and regional vegetation from drought-prone northeast Mexico and subsequently compared them with the paleoclimate of the central-southern United States. Our study suggests that congruent changes occurred in both parts during ~13.5–9.5 cal ka BP, an interval with no warm pool in the northern Gulf of Mexico. The precipitation and erosion responded to temperature-modulated variations in positions of the Inter-Tropical Convergence Zone (ITCZ). Conditions were wetter than today in the subsequent warmer interval (~9.5–8.2 cal ka BP) with generally stable ITCZ and the highest summer insolation. Hydroclimate changes of both parts lacked congruency during ~8.2–6.8 cal ka BP as the northern Gulf of Mexico began hosting a warm pool. Similar to the modern conditions, this warm pool might have modified trajectories of the tropical storms. Erosion and abundance of C₃ plants decreased in northeast Mexico. Higher wetness in the Mississippi River Basin and the southern Great Plains during this interval suggested that the storms made landfall more frequently in the central-southern United States. © 2019 John Wiley & Sons, Ltd.     

Optical properties of chromophoric dissolved organic matter (CDOM) in surface and pore waters adjacent to an oil well in a southern California salt marsh

Chromophoric dissolved organic matter (CDOM) optical properties were measured in surface and pore waters as a function of depth and distance from an oil well in a southern California salt marsh. Higher fluorescence and absorbances in pore vs. surface waters suggest soil pore water is a reservoir of CDOM in the marsh. Protein-like fluorophores in pore waters at distinct depths corresponded to variations in sulfate depletion and Fe(II) concentrations from anaerobic microbial activity. These variations were supported by fluorescence indexes and are consistent with differences in optical molecular weight and aromaticity indicators. Fluorescence indices were consistent with autochthonous material of aquatic origin in surface waters, with more terrestrial, humified allochthonous material in deeper pore waters. CDOM optical properties were consistent with significantly enhanced microbial activity in regions closest to the oil well, along with a three-dimensional excitation/emission matrix fluorescence spectrum peak attributable to oil, suggesting anaerobic microbial degradation of oil.     

Rehabilitation of earthquake damaged external RC beam‐column joints by joint enlargement using prestressed steel angles

The effectiveness of a rehabilitation method based on joint enlargement using prestressed steel angles to enhance the seismic behavior of damaged external reinforced concrete beam‐column joints was experimentally investigated. Three half‐scale joints having either non‐seismic or seismic reinforcement details were tested both before and after rehabilitation by applying lateral cyclic loading of increasing amplitudes. Two defects were considered for the two non‐seismic units, being the absence of transverse steel hoops and insufficient bond capacity of beam bottom steel reinforcing bars in the joint panel zone. The damaged specimens were rehabilitated by injecting epoxy grout into existing cracks and installing stiffened steel angles at the re‐entrant corners of the beam‐column joint, both above and below the beam, that were mounted and held in place using prestressed high‐tensile strength bars. The test results indicated that the seismic performance of the rehabilitated specimens in terms of strength, stiffness, and ductility was fully recovered and comparable with the performance of the seismically detailed specimen. Copyright © 2016 John Wiley & Sons, Ltd.     

Insight into sediment transport processes on saline rangeland hillslopes using three‐dimensional soil microtopography changes

In arid and semi‐arid rangeland environments, an accurate understanding of runoff generation and sediment transport processes is key to developing effective management actions and addressing ecosystem response to changes. Yet, many primary processes (namely sheet and splash and concentrated flow erosion, as well as deposition) are still poorly understood due to a historic lack of measurement techniques capable of parsing total soil loss into these primary processes. Current knowledge gaps can be addressed by combining traditional erosion and runoff measurement techniques with image‐based three‐dimensional (3D) soil surface reconstructions. In this study, data (hydrology, erosion and high‐resolution surface microtopography changes) from rainfall simulation experiments on 24 plots in saline rangelands communities of the Upper Colorado River Basin were used to improve understanding on various sediment transport processes. A series of surface change metrics were developed to quantify and characterize various erosion and transport processes (e.g. plot‐wide versus concentrated flow detachment and deposition) and were related to hydrology and biotic and abiotic land surface characteristics. In general, erosivity controlled detachment and transport processes while factors modulating surface roughness such as vegetation controlled deposition. The extent of the channel network was a positive function of slope, discharge and vegetation. Vegetation may deflect runoff in many flow paths but promoted deposition. From a management perspective, this study suggests that effective runoff soil and salt load reduction strategies should aim to promote deposition of transported sediments rather than reducing detachment which might not be feasible in these resource‐limited environments. Copyright © 2016 John Wiley & Sons, Ltd.     

<Emphasis Type="Italic">Acanthaster planci</Emphasis> outbreaks in Vanuatu coincide with ocean productivity,furthering trends throughout the pacific ocean

This study identifies linkages between regional ocean productivity and the emergence of large Acanthaster planci starfish populations in Vanuatu. Positive correlations were found between wind stress, chlorophyll-a, and upwelling during January–February 2009, corresponding with coral-eating starfish occurrences. Further, temporal associations have existed between monthly wind stress and upwelling since 2000, and were predictors of past starfish events. Links between starfish emergence and oceanographic features are discussed, drawing upon evidence from other asteroid echinoderms. High regional productivity associated with anomalous oceanographic conditions in Vanuatu, and globally, can be used as early warning indicators of probable, future starfish emergence to aid the foundation and success of local management efforts.     

Geospatial analysis of controls on subglacial bedform morphometry in the New York Drumlin Field – implications for Laurentide Ice Sheet dynamics

The outline and trend of 6566 subglacial bedforms in the New York Drumlin Field have been digitized from digital elevation data. A spatial predictive model has been used to extend values of bedform elongation over an area measuring 200 km × 110 km. The resulting surface is used in conjunction with depth‐to‐bedrock data and an assumed duration of ice residence to test three proposed controls on bedform elongation. Upon comparison, the resulting display of morphometry is best explained by differences in ice velocity across the field of study. The existence of multiple zones of fast‐moving ice located along the southern margin of the Laurentide Ice Sheet is implied by the observed patterns of bedform elongation and orientation. We present two interpretations that are consistent with the observations. First, enhanced basal sliding caused by decreasing effective pressure near a calving margin is suggested as a possible mechanism by which localized fast ice flow is initiated and maintained. Second, topographically controlled ice streams likely occupied the fjord‐like troughs of the Appalachian Upland northern rim. Contrary to previous understanding of the Laurentide southern margin in New York State, the resulting palaeoglaciological reconstruction illustrates a dynamic mosaic of ice stream and/or outlet glacier activity. Copyright © 2009 John Wiley & Sons, Ltd.     

Prescribed‐fire effects on rill and interrill runoff and erosion in a mountainous sagebrush landscape

Changing fire regimes and prescribed‐fire use in invasive species management on rangelands require improved understanding of fire effects on runoff and erosion from steeply sloping sagebrush‐steppe. Small (0·5 m²) and large (32·5 m²) plot rainfall simulations (85 mm h^–1, 1 h) and concentrated flow methodologies were employed immediately following burning and 1 and 2 years post‐fire to investigate infiltration, runoff and erosion from interrill (rainsplash, sheetwash) and rill (concentrated flow) processes on unburned and burned areas of a steeply sloped sagebrush site on coarse‐textured soils. Soil water repellency and vegetation were assessed to infer relationships in soil and vegetation factors that influence runoff and erosion. Runoff and erosion from rainfall simulations and concentrated flow experiments increased immediately following burning. Runoff returned to near pre‐burn levels and sediment yield was greatly reduced with ground cover recovery to 40 per cent 1 year post‐fire. Erosion remained above pre‐burn levels on large rainfall simulation and concentrated flow plots until ground cover reached 60 per cent two growing seasons post‐fire. The greatest impact of the fire was the threefold reduction of ground cover. Removal of vegetation and ground cover and the influence of pre‐existing strong soil‐water repellency increased the spatial continuity of overland flow, reduced runoff and sediment filtering effects of vegetation and ground cover, and facilitated increased velocity and transport capacity of overland flow. Small plot rainfall simulations suggest ground cover recovery to 40 per cent probably protected the site from low‐return‐interval storms, large plot rainfall and concentrated flow experiments indicate the site remained susceptible to elevated erosion rates during high‐intensity or long duration events until ground cover levels reached 60 per cent. The data demonstrate that the persistence of fire effects on steeply‐sloped, sandy sagebrush sites depends on the time period required for ground cover to recover to near 60 per cent and on the strength and persistence of ‘background’ or fire‐induced soil water repellency. Published in 2009 by John Wiley & Sons, Ltd.     

Variability of soil physical and hydraulic properties at the Mojave Global Change Facility,Nevada: Implications for water budget and evapotranspiration

The purposes of this study, conducted at the Mojave Global Change Facility in Nevada, USA, were to examine whether, and to what extent, spatial structure in soil physical properties would manifest as similar spatial structure in hydraulic properties and in spatial differences of evapotranspiration (ET) predictions. Soil samples were collected from 0 to 5 and 5 to 10 cm depths at each of 96 test plots. Hydraulic properties were estimated using textural properties and bulk density in the pedotransfer function method and then examined for spatial structure using variogram analysis. Results show a fining of texture at both depths in the northeast (downslope) quadrant, which led to higher water-holding capacity. Weak spatial structure was observed for most properties (ranges ～75 m). Hydraulic property data were then used as input to a one-dimensional numerical model (HYDRUS-1D) applied at each of the 96 sites for 159 days. Four different shrub cover percentages (0, 10, 20, and 30%) were used. The results showed a predominant soil evaporation-dominated water balance in the northeast quadrant of the site, regardless of plant cover. Elsewhere, plant transpiration was more dominant as percent cover decreased. Results revealed the important role that soil hydraulic properties play in near-surface water balance.     

Regional beryllium-10 production rate calibration for late-glacial northeastern North America

The major uncertainty in relating cosmogenic-nuclide exposure ages to ages measured by other dating methods comes from extrapolating nuclide production rates measured at globally scattered calibration sites to the sites of unknown age that are to be dated. This uncertainty can be reduced by locating production rate calibration sites that are similar in location and age to the sites to be dated. We use this strategy to reconcile exposure age and radiocarbon deglaciation chronologies for northeastern North America by compiling ¹⁰Be production rate calibration measurements from independently dated late-glacial and early Holocene ice-marginal landforms in this region. ¹⁰Be production rates measured at these sites are 6–12% lower than predicted by the commonly accepted global ¹⁰Be calibration data set used with any published production rate scaling scheme. In addition, the regional calibration data set shows significantly less internal scatter than the global calibration data set. Thus, this calibration data set can be used to improve both the precision and accuracy of exposure dating of regional late-glacial events. For example, if the global calibration data set is used to calculate exposure ages, the exposure-age deglaciation chronology for central New England is inconsistent with the deglaciation chronology inferred from radiocarbon dating and varve stratigraphy. We show that using the regional data set instead makes the exposure age and radiocarbon chronologies consistent. This increases confidence in correlating exposure ages of ice-marginal landforms in northeastern North America with glacial and climate events dated by other means.     

Controls on accumulation of anomalously thick coals: Implications for sequence stratigraphic analysis

Coal seams preserve high-resolution records of ancient terrestrial water table (base level) fluctuations in ancient peat accumulations, but little is known about base level change in anomalously thick coal seams. Using the Early Cretaceous 91 m anomalously thick No. 6 coal (lignite) seam in the Erlian Basin (north-east China) as a case study, the origin and evolution of peat accumulation in a continental faulted basin is revealed by sedimentological, sequence stratigraphic and coal petrological analyses. The lignite is dominated by huminite, indicating oxygen-deficient and waterlogged conditions in the precursor mire. Four types of key sequence stratigraphic surfaces are recognized, including paludification, terrestrialization, accommodation-reversal and give-up transgressive surfaces. Vertically, the No. 6 coal seam consists of fourteen superimposed wetting-up and drying-up cycles separated by key sequence stratigraphic surfaces, with each of these cycles having a mean duration of about 156 to 173 kyr. In a high accommodation peat swamp system, the wetting-up cycles are generally characterized by an upward increase in mineral matter and inertodetrinite and an upward decrease in huminite with the paludification surface as their base and the give-up transgressive surface or accommodation-reversal surface as their top, representing a trend of upward-increasing accommodation. In contrast, the drying-up cycles are generally characterized by an upward decrease in mineral matter and inertodetrinite and an upward increase in huminite, with the terrestrialization surface as their base and the accommodation-reversal surface as their top, representing a trend of upward-decreasing accommodation. A multi-phase mire stacking model for accumulation of the coal seam is proposed based on high-frequency accommodation cycles and the stratigraphic relationships between coal and clastic sediments. High-frequency accommodation cycles in the coal are closely related to water table fluctuations in the precursor mires and are driven by high-frequency climate via changes in the intensity and seasonality of precipitation in a relatively stable subsidence regime. Recognition that the No. 6 coal seam is composed of multiple stacked mires has implications for studies addressing palaeoclimatic inferences and genesis of anomalously thick coals seams.