Effect of storms during drought on post‐wildfire recovery of channel sediment dynamics and habitat in the southern California chaparral,USA

Current global warming projections suggest a possible increase in wildfire and drought, augmenting the need to understand how drought following wildfire affects the recovery of stream channels in relation to sediment dynamics. We investigated post‐wildfire geomorphic responses caused by storms during a prolonged drought following the 2013 Springs Fire in southern California (USA), using multi‐temporal terrestrial laser scanning and detailed field measurements. After the fire, a dry‐season dry‐ravel sediment pulse contributed sand and small gravel to hillslope‐channel margins in Big Sycamore Creek and its tributaries. A small storm in WY 2014 generated sufficient flow to mobilize a portion of the sediment derived from the dry‐ravel pulse and deposited the fine sediment in the channel, totaling ~0.60 m³/m of volume per unit length of channel. The sediment deposit buried step‐pool habitat structure and reduced roughness by over 90%. These changes altered sediment transport characteristics of the bed material present before and after the storm; the ratio of available to critical shear stress (τ_o/τ_c) increased by five times. Storms during WY 2015 contributed additional fine sediment from tributaries and lower hillslopes and hyperconcentrated flow transported and deposited additional sediment in the channel. Together these sources delivered sediment on the order of six times that in 2014, further increasing τ_o/τ_c. These storms during multi‐year drought following wildfire transformed channel dynamics. The increased sediment transport capacity persisted during the drought period characterized by the longer residence time of relatively fine‐grained post‐fire channel sedimentation. This contrasts with wetter years, when post‐fire sediment is transported from the fluvial system during the same season as the post‐fire sediment pulse. Results of this short‐term study highlight the complex and substantial effects of multi‐year drought on geomorphic responses following wildfire. These responses influence pool habitat that is critical to longer‐term post‐wildfire riparian ecosystem recovery. Copyright © 2017 John Wiley & Sons, Ltd.     

Gas hydrate disturbance fabrics of southern Hydrate Ridge sediments (ODP Leg 204): Relationship with texture and physical properties

Soupy and mousse-like fabrics are disturbance sedimentary features that result from the dissociation of gas hydrate, a process that releases water. During the core retrieval process, soupy and mousse-like fabrics are produced in the gas hydrate-bearing sediments due to changes in pressure and temperature conditions. Therefore, the identification of soupy and mousse-like fabrics can be used as a proxy for the presence of gas hydrate in addition to other evidence, such as pore water freshening or anomalously cool temperature. We present here grain-size results, mineralogical composition and magnetic susceptibility data of soupy and mousse-like samples from the southern Hydrate Ridge (Cascadia accretionary complex) acquired during Leg 204 of the Ocean Drilling Program. In order to study the relationship between sedimentary texture and the presence of gas hydrates, we have compared these results with the main textural and compositional data available from the same area. Most of the disturbed analyzed samples from the summit and the western flank of southern Hydrate Ridge show a mean grain size coarser than the average mean grain size of the hemipelagic samples from the same area. The depositional features of the sediments are not recognised due to disturbance. However, their granulometric statistical parameters and distribution curves, and magnetic susceptibility logs indicate that they correspond to a turbidite facies. These results suggest that gas hydrates in the southern Hydrate Ridge could form preferentially in coarser grain-size layers that could act as conduits feeding gas from below the BSR. Two samples from the uppermost metres near the seafloor at the summit of the southern Hydrate Ridge show a finer mean grain-size value than the average of hemipelagic samples. They were located where the highest amount of gas hydrates was detected, suggesting that in this area the availability of methane gas was high enough to generate gas hydrates, even within low-permeability layers. The mineralogical composition of the soupy and mousse-like sediments does not show any specific characteristic with respect to the other samples from the southern Hydrate Ridge.     

Palynological Evidence for Climatic Change and Human Activity during the Holocene on Minorca (Balearic Islands)

Four pollen diagrams from Minorca (Balearic Islands) have been correlated with other previously studied sequences from Majorca and Minorca to define a Holocene landscape sequence for the region from 8000 yr B.P. to the present. The lower part of the pollen diagrams reflects a climatic phase with more rain and less-marked seasonality than today. Significant quantities ofCorylus, Buxus,and mesophilous taxa are found. In the middle part, between 5000 and 4000 yr B.P., a strong change is recorded in composition and structure of the vegetational landscape, with vegetation appearing that was adapted to Mediterranean conditions. This episode coincided with the first human colonization of the island and also with a widespread climatic change in the western Mediterranean region. The change in taxa was complex and some sclerophyllous taxa suchOleaplayed an important role in the transformation of the landscape physiognomy from the mid-Holocene until the present. Although human activities have removed much of the Mediterranean vegetation on the Balearic Islands, it seems clear that the changes have been brought about, in part, by increasing dryness.     

A crystal-chemical classification of borate structures with emphasis on hydrated borates

The rules governing formation of hydrated borate polyanions that were proposed by C.L. Christ in 1960 are critically reviewed and new rules added on the basis of recent crystal structure determinations. Principles and classifications previously published by others are also critically reviewed briefly. The fundamental building blocks from which borate polyanions can be constructed are defined on the basis of the number n of boron atoms, and the fully hydrated polyanions are illustrated. Known structures are grouped accordingly, and a shorthand notation using n and symbols Δ = triangle, T = tetrahedron is introduced so that the polyanions can be easily characterized. For example, 3:Δ+2T describes [B₃O₃(OH)₅]^2?. Correct structural formulas are assigned borates with known structures whereas borates of unknown structure are grouped separately.     

Constraining Holocene sea levels using U‐Th ages of phreatic overgrowths on speleothems from coastal caves in Mallorca (Western Mediterranean)

Phreatic overgrowths on speleothems (POS) are carbonate formations deposited at the water table of caves in unique karstic coastal settings having morphologies that can be directly related to sea level at the time of formation. The U‐Th ages of calcite and aragonite overgrowths collected from the modern water table in coastal caves on Mallorca (Cova de Cala Varques A and Cova des Pas de Vallgornera) were determined using high‐precision MC‐ICPMS techniques. U‐Th ages indicate that phreatic carbonate deposition occurred between ca 2·8 and at least 0·6 ka BP and are in accord with an archeologically estimated age of 3·7–3·0 ka BP for a drowned prehistoric construction at a depth of 1 m below current sea level in a cave from the same area. Speleothem δ¹³C and δ¹⁸O and chemical composition of cave pools provide supportive evidence that POS reflect mixing between seawater and brackish water table. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaporation of perennial semi‐arid woodland in southeastern Australia is adapted for irregular but common dry periods

Measurements of water vapour flux from semi‐arid perennial woodland (mallee) were made for 3 years using eddy covariance instrumentation. There have been no previous long‐term, detailed measures of water use in this ecosystem. Latent energy flux (LE) on a half hourly basis was the measure of the combined soil and plant evaporation, ‘evapotranspiration’ (E_LE) of the site. Aggregation over 3 years of the site measured rain (1136 mm) and the estimated evaporation (794 mm) suggests that 342 mm or 30% of rain had moved into or past the root zone of the vegetation. Above average rainfall during 2011 and the first quarter of 2012 (633 mm, 15 months) would likely have been the period during which significant groundwater recharge occurred. At times immediately after rainfall, E_LE rates were the same or exceeded estimates of potential E calculated from a suitably parameterized Penman–Monteith (E_PMo) equation. Apparent free water E from plant interception and soil evaporation was about 2.3 mm and lasted for 1.3 days following rainfall in summer, while in autumn, E was 5.1 mm that lasted over 5.4 days. The leaf area index (LAI) needed to adjust a wind function calibrated Penman equation (E_PMe) to match the E_LE values could be back calculated to generate seasonal change in LAI from 0.12 to 0.46 and compared well with normalized difference vegetation index; r = 0.38 and p = 0.0213* and LAI calculated from digital cover photography. The apparently conservative response of perennial vegetation evaporation to available water in these semi‐arid environments reinforces the conclusion that these ecosystems use this mechanism to survive the reasonably common dry periods. Plant response to soil water availability is primarily through gradual changes in leaf area. Copyright © 2015 John Wiley & Sons, Ltd.     

Postglacial climates inferred from a lake at treeline,southwest Yukon Territory,Canada

Pollen, chironomid, and ostracode records from a lake located at alpine treeline provide regional paleoclimate reconstructions from the southwest Yukon Territory, Canada. The pollen spectra indicate herbaceous tundra existed on the landscape from 13.6–11 ka followed by birch shrub tundra until 10 ka. Although Picea pollen dominated the assemblages after 10 ka, low pollen accumulation rates and Picea percentages indicate minimal treeline movement through the Holocene. Chironomid accumulation rates provide evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Ostracodes were found in the late glacial and early Holocene, but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape. Inferred mean July air temperature, total annual precipitation, and water depth indicate a long-term cooling with increasing moisture from the late glacial through the Holocene. During the Younger Dryas (12.9–11.2 ka), cold and dry conditions prevailed. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4 ka, and warm, dry conditions since the end of the Little Ice Age.     

Spatial variability of snowmelt timing from AMSR‐E and SSM/I passive microwave sensors,Pelly River,Yukon Territory,Canada

Spring snow melt run‐off in high latitude and snow‐dominated drainage basins is generally the most significant annual hydrological event. Melt timing, duration, and flow magnitude are highly variable and influence regional climate, geomorphology, and hydrology. Arctic and sub‐arctic regions have sparse long‐term ground observations and these snow‐dominated hydrologic regimes are sensitive to the rapidly warming climate trends that characterize much of the northern latitudes. Passive microwave brightness temperatures are sensitive to changes in the liquid water content of the snow pack and make it possible to detect incipient melt, diurnal melt‐refreeze cycles, and the approximate end of snow cover on the ground over large regions. Special Sensor Microwave Imager (SSM/I) and Advanced Microwave Scanning Radiometer for EOS (AMSR‐E) passive microwave brightness temperatures (T_b) and diurnal amplitude variations (DAV) are used to investigate the spatial variability of snowmelt onset timing (in two stages, ‘DAV onset’ and ‘melt onset’) and duration for a complex sub‐arctic landscape during 2005. The satellites are sensitive to small percentages of liquid water, and therefore represent ‘incipient melt’, a condition somewhat earlier than a traditional definition of a melting snowpack. Incipient melt dates and duration are compared to topography, land cover, and hydrology to investigate the strength and significance of melt timing in heterogeneous landscapes in the Pelly River, a major tributary to the Yukon River. Microwave‐derived melt onset in this region in 2005 occurred from late February to late April. Upland areas melt 1–2 weeks later than lowland areas and have shorter transition periods. Melt timing and duration appear to be influenced by pixel elevation, aspect, and uniformity as well as other factors such as weather and snow mass distribution. The end of the transition season is uniform across sensors and across the basin in spite of a wide variety of pixel characteristics. Copyright © 2007 John Wiley & Sons, Ltd.