Structural transformations in natrolite and edingtonite

Results of structural transformation studies in the natural fibrous zeolites natrolite and edingtonite are presented. The minerals were studied in situ in a wide range of temperatures, pressures and compositions by differential scanning microcalorimetry, thermogravimetry, dilatometry, X-ray diffractometry, nuclear magnetic resonance (NMR)and Raman-spectroscopy. The high-pressure experiments were done in diamond anvil cell and Be-bronze bomb for NMR using liquids with various dimensions of molecules as pressure-transmitting media. A number of structural transformations in natrolites and edingtonite have been found with constant or changing water content during transformation. Under high water pressures, some additional H₂O molecules entered the framework channels causing a framework deformation and an anisotropic “swelling” of the crystal. Under compression in nonpenetrating liquids no transformations were detected and above 70 kbar amorphization of the minerals was observed. The same displacive-tilt transformations were observed in zeolites at elevated temperatures as a result of the dehydration, e.g. natrolite ? α-metanatrolite at 280° C. Fully reversible phase transitions at constant H₂O content were observed in natrolites and edingtonite at low temperatures (down to -120° C). These are connected with a variation in the mobility and position of exchange cations and water molecules within the framework channels and are followed by significant volume and thermal effects. In dehydrated zeolites, the transformations were found to be similar to the α ? β transition in quartz (α-metanatrolite ? β-metanatrolite). Heating of fibrous zeolites above 500 ÷ 700° C causes their amorphization and formation of porous quasiglass. The principal difference in structural behaviour of microporous crystals under compression in penetrating and nonpenetrating media has essential geochemical implications. Structural transformations of zeolites in P-T-X space demonstrate crystal chemical analogy of these parameters. Some deviations from this analogy depend on complex interactions between channel “filling”, H₂O and cations, and the [(Al,Si)-O_4/2] framework.     

Solid solutions of (Mg,Fe<Superscript>2+</Superscript>)-cordierite: Synthesis,water content,and magnetic properties

The dependence of water concentration in synthetic (Mg, Fe²⁺)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe²⁺/(Fe²⁺ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H₂O molecules in the structural channels of alkali-free (Mg, Fe²⁺)-cordierite indicate that the H-H vector of some H₂O molecules (H₂O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe²⁺)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.     

Chemical structural studies of natural lignin by dipolar dephasing solid-state 13C nuclear magnetic resonance

Two natural lignins, one from a gymnosperm wood the other from angiosperm wood, were examined by conventional solid-state and dipolar dephasing ¹³C nuclear magnetic resonance (NMR) techniques. The results obtained from both techniques show that the structure of natural lignins is consistent with models of softwood and hardwood lignin. The dipolar dephasing NMR data provide a measure of the degree of substitution on aromatic rings which is consistent with the models.     

Infrared investigation of CO2-bearing cordierites

CO₂ molecules were introduced experimentally into the structural channels of synthetic well ordered Mgcordierite (Mg₂ ^[VI][(Al₄Si₅)^[IV]O₁₈]) at temperatures of 600 and 800° C, and pressures of 7, 8, 10, 12, and 25 kbar. Powder infrared spectra of the run products show five absorption bands in the region of the asymmetric stretching mode of CO₂. Two of them, strong and sharp, occurring at 2353 cm^–1 {2} and 2348 cm^–1 {3}, are related to two different types of CO₂ molecules. The relative intensity of the band {2} (type I) increases with the cell parameter c_o whereas the relative intensity of the band {3} (type II) increases with the parameter a₀ of the crystal. It is concluded that CO₂ molecules of type I may be oriented with their elongation parallel to the c-axis of the crystal, while CO₂ molecules of type II lie with their O-C-O vector parallel to the a-axis. Analytical data indicate that the intensity ratio Z of these two bands ({2}/{3}) is a linear function of the CO₂ content of cordierite. This ratio depends also on the temperature and, to a less extent, on the pressure under which cordierite entrapped CO₂ molecules. It is proposed to combine this infrared parameter Z together with an estimate of the P-T conditions of the incorporation of CO₂ into the channels, in order to determine the CO₂ content of natural cordierites. The samples do not need to be of high purity and only small amounts (<5 mg) are necessary. This semi-empirical analytical method, which does not require complicated data treatments, is suitable for CO₂-rich cordierites of granulite facies rocks.     

Intelligent methods for predicting nuclear magnetic resonance of porosity and permeability by conventional well-logs: a case study of Saharan field

In the well-log data processing, the principal advantage of the nuclear magnetic resonance (NMR) method is the measurement of fluid volume and pore size distribution without resorting to parameters such as rock resistivity. Preliminary processing of the well-log data allowed first to have the petrophysical parameters and then to evaluate the performances of the transverse relaxation time T ₂ NMR. Petrophysical parameters such as the porosity of the formation as well as the effective permeability can be estimated without having recourse the fluid type. The well-log data of five wells were completed during the construction of intelligent models in the Saharan oil field Oued Mya Basin in order to assess the reliability of the developed models. Data processing of NMR combined with conventional well data was performed by artificial intelligence. First, the support vector regression method was applied to a sandy clay reservoir with a model based on the prediction of porosity and permeability. NMR parameters estimated using intelligent systems, i.e., fuzzy logic (FL) model, back propagation neural network (BP-NN), and support vector machine, with conventional well-log data are combined with those of NMR, resulting in a good estimation of porosity and permeability. The results obtained during the processing are then compared to the FL and NN regression models performed by the regression method during the validation stage. They show that the correlation coefficients R ² estimated vary between 0.959 and 0.964, corresponding to the root mean square error values of 0.20 and 0.15.     

Geochemistry,Petrography and Spectroscopy of Organic Matter of Clay‐Associated Kerogen of Ypresian Series: Gafsa‐Metlaoui Phosphatic Basin,Tunisia

This work presents geochemical characterization of isolated kerogen out of clay fraction using petrography studies, infrared absorption and solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy, with N‐alkane distributions of saturated hydrocarbon. Mineralogical study of clay mineral associations was carried out using X‐ray diffraction (XRD), on Ypresian phosphatic series from Gafsa‐Metlaoui basin, Tunisia. The XRD data indicate that smectite, palygorskite and sepiolite are the prevalent clay minerals in the selected samples. In this clay mineral association, the N‐alkane (m/z = 57) distribution indicates that the marine organic matter is plankton and bacterial in origin. The kerogens observed on transmitted light microscopy, however, appear to be totally amorphous organic matter, without any appearance of biological form. The orange gel‐like amorphous organic matter with distinct edges and homogenous texture is consistent with a high degree of aliphaticity. This material has relatively intense CH₂ and CH₃ infrared bands in ¹³C NMR peaks. This aliphatic character is related to bacterial origin. Brown amorphous organic matter with diffuse edges has a lower aliphatic character than the previous kerogen, deduced from relatively low CH₂ and CH₃ infrared and ¹³C NMR band intensities.     

Mineralogical Significance of Fluids in Channels of Colombian Emeralds: A Mass-Spectrometric Study

Quadrupolar mass-spectrometric study of fluids trapped in inclusions and channels is used to characterize the qualitative and quantitative composition of fluids within emeralds from 10 deposits located on the eastern and western margins of the Eastern Cordillera of Colombia. Fluids in channels and in fluid inclusions have the same qualitative composition. The bulk composition of the fluid phase is: H₂O (80 to 92 mol%), N₂ (3 to 10%), CO₂ (2.5 to 5%), H₂ (1 to 5%), CO (0.1 to 1.5%), CH₄ (<0.5%), organic compounds (<0.05%), and inert gases. Hydrogen and carbon monoxide in large part come from water and carbon dioxide reduction. Fluids trapped during emerald growth belong to an aquo-carbonic fluid phase with CO₂/N₂ ratios near 1 (0.8 to 0.84 in fluid inclusions, 1 to 1.3 in channels). Fluid contents in channels are higher by ～20% in emeralds from the western group compared to those in the eastern deposits. The concentration of H₂O in channels in emeralds from the eastern group is lower (1.35 < H₂O < 1.45 wt%) than in those of the western group (1.63 < H₂O < 2.19 wt%), and it corresponds to the lowest contents of H₂O found worldwide for emeralds from different deposit types. Colombian emeralds are quite different because their hydrothermal-sedimentary genesis is unique.

Emeralds from the western zone also have higher Na₂O contents than those from the eastern zone. A strong correlation exists between channel H₂O and Na₂O in the emeralds, which also is the case for beryls and emeralds from different geological environments. Such an enrichment is indicative of the presence of Na⁺ in the structural channels of beryl. This correlation between channel H₂O and Na₂O of the crystal is evidence for the introduction of channel H₂O as Type-II water molecules in beryls.     

The hafnium isotope composition of Pacific Ocean water

The first Hf isotope data for seawater are reported for a series of stations in the Northwestern Pacific and define a range from ε_Hf = 3.5 ± 1.4 to 8.6 ± 1.6. Most samples have values within error of the average of ε_Hf = 5.9, but significant variations are found in intermediate waters at a depth of 600 m, as well as in deep waters. The Nd and Hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region, confirming that Hf in the Fe-Mn crusts has been derived from the overlying water column, which thus provide an archive of past seawater compositions. Although the seawater samples are generally close to the global ε_Nd-ε_Hf correlation obtained from ferromanganese crusts, there are significant deviations from this correlation indicating that there is some additional decoupling between Nd and Hf isotope signals, most likely caused by local water mass mixing and differences in residence times. This is not resolved in the crust samples, which integrate seawater signals over 10⁴ years. The combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans. Studies of the distribution of oceanic Hf isotope compositions that have been confined to deep water and boundary waters, as recorded in seafloor ferromanganese crusts, can now be extended and aimed at characterising the entire present-day water column. Average Hf concentrations measured in this study are somewhat lower than previously reported, suggesting a shorter residence time for Hf in the global oceans, although the uncertainty in the extent of Hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined.     

Geometry and dynamics of braided channels and bars under experimental density currents

Submarine channels convey turbidity currents, the primary means for distributing sand and coarser sediments to the deep ocean. In some cases, submarine channels have been shown to braid, in a similar way to rivers. Yet the strength of the analogy between the subaerial and submarine braided channels is incompletely understood. Six experiments with subaqueous density currents and two experiments with subaerial rivers were conducted to quantify: (i) submarine channel kinematics; and (ii) the responses of channel and bar geometry to subaerial versus submarine basin conditions, inlet conditions and the ratio of ‘flow to sediment’ discharge (Q _w/Q _s). For a range of Q _w/Q _s values spanning a factor of 2·7, subaqueous braided channels consistently developed, were deeper upstream compared to downstream, and alternated with zones of sheet flow downstream. Topographic analyses included spatial statistics and mapping bars and channels using a reduced‐complexity flow model. The ratio of the estimated depth‐slope product for the submarine channels versus the subaerial channels was greater than unity, consistent with theoretical predictions, but with downstream variations ranging over a factor of 10. For the same inlet geometry and Q _w/Q _s, a subaqueous experiment produced deeper, steeper channels with fewer channel threads than its subaerial counterpart. For the subaqueous cases, neither slope, nor braiding index, nor bar aspect ratio varied consistently with Q _w/Q _s. For the subaqueous channels, the timescale for avulsion was double the time to migrate one channel width, and one‐third the time to aggrade one channel depth. The experiments inform a new stratigraphic model for submarine braided channels, wherein sand bodies are more laterally connected and less vertically persistent than those formed by submarine meandering channels.     

The system NaAlSi3O8−H2O−H2 (1200° C, 2 kbar): the solubility and interaction mechanism of fluid species with melt

The H₂O and H₂ solubilities in an albite melt at 1200° C and 2 kbar over the entire range of gas phase composition, from pure hydrogen to pure water were studied in gas-media pressure vessels. The water solubility initially increases with increasing hydrogen content until a maximum of 9.19 wt% H₂O atXH ₂ ^v =0.1 is reached, withXH ₂ ^v >0.1 the water solubility decreases. The hydrogen solubility curve has a maximum atXH ₂ ^v =0.42 where the concentration reaches 0.206 wt% H₂O. Over the entire compositional range¹H NMR (nuclear magnetic resonance) spectra show distinct absorption lines due to protons bound to OH groups and to isolated firmly bound water molecules. In NMR and Raman spectra there were no bands attributable to the H–H vibrations of molecular hydrogen. The X-ray photo-electronic spectra of hydrogen-bearing glasses show the Si2p (99 eV) band which corresponds to the zero-valency silicon. The formation of OH groups and molecular water during interaction between hydrogen-bearing fluids and melts under reducing conditions has a qualitative effect, the same as for water dissolution. Another point of interest is that hydrogen-bearing melts undergo more depolymerization than do hydrous melts.     

The dehydration process in the zeolite laumontite: a real-time synchrotron X-ray powder diffraction study

The dehydration process of the natural zeolite laumontite Ca₄Si₁₆Al₈O₄₈ · 18 H₂O has been studied in situ by means of powder diffraction and X-ray synchrotron radiation. Powder diffraction profiles suitable for Rietveld refinements were accumulated in time intervals of 5 minutes using a position sensitive detector (CPS-120 by INEL), while the temperature increased in steps of about 5 K. The synchronization of accumulation time and temperature plateau allowed collection of 62 temperature-resolved powder patterns in the range 310–584 K, whose analysis produced a dynamic picture of the laumontite structure response to dehydration. The first zeolitic water molecules diffusing out of the channels are those not bonded to the Ca cations and located in the W(1) site, whose occupancy drops smoothly to 10% during heating to 349 K, while the sample in the capillary is still submerged in water. The remaining W(1) and 60% of W(5) water molecules are expelled rather sharply at about 370 K. At this temperature all remaining water submerging the powder crystallites is lost, the structure contains about 13 water molecules/cell, and the crystal structure is that of leonhardite. On continued heating 80% of the water molecules from the W(2) site are lost between 420 and 480 K, while a small amount of the diffusing water is reinserted in the W(5) site. The occupancy factor of the W(8) site decreases starting at 480 K, and reaches a maximum loss of 20% at 584 K. The combined occupancy of the Ca-coordinated W (2) and W (8) water sites never falls much below two, so that the Ca cations in the channels, which are bonded to four framework oxygen atoms, are nearly six-coordinated in the explored temperature range. The water loss is accompanied by large changes in the unit cell dimensions. Except at 367 K, where the excess surrounding water is leaving, all changes in cell dimensions are gradual. The loss of the hydrogen bonded W(1) and W(5) water molecules is related to most of the unit cell volume reduction below 370 K, as shown by the contraction of the a-, b- and c-axes and the increase in the monoclinic angle. Loss of the Ca-coordinated W(2) and W(8) water molecules has a small effect on the unit cell volume as the continued contraction of the a- and c-axes is counter-balanced by a large expansion in the b-axis and a decrease in the monoclinic β angle.     

Nuclear magnetic resonance spectroscopy of soils and related materials. Relaxation of 13C nuclei in cross polarization nuclear magnetic resonance experiments

Relaxation of ¹³C nuclei in a peat, a soil, and three soil fractions have been investigated in order to improve structural resolution and to investigate quantification of various carbon types. Rotating frame spin lattice relaxation times (T_1?'s) and transverse relaxation times (T₂′'s) are similar to those observed for coals. T₂′'s of carbons in different magnetic environments differ sufficiently that spectra can be obtained containing only nonprotonated carbon and methyl substituents if a 40 μsec delay without decoupling is inserted into the pulse programme before data acquisition (dipolar dephasing). Provided quantitative data is obtained in simple cross polarization experiments and allowance is made for loss in signal intensity of nonprotonated carbon during dipolar dephasing, then the fraction of aromatic carbon which is protonated in the samples can be determined.     

Water molecules in beryl and cordierite: high-temperature vibrational behavior,dehydration, and coordination to cations

Natural samples of typical cyclosilicates beryl and cordierite include water and carbon dioxide molecules in channels formed by the open cavities. Water molecules in the channels have two forms that are distinguished by whether they coordinate to extra-framework cations (type II) or not (type I). We measured polarized infrared (IR) spectra for thin sections of the (100) plane of beryl or the (100) and (010) planes (cb and ca planes) of cordierite under various temperature conditions. The spectral features of major bands clearly showed the distinguishable behavior of types I and II water molecules under high temperature as follows. Over the temperature range from room temperature to 800°C where rapid dehydration did not occur, the decrease in band heights for type II water molecules were smaller than those for type I, and band shifts were more predominant for type II water molecules. The decrease in band heights and band shifts of type I/II bands differed also for beryl and cordierite, which arises from the different ways in which water molecules are fixed in the channels. Dehydration was enhanced at 850°C. The IR spectra at room temperature quenched from 850°C both for beryl and cordierite showed that the vibrational bands related to type II water molecules were stable after those related to type I water molecules disappeared. In addition, frequency changes of type II bands were observed, possibly because of changes of coordination states of type II water molecules to extra-framework cations in the channels.     

Chemical structures of coal lithotypes before and after CO2 adsorption as investigated by advanced solid-state C nuclear magnetic resonance spectroscopy

Four lithotypes (vitrain, bright clarain, clarain, and fusain) of a high volatile bituminous Springfield Coal from the Illinois Basin were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The NMR techniques included quantitative direct polarization/magic angle spinning (DP/MAS), cross polarization/total sideband suppression (CP/TOSS), dipolar dephasing, CH_n selection, and recoupled C-H long-range dipolar dephasing techniques. The lithotypes that experienced high-pressure CO₂ adsorption isotherm analysis were also analyzed to determine possible changes in coal structure as a result of CO₂ saturation at high pressure and subsequent evacuation. The main carbon functionalities present in original vitrain, bright clarain, clarain and fusain were aromatic carbons (65.9%-86.1%), nonpolar alkyl groups (9.0%-28.9%), and aromatic C-O carbons (4.1%-9.5%). Among these lithotypes, aromaticity increased in the order of clarain, bright clarain, vitrain, and fusain, whereas the fraction of alkyl carbons decreased in the same order. Fusain was distinct from other three lithotypes in respect to its highest aromatic composition (86.1%) and remarkably small fraction of alkyl carbons (11.0%). The aromatic cluster size in fusain was larger than that in bright clarain. The lithotypes studied responded differently to high pressure CO₂ saturation. After exposure to high pressure CO₂, vitrain and fusain showed a decrease in aromaticity but an increase in the fraction of alkyl carbons, whereas bright clarain and clarain displayed an increase in aromaticity but a decrease in the fraction of alkyl carbons. Aromatic fused-rings were larger for bright clarain but smaller for fusain in the post-CO₂ adsorption samples compared to the original lithotypes. These observations suggested chemical CO₂-coal interactions at high pressure and the selectivity of lithotypes in response to CO₂ adsorption.     

Dynamics Of Karstification: A Model Applied To Hydraulic Structures In Karst Terranes

To model the development of karst channels from primary fissures in limestone, a computer simulation of solutional widening of a fracture by calcite agressive water is proposed. The parameters defining the problem are the initial width a₀ of the fracture, its length l, and the hydraulic gradient i driving water through it. The dissolution rates limestone determine how fast enlargement of the fractures proceeds. At a calcite concentration, c, far from equilibrium, the dissolution follows a first-order rate law, F⁽¹⁾=α₀(c_eq-c); close to the equilibrium concentration, c_eq, a slow fourth-order rate law F⁽⁴⁾=β₀(c_eq-c)⁴ is valid. The results show that, at the time of initiation, the water flow through the karst channels increases slowly in time until an abrupt increase occurs. After this moment of breakthrough, the channel enlarges rapidly and evenly over its entire length by first-order kinetics. Breakthrough times have been calculated for karstification under natural conditions for low hydraulic gradients as functions of a₀, l, and i. Special attention is given to karstification in the vicinity of hydraulic structures where hydraulic gradients are high (>0.5) and channel lengths are below 200 m. We find that the breakthrough event will occur in less than 100 years, if: (i/l) > (5.3·10^?8a₀ ^?2.63P_CO2 ^?0.77) where l is in m and a₀ is in cm, (i/l) is given in m^-1, and P_CO2[atm] is the CO₂ pressure of the water entering the fracture. After this event, the channels will widen to a width of about 1 cm within only 10 years, which can cause considerable leakage near or through hydraulic structures. Finally, critical values of the parameters i, l, a₀, which give the conditions of failure in various types of hydraulic structures are discussed.     

Structure vs. composition: A solid-state H and Si NMR study of quenched glasses along the Na2O-SiO2-H2O join

A suite of six hydrous (7 wt.% H₂O) sodium silicate glasses spanning sodium octasilicate to sodium disilicate in composition were analyzed using ²⁹Si single pulse (SP) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, ¹H-²⁹Si cross polarization (CP) MAS NMR, and fast MAS ¹H-NMR. From the ²⁹Si SPMAS data it is observed that at low sodium compositions dissolved water significantly depolymerizes the silicate network. At higher sodium contents, however, dissolved H₂O does not affect a significant increase in depolymerization over that predicted based on the Na/Si ratio alone. The fast MAS ¹H-NMR data reveal considerable complexity in proton environments in each of the glasses studied. The fast MAS ¹H-NMR spectra of the highest sodium concentration glasses do not exhibit evidence of signficantly greater fractions of dissolved water as molecular H₂O than the lower sodium concentration glasses requiring that the decrease in polymerization at high sodium contents involves a change in sodium solution mechanism. Variable contact time ¹H-²⁹Si cross polarization (CP) MAS NMR data reveal an increase in the rotating frame spin lattice relaxation rate constant (T_1ρ*) for various Qⁿ species with increasing sodium content that correlates with a reduction in the average ¹H-²⁹Si coupling strength. At the highest sodium concentration, however, T_1ρ* drops significantly, consistent with a change in the Na₂O solution mechanism.     

Mechanism of water dissolution in sodium-silicate melts and glasses: Structural interpretation of spectroscopic data

Sodium-silicate glasses with varying water contents were studied by ²³Na NMR and ¹H NMR spectroscopy. The ²³Na NMR spectrum is made up of two Gaussian-Lorentzian components corresponding to rigidly bound and free Na ions. The rigidly bound Na is allocated in the disilicate-like domains corresponding to Q₃ species of sodium-silicate glasses. Unbound Na is associated with Q₂ and Q₁ species. It was shown that, during water dissolution, some hydroxyls are incorporated into the disilicate unit of the structure to form NaHSi₂O₅, while others hydrate silica (Q₄species). Our ²³Na NMR data are consistent with available data on Q speciation and the proportions of water species in sodium-silicate glasses in the frameworks of a proposed detailed structural scheme of water dissolution.     

Pressure-induced migration of zeolitic water in laumontite

A polycrystalline sample of natural laumontite (Pleasant Valley, Connecticut) was studied up to 6.8 (1) GPa at room temperature using monochromatic synchrotron X-ray powder diffraction and a diamond-anvil cell. A methanol: ethanol: water mixture was used as a penetrating pressure-transmitting fluid. A dry sample measured before adding the pressure fluid inside the diamond-anvil cell contained ~12 H₂O per formula unit, consistent with the water content of partially dehydrated laumontite. Upon increasing the pressure to 0.2 (1) GPa, fully hydrated laumontite with 18 H₂O per unit cell formed and the unit-cell volume expanded by 2.6%. Further pressure increase up to 2.4 (1) GPa resulted in a gradual contraction of the unit-cell volume and individual cell lengths. During this process, a successive order–disorder transition of hydrogen-bonded water molecules from their symmetry sites was observed, concomittent with an inflectional behavior of the monoclinic beta angle and the channel ellipticity. Above 3 GPa, a tripling of the b axis was detected. Thereafter, up to 6.8 (1) GPa, the compression behavior was reversed for the c axis length and the monoclinic beta angle which showed a gradual increase and decrease, respectively, without any apparent volume discontinuity. We suspect that different ordering of the water molecules or Ca cations inside the channels along the b axis may be responsible for the observed supercell transition above 3 GPa.     

Fish assemblage composition in constructed and natural tidal marshes of San Diego Bay: Relative influence of channel morphology and restoration history

This study evaluated the use by fish of restored tidal wetlands and identified links between fish species composition and habitat characteristics. We compared the attributes of natural and constructed channel habitats in Sweetwater Marsh National Wildlife Refuge, San Diego Bay, California, by using fish monitoring data to explore the relationships between channel environmental characteristics and fish species composition. Fishes were sampled annually for 8 yr (1989–1996) at eight sampling sites, four in constructed marshes and four in natural marshes, using beach seines and blocking nets. We also measured channel habitat characteristics, including channel hydrology (stream order), width and maximum depth, bank slope, water quality (DO, temperature, salinity), and sediment composition. Fish colonization was rapid in constructed channels, and there was no obvious relationship between channel age and species richness or density. Total richness and total density did not differ significantly between constructed and natural channels, although California killifish (Fundulus parvipinnis) were found in significantly higher densities in constructed channels. Multivariate analyses showed fish assemblage composition was related to channel habitat characteristics, suggesting a channel’s physical properties were more important in determining fish use than its restoration status. This relationship highlights the importance of designing restoration projects with natural hydrologic features and choosing proper assessment criteria in order to avoid misleading interpretations of constructed channel success. We recommend that future projects be designed to mimic natural marsh hydrogeomorphology and diversity more closely, the assessment process utilize better estimates of fish habitat function (e.g., individual and community-based species trends, residence time, feeding, growth) and reference site choice, and experimental research be further incorporated into the restoration process.     

Channel evolution of Des Moines Lobe till drainage ditches in southern Minnesota (USA)

Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R ² (0.82, 0.68) for channel CSA and width, a weaker R ² (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/or bed particle size distribution. However, stepwise regression determined that D₈₄, width/depth ratio and mean bankfull depth explained 83?% of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability.