Removal of ammonia toxicity in marine sediment TIEs: a comparison of Ulva lactuca,zeolite and aeration methods

Toxicity Identification Evaluations (TIEs) can be used to determine the specific toxicant(s), including ammonia, causing toxicity observed in marine sediments. Two primary TIE manipulations are available for characterizing and identifying ammonia in marine sediments: Ulva lactuca addition and zeolite addition. In this study, we compared the efficacy of these methods to (1) remove NH(x) and NH(3) from overlying and interstitial waters and (2) reduce toxicity to the amphipod Ampelisca abdita and mysid Americamysis bahia using both spiked and environmentally contaminated sediments. The utility of aeration for removing NH(x) and NH(3) during a marine sediment TIE was also evaluated preliminarily. In general, the U. lactuca and zeolite addition methods performed similarly well at removing spiked NH(x) and NH(3) from overlying and interstitial waters compared to an unmanipulated sediment. Toxicity to the amphipod was reduced approximately the same by both methods. However, toxicity to the mysid was most effectively reduced by the U. lactuca addition indicating this method functions best with epibenthic species exposed to ammonia in the water column. Aeration removed NH(x) and NH(3) from seawater when the pH was adjusted to 10; however, very little ammonia was removed at ambient pHs ( approximately 8.0). This comparison demonstrates both U. lactuca and zeolite addition methods are effective TIE tools for reducing the concentrations and toxicity of ammonia in whole sediment toxicity tests.     

From partial melting to retrogression in the Pointe Geologie migmatitic complex: a history of heterogeneous distribution of fluids

In the Pointe Géologie area (66°40 S; 140°00 E; Terre Adélie, East Antarctica), the Paleoproterozoic basement consists in a migmatitic complex of metasedimentary origin. Metasediments underwent a thermal event, leading to the high-grade amphibolite facies assemblages biotite–cordierite–sillimanite and to dehydration melting reactions at 4–6 kbar and 700±50 °C, followed by retrogression in greenschist facies.

In most of the archipelago, K-feldspar gneisses (KFG) are characterized by a Sil+Crd+Kfs+Bt assemblage and many K-feldspar-rich leucosomes. Locally, a spectacular rock type occurs as North dipping bands of about 10 m thick and consists in nodular gneisses (NG) that display less abundant, K-feldspar-poor leucosomes.

Commonly, the retrograde imprint facies is quite weak in KFG and only expressed by sporadic Bt–Ms±And equilibrium assemblage, whereas it developed more extensively in NG. A pseudosection calculated at constant P=4 kbar shows that the differences between NG and KFG assemblages can be considered to be mainly driven by difference in H₂O proportions and much less by differences in FeO/MgO or K₂O/MgO ratios. The hydrated assemblage (Bt–Ms nodules) in NG requires at least 10–20% more H₂O than the Crd+Kfs+Sil/And assemblage does in KFG. Parageneses and mineral compositions indicate that this difference in H₂O occurred early in the history, at least as early as the anatectic stage. Therefore, differences between NG and KFG are related to the variation in partial melting features (water distribution, proportion of melt extraction), which appears to be spatially controlled by cryptic tectonic structures. The particular shape and orientation of NG bands are interpreted as a complex history of melt extraction in the Pointe Géologie area which could involve a two stage melting process.     

Iron distribution in the octahedral sheet of dioctahedral smectites. An Fe K-edge X-ray absorption spectroscopy study

The distribution of iron atoms in the octahedral sheet of a series of dioctahedral smectites with varying unit-cell composition and iron content was investigated by Fe K-edge XAS spectroscopy. First-step analysis reveals that the patterns corresponding to backscattering by atoms located between 3 and 4 Å from the absorbing atom are very sensitive to the relative amount of light (Si, Al, Mg) and heavy (Fe) atoms. Detailed modelling of this domain then provides valuable information on the number of iron atoms surrounding octahedral iron. By comparing the number of iron neighbours deduced from EXAFS with that determined from unit-cell composition assuming a statistical distribution, three groups of montmorillonites can be distinguished: (1) clay samples from Wyoming display an ordered distribution of iron atoms; (2) clay samples from Georgia, Milos, China and Washington exhibit a close to random distribution of iron atoms; (3) clay samples from North Africa, Germany, Texas and Arizona display extensive iron clustering. These results complement previously obtained IR results and show that the combination of these two spectroscopic techniques could provide an additional crystal-chemistry-based framework for typological analysis of montmorillonite deposits.     

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen-Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44-2.8 μg g⁻¹) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g⁻¹), depleted in Li (most values below 1 μg g⁻¹) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4-65.0 μg g⁻¹), H₂O and Cl contents and to lower Li contents (0.07-3.37 μg g⁻¹) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock-water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17-307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25-904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.     

Sea-to-air flux of contaminants via bubbles bursting. An experimental approach for tributyltin

Although seawater concentration of tributyltin (TBT) should decrease when the direct inputs from ship hulls will cease after the incoming world ban of organotin-based antifouling paints in 2003 or later, the TBT environmental issue will still be present for decades as contaminated sediments in shallow waters will be acting as a long-lasting reservoir for TBT and its degradation products. The lost of TBT to the atmosphere by volatilization has already been proposed as a part of its molecular motion through the aquatic environment but most recent calculated values of water-to-air rate of exchange of TBT (from 20 to 510 nmol m⁻² year⁻¹) do not take into account the potential contribution of aerosols ejection to the atmosphere upon bubbles bursting, an important process for pollutants transport in the marine environment. In this work, an experimental approach to measure the seawater-to-air flux of TBT mediated by bubbles bursting is described, and the influence of phytoplankton cells and dissolved organic matter from exudates and culture weathering on flux rates was assessed. The results demonstrate that TBT can be transferred from water to air via the ejection of droplets from bubbles bursting and that cell density strongly affected the transfer. Under a bubbling regime, the water-to-air flux (pmol TBT cm⁻² min⁻¹ level) is estimated up to 1000-fold the flux measured for the molecular diffusion and volatilization under static quiescent conditions. The surface microlayer acted as a transient boundary between the water column and the atmosphere where the dynamic of TBT accumulation has the same trend as the dynamic of TBT ejection. This physical transfer mechanism might be of high significance in nearshore environments, harbors, and shallow channels where clouds of bubbles generated in the wake of large ships play an important role for the atmosphere/seawater chemical exchanges.     

Desert pavement dynamics: numerical modeling and field‐based calibration

Desert pavements are widely used as a relative surface‐dating tool because they are progressively better developed on surfaces ranging from thousands to hundreds of thousands of years in age. Recent work, however, has highlighted the dynamic nature of pavements and undermined their use as surface‐age indicators. Quade (2001) proposed that latest Pleistocene vegetation advances destroyed all Mojave Desert pavements above 400 m elevation, making all such pavements Holocene in age. In an effort to reconcile young‐pavement evidence with their widespread use as Pleistocene surface‐age indicators, we developed a numerical model based on the classic conceptual model in which pavements co‐evolve with their underlying eolian epipedons over millennial timescales. In this co‐evolutionary process, fine‐grained eolian deposition and A_v‐horizon development within the eolian epipedon promotes surface clast motion and pavement development, enhancing the eolian‐sediment‐trapping ability of the pavement in a positive feedback. Model results illustrate the multi‐scale nature of pavement dynamics: pavements may require tens of thousands of years to fully develop from a newly abandoned alluvial surface, but may heal over timescales of decades to centuries if a mature eolian epipedon is present. As such, there is no inconsistency between rapid pavement healing and a Pleistocene age for the underlying alluvial surface. To calibrate the model, we conducted surficial geologic mapping and pavement‐sedimentological analysis on two desert piedmonts. Our study areas include both proximal and distal fan environments, illustrating the role of parent‐material texture in controlling the mode of pavement formation. Using available geochronology, our work provides a rigorous calibration of pavement formation rates in our study areas and provides evidence supporting the use of pavements as local relative surface‐age indicators over Holocene to late Pleistocene timescales. Copyright © 2007 John Wiley & Sons, Ltd.     

Crack propagation by differential insolation on desert surface clasts

In the southwest U.S., cracks in alluvial fan surface clasts have a preferred orientation independent of rock fabric and shape. In this paper, we show that differential insolation of incipient cracks of random orientations predicts a distribution of crack orientations consistent with field observations. In this model, crack growth by hydration and/or thermal weathering is primarily a function of local water content at the crack tip. Crack tips that experience minimal solar insolation maintain a greater average moisture and, hence, weather more rapidly than cracks that experience greater solar insolation. To show this, we used a numerical radiative transfer code to quantify the solar insolation of rectangular cracks at 35° N. latitude with a range of depths and orientations. The amount of solar energy reaching the bottom of each crack was calculated at 5-min intervals over the day for several days of the year to determine hourly, daily, seasonal, and annual energy deposition as a function of crack depth and orientation. By assuming that only crack orientations that effectively shield their interiors and minimize their water loss are able to grow, the pattern of cracks produced by the model is consistent with field observations. The annual average insolation, which controls water retention, is associated with the two primary modes of crack orientation. The effect of daily recharge by summer rains of the North American monsoon system is consistent with the observed deviations from these primary modes. Model results suggest that both the annual average insolation and the daily pattern of rainfall is recorded in the preferred crack orientations of surface clasts in the southwest U.S.     

Topographic correlations with soil and regolith thickness from shallow‐seismic refraction constraints across upland hillslopes in the Valles Caldera,New Mexico

How rock is weathered physically and chemically into transportable material is a fundamental question in critical‐zone science. In addition, the distribution of weathered material (soil and intact regolith) across upland landscapes exerts a first‐order control on the hydrology of watersheds. In this paper we present the results of six shallow seismic‐refraction surveys in the Redondo Mountain region of the Valles Caldera, New Mexico. The P‐wave velocities corresponding to soil (≤ 0.6 km s^?1) were inferred from a seventh seismic survey where soil‐thickness data were determined by pit excavation. Using multivariable regression, we quantified the relationships among slope gradient, aspect, and topographic wetness index (TWI) on soil and regolith (soil plus intact regolith) thicknesses. Our results show that both soil and regolith thicknesses vary inversely with TWI in all six survey areas while varying directly with slope aspect (i.e. thicker beneath north‐facing slopes) and inversely with slope gradient (i.e. thinner beneath steep slopes) in the majority of the survey areas. An empirical model based on power‐law relationships between regolith thickness and its correlative variables can fit our inferred thicknesses with R² ‐values up to 0.880 for soil and 0.831 for regolith in areas with significant topographic variations. These results further demonstrate the efficacy of shallow seismic refraction for mapping and determining how soil and regolith variations correlate with topography across upland landscapes. Copyright © 2016 John Wiley & Sons, Ltd.     

Cosmogenic 3He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor,Arizona, USA

Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic ³He (³He_c) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, < 350 ka, < 150 ka, and < 63 ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2 Ma. ³He_c data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in ³He_c ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley.