Lacustrine Clay Research Articles

A Pleistocene landslide-dammed lake, Jinsha River, Yunnan, China

On both sides of the Jinsha River valley near Benzilan, Dêqên, northwest of Yunnan, China, a series of varied lacustrine deposits have been discovered under the fourth terrace (Q 3 2). Landslide debris has been identified underlying the lacustrine deposits, indicating that the lacustrine deposits were caused by a landslide damming the Jinsha River. U-series dating shows that the age of the upper and middle portions of the lacustrine deposits on the right bank were 55.4 ± 3.5 and 82.1 ± 6.6 ka respectively. Lacustrine deposits on the left bank have been dated to 122.0 ± 12.4 ka at the bottom, which indicates that the large-scale landslide was formed before 122.0 ± 12.4 ka and the paleo-lake was developed within the early stage to the mid-stage of the Late Pleistocene. The characteristic of spore-pollen and high content of CaCO 3 in the lacustrine clays reveal that the paleoclimate was semi-arid with less heavy rainfall. Considering the regional neotectonic background, the authors prefer to interpret that the Benzilan ancient landslide-dammed lake was caused by an ancient earthquake. The discovery of the ancient landslide-dammed lake provides significance to the understanding of the formation and evolvement of calamitous geological hazards along large rivers.

Pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern Baltic Sea

Abstract: Holocene postglacial lacustrine clays in the northern Baltic Sea were studied for ichnofossils, petrography, and microscale elemental and 34 S/ 32 S composition to understand the impact of benthos-produced organic matrices on the early diagenesis of sediments. X-radiographs of these clays display intense burrow mottling with discrete Palaeophycus , ‘Mycellia' and rare Arenicolites . Small burrow-like and irregularly shaped concretions were separated from these clays. The burrow-like concretions are composed of framboidal pyrite and poorly crystalline FeS 2 cement. They formed within worm-produced burrows by the decomposition of mucous coatings on the burrow walls by sulphate reduction. δ 34 S CDT composition of the framboids indicates saline inflows from the North Sea as the sulphate source. The irregular concretions are microcrystalline pyritic masses that formed in pore spaces with less reactive organic compounds. Their δ 34 S CDT values reflect precipitation from the ambient porewater sulphate by sulphate reduction. The burrow-like and irregular concretions are microcrystalline barite in the lower parts of the clays. ‘Mycellia' are mineral-replaced filaments produced by large sulphur-oxidizing bacteria or mycelial fungi. They concentrate in monosulphide bands in the lower substrate levels, but are associated with organic patches upward. This study demonstrates that substantial amounts of pyrite formed within organic matrices close to the sediment–water interface. This differs from previous studies that proposed a deeper-burial origin and allochthonous sulphur source. Supplementary material: Sulphur isotope data are available at http://www.geolsoc.org.uk/SUP18426 .

Paleotopographic control of landslides in lacustrine deposits (Trièves plateau, French western Alps)

Paleotopography in Quaternary sedimentary environments can be an important factor that controls landslide movement. This study investigates the relation between paleotopography and landslide activity in two adjacent landslides in glaciolacustrine sediments located in the Trièves area (French western Alps). Although both are in slopes underlain by the same lacustrine deposits, the Avignonet and Harmalière landslides exhibit major differences in morphology and displacement rates. Through a combination of geological mapping, airborne light detection and ranging (LiDAR) data, aerial photographs, global positioning system (GPS), and seismic noise measurements, a three-dimensional impression was made of both landslides. The analysis reveals that the difference in kinematics between the two mass movements can be traced back to at least 50 years ago. The results show that the Harmalière slide, which failed catastrophically in 1981, is still much more active than the Avignonet landslide. The fear was that the Avignonet landslide might develop in a similar catastrophic manner, threatening a number of houses constructed on the landslide. A geophysical survey based on ambient noise measurements allowed us to map the base of the lacustrine clays, and the results indicate the presence of a N–S ridge of hard sediments (Jurassic bedrock and/or compact alluvial layers) on the eastern side of the Avignonet landslide. This ridge disappears when approaching the Harmalière landslide and makes a place to what can be interpreted as a NW–SE oriented paleovalley of the river Drac. We proposed that the ridge acts as a buttress that could mechanically prevent the Avignonet landslide from evolving as fast as the Harmalière. Furthermore, the NW–SE paleovalley located under the Harmalière landslide corresponds to the motion direction of the slide. Therefore, the different behaviour of the two landslides is partly controlled by the paleotopographic setting of Lake Trièves during the last Glacial Maximum extension. These results suggest a major influence of the bedrock paleotopography on the kinematics of the landslides.

Efficient Monte Carlo Simulation of parameter sensitivity in probabilistic slope stability analysis

Monte Carlo Simulation (MCS) method has been widely used in probabilistic analysis of slope stability, and it provides a robust and simple way to assess failure probability. However, MCS method does not offer insight into the relative contributions of various uncertainties (e.g., inherent spatial variability of soil properties and subsurface stratigraphy) to the failure probability and suffers from a lack of resolution and efficiency at small probability levels. This paper develop a probabilistic failure analysis approach that makes use of the failure samples generated in the MCS and analyzes these failure samples to assess the effects of various uncertainties on slope failure probability. The approach contains two major components: hypothesis tests for prioritizing effects of various uncertainties and Bayesian analysis for further quantifying their effects. Equations are derived for the hypothesis tests and Bayesian analysis. The probabilistic failure analysis requires a large number of failure samples in MCS, and an advanced Monte Carlo Simulation called Subset Simulation is employed to improve efficiency of generating failure samples in MCS. As an illustration, the proposed probabilistic failure analysis approach is applied to study a design scenario of James Bay Dyke. The hypothesis tests show that the uncertainty of undrained shear strength of lacustrine clay has the most significant effect on the slope failure probability, while the uncertainty of the clay crust thickness contributes the least. The effect of the former is then further quantified by a Bayesian analysis. Both hypothesis test results and Bayesian analysis results are validated against independent sensitivity studies. It is shown that probabilistic failure analysis provides results that are equivalent to those from additional sensitivity studies, but it has the advantage of avoiding additional computational times and efforts for repeated runs of MCS in sensitivity studies.

Field evaluation of deformation characteristics of a lacustrine clay deposit using seismic piezocone tests

The use of the seismic piezocone test (SCPTU) in geotechnical site investigation offers direct field measurement on stratigraphy and soil behaviour. Compared with some traditional investigation methods such as drilling, sampling and field inspecting method or laboratory test procedures, SCPTU can greatly accelerate the field work and hereby reduce corresponding operation cost. This paper presents the methods review to predict the constrained modulus ( M) and to measure small strain shear modulus ( G max) from SCPTU data. The capacity of current interpretation methods to predict the constrained modulus ( M) was evaluated for Lixia River lagoonal soils. Several sites in Lixia River area were selected for this study. In each site, SCPTU tests were performed and the soundings of cone tip resistance ( q c), sleeve friction ( f s), pore pressure ( u 2), and shear wave velocity ( V s ) were recorded. The predicted M values obtained from the SCPTU interpretation methods were compared with the reference values determined from the laboratory consolidation tests. Results of this study showed that the Sanglerat method was able to predict M better than the other methods. Two new correlations were also developed to predict M from the corrected cone tip resistance ( q t) and the net cone tip resistance ( q n). Small strain shear stiffness can be tentatively estimated in the absence of direct measurement of shear wave velocity. The reliable result was obtained using an association between G max, q t and pore pressure parameter B q . Compared to field downhole wave velocity measurements, SCPTU can lead to a more reliable measurement of maximum stiffness profile with depth. The stress–strain–strength response of the clays was evaluated by the maximum shear stiffness and undrained shear strength.

Full‐waveform inversion of cross‐hole ground‐penetrating radar data to characterize a gravel aquifer close to the Thur River, Switzerland

ABSTRACTCross‐hole radar tomography is a useful tool for mapping shallow subsurface electrical properties viz. dielectric permittivity and electrical conductivity. Common practice is to invert cross‐hole radar data with ray‐based tomographic algorithms using first arrival traveltimes and first cycle amplitudes. However, the resolution of conventional standard ray‐based inversion schemes for cross‐hole ground‐penetrating radar (GPR) is limited because only a fraction of the information contained in the radar data is used. The resolution can be improved significantly by using a full‐waveform inversion that considers the entire waveform, or significant parts thereof. A recently developed 2D time‐domain vectorial full‐waveform cross‐hole radar inversion code has been modified in the present study by allowing optimized acquisition setups that reduce the acquisition time and computational costs significantly. This is achieved by minimizing the number of transmitter points and maximizing the number of receiver positions. The improved algorithm was employed to invert cross‐hole GPR data acquired within a gravel aquifer (4–10 m depth) in the Thur valley, Switzerland. The simulated traces of the final model obtained by the full‐waveform inversion fit the observed traces very well in the lower part of the section and reasonably well in the upper part of the section. Compared to the ray‐based inversion, the results from the full‐waveform inversion show significantly higher resolution images. At either side, 2.5 m distance away from the cross‐hole plane, borehole logs were acquired. There is a good correspondence between the conductivity tomograms and the natural gamma logs at the boundary of the gravel layer and the underlying lacustrine clay deposits. Using existing petrophysical models, the inversion results and neutron‐neutron logs are converted to porosity. Without any additional calibration, the values obtained for the converted neutron‐neutron logs and permittivity results are very close and similar vertical variations can be observed. The full‐waveform inversion provides in both cases additional information about the subsurface. Due to the presence of the water table and associated refracted/reflected waves, the upper traces are not well fitted and the upper 2 m in the permittivity and conductivity tomograms are not reliably reconstructed because the unsaturated zone is not incorporated into the inversion domain.

Application and validation of a GIS-based stream bank stability tool for the Great Lakes region

W atershed sediment contributions are a continual source of pollution to the Great Lakes. Sediment from upstream sources may deliver contaminants such as polychlorinated biphenyls (PCBs) and excess nutrients such as phosphorus to the lakes. The excess nutrients often contribute to algal blooms and degradation of freshwater habitat. The sediment loading from agricultural watersheds is often aggravated by ditch maintenance activities. Many of the agricultural lands draining into the Great Lakes lie on lacustrine clay and till plains and consequently have very little relief, resulting in poor drainage. Tile drains and ditches are often constructed in these areas to accelerate drainage and facilitate utilization of land for agriculture. Ditch maintenance activities include removal of riparian vegetation and dredging to increase conveyance. Section 516(e) of the Water Resources Development Act of 1996 directed the US Army Corps of Engineers (USACE) to develop sediment transport models for major Great Lakes tributaries contributing sediment to federal navigation projects (harbors, channels, etc.) or Areas of Concern (AOCs). Under this legislation, the models are developed to assist watershed stakeholders with the development of watershed management approaches to soil conservation and sediment management to reduce sediment loads, dredging costs, and the need for other sediment-based remediation activities.

Arsenic Management Through Well Modification and Simulation

Arsenic concentrations can be managed with a relatively simple strategy of grouting instead of completely destroying a selected interval of well. The strategy of selective grouting was investigated in Antelope Valley, California, where groundwater supplies most of the water demand. Naturally occurring arsenic typically exceeds concentrations of 10 microg/L in the water produced from these long-screened wells. The vertical distributions of arsenic concentrations in intervals of the aquifer contributing water to selected supply wells were characterized with depth-dependent water-quality sampling and flow logs. Arsenic primarily entered the lower half of the wells where lacustrine clay deposits and a deeper aquifer occurred. Five wells were modified by grouting from below the top of the lacustrine clay deposits to the bottom of the well, which reduced produced arsenic concentrations to less than 2 microg/L in four of the five wells. Long-term viability of well modification and reduction of specific capacity was assessed for well 4-54 with AnalyzeHOLE, which creates and uses axisymmetric, radial MODFLOW models. Two radial models were calibrated to observed borehole flows, drawdowns, and transmissivity by estimating hydraulic-conductivity values in the aquifer system and gravel packs of the original and modified wells. Lithology also constrained hydraulic-conductivity estimates as regularization observations. Well encrustations caused as much as 2 microg/L increase in simulated arsenic concentration by reducing the contribution of flow from the aquifer system above the lacustrine clay deposits. Simulated arsenic concentrations in the modified well remained less than 3 microg/L over a 20-year period.

Ultrafine Clay Minerals of the Pleistocene Olorgesailie Formation, Southern Kenya Rift: Diagenesis and Paleoenvironments of Early Hominins

AbstractThe Pleistocene Olorgesailie Formation in the southern Kenya Rift has yielded a remarkable assemblage of Acheulean artifacts and vertebrate fossils, including hominin specimen KNM-OG 45500. Members 1 and 7 both contain clay-rich deposits that have been pedogenically modified into paleosols (UM1p and UM7p, respectively). This study provides the first detailed mineralogical and geochemical analyses of the clays of this important Pleistocene basin. The smectitic clays, which show abundant evidence for pedogenesis, were apparently originally deposited under lacustrine conditions. They have an average structural formula of (Ca0.01Na0.32K0.26)(Si3.76Al0.24)(Al0.86Ti0.04Fe0.68Mg0.42)O10(OH)2. The high layer charge clays indicate diagenetic alteration of detrital clay derived from the volcanic drainage basin, probably involving alkaline waters of variable salinity. Despite overall lower salinity compared to other Plio–Pleistocene basins of the region (e.g. Olduvai Gorge), the basin still shows evidence for authigenic clay mineral precipitation. Clay chemistry and bulk geochemical indicators of pedogenesis imply that UM1p clays more closely reflect depositional paleo-waters, whereas UM7p clays have been more pedogenically altered due to subaerial exposure. UM1p smectites show some Mg enrichment near the western Lava Hump locality, consistent with discharge of Mg-bearing paleo-waters from a volcanic aquifer into a siliceous and alkaline (though not highly saline) paleo-lake. UM7p smectites were deposited in a more saline paleo-lake, but have lost substantial amounts of Mg due to post-depositional weathering. Locally abundant artifacts and vertebrate fossils found in these deposits accumulated at times following deposition of the lacustrine clay, probably concurrent with pedogenesis. The limnological conditions associated with initial clay deposition, therefore, preceded hominin occupation of the exposed surfaces.

Non-Linear Elasto-Plastic Behaviour of Lacustrine Clay

The paper presents the experimental investigation and analysis of the non-linear elasto-plastic stress–strain behaviour of normally consolidated lacustrine clay. Drained triaxial stress path tests were performed on natural block samples of Swiss lacustrine clay. Data were analysed using plasticity theory and the shape and extent of kinematic yield and bounding surfaces were determined and found to be elliptical but not congruent. Cross-anisotropic elasticity was used to quantify elastic strains to permit plastic strain increment vectors and hence a plastic potential surface to be defined.

RESPUESTA SÍSMICA EN EL LAGO DE TEXCOCO. RESULTADOS A PARTIR DE REGISTROS DE MOVIMIENTOS FUERTES

En 1995 se instaló un arreglo acelerométrico constituido por seis estaciones en superficie, que formaron dos triángulos con aperturas de 30 y 400 m y un arreglo vertical con instrumentos en cinco profundidades, que alcanzaron los 40 m. La base de datos de este arreglo consta de ocho eventos de movimientos fuertes (5.9 < M < 7.9) que analizamos para obtener la respuesta sísmica, el modelo de velocidades y el espectro de respuesta de sitio. Los resultados de este estudio muestran que la respuesta de sitio está caracterizada por la frecuencia fundamental en 0.37 Hz. El análisis de los registros en pozo permitió obtener un modelo de velocidades que está constituido por dos capas, con factores de atenuación Q distintos para cada una; la amplificación dinámica, debida a las propiedades físicas de las arcillas lacustres, domina la respuesta de sitio. Finalmente, el espectro de respuesta muestra que las seudoaceleraciones máximas en el sitio se encuentran en dos intervalos de periodo: entre 0 y 1 s y entre 2 y 3 s, con ordenadas espectrales de 0.6 g y 0.74 g, respectivamente.

Space geodetic imaging of rapid ground subsidence in Mexico City

Since the late 1950s, several areas of Mexico City have undergone accelerated ground subsidence and have developed associated fracturing and faulting. New interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) data indicate that rates of current land subsidence in Mexico City exceed 350 mm/yr. These rates are close to historical maximum levels of the mid-twentieth century, when mitigation efforts were fi rst undertaken to reduce damage to urban infrastructure. The locus of maximum subsidence has shifted from its historical location in the old city center to the east. Correlation of our InSAR results with seismically mapped stratigraphic units suggests that subsidence is primarily controlled by compaction of Quaternary lacustrine clays and silts. We also evaluate spatial gradients in subsidence and suggest that this, rather than subsidence magnitude, is the key factor in risk assessment. Subsidence represents a major geologic risk for Mexico City and imposes serious constraints to any further urban development.

Interplay between tectonics and compaction in a rift‐margin, lacustrine delta system: Miocene of the Eger Graben, Czech Republic

AbstractThe Early Miocene Bílina Palaeodelta consists of fluvio‐deltaic and lacustrine clastics deposited along the south‐eastern margin of the extensional Most Basin, part of the Eger Graben in north Bohemia (Czech Republic). The Bílina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault. Exposures up to ca 4·5 km long in the Bílina open‐cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small‐scale and basin‐scale stratal geometries to be evaluated. The Bílina Palaeodelta deposits include sand‐dominated, fluvial channel fills and heterolithic sheets interpreted as delta plain strata, sand‐dominated mouth‐bar wedges and heterolithic sheets of prodeltaic deposits, passing distally into lacustrine clays. The depositional environment is interpreted as a fluvial‐dominated, mixed‐load, lacustrine delta with a high degree of grain‐size segregation at the feeder‐channel mouths. On the largest temporal and spatial scales, variable tectonic subsidence controlled the overall advance and retreat of the delta system. The medium‐term transgressive‐regressive history was probably driven by episodes of increased subsidence rate. However, at this temporal scale, the architecture of the deltaic sequences (deltaic lobes and correlative lacustrine deposits) was strongly affected by: (i) compaction of underlying peat and clay which drove lateral offset stacking of medium‐term sequences; and (ii) growth of a fault‐propagation fold close to the active Bílina Fault. At the smallest scale, the geometries of individual mouth bars and groups of mouth bars (short‐term sequences) reflect the interaction among sediment loading, compaction and growth faulting that produced high‐frequency relative lake‐level fluctuations and created local accommodation at the delta front.

Elucidating the crystal-chemistry of Jbel Rhassoul stevensite (Morocco) by advanced analytical techniques

AbstractThe composition of Rhassoul clay is controversial regarding the nature of the pure-mineral clay fraction which is claimed to be stevensite rather than saponite. In this study, the raw and mineral fractions were characterized using various techniques including Fourier transform infrared spectroscopy and magic angle spinning nuclear magnetic resonance (MAS NMR). The isolated fine clay mineral fraction contained a larger amount of Al (>1 wt.%) than that reported for other stevensite occurrences. The27Al MAS NMR technique confirmed that the mineral is stevensite in which the Al is equally split between the tetrahedral and octahedral coordination sites. The29Si NMR spectrum showed a single unresolved resonance indicating little or no short-range ordering of silicon. The chemical composition of the stevensite from Jbel Rhassoul was determined to be ((Na0.25K0.20)(Mg5.04Al0.37Fe0.20☐0.21)5.61(Si7.76Al0.24)8O20(OH)4). This formula differs from previous compositions described from this locality and shows it to be an Al-bearing lacustrine clay mineral.

Gill structure and relationships of the Triassic cycloid crustaceans

Unusually well-preserved fossils of a Halicyne-like cycloid crustacean frequently occur in the early Late Triassic lacustrine clay bed at Krasiejów in Opole Silesia, southern Poland. Its gill-like structures form a horseshoe-shaped pair of units composed of numerous calcified blades with reverse U-shaped cross-section. Originally, these were parallel slits opening on the ventral surface of the carapace. Lobation of the posterior margin of the carapace, of unusually large mature size for the group, make the animal different from other members of Halicynidae, and the new name Opolanka decorosa gen. et sp. nov. is proposed for it. More completely preserved specimens of cycloids from Vosges, France, and Madagascar show that the slit openings were located above radially arranged coxae of the walking appendages and a reduced abdomen. The disposition and arrangement of the cycloid gills suggest at least close analogy, and possibly homology, with the "respiratory areas" of the Branchiura, serving mostly as ion-exchange organs. It is proposed that they originated, in connection with the body size increase and adaptation to fresh-water environment, as radially arranged infoldings of the respiratory areas cuticle, with strongly calcified rigid dorsal parts suspended from the carapace. At least three ecologically and anatomically distinct lineages were represented in the order Cyclida, which was probably initially confined to marine environments and gradually adapted to life in continental waters. New taxa Schraminidae fam. nov. (with Schramine gen. nov.) and Americlidae fam. nov. (with Americlus gen. nov.) are proposed.

Economic Sampling and Extraction of Undisturbed, High Quality Samples in Normally Consolidated Lacustrine Clays Using a Large Diameter Tube

This paper describes the development, design and use of a large diameter sampling tube. High quality test specimens are essential for the investigation of mechanical properties of a soil for high risk projects and when complex and expensive testing methods are to be used. Block sampling is recommended to give the highest sample quality for clayey soils, however, extracting blocks of normally consolidated lacustrine silty clay without excessive disturbance was challenging due to the inherent structure of the soft varved silty clay and difficulty in maintaining Ko conditions, as well as no vertical strain, in the sample. A new sample tube, with an inner diameter of 196 mm, an area ratio of 4% and an outer cutting-edge angle of 11° was designed to offer a larger cross sectional area than conventional thin walled sampling tubes, to provide the necessary side support and to prevent water ingress at the sides of the sample. The length-diameter aspect ratio was 1.275 to optimise the amount of clay sampled for subsequent testing and in an attempt to minimize the pressure in front of the tube. Samples were taken in initially newly excavated trenches at a depth of c. 1 m with this new sampler and with conventionally sampled soil specimens, prior to the main testing programme with samples from 6 m depth. A comparative study was then performed including preliminary unconsolidated unconfined compression tests followed by anisotropically consolidated undrained triaxial compression tests. It was important to establish whether this approach had led to an improvement in sample quality prior to embarking on an extensive triaxial stress path testing programme on this varved soil (Messerklinger, Non-linearity and small strain behaviour in lacustrine clay, 2006; Messerklinger and Springman, Geotech Test J 30(6), 2007; Messerklinger and Springman, Geotech Geol J, 2008). The results showed that the undrained shear strength of the specimens from the new sampler was consistently around 20% higher than that of specimens extruded from conventional thin walled tube samplers. This confirmed that samples with a significantly higher quality could be extracted from normally consolidated, fine grained, varved lacustrine deposits with this large diameter ‘block’ sampling tube.

Fractal Mapping of Pore and Solid Attributes

Soil structure depends on its genesis and consists of highly variable pore and solid networks. Several internal and external factors affect the attributes of these networks, with water being the most aggressive agent. In this study, we used selected fractal parameters (called descriptors) to quantify the basic topological attributes—compactness and connectedness—as well as lacunarity and roughness of porous materials, with special attention to sampling error and population variance dynamics. Four microhorizons were sampled from a 1.0‐ by 1.0‐ by 1.6‐m monolith during a long‐term drying period under controlled conditions. A comparative fractal analysis of selected multitemporal (11 mo) and multiscale (50×, 500×, 1000×, and 5000×) scanning electron microscopy (SEM) images was accomplished for the following layers with contrasting texture or structure: loam topsoil, pure basaltic compacted sand, and two amorphous lacustrine clays. The scale invariance (self‐similarity) of pore and solid networks was documented. A complex relationship was found between the descriptor mean values and corresponding variances. Simple power‐law relations were established between the standard deviation (and variance) and layer depth, as well as scale and sampling time. Pearson's r correlation criterion was applied to measure the degree of association between multiobjective fractal descriptors. Systematic and standardized fractal analysis of multiscale images is recommended for the statistical quantitative description of complex and dynamic pore and solid networks.

Phosphorus forms and reactive iron in lateglacial, postglacial and brackish-water sediments of the Archipelago Sea, northern Baltic Sea

Glacial transport is an important agent of P and reactive Fe delivery to the ocean margins. However, whereas P may travel through salinity gradients, reactive Fe is substantially trapped in estuaries. Studies of glaciolacustrine to brackish-water depositional successions elucidate processes associated with the glacial P and reactive Fe flux into the oceans. The sedimentary P species, ‘reactive’ Fe-oxide-associated Fe (and Al and Mn), C, N and S geochemistry for sediment cores from three sites in the Archipelago Sea (northern Baltic Sea) were determined in order to understand the distribution and coupling of these geochemical components in the glaciolacustrine to post-glacial lacustrine to brackish-water depositional succession. The up to 6 m long cores record deposition after 11 350 cal. BP. Previous studies suggest that the studied depositional succession records development common for the deglaciation phase of large, low relief, epicontinental basins. All the studied geochemical components are strongly coupled in the basal glaciolacustrine rhythmites, which reflect their rapid deposition by glacial meltwaters. Low C org/P reac ratios suggest that a part of the rhythmite material is reworked from pre-glacial deposits. Delivery rates are drastically lower in the successive post-glacial lacustrine setting due to the decrease of sedimentation rate by an order of magnitude. Decoupling of reactive Fe and associated components in the post-glacial lacustrine clays reflect increasing contribution of sediment from the emerging shoreline and rivers, but also post-depositional overprinting by sulphidization. The onset of brackish-water conditions soon after 7600 cal. BP led to 1) decoupling of reactive Fe from the detrital flux due to the establishment of salinity gradients at the river mouths, and 2) increased nutrient availability and production in water, as the intensified accumulation and burial of reactive P species, organic C and N indicate. The higher organic deposition led to deteriorated seafloor oxygen conditions; alternating thinly-laminated to strongly-bioturbated lithofacies characterise the brackish-water muds, and record temporal changes in the seafloor oxygenation. The temporal changes in the seafloor oxygen conditions appear to be broadly linked to the Holocene climate variability; brackish-water muds at the northern end of Paimionlahti Bay record anoxia during the Medieval Warm Period (1250–450 cal. BP), oxic conditions during the ‘Little Ice Age’ (450–300 cal. BP), and anoxia until the present day. On a finer scale, the redox histories in adjacent sub-basins differ considerably due to patchy topography, sluggish bottom water exchange and high organic deposition in the area. Terrestrial organic influx can temporally overprint marine organic deposition in areas close to river mouths.

Influence of nested groundwater systems on reduction–oxidation and alkalinity gradients with implications for plant nutrient availability in four New York fens

Summary This study compares hydrologic investigations of four New York fens designed to characterize spatial variability in groundwater flow (GWF) and the resulting hydrochemical patterns that influence plant nutrient availability. At the marl fen and two rich fen sites, hydrometric data and pore water samples collected from nested piezometers set along the GWF-path showed spatial patterns consistent with a mixing gradient established by the intersection of local (i.e., hill slope), relatively dilute groundwater discharge and mineral-rich groundwater discharged from a larger-scaled (i.e., intermediate) system. Distinct redox and carbonate gradients along the GWF-paths of three rich fens reflected strong interactions between groundwater supply of terminal electron acceptors and microbial processes affecting speciation of iron, nitrogen and phosphorus. Decreased nitrate concentrations, increased dissolved iron, and removal of sulfate along flowpaths indicated that redox conditions became more reduced as groundwater moved down-gradient. Nitrate removal occurred across the interface between the upland mineral soil and the organic-rich fen peat. Lack of ammonium accumulation indicated that denitrification limits N-supply to plants in down-gradient fen areas. Further down-gradient, sulfate-enriched groundwater discharge sustained sub-oxic conditions and enhanced phosphorus mobility through effects on iron–sulfur–phosphorus dynamics, alkalinity generation, and dissolution of mineral-bound P. In contrast, the poor fen was sustained by an isolated surficial groundwater system; a thick lacustrine clay aquitard limited intermediate or regional-scaled groundwater discharge. As a result, water table fluctuations were more responsive to short-term weather events and imposed the strongest influence on redox status. A redox gradient, evidenced by increasing iron and ammonium concentrations, extended downward with depth from the aerated peat surface. The dilute water supply and lack of additional internal alkalinity generation limited buffer capacity and decomposition. Overall, ammonium accumulation increased N-availability whereas reduced decomposition rates further limited P availability. Our results show that interaction of multiple GWF systems within the fens was the critical determinant of water table stability (rich fens) or fluctuation (poor fen), as well as the supply of dissolved ions that influence geochemical processes affecting plant nutrient availability. Together these factors accounted directly or indirectly for observed spatial patterns in redox and alkalinity gradients, which in turn controlled speciation of phosphorus-reactive minerals. Differences among fens in spatial patterns could be accounted for by differences in the geomorphometry of the basins within which the fens developed and the stratigraphy and hydraulic characteristics of underlying sediments. Results suggest that Fe–S dynamics, rather than carbonate precipitation, likely influences inorganic phosphorus pools and release mechanisms.

Correcting distorted paleosecular variation in late glacial lacustrine clay

An undisturbed, horizontal chronostratigraphic marker horizon of laminated red glacio-lacustrine clay crops out over ∼25,000 km 2 in northern Ontario, Canada. The primary, clastic hematite laminae possess two stable vector components of magnetization. We sampled a 1.6 m vertical section at overlapping 2 cm intervals in cubic specimens (8 cm 3, n = 106), precisely oriented in geographic coordinates which permitted measurement of inclination and declination. Alternating field demagnetization (12–17 steps per specimen) isolated a characteristic (ChRM), primary component (coercivity ≥40 mT) approximately 30° shallower than the mean geomagnetic field inclination at this latitude, with distorted paleosecular secular variation (PSV). A lower coercivity overprint (20–40 mT) is 6° shallower than the present geomagnetic field and similarly inclined to the depositional-remanence deflection when the clay was re-sedimented in the laboratory. We believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition, caused by the magnetic anisotropy of the clay. Using this as a proxy for the initial inclination, the ChRM (hard-component) inclinations were restored to their original values assuming vertical compaction, which averages to 51%. The restored inclinations are compatible with site-latitude and the restored secular-variation loops centre reasonably on the geographic North Pole. The structural correction based on homogeneous vertical shortening over-simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape, size and packing and effectively combines influences of compaction and depositional settlement. Nevertheless, this correction makes the PSV data more useful and interpretable in terms of paleopole migration. The more logical correction technique using anisotropy of anhysteretic remanence (AARM; Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., Gruber, W., 1991. Detrital remanence, inclination errors, and anhysteretic remanence anisotropy: quantitative model and experimental results. Geophys. J. Int. 104: 95–103) is foiled here due to the high-coercivity of the remanence-bearing grains and their unknown orientation-distribution and shape-distribution.