Detrital Aluminosilicates Research Articles

Strontium Isotope Composition of Triassic Coastal and Shallow Marine Carbonates in the Western Balkanides, NW Bulgaria

Whole-rock samples of Triassic coastal and shallow marine carbonates were analyzed for their Sr isotope signatures and trace element concentrations were measured in order to assess the possible impact of diagenetic alteration. The 87Sr/86Sr ratios of Anisian to lower Carnian calcite micrites reproduce the global Triassic seawater variations fairly well. The recorded higher values can be explained by an influence from detrital aluminosilicate phases and a minor postdepositional alteration. The samples of tidal-flat dolomicrites from the base of the Anisian show no significant diagenetic overprint but have considerably more radiogenic 87Sr/86Sr signatures compared to the global Sr isotope curve that corresponds to the Triassic (Mesozoic) maximum. These anomalously high values may have resulted from some interaction between the dolomitizing brines and the underlying fluvial siliciclastics (Buntsandstein facies), but more likely from the high 87Sr/86Sr river waters discharged from a silicate-dominated drainage basin to the coastal area at the beginning of the Middle Triassic. This study demonstrates that bulk micrite can be used as a proxy for seawater 87Sr/86Sr after proper sample selection and diagenetic screening. It also suggests that the 87Sr/86Sr ratios of ancient carbonates precipitated in coastal settings should be interpreted with caution, considering the possible influence of continentally-derived fluids.

Authigenic clay mineral constraints on spatiotemporal evolution of restricted, evaporitic conditions during deposition of the Ediacaran Doushantuo Formation

The carbonate-rich shelf facies of the Ediacaran Doushantuo Formation (South China) have produced a broad array of microfossil and isotope proxy records which underpin much of our understanding of environmental change and biospheric evolution during this key time period. Recent reports of locally abundant authigenic, Mg-rich saponite clay in the Yangtze Gorges Area hint at potentially widespread evaporitic conditions in a lagoonal setting. Despite the implications for interpretation of proxy records and the environmental setting of early metazoans, the spatiotemporal extent of restricted, evaporitic conditions across the Yangtze Block remains largely unexplored. Here we use mineralogical and petrographic techniques to document the spatial and stratigraphic distribution of authigenic clay minerals across seven sites, representing a transect from shallow proximal shelf to deep basinal settings. Our results demonstrate the widespread and persistent occurrence of authigenic saponite in proximal shelf settings, whereas talc is identified at more distal shelf sites, consistent with Mg-clay authigenesis in an evaporitic lagoon with spatially variable detrital aluminosilicate input. Slope and basinal sites contain no Mg-clays and are instead characterized by abundant authigenic illite, consistent with an open marine setting. Stratigraphically, illite was found in the basal cap carbonate and uppermost Doushantuo Formation, with Mg-clays only present in the interval in-between, suggesting gradual development of an extensive carbonate rimmed lagoon over the Yangtze shelf that restricted seawater exchange, followed by stronger marine influence due to erosion of the carbonate rim and subsequent marine transgression. We conclude that restricted, evaporitic conditions were much more expansive than previously assumed, potentially providing favorable conditions for the Doushantuo Biota. Further, the widespread presence of authigenic clays across the shelf supports the case for enhanced reverse weathering before the advent of biosilicification.

Organic-rich bimineralic ooids record biological processes in Shark Bay, Western Australia.

Marine ooids have formed in microbially colonized environments for billions of years, but the microbial contributions to mineral formation in ooids continue to be debated. Here we provide evidence of these contributions in ooids from Carbla Beach, Shark Bay, Western Australia. Dark 100-240 μm diameter ooids from Carbla Beach contain two different carbonate minerals. These ooids have 50-100 μm-diameter dark nuclei that contain aragonite, amorphous iron sulfide, detrital aluminosilicate grains and organic matter, and 10-20 μm-thick layers of high-Mg calcite that separate nuclei from aragonitic outer cortices. Raman spectroscopy indicates organic enrichments in the nuclei and high-Mg calcite layers. Synchrotron-based microfocused X-ray fluorescence mapping reveals high-Mg calcite layers and the presence of iron sulfides and detrital grains in the peloidal nuclei. Iron sulfide grains within the nuclei indicate past sulfate reduction in the presence of iron. The preservation of organic signals in and around high-Mg calcite layers and the absence of iron sulfide suggest that organics stabilized high-Mg calcite under less sulfidic conditions. Aragonitic cortices that surround the nuclei and Mg-calcite layers do not preserve microporosity, iron sulfide minerals nor organic enrichments, indicating growth under more oxidizing conditions. These morphological, compositional, and mineralogical signals of microbial processes in dark ooids from Shark Bay, Western Australia, record the formation of ooid nuclei and the accretion of magnesium-rich cortical layers in benthic, reducing, microbially colonized areas.

Control of variability of primary grain assemblages on the stratigraphic differences in diagenetic processes and products in organic-rich sediments

Diagenesis exerts a key control on the evolution of mineralogical systems in shales, and by extension, on the development of rock properties amenable to unconventional reservoir prospectivity. To develop an understanding of the control of primary grain assemblage on the diagenetic pathways and products in shales, integrated high-resolution petrographic and multiple geochemical studies were conducted on samples from a vertical transect of oil-gas mature (vitrinite reflectance ranging from 0.98 to 2.72%) Whitehill Formation, a shale unit identified as the main potential gas reservoir in South Africa. Results show that the formation comprises three intervals with contrasting primary grain assemblages, including organic matter content, maceral type, and detrital mineral grains, as well as the submillimeter-scale arrangement of these constituents. The lower interval occurs as parallel-laminated black shale dominated by components derived from primary production, such as Tasmanites cysts, colonial algae, radiolarian tests, and amorphous organic matter with up to 5.78% TOC, and depleted in siliciclastic detritus. The mid and upper intervals contain biologically produced components (up to 16.57% TOC) and high quantities of siliciclastic mud dominated by clay minerals and micas, and are organized into thick discontinuous gray laminae. Petrographic data suggest that the high organic content supported a variety of bacterial metabolic pathways, which caused the precipitation of mineral cements in the sediment pore spaces prior to compaction. However, the early diagenetic reactions and products in these intervals display systematic variations that reflect heterogeneity in their primary grain assemblages. The presence of framboidal pyrite throughout the lower interval suggests sulfate reduction as the predominant metabolic pathway. In absence of significant volumes of siliciclastics, the acidity associated with this process was poorly buffered, and the pore waters were reducing. These conditions led to the dissolution of opaline radiolarian tests, which recrystallized within Tasmanites cysts and interstitial pores to quartz. On the contrary, carbon isotopes indicate that organic oxidation in the mid and upper sections was mainly promoted by methanogenic archaea and that the associated acidity was effectively buffered by the detrital aluminosilicates. The resulting bicarbonate alkalinities allowed for the formation of interstitial carbonate cements (dolomite, calcite). Aluminum released during the acid-consumption reactions recrystallized within sheltered and interstitial pores to chlorite. Considering that many other shale successions exhibit similar stratigraphic variability of primary grain assemblages as the Whitehill Formation, this type of study will improve the characterization of shale mineralogical systems and bulk properties amenable to sustainable unconventional exploration.

Case studies on the utility of sequential carbonate leaching for radiogenic strontium isotope analysis

Radiogenic strontium isotopes (87Sr/86Sr) have been extensively used as a tool to explore a diversity of Earth system problems, including long-term global weathering rates and global sequence correlation. Strontium isotopes are measured on a range of geological materials (e.g., calcite fossils, barites, limestone micrites), but whole-rock limestones are by far the most abundant of these materials within the geological record for paleo-seawater 87Sr/86Sr work. Whole-rock limestones, however, have a poor track record of recording primary seawater 87Sr/86Sr values. Alteration of the limestone during diagenesis and contamination from detrital aluminosilicate phases during carbonate extraction have been consistent problems. Various preparation and quality control methods have been applied to whole-rock 87Sr/86Sr work, yet there remains no consistent framework used to separate and identify both contamination and alteration simultaneously. The lack of consistent and systematic methods has made it difficult to gauge the accuracy and fidelity of much of the previously generated whole rock limestones 87Sr/86Sr data, especially for Precambrian sequences. Building on previous work, we explore a sequential leaching method designed to systematically isolate least-altered carbonate phases from detrital aluminosilicate Sr contamination and present several case studies that demonstrate the advantages of this approach. In the first case study, we use the Mid-Carboniferous Bird Spring Formation to empirically validate the accuracy of this sequential leaching method. Comparing least-altered sequentially leached whole-rock 87Sr/86Sr values with well-preserved calcite brachiopod 87Sr/86Sr values from the same section, we find near identical values. Following this first case, we studied the Neoproterozoic Dhaiqa Formation and the mid-Proterozoic Jixian Group and Muskwa Assemblage to outline a framework for identifying least-altered leachate fractions in Proterozoic carbonate samples. As a whole, we find that with this method it is possible to better identify whole-rock samples primary or least-altered carbonate fractions, and better account for alteration, providing a means to back-calculate a samples primary and least-altered marine 87Sr/86Sr value.

The reliability of ∼2.9 Ga old Witwatersrand banded iron formations (South Africa) as archives for Mesoarchean seawater: Evidence from REE and Nd isotope systematics

Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm–Nd isotope data of ∼2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm–Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between −3.95 and −2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the “near-shore” Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton.

Metal deposition in deep sediments from the Central and South Adriatic Sea

Geochemical analysis and trace element distribution were performed on the marine sediments from short cores (30 to 50 cm) from the Middle (Jabuka and Palagruža pits, depth 230 m and 170 m respectively) and South Adriatic Sea (depth 1030 m) and the Albanian offshore (50 m depth). The distribution and mineralogy of trace elements (As, Co, Cd, Cu Cr, Ni, Zn, Hg, and Pb) and major elements (Al, Ca and Mn) in the sediments is presented. Sediments are highly heterogeneous and consist of carbonate and detrital aluminosilicates. The main mineral phase is calcite, followed by quartz, feldspars, micas and clay minerals (smectite, chlorite, illite and kaolinite). The cores were dated using 137Cs. The cores from the South Adriatic Pit and Palagruža Sill gave estimated sedimentation rates of 1.8 mm y-1 and 3.1 mm y-1 in Jabuka Pit. Distributions of Ni and Cr showed that they can be used as tracers of sediment provenance along the southern part of the Eastern Adriatic Current. Calculated enrichment factors for Pb, Cd and Hg are highest in the top 2 cm of the cores. Mercury shows the highest degree of enrichment in 0-2 cm sediment intervals (the highest in the Albania core). Generally the estimated surface enrichment follows the order: Hg>Pb>Cd. No enrichment was found for Zn, Ni and Cr.

Redox-dependent changes in manganese speciation in Baltic Sea sediments from the Holocene Thermal Maximum: An EXAFS, XANES and LA-ICP-MS study

Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca–Mn–carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e.g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000–4000cal.yrBP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum (Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe–S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters.

A Holocene record of endogenic iron and manganese precipitation and vegetation history in a lake-fen complex in northwestern Minnesota

Little Shingobee Lake and Fen are part of the extensive network of lakes and wetlands in the Shingobee River headwaters of northwestern Minnesota, designed to study the interactions between surface and ground waters. Prior to about 11.2 cal. ka, most of these lakes and wetlands were interconnected to form glacial Lake Willobee, which apparently formed when a debris flow dammed the Shingobee River. Between 11.2 and 8.5 cal. ka, the level of Lake Willobee fell as a result of breaching of the dam, transforming the deep lake into the existing lakes and wetlands. Analyses of a 9-m core from Little Shingobee Lake (LSL-B), and lacustrine sediments under 3.3 m of peat in a 17-m core from Little Shingobee Fen (LSF-10), show that the dominant components are allogenic clastic material, and endogenic CaCO3 and organic matter. In both cores almost all of the iron (Fe) and manganese (Mn) are incorporated in endogenic minerals, presumed to be X-ray amorphous oxyhydroxide minerals, that occur in significant quantities throughout the cores; almost no Fe and Mn are contributed from detrital aluminosilicate minerals. This suggests that, for most of the Holocene, the allogenic watershed contributions to lake chemistry were minor compared to the dissolved mineral load. In addition, prior to 3.5 cal. ka, pollen zone boundaries coincide with large changes in lake-sediment mineralogy, indicating that both landscape and climate processes were linked to early- and mid-Holocene lake chemistry. The pollen time series, with sequential domination by spruce, pine, sagebrush-oak, birch-oak and, finally, white pine is typical of the region and reflects the changing location of the prairie-forest transition zone over time. These changes in vegetation had some profound effects on the geochemistry of the lake waters.

Metalliferous sediments from the H.M.S. Challenger voyage (1872–1876)

The legendary cruise of H.M.S. Challenger (1872–1876) around the globe must always occupy an eminent place in the annals of oceanography, as being the first systematic attempt made on a global scale to explore the ocean. This expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise. Sediment with high content of metals (later called “metalliferous”) was among the enigmatic findings taken onboard. Although the nature of metalliferous sediments is well known today, the very first sampled sediments of this type have not been studied to date. Motivated by the historical value of Challenger’s metalliferous sediment collection we undertook an investigation addressing two questions: (1) the composition of sediments from seafloor for which we have very limited data; (2) Sr–Nd–Pb–Fe–Zn-isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century. The SE Pacific metalliferous sediments sampled by the Challenger’s explorers are of 2 types: (1) metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth (CCD); and (2) stripped of CaCO 3 metalliferous sediments located beneath the CCD in the deeps near the mid-ocean ridges. The abiogenic part of these sediments is composed mainly of poorly-crystalline to X-ray amorphous Fe–Mn-oxyhydroxides, and an amorphous silicate phase. These sediments have geochemical features similar to those of all the other metalliferous sediments: very high Fe and Mn content (on abiogenic basis), very low Al/(Al + Fe + Mn), and high content (on abiogenic basis) of As, Ba, Be, Bi, Cd, Co, Cu, Mo, Ni, Pb, Sb, Th, Tl, U, V, W, Y, Zn and Zr. Their REE distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint (weak positive or no Eu anomaly). Seawater and/or terrigenous input from South America control the Sr–Nd–Pb-isotope signature of the Challenger metalliferous sediments and have almost completely obliterated any original MORB-derived hydrothermal signal. Zn isotopes are mainly contributed from seawater although other Zn sources (hydrothermal fluid and detrital aluminosilicates, barite and volcanic glass) are necessary to fully explain Zn-isotope ratios. Fe isotopes indicate relatively slow Fe 2+ to Fe 3+ oxidation in the non-buoyant plume, thus producing relatively lighter Fe-isotope signature of the FeOOH particles that formed the studied metalliferous sediments.

Diffuse flow hydrothermal manganese mineralization along the active Mariana and southern Izu‐Bonin arc system, western Pacific

Abundant ferromanganese oxides were collected along 1200 km of the active Izu‐Bonin‐Mariana arc system. Chemical compositions and mineralogy show that samples were collected from two deposit types: Fe‐Mn crusts of mixed hydrogenetic/hydrothermal origin and hydrothermal Mn oxide deposits; this paper addresses only the second type. Mn oxides cement volcaniclastic and biogenic sandstone and breccia layers (Mn sandstone) and form discrete dense stratabound layers along bedding planes and within beds (stratabound Mn). The Mn oxide was deposited within coarse‐grained sediments from diffuse flow systems where precipitation occurred below the seafloor. Deposits were exposed at the seabed by faulting, mass wasting, and erosion. Scanning electron microscopy and microprobe analyses indicate the presence of both amorphous and crystalline 10 Å and 7 Å manganate minerals, the fundamental chemical difference being high water contents in the amorphous Mn oxides. Alternation of amorphous and crystalline laminae occurs in many samples, which likely resulted from initial rapid precipitation of amorphous Mn oxides from waxing pulses of hydrothermal fluids followed by precipitation of slow forming crystallites during waning stages. The chemical composition is characteristic of a hydrothermal origin including strong fractionation between Fe (mean 0.9 wt %) and Mn (mean 48 wt %) for the stratabound Mn, generally low trace metal contents, and very low rare earth element and platinum group element contents. However, Mo, Cd, Zn, Cu, Ni, and Co occur in high concentrations in some samples and may be good indicator elements for proximity to the heat source or to massive sulfide deposits. For the Mn sandstones, Fe (mean 8.4%) and Mn (12.4%) are not significantly fractionated because of high Fe contents in the volcaniclastic material. However, the proportion of hydrothermal Fe (nondetrital Fe) to total Fe is remarkably constant (49–58%) for all the sample groups, regardless of the degree of Mn mineralization. Factor analyses indicate various mixtures of two dominant components: hydrothermal Mn oxide for the stratabound Mn and detrital aluminosilicate for the Mn‐cemented sandstone; and two minor components, hydrothermal Fe oxyhydroxide and biocarbonate/biosilica. Our conceptual model shows that Mn mineralization was produced by hydrothermal convection cells within arc volcanoes and sedimentary prisms that occur along the flanks and within calderas. The main source of hydrothermal fluid was seawater that penetrated through fractures, faults, and permeable volcanic edifices. The fluids were heated by magma, enriched in metals by leaching of basement rocks and sediments, and mixed with magmatic fluids and gases. Dikes and sills may have been another source of heat that drove small‐scale circulation within sedimentary prisms.

Contribution of cold seep barite to the barium geochemical budget of a marginal basin

We investigated barium partitioning in a marginal basin where large barite structures occur associated with cold seeps on the San Clemente fault, off the coast of southern California. The barium flux to San Clemente basin sediments is two to three times greater than fluxes measured in surrounding California Borderland basins where there is no evidence of seeps. Barium is accumulating in San Clemente sediments at an average rate of approximately 11 μmol m −2 day −1. Analyses of sediment trap material, water column samples, sediments and pore water indicate that expected barium-bearing mineral phases (e.g. detrital aluminosilicates, biogenic material and Fe/Mn oxyhydroxides) cannot account for all the barium accumulating in the San Clemente Basin. The excess barium, which represents transport of cold seep barite, is comparable to the combined flux of detrital and biogenic material to the basin floor. The magnitude of this flux and the occurrence of cold seeps throughout the oceans necessitate consideration of this phase in the development of barium geochemical budgets and the variety of paleoproxies involving barium in marine sediments.

Geochemical Controls on Contaminant Uranium in Vadose Hanford Formation Sediments at the 200 Area and 300 Area, Hanford Site, Washington

Long‐term historic spills of uranium at the 300 Area fuel fabrication site (58,000 kg of disposed uranium over 32 yr) and at the 200 East Area BX tank farm (7000 kg of spilled uranium in one event), both within the Hanford formation in the Hanford Site, Washington State, were investigated by subsurface sampling and subsequent microscale investigations of excavated samples. The 200 Area sediments contained uranyl silicate mineralization (sodium boltwoodite) in restrictive microfractures in granitic clasts, in the vadose zone over a narrow range in depth. Well logging and column experiments indicated that tank wastes migrated deeper than observed in core samples. The 300 Area sediments included metatorbernite and uranium at low concentrations associated with detrital aluminosilicates, along with other mineral phases that could accommodate uranyl, such as uranophane and calcium carbonate. The association of contaminant uranyl with Hanford formation sediments provided a persistent source of uranium to groundwater. The results of both studies suggest that the formation of secondary solid uranyl‐bearing phases influences the subsequent release of uranium to the environment and that our understanding of these processes and individual waste sites is incomplete.

Uranium-series isotopes in colloids and suspended sediments: Timescale for sediment production and transport in the Murray–Darling River system

We have measured 238U– 234U– 230Th radioactive disequilibria in five size fractions of colloids and suspended sediments for four rivers from the Murray–Darling River basin (SE Australia). Continuous compositional trends of composition are observed for ( 234U/ 238U) and ( 230Th/ 238U) activity ratios over the range of size fractions studied. They are explained by the variable contribution of two components: detrital alumino-silicates and organic matter. Using relationships between activity ratios and the loss-on-ignition, it is possible to estimate the U-series isotopic composition of both detrital and organic end-members, which provide the best estimate for the composition of the true detrital phase (i.e. controlled only by the removal of elements through weathering) and the true dissolved phase (i.e. colloid-free), respectively. When performing a single filtration at 0.2 μm, it appears that size fractions > and < 0.2 μm underestimate the disequilibrium in both the true detrital and dissolved phases, respectively. We show that erosion does not currently operate in steady state in the Murray–Darling basin and catchment soils are currently being degraded. This could be a consequence of the intensification of agriculture and deforestation over the past 100 yr. The residence time of sediments in the different sub-basins is calculated by computing the composition of the detrital phase with a continuous weathering model. It yields 11 ± 2 kyr for the Darling River basin, only 2–3 ± 1 kyr for the Murray River and 7 ± 2 kyr for the Goulburn River. These values are correlated with the age of changes in river dynamics, in turn probably controlled by climate fluctuation, and indicate the age of the modern river system for each sub-basin.

Origin of fibrous clays in Tunisian Paleogene continental deposits

This paper presents mineralogical and geochemical data from several continental sequences located in Central (Ain Ghréwiss and Kébar) and Central-Southern Tunisia (Selja, Kef Schefeir, Shib, Oum El Kcheb and Haidoudi). These sequences vary in age from Late Palaeocene to Early Oligocene and contain considerable quantities of fibrous clays (up to 75% palygorskite and 90% sepiolite). These clays appear in assocation with carbonates (mainly dolomite), detrital aluminosilicates (illite, Al smectites, mixed-layers illite–smectite and kaolinite), quartz and lesser quantities of gypsum and halite. The textural characteristics observed by electron microscopy, the trace and rare earth elements contents and their distribution in the various mineral phases, together with the isotopic composition of dolomite and fibrous clays, provide good clues as to the genesis of the neoformed minerals. Thus, the sepiolite would have precipitated directly in lacustrine, playa-lake or sebka environments under alkaline conditions, high Si and Mg and low Al activity, and arid to semiarid climate. On the other hand, the palygorskite would have formed by transformation of already existing illite and/or smectite type aluminosilicates in solutions in equilibrium with isotopically heavier and, therefore, more evaporated solutions than the sepiolite.

Temporal and spatial variations in the geochemistry of major and minor particulate and selected dissolved elements of Thermaikos Gulf, Northwestern Aegean Sea

The objectives of this study were to examine both spatial and temporal changes of particulate major elements and minor metals, as well as dissolved Mn and Cd, in the waters of Thermaikos Gulf. Collections of water and suspended particulate matter (SPM), as depth profiles (5–8 depths), were undertaken at 10 principal stations, essentially on a N–S traverse of the western side of the Gulf. One of the principal aims of the study was to observe if there was any change in the patterning of the elements between the three occupations of the stations: (a) in September 2001, immediately before the commencement of trawling; (b) in October 2001, whilst fishing was active; and (c) in winter/early spring conditions (February 2002), when fishing was still active, but after a change of river/atmospheric conditions. Bottom (∼20 m) waters were dominated by sediment resuspension; this was identified by concentration changes in the aluminosilicate elements (e.g. Al, Ti, K, Fe) of the SPM. A two- to three-fold increase occurred between September and October, caused probably by trawling; this was sustained at the offshore stations, in February. During February, the western inshore stations showed little sediment resuspension, caused by extreme winter cooling and the sinking of water. Consequently, a N–S density discontinuity existed at all water depths, which prevented the thermohaline cyclonic circulation from penetrating into the western seaboard of the Gulf. The distribution of dissolved and particulate Mn in the lower waters was due to redox cycling of the element at the benthic boundary; this was more intense in the north, where the organic supply was higher. Biogenic element concentrations and Ca/Al, Si/Al ratios showed no evidence that trawling activity promoted higher biological production. Strong correlations of Co, Cr, Ni and V, with Al and K, showed that these elements were associated strongly with detrital aluminosilicates. However, the variable association of Cd, Pb and Zn, with K (and Al), especially in the upper waters, implied an anthropogenic source derived from the rivers and the city of Thessaloniki. Examination of the Kd's of Cd showed a two-order of magnitude decrease with depth, caused by resuspension and possible advection of relatively unpolluted sediments, into the western Gulf.

The Significance of Ground Water to the Accumulation of Iron and Manganese in the Sediments of Two Hydrologically Distinct Lakes in North‐Central Minnesota: A Geological Perspective

AbstractWilliams and Shingobee lakes are at opposite ends of the local ground water flow system in the Shingobee River Headwaters Area (SRHA) in north‐central Minnesota. Williams Lake, situated near the highest point in the flow system, has no surface inlet or outlet, and ground water and precipitation are the only sources of water. Shingobee Lake, situated at the lowest point in the flow system, has the Shingobee River as an inlet and outlet. Ground water directly contributes an estimated one‐fourth of the water input to Shingobee Lake. The Shingobee River also receives large amounts of ground water discharge along its reach to Shingobee Lake providing a large, indirect source of ground water to the lake. Differences in nutrient concentrations reflect the residence times and nutrient supplies of these two lakes. The average phosphorus content of Shingobee Lake is about twice that of Williams Lake. Consequently, phy‐toplankton productivity in Shingobee Lake is much higher than in Williams Lake, leading to an oxygen‐deficient (&lt;1 ppm dissolved oxygen) hypolimnion within a month after overturn in both the spring and fall. Because of the extreme reducing conditions in the hypolimnion of Shingobee Lake, high concentrations of dissolved iron and manganese are present there during summer stratification. In some years, the manganese concentration in the hypolimnion of Shingobee Lake remains high throughout the year. Precipitation of iron and manganese minerals, presumed to be X‐ray amorphous oxyhydroxides, at periods of fall and spring overturn result in concentrations of iron and manganese in surface sediments of Shingobee Lake that are seven times and 27 times higher, respectively, than can be explained by contributions of iron and manganese from detrital aluminosilicates. These findings indicate that the source and amounts of this excess iron and manganese found in the sediments are correlated to the amount of iron‐ and manganese‐rich ground water discharging to Shingobee Lake. Because iron and manganese oxyhydroxides are efficient adsorbers of phosphorus, concentrations of phosphorus are also high in the sediments of Shingobee Lake. Without this sequestration of phosphorus, the productivity of Shingobee Lake would probably be much higher.

Direct barite determination using SEM/EDS-ACC system: implication for constraining barium carriers and barite preservation in marine sediments

Barite (BaSO 4) in marine sediments is considered as a proxy of surface ocean productivity and is widely used for paleo-productivity reconstructions. However, direct barite determination is not achieved by currently used methods, which rely on the measurement of total barium by inductively coupled plasma–mass spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA) and correction from the allogenic Ba contribution. We report here on a technique that directly counts individual barite crystals using a scanning electron microscope (SEM) coupled to an energy dispersive spectrometer (EDS) equipped with an automated particle counting and classification (ACC) system. We show that barite–Ba concentrations can be deduced from the abundance, shape and size distributions of barite crystals with current precision and sensitivity as high as ±5 wt.% (2 σ confidence limit) and 10 ppm, respectively. This technique, coupled with ICP-MS and INAA techniques for total Ba measurements, was applied to sediment samples from the tropical North Atlantic and the Central North Pacific oceans. Results show that: (1) barite and detrital aluminosilicates are the two main carriers of Ba in the investigated sediments; (2) calculated Ba/Al ratios range from 0.005 to 0.008 with an average value around 0.0066, in close agreement with previously reported values; (3) barite might not be the major source for released barium out of sediments and we suggest instead adsorbed barium as a potential source.

Significance of early-diagenetic water-rock interactions in a modern marine siliciclastic/evaporite environment: Salina Ometepec, Baja California

AbstractAlthough marine brines are a significant component of pore waters in sedimentary basins, there are few geochemical studies of modern analogues of such systems, especially in siliciclastic settings. For these reasons, we chose the evaporite-associated siliciclastic sediments deposited in the salt flats of the Salina Ometepec, Baja California, for an integrated investigation of sediment, pore-water, and overlying brine geochemistry. Here, the detrital components include quartz, K-feldspar, plagioclase, chlorite, biotite, and smectite, and authigenic minerals are dominated by halite, gypsum, and K-rich magnesium smectite.Thermal and saline stresses on the sediments of the Salina Ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments. Sediment pore waters exhibit little microbial sulfate reduction, and dissolved inorganic C contents are also very low. As a result, we did not observe carbonate and sulfide mineral authigenesis in the Salina Ometepec sediments. Instead, pore-water geochemical evolution is largely controlled by evaporative concentration of seawater, evaporite-mineral dissolution and recrystallization, and diagenetic alteration of detrital aluminosilicates.Evaporite-mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment. Specifically, Na+ and Cl− concentrations are increased owing to halite dissolution. We see significant Br− enrichment relative to expected seawater evaporation trends in near-surface pore water, secondary to dissolution of K- and Mg salts. Because bacterial sulfate reduction is inhibited in the Salina Ometepec sediments, sulfate concentrations are more accurate indicators of the degree of evaporation than Br−, a usually conservative element during geochemical reactions.Pore waters exhibit down-core increases in dissolved Mg2+, K+, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(H_{4}SiO_{4}^{0}\) \end{document} over the upper 1 m. Authigenic K-rich Mg-smectite formation is promoted by the concurrent processes of brine concentration, selective dissolution of K- and Mg-bearing salts, and dissolution of detrital aluminosilicates. Pore waters at a depth of 1 m have 87Sr/86Sr ratios that require input of Sr that is less radiogenic than that of Gulf of California seawater. This Sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources. These results show that significant chemical interactions among marine brines, evaporite minerals, and detrital aluminosilicates can occur relatively soon after sedimentation.

Sequences of silica phase transitions: effects of Na, Mg, K, Al, and Fe ions

Hydrothermal experiments on silica phase transitions in artificial solutions tested the effects of dissolved aluminum and iron on the evolution of solution composition and the mineral compositions of solid phases, Dissolved aluminum and iron come from detrital aluminosilicate phases, including their coatings. Aluminosilicate phases retard formation of opal-CT, a metastable silica phase, but they may promote transformation of opal-CT to quartz, the stable phase. The four starting solutions all had alkalinity greater than seawater, either 90 or 8 mM. The four solutions were dominated by different cations: one solution contained magnesium ion, one magnesium and sodium ions, one potassium ion, and one sodium ion. Compositions of the solutions changed within 4 weeks. Quartz formed under a higher initial alkalinity of ~90 mM, with or without magnesium. Minor phases, including opal-CT, clay minerals and melanophlogite, were also observed in some experiments. Opal-CT formed in the presence of magnesium and an initial alkalinity above 8 mM. Dissolved aluminum or iron reduced concentrations of dissolved silica, causing precipitation of an initially more ordered opal-CT in the presence of magnesium, and quartz in its absence. These effects influence the rates and sequences of silica diagenesis.