Brown Clay Research Articles

북동태평양 클라리온-클리퍼톤 균열대 KODOS 지역 심해저 퇴적물의 지질공학적 특성에 따른 유형분류

Deep-sea surface sediments, acquired from 1997 to 2002 in the Clarion-Clipperton fracture zone of the northeast equatorial Pacific, were analyzed for index and geotechnical properties to provide background information for the design of manganese nodule minor. The sediments were classified into 16 types based on the measured properties and evaluated in terms of miner maneuverabillity and potential environmental impacts arising from mining activities. It was found that the middle part of the study area covered with coarse siliceous sediments is more favorable to the commercial production than the northern part of pelagic red clay. In particular, Area B2 in the middle part is considered the best mining site since it shows the highest abundance as well as it consists mostly of normally to over consolidated (types B, C, D) coarse siliceous sediments that are appropriate for effective minor movement and accompany weak environmental impacts. Taking account of all the analyzed core logs, the average shear-strength values are proposed as a practical guideline fur movements of a manganese nodule miner: 6.0 kPa at 10cm and 7.0kPa at 40cm below the seabed.

The mass balance of dissolved thallium in the oceans

This study provides a comprehensive and quantitative discussion of the inventory and mass balance of dissolved thallium (Tl) in the oceans. The calculations are based on a critical evaluation of recently published analytical data. Thallium is estimated to have a mean seawater concentration of 65 pmol/kg and this implies a total oceanic inventory of about 8.76×10 4 Mmol Tl. The five primary sources of dissolved Tl in the oceans are rivers, hydrothermal fluids, eolian contributions from volcanic emanations and mineral aerosols, and benthic fluxes from continental margin sediments. Combined, these sources generate a Tl flux of between 1.4 and 12 Mmol/year, with a best estimate of 5.2 Mmol/year. Based on this, an oceanic residence time of 7–63 ky is calculated for Tl, with a best estimate of 17 ky. The two main oceanic sinks of Tl are scavenging from seawater by the authigenic phases of pelagic clays and uptake of Tl during low-temperature alteration of oceanic crust. These sinks remove about 1.6–7.7 Mmol/year of Tl, with a best estimate of 4.2 Mmol/year. This indicates an oceanic residence time of 11–56 ky, with a best estimate of 21 ky. The mass balance yields input and output fluxes which are identical, within the uncertainties, and this is in accord with a budget that is in steady state.

Microbial communities associated with geological horizons in coastal subseafloor sediments from the sea of okhotsk.

Microbial communities from a subseafloor sediment core from the southwestern Sea of Okhotsk were evaluated by performing both cultivation-dependent and cultivation-independent (molecular) analyses. The core, which extended 58.1 m below the seafloor, was composed of pelagic clays with several volcanic ash layers containing fine pumice grains. Direct cell counting and quantitative PCR analysis of archaeal and bacterial 16S rRNA gene fragments indicated that the bacterial populations in the ash layers were approximately 2 to 10 times larger than those in the clays. Partial sequences of 1,210 rRNA gene clones revealed that there were qualitative differences in the microbial communities from the two different types of layers. Two phylogenetically distinct archaeal assemblages in the Crenarchaeota, the miscellaneous crenarchaeotic group and the deep-sea archaeal group, were the most predominant archaeal 16S rRNA gene components in the ash layers and the pelagic clays, respectively. Clones of 16S rRNA gene sequences from members of the gamma subclass of the class Proteobacteria dominated the ash layers, whereas sequences from members of the candidate division OP9 and the green nonsulfur bacteria dominated the pelagic clay environments. Molecular (16S rRNA gene sequence) analysis of 181 isolated colonies revealed that there was regional proliferation of viable heterotrophic mesophiles in the volcanic ash layers, along with some gram-positive bacteria and actinobacteria. The porous ash layers, which ranged in age from tens of thousands of years to hundreds of thousands of years, thus appear to be discrete microbial habitats within the coastal subseafloor clay sediment, which are capable of harboring microbial communities that are very distinct from the communities in the more abundant pelagic clays.

The Hawaii-2 Observatory: Observation of Nanoearthquakes

The Hawaii-2 Observatory (H2O; http://www.iris.edu/GSN/) was established on the seafloor between Hawaii and California (Figure 1) in 1998 using the retired ATT Duennebier et al. , 2002; Harris and Duennebier, 2002; Pettit et al. , 2002). This is the first undersea seismic station in the Global Seismographic Network. The on-site instrumentation includes a broadband Guralp CMG-3 and 4.5 Hz Geospace HS1 geophones, buried 0.5 m into the sea-floor mud, and an OAS E2PD hydrophone 0.5 m above the seafloor. All of the data are natively sampled at 160 sps continuously and sent via the Hawaii-2 cable to Oahu, where they are distributed by the University of Hawaii to IRIS. Data are available in real-time via a Live Internet Seismic Server (LISS) server and are being used by the Pacific Tsunami Warning Center and the National Seismic Network. In early 2002, the Ocean Drilling Program drilled a borehole through 29±1 m of sediment into basalt about 1.5 km northeast of the current H2O instrumentation as a site for a future borehole sensor (Stephen et al. , 2003). Figure 1. (left) The Hawaii-2 Observatory at 27° 52.916 N, 141° 59.504 W is sited 1,750 km east-northeast of Honolulu (Butler et al. , 2000). The site lies between the Murray and Molokai Fracture Zones at 4,979 m depth on predominantly terrigeneous, pelagic clay. The oceanic crust is of Eocene age (45–50 m.y.) and is normal but fast-spreading with a spreading half-rate of 7 cm/y. (top, center) The Hawaii-2 cable terminates in western Oahu and passes between Kauai and Oahu before traversing northeast toward California, (right) The abyssal bathymetry is locally flat in a ∼5,000 km2 region with abyssal hills rising less than 500 m high at about 30 …

Platinum-group elements (PGE) and rhenium in marine sediments across the Cretaceous–Tertiary boundary: constraints on Re-PGE transport in the marine environment

The nature of Re–platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous–Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ∼1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ∼95% relative to chondritic Ir proportions. A similar depletion in Os (∼90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ∼1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ∼65 Ma, the effective diffusivities are ∼10 −13 cm 2/s, much smaller than that of soluble cations in pore waters (∼10 −6 cm 2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios ≥1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ∼25% of the K-T impactor’s Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.

PALEOENVIRONMENTAL SIGNIFICANCE OF A PALEOSOL COMPLEX IN CHINESE LOESS

Several studies of loess-paleosol sequences have been conducted with the objective of reconstructing the monsoonal climate changes in East Asia, but pedocomplexes have not been identified and studied. In this study, a pedocomplex was studied with the objective of evaluating paleoenvironmental changes. The data show that the paleosol S5 of the Brunhes (780,000–0 yr B.P.), which developed over about 100 ka, is a pedocomplex and varies in thickness from 2.2 to 6.5 m. The paleosol is located at the Liujiapo near Xi’an, Renjiacun, near Baoji and Yaergou near Changwu and consists of four to five horizons. The first horizon (uppermost) is a well developed reddish brown clay horizon (Bts); the second horizon is a light reddish brown weathering-cracked loess (Ccs); the third is a dark yellowish brown weathering-cracked loess (Cs); the fourth is a light yellowish brown weathering-cracked loess (Cl); and the fifth horizon consists of CaCO3 nodules (Ck). Weathering-cracked loess horizons have not previously been found in Chinese paleosol. The clay component is montmorillonite-illite with some kaolinite minerals. The pedocomplex suggests that a moist subtropical climate prevailed from 570,000 to 470,000 years B.P. in the central and southern parts of the Chinese Loess Plateau. Its profile is Bts-Ccs-Cs-Cl-Ck-Co in the Xi’an, Baoji areas and Bts-Cs-Cl-Ck-Co in the Changwu area. When the pedocomplex developed in the Xi’an and Baoji areas (southeastern Loess Plateau), the mean annual temperature and precipitation were about 16 °C and 1100 mm, respectively, about 4 °C higher and 500 mm more than at present. In the Changwu area (central Loess Plateau), the mean annual temperature and precipitation were 15 °C and 900 mm, about 6 °C higher and 300 mm more than at present. In the typical interglacial developing the S5, the Qinling Mountains lost their function as the boundary between the subtropical and temperate zones of China.

The source of raw materials for Roman pottery from Leptiminus, Tunisia

AbstractLeptiminus, a Roman port city on the west coast of Tunisia, North Africa, exported olive oil and a garum fish paste to Rome. Excavations have uncovered many facilities including kilns and a potter's workshop, indicating an extensive ceramic industry. The vessels, manufactured at Leptiminus, included African red‐slip fineware, coarseware, and amphorae. A petrographic study of pottery sherds showed them to contain very similar temper, rounded aeolian sand grains and limestone, but varying in proportions to produce different textures. An investigation of the source of raw materials for the pottery found three distinct types of clay within a 50 km radius of Leptiminus: grey Miocene, brown Pliocene, and green Late Pliocene clay. Statistical analysis of trace element compositions, using induced neutron activation, of clays and sherds showed that the Pliocene brown clay was used to create all types of pottery. The specific clay horizon, used in Roman times, has apparently been removed by quarrying. However, a lateral continuation of this bed was found 7 m beneath the Leptiminus site 290. © 2002 Wiley Periodicals, Inc.

Ichthyolith strontium isotope stratigraphy of a Neogene red clay sequence: calibrating eolian dust accumulation rates in the central North Pacific

Cenozoic pelagic (‘red’) clays of predominantly eolian and hydrogenous origin blanket much of the central North Pacific ocean basin. The eolian component is a key indicator of past paleoclimatic conditions; thus, Cenozoic atmospheric circulation can potentially be reconstructed through provenance studies of Pacific red clays, provided there are precise age controls. Methods commonly employed in the past to date red clay cores have included cobalt accumulation rates, ichthyolith biostratigraphy, magnetostratigraphy, and ichthyolith strontium isotope stratigraphy. The first two dating methods yield ages with large uncertainties, while magnetostratigraphy is only relevant to cores with accumulation rates in excess of 1 mm/kyr. Ichthyolith strontium isotope stratigraphy has shown promise as a chronological tool in marine studies, but has been only sparingly employed in the dating of marine red clay sequences. In this study, we present a complete age–depth profile for a large diameter piston core from the central North Pacific Ocean (EW9709 PC-01, 32.5°N, 141.2°W), consisting of 11 m of primarily wind-deposited dust. To generate this age–depth profile, strontium isotopic compositions were determined on fish teeth ichthyoliths previously cleaned of contaminants using a newly modified reductive cleaning procedure. Ages were determined by reference to the recently refined Sr isotope curve for Neogene seawater. Comparison with nearby giant piston core LL44-GPC3 reinforces the accuracy of our methods. The data for EW9709 PC-01 indicate a fairly constant sediment accumulation rate of ∼0.45 mm/kyr over most of the 24 Myr time period represented by this core.

Geochemistry and origin of piemontite-quartz schists in the Sanbagawa Metamorphic Belt, central Shikoku, Japan

In the Sanbagawa Metamorphic Belt of central Shikoku, geochemical characteristics are used to demonstrate that piemontite-quartz schists had a manganiferous sedimentary protolith. The Sanbagawa Metamorphic Belt was formed during a Cretaceous regional intermediate high-pressure type of metamorphism which is present throughout Southwest Japan. Based on a detailed mineralogical study of the Asemi-gawa and Besshi areas in central Shikoku, the Sanbagawa piemontite-quartz schists can be divided into talc-bearing and talc-free types. Phlogopite occurs only in talc-bearing assemblages and the modal percentage of piemontite in talc-bearing types is higher than in talc-free types. Plotting analyses of piemontites and garnets from talc-bearing and talc-free types in the Al 2O 3–Fe 2O 3–Mn 2O 3–MnO(Al–Fe 3+–Mn 3+–Mn 2+) diagram, for the Asemi-gawa and Besshi areas separately, shows that the piemontite which accompanies talc is richer in Mn 3+ and poorer in Fe 3+. Also, garnets from talc-bearing assemblage contain a slightly higher amount of Mn 2+ and a lower amount of Al than talc-free types. The Al 2O 3–SiO 2–Fe 2O 3+FeO plot shows that Sanbagawa piemontite-quartz schists have a composition between deep-sea pelagic clay and pure silica. By the use of Pearson's product moment correlation coefficient method, and plotting elements against each other after removing silica and volatiles, it is possible to distinguish four components which are able to explain the variance of the samples. These components are: (first component: Al 2O 3, TiO 2, MgO, Na 2O, K 2O, Zr, Rb, Y, Nb and to lesser degree Fe 2O 3, FeO, Ni, Th, Ba); (second component: SiO 2); (third component: MnO) and (fourth component: CaO). Al 2O 3, TiO 2, Zr and other elements with positive coefficient relations with them have a detrital origin. Biogenic origins are considered for CaO and SiO 2 and it was concluded that MnO was derived from an hydrothermal source. Because the Sanbagawa Metamorphic Belt has been subjected to much detailed research and also contains talc-bearing and talc-free piemontite-quartz schists, it provides the best opportunity to find an explanation for the occurrence of talc in some types of piemontite schist and its absence from others. Mineralogical and geochemical evidences show that the oxygen fugacity for talc-bearing and talc-free types is similar. Oxygen fugacity is so high that almost all of the iron is present as Fe 3+ in hematite. Pressure and temperature is considered to have been constant for talc-bearing and talc-free types in each locality because the samples were collected from a known metamorphic context. The talc absent/present problem is discussed using phase relations of the samples with the greatest number of phases in each locality. Consideration of talc, clinochlore, piemontite and garnet in the system Al 2O 3–Mn 2O 3/MnO–MgO–CaO(ACMMn), with excess phases albite, quartz, hematite, phengite and water, shows that talc coexists with piemontite and garnet which are richer in the piemontite and spessartine end-members, respectively.

Early diagenesis in an organic-rich turbidite and pelagic clay sequence from the Cape Verde Abyssal Plain, NE Atlantic: magnetic and geochemical signals

Abstract Processes of early diagenesis in sediments from the Cape Verde Abyssal Plain are investigated using the complimentary techniques of environmental magnetism and solid-phase geochemistry. The sequence captured by a 2 m-long box core comprises an organic-rich turbidite derived from the NW African continental margin (emplaced ca. 330 ka BP), overlain by brown, pelagic clay. Diagenesis in the pelagic clay mainly involves slow, in-situ oxidation of the iron oxide phases present, which Mossbauer-effect data suggest are detrital haematite and magnetite (possibly maghaemitised). In the turbidite, diagenesis has proceeded in two stages: the first involving suboxic (reductive) processes, following depletion of porewater O2, and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of O2 from the overlying seawater. Carbonate-free magnetic parameters, interparametric quotients, and remanence acquisition, demagnetisation and coercivity measurements of the core, reveal that significant depletion of detrital ferrimagnetic iron (Fe2+/Fe3+) oxide grains like (titano)magnetite has occurred within the turbidite during suboxic diagenesis, and confirm previous suggestions that reductive dissolution of such components is a grain size-selective process. However, the higher oxidation state (Fe3+), canted-antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis. Remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic Fe/Mn micronodules in a distinct layer immediately overlying the front, and above this, a further interval of Mn-enriched, “diagenetic laminae”. However, within the suboxic, organic-rich zone of the turbidite, well below the oxidation front, there is clear evidence for the presence of a significant concentration of fine (single-domain) ferrimagnetic iron oxide grains, which are most probably associated with a population of live magnetotactic bacteria. Between this zone and the oxidation front, there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly, and fossil magnetosomes in the sediment are being reductively dissolved progressively up-sequence. These results are broadly similar, though differing in some respects, to magnetic and geochemical studies of hemipelagic and high-productivity pelagic sediments in which Mn- and Fe-redox boundaries have been recognised, and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind.

Ancient denudational surfaces in the Gondwana sequences of Peninsular India

Gondwana basins of Peninsular India contain a thick pile of clastic sediments of predominantly fluvial origin. As expected in such a depositional milieu, the sedimentary pile is punctuated by numerous episodes of contemporaneous uplift, non-deposition and erosion. Most of these events are localised in nature and hence not correlatable from one basin to another. However, a few events of regional nature can be recognized. They comprise (a) basement surface with glaciated features like polishing, striations and frost wedging below the basal Talchir strata; (b) surface of angular and erosional unconformity between Permian/Triassic and Early Triassic/infra - Late Norian and (c) a major diastrophic episode resulting in reversal of palaeoslope between Late Jurassic and Early Cretaceous. Besides these, localized events represented by surfaces of contemporaneous subaerial exposure and oxidation showing characters of pedogenic (palaeosol) and lateritic (latosol) profiles have been recorded. These include Lower Barakar (Early Permian) of Hutar basin of Koel valley, Barren Measures (Late Permian) and Triassic/Jurassic contact of Rewa and Satpura basins, and Late Cretaceous Lometa Beds in Rewa Basin. Breaks represented by conglomerate beds occur in Early Permian Barakar strata of Talchir coalfield and Late Mesozoic Bagra Formation of Satpura basin. The palaeosols vary in thickness from 4 m to 25 m and are characterised by red brown clay with mottles, rootlets, slickensides, ferruginous concretions and calcareous veinlets. The palaeosols are hydromorphic and are developed under poorly drained to well drained groundwater conditions. Even where the palaeosols are not present, the bedrock displays evidences of deep weathering. These probably represent the earlier topographic positive areas affected by physical weathering and intense groundwater activity.

Technical note study on the flotation selectivity of a problem phosphate ore

Three types of mineralisation are present in the phosphate ore mined by Serrana in Araxá, Brazil: oxidised, micaceous and silicate—carbonate. The micaceous and silicate—carbonate are considered problem ores. This investigation deals with characterisation and fundamental studies performed on fractions resulting from the purification of five samples from the silicate—carbonate type collected in the mine, representing portions with visually different aspects, designated as: barite, carbonatite, green micaceous, brown—green micaceous and brown clay. Each type was purified in the laboratory and then submitted to mineralogical characterisation, zeta potential determinations, and microflotation experiments with anionic and cationic collectors. The samples, with the exception of the barite, are mineralogically complex and the selectivity between apatite and the gangue minerals is poor. The only feasible route for processing the silicate—carbonate ore is blending it with the standard oxidised ore.

Prediction of Abnormally High Formation Pressures (AHFP) in petroliferous salt-bearing sections

Prediction of overpressures [Abnormally High Formation Pressures (AHFP)] in the evaporite sequences is indeed a challenging problem. Several techniques are suggested here for the solution of this forecasting problem. For a reliable AHFP forecast it is necessary to delineate the salt plugs. A method of accomplishing this is presented here. In many cases there is evidence of vertical gas migration along flanks of salt plugs and associated fractured zones (and faults). The composition of gases in the secondary traps in the sealing evaporite sequence is the same as that of gas in the massive, major accumulation below. Predictive precursors of the AHFP include lithological barrier (e.g., anhydrite) with decreasing drilling rates and appearance of “gas clay” in circulating mud (i.e., plastic greenish-gray and brown clay with gas bubbles).

The geochemistry of banded iron formations in the sukumaland greenstone belt of Geita, northern Tanzania: evidence for mixing of hydrothermal and clastic sources of the chemical elements

Major and trace element compositions of samples of Banded Iron Formations (BIF) from the Neoarchaean Sukumaland Greenstone Belt of Geita in northern Tanzania reveal that the BIF precipitated from hydrothermal solutions. Fe-Ti-Al-Mn systematics suggest that the hydrothermal deposits have been contaminated, by up to 20% by weight, with detrital material having a composition similar to modern deep-sea pelagic clays. SiO2 and Fe2O3 contents are 48.2 to 88.5% and 8.9 to 49.1% respectively. Al2O3 contents lie between 0.33 and 2.1% and show no correlation with either Fe2O3 or SiO2. Al2O3 is, however, positively correlated with Ti, Ga, Hf, Rb,Th, Zr and Sr but not with CaO, the alkalies and the total Rare Earth Elements (REE). The other major element oxides are generally present in negligible amounts. The samples are characterised by mean Zr/Hf and 144Sm/143Nd ratios of 48± 5 (2 SE) and 0.10±0.01 (2 SE) respectively, similar to mean upper continental crustal values. Shale-normalised REE patterns are nearly flat, except for small positive Eu and very slight negative Ce anomalies and reveal that, compared to average upper crust, the abundances of the REE in the BIF are up to an order of magnitude lower. Chondrite-normalised patterns are characterised by light (L) REE enrichment, flat to slightly depleted heavy (H) REE, slightly positive Eu anomalies and very small negative Ce anomalies. The HREE-depleted patterns are similar to patterns derived from granite-dominated upper continental crust and indicate that the bulk of the REE in the Geita BIF can not have been derived from a mixture of Neoarchaean sea water and bottom hydrothermal solutions. The trace element data, and the REE in particular, indicate that, despite their relatively low proportions, granitic detritus probably derived from contemporaneous felsic flows and pyroclastics are the cause of the dominant trace element geochemical signature of the BIF. Tanzania Journal of Science Volume 27 (2001), pp. 21-36

Palaeoenvironmental implications of palygorskite clays in Eocene deep-water sediments from the western Central Atlantic

Abstract Clay mineral analyses were performed on Eocene sediments from drill sites in the western Central Atlantic. The investigated sites cover the full range of early Palaeogene deep waters above and below the calcite compensation depth (CCD), but otherwise represent different depositional and hydrographic regimes. Palygorskite clays with authigenic microstructures were discovered in Lower Eocene hemipelagic sediments from the distal end of the Blake Nose depth transect and in pelagic clays of the same age from the distal Nares Abyssal Plain, where terrigenous input was reduced. Palygorskite clays were not detected in coeval sediments from a distal near-CCD setting on Bermuda Rise that received major terrigenous input. The distribution of palygorskite clays at these sites, the microstructures of the constituent minerals, their absence from contemporaneous deposits on the American margin, and the position of the northerly sites outside the range of a potential African aeolian supply strongly suggest an authigenic origin of these clays at the early Eocene sea floor. Palygorskite clays are widely distributed in lower Eocene sediments from about 50° N to 50° S palaeolatitude. The most widespread distributions and peak abundances in Atlantic oceanic sediments are reported from shelf to deep-water sites of the palaeo-tropical and -subtropical belt and correlate with the Early Eocene period of extreme warmth. Marine authigenic palygorskite clay may provide an indication of the localities and the time periods that were characterized by high bottom-water temperatures, by elevated alkalinity, silica and magnesium concentrations, and by reduced sediment accumulation rates.

Orbital control on pelagic clay sedimentology; the case of the late Albian "Amadeus Segment" (central Italy)

Abstract The Amadeus Segment which falls within the Oceanic Anoxic Event 1c, is a composite stratigraphic interval of late Albian age, composed by frequent alternation of black shales characterised by organic carbon content usually lower than 1% and carbonate-richer sediments. Geochemical and clay mineral signatures indicate a coarser grain size for the black shales, and geochemical data from literature indicate a continental origin of the organic matter. Most of the geochemical variables analyzed show a record which follows an orbital cyclicity, with the exception of Mn and Ba, whose mobility after burial was identified by independent compositional criteria. The spectral analysis succeeded in identifying such compositional peculiarities, and also enabled to correlate the Amadeus Segment with the Segment-1 of the nearby Piobbico Core. The Milankovitch cyclicity (low eccentricity and obliquity) deeply affects the sedimentation of the Amadeus Segment, because orbital frequencies characterize the distribution of foraminiferal and compositional variables, which are both controlled by organic productivity and by detrital supply (dilution effect excluded). The high frequency changes from carbonate-rich layers to black shales are interpreted as changes in oceanographic regimes which in turn caused variations in the dispersal regime of continental inputs and in productivity.

Ferromanganese phases and the chemistry of contemporary seawater

We report on a correlation between oceanic residence times of chemical elements and their enrichment in marine ferromanganese deposits. When a logarithmic plot of oceanic residence time vs. atomic number is placed above a similar plot of enrichment factor vs. atomic number, the plots exhibit a clear mirror image relationship. We suggest that this relationship is not accidental, but rather implies an important role for ferromanganese phases in the regulation of seawater chemistry, especially the residence times of the minor elements. The formation of ferromanganese phases, as nodules, crusts, particles and coatings on pelagic clays, is dependent upon oxic seawater conditions similar to those existing today. The absence of oxic conditions throughout much of geologic time insures that ferromanganese minerals would have been rare or nonexistent sedimentary deposits. If these phases are as important as contemporary residence time correlations suggest, the minor element chemistry of earlier oceans must have been radically different from that of the present. Moreover, in times of deep water stagnation over the Phanerozic, conditions would have been thermodynamically unsuitable for ferromanganese mineral formation and hence suitable for the accumulation of high concentrations of certain toxic metals. We suggest that metal poisoning resulting from deep water turnover may have contributed to some of the great marine extinctions occurring over Phanerozoic Time.

Dehydration and melting experiments constrain the fate of subducted sediments

Geochemical tracers demonstrate that elements are cycled from subducted sediments into the arc melting regime at subduction zones, although the transfer mechanism is poorly understood. Are key elements (Th, Be, Rb) lost during sediment dehydration or is sediment melting required? To investigate this question, we conducted phase equilibria and trace element partitioning experiments on a pelagic red clay for conditions appropriate to the slab beneath arc volcanoes (2–4 GPa, 600°–1000°C). Using both piston cylinders and multianvils, we determined the solidus, phase stabilities, and major element compositions of coexisting phases. The solidus (H2O + Cl fluid‐saturated) was located at 775 ± 25°C at 2 GPa, 810 ± 15°C at 3 GPa, and 1025 ± 25°C at 4 GPa with noevidence for complete miscibility between melt and fluid. This sediment composition produces a profusion of phases both above and below the solidus: garnet, jadeitic pyroxene, alkali‐rich amphibole, phengite, biotite, magnetite, coesite, kyanite, apatite, zircon, Cl‐rich fluids, and peraluminous to peralkaline granitic melts. At 2 GPa the phengite dehydration solidus is at 800°–825°C, while biotite breaks down between 850° and 900°C. To explore trace element partitioning across the solidus at 2 GPa, we used diamonds to trap fluids and melts. Both the bulk sediment residues and diamond traps were analyzed postexperiment by inductively coupled plasma–mass spectrometry (ICP‐MS) and inductively coupled plasma–atomic emission spectrometry (ICP‐AES) for 40 elements for which we calculated bulk partition coefficients (D = Csolid/Cfluid). Below the solidus, Rb, Sr, Ba, and Pb showed the greatest mobility (D ∼ 0.5–1.0), while at the solidus, Th and Be became notably partitioned into the melt (D values changing from &gt;2.0 to &lt;1.0). K and Rb D values fall below 1.0 when the micas breakdown. Only at the solidus do Th and Rb attain similar partition coefficients, a condition required by arc data. Taken together, the experimental results indicate that critical elements (Th and Be) require sediment melting to be efficiently transferred to the arc. This conclusion is at odds with most thermal models for subduction zones, which predict slab temperatures more than 100°C lower than sediment solidi. Thus the condition of sediment melting (with oceanic crust dehydration) may provide new constraints on the next generation of thermal/geodynamical models of subduction zones.

The osmium isotopic composition change of Cenozoic sea water as inferred from a deep-sea core corrected for meteoritic contributions

A detailed study was made of the oceanic 187Os/ 186Os variation through the Cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the North Pacific (Long Lines-44 GPC3). A 6% H 2O 2 solution was used. The range of 187Os/ 186Os obtained by this procedure was between 2 and 8.2. A milder leach (0.15% H 2O 2) in three out of four samples yielded a slightly higher 187Os/ 186Os value than the stronger leach implying that the stronger leach released some meteoritic Os with 187Os/ 186Os of 1. Using published 3He concentrations in GPC3 and cosmic dust 3He and Os fluxes to deep-sea sediments, the extraterrestrial Os concentration was estimated for each core segment and subtracted to yield “corrected” values of 187Os/ 186Os. The results based on the milder leaching protocol and the 3He correction protocol yield similar values. The “corrected” ocean water 187Os/ 186Os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire Cenozoic. The variation with time in GPC3, uniquely, provides a statigraphic signature for the Paleogene.

Stratigraphy of sediments from the Håkon Mosby Mud Volcano Area

Studies of four gravity cores from the Hakon Mosby Mud Volcano area have shown the existence of three lithologic–stratigraphic units: I – brown clay (0–10 ka), II – gray clay (10–30 ka) and III – gray silt (30–61 ka). The units reflect different paleographic sedimentation and conditions, including the influence of mud volcanic activity. Two periods of mud volcanic activity can probably be discerned: 10–30 ka and Recent. The coexistence of Pleistocene and Late Pliocene benthic foraminifera suggests that the source depth of the volcano is located below the Pliocene–Pleistocene boundary.