Deep Basin Sediments Research Articles

Laboratory Determination of Molybdenum Accumulation Rates as a Measure of Hypoxic Conditions

Enrichment of molybdenum (Mo) in reducing sediments due to authigenic fixation in anoxic interstitial waters could provide an indicator of hypoxic conditions that integrates over substantial temporal extents. Sediments maintained under controlled dissolved oxygen (DO) conditions showed elevated concentrations of Mo when exposed to low DO concentrations. Mo accumulation was linearly related to time of exposure in treatments below 2.8 mg O2/L, with less or no accumulation at higher concentrations. Rates of accumulation were independent of DO concentration below 2.8 mg/L. Accumulation occurred at DO concentrations higher than those limiting accumulation in field settings, with rates in the lowest treatments similar to those in sediments of deep basins with restricted circulation and low DO concentrations.

Geophysical evidence for 'blind' magmatism associated with Devonian rifting, Lachlan Orogen, New South Wales

The Silurian?Middle Devonian history of the Lachlan Orogen is characterised by the formation of rift basins and the emplacement of large amounts of granite. Many rift basins contain felsic or mixed felsic and mafic volcanic rocks, indicative of crustal as well as mantle melting being involved in lithospheric extension. However, there are several large rift basins that are filled by siliciclastic sedimentary rocks in which volcanics occupy < 1 % of the basin fill and may be buried. For the latter basins the question is: was rifting amagmatic, or are products of melting present at depth below the surface, either in deep basin sediments or in basement below the basin. In this paper, we attempt to address this question for sedimentary basins in the Cobar?Louth region of western New South Wales. For the Late Silurian?Early Devonian Cobar Basin, we use 1989 explosion?generated seismic reflection data that have been reprocessed using a new semblance filtering technique to improve the data quality. For the Nelyambo Trough, part of the Devonian Darling Basin in western NSW, we used Vibroseis deep seismic reflection data recently acquired in cooperation with Geoscience Australia and the Predictive Mineral Discovery Cooperative Research Centre. Gravity profiles were acquired along the Cobar lines. For the Nelyambo Basin, gravity data were extracted from a statewide dataset to match the seismic lines. The combined seismic and gravity data sets suggest that bright reflectors in the seismic sections represent mafic volcanics. These reflectors lie within inferred rift fill near the base of the Nelyambo Trough, but also occur in basement under the southwestern margin of the Cobar Basin.

Retreat history of the Ross Ice Sheet (Shelf) since the Last Glacial Maximum from deep-basin sediment cores around Ross Island

Radiocarbon-dated sediment cores from deep basins beneath the McMurdo Ice Shelf, and seasonally open water north of Ross Island, McMurdo Sound, display a characteristic succession of sedimentary facies that document the retreat of the Ross Ice Sheet at the Last Glacial Maximum to its present ice shelf configuration. The facies succession records a transition from a nearly-grounded ice sheet to open marine environments (north of Ross Island) that comprises in ascending stratigraphic order: (1) slightly consolidated, clast-rich muddy diamict dominated by basement clasts from the Transantarctic Mountains, and interpreted as melt-out from the basal layer debris proximal to a retreating grounding zone; (2) sparsely-fossiliferous (reworked diatom frustules) and non-bioturbated mud lacking lonestones, interpreted as a sub-ice shelf facies; and (3) diatom mud and diatom ooze indicative of open marine conditions with evidence of iceberg rafting. The succession in the open marine Lewis Basin north of Ross Island is similar, though the diamict is much sandier and sedimentation rates 1–2 orders of magnitude higher. We present a radiocarbon chronology from total organic carbon which implies that lift-off of grounded ice in the 900 m-deep marine basins surrounding Ross Island occurred by ~10,100 14C years BP. Following lift-off, an ice shelf was maintained to the north of Ross Island until ~8900 14C years BP. We identify a phase of accelerated retreat at that time between the Drygalski Trough and Ross Island, immediately preceding the timing of Meltwater Pulse 1b. At ~8900 14C years BP the calving line became pinned to Ross Island, significantly decoupling from the grounding line, and marking the transition from a retreating ice sheet to the development of the present ice shelf.

Oil in the lake of world heritage

Lake Baikal is the oldest and deepest lake in the world. It contains approximately 20% (23 · 10 3 km 3 ) of the world’s surface reserves. Since the community of diverse endemic organisms of the lake has not suffered from human impact at its recent stage of development [1], it is included into the UNESCO world heritage list. Unique features of Lake Baikal include natural oil seeps in its shelf zone, which have been known since the 18th century [2]. At oil shows in the middle and south sections of Lake Baikal, oil occurs as bitumen in shore cliffs or rises from the lake bottom as floating spherules of viscous hydrocarbons that make up spills (up to 1.5 m across) in an area of approximately 1 km 2 in summer. In winter, the floating oil is accumulated under ice as bituminous films and inclusions in narrow fissures. The oil composition corresponds to the biodegraded type. Its origin and age were debatable until recently [3]. In 2005, a new natural oil occurrence was discovered in deep-water settings of Lake Baikal that differs from the previously known ones. The discovery poses a problem concerning the role of natural oil occurrences in the ecosystem of the unique basin and its petroleum potential. The new oil occurrence was discovered in Middle Baikal (Cape Gorevoi Utes) owing to satellite observations of a dark spot (~1km across) on the lake surface in the spring of 2003. Observations of 2004 and 2005 showed that such a spot appears in this area every spring. In July 2005, numerous oil spots (up to 1 m in diameter) were recorded on the lake surface in an area of approximately 1 km 2 . Echo sounding revealed an underwater acoustic anomaly in the form of a gas flare ~500 m high. The oil sampled from the lake surface was studied using the gas chromatography/mass spectrometry method (Fig. 1). The samples contained n- alkanes, acyclic isoprenides, and polycyclic aromatic hydrocarbons. Based on the database including characteristics of molecular indicators for 76 genetically different oil fields [4], the ratios of molecular indicators (pristane/phytane 6.7; dibenzothiophene/phenathrene <0/1), and the low sulfur content (<0.08%), we concluded that the samples correspond to oils formed in sediments of deep freshwater basins during the Oligocene‐Early Miocene. This age estimate obtained for the Baikal oil based on molecular indicators is consistent with one hypothesis, according to which the Baikal oil is not older than the Cretaceous and it originates from organic matter buried in a freshwater basin [5]. The bottom sediments were sampled at the center of the gas flare and at its periphery (at a distance of 200 m). The core taken in the central part (0‐40 cm) is composed of dark gray reduced massive clayey‐silty sediment. The Holocene diatomaceous ooze layer typical of Baikal is missing at the surface. The sediment was mixed with oil and saturated with gas. The oil content exceeds 10% of the dry sediment. The second core demonstrates distinct bedding along its entire length. According to biostratigraphic data ( Cyclotella minuta, Aulacoseira baicalensis , and others), the sediment represents the Holocene diatomaceous ooze. The surface sediment (upper 4 cm with a brown color) is oxidized, while the remainder of the core is reduced (gray in color). The interval of 10‐40 cm hosts lenses and interbeds of silt and fine- to medium-grained sand with small brown oil droplets (up to 3 mm across) fringed by small gas bubbles. The oil content in this core interval is approximately 1% of dry sediment. The difference between two sampled cores in terms of oil-and-gas occurrence and lithology is probably explained by different intensities of their influx. The central part of the gas flare is characterized by permanent fluxes of gas, oil, and groundwater, which provide

Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Abstract Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sediments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly by porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH &lt; 8, porewater Fe 2+ = 10 −6 g cm −3 ) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100–1000 μg cm −2 a −1 . The addition of reactive iron to deep-basin sediments is determined by the magnitude of this diffusive flux, the export efficiency (ɛ) of recycled iron from the shelf, the ratio of source area ( S ) to basin sink area ( B ) and the trapping of reactive iron in the deep basin. Values of ɛ are poorly constrained but modern enclosed or semi-enclosed sedimentary basins show a wide variation in S/B ratios (0.25–13) where the shelf source area is defined as sediments at less than 200 m water depth. Diffusive fluxes in the range 100–1000 μg cm −2 a −1 are able to produce the observed reactive iron enrichments in the Black Sea, the Cariaco Basin and the Gotland Deep for values of ɛ × S/B from 0.1–5. Transported reactive iron can be trapped physically and/or chemically in deep basins. Physical trapping is controlled by basin geometry and chemical capture by the presence of euxinic bottom water. The S/B ratios in modern basins may not be representative of those in ancient euxinic/semi-euxinic sediments but preliminary data suggest that ɛ × S/B in ancient euxinic sediments has a similar range as in modern euxinic sediments.

Late Holocene lake level dynamics inferred from magnetic susceptibility and stable oxygen isotope data: Lake Elsinore, southern California (USA)

Southern California faces an imminent freshwater shortage. To better assess the future impact of this water crisis, it is essential that we develop continental archives of past hydrological variability. Using four sediment cores from Lake Elsinore in Southern California, we reconstruct late Holocene (� 3800 calendar years B.P.) hydrological change using a twentieth-century calibrated, proxy methodology. We compared magnetic susceptibility from Lake Elsinore deep basin sediments, lake level from Lake Elsinore, and regional winter precipitation data over the twentieth century to calibrate the late Holocene lake sediment record. The comparison revealed a strong positive, first-order relationship between the three variables. As a working hypothesis, we suggest that periods of greater precipitation produce higher lake levels. Greater precipitation also increases the supply of detritus (i.e., magnetic-rich minerals) from the lake's surrounding drainage basin into the lake environment. As a result, magnetic susceptibility values increase during periods of high lake level. We apply this modern calibration to late Holocene sediments from the lake's littoral zone. As an independent verification of this hypothesis, we analyzed � 18 O(calcite), interpreted as a proxy for variations in the precipitation:evaporation ratio, which reflect first order hydrological variability. The results of this verification support our hypothesis that magnetic susceptibility records regional hydrological change as related to precipitation and lake level. Using both proxy data, we analyzed the past 3800 calendar years of hydrological variability. Our analyses indicate a long period of dry, less variable climate between 3800 and 2000 calendar years B.P. followed by a wet, more variable climate to the present. These results suggest that droughts of greater magnitude and duration than those observed in the modern record have occurred in the recent geological past. This conclusion presents insight to the potential impact of future droughts on the over-populated, water-poor region of Southern California.

Sources and mechanisms for the enrichment of highly reactive iron in euxinic Black Sea sediments

The deep basinal euxinic sediments of the Black Sea are enriched in iron that is highly susceptible to sulfidization compared to oxic/suboxic continental margin sediments in the Black Sea and oxic/dysoxic continental margin and deep-sea sediments worldwide. A mass balance treatment of iron speciation data from three deep basin sediment cores shows that this enrichment is due to a combination of (1) highly reactive iron-bearing phases (sulfides and oxides) whose ultimate source was by diagenetic mobilization from the shelf (Wijsman and others, 2001) and (2) enhanced iron reactivity in the lithogenous component of deep basinal sediments. The cause of the enhanced reactivity of lithogenous iron is problematical. Possible mechanisms include microbial oxidation of non-reactive iron silicates and the preferential deposition of a fine-grained, reactive iron-enriched lithogenous component in the deep basin by the fractionation of the lithogenous flux during transport across the shelf. The application of paleoredox indicators based on iron reactivity (Degree of Pyritization, Indicator of Anoxicity) should take into account that the availability of highly reactive iron, and hence the concentration of reactive iron phases in the sediment, is controlled by a variety of chemical, physical and biological factors, some of which are not related directly to redox conditions in the water column.

Oxygen minimum zone controlled Mn redistribution in Arabian Sea sediments during the late Quaternary

Accumulation of labile Mn is related to bottom water oxygen concentrations. In this study we investigate the relation between oxygen minimum zone (OMZ) intensity and Mn burial in Arabian Sea during the late Quaternary. Mn carbonates in sediments of the deep Arabian Basin indicate that this part of the basin remained oxygenated for at least the last 185 kyr. Sequential leaching showed manganese carbonate enrichments with concentrations up to 3750 ppm in deep basin sediments. These Mn spikes must have formed diagenetically during periods of increased Mn oxide fluxes to the deep basin, associated with vertical extension of the OMZ. This extension caused remobilization of Mn oxides from a previously oxic slope, resulting in increased Mn accumulation in the deep basin. Mn relocation is evidenced by synchronous Mn depletions in organic‐rich intervals in slope sediments. Mn records from the deep basin can thus be used as a qualitative proxy to reconstruct intermediate water oxygenation.

Enhanced regeneration of phosphorus during formation of the most recent eastern Mediterranean sapropel (S1)

Phosphorus regeneration and burial fluxes during and after formation of the most recent sapropel S1 were determined for two deep-basin, low-sedimentation sites in the eastern Mediterranean Sea. Organic C/P ratios and burial fluxes indicate enhanced regeneration of P relative to C during deposition of sapropel S1. This is largely due to the enhanced release of P from organic matter during sulfate reduction. Release of P from Fe-bound P also increased, but this was only a relatively minor source of dissolved P. Pore-water HPO42− concentrations remained too low for carbonate fluorapatite formation. An increased burial of biogenic Ca-P (i.e., fish debris) was observed for one site. Estimated benthic fluxes of P during sapropel formation were elevated relative to the present day (∼900 to 2800 vs. ∼70 to 120 μmol m−2 yr−1). The present-day sedimentary P cycle in the deep-basin sediments is characterized by two major zones of reaction: (1) the zone near the sediment-water interface where substantial release of HPO42− from organic matter takes place, and (2) the oxidation front at the top of the S1 where upward-diffusing HPO42− from below the sapropel is sorbed to Fe-oxides. The efficiency of aerobic organisms in retaining P is reflected in the low organic C/P ratios in the oxidized part of the sapropel. Burial efficiencies for reactive P were significantly lower during S1 times compared with the present day (∼7 to 15% vs. 64 to 77%). Budget calculations for the eastern Mediterranean Sea demonstrate that the weakening of the antiestuarine circulation and the enhanced regeneration of P both contributed to a significant increase in deep-water HPO42− concentrations during sapropel S1 times. Provided that sufficient vertical mixing occurred, enhanced regeneration of P at the seafloor may have played a key role in maintaining increased productivity during sapropel S1 formation.

Phosphorus regeneration vs. burial in sediments of the Arabian Sea

Abstract Solid-phase phosphorus (P) speciation and benthic phosphate fluxes have been determined in Arabian Sea sediments. Benthic phosphate fluxes are highest in the continental slope sediments, underlying bottom waters with low oxygen concentrations. Organic matter degradation and phosphate desorption from iron oxides do not produce sufficient phosphate to explain these high phosphate fluxes. The potentially high deposition of P associated with fish debris (Pfish) in the Arabian Sea, and a good correlation between benthic phosphate fluxes and Pfish accumulation rates suggest that benthic phosphate fluxes in these sediments are to a large extent governed by dissolution of biogenic apatite. Factors controlling dissolution and preservation of fish debris, therefore, may play an important role in the burial and regeneration of P in continental margin sediments. A sharp decrease of the reactive P accumulation rate with increasing water depth, in combination with rather constant primary productivity rates throughout the northern Arabian Sea, indicates that P burial in continental margin sediments located within the OMZ is more efficient than in deep basin sediments. The effectiveness of P burial is to a large extent regulated by P regeneration occurring in the water column and redeposition processes. Sedimentary phosphorus burial efficiencies, thus, should be interpreted with caution in terms of the environmental conditions.

Variations in pyrite texture, sulfur isotope composition, and iron systematics in the Black Sea: evidence for Late Pleistocene to Holocene excursions of the o 2-h 2s redox transition

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O 2-H 2S boundary. Sediments from the basin margin collected below the impingement of the O 2-H 2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing <15 ka. Sediment fabric and framboid size distributions at a shallow, basin-margin site suggest that during two extended periods of Unit II deposition, the O 2-H 2S transition retreated to depths below 200 m. However, during these same intervals, the δ 34S of pyrite remains uniform compared to overlying and underlying laminated sediments, implying that this chemical signature may not be uniquely tied to the position of the O 2-H 2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.

Spatial variations and chronologies of aliphatic hydrocarbons in Lake Michigan sediments.

Four sediment cores were collected in fine-grained depositional areas of the southern basin of Lake Michigan. Spatial variations of aliphatic hydrocarbons in surficial sediments were consistent with a lakeward movement of riverine sediments in a series of resuspension-settling cycles in which an unresolved complex mixture (UCM) of hydrocarbons associated with dense sediments is deposited in nearshore areas, fine-grained sediments of terrestrial origin accumulate in the deep basin, and planktonic hydrocarbons are depleted by microbial degradation during transport to the deep basin. The rate of accumulation of the UCM (a marker of petroleum residues) in deep basin sediments has increased by more than an order of magnitude since 1880, from 60 microg m(-2) x a(-1) to approximately 960 microg m(-2) x a(-1) in 1980. Crude estimates of the atmospheric loading of the UCM (1100 microg m(-2) x a(-1)) indicate that accumulations in deep-basin sediments might be supported by atmospheric deposition. Agreement was poor between the atmospheric flux of the terrestrial n-alkanes (sigmaC25, C27, C29, C31) to the deep basin (3200 microg m(-2) x a(-1)) and the sediment accumulation rate (660 microg m(-2) x a(-1)). Understanding of atmospheric fluxes, estimated from the very few available data, would be improved by more frequent measurement of the levels of aliphatic hydrocarbons in air and precipitation and a better knowledge of the particle deposition velocities and precipitation scavenging coefficients.

Thermal buffering of sedimentary basins by basement rocks: implications arising from numerical simulations

The Earth’s crust releases or absorbs heat energy in response to changes in the thermal regime. Numerical simulations of basin-scale heat transport which use the sediment–basement interface as a thermal boundary assume that near-surface temperature changes have no effect on basement rocks and vice versa (unbuffered model). We test this assumption by comparing numerical models of transient fluid flow and heat transport in the Arkoma foreland basin with and without thermal buffering by basement rocks. Thermal buffering by basement rocks reduces cooling (by fresh water recharge) of deep basin sediments near the fold–thrust belt and reduces warming (by upward fluid discharge) of basin margin sediments. The unbuffered model predicts a temporary warming of basin margin sediments which is largely an artefact of the model thermal boundary conditions. In all cases, the buffered model produces no significant thermal transient. Exaggerated temperature predictions also can occur in numerical simulations of uplift and erosion or heat refraction.

00/03045 Rheological properties and model of the two-phase structure of coal

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O2-H2S boundary. Sediments from the basin margin collected below the impingement of the O2-H2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing <15 ka. Sediment fabric and framboid size distributions at a shallow, basin-margin site suggest that during two extended periods of Unit II deposition, the O2-H2S transition retreated to depths below 200 m. However, during these same intervals, the δ34S of pyrite remains uniform compared to overlying and underlying laminated sediments, implying that this chemical signature may not be uniquely tied to the position of the O2-H2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.

00/03044 Relation between variation of parameters and the softening and fusibility behaviors of coals during heating

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O2-H2S boundary. Sediments from the basin margin collected below the impingement of the O2-H2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing <15 ka. Sediment fabric and framboid size distributions at a shallow, basin-margin site suggest that during two extended periods of Unit II deposition, the O2-H2S transition retreated to depths below 200 m. However, during these same intervals, the δ34S of pyrite remains uniform compared to overlying and underlying laminated sediments, implying that this chemical signature may not be uniquely tied to the position of the O2-H2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.

Magnetic stratigraphy and tectonic rotation of the Eocene‐Oligocene Keasey Formation, northwest Oregon

The upper Eocene‐lower Oligocene Keasey Formation in northwestern Oregon consists of ∼700 m of deep marine forearc basin sediments exposed in stream beds and roadcuts. The abundant molluscs of this unit were the basis for the Keasey Stage of the Weaver et al. [1944] timescale, but the precise age of this fauna and formation have long been controversial. Analysis of over 130 magnetic samples from the Keasey in five sections yielded a stable remanence held mainly in magnetite which passed a reversal test. On the basis of calibration by benthic foraminifera and molluscs the Keasey Formation is correlative with Chrons C15r–C12r (35.0–33.0 Ma), so the unit spans the latest Eocene and early Oligocene. The magnetic directions suggest a tectonic rotation of 73.7°±4.7°, which is consistent with rotation of the nearest volcanic basement rock reported by Wells and Coe [1985].

A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mossbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Distribution of Rare Earth Elements in Early Permian Reef‐Island Ocean Sediments in Eastern Kunlun

Abstract: Based on the study of stratigraphy and fossils, the Early Permian ocean in eastern Kunlun is recognized as a kind of reef‐island ocean, in which there exist many different kinds of sediment, including patch carbonate platform, reef facies, transitional facies and deep basin sediments. It has been found that the total contents of REEs increase gradually from carbonate platform facies to deep basin facies. Meanwhile, sediments of different facies have different REE distribution patterns and different Ce anomalies. Most of the sediments of patch carbonate platform facies or reef facies are characterized by extremely negative Ce anomalies or moderately negative Ce anomalies (Ce/Ce*=0.33 to 0.55), and medium or thin‐bedded limestones of transitional facies by moderately negative Ce anomalies (Ce/Ce*=0.49 to 0.60). However, sediments of deep basin facies show weak or no negative Ce anomalies (Ce/Ce*=0.69 to 1.47), among which the value of Ce/Ce* in the radiolarian chert is 1.47.

Reconciling rapid strain accumulation with deep seismogenic fault planes in the Ventura Basin, California

Global Positioning System measurements across the east central Ventura basin, Transverse Ranges, southern California, before the nearby 1994 Northridge earthquake show high strain rates. Interpreting this rapid strain accumulation using the usual model of deep slip on a dislocation in a uniform elastic half‐space requires slip to extend to within 5 km of the surface. Such shallow slip is difficult to reconcile with the substantial coseismic displacement at depths from 7 to 19 km during the Northridge earthquake. Here we model the displacement and velocity fields throughout the earthquake cycle using a two‐dimensional finite element model with a viscoelastic rheology. Displacements are driven by far‐field and basal velocity boundary conditions and by imposed periodic earthquakes on the thrust faults bounding the basin. The thrust faults rupture through an elastic upper crust to a depth of 15 km. After a transient stage, during which stresses and strains build up to quasi‐equilibrium values, the behavior of the model becomes periodic. The sum of the coseismic displacement divided by the repeat interval, plus the average interseismic velocity, is equal to the geologic velocity. The temporal variation in surface velocity depends mainly on the Elsasser relaxation time (proportional to the product of the Maxwell time of the lower crust and the ratio of the thicknesses of the entire crust and viscoelastic lower crust). We are able to match the observed high strain rate only if we include the observed variations in elastic modulus associated with the deep basin sediments. The model reconciles geologic, geodetic, and seismological observations of deformation. There are trade‐offs among the far‐field convergence rate, the Elsasser time, the earthquake repeat time, and the time into the earthquake cycle. Acceptable convergence rates range from 8 mm/yr, for a relaxation time of the lower crust of 300 years, to 12 mm/yr, for a 30‐year relaxation time.

90Sr and 137Cs in the Black Sea after the Chernobyl NPP accident: inventories, balance and tracer applications

The paper summarizes studies of the distributions of 90Sr and 137Cs in the water and sediments of the Black Sea carried out during a 10-year period following the 1986 accident at the Chernobyl Nuclear Power Plant. Its goal is to assess the temporal evolution of radionuclide inventories and balances and to evaluate the mixing of water masses and the sedimentation processes using man-made radionuclides as tracers. Using mathematical models and field data, mixing time-scales of 5, 16 and 24 years have been estimated, respectively, for the water layers of depths 0–50, 0–100 and 0–200 m. For the Central Basin the ventilation time of the lower pycnocline is estimated at 15–25 years. 137Cs has been used to date shelf and deep-basin sediments, providing the history of chemical and radioactive pollution and of eutrophication during the past 50 years.