Post-depositional Remobilization Research Articles

Evidence of postdepositional remobilization of redox-sensitive metals across sapropel boundaries: New insights from LA-ICP-MS and EDX mapping analyses

Deoxygenation in marine systems is globally increasing due to anthropogenic pressures and climate change impacts, leading to significant ecological and environmental consequences. Understanding past deoxygenation events is crucial for assessing the response of ocean stressors to climate variability and for predicting and managing the impacts of current and future changes. Deep-marine basins with organic-rich deposits serve as valuable palaeo-archives, preserving evidence of profound palaeoceanographic changes involving deoxygenation events. Such events have been broadly studied and reconstructed using redox-sensitive trace metals (RSTMs) as proxies for oxygenation conditions. Assessing these proxies is therefore crucial for further understanding deoxygenation events. To do so, this study focuses on RSTMs distribution across the most recent sapropel, S1, which is a well-preserved organic-rich sediment layer that represents the most recent large-scale deoxygenation event in the Eastern Mediterranean. High-resolution geochemical microanalyses, including Laser Ablation ICP-MS and SEM-EDX, were conducted to investigate sub-mm scale geochemical signals and post-depositional remobilization of RSTMs within S1 and surrounding sediments. Statistical analyses have also helped to identify the geochemical fractions within S1, including detritic, carbonate, and authigenic, enriched in RSTMs and organic matter. The study revealed distinct specific geochemical intervals, such as the marker bed, oxidized interval, unoxidized interval, and synsapropel interval. These intervals provided insights into the diagenetic processes and post-depositional reactions occurring within and across sapropel S1 boundaries. The enrichment of RSTMs within pyrite aggregates in the synsapropel interval was primarily attributed to post-depositional processes rather than the initial syndepositional conditions of S1. Large pyrite surfaces below S1 and the precipitation of Mn-oxyhydroxides over S1 played a significant role in the fixation and absorption of dissolved RSTMs derived from postdepositional oxidation. Furthermore, the sub-mm scale analysis revealed spatial heterogeneities in mineralogical composition and elemental distribution, providing valuable information on the complexity of post-depositional reactions and processes. The findings highlight the importance of microscale geochemical analyses for understanding the dynamics of RSTM fixation and enrichment processes in marine sediments. By differentiating between syndepositional and post-depositional geochemical signals, this study contributes to the interpretation of palaeoenvironmental conditions and enhances our understanding of the spatial complexity of deoxygenation processes. Such insights are crucial for accurately assessing the impacts of deoxygenation on biogeochemical cycling and ecosystem functioning in marine environments.

Mediation of arsenic mobility by organic matter in mining-impacted sediment from sub\u2010Arctic lakes: implications for environmental monitoring in a warming climate

Arsenic (As) is commonly sequestered at the sediment–water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.

U-Th dating of lake sediments: Lessons from the 700 ka sediment record of Lake Junín, Peru

Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has been often limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ∼100 m drill core from Lake Junín, Peru. The results form the foundation of an age-depth model spanning ∼700 kyrs. High uranium concentrations (0.3–4 ppm) of these sediments allow us to date smaller amounts of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest factor limiting the success of previous U-Th dating efforts in other lake basins. Despite this advantage, the dates from 174 analyses on 55 bulk carbonate samples reveal significant scatter that cannot be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and δ234Uinitial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. Alongside simulations of the isotopic evolution of our samples, we find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. Examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that geologic context is equally as important as the accuracy and precision of analytical measurements. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junín is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates.

Use of Arcellinida (testate lobose amoebae) arsenic tolerance limits as a novel tool for biomonitoring arsenic contamination in lakes

Abstract Arsenic (As) contamination from legacy gold mining in subarctic Canada poses an ongoing threat to lake biota. With climatic warming expected to increase As bioavailability in lake waters, developing tools for monitoring As variability becomes essential. Arcellinida (testate lobose amoebae) is an established group of lacustrine bioindicators that are sensitive to changes in environmental conditions and lacustrine ecological health. In this study, As-tolerance of Arcellinida (testate lobose amoebae) in lake sediments (n = 93) in subarctic Northwest Territories, Canada was investigated. Arcellinida assemblage dynamics were compared with the intra-lake As distribution to delineate the geospatial extent of legacy As contamination related to the former Giant Mine (Yellowknife). Cluster analysis revealed five Arcellinida assemblages that correlate strongly with ten variables (variance explained = 40.4%), with As (9.4%) and S1-carbon (labile organic matter; 8.9%) being the most important (p-value = 0.001, n = 84). Stressed assemblages characterized proximal lakes 750 ppm). The predictive capability of the Low-Moderate and Extreme tolerance groups is particularly strong, correlating with As concentrations in 66.6% (n = 20/30) of a test dataset. We propose that As influences the spatial distribution of the more nutrient-sensitive Arcellinida taxa (e.g., Cucurbitella tricuspis and Difflugia oblonga strain “oblonga”) through suppression of preferred microbial food sources. These findings, which indicate that there is a variable species-level arcellinidan response to As contamination, showcases the potential of using the group as a reliable tool for inferring historical variability in As concentrations in impacted lakes, not possible using As itself due to the redox driven sensitivity of the metalloid to post-depositional remobilization. Arcellinida can also provide insight into the overall impact of As contamination on the ecological health of lakes, a metric not readily captured using instrumental analyses. Lakes with As-stressed arcellinidan faunas and high As concentrations may then be targeted for further As speciation analysis to provide additional information for risk assessment.

The Alba Field, Block 16/26a, UK North Sea

AbstractThe Alba Field is a relatively heavy oil accumulation lying in an Eocene deep-water channel complex in Block 16/26a of the Central North Sea. With an estimated 880 MMbbl in place, the reservoir is characterized by thick, high net/gross sands with excellent reservoir properties and rock physics favourable for seismic property detection. The field has been developed by horizontal production wells, with pressure support provided by seawater injectors. After 24 years of production, more than 427 MMbbl have been recovered.Over the course of the development, the results of development drilling and improved reservoir imaging from seismic have revealed greater reservoir complexity than anticipated at sanction. The highly irregular reservoir geometry is likely to reflect the internal stacking patterns of channel elements within the channel complex that are locally overprinted by post-depositional remobilization. This increased reservoir complexity has required more wells to effectively drain the expected volumes. Despite this, recovery has exceeded estimates from the initial field development plan, reflecting an extremely efficient waterflood. 4D seismic spectacularly images extensive sweep away from injectors and excellent reservoir connectivity. Throughout the development, the application of seismic technologies has been a key enabler for effective reservoir management and, looking forward, maximizing value.

The Mariner Field, Block 9/11a, UK North Sea

AbstractThe Mariner Field is located on the East Shetland Platform in UK Block 9/11, 320 km to the NE of Aberdeen. The 9/11-1 well was drilled in 1981, discovering oil in the Paleocene Heimdal Sandstone Member and the Paleocene Maureen Formation.The deeper Maureen Formation was deposited on a submarine slope as canyon fill, and overspill canyon sands and debrites. The Heimdal Sandstone Member was deposited above the Maureen Formation on the same inherited, but younger, submarine slope system as deep-water slope channels, and is encased within suspension-deposited Lista Formation claystones. These slope channel sands have undergone post-depositional remobilization and sand-injection processes, leading to the complex sandstone geometries identified and mapped on seismic data.During a 36-year period, 28 exploration, appraisal and production test wells were drilled in UK Block 9/11, but only now is the field finally undergoing development. The long interval between discovery and development is due to several key issues, including the fact that development of heavy oil is technically and commercially challenging, and that the seismic imaging of the Heimdal Sandstone Member has been difficult or not possible until recently. These challenges are being overcome and the Mariner Field started producing in August 2019.

Geochemical footprint of the Millennium unconformity-type uranium deposit, Canada: implications for vectoring new targets

The Millennium deposit, a c. 650-m deep monomineralic uranium deposit, is located in the southeastern Athabasca Basin, Saskatchewan, a region containing numerous high-grade unconformity-type U deposits. 3D modelling of the whole-rock lithogeochemistry of sandstones above the deposit reveals a distinct footprint with select major and trace elements showing increased concentrations towards mineralization. Molar Mg/K ratios increase from background levels 10 km north of the deposit, along the B1 conductive trend, whereas Mo, Co, Rb, and Ga exhibit elevated concentrations immediately above the deposit, extending vertically from the unconformity to surface. Lead, Ag, Bi, Sb, REE, and Y exhibit elevated concentrations up to 650 m above the main mineralized body. Increasing Mg/K values indicate the transition from diagenetic to hydrothermal alteration with shifts from illitic (K-dominant) to chloritic and dravitic (Mg-dominant) alteration, with the latter increasing more proximal to mineralization. Trace element enrichment patterns highlight that fractures and faults were conduits for fluid flow from the basement into the basin, both during ore formation and through tectonically driven post-depositional remobilization. Key indicators such as molar element ratios (Mg-K-Al) and trace elements related to redox reactions provide scalable vectors at the Millennium deposit that are likely applicable to similar unconformity-type U deposits elsewhere.

Hypoxia in the Holocene Baltic Sea: Comparing modern versus past intervals using sedimentary trace metals

Anthropogenic nutrient input has caused a rapid expansion of bottom water hypoxia in the Baltic Sea over the past century. Two earlier intervals of widespread hypoxia, coinciding with the Holocene Thermal Maximum (HTMHI; 8–4 ka before present; BP) and the Medieval Climate Anomaly (MCAHI; ~1200–750 years BP), have been identified from Baltic Sea sediments. Here we present sediment records from two sites in the Baltic Sea, and compare the trace metal (As, Ba, Cd, Cu, Mo, Ni, Pb, Re, Sb, Tl, U, V, Zn) enrichments during all three hypoxic intervals. Distinct differences are observed between the intervals and the various elements, highlighting the much stronger perturbation of trace metal cycles during the modern hypoxic interval. Both Mo and U show a strong correlation with Corg and very high absolute concentrations, indicative of frequently euxinic bottom waters during hypoxic intervals. During the modern hypoxic interval (ModernHI) comparatively less Mo is sequestered relative to Corg than in earlier intervals. This suggests partial drawdown of the water column Mo inventory in the modern water column due to persistent euxinia and only partial replenishment of Mo through North Sea inflows. Molybdenum contents in modern sediments are likely also affected by the recent slowdown in input of Mo in association with deposition of Fe and Mn oxides. Strong enrichments of U in recent sediments confirm that the ModernHI is more intense than past intervals. These results suggest that U is a more reliable indicator for the intensity of bottom water deoxygenation in the Baltic Sea than Mo. Sedimentary Re enrichment commences under mildly reducing conditions, but this element is not further enriched under more reducing conditions. Enrichments of V are relatively minor for the MCAHI and ModernHI, possibly due to strong reservoir effects on V in the water column, indicating that V is unreliable as an indicator for the intensity of bottom water hypoxia in this setting. Furthermore, Ba profiles are strongly influenced by post-depositional remobilization throughout the Holocene. The strong relationship between Corg and Ni, Tl and particularly Cu suggests that these trace metals can be used to reconstruct the Corg flux into the sediments. Profiles of As, Sb and Cd and especially Pb and Zn are strongly influenced by anthropogenic pollution.

Iridium profiles and delivery across the Cretaceous/Paleogene boundary

We examined iridium (Ir) anomalies at the Cretaceous/Paleogene (K/Pg) boundary in siliciclastic shallow marine cores of the New Jersey Coastal Plain, USA, that were deposited at an intermediate distance (∼2500 km) from the Chicxulub, Mexico crater. Although closely spaced and generally biostratigraphically complete, the cores show heterogeneity in terms of preservation of the ejecta layers, maximum concentration of Ir measured (∼0.1–2.4 ppb), and total thickness of the Ir-enriched interval (11–119 cm). We analyzed the shape of the Ir profiles with a Lagrangian particle-tracking model of sediment mixing. Fits between the mixing model and measured Ir profiles, as well as visible burrows in the cores, show that the shape of the Ir profiles was determined primarily by sediment mixing via bioturbation. In contrast, Tighe Park 1 and Bass River cores show post-depositional remobilization of Ir by geochemical processes. There is a strong inverse relationship between the maximum concentration of Ir measured and the thickness of the sediments over which Ir is spread. We show that the depth-integrated Ir inventory is similar in the majority of the cores, indicating that the total Ir delivery at time of the K/Pg event was spatially homogeneous over this region. Though delivered through a near-instantaneous source, stratospheric dispersal, and settling, our study shows that non-uniform Ir profiles develop due to changes in the regional delivery and post-depositional modification by bioturbation and geochemical processes.

Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

The Yangtze Platform in South China is considered a key site for studying Neoproterozoic ocean oxygenation. Comprehensive Cr concentration and isotopic signatures are presented from cap dolostones of the Doushantuo Formation from a deep water and a shallow marine carbonate section, that have previously been used to constrain the redox evolution of Ediacaran seawater in space and time employing major and trace element abundances as well as Sr-, O- and C-isotope compositions. The majority of the studied samples yield elevated Cr concentrations and negatively fractionated δ53Cr signals due to (i) detrital contamination consistent with Cr signatures of other post-Marinoan cap dolostones, (ii) diagenetic carbonate alteration and post-depositional remobilization of Cr. Al concentrations were used as indicator of detrital contamination and to calculate authigenic Cr concentration and isotope signatures when the detrital contribution of Cr was <40%, which generally resulted in slightly lower Cr concentrations and slightly higher Cr isotopic signals in these samples. Diagenetic alteration of a majority of the studied cap dolostones is evident from low Sr concentrations and light δ18Ocarb signals that correlate with low Cr concentrations and negatively fractionated δ53Cr values. Post-depositional remobilization of Cr results in the loss of heavy Cr isotopes and consequently light Cr isotopes are retained during precipitation of secondary carbonates. However, some samples might still show pristine Ediacaran seawater signals. Two samples pertaining to the deep water cap dolostone section at Yanwutan and one sample pertaining to the shallow water section at Jijiawan yield slightly positively fractionated δ53Cr signals, particularly when corrected to presumably authigenic signals (δ53Crauth). These values are consistent with those from other Neoproterozoic carbonate sections deposited in an oxygenated environment. This short sequence of positively fractionated δ53Crauth is concurrent with a cap dolostone 87Sr/86Sr signal close to Ediacaran seawater values with relatively high Sr and Cr concentrations, with decreased detrital contamination and with a pronounced negative Ce anomaly. They indicate either light pulses of enhanced oxidative weathering or, inferred from a few least-altered samples, prevailing oxic conditions during cap dolostone deposition. The Cr isotopic composition of Ediacaran cap dolostones pertaining to the Doushantuo Formation reveals fine-scale fluctuations in post-glacial detrital contamination and diagenesis balanced against a clear signal of oxidative weathering. Cr isotopes have the potential to further enhance our understanding of Neoproterozoic weathering regimes and past ocean oxygenation, however, before the Cr isotope system can be faithfully applied to delineate changes in redox conditions recorded in ancient marine carbonates, possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration.

Continental margin molybdenum isotope signatures from the early Eocene

Molybdenum (Mo) isotope compositions of marine sedimentary deposits that span the Paleocene–Eocene Thermal Maximum (PETM, approximately 56 Ma) are presented from two Tethys Ocean sites (Guru Fatima, Tajikistan and Kheu River, Georgia). Local redox indicators suggest that both locations experienced anoxia and intervals of euxinia (with hydrogen sulphide present in seawater) during the early part of the PETM. However, the Mo-isotope compositions (expressed as δMo98/95) for each site differ significantly. Local redox conditions were very stable at Guru Fatima, where δMo98/95 reached a maximum of 0.96‰, which is ∼0.7‰ lower than for early Eocene seawater as recently inferred from euxinic Arctic Ocean deposits. This observation supports the argument that a ∼0.7‰ difference between seawater and anoxic continental margin marine sediments documented at the present day might also be found in the paleo-record. In contrast, local redox conditions at Kheu River were not stable over the study interval, and δMo98/95 were much lower than at Guru Fatima. The low δMo98/95 values at Kheu River are attributed to the influence of post-depositional remobilization of Mo by repeated adsorption and dissolution of Fe–Mn oxides during brief intervals of bottom water oxidation. The data highlight the importance of obtaining multi-proxy constraints on both local redox and paleoceanographic setting before the Mo-isotope compositions of sedimentary deposits can be interpreted accurately.

The Legacy of Arsenic Contamination from Mining and Processing Refractory Gold Ore at Giant Mine, Yellowknife, Northwest Territories, Canada

The case of the Giant mine illustrates how a large, long-lived Au mine has resulted in a complex regional legacy of As contamination and an estimated remediation cost of almost one billion Canadian dollars (AANDC 2012). The mine, located a few km north of the city of Yellowknife on the shore of Great Slave Lake (Figs. 1, 2) produced more than 7 million troy ounces of Au (approximately 220 tonnes) from a largely underground operation. Giant mine was the largest producer in the Yellowknife greenstone belt, which produced more than12 million troy ounces (~370 tonnes) in total (Bullen and Robb 2006). Arsenopyrite-bearing Au ore was roasted from 1949 to 1999 as a pretreatment for cyanidation (Fig. 3a). Poor or nonexistent emission controls in the early years resulted in the release of an estimated 20,000 tonnes of roaster-generated As2O3 to the surrounding environment through stack emissions (CPHA 1977; Wrye 2008). Over the lifetime of the mine, however, most of the As2O3 (237,000 tonnes) was stored in underground chambers (Fig. 3b) and is a now an ongoing source of As to groundwater and surface water (INAC 2007; Jamieson et al. 2013). Other roaster products include As-bearing maghemite and hematite (calcine) were deposited with tailings and re-mobilized into creek and lake sediments. Under reducing conditions, post-depositional remobilization of As associated with roaster-generated Fe oxides results in release of As to sediment pore water and reprecipitation of some As as a sulfide phase (Fawcett and Jamieson 2011). However, As(III) in maghemite and As2O3 persists in the oxidizing conditions of near-surface tailings and soils (Walker et al. 2005; Jamieson et al. 2013). Ore roasting increases the solubility, toxicity, and bioaccessibility of As by converting sulfide-hosted As to oxide-hosted As. At Giant, …

Heavy mineral sorting in downwards injected Palaeocene sandstone, Siri Canyon, Danish North Sea

Post-depositional remobilization and injection of sand are often seen in deep-water clastic systems and have been recently recognised as a significant modifier of deep-water sandstone geometry. Large scale injectite complexes have been interpreted from borehole data in the Palaeocene Siri Canyon near the Danish Central Graben of the North Sea hydrocarbon province. The emplacement of large scale injectite complexes has been commonly attributed to seismic activity and consequent sand liquefaction. However, due to very small differences in textural and compositional properties, and the lack of depositional structures in deep-water sandstones, the distinction between “in situ” and injected or remobilized sandstones is often ambiguous. Large scale heavy mineral sorting (in 10 m thick units) is observed in several reservoir units in the Siri Canyon and has been interpreted to represent the depositional sorting. In this study we describe an example of effective shear-zone sorting of heavy minerals in a thin downwards injected sandstone dyke which was encountered in one of the cores in the Cecilie Field, Siri Canyon. Differences in sorting pattern of heavy minerals are suggested as a tool for petrographic/geochemical distinction between “in situ” sandstones and their related injectites, especially where primary sedimentary structures are removed by fluidization or minor remobilization.

Present environment of Dam Lake Sambe, southwestern Japan: a geochemical study of bottom sediments

Geochemical analyses of lakebed and core sediments from Lake Sambe on the outskirts of Oda City in Shimane prefecture in southwestern Japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements. The lake water showed a stratified condition with respect to dissolved O2, and As, Fe, and Mn concentrations in the bottom layers which increased in the summer. The chemical composition of the sediments, as measured by X-ray fluorescence, included major and trace elements (P, Ca, Sc, Ti, V, Cr, Fe, Ni, Cu, Zn, As, Sr, Zr, Pb, and Th), and total sulfur (TS). Elevated values of As, Zn, V, Fe, P, and TS were present in several layers of the upper cores (from 0 to 5 cm) and other surface sediments. Increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments, fine-grained organic rich sediments, and post-depositional diagenetic remobilization. Moreover, correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto Fe (oxy)hydroxides, whereas correlations with sulfur indicate that they were precipitated as Fe-sulfides. The average abundances of As, Pb, Zn, and Cu exceeded the lowest effect level and Interim Sediment Quality Guideline values that the New York State Department of Environmental Conservation and the Canadian Council of Ministers of the Environment determined to have moderate impact on aquatic organisms. In addition, concentrations of As and Zn exceeded the Coastal Ocean Sediment Database threshold value, indicating potentially toxic levels. Therefore, the presence of trace metals in the lake sediments may result in adverse effects on biota health.

A non-magmatic iron projectile for the Gardnos impact event

The goal of this study is to identify the type of projectile responsible for the formation of the late Precambrian Gardnos impact structure in Norway. Fifteen impactite samples, predominantly impact breccias and suevites from the central and northeastern part of the structure, were analyzed for platinum group elements (PGE) and Au using nickel-sulfide fire assay combined with inductively coupled plasma mass spectrometry (ICP-MS). Major and trace elements were measured in the same samples using X-ray fluorescence (XRF). In addition, the concentrations of siderophile elements Ni, Cr, and Co were determined by ICP-MS after acid digestion. The samples collected at the contact between suevite and the sedimentary infill yielded the highest PGE concentrations (Ir = 1.926 ng/g, Ru = 3.494 ng/g, Pt = 4.716 ng/g, Rh = 0.766 ng/g, Pd = 2.842 ng/g for GC6). The CI-normalized PGE patterns are characterized by Ru and Rh enrichments suggesting a non-chondritic impactor. Concentration plots of the different PGE display an excellent correlation ( R > 0.99), indicative of a single source for the PGE enrichment. The Ni/Cr ratio of the Gardnos impactor (2.56 ± 0.20) agrees with that of chondrites (2 to 7), whereas Ir is depleted relative to Ni in this projectile (Ni/Ir ratio of 92 000 ± 8000 compared to an average Ni/Ir ratio of 23 150 ± 4250 for chondrites). There is no clear indication of selective post-depositional remobilization of the characteristic highly siderophile elements. The Ni/Ir and Cr/Ir data combined with the non-chondritic PGE ratios probably indicate a differentiated projectile. Based on (1) the similarity of the inter-element ratios of the impactor with the iron phase of non-magmatic iron meteorites and (2) the presence of characteristics of both chondrites and iron meteorites (Ni/Cr and Ni/Ir ratios), an IA or IIIC non-magmatic iron meteorite is a very plausible impactor.

Development of Rheological Heterogeneity in Clay-Rich High-Density Turbidity Currents: Aptian Britannia Sandstone Member, U.K. Continental Shelf

Abstract Upper Aptian deep-water sandstones in the Witch Ground Graben of the North Sea have been extensively drilled and cored in the Britannia Field. The upper part of the Britannia Sandstone Member comprises a series of thick (~ 3.8 to 16.5 m) sandstone beds that thin and pinch out against a paleoslope formed by the Fladen Ground Spur to the north. Deposition was from unusually large-volume flows that carpeted the deep basin floor and base of the lateral slope with sand. The flows were able to heal significant seafloor topography produced during periods of local slope collapse. The recognition of widespread early remobilization is new and results in a revised event-bed correlation scheme and casts doubt on relationships between spatial patterns of grain-size distribution, facies distribution, and paleoflow used to underpin previous models of deposit emplacement. The beds are here interpreted as the deposits of large-volume, axially dispersing turbidity currents, probably some 70 km from source, and within 25 to 35 km of their run out limit to the south-southeast of the Britannia Field. Single event beds contain a wide range of sand-rich facies (deposited progressively from turbulent suspension), and clay-rich facies, previously termed slurry deposits (deposited in cycles of en masse deposition from dilute laminar flow alternating with phases of suspension fallout from turbulent flow). Along the northern fringe of the system clay-rich facies dominate, but pass southward over six or seven km into progressively cleaner sand-rich facies. This trend in facies distribution is at a high angle to the inferred SE-dispersing paleoflow and is thought to reflect lateral suppression of flow turbulence from flow axes to flow margins controlled by relative changes in flow depth. In addition, the vertical arrangement of facies within beds always shows sand-rich facies at bed bases, passing upward often via distinctive banded facies into clay-rich facies, indicating a temporal flow transformation from dominantly turbulent to dominantly laminar, with an intermediate phase of alternating laminar and turbulent flow. This temporal transition in inferred flow conditions is similar to the inferred on-axis to off-axis spatial transition in conditions. Thus single clay-rich sediment gravity flows can evolve in both time and space to develop different rheological zones, of turbulent, transitional, and laminar character. In addition, certain debritic, clay-rich facies are now interpreted to result from post-depositional remobilization from off the confining slope to the north, and must be differentiated from the primary suite of depositional facies.

Flocculation and the loss of sediment from the Po River plume

In October 2000, a 100-year flood event in the Po River resulted in the formation of a fine-grained sediment deposit extending up to 10 km from the river mouth. Soon after this event, and for a subsequent period of 2 years, box cores were collected on a grid of stations off of the Po Delta to observe the evolution of the flood deposit. Using a process-based parameterization of the disaggregated inorganic grain size distribution, the evolution of the surficial sediment on the Po shelf since the 2000 flood has been interpreted in the context of particle flocculation dynamics. This method produces estimates of floc limit, the diameter at which the flux of single grains to the seabed equals the flux of flocs, and floc mass fraction, the amount of material deposited as flocs. Floc limit depends on the extent of flocculation in suspension, and floc mass fraction describes the extent of flocculation in the sediment. Immediately after the flood, these two parameters were high at stations located beneath the path of the flood plume as observed in satellite images. The occurrence of a highly flocculated deposit below the path of the plume leads to two hypotheses: (1) the high sediment concentration in the river flood plume produced extensive sediment flocculation in the plume and (2) post-depositional remobilization of sediment delivered from the plume to the seabed was limited in the relatively low-energy environment of the Po prodelta. Floc limit and floc mass fraction estimated from bottom sediment sampling 3 and 10 months after the initial sampling were lower, indicating that during normal discharge, flocculated fine-grained sediment from the Po River settles close to the mouth, leaving only a small amount of material in suspension in the plume for direct deposition onto the prodelta. These findings are consistent with laboratory studies of suspended sediment that show that sediment concentration and turbulent energy exert dominant control on the extent of flocculation and the loss of sediment from suspension.

A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic hazard assessment

A continuous ∼5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

The role of prokaryotes in subsurface weathering of hydrothermal sediments: A combined geochemical and microbiological investigation

A detailed geochemical and microbiological study of a ∼2 m sediment core from the inactive Alvin mounds within the TAG hydrothermal field was conducted to examine, for the first time, the role of prokaryotes in subsurface weathering of hydrothermal sediments. Results show that there has been substantial post-depositional remobilisation of metal species and diagenetic overprinting of the original high-temperature hydrothermal minerals, and aspects have involved prokaryotic processes. Prokaryotic enumeration demonstrates the presence of a population smaller than the average for deep sea sediments, probably due to the low organic carbon content, but not inhibited by (and hence adapted to) the metal rich environment. There was a small but significant increase in population size associated with the active redox boundary in an upper metal sulphide layer (50–70 cm) around which active metal remobilisation was concentrated (Cu, Au, Cd, Ag, U, Zn and Zn). Hence, subsurface prokaryotes were potentially obtaining energy from metal metabolism in this near surface zone. Close association of numbers of culturable Mn and Fe reducing prokaryotes with subsurface Fe 2+ and Mn 2+ pore water profiles suggested active prokaryotic metal reduction at depth in core CD102/43 (to ∼175 cm). In addition, a prokaryotic mechanism, which is associated with bacterial sulphate reduction, is invoked to explain the U enrichment on pyrite surfaces and Zn and Pb remobilisation in the upper sediment. Although prokaryotic populations are present throughout this metalliferous sediment, thermodynamic calculations indicated that the inferred low pH of pore waters and the suboxic/anoxic conditions limits the potential energy available from Fe(II) oxidation, which may restrict prokaryotic chemolithotrophic biomass. This suggests that intense prokaryotic Fe oxidation and weathering of seafloor massive sulphide deposits may be restricted to the upper portion of the deposit that is influenced by near neutral pH and oxic seawater unless there is significant subsurface fluid flow.

Managing reservoir uncertainty after five years of field life: Britannia Field

The Britannia Field, located 210 km northeast of Aberdeen, is the largest Lower Cretaceous condensate gas accumulation in the North Sea. The field spans four UK blocks (250km 2 ) and is a combination structural/stratigraphic trap. Assessing the remaining uncertainty in a field the size of Britannia is crucial to future development planning scenarios. After five years of field life, it has proved vital to take the ‘reservoir pulse’ and to evaluate the key reservoir parameters that impact the future development of the field. The Britannia reservoir consists of deep-water mass-flow sandstones, ranging from high density ‘clean’ turbidites to non-reservoir facies consisting of ‘mixed-slurry’ and debris flows. Post-depositional remobilization and slumping have modified the original depositional fabric; however, a wealth of core data (over 20000 ft), along with 63 logged wells and dynamic production data, have allowed refining of the flow unit definition within the reservoir. A rigorous re-correlation study based on the integration of biostratigraphy, chemostratigraphy and pressure data has defined an overall sheet-like reservoir architecture, with some units exhibiting a more complex channel-like geometry (Zones 30/40). The Britannia reservoir teamhas capitalized on recent advances in reservoir modelling technology, including a more rigorous approach to uncertainty analysis in both the static and dynamic realm. The results from a dynamic uncertainty study have provided key information on the range and magnitude of key petrophysical parameters and their impact on reservoir performance and ultimate recovery. Weighing the impact of a deterministic versus stochastic approach has also been a critical factor in the facies distribution. The major uncertainties in the dynamic reservoir model that impact reserves and recovery factor are (1) original gas in place, (2) fault and/or stratigraphic transmissibility ‘baffles’, (3) effective permeability, and, (4) condensate banking. In the static geological model, Jacta – a Gocad uncertainty module – has led to over 300 realizations of the full field model. In the dynamic realm, experimental design was used to monitor reservoir performance by capturing the variables having largest impact on dynamic flow behaviour and gas recovery.