Sediment and ice characteristics at a buried ice-wedge system at Barrow

Barrow, the northernmost point in Alaska, is one of the most intensively studied areas in the Arctic. However, paleoenvironmental evidence is limited for northern Alaska for the Lateglacial-Holocene transition. For a regional paleoenvironmental reconstruction, we investigated a permafrost ice-wedge tunnel near Barrow, Alaska. The studied site was first excavated in the early 1960s and intercepts a buried ice-wedge system at 3-6 m depth below the surface. A multi-methodological approach was applied to this buried ice-wedge system and the enclosing sediments, which in their combination, give new insight into the Late Quaternary environmental and climate history. Results of geochronological, sedimentological, cryolithological, paleoecological, isotope geochemical and microbiological studies reflect different stages of mid to late Wisconsin (MW to LW), Allerod (AD), Younger Dryas (YD), Preboreal (PB), and Late Holocene paleoenvironmental evolution. The LW age of the site is indicated by AMS dates in the surrounding sediments of 21.7 kyr BP at the lateral contact of the ice-wedge system as well as 39.5 kyr BP below the ice-wedge system. It is only recently that in this region, stable isotope techniques have been employed, i.e. to characterize different types of ground ice. The stable isotope record (oxygen: d18O; hydrogen: dD) of two intersecting ice wedges suggests different phases of the northern Alaskan climate history from AD to PB, with radiocarbon dates from 12.4 to 9.9 kyr BP (ranging from 14.8 to 10.6 kyr cal BP). Stable isotope geochemistry of ice wedges reveals winter temperature variations of the Lateglacial-Holocene transition including a prominent YD cold period, clearly separated from the warmer AD and PB phases. YD is only weakly developed in summer temperature indicators (such as pollen) for the northern Alaska area, and by consequence, the YD cold stadial was here especially related to the winter season. This highlights that the combination of winter and summer indicators comprehensively describes the seasonality of climate-relevant processes in discrete time intervals. The stable isotope record for the Barrow buried ice-wedge system documents for the first time winter climate change at the Lateglacial-Holocene transition continuously and at relatively high (likely centennial) resolution.

Geochemistry and stable isotope composition of fresh alkaline porphyry copper tailings: Implications on sources and mobility of elements during transport and early stages of deposition

Prevention of acid mine drainage (AMD) in sulfide-containing tailings requires the identification of the geochemical processes and element pathways in the early stages of tailing deposition. However, analyses of recently deposited tailings in active tailings impoundments are scarce because mineralogical changes occur near the detection limits of many assays. This study shows that a detailed geochemical study which includes stable isotopes of water (delta H-2, delta O-18), dissolved sulfates (delta S-34, delta O-18) and hydrochernical parameter (pH, Eh, DOC, major and trace elements) from tailings samples taken at different depths in rainy and dry seasons allows the understanding of weathering (oxidation, dissolution, sorption, and desorption), water and element pathways, and mixing processes in active tailings impoundments. Fresh alkaline tailings (pH 9.2-10.2) from the Cu-Mo porphyry deposit in El Teniente, Chile had low carbonate (0.8-1.1 Wt-% CaCO3 equivalent) and sulfide concentrations (0.8-1.3 wt.%, mainly as pyrite). In the alkaline tailings water, Mo and Cu (up to 3.9 mg/L Mo and 0.016 mg/L Cu) were mobile as MoO42- and Cu (OH)(2)(0). During the flotation, tailings water reached equilibrium with gypsum (up to 738 mg/L Ca and 1765 mg/ L SO4). The delta S-34 VS. delta O-18 covariations of dissolved sulfate (2.3 to 4.5% delta S-34 and 4.1 to 6.0 % delta O-18) revealed the sulfate sources: the dissolution of primary sulfates (12.0 to 13.2%. delta S-34, 7.4 to 10.9%.delta O-18) and oxidation of primary sulfides (-6.7 to 1.7%. delta S-34). Sedimented tailings in the tailings impoundment can be divided into three layers with different water sources, element pathways, and geochemical processes. The deeper sediments (> 1 m depth) were infiltrated by catchment water, which partly replaced the original tailings water, especially during the winter season. This may have resulted in the change from alkaline to near-neutral pH and towards lower concentrations of most dissolved elements. The neutral pH and high DOC (up to 99.4 mg/L C) of the catchment water mobilized Cu (up to 0.25 mg/L) due to formation of organic Cu complexes; and Zn (up to 130 mg/L) due to dissolution of Zn oxides and desorption). At I m depth, tailings pore water obtained during the winter season was chemically and isotopically similar to fresh tailings water (pH 9.8-10.6, 26.7-35.5 mg/L Cl, 2.3-6.0 mg/L Mo). During the summer, a vadose zone evolved locally and temporarily up to 1.2 m depth. resulting in a higher concentration of dissolved solids in the pore water due to evaporation. During periodical new deposition of fresh tailings, the geochemistry of the surface layer was geochemically similar to fresh tailings. In periods without deposition, sulfide oxidation was suggested by decreasing pH (7.7-9.5), enrichment of MoO42- and SO42-, and changes in the isotopic composition of dissolved sulfates. Further enrichment for Na, K, Cl, SO4, Mg, Cu, and Mo (up to 23.8 mg/L Mo) resulted from capillary transport towards the surface followed by evaporation and the precipitation of highly soluble efflorescent salts (e.g., mirabilite, syngenite) at the tailing surface during summer. (C) 2008 Elsevier B.V. All rights reserved.

Controls on ostracod valve geochemistry: Part 2. Carbon and oxygen isotope compositions

The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in 18O of more than 30/00 relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in 18O for these species of 1.7 to 2.30/00. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens' δ13C values are mainly controlled by seasonal variations in δ13CDIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope 'vital effects' in ostracods. Two mechanisms that might enrich the 18O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation.

Silver-base metal epithermal vein and listwaenite types of deposit Crnac, Rogozna Mts., Kosovo. Part I: Ore mineral geochemistry and sulfur isotope study

Pb-Zn-Ag vein and listwaenite types of mineralization in Crnac deposit, Western Vardar zone, were deposited within several stages: (i) the pre-ore stage comprises pyrite, arsenopyrite, pyrrhotite, quartz, kaolinite and is followed by magnetite-pyrite; (ii) the syn-ore stage is composed of galena, sphalerite, tetrahedrite and stefanite; and (iii) the post-ore stage is composed of carbonates, pyrite, arsenopyrite and minor galena. The vein type mineralization is hosted by Jurassic amphibolites and veins terminate within overlying serpentinites. Mineralized listwaenites are developed along the serpentinite-amphibolite interface. The reserves are estimated to 1.7 Mt of ore containing in average 7.6% lead, 2.9% zinc, and 102 g/t silver. Sulfides from the pre- and syn-mineralization assemblage of the vein- and listwaenite-types of mineralization from the Crnac Pb-Zn-Ag deposit have been analyzed using microprobe, crush-leachates and sulfur isotopes. The pre-ore assemblage precipitated under high sulfur fugacities (f(S(2)) = 10(-8)-10(-6) bar) from temperatures ranging between 350 degrees C and 380 degrees C. Most likely water-rock reactions, boiling and/or increase of pH caused an increase of delta(34)S of pyrite toward upper levels within the deposit. The decomposition of pre-ore pyrrhotite to a pyrite-magnetite mixture occurred at a fugacity of sulfur from f(S(2)) = 8.7 x 10(-10) to 9.6 x 10(-9) bar and fugacity of oxygen from f(O(2)) = 2.4 x 10(-30) to 3.1 x 10(-28) bars, indicating a contribution of an oxidizing fluid, i.e. meteoric water during pre-ore stages of hydrothermal activity. The crystallization temperatures obtained by the sphalerite-galena isotope geothermometer range from 230 to 310 degrees C. The delta(34)S values of pre- and syn-ore sulfides (pyrite, galena, sphalerite, delta(34)S = 0.3-5.9 parts per thousand) point to magmatic sulfur. Values of delta(34)S of galena and sphalerite are decreasing upwards due to precipitation of early formed sulfide minerals. Post-ore assemblage precipitated at temperature below 190 degrees C. Based on data presented above, we assume two fluid sources: (i) a magmatic source, supported by sulfur isotopic compositions within pre- and syn-ore minerals and a high mol% of fluorine found within pre- and syn-ore leachates, and (ii) a meteoric source, deduced by coincident pyrite-magnetite intergrowth, sulfur isotopic trends within syn-ore minerals and decrease of crystallization temperatures from the pre-ore stage (380-350 degrees C), towards the syn-ore (310-215 degrees C) and post-ore stages (<190 degrees C). Post-ore fluids are Na-Ca-Mg-K-Li chlorine rich and were modified via water-rock reactions. Simple mineral assemblage and sphalerite composition range from 1.5 to 10.1 mol% of FeS catalog Crnac to a group of intermediate sulfidation epithermal deposit. (C) 2011 Elsevier B.V. All rights reserved.

Genesis of the Jurassic carbonate-hosted Pb-Zn deposits of Jebel Ressas (North-eastern Tunisia): Evidence from mineralogy, petrography and trace metal contents and isotope (O, C, S, Pb) geochemistry

The Jebel Ressas Pb-Zn deposits in North-Eastern Tunisia occur mainly as open-space fillings (lodes, tectonic breccia cements) in bioclastic limestones of the Upper Jurassic Ressas Formation and along the contact of this formation with Triassic rocks. The galena-sphalerite association and their alteration products (cerussite, hemimorphite, hydrozincite) are set within a calcite gangue. The Triassic rocks exhibit enrichments in trace metals, namely Pb, Co and Cd enrichment in clays and Pb, Zn, Cd, Co and Cr enrichment in carbonates, suggesting that the Triassic rocks have interacted with the ore-bearing fluids associated with the Jebel Ressas Pb-Zn deposits. The delta(18)O content of calcite associated with the Pb-Zn mineralization suggests that it is likely to have precipitated from a fluid that was in equilibrium with the Triassic dolostones. The delta(34)S values in galenas from the Pb-Zn deposits range from -1.5 to +11.4%, with an average of 5.9% and standard deviation of 3.9%. These data imply mixing of thermochemically-reduced heavy sulfur carried in geothermal- and fault-stress-driven deep-seated source fluid with bacterially-reduced light sulfur carried in topography-driven meteoric fluid. Lead isotope ratios in galenas from the Pb-Zn deposits are homogenous and indicate a single upper crustal source of base-metals for these deposits. Synthesis of the geochemical data with geological data suggests that the base-metal mineralization at Jebel Ressas was formed during the Serravallian-Tortonian (or Middle-Late Miocene) Alpine compressional tectonics.

Aqueous geochemistry and oxygen isotope compositions of acid mine drainage from the Río Tinto, SW Spain, highlight inconsistencies in current models

The Rio Tinto river in SW Spain is a classic example of acid mine drainage and the focus of an increasing amount of research including environmental geochemistry, extremophile microbiology and Mars-analogue studies. Its 5000-year mining legacy has resulted in a wide range of point inputs including spoil heaps and tunnels draining underground workings. The variety of inputs and importance of the river as a research site make it an ideal location for investigating sulphide oxidation mechanisms at the field scale. Mass balance calculations showed that pyrite oxidation accounts for over 93% of the dissolved sulphate derived from sulphide oxidation in the Rio Tinto point inputs. Oxygen isotopes in water and sulphate were analysed from a variety of drainage sources and displayed delta O-18((SO4-H2O)) values from 3.9 to 13.6 parts per thousand, indicating that different oxidation pathways occurred at different sites within the catchment. The most commonly used approach to interpreting field oxygen isotope data applies water and oxygen fractionation factors derived from laboratory experiments. We demonstrate that this approach cannot explain high delta O-18((SO4-H2O)) values in a manner that is consistent with recent models of pyrite and sulphoxyanion oxidation. In the Rio Tinto, high delta O-18((SO4-H2O)) values (11.2-13.6 parts per thousand) occur in concentrated (Fe = 172-829 mM), low pH (0.88-1.4), ferrous iron (68-91% of total Fe) waters and are most simply explained by a mechanism involving a dissolved sulphite intermediate, sulphite-water oxygen equilibrium exchange and finally sulphite oxidation to sulphate with O-2. In contrast, drainage from large waste blocks of acid volcanic tuff with pyritiferous veins also had low pH (1.7). but had a low delta O-18((SO4-H2O)) value of 4.0 parts per thousand and high concentrations of ferric iron (Fe(III) = 185 mM, total Fe = 186 mM), suggesting a pathway where ferric iron is the primary oxidant, water is the primary source of oxygen in the sulphate and where sulphate is released directly from the pyrite surface. However, problems remain with the sulphite-water oxygen exchange model and recommendations are therefore made for future experiments to refine our understanding of oxygen isotopes in pyrite oxidation. (C) 2009 Elsevier B.V. All rights reserved.

Geochemistry and oxygen and iron isotope ratios of mafic rocks

Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (d18O) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5 ± 0.4 per mil) and provide support for this hypothesis, as it is widely assumed that most variations in d18O are produced by near-surface low-temperature processes. Here we show a significant linear relationship between d18O and stable iron isotope ratios (d57Fe) in a suite of pristine eclogite xenoliths. The d18O values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between d18O and d57Fe is unlikely to be inherited from oceanic crust, as d57Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with d18O. It is proposed that the correlated d57Fe and d18O variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample d57Fe and Cr content and bulk sample and garnet d18O and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both d57Fe and d18O by melting processes and specifically the production of low-d18O signatures in mafic rocks implies that some of the isotopically light d18O values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components.

Sedimentology and geochemistry of Oxygen Isotope Stage 3 and Holocene sediments from Laguna Potrok Aike, Patagonia

Seismic reflection studies in the maar lake Laguna Potrok Aike (51°58? S, 70°23? W) revealed an erosional unconformity associated with a sub-aquatic lake-level terrace at a water depth of 30m. Radiocarbon-dated, multi-proxy sediment studies of a piston core from this location indicate that the sediment below this discontinuity has an age of 45kyr BP (Oxygen Isotope Stage 3), and was deposited during an interval of high lake level. In comparison to the Holocene section, geochemical indicators of this older part of the record either point towards a different sediment source or to a different transport mechanism for Oxygen Isotope Stage 3 sediments. Holocene sedimentation started again before 6790cal. yr BP, providing a sediment record of hydrological variability until the present. Geochemical and isotopic data indicate a fluctuating lake level until 5310cal. yr BP. During the late Holocene the lake level shows a receding tendency. Nevertheless, the lake level did not drop below the 30m terrace to create another unconformity. The geochemical characterization of volcanic ashes reveals evidence for previously unknown explosive activity of the Reclús and Mt. Burney volcanoes during Oxygen Isotope Stage 3.

Texture, geochemistry, isotope composition, petrography and epoch at DSDP Site 93-603

Abundant Fe-Mn carbonate concretions (mainly siderite, manganosiderite, and rhodochrosite) were found in the hemipelagic claystones of Site 603 on the eastern North American continental rise. They occur as nodules, micronodules, or carbonate-replaced burrow fills and layers at a subbottom depth of between ~ 120 (Pliocene) and 1160 m (Albian-Cenomanian). In general, the Fe-Mn carbonate concretions form from CO3- produced by the microbiological degradation of organic matter in the presence of abundant Fe + or Mn + and very low S- concentrations. However, there is also some evidence for diagenetic replacement of preexisting calcite by siderite. The carbon isotope composition of diagenetic Fe-Mn carbonate nodules is determined by CO2 reduction during methanogenesis. Carbonate nodules in Cretaceous sediments at sub-bottom depths of 1085 and 1160 m have distinctly lower d13C values (- 12.2 and - 12.9 per mil) than Neogene siderites, associated with abundant biogenic methane in the pore space (-8.9 to 1.7 per mil between 330 and 780 m depth). Since no isotopic zonation could be detected within individual nodules, we assume that the isotopic composition reflects more or less geochemical conditions at the present burial depth of the carbonate nodules. Carbonates did not precipitate within the zone of sulfate reduction (approximately 0.01 to 10 m), where all of the pyrite was formed. The oxygen isotope composition indicates precipitation from seawater-derived interstitial waters. The d18O values decrease with increasing burial depth from + 5.1 to - 1.2 per mil, suggesting successively higher temperatures during carbonate formation.

Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

Stable isotope and trace element geochemistry of carbonate sediments at DSDP Holes 87-577 and 6-47.2

Detailed analyses of well-preserved carbonate samples from across the Cretaceous/Tertiary boundary in Hole 577 have revealed a significant decline in the d13C values of calcareous nannoplankton from the Maestrichtian to the Danian Age accompanied by a substantial reduction in carbonate accumulation rates. Benthic foraminifers, however, do not exhibit a shift in carbon composition similar to that recorded by the calcareous nannoplankton, but actually increase slightly over the same time interval. These results are similar to the earlier findings at two North Pacific Deep Sea Drilling Project locations, Sites 47.2 and 465, and are considered to represent a dramatic decrease in oceanic phytoplankton production associated with the catastrophic Cretaceous/Tertiary boundary extinctions. In addition, the change in carbon composition of calcareous nannoplankton across the Cretaceous/Tertiary boundary at Hole 577 is accompanied by only minor changes in the oxygen isotope trends of both calcareous nannoplankton and benthic foraminifers, suggesting that temperature variations in the North Pacific from the late Maestrichtian to the early Danian Age were insignificant.

Geochemistry and stable carbon isotope composition of kerogen of DSDP Site 124-767

Celebes Basin sediments from Ocean Drilling Program Site 767 (Leg 124) containing both marine and terrestrial organic matter have been investigated through palynofacies and geochemical analyses. The main degradation processes affecting or having affected organic matter are recorded in the sedimentary column as shown by ammonium, phosphate and sulfate pore-water profiles, and by petrographic and geochemical analyses of sediments. In the upper part of the sedimentary section (down to 200 mbsf), the decrease of the ratio of total organic carbon to sulfur (TOC/S) with depth, generally related to the sulfate reduction process, is accompanied by an increase of framboidal pyrite content in the marine organic matter, and by an increasing amount of amorphous marine organic matter relative to the total organic matter. However, as the terrestrial organic input also varies with depth, dilution effects are superimposed on diagenesis. This continental supply affects the TOC/S ratio by increasing total organic carbon and decreasing the ability of the bulk organic matter to be metabolized through sulfate reduction. A positive relationship between the TOC/P ratio and the amount of degraded organic matter of marine origin clearly displays the effect of an organic source on the composition of the sediment. Each lithostratigraphic unit possesses its own characteristics in terms of composition and preservation of organic matter. The effects of diagenesis can only be appreciated within a single lithostratigraphic unit and mainly affect the less-resistant marine organic matter.

Geochemistry of tropical Atlantic sediments of Marine Isotope Stage 4

Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean (SO) to the low-latitude surface ocean and therefore have an important control on low-latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pCO2. This mechanism is known as the "silicic acid leakage hypothesis" (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage 4 (~60-70 ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development.