Down-core Trends Research Articles

HPLC approach for revealing age-related changes of aquatic dissolved organic matter in sediment core

Analytical method based on HPLC has been used to characterize aquatic dissolved organic matter (DOM) from sediment core of Lake Võrtsjärv, South- Estonia. High-performance size exclusion chromatography (HPSEC) as separation method was coupled with diode-array detection (DAD) and separated molecular fractions of DOM were subject to qualitative and semi-quantitative analysis. Qualitative analysis based on UV- spectra revealed the presence of proteins and humic constituents in separated high molecular weight fraction and aromatic constituents in low molecular weight fraction. Statistical data treatment methods enabled clustering sediment layers into 4 periods according to sediment depth and age. The upper 0-30 cm sediment DOM had statistically relevant differences in comparison to other periods as revealed by lower total peak and humic substances (HS) fraction areas, and molecular weights. Samples from 80-120 cm depth differed from others by decreased low molecular weight (LMW) fraction content. The observed down-core trends suggest polymerization of LMW organic constituents and increasing humification. The statistical analyses revealed that some chromatographic and spectrometric parameters can be used to differentiate between sediment layers and to evaluate environmental changes.

Effects of carbon dioxide sequestration on California margin deep-sea foraminiferal assemblages

Deep-sea sequestration of CO 2 is being considered as a possible mitigation tool to decrease atmospheric CO 2 concentrations and its associated negative effects. This study investigated potential effects of liquid carbon dioxide (CO 2) injection on deep-sea foraminiferal assemblages. Foraminifera are ideal for this ecological impact investigation because of differing test composition (calcareous and non-calcareous) and thickness, and diverse epifaunal and infaunal depth preferences. The experiment was conducted on August–September 2003, at 3600 m off the coast of Monterey Bay, California, aboard the R/V Western Flyer using the ROV Tiburon. The pH of the site was monitored throughout the experiment. Sediment push-cores were collected (both from the experimental and control sites) and stained to distinguish live (stained) from dead (unstained) individuals. Effects of CO 2 injection on assemblages have been tracked both vertically (to 10 cm depth below sea floor) and horizontally (up to 10 m from CO 2 injection sites), as well as between live and dead individuals. Within corrals (containing the injected CO 2) and their underlying sediments, severe pH changes (near 4.0 units) were recorded. This compares with a record of small average reductions in ocean pH (− 0.05 units) combined with large episodic excursions (− 1.7 units) over the experimental area due to the injection of CO 2. Exposure to this gradient of low pH caused increased mortality and dissolution of calcareous forms within corrals, as far as 5 m from the injection site, and to at least 10 cm depth in the sediments. This experiment revealed several major effects of CO 2 injection on foraminiferal assemblages in surficial sediments: 1) total number of foraminifera in a sample decreases; 2) foraminiferal species richness decreases in both stained and unstained specimens; and 3) relative percentage of stained (live) forms in the remaining tests increases. Down-core trends (to 10 cm below sea floor) have revealed: 1) percent agglutinated forms decline and calcareous forms increase with depth; 2) agglutinated species richness decreases with depth; and 3) experimental core assemblages become increasingly similar with depth to those in control cores not subjected to CO 2 injection. These results imply almost complete initial mortality and dissolution in the upper 10 cm throughout the corrals following liquid CO 2 injection. Since calcareous foraminifera represent more than 50% of the total assemblages, this clearly indicates that emplacement of CO 2 will result in negative effects to diversity and survivorship of the deep-sea benthic meiofauna.

Elemental and isotopic compositions of organic carbon and nitrogen of recently deposited organic matter in Empakai crater and its implication for climatic changes in northern Tanzania

A 29 cm long core recovered from a water depth of 5 m in a small closed lake located in the Empakai crater northern Tanzania, is used to document the contents of organic carbon and nitrogen, stable isotopes composition of organic carbon and nitrogen, and C/N ratios and to infer climatic changes from these data. The &#948 13C and &#94815N values for this core, which averages -16.5±2.1l and 11.0±5.6l respectively, show a general downcore decrease below 5 cm. However, within this general trend, two peaks enriched in 13C are observable at about 8 cm and 13 cm. The peak at about 13 cm corresponds also to a zone depleted in 15N values. The general downcore trend can be attributed to systematic changes in relative proportion between C3 and C4 types of organic matter (OM), resulting from climatic changes or nutrient changes and shift between algae and higher plants. The lower most section containing the most depleted values can be attributed either to high input of autochthonous material owing to wetter/humid conditions or high contribution of C3 high land plant materials in response to increase in forest cover. The general downcore decrease in 13C values can also be interpreted to indicate that precipitation and vegetation cover in the area has been declining over time, and occasionally drought has been intense particularly during two zones that are characterized by enrichment in 13C. The upper 5 cm is characterized by near constant values of 13C and 15N with isotope values as high as -13.15‰ and 22.15‰ respectively. High enrichment in 15N in the upper 5 cm could be attributed to denitrification and ammonium volatilization processes, while enrichment in 13C in the upper 5 cm could be attributed to high proportion of C4 type of OM and/or utilization of HCO3- by phytoplankton during synthesis of OM. This site is also characterized by high concentrations of organic carbon (averaging 7.8±2.0%) and nitrogen (averaging 0.8±0.3%). A smell of hydrogen sulfide detected in water samples collected at approximately 20 m, suggest that there is enhanced preservation of OM in the lake under anoxic condition. The C/N ratios values, which average 12.2±2.3, increase downcore to the base of the core owing to preferential loss of nitrogen containing compounds during diagenesis. Tanzania Journal of Science Vol. 30 (2) 2004: pp. 87-96

Isotope composition of bulk carbon in replicated Sphagnum peat cores from three Central European high-elevation wetlands

Several processes may contribute to systematic downcore trends in δ13C of bulk Sphagnum peat. Whereas changes in water availability during C assimilation may change δ13C values in both a negative and positive direction, other processes would always cause a uni-directional shift in δ13C. Selective preservation of isotopically light lignin C may lead to more negative δ13C values with an increasing depth and age of peat. Anthropogenic change toward lower δ13C of atmospheric CO2 due to massive coal burning since the beginning of the Industrial Revolution would result in lower δ13C of the youngest peat layers, and in higher δ13C of older peat layers. Emissions of low-δ13C methane from wetlands should result in a progressive enrichment of the residual peat substrate in the heavier isotope 13C. Consequently, deeper peat would have higher δ13C. In a specific peat profile, the downcore trend in δ13C will be the result of an interplay between all these isotope-sensitive processes. Most Central European wetlands studied previously show a 13C enrichment (i.e., higher δ13C values) with an increasing depth and age. Here we focus on sites which showed lower δ13C with an increasing depth and age when a single peat core was taken. Replication did not confirm this negative downcore δ13C shift. A positive downcore δ13C shift is more widespread than previously believed. We suggest that decreasing δ13C of atmospheric CO2 and emissions of low-δ13C methane belong to the main controls of the downcore δ13C trends in young peat substrate.

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50–150mgdm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4–99.2mmol CO2m−2year−1) are lower than the marine weathering rates calculated from the solid phase data (198–245mmol CO2m−2year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22–15.0mmol CO2m−2year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5–20Tmol CO2year−1 is as high as the global rate of continental silicate weathering.

Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt

The analysis and interpretation of changes in mineral magnetic signatures from a long (ca. 8.2 m) sedimentary sequence recovered from Lake Qarun, Middle Egypt in 2003 spanning a timescale of approximately the last 2,000 years is reported. A suite of mass specific susceptibility and magnetic remanence measurements were made at irregular intervals downcore on 39 samples. These samples were selected on the basis of trends and abrupt changes in whole-core magnetic susceptibility measured using a Bartington® MS2E sensor and were analysed for low and high temperature loss on ignition and their particle size distribution. Trends in all mineral magnetic concentration parameters are remarkably similar and were initially used to divide the core into three magnetically distinct zones. The upper and lower sections of the core (0–119 cm and 445–822 cm depth) are characterised by low values for all magnetic concentration parameters. Between 153 and 380 cm depth, concentration parameters are considerably higher, although somewhat variable. The S ratio and percentage loss of remanence after 24 h (IRMloss) follow a different trend and are inversely related to each other. A low S ratio ( 4%. On the basis of these parameters, the core can be divided into four zones, and differences in magnetic mineralogy between these four zones were confirmed by measurement of IRM acquisition curves. The major difference between concentration parameters and ratios or percentage loss of IRM lies in the identification of an additional zone below 619 cm depth where the S ratio is high and IRMloss is low. There is little evidence to suggest that the magnetic signatures are controlled by particle size or by trends in organic matter and/or carbonate content. The signatures appear to be predominantly detrital and show little evidence of post-depositional alteration through dissolution or authigenic addition of bacterial magnetite or greigite. Analysis of Saharan dust deposition rates in Northern Egypt suggests that atmospheric fallout is likely to make only a very minor contribution (<1%) to sedimentation rates in Lake Qarun. The downcore trends therefore appear to reflect major changes in fluvial sediment sources over the ca. 2,000 year time period spanned by this ∼8 m core. Preliminary mineral magnetic characterisations of potential local sources suggest that these cannot account for the range of signatures recorded in the Qarun sediments and it is hypothesised that these sediments are derived from Nile river floods.

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analysed for biogenic opal, organic carbon, carbonate, sulphur, major element concentrations, mineral contents and dissolved substances including nutrients, sulphate, methane, major cations, humic substances and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration.

New spatial Mg/Ca‐temperature calibrations for three Arctic, benthic foraminifera and reconstruction of north Iceland shelf temperature for the past 4000 years

We have developed cold‐end Mg/Ca‐temperature calibrations for three common Arctic benthic foraminifera, Islandiella norcrossi/helenae, Melonis barleeanus, and Cassidulina neoteretis, and compare the three calibrations in a late Holocene downcore record (0–4000 cal yr B.P.). The calibration and downcore trends for the three Arctic species extend the observation that Mg incorporation into benthic foraminifera is species specific. For the calibration we use a set of CTD casts, bottom water δ18Oseawater measurements, and surface grab‐samples collected from the Iceland margin (cruise B997) and the Greenland margin (cruise BS1191). Water depth of sites used ranges from 165 to 656 m, while spatial bottom temperature ranges from 0 to 7°C. Mg/Ca values ranged from 1.02 to 1.47 for I. norcrossi/helenae, 0.64 to 2.21 for M. barleeanus, and 0.93 to 1.38 mmol/mol for C. neoteretis. We calibrated Mg/Ca content against isotopic calcification temperature (calculated using T = 16.9 − 4.0*(δ18Ocalcite corrected for vital effect − δ18Oseawater)). Exponential calibrations for the three species are as follows: I. norcrossi/helenae Mg/Ca = 1.051 ± 0.03 * exp(0.060 ± 0.011 * T), M. barleeanus Mg/Ca = 0.658 ± 0.07 * exp(0.137 ± 0.020 * T), and C. neoteretis Mg/Ca = 0.864 ± 0.07 * exp(0.082 ± 0.020 * T). On the basis of Mg/Ca in these benthic species the downcore record from core MD99‐2269 is reconstructed. Bottom temperature values are interpreted to reflect variable inflow of Atlantic and Arctic water to the north Iceland shelf during the last 4000 cal yr B.P. All three reconstructions show a decline by 0.1°C per century from circa 1500‐0 cal yr B.P., which coincides with an increase in Arctic benthic foraminifera abundances and a rise in sea ice proxies in the same core. Intriguingly, C. neoteretis diverges periodically to higher average temperature (Atlantic water conditions) than shown by M. barleeanus or I. norcrossi/helenae (which both show Arctic water temperature) circa 1500–4000 cal yr B.P.

Natural and anthropogenic geochemical signatures of floodplain and deltaic sedimentary strata, Sacramento–San Joaquin Delta, California, USA

The geochemical history of an upper deltaic plain pending tidal wetland restoration was reconstructed to assess remobilization of redox-sensitive constituents in sediment, identify depositional processes promoting geochemical retention, and determine the extent of contamination with Hg, As, Pb, Cu, and Zn. Three 12–14-m sediment cores were analyzed for bulk sediment geochemistry using ICP-AES. Rather than showing similar stratigraphic and geochemical down-core trends, cores had a unique record indicative of strong spatial gradients in deposition processes. Each strata type (e.g. basal clay, sand channel, distal floodplain, and agriculturally impacted surficial horizon) had a unique geochemical “fingerprint”. The agriculturally impacted surficial layer showed high [Hg], [As], and [Pb]. The significance is that a restored upper delta will have a complex geomorphology defying conventional criteria of “success” in a restoration framework. Also, there is a significant risk of generating toxic, bio-available CH 3Hg + that would be hazardous to fish.

Chrysophyte cyst relationships to water chemistry in Pyrenean lakes (NE Spain) and their potential for environmental reconstruction

Chrysophyte cysts were identified from the surface sediment of 105 mountain lakes in the Pyrenees (NE Spain), and their statistical relationship to water chemistry was examined using canonical correspondence analysis (CCA). The chemical parameters that explained significant and independent amounts of variability were alkalinity, pH, potassium, nitrate and magnesium. In a CCA using these parameters, the first canonical axis was related to a gradient of alkalinity and pH, which reflected the varying nature of the watershed bedrock in the Pyrenees, while the second axis was correlated with potassium (negatively) and nitrate (positively). The potential for environmental reconstructions of the five chemical parameters was further studied by: (i) analyzing the distribution of optima and tolerances calculated by weighted-averaging (WA); (ii) carrying out detrended canonical correspondence analysis (DCCA) with a single environmental variable; and (iii) examining the performance of WA-PLS transfer functions. Acceptable transfer functions were obtained for alkalinity, pH and nitrate. However, for potassium and magnesium the tolerance of cysts was too broad and the distribution of optima too skewed, respectively. The possibility of reconstructing nitrogen-related issues using chrysophyte cysts is particularly interesting because of the lack of direct chemical records of nitrogen compounds in sediments. Nitrate reconstructions using transfer functions may be complemented by a holistic reconstruction using partial CCA, where, after subtracting the effects of other chemicals, samples are ordered on a plain defined by potassium and nitrate. This ordination could show down-core trends in lake productivity and renewal time.

Changes in sediment sources and floodplain deposition rates in the catchment of the River Tweed, Scotland, over the last 100 years: the impact of climate and land use change

AbstractEvidence from floodplain cores collected from three sites in the middle reaches of the Tweed basin in Scotland is used to reconstruct changes in sediment sources and overbank floodplain deposition rates over the last c. 100 years. Core chronologies and sedimentation rates are established using 137Cs and unsupported 210Pb measurements. The average sedimentation rates since 1963 range from 1·9 ± 0·2 to 2·2 ± 0·2 kg m−2 a−1 and are lower than the average rates for the period 1894/95 to 1963, which range from 2·7 ± 0·6 to 5·9 ± 0·9 kg m−2 a−1. There is also evidence of significant downcore variations in sediment source, defined in terms of both type (i.e. topsoil or channel bank/subsoil material) and spatial location (i.e. main geological/topographic zones). There is no clear link between the changes in overbank sedimentation rates and sediment sources and the trends shown by precipitation, weather pattern and river flow records over the past 100 years, suggesting that changes in climate alone cannot explain the downcore trends. Instead, the temporal changes in overbank sedimentation rates and sediment source appear to be linked more closely to changes in land use and land management over the past 100 years and, more particularly, the introduction of land drainage at the end of the 19th century, the rapid increase in afforestation since the 1940s and the post‐war conversion of grassland to arable land. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Sources and diagenetic alteration of organic matter in coastal waters at the vicinity of Dar es Salaam, Tanzania.

The stable isotope compositions of organic carbon (OC) and nitrogen, contents of OC and nitrogen, and C/N ratios for four short cores, which were recovered from Msansani Bay (WO 57), off Msimbazi River (WO 76) and Dar es Salaam harbour (WO 62 and WO 74), are used to document temporal variation in the sources and diagenetic alteration of organic matter (OM). The δ15N and δ13C values at sites WO 57, WO 62 and WO 74 show a trend of down-core decrease to the base of the core, and such a down-core trend could be related to diagenetic loss of compounds enriched in 13C and 15N. At site WO 76, the δ13C values show two zones that are depleted in 13C. These two zones correspond to black-greasy layers that were observed during sampling, and low δ13C values probably is an indication that the material was derived from the continent. Furthermore, these two zones have elevated values of OC, nitrogen and C/N ratios. The stable isotopes of OC and nitrogen show inter-sites differences with enrichment in 15N in the following order: WO 57 Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 39-55

Long-term history of chemoautotrophic clam-dominated faunas of petroleum seeps in the Northwestern Gulf of Mexico

Chemoautotrophic clam-dominated assemblages are commonly associated with petroleum seepage on the continental slope of the Gulf of Mexico. We examine the persistence and resilence of these communities by evaluating downcore trends in abundance, biomass, and trophodynamics in communities from four separate petroleum seep sites on the Louisiana continental slope. Some petroleum seep sites retained optimal habitat for some species continuously over geologically-relevant periods of time. More commonly, however, habitat optimality varied substantially over time scales of hundreds of years. Thus, one important characteristic of these sites was the degree of persistence of the chemoautotrophic biota. A fauna typically was persistent over a time span of a few hundred years, but was typically not persistent over a longer time span. The mechanisms producing local extinction remain unclear, however temporal variations in juvenile survivorship seem to be substantially larger than temporal variations in larval settlement, to the extent that the heavily taphonomically-biased record of juvenile individuals permits such a conclusion. When local extinctions occurred in the chemoautotrophic biota, the biota was replaced by a normal slope biota or a mixture of a normal slope biota and the juveniles of chemoautotrophic species that failed to survive to adulthood. Thus, the only faunal transitions were between specific chemoautotrophic faunas and the non-chemoautotrophic fauna. Not one distinctive faunal transition between two chemoautotrophic faunas was observed. Accordingly, each discrete chemoautotrophic fauna was resilient over long time scales; time scales of geological importance.

Bioturbation, geochemistry and geotechnics of sediments affected by the oxygen minimum zone on the Oman continental slope and abyssal plain, Arabian Sea

We investigate the way the oxygen minimum zone (OMZ) alters interactions between bioturbation and sediment geochemistry, and geotechnical properties. Sediments are compared within and below the OMZ on the Oman continental slope and adjacent abyssal plain during the post monsoonal autumn season. Quantitative measurements were made of Eh and pH, of total organic matter (TOM) and carbonate, of water content and shear strength, and of bioturbation structures in vertical profiles of subcores taken from spade-box core samples. The OMZ stations had distinctively low redox conditions and high carbonate content, and different geotechnical properties and different bioturbation structures than stations below the OMZ on the abyssal plain. These differences are related to the degree of anoxia and to water depth. Within the OMZ, Eh, pH and carbonate increased with water depth, and TOM and water content decreased. We also noted the presence of subsurface sediment heterogeneity on the continental slope within the OMZ. In the OMZ, Eh, water content and bioturbation decreased with increasing sediment depth. There was a slight decrease in pH in the top 5 cm at all stations. Shear strength nearly always increased with increasing sediment depth. At each water depth correlations show down-core trends in these parameters, while across all water depths correlations were significant at deeper sediment depths (20–30 cm). An Eh–pH diagram identified two water-depth groupings: 391–1008 and 1265–3396 m. Cluster analysis showed the upper and lower sediment depths form separate clusters, the break occurring at 4–7.5 cm; while there are also distinct clusters related to water depth. We relate our results to bottom-water oxygen concentrations reported by other investigators, and to regional-scale geochemical processes.

Variations in late Quaternary upper ocean structure of Okinawa Trough: A nannofossil approach

The relative abundance ofFlorisphueru profunda, a calcareous nannofossil species in lower euphotic zone, affords an opportunity to study variations in upper ocean structure such as thickness of mixed layer. As revealed by the nannofossil analysis in sediments since 20 ka B. P. from three Okinawa Trough cores, the percentage ofF. profunda increased significantly from glacial to Holocene, with a declining event ca 4 ka B. P. in its southern part. Its downcore trend is similar to that ofPulleniatina obliquiloculuta, a tropical planktonic foraminifera1 species which lives in subsurface water below thermocline in the modern ocean. Their similar variations imply an eastward shift of the Kuroshio Current, a reduction in thickness of the mixed layer, a shallowing of thermocline and a possible increase of turbidity in the upper ocean during the glaciation. The eastward shift of the Kuroshio might be repeated again about 4 ka B.P.

Sulfur isotopic trends and pathways of iron sulfide formation in upper Holocene sediments of the anoxic Black Sea

Down-core trends for extents of iron sulfidation in upper Holocene microlaminated deposits of the Black Sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and/or very close to the sediment-water interface. Sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core, with a mean δ 34S CDT value of −37.2 ± 0.9‰ (1σ, n = 18). These isotopic trends support the hypothesis of early pyrite formation. Isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and toδ 34S values for water-column dissolved sulfide within a narrow zone immediately below the O 2/H 2S interface. These three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column (>175 m), with δ 34S values for the deep dissolved sulfide reservoir being consistently lower by ∼2 to 4‰. These isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow, shallow region at the top of the sulfidic water mass. Isotopic data from a fine-grained turbidite collected in the deep basin and from rapidly accumulating, iron monosulfide-rich muds on the anoxic upper slope are enriched in 34S relative to the microlaminated deposits. The enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial. A broad suite of geochemical data, including sulfur isotopic results, are consistent with an upper-slope sediment source for the ubiquitous muddy turbidites of the deep basin. The collective effects of reworking, transport, and redeposition result in a complete conversion of AVS to pyrite with little additional iron sulfidation. This conclusion is supported by a detailed isotopic mass balance. The present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin. Sediments of the upper slope and the genetically-linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep-basin floor, suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin. Importantly, isotopic patterns from the modern Black Sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record.

The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions

Pyrite framboids are densely packed, generally spherical aggregates of submicron-sized pyrite crystals. In this study, a survey was made of framboid size distributions in recently deposited sediments from euxinic (Black Sea; Framvaren Fjord, Norway; Pettaquamscutt River Estuary, Rhode Island, USA), dysoxic (Peru Margin), and oxic (Wallops Island, Virginia, USA; Great Salt Marsh, Delaware, USA) environments. Pyrite framboids in sediments of modern euxinic basins are on average smaller and less variable in size than those of sediments underlying dysoxic or oxic water columns. Down-core trends indicate framboid size distribution is a sediment property fixed very early during anoxic diagenesis, generally within the top few centimeters of burial. Size distributions in modern sediments are comparable with those in ancient sedimentary rocks, evidence that framboid size is preserved through advanced stages of diagenesis and lithification. It is proposed that where secondary pyrite growth is limited, as to preserve primary pyrite textures, framboid size distribution may be used to indicate whether fine-grained sedimentary rocks were deposited under oxic or anoxic conditions. The Crystal Size Distribution Theory relates framboid size to growth time and rate. On the basis of this theory, the characteristic smaller sizes of framboids in sediments of modern euxinic basins reflect shorter average growth times relative to oxic or dysoxic environments. In euxinic environments, framboid nucleation and growth occurs within anoxic water columns, and growth times are, on average, shorter because of hydrodynamic effects than when framboid nucleation and growth occurs within anoxic sediment porewaters underlying oxic water columns. A maximum framboid growth time of 0.4 years is indicated for framboids forming in the water columns of euxinic basins.

Porosity-Permeability Trends, Sparta B Sandstones (Eocene), Fordoche Field, Pointe Coupee Parish, Louisiana

Fordoche field, Pointe Coupee Parish, Louisiana (44 wells) contains at least ten hydrocarbon-bearing zones. Major reserves of bypassed oil probably exist in the shallowest reservoir, Sparta B, estimated to contain {approximately}5.6 mm STB OIP and 2.0 bcf GIP. Sparta B pay thicknesses average 15 ft with 24.7% porosities and 200 md permeabilities. Other averages include: 7,980 PSI BHP (abnormal range), 228F BHT, 2,075 PSIG bubble point pressure, gas-oil ratio 500 SCF/bbl, oil gravities 40-44 API. Petrographic, SEM, and EDX study of 20 thin sections from these units discloses the following down-core trends and petrofacies: (1) high porosity (20-25%)-permeability (250-500+ md), fine- to very fine-grained quartzarenites whose pore throats average about 10 microns comprise the storm washover deposits. Vermicular chlorite begins to occlude porosity near the bottom of this unit (11,338.25 ft); (2) pore throats of 10-20 microns occur in quartzarenitic, upper shoreface sandstones that display slightly lower porosity and permeability than overlying units. Kaolinite and chlorite begin to occlude porosity in this facies (11,346.00 ft and 11,347.50 ft); calcite completely occludes porosity at 11,350.46 ft; (3) lower shoreface sandstones are increasingly organic, mostly very fine sand size. Organic lamellae parallel to bedding are characteristic with pore throats in the 10-15more » micron-size range; chlorite and calcite occlude porosity at 11,360.50 ft and 11,365.00 ft, respectively; (4) inner shelf lithologies are very fine-grained, highly organic, micaceous, poorly sorted quartzarenites with virtually no permeability. Pore throats are small ({approximately}10 microns). Biotite occludes porosity at 11,374.67 ft, muscovite at 11,375.79 ft.« less

Late Quaternary history of the Lake O'Hara region, British Columbia – an evaluation of sedimentation rates and bulk amino acid ratios in lacustrine records

Lake O'Hara (subalpine) and Opabin Lake (alpine) are situated directly adjacent to a high section of the Continental Divide in the central Canadian Rocky Mountains. Core samples recovered from the lakes show a consistent stratigraphy comprising gyttja and underlying inorganic clastic sediments. The gyttja contains Bridge River (2350 years BP) and Mazama (6800 years BP) tephras and is separated from the lower clastic sediments by a sharp, conformable contact. Radiocarbon dates obtained from conifer needles, extracted from directly above the contact, indicate that deglaciation had proceeded upvalley from the O'Hara basin priorto ca. 10 100 years BP. Preliminary palaeobotanical and macrofossil data suggest that a Pinus–Abies forest with lesser Picea was established in the vicinity of Lake O'Hara by this time. Consequently, the minimum age of moraine systems situated downvalley from Lake O'Hara is Late Wisconsinan.Mean annual sedimentation rates were derived from sediment thickness data from 14 Lake O'Hara and 2 Opabin Lake cores. Averaged total sedimentation rate values from the Lake O'Hara cores are 0.13 mm/year (post-Bridge River), 0.13 mm/year (Mazama – Bridge River) and 0.05 mm/year (11 000 years BP – Mazama). Averaged total sedimentation rate values from the Opabin Lake cores are 0.19 mm/year (post-Bridge River), 0.07 mm/year (Mazama – Bridge River), and 0.06 mm/year (8530 years BP – Mazama). Higher total sedimentation rates in post-Bridge River sediments of Opabin Lake are presumably related to climatic conditions associated with more extensive upvalley ice during the last ca. 2300 years. Highly variable sedimentation rate data obtained from the Lake O'Hara cores suggest that the use of sedimentation rate data as a proxy record of upvalley glacial activity is inappropriate in the Lake O'Hara setting where inflowing glacial stream systems are interrupted by upvalley lake basins.Aspartic acid D/L ratios were derived from bulk gyttja samples of known age from seven Lake O'Hara and one Opabin Lake core. In all but two cases, aspartic acid D/L ratios increase consistently with respect to sediment age. The increasing downcore trends in the aspartic acid D/L ratios suggest the possibility of using amino acid data from bulk gyttja samples as a check for reworking in cases where chronostratigraphic markers are absent.