Hydrothermal petroleum of Lake Chapala,Citala Rift,western Mexico: Bitumen compositions from source sediments and application of hydrous pyrolysis

Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250 °C over brief geological times.     

Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift.

Cape Kalamba oil seeps occur at the south end of the Ubwari Peninsula, at the intersection of faults controlling the morphology of the northern basin of the Tanganyika Rift, East Africa. Oil samples collected at the surface of the lake 3-4 km offshore from Cape Kalamba have been studied. The aliphatic hydrocarbon and biomarker compositions, with the absence of the typical suite of polynuclear aromatic hydrocarbons, indicate an origin from hydrothermal alteration of immature microbial biomass in the sediments. These data show a similarity between a tar sample from the beach and the petroleum from the oil seeps, and confirm that the source of these oils is from organic matter consisting mainly of bacterial and degraded algal biomass, altered by hydrothermal activity. The compositions also demonstrate a < 200 degrees C temperature for formation/generation of this hydrothermal petroleum, similar to the fluid temperature identified for the Pemba hydrothermal site located 150 km north of Cape Kalamba. The 14C age of 25.6 ka B.P. obtained for the tar ball suggests that Pleistocene lake sediments could be the source rock. Hydrothermal generation may have occurred slightly before 25 ka B.P., during a dry climatic environment, when the lake level was lower than today. These results also suggest that the Cape Kalamba hydrothermal activity did not occur in connection with an increased flux of meteoric water, higher water tables and lake levels as demonstrated in the Kenya Rift and for the Pemba site. Hydrothermal petroleum formation is a facile process also in continental rift systems and should be considered in exploration for energy resources in such locales.     

Radiocarbon age inversions and progression: source and causes in Late Holocene sediments from Lake Chapala,western Mexico

The results of eight radiocarbon datings of Lake Chapala sediments (site T46) are presented, the age inversions (AI) observed and their age progression discussed. As deduced from some AIs and the ²¹⁰Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5–25 cm. The linear sedimentation rates (LSRs) calculated from ¹⁴C ages do not match the LSR calculated from unsupported ²¹⁰Pb activity for the upper sediments. This demonstrates the usefulness of dating sediments with complementary radiometric techniques such as short-lived isotope counting (SLIC), i.e., ²¹⁰Pb and ¹³⁷Cs. This approach leads to the following conclusions: (1) The incorporation of detrital particles with ancient carbon into the sedimentary column of the lake occurred by a combination of: (a) the presence of outcrops of hydrothermal petroleum with ages >40 ka (ka = thousands of years) in the lake, and (b) mass transport due to the presence of two elongated gyre circulation patterns integrated by cyclonic circulation (counterclockwise) in the north portion of the lake and anticyclonic circulation in the southern part. (2) Consequently, the ¹⁴C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the ²¹⁰Pb method. (3) A bioturbation mechanism is not necessary to explain the ¹⁴C AI in the top 70 cm and from 110 to 150 cm depth of the sediments. (4) According to the biological proxies data for the last 600 years B.P., the paleoclimate at Lake Chapala has changed from sub-humid to dry environmental conditions, and eutrophication has increased over the past 100 years due to local input from ongoing agricultural activities.     

Composition and sources of extractable organic matter from a sediment core in Lake Kivu,East African rift valley

Lake Kivu is a gas-charged East African rift lake with currently anoxic bottom water. The extractable compounds and residual organic matter of a short sediment core have δ¹³C values typical of lacustrine microbial detritus. The total extracts consist primarily of polar compounds such as n-alkanoic acids, hydroxyalkanoic acids, triterpenoids, steroids and monosaccharides, with minor amounts of n-alkanes and n-alkanols. These tracer compounds and δ¹³C values indicate that the organic matter in the surficial and deeper sedimentary record was dominated by bacterial sources. The sapropelic sediment between these horizons contains organic matter from primarily algal with lesser bacterial input. Terrestrial organic markers are minor in all samples. The major fractions of the compounds in the total extracts were oxidized in the upper water column prior to transit through the anoxic bottom water to sedimentary deposition. The sapropelic horizon may reflect lake water turnover with ventilation or hydrothermal activity and consequently increased algal blooms.     

Biogeochemical controls on reaction of sedimentary organic matter and aqueous sulfides in holocene sediments of Mud Lake, Florida

The distribution and quantity of organic sulfur and iron sulfur species were determined in the Holocene sediments from Mud Lake, Florida. The sediments of this shallow, sinkhole lake are characterized by high sulfur and organic carbon contents as well as active sulfate reduction. They record a shift from a basal peat (below 2 m) comprised of water lily-dominated organic matter to the present cyanobacterial/algal-dominated lake deposit (upper 1 m). This shift in depositional environment and subsequent organic matter source was accompanied by variation in the amount of reactive iron delivered to the sediments, which in turn influenced the type and extent of organic matter sulfurization. Extractable intramolecular organic sulfur is principally found as C₂₅ highly branched isoprenoid (HBI) thiolanes. Extractable polysulfide-linked lipids, determined by selective chemical cleavage with MeLi/MeI and analyzed as methylthioethers (MTE), are dominated by n-alkanes with sulfur attachments at position 1 and 2, as well as lower amounts of C₂₅ HBI-MTE. The δ¹³C values and carbon-chain length distribution of both series of n-alkylMTE indicate that they are derived from distinct biological precursors. Among the n-alkylMTE with sulfur attachment at position 1 there are three homologous series: one saturated and two with both cis and trans enethiol isomers. The identification of the enethiol in the sulfur-linked macromolecules indicates that n-alkylaldehydes are precursors lipids. The intervals of high concentration of bulk organic sulfur and sulfurized lipids coincide with the intervals of high mineral sulfur content (acid volatile sulfide and chromium reducible sulfur). We suggest that the main control on the enhanced addition of sulfur to the organic matter in Mud Lake was the increased formation of polysulfides during the reduction of iron hydroxides and the subsequent reaction of those polysulfides with mildly oxidized sedimentary organic matter.     

Long-term cooling history of the Albertine Rift: new evidence from the western rift shoulder,D.R. Congo

To determine the long-term landscape evolution of the Albertine Rift in East Africa, low-temperature thermochronology was applied and the cooling history constrained using thermal history modelling. Acquired results reveal (1) “old” cooling ages, with predominantly Devonian to Carboniferous apatite fission-track ages, Ordovician to Silurian zircon (U–Th)/He ages and Jurassic to Cretaceous apatite (U–Th–Sm)/He ages; (2) protracted cooling histories of the western rift shoulder with major phases of exhumation in mid-Palaeozoic and Palaeogene to Neogene times; (3) low Palaeozoic and Neogene erosion rates. This indicates a long residence time of the analysed samples in the uppermost crust, with the current landscape surface at a near-surface position for hundreds of million years. Apatite He cooling ages and thermal history models indicate moderate reheating in Jurassic to Cretaceous times. Together with the cooling age distribution, a possible Albertine high with a distinct relief can be inferred that might have been a source area for the Congo Basin.     

A millennial‐scale Late Pleistocene–Holocene palaeoclimatic record from the western Chihuahua Desert,Mexico

We present elemental concentrations and magnetic susceptibility data from a new 270‐cm‐long sediment core collected from the western part of palaeolake Babicora and infer millennial‐scale hydrological variations over the last 27 cal. ka in the western Chihuahua Desert. Variations in the available water content at the sediment–air interface of the watershed, lake salinity and lake productivity are inferred from values of the chemical index of alteration (CIA), CaCO ₃ and C_org, respectively. An abrupt increase in runoff at c. 24 cal. ka BP appears correlative with the Heinrich 2 (H2) event. Except for this event, diminished runoff between c. 27 and 19 cal. ka BP indicates lower annual precipitation (weak summer rainfall) during the Last Glacial Maximum. The deposition of chemically altered sediments between c. 25 and 22 cal. ka BP results from the higher sediment–water interaction in the watershed owing to lower evaporation, cooler conditions and higher precipitation during the H2 event. Since 19 cal. ka BP the runoff has been characterized by high‐amplitude fluctuations with intervals of reduced precipitation identified at c. 19, 18, 17.5, 13–14, 11.5, 10, 7.5 and 3 cal. ka BP.     

Molecular characterization of organic matter in sediments from the Keg River formation (Elk Point group), western Canada sedimentary basin

Distributions of free and sulfur-bound biomarkers in organic-rich sediments from the lower and upper Keg River formation (Elk Point group, western Canada) reveal deposition of these sediments under reducing conditions, in agreement with the geological data. The predominance of aryl isoprenoids of Chlorobiaceae origin indicates that photic zone anoxia occurred during time of deposition. A series of novel aryl isoprenoids with a carboxylic acid function was identified by GC–MS and the structure of the predominant 3-methyl-5-(2′,3′,6′-trimethylphenyl)pentanoate was proven by synthesis of an authentic standard. The occurrence of these acids gives evidence that oxidative breakdown at least partially accounts for the diagenetic fate of aromatic carotenoids. Li/EtNH₂ desulfurization of two kerogen concentrates yielded only low amounts of sulfur-bound hydrocarbons, and suggests that either sulfur sequestration was not a predominant process or, more probably, that a major part of the hydrocarbons, initially sulfurized, were released upon maturation, giving further evidence that (poly)sulfide bonds are cleaved at relatively low levels of thermal stress.     

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.     

近70年滇池有机质来源和生产力变化的沉积记录

总有机碳(TOC)、稳定碳同位素(δ¹³C)、岩石热解分析(Rock-Eval)和甾醇生物标志物等有机地球化学指标用于研究滇池过去70年沉积物有机质的来源及水体生产力的变化。C/N、δ¹³C、氢指数(HI)表明滇池沉积物有机质主要为水生来源, 且主要为藻类有机质。初级生产力指标(热解烃峰参数(S2)、HI、δ¹³C)和浮游植物甾醇(菜籽甾醇、甲藻甾醇)含量从2000年以后快速增加, 而从近几年开始增速放缓或开始下降, 说明滇池水体生产力在过去几十年不断上升, 不过在最近得到一定控制, 这与滇池沉积物营养盐的变化趋势相似。对生产力指标和浮游植物甾醇与环境因子如5年移动平均气温(T₅)、总氮(TN)、总磷(TP)进行相关分析, 结果表明滇池生产力变化与环境因子呈现显著的正相关关系, 说明气候变暖和营养盐增加均促进该水域藻类的繁殖; 此外, 多元线性回归分析结果表明, TN是影响滇池生产力变化的主要环境因子。研究结果对探寻水体富营养化过程及影响因子, 并对湖泊管理和水华防治工作提供科学依据。

     

The early diagenesis of aliphatic hydrocarbons and organic matter in sedimentary particulates from Dabob Bay,Washington

Aliphatic hydrocarbon compositions were quantitatively characterized in plankton, sediment trap-collected particulate materials and sediments from Dabob Bay using high resolution glass capillary gas chromatography. The average net accumulation of individual hydrocarbons measured in a 1-yr series of sediment traps was compared with the net accumulation of corresponding compounds measured in three depth intervals of ²¹⁰Pb-dated bottom sediments. Systematic and rapid decreases in the net accumulation of individual hydrocarbons were observed from the sediment traps to the sediments. Most pronounced decreases were measured for planktonically derived hydrocarbon constituents (e.g. pristane and two unsaturated compounds) which are rapidly remineralized at or near the sediment-water interface. Consequently, the amount of each compound measured in deposited sediments is not necessarily a quantitative indication of its initial flux to the sediments. The n-alkanes (C_25,27,29,31). characteristic of terrestrial plant waxes, are the predominant hydrocarbons measured by 4–6 cm depth in these sediments and show reasonably constant net accumulation below this interval.Significant diagenetic alteration of the bulk organic matter contained in the average sediment trap particulate material is also noted through comparison with bottom sediments on the basis of organic C/N and δ¹³C measurements. Organic matter elementally similar to marine plankton is preferentially remineralized upon deposition of the sedimentary particulates. The residual organic matter remaining and buried in the bottom sediments closely resembles terrestrial organic matter.     

Structural determination of purpurin-18 (as methyl ester) from sedimentary organic matter

We report the isolation and structural determination (RP-18 HPLC, UV–VIS, MS, ¹H NMR and two-dimensional ¹H NMR (COSY and ROESY)) of purpurin-18 (as methyl ester) from geological samples. Existing data suggest that purpurins-18 are derived from chlorophyll or its transformation products by postdepositional, microbially mediated processes in sediments. Diagenetic transformations of purpurins-18 can lead through fossilization to pyrroporphyrins and some ETIO porphyrins isolated from sediments.     

Origin and transport of sedimentary organic matter in the Yalujiang estuary,North China

The biogeochemistry of organic matter (OM) in a macrotidal estuary, the Yalujiang River, was studied during two cruises: the flood season in August 1994 and the dry season in April 1996. Surface sediments were collected in the riverine zone (RZ), the turbidity maximum zone (TMZ), and the marine zone (MZ). The molecular distribution of the n-alkanes and fatty acid series and bulk sediment characteristics, such as C:N and δ¹³C, were used to assess differences in OM source and transport from the river upstream to the marine end member. Higher C:N values typical for terrestrial sources were observed at the upper reach for both seasons. The δ¹³C of OM in surface sediments varied from −27.3‰ to −21.6‰ in the flood season and from −26.8‰ to −31‰ in the dry season. The concentrations of n-alkanes varied between 0.3–21.4 μg g⁻¹ and the variation of fatty acids was 4.8–32.9 μg g⁻¹. The data showed mixing of terrestrial and autochthonous OM in the middle and lower reaches. The distribution of lipids (n-alkanes and Carbon Preference Index) encountered in this study confirmed the importance of terrestrial OM in the sediment samples from degraded soil material. The distribution of fatty acids suggested important phytoplankton, zooplankton, and microbial signals (short-chain and unsaturated acids; ≤C₂₀). Branched fatty acids, such as the iso- and anteiso-C₁₅ and C₁₇ compounds, relfect bacterial contributions. All samples were characterized by a high proportion of mixture inputs in both seasons. A slight decreasing trend was observed with increasing salinity except for the highest percentage of mixed fatty acids in the TMZ of the flood season. Terrestrial fatty acids were approximately 20% in the flood season and 27–46% in the dry season. Differences in hydrological conditions and primary production between the TMZ, RZ, and MZ resulted in different OM distributions, which are reflected in the sources and degree of diagenesis of the sedimentary OM. Seasonal variation may be strongly influenced by hydrological characteristics rather than primary productivity and anthropogenic activities in the Yalujiang region.     

Research on the carbon isotopic composition of organic matter from Lake Chenghai and Caohai Lake sediments

The carbon isotopic composition of organic matter from lake sediments has been extensively used to infer variations in productivity. In this paper, based on the study of the contents and δ13C values of organic matter in different types of lakes, it has been found that δ13C values of organic matter have different responses to lake productivity in different lakes. As to the lakes dominated by aqutic macrophytes such as Lake Caohai, organic matter becomes enriched in 13C with increasing productivity. As to the lakes dominated by aquatic algae such as Lake Chenghai, δ13C values of organic matter decrease with increasing productivity, and the degradation of aquatic algae is the main factor leading to the decrease of δ13C values of organic matter with increasing productivity. Therefore, we should be cautious to use the carbon isotopic composition of organic matter to deduce lake productivity.     

Molecular composition and indigenity of organic matter in Late Neoproterozoic sedimentary rocks from the Yangtze region, South China

1 Introduction As a linkage between the biosphere and the geosphere, organic geochemistry, especially molecular markers, has become a powerful tool for investigating important geological events and the evolutionary history of ancient life on Earth (Kvenvo…