Palaeoenvironmental conditions during deposition of the Upper Cretaceous oil shale sequences in the Songliao Basin (NE China): Implications from geochemical analysis

Excellent hydrocarbon source rocks (oil shales), containing Type I organic matter (OM), were deposited in the continental Songliao rift basin during the Late Cretaceous. A major contribution of aquatic organisms (dinoflagellates, green algae, botryococcus) and minor input from macrophytes and land plants to OM accumulation is indicated by n-alkane distribution, steroid composition and δ¹³C values of individual biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria and chemoautotrophic bacteria, as well as purple and green sulfur bacteria. The presence of methanotrophic bacteria is indicated by ¹³C-depleted methyl hopane. The sediments were deposited in a eutrophic, alkaline palaeolake. Highly reducing (saline) bottom water conditions and a stratified water column existed during OM accumulation of the Qingshankou Formation and Member 1 of the Nenjiang Formation. This is indicated by low pristane/phytane, gammacerane index and MTTC ratios, and the presence of β-carotane and aryl isoprenoids. However, an abrupt change in environmental conditions during deposition of Member 2 of the Nenjiang Formation is indicated by significant changes in salinity and redox-sensitive biomarker ratios. A freshwater environment and suboxic conditions in the deep water prevailed during this period. Higher input of terrigenous OM occurred during deposition of the upper Nenjiang Formation.Good oil-to-source rock correlation was obtained using biomarker fingerprints of oil-stained sandstone from the Quantou Formation and oil shales from the Qingshankou Formation. Based on the extent of isomerisation of C₃₁ hopanes, the oil was most probably derived from oil shales of the Qingshankou Formation in deeper parts of the basin.     

Geochemical Characteristics of Organic Matter from Maikop Rocks of Eastern Azerbaijan

Based on investigation of more than 170 samples taken from natural outcrops of the Maikop Formation (Oligocene–Lower Miocene) in eastern Azerbaijan, the genetic hydrocarbon potential and the organic matter (OM) maturity of these rocks were estimated. In the study region, sedimentary rocks of this formation were deposited under reductive or weakly oxidative conditions. Possessing a relatively high (1.9%, on the average) content of organic matter of a mixed (continental–marine) OM, these rocks are able to generate both liquid and gaseous hydrocarbons under favorable conditions. Contributions of both the continental and marine components to the total organic carbon (TOC) varied in time and space. The upper and lower subformations of the Maikop Formation differ in the qualitative and quantitative compositions of OM. Oligocene rocks have a relatively lower OM content and are characterized by better oil-generating properties, as compared to lower Miocene rocks.     

Tectonic and climate control of oil shale deposition in the Upper Cretaceous Qingshankou Formation (Songliao Basin, NE China)

Oil shales were deposited in the Songliao Basin (NE China) during the Upper Cretaceous period, representing excellent hydrocarbon source rocks. High organic matter (OM) contents, a predominance of type-I kerogen, and a low maturity of OM in the oil shales are indicated by bulk geochemical parameters and biomarker data. A major contribution of aquatic organisms and minor inputs from terrigenous land plants to OM input are indicated by n-alkane distribution patterns, composition of steroids, and organic macerals. Strongly reducing bottom water conditions during the deposition of the oil shale sequences are indicated by low pristane/phytane ratios, high C₁₄-aryl-isoprenoid contents, homohopane distribution patterns, and high V/Ni ratios. Enhanced salinity stratification with mesosaline and alkaline bottom waters during deposition of the oil shales are indicated by high gammacerane index values, low MTTC ratios, high β-carotene contents, low TOC/S ratios, and high Sr/Ba ratios. The stratified water column with anoxic conditions in the bottom water enhanced preservation of OM. Moderate input of detrital minerals during the deposition of the oil shale sequences is reflected by titanium concentrations. In this study, environmental conditions in the paleo-lake leading to OM accumulation in the sediments are related to sequence stratigraphy governed by climate and tectonics. The first Member of the Qingshankou Formation (K₂qn₁) in the Songliao Basin, containing the oil shale sequence, encompasses a third-order sequence that can be divided into three system tracts (transgressive system tract—TST, highstand system tract—HST, and regressive system tract—RST). Enrichment of OM changed from low values during TST-I to high-moderate values during TST-II/III and HST-I/II. Low OM enrichment occurs during RST-I and RST-II. Therefore, the highest enrichment of OM in the sediments is related to stages of mid-late TST and early HST.     

Compositions,sources and depositional environments of organic matter from the Middle Jurassic clays of Poland

The comprehensive biomarker characteristics from previously undescribed Middle Jurassic clays of Poland are presented. The molecular composition of the organic matter (OM) derived from clays of Aalenian to Callovian age has not changed significantly through time. High relative concentrations of many biomarkers typical for terrestrial material suggest a distinct dominance of OM derived from land plants. Increasing concentrations of C₂₉-diaster-13(17)-enes towards the northern part of the basin indicate an increase in terrestrial input. This terrestrial material would have originated from the enhanced transport of organic matter from land situated at the northern bank of the basin, i.e., the Fennoscandian Shield. The organic matter was deposited in an oxic to suboxic environment, as indicated by relatively low concentrations of C₃₃–C₃₅ homohopanes, moderate to high Pr/Ph ratio values, an absence of compounds characteristic for anoxia and water column stratification, such as isorenieratane, aryl isoprenoids and gammacerane, as well as common benthic fauna and burrows. δ¹⁸O measurements from calcitic rostra of belemnites suggest that the mean value of the Middle Jurassic sea-water temperature of the Polish Basin was 13.1 °C. It is suggested that this mirrored the temperature of the lower water column because belemnites are considered here to be necto-benthic. The organic matter from the Middle Jurassic basin of Poland is immature. This is clearly indicated by a large concentration of biomarkers with the biogenic configurations, such as ββ-hopanes, hop-13(18)-enes, hop-17(21)-enes, diasterenes and sterenes. The identification of preserved, unaltered biomolecules like ferruginol, 6,7-dehydroferruginol and sugiol in Protopodocarpoxylon wood samples from these sediments present particularly strong evidence for the presence of immature OM in the Middle Jurassic sediments. Moreover, the occurrence of these polar diterpenoids is important due to the fact that they are definitely the oldest known natural products detected in geological samples.     

Organic matter in Oligocene Maikop Sequence of the North Caucasus

The results of the thorough study of organic matter (OM) in the Oligocene-Miocene Maikop sequence of the North Caucasus are considered. It is shown that its distribution within the Maikop sequence is characterized by irregular patterns. Despite that these rocks show the elevated TOC content, their hydrocarbon-generating potential is, as a rule, low. The low quality of OM is partly related to its intense anaerobic decomposition in anoxic environments with terrigenous sedimentation. The elevation of OM concentrations and hydrocarbon-generating potential could be connected with the reinforced phytoplankton influx into sediments. Sediments deposited in the second half of the Early Oligocene were synchronously enriched in OM. This enrichment chronologically corresponded to the restoration of connection with the World Ocean of the formerly partly isolated and freshened basin. Most probably, this event resulted from the sharp increase in biological productivity.     

A biogeochemical study of sediments from the eutrophic Lake Lugano and the oligotrophic Lake Brienz,Switzerland

The biomarker composition and stable isotope (C, O) ratio values of organic matter (OM) and carbonate from sediment cores from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both in Switzerland) are compared, in order to obtain information about OM sources and transformation processes. Eutrophic conditions at Lake Lugano are reflected in elevated total organic carbon (TOC) content and hydrogen index (HI) values, as well as higher lipid concentrations. Parallel down core trends in δ¹³C values of TOC and calcite in the Lake Lugano sediments reflect bioproductivity cycles. Variations in δ¹⁸O values of calcite are consistent with changes in mean summer temperature over the time interval covered by the core. In contrast, such a correlation does not exist for Lake Brienz and there the stable isotope composition of calcite reflects its allochthonous origin. In the sediments of both lakes, fatty acid (FA) distributions and the composition of n-alkanols and n-alkanes indicate highly variable proportions of autochthonous OM sources (algae, zooplankton, bacteria) and OM from land plants. Enhanced in situ microbial synthesis during sediment deposition in Lake Lugano is suggested by the higher TOC-normalised concentrations of branched chain FAs (C₁₅–C₁₇), hopanoic acids and triterpenoid alcohols (i.e. tetrahymanol, diplopterol). Variations in the concentrations of cholesterol are related to contributions from zooplankton and/or green algae, while sitosterol concentrations reflect the input of vascular plants. Periods of increased input of OM from diatoms are evidenced by high 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol) and/or highly branched isoprenoid (HBI) alkenes concentrations. High relative concentrations of diplopterol in Lake Lugano sediments are consistent with the predominance of cyanobacteria commonly observed in eutrophic lakes. The presence of archeol and hydroxyarcheol in very low concentrations in the Lugano sediments argues for the activity of methanogens and/or anaerobic methanotrophs.Differences in OM degradation processes are reflected in higher chlorin index values in the Brienz sediments but higher saturated vs. unsaturated n-FAs in the core from Lugano. Higher concentrations of branched chain FAs and 16:1ω7 n-FA, as well as enhanced 18:1ω7/18:1ω9 n-FA, are consistent with enhanced bacterial biomass in the Lugano water column or sediments. The preservation of phytol seems to be enhanced in sediments with a high relative contribution of land plant OM. Major factors affecting OM accumulation in the lakes are differences in OM sources (i.e. terrestrial OM vs. autochthonous production), extent of bacterial activity and most likely oxygen availability in the water column.     

Geochemical technique of organic matter research in deposits enrich in kerogene (the Bazhenov Formation,West Siberia)

The international interest in shale oil has recently provoked special attention to the Russian unconventional oil-source formations, including the Bazhenov Formation of West Siberia, domanik deposits in the Volga–Ural region, and the lower Maikop Group of the Cis-Caucasus. High contents of para-autochthonous soluble organic matter (bitumen) in clayey–carbonate, clayey–siliceous, carbonate–clayey–siliceous rocks with low filtration–capacity properties results in significant uncertainties in the identification of the generation potential of organic matter (OM). Examination of a large database on the OM of the Bazhenov Formation allowed us to propose an optimum complex technique for study of the source rock potential and assessment of the amount of produced hydrocarbons in the kerogen-rich sediments. The investigations include a combination of Rock Eval pyrolysis prior to and after extraction with different solvents, the comparison of bituminological and pyrolytic characteristics, and the determination of the group composition of soluble organic matter, as well as chromatography and chromato-mass spectrometry.     

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.     

Transformation of particulate organic matter at the water-bottom boundary in the Russian Arctic seas: Evidence from isotope and radioisotope data

Complex biogeochemical studies including the determination of isotopic composition of C_org in both suspended particulate matter and surface horizon (0–1 cm) of sediments (more than 260 determinations of δ¹³C-C_org) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi. The aim of this study is to elucidate causes that change the isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that the isotopic composition of C_org in sediments from seas with a high river runoff (White, Kara, and East Siberian) does not inherit the isotopic composition of C_org in particles precipitating from the water column, but is enriched in heavy ¹³C. Seas with a low river runoff (Barents and Chukchi) show insignificant difference between the value of δ¹³C-C_org in both suspended load and sediment because of a low content of the isotopically light allochthonous organic matter (OM) in particulates. Complex biogeochemical studies with radioisotope tracers (¹⁴CO₂, ³⁵S, and ¹⁴CH₄) revealed the existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents the mass influx of products of OM decomposition into water column, as well as reduces the influx of a part of OM contained in the suspended particulate matter from water into sediment.     

Investigating pathways of diagenetic organic matter sulfurization using compound-specific sulfur isotope analysis

We present the results of compound-specific sulfur isotope analyses performed on organic sulfur compounds (OSCs) isolated from sediments deposited in the euxinic Cariaco Basin, Venezuela. Individual OSCs (sulfurized highly branched isoprenoids and malabaricatriene) have sulfur isotope compositions of ca. −15‰, which is ³⁴S enriched by 5-15‰ relative to coeval bulk organic and inorganic sulfur pools. These observed differences in the sulfur isotope composition of bulk organic sulfur in the kerogen and bitumen pools and individual OSCs demonstrate that there are multiple pathways of organic sulfur formation operating simultaneously in marine sediments. Comparison of our measured compound-specific sulfur isotope data with values predicted using simple isotopic mass balance assumptions suggests that the sulfurization process likely involves multiple sources of inorganic sulfur. Further, the isotopic composition of these various precursor inorganic sulfur species and the specific pathway of sulfur incorporation into organic matter (OM) impart distinct isotopic compositions to the resulting organic sulfur compounds. These data represent the first compound-specific sulfur isotope measurements made in marine sediments, and demonstrate the utility of compound-specific sulfur isotope analysis in identification of inorganic sulfur sources for OM sulfurization and tracking pathways of sulfur incorporation, which will lead to a more complete understanding of diagenetic sulfurization of OM.     

A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).     

Organic matter from the Callovian (Middle Jurassic) deposits of Lithuania: Compositions,sources and depositional environments

This study presents the first organic geochemical and petrographical investigation of the Callovian deposits of the eastern part of the Central European Basin. It is shown that in both the terrigenous Papil? Formation (Lower Callovian) and shallow- to deeper-marine facies of the Papartin? and Skinija formations (Middle and Upper Callovian, respectively), terrestrial organic matter predominates. This is reflected by the carbon preference index values higher than 1.2 for all samples and in some cases higher than 2, as well as the occurrence of characteristic higher plant biomarkers like cadalene, dehydroabietane, simonellite and retene. Moreover, in the case of the Papil? Formation, sugiol – a natural product terpenoid produced by distinct conifer families, has been detected in clay sediments. The occurrence of such a biomolecule in the Middle Jurassic clays is reported for the first time. Its occurrence is probably connected with the presence of small wood debris in the clay sediments. In samples of the Papil? Formation, charcoal fragments co-occurring with unsubstituted polycyclic aromatic hydrocarbons were detected, indicating that wildfires took place during the Early Callovian of Lithuania and/or neighbouring areas. In the Middle and Upper marine Callovian sediments of Lithuania there is no evidence of anoxic conditions occurring in the water column. However, periodic anoxic or strongly dysoxic episodes may have occurred, most probably below the photic zone, during the deepest phase of the Late Callovian transgression, as is evidenced from pyrite framboid diameter distribution and general impoverishment of benthic fauna. Huminite reflectance (R_r) values for the investigated area are in the range of 0.21–0.31%, suggesting the occurrence of immature organic matter. Such values indicate that these investigated deposits were close to the surface during their whole diagenetic history, and the thickness of younger cover did not exceed ca. 500 m. This is also supported by a biomarker analysis in which less thermally stable ββ-hopanes and hopenes significantly dominated.     

Identification of aryl isoprenoids in source rocks and crude oils: Biological markers for the green sulphur bacteria

A series of C₁₃ to C₃₁ aryl isoprenoids (1-alkyl,2,3,6-trimethylbenzenes) have been identified in reef-hosted oils and their source rocks from the Middle and Upper Silurian of the Michigan Basin and Middle Devonian of the Alberta Basin, Canada. Their structure has been confirmed by unambiguous synthesis of the C₁₄ member of the series. Their structure and isotopic composition indicate that they are derived from isorenieratene from the Chlorobiaceae family of sulphur bacteria. These results are consistent with geological and geochemical studies that show that the source rocks were deposited under metahaline to hypersaline sulphate and sulphide rich water columns. The distribution of other biomarkers in these oils and source rocks indicates that a diverse biota contributed organic matter to the source environment. In conjunction with the aryl isoprenoids, they show that there is a remarkable similarity in composition between the two sets of oils and source rocks despite their great temporal and geographic separation. This reflects the similarity of their environments and emphasizes the importance of sedimentary facies in controlling the composition of organic matter in source rocks and their derived oils.     

Composition and genesis of the organic matter in the bottom sediments of the East Siberian Sea

The chemical composition of organic matter (C_org, N_org, δ¹³C, δ¹⁵N, and n-alkanes) was studied in the top layer of bottom sediments of the East Siberian Sea. Possible ways were proposed to estimate the amount of the terrigenous component in their organic matter (OM). The fraction of terrigenous OM estimated by the combined use of genetic indicators varied from 15% in the eastern part of the sea, near the Long Strait, to 95% in the estuaries of the Indigirka and Kolyma rivers, averaging 62% over the sea area.     

Changes of palaeoenvironmental conditions recorded in Late Devonian reef systems from the Canning Basin,Western Australia: A biomarker and stable isotope approach

Although the Late Devonian extinctions were amongst the largest mass extinction events in the Phanerozoic, the causes, nature and timing of these events remain poorly restrained. In addition to the most pronounced biodiversity loss at the Frasnian–Famennian (F–F) boundary and the end Famennian, there were also less extensively studied extinction pulses in the Middle to Late Givetian and the Frasnian. Here we used a combination of palynological, elemental, molecular and stable isotope analyses to investigate a sedimentary record of reef-systems from this time period in the Canning Basin, Western Australia.The acquired data generally showed distinct variations between sediments from (i) the time around the Givetian–Frasnian (G–F) boundary and (ii) later in the Frasnian and indicated a distinct interval of biotic stress, particularly for reef-builders, in the older sediments. Alterations of pristane/phytane ratios, gammacerane indices, Chlorobi biomarkers, δD_kerogen and chroman ratios describe the change from a restricted marine palaeoenvironment with an anoxic/euxinic hypolimnion towards a presumably open marine setting with a vertically mixed oxic to suboxic water column. Simultaneous excursions in δ¹³C profiles of carbonates, organic matter (OM) and hydrocarbons in the older sediments reflect the stratification-induced enhancement of OM-recycling by sulfate reducing bacteria. Alterations in sterane distributions and elevated abundances of methyltrimethyltridecylchromans (MTTCs) and perylene indicate an increased terrigenous nutrient input via riverine influx, which would have promoted stratification, phytoplankton blooms and the development of lower water column anoxia.The detected palaeoenvironmental conditions around the G–F boundary may reflect a local or global extinction event. Our data furthermore suggest a contribution of the higher plant-expansion and photic zone euxinia to the Late Devonian extinctions, consistent with previous hypotheses. Furthermore, this work might contribute to the understanding of variations in Devonian reef margin and platform-top architecture, relevant for petroleum exploration and development in the global Devonian hydrocarbon resources.     

Geochemical analysis of mixed oil in the Ordovician reservoir of the Halahatang Depression, Tarim Basin, China

In this study,12 crude oil samples were collected and analyzed from the Ordovician reservoir in the Halahatang Depression,Tarim Basin,China.Although the density of oil samples varies considerably,based on saturated hydrocarbon gas chromatographic(GC),saturated and aromatic hydrocarbon gas chromatographic-mass spectrometric(GC/MS) and stable carbon isotopic composition analyses,all the samples are interpreted to represent a single oil population with similar characteristics in a source bed or a source kitchen,organic facies and even in oil charge history.The co-existence of a full suite of n-alkanes and acyclic isoprenoids with UCM and 25-norhopanes in the crude oil samples indicates mixing of biodegraded oil with fresher non-biodegraded oil in the Ordovician reservoir.Moreover,according to the conversion diagram of double filling ratios for subsurface mixed crude oils,biodegraded/non-biodegraded oil ratios were determined as in the range from 58/42 to 4/96.Based on oil density and oil mix ratio,the oils can be divided into two groups:Group 1,with specific density>0.88(g/cm3) and oil mix ratio>1,occurring in the north of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines,and Group 2,with specific density<0.88(g/cm3) and oil mix ratio<1,occurring in the south of the pinchout lines.Obviously,Group 2 oils with low densities and being dominated by non-biodegraded oils are better than Group 1 oils with respect to quality.It is suggested that more attention should be paid to the area in the south of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines for further exploration.     

四川广元上寺剖面上二叠统大隆组有机质分布特征与富集因素

四川盆地上二叠统大隆组富有机质硅质泥岩是页岩气勘探的一套重要目的层系.利用扫描电子显微镜和元素地球化学等方法探究大隆组有机质类型、分布特征及富集因素,对该地区非常规油气勘探具有指导意义.根据扫描电镜图片,上寺剖面大隆组有机质按形态可分为形态有机质、弥散有机质和沥青.三者成因不同,形态有机质是选择性保存的结果,以离散状分布在所有岩性样品中;弥散有机质是大隆组有机碳总量(total organic carbon,TOC)的主体,主要分布在硅质泥岩样品中,硅质灰岩样品中极少,体现了粘土矿物的吸附作用;沥青是可溶有机质运移进入孔、缝系统,经高热演化后形成的块状有机质,主要分布于硅质灰岩和灰岩样品中.该剖面地球化学数据显示缺氧沉积环境有利于有机质保存,但海洋表层生产力才是控制TOC含量变化的主要因素.研究表明,晚二叠世海平面上升导致上寺剖面大隆组表层生产力增加及底层海水缺氧,该组富有机质黑色硅质泥岩段高TOC是沉积环境与矿物吸附共同作用的结果,同时也反映了原生有机质保存情况.更好的理解泥质烃源岩中有机质赋存类型有助于烃源岩评价及非常规油气勘探开发工作.      

Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland)

We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ¹³C_TOC and δ¹⁵N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ¹³C_DIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH₄ and CO₂ in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO₃.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes.     

How depositional conditions control input, composition, and degradation of organic matter in sediments from the Chilean coastal upwelling region

In a comprehensive study, we compared depositional conditions, organic matter (OM) composition, and organic carbon turnover in sediments from two different depositional systems along the Chilean continental margin: at ∼23° S off Antofagasta and at ∼36° S off Concepción. Both sites lie within the Chilean coastal upwelling system and have an extended oxygen minimum zone in the water column. However, the northern site (23° S) borders the Atacama Desert, while the southern site (36° S) has a humid hinterland. Eight surface sediment cores (up to 30 cm long) from water depths of 126-1350 m were investigated for excess ²¹⁰Pb (²¹⁰Pb_xs) activity, total organic and total inorganic carbon concentrations (TOC and TIC, respectively), C/N-ratios, organic carbon isotopic compositions (δ¹³C), chlorin concentrations, Chlorin Indices (CI), and sulfate reduction rates (SRR). Sediment accumulation rates obtained from ²¹⁰Pb-analysis were similar in both regions (0.04-0.15 cm yr⁻¹ at 23° S, 0.10-0.19 cm yr⁻¹ at 36° S), although total ²¹⁰Pb_xs fluxes indicated that the vertical particle flux was higher at 36° S than at 23° S. We propose that sediment focusing in isolated deposition centers led to high sediment accumulation rates at 23° S. Furthermore, there were no indications for sediment mixing at 23° S, while bioturbation was intense at 36° S. δ¹³C-values (−24.5‰ to −20.1‰ vs. VPDB) and C/N-ratios (molar, 8.6-12.8) were characteristic of a predominantly marine origin of the sedimentary OM in both investigated areas. The extent of OM alteration in the water column was partly reflected in the surface sediments as chlorin concentrations decreased and C/N-ratios and CI increased with increasing water depth of the sampling site. SRR were lower at 23° S (areal SRR 0.12-0.60 mmol m⁻² d⁻¹) than at 36° S (areal SRR 0.82-1.18 mmol m⁻² d⁻¹), which was partly due to the greater water depth of most of the sediments investigated in the northern region and consistent with a lower quality of the sedimentary OM at 23° S. Reaction rate constants for TOC degradation that were obtained from measured SRR (k_SRR; 0.0004-0.0022 yr⁻¹) showed a good correspondence to k_TOC that were derived from the depth profiles of TOC (0.0003-0.0014 yr⁻¹). Both, k_SRR and k_TOC, reflect differences in OM composition. At 36° S they were related to the degradation state of bulk OM (represented by C/N-ratios), whereas near 23° S they were related to the freshness of a small fraction of labile OM (represented by CI). Our study shows that although rates of organic carbon accumulation were similar in both investigated sites, the extent and kinetics of organic carbon degradation were closely linked to differing depositional conditions.     

Organic matter of the World Ocean

Available data on synthesis, input, and decomposition of organic matter (OM) in the water column and recent bottom sediments of the World Ocean are generalized. The most reliable values of OM production and masses in the ocean, the total supply of organic carbon, and the input of terrigenous OM with coastal erosion, river runoff, and eolian matter are estimated. Maps of fossilization coefficients, distribution, and accumulation of OM in recent bottom sediments of the World Ocean are presented. A numerical expression is proposed for the main circumcontinental pattern of OM accumulation in the ocean. The group and elemental compositions of living matter of the ocean, land, and the Earth as a whole and the organic composition of bottom sediments are briefly considered.