Amounts Of Terrigenous Organic Matter Research Articles

Organic petrographic and geochemical insights into organic matter derived from land plants and marine algae in the Lark Formation, Danish North Sea

Climatic fluctuations from the Eocene to the Miocene highlight the importance of investigating the paleoenvironment of the latest Eocene to the Middle Miocene Lark Formation in the Danish North Sea. This study investigates immature sedimentary organic matter in the Lark Formation using 54 cuttings samples and one core sample collected from seven wells in the eastern North Sea Basin. Organic petrography and molecular geochemistry analyses were performed to determine the variations in the quantity and origin of allochthonous and autochthonous organic matter. Additionally, the study assesses the impact of climate fluctuations on marine productivity in the eastern North Sea Basin and land plant vegetation at the basin margins during the latest Eocene to the Middle Miocene.The organic matter in the Lark Formation originated from mixed sources, primarily land plants, with a secondary contribution from marine algae. This is indicated by the maceral composition and the types and abundance of aliphatic and aromatic hydrocarbon biomarker compounds. Moreover, the presence of diterpenoids (gymnosperm biomarkers) and non-hopanoid triterpenoids (angiosperm biomarkers) reveals that the allochthonous organic matter originated from conifers and angiosperms.Climatic impacts on land plants and marine algae during the latest Eocene to the Middle Miocene are revealed by several parameters: the Averaged Chain Length (ACL) of land plant waxes, the proportion of coniferous contribution (C/(C + A)), and the whole rock volume percentages of huminite, inertinite (H + I, vol%) and liptinite (L, vol%). The shifts to cooler and drier climates highlighted the cold adaptation of onshore conifers and resulted in the input of higher molecular weight waxy components into the sediments. However, under these conditions, reduced precipitation and runoff resulted in lower amounts of terrigenous organic matter supplied to the basin. Additionally, the drop in water temperature and the warm-affinity of local algae assemblage led to reduced marine productivity. Together, these factors contributed to an overall decrease in organic richness. In contrast, during shifts to warmer and more humid climates, the trend reversed. The contribution of conifers to the floral assemblage diminished, but higher amounts of terrigenous organic matter were transported to the basin due to increased precipitation and runoff. This was accompanied by warmer water temperatures, boosting the productivity of organic-walled microplankton in the marine environment and contributing to greater organic richness.

Lignin-oxidizing and xylan-hydrolyzing Vibrio involved in the mineralization of plant detritus in the continental slope

A large amount of terrigenous organic matter (TOM) is constantly transported to the deep sea. However, relatively little is known about the microbial mineralization of TOM therein. Our recent in situ enrichment experiments revealed that Vibrio is especially enriched as one of the predominant taxa in the cultures amended with natural plant materials in the deep sea. Yet their role in the mineralization of plant-derived TOM in the deep sea remains largely unknown. Here we isolated Vibrio strains representing dominant members of the enrichments and verified their potential to degrade lignin and xylan. The isolated strains were closely related to Vibrio harveyi, V. alginolyticus, V. diabolicus, and V. parahaemolyticus. Extracellular enzyme assays, and genome and transcriptome analyses revealed diverse peroxidases, including lignin peroxidase (LiP), catalase-peroxidase (KatG), and decolorizing peroxidase (DyP), which played an important role in the depolymerization and oxidation of lignin. Superoxide dismutase was found to likely promote lignin oxidation by supplying H2O2 to LiP, DyP, and KatG. Interestingly, these deep-sea Vibrio strains could oxidize lignin and hydrolyze xylan not only through aerobic pathway, but also through anaerobic pathway. Genome analysis revealed multiple anaerobic respiratory mechanisms, including the reductions of nitrate, arsenate, tetrathionate, and dimethyl sulfoxide. The strains showed the potential to anaerobically reduce sulfite and metal oxides of iron and manganese, in contrast the non-deep-sea Vibrio strains were not retrieved of genes involved in reduction of metal oxides. This is the first report about the lignin oxidation mechanisms in Vibrio and their role in TOM mineralization in anoxic and oxic environments of the marginal sea.

Biodegradation of Terrigenous Organic Matter in a Stratified Large-Volume Water Column: Implications of the Removal of Terrigenous Organic Matter in the Coastal Ocean.

Large amounts of terrigenous organic matter (TOM) are delivered to the ocean every year. However, removal processes of TOM in the ocean are still poorly constrained. Here, we report results from a 339-day dark incubation experiment with a unique system holding a vertically stratified freshwater-seawater column. The quality and quantity of dissolved organic matter (DOM), RNA-based size-fraction microbial communities, and environmental factors were high-frequency-monitored. Microbial processes impacted TOM composition, including an increased DOM photobleaching rate with incubation time. The mixed layer had changed the bacterial community structure, diversity, and higher oxygen consumption rate. A two-end member modeling analysis suggested that estimated nutrient concentrations and prokaryotic abundance were lower, and total dissolved organic carbon was higher than that of the measured values. These results imply that DOM biodegradation was stimulated during freshwater-seawater mixing. In the bottom layer, fluorescent DOM components increased with the incubation time and were significantly positively related to highly unsaturated, oxygenated, and presumably aromatic compound molecular formulas. These results suggest that surfaced-derived TOM sinking leads to increased DOM transformation and likely results in carbon storage in the bottom water. Overall, these results suggest that microbial transforming TOM plays more important biogeochemical roles in estuaries and coastal oceans than what we know before.

SOURCE ROCK QUALITY VARIATIONS OF UPPER JURASSIC – LOWERMOST CRETACEOUS MARINE SHALES AND THEIR RELATIONSHIP TO OILS IN THE VALDEMAR FIELD, DANISH NORTH SEA

The main source rocks for the hydrocarbons at the Valdemar field (Danish North Sea) are the Upper Jurassic – lowermost Cretaceous organic‐rich marine shales of the Farsund Formation. However, geochemical analyses of retained petroleum in reservoir cores show variations in oil type and maturity which indicate a complex charging history. This paper reviews the organofacies and source rock quality variations in 55 samples of the Farsund Formation from the North Jens‐1 well (Valdemar field) within a sequence stratigraphic framework in order to discuss the source of the hydrocarbons. Petrographic and geochemical data, including biomarker analyses, were integrated in order to characterize the kerogen composition, original source rock potential and depositional environment of the Farsund Formation.The thermal maturity, source rock quality and kerogen quality all vary at the sequence level, and in general change upwards from early mature, primarily gas‐prone Type II kerogen in the Kimmeridgian Kimm‐2 and Kimm‐3 sequences to immature, highly oil‐prone sapropelic Type II kerogen in the Volg‐4 and Ryaz‐1 sequences (Volgian, Ryazanian). The kerogen has a maceral composition dominated by fluorescing amorphous organic matter (AOM) and liptodetrinite, with variable but generally minor amounts of terrigenous organic matter. The stratigraphic distribution of organic matter is similar to that in regional observations from the Danish Central Graben but minor differences occur, especially in the amount of fluorescing AOM in the Kimmeridgian sequences. The decrease in terrigenous input (vitrinite) upwards through the marine shale succession likely reflects a marine transgression of the Danish Central Graben area during Late Jurassic time. The source potential of the Upper Jurassic – lowermost Cretaceous shales in the North Jens‐1 well is generally lower than that observed regionally, including an absence of relatively organic‐rich, oil‐prone intervals in the older part of the succession which have been demonstrated to occur elsewhere in the Danish Central Graben. However, in agreement with the regional trend, back‐calculated source rock data and calculated Ultimate Expulsion Potentials show that the uppermost Volgian (Volg‐4) and Ryazanian (Ryaz‐1) sequences are the most oil‐prone intervals. The Ryaz‐1 sequence represents a condensed section formed during a period characterised by low sedimentation rates and high preservation of algal organic matter.Biomarker compositions from source rock extracts from the North Jens‐1 well cannot be directly correlated to Valdemar reservoir oils, suggesting that the mature organofacies which charged the oils are not represented in the samples from North Jens‐1.

Source rock potential of the Early Cretaceous intervals in the Darquain field, Abadan Plain, Zagros Basin, SW Iran

The Darquain field is located in the Abadan Plain SW Iran, which has a high potential for exploration and production of hydrocarbon resources with poorly defined petroleum systems. The goal of this paper is to characterize the Cretaceous source rock potential in the Darquain field. A gamma-sonic log was used as a proxy to identify probable source rock zones and to select sample depths. A total of 37 samples were analyzed by Rock-Eval, elemental, Gas Chromatography (GC) and gas chromatographymass spectrometry (GC-MS) techniques. The petroleum generation potential, as well as origin of the organic matter of samples, have been investigated. The results of this study indicate that, in the Early Cretaceous successions, there are favorable source zones in Garau, Gadvan, and Kazhdumi Formations. These source zones predominantly contain organic matter from marine origin, suggesting kerogen type II, while some samples contain terrestrial organic matters, suggesting kerogen type III. The Lower Garau zone, with 1.14 to 14.29% total organic carbon (TOC), is an excellent source rock with a fair genetic potential in the range of 2.74 and 9.5 mg/g. Elemental analysis results in the Lower Garau zone allow the classification of kerogens as high-sulfur. There are very good source zones in the Kazhdumi Formation, with fair to good genetic potential. The results of Rock-Eval and petrophysical analyses show that gamma ray data can be used as a useful tool to determine the potential source zones, although where TOC is less than 1% there is inconsistency. The results obtained from the biomarker characteristics are in agreement with the results of Rock-Eval pyrolysis and indicated carbonate source rocks, which contain a mixture of marine organic matter with a minor amount of terrigenous organic matter. Biomarker characteristics also suggest that the organic matter was deposited in a marine environment under reducing conditions, and that Early Cretaceous source zones are at the stage of early oil window to peak of oil generation.

Sedimentology, geochemistry and paleoenvironmental reconstruction of the Cretaceous Yolde formation from Yola Sub-basin, Northern Benue Trough, NE Nigeria

Sedimentology, organic and inorganic geochemical studies were applied on sediments of the Cretaceous Yolde formation from Yola Sub-basin, Northern Benue Trough, northeastern Nigeria with an attempt to determine the sedimentary facies, paleo-redox condition and to re-construct the palaeo-depositional environments. Eight (8) facies were identified on the basis of lithology, grain size, ichnofossils/degree of bioturbation and sedimentary structures. These facies constitute four facies associations; the FA-1 (offshore marine), FA-2 (offshore transition to lower shoreface), FA-3 (middle shoreface) and FA-4 (upper shoreface). The succession of these facies associations indicates storm and wave influenced offshore/shoreface depositional environments for the formation. Molecular organic geochemical investigation suggests deposition under sub-oxic marine environment with major contribution of phytoplankton, bacteria and minor amount of terrigenous organic matter. XRD mineralogical assessment, and major and trace elements geochemistry revealed that the Yolde formation sediments were sourced from continental areas of passive continental margin setting and deposited in semiarid climate under suboxic shallow marine conditions related to the first marine transgression that occurred in the Benue Trough during the Late Cretaceous period.

Geochemical and petrographic characterization of Late Jurassic–Early Cretaceous Chia Gara Formation in Northern Iraq: Palaeoenvironment and oil-generation potential

The marine Late Jurassic to Early Cretaceous Chia Gara Formation is well exposed in northern Iraq. It is built of organic-rich limestones and calcareous shales. The organic-rich sediments have been investigated to determine the type and origin of the organic matter as well as their petroleum-generation potential.Kerogen microscopy shows that these sediments are characterized by large amounts of predominantly amorphous organic matter with a Total Organic Carbon content of 7.42%. The large amounts of organic matter are mainly due to good preservation under anoxic conditions, as evidenced by numerous pyritized fragments and biomarkers that are diagnostic for the depositional environment. The investigated biomarkers are characterized by a dominance of low to medium molecular weight compounds, a low Pr/Ph ratio (<1.0), a composition of C27–C29 regular steranes, and the presence of tricyclic terpanes, indicating a strong decay of marine organic matter preserved under reducing conditions. A small amount of terrigenous organic matter is, according to the n-alkane distribution, also present.The Chia Gara sediments thus have a high oil- but a low gas-generation potential due to the high content of hydrogen-rich Type II and mixed Type II–III kerogens with a minor contribution of Type III kerogen.

Stable carbon isotopic compositions of individual aromatic hydrocarbons as source and age indicators in oils from western Australian basins

The present study aims to establish the factors controlling the stable carbon isotopic compositions (δ 13C) of individual aromatic hydrocarbons analysed by compound specific isotope analysis (CSIA) in crude oils from western Australian petroleum basins of varying age and facies type. This paper reports δ 13C values of individual aromatic hydrocarbons, like alkylbenzenes, alkylnaphthalenes, alkylphenanthrenes and methylated biphenyls. The main aims are to confirm the origin (source) and age of these oils based on CSIA of selected aromatic compounds and to understand why the Sofer plot is ineffective in establishing the source of western Australian petroleum systems. The bulk δ 13C of saturated and aromatic hydrocarbon fractions of crude oils have been previously used to differentiate sources, however, many Australian crude oils are not classified correctly using this method. The oils were classified as marine by the δ 13C values of individual aromatic compounds and as terrigenous based on the bulk δ 13C data (Sofer plot). The oils where the δ 13C values of 1,6-DMN and 1,2,5-TMN isomers are most negative are indicative of a marine source, whereas oils with a less negative values for the 1,6-DMN and 1,2,5-TMN isomers are derived from marine source rocks that contain a significant terrigenous component. Similarly, oils with the least negative δ 13C values for the 1-MP and 1,9-DMP isomers reflect varying inputs of terrigenous organic matter to the their marine source rocks. Plots of P/DBT and Pr/Ph concentration ratios versus δ 13C values of DMP, 1,6-DMN, 1,2,5-TMN, 1-MP and 1,9-MP are constructed to establish the relative amount of terrigenous organic matter contributing to the source rock of a series of marine oils. The ratios of P/DBT and Pr/Ph plotted against the δ 13C values of the aromatic isomers (such as 1,6-DMN, 1,2,5-TMN, 1-MP and 1,9-MP) provide a novel and convenient way to discriminate crude oils derived from different source rocks that contain varying amounts of marine and terrigenous organic matter.

Understanding shallow gas occurrences in the Gulf of Lions

New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95,700 ppm) off the Rhone prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004-2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions, and its association with recent sedimentary dynamics. The Rhone prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhone delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon. In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 5,000 ppm methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples at the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.

Shallow gas off the Rhône prodelta, Gulf of Lions

Sediment cores acquired in 2004 off the Rhône prodelta show consistent anomalous methane concentrations of up to 87,440 ppm. Methane compositional and isotopic data support a biogenic origin, although there are a few sites that show strongly depleted δ 13C values (− 53‰ PDB) suggesting a mixed source for the gas (biogenic and thermogenic). Anomalous methane concentrations (samples with more than 90 ppm) are discussed and integrated with organic carbon data, sedimentary rates and ADCP profiles. Highest gas concentrations were found directly off the river mouth (20–40 m water depth) and where the IFREMER models point to the thickest accumulation (> 2 m) in response to the Rhône flood event. In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met; in these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the sub-surface. We propose that in the Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake and leading to the potential for methanogenesis with burial.

Shallow gas and flood deposition on the Po Delta

Sediment cores acquired on the Po delta, northwestern Adriatic Sea, in water depths of 10 to 25 m yielded anomalous concentrations of methane up to 41,300 ppm. Of the 19 unique sites analyzed 5 sites (9 cores) had more than 90 ppm of CH 4 and of those, 3 sites (4 cores) had more than 13,800 ppm methane. Compositional and isotopic analyses of the gas support a bacterial origin. Anomalous methane concentrations were found in kasten, gravity, and box cores; where more than one core type was acquired at a single location, all cores contained anomalous methane. Despite the regional high productivity in the Adriatic, all of the highest gas concentrations were found in the region associated with the thickest accumulation of recent flood deposits off the most active channel of the Po delta (Po di Pila). Cores acquired in this region contain primary sedimentary structures that indicate rapid burial of thick (> 10 cm) flood deposits and a relative lack of bioturbation down to the base of the flood layer. We propose that in the Po delta, flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried on the prodelta, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. In areas unaffected by this high flux of organic matter and rapid/thick flood deposition, or in between flood events, our data indicate that the conditions for methanogenesis and gas accumulation have not been met. We suggest that in these areas, the physical and biological reworking of the surficial sediment between flood events may effectively oxidize and mineralize organic matter (derived from both marine and terrestrial sources) and limit bacterial methanogenesis in the subsurface.

The molecular composition of kerogen in Pliocene Mediterranean sapropels and associated homogeneous calcareous ooze

The organic matter of three time-equivalent Pliocene sapropels was characterized as being predominantly Type II kerogen using RockEval pyrolysis. It is dominated by amorphous organic matter, originating mainly from marine organisms, as indicated by the relatively high abundances of algaenan- and chlorophyll-derived pyrolysis products in the Curie-point pyrolysates of the kerogen of the sapropels. However, a small amount of terrigenous organic matter was also observed in the form of pollen and spores that most probably yielded the oxygenated aromatic pyrolysis products known to derive from sporopollenin. Lignin pyrolysis products, such as methoxyphenol and catechol (1,2-benzenediol), commonly used as unequivocal evidence for a contribution of terrestrial higher plants in the form of woody material to the kerogen, were absent from the kerogen pyrolysates. No major spatial and temporal differences in the kerogen composition of the three time-equivalent sapropels were observed. Palynological studies revealed that organic matter in the homogeneous intervals, below and above the sapropels, consisted of pollen, spores and amorphous organic matter. This was confirmed by Curie-point pyrolysis – GC/MS results showing a relatively high abundance of algaenan-derived compounds and the presence of oxygenated aromatic pyrolysis products. The differences observed between the kerogen pyrolysates of the sapropels and their homogeneous intervals are suggested to be governed primarily by enhanced preservation conditions, resulting in increased preservation of marine organic matter in the sapropels.

Source-Rock Hydrocarbon Potential of the Middle Triassic—Lower Jurassic Cudi Group Units, Eastern Southeast Turkey

The southern part of Southeast Turkey constitutes the northern margin of the Arabian plate. The region contains rock units ranging in age from Paleozoic to Mesozoic. The source-rock potential of the Middle Triassic-Lower Jurassic Cudi Group in the eastern part of Southeast Turkey was investigated. Samples were obtained from six wells (çilesiz-1, B. Yolaçan-1, G. Dinçer-1, çamurlu-101, çamurlu-102, and Yolaçan-3). The source-rock potential of these units was evaluated by organicgeochemical and petrographic analyses. Sediments have an average of 0.15% total organic carbon (TOC). Commonly, TOC is higher in carbonate-rich facies than in evaporitic deposits. TOC analyses of the Cudi Group shows that these formations are poor to moderate in organic matter. The S1 (average 0.25 mgHC/gTOC), S2 (0.40 mgHC/gTOC), hydrogen index (HI) (average 235 mgHC/gTOC), oxygen index (OI) (average 231 mgCO2/gTOC), and Tmax (average 422°C) values from the Rock-Eval pyrolysis indicate that the dominant organic matter consists of types II and III kerogen, which have gas generative potential. Production index (PI) values indicate samples containing secondarily migrated hydrocarbons. Optical investigation demonstrates that the organic matter is mainly amorphous (70-90%) and of marine origin, but includes small amounts of terrigenous organic matter (10-30%). Results suggest that these beds probably do not constitute a source of liquid hydrocarbons but may represent a source of gas.

Composition of organic matter in subducted and unsubducted sediments off the Nicoya peninsula, Costa Rica (ODP Leg 170, Sites 1039 and 1040)

Investigation of sediment samples from Sites 1039 and 1040 (ODP Leg 170) drilled off the Nicoya peninsula (Costa Rica) by organic geochemical and organic petrological methods has revealed that subduction has only little influence on the composition of the sedimentary organic matter. Organic carbon contents reached 1.5% in the Pleistocene samples but Miocene and Pliocene sediments had an average organic carbon content of less than 0.5%. Organic carbon/sulfur ratios are generally below 2.8, reflecting an intense sulfate reduction in the uppermost sediments which was enhanced by sulfate supply both from sea water and deeper strata. Microscopical examinations indicate that the organic matter is mainly derived from marine sources. A small amount of terrigenous organic matter is, however, present as well according to n-alkane and fatty acid distributions. The alkenone unsaturation index U37k′ shows only a slight decrease during the Miocene and Pliocene, and stronger variations in the Pleistocene, probably indicating more stable sea surface temperatures during the Miocene and Pliocene. Variations in the Pleistocene can possibly be related to glacial/interglacial changes.

Geochemical characteristics of oils from Taiwan

Geochemical studies on a number of oils from Taiwan have shown that they are sourced predominantly from terrigenous organic matter. Biomarker characteristics include the presence of high amounts of long-chain regular isoprenoids, bicyclic sesquiterpanes, diterpanes, low concentrations or absence of tricyclic terpanes, presence of several C 24 tetracyclic terpanes and some source-specific pentacyclic terpanes and a predominance of C 29 steranes. For most of the oils, the n-alkanes extended to C 30 or above and have CPI values of approximately 1, indicating the mature nature of these oils. The pristane/phytane ratios, generally in excess of 5, suggest the likely prevalence of a highly oxic, nearshore, marine-open peat swamp depositional environment which incorporated large amounts of terrigenous organic matter. The long-chain isoprenoids extending to C 42, and possible higher, are proposed to be regular head-to-tail isoprenoids possibly derived from polyprenols in higher plants. The sesquiterpanes present in the oils range from C 15 and C 17 and are dominated by 8β(H)-drimane and 8β (H)-homodrimane. The diterpanes include C 17-C 20 tricyclic and C 19 and C 20 tetracyclic diterpanes. The presence of diterpanes in these oils suggests that resins derived from conifers contributed to the source of these oils. At least nine tetracyclic terpanes with carbon numbers ranging 24 to 27 were detected in Taiwan oils. Two series of tetracyclic terpanes are apparent, one is the de-A-oleanane/ursane series and the other is the 17,21-secohopane series. The former is more source-specific than the latter and is probably related to the presence of oleanane. A number of pentacyclic terpanes, including oleanane, have been detected and many of these compounds may be of terrigenous origin and source-specific. Oleanane, de-A-oleanane/ursane and de-A-lupane can be used as indicators for angiosperms which have evolved since the late Cretaceous. Sterane distributions in the Taiwan oils are dominated by the C 29 steranes and include regular and rearranged steranes, indicating the terrigenous nature of the organic matter in the source rocks. The significance of the variations observed in the biomarker distributions of the oils from Taiwan is discussed in some detail in this paper.

Organic carbon, sulphur, and iron in recent semi-euxinic sediments of Kau Bay, Indonesia

Kau Bay (island of Halmahera, Eastern Indonesia) is a 470 m deep basin separated from the Pacific Ocean by a sill that is at present only 40 m below sea-level. During Weichselian time, the sea-level dropped below the depth of the sill and freshwater sediments were deposited. These sediments are rich in organic matter ( C org = 2–6 wt% ) with terrigenous isotope characteristics ( δ 13C org = −28 to −30%. ), and are also rich in sulphur (S ≈ 4 wt%). This sulphur enrichment is interpreted to be a late diagenetic feature related to diffusion of dissolved sulphate and sulphide from the marine sediments into the freshwater sediments; the sulphur in these sediments has a δ 34S value of + 15%.. The overlying Holocene sediments consist of marine muds deposited in alternating low-oxygen and H 2S-bearing bottom waters. The marine sediments are also rich in organic matter (C org = 2–6 wt%), but of mixed terrigenous and marine origin ( δ 13C = −25 to −19%. ). Moreover, the sulphur content of these Holocene sediments is lower (S = 1–2 wt%) and isotopically lighter ( δ 34S = −20%. ). In addition to pyrite, large amounts of acid-volatile sulphur (AVS) compounds accumulate in Kau Bay. The reactive iron content of the sediment limits the amount of iron that is sulphidized, while the supply of oxidants controls the limited conversion of AVS to pyrite. Total and reactive iron contents are strongly correlated with the amount of terrigenous organic matter as a consequence of their common provenance and depositional histories. The marine sediments in Kau Bay show no simple relation between organic carbon and sulphur.