Terrigenous Organic Matter Input Research Articles

Hydroxylated isoprenoid GDGTs in Chinese coastal seas and their potential as a paleotemperature proxy for mid-to-low latitude marginal seas

Hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (OH-GDGTs), with one or two hydroxyl groups in one of the biphytanyl moieties, are major compounds in planktonic Thaumarchaeota, and occur widely in marine and lacustrine sediments. In order to examine their potential as an indicator of local variation in sea surface temperature (SST), we collected 70 surface sediments from the Chinese coastal seas (CCSs), including 16 from the Yangtze Estuary, to determine their tetraether lipid composition. The proportion of OH-GDGTs relative to total isoprenoid GDGTs increased with latitude from 1.8% to 12.8% and correlated strongly with SST, as indicated from redundancy analysis. The variation in OH-GDGT composition was best captured by the weighted average number of cyclopentane moieties in OH-GDGT-1 and -2 (RI-OH), which correlated significantly with both summer and autumn SST and was associated with minimum residual SST relative to remote sensing data. The large input of terrigenous organic matter to the Yangtze Estuary appears to have no significant influence on the OH-GDGT based proxy. Our study suggests that RI-OH could serve as a useful proxy for recording warm seasonal SST in the CCSs. A global surface sediment compilation further indicates that RI-OH and its derivative, RI-OH′, which additionally contains OH-GDGT-0, may be used to estimate SST in warm (ca.>15°C annual mean SST) and cold (ca.<15°C) environments, respectively.

Holocene Climatic Optimum centennial-scale paleoceanography in the NE Aegean (Mediterranean Sea)

Combined micropaleontological and geochemical analyses of the high-sedimentation gravity core M-4G provided new centennial-scale paleoceanographic data for sapropel S1 deposition in the NE Aegean Sea during the Holocene Climatic Optimum. Sapropel layer S1a (10.2–8.0 ka) was deposited in dysoxic to oxic bottom waters characterized by a high abundance of benthic foraminiferal species tolerating surface sediment and/or pore water oxygen depletion (e.g., Chilostomella mediterranensis, Globobulimina affinis), and the presence of Uvigerina mediterranea, which thrives in oxic mesotrophic-eutrophic environments. Preservation of organic matter (OM) is inferred based on high organic carbon as well as loliolide and isololiolide contents, while the biomarker record and the abundances of eutrophic planktonic foraminifera document enhanced productivity. High inputs of terrigenous OM are attributed to north Aegean borderland riverine inputs. Both alkenone-based sea surface temperatures (SSTs) and δO18G. bulloides records indicate cooling at 8.2 ka (S1a) and ~7.8 ka (S1 interruption). Sapropelic layer S1b (7.7–6.4 ka) is characterized by rather oxic conditions; abundances of foraminiferal species tolerant to oxygen depletion are very low compared with the U. mediterranea rise. Strongly fluctuating SSTs demonstrate repeated cooling and associated dense water formation, with a major event at 7.4 ka followed by cold spells at 7.0, 6.8, and 6.5 ka. The prominent rise of the carbon preference index within the S1b layer indicates the delivery of less degraded terrestrial OM. The increase of algal biomarkers, labile OM-feeding foraminifera and eutrophic planktonic species pinpoints an enhanced in situ marine productivity, promoted by more efficient vertical convection due to repeated cold events. The associated contributions of labile marine OM along with fresher terrestrial OM inputs after ~7.7 ka imply sources alternative/additional to the north Aegean riverine borderland sources for the influx of organic matter in the south Limnos Basin, plausibly related to the inflow of highly productive Marmara/Black Sea waters.

Shale oil potential and thermal maturity of the Lower Toarcian Posidonia Shale in NW Europe

A suite of drilling cores and outcrop samples of the Lower Toarcian Posidonia Shale (PS) were collected from multiple locations including the Swabian Alb and Franconian Alb of Southwest-Germany, Runswick Bay of UK and Loon op Zand well (LOZ-1) of the West Netherlands Basin. In order to assess the thermal maturity and quantify the petroleum generation potential of the rocks, elemental analysis, Rock-Eval pyrolysis, organic petrographic investigations and molecular hydrocarbon analyses were performed. Available literature data of the Posidonia Shale from Luxembourg and northwest Germany were used for comparison in the discussion.The Lower Toarcian Shale is more carbonate-rich (about 30% on average) in SW-Germany, NW-Germany and Luxembourg, and is instead more silicate-rich in UK and NL with carbonate contents of about 15% on average. This coincides with the relative distances of the depositional settings to the clastic source areas on the continents during the early Jurassic. However, HI values are similar, approx. 500–700mgHC/g TOC on average at all sampling localities, exhibiting typical type II kerogen with excellent hydrocarbon generation potential. Differences in relative abundance of terrigenous organic matter input are deduced from molecular indicators, Rock-Eval data and organic petrography. The highest abundance of terrigenous organic matter is evident for Runswick Bay, UK.Samples from outcrops of the Lower Toarcian Shale from Runswick Bay, UK, have reached the early stage of the oil window, with quite uniform Tmax values (425–438°C, avg. 433°C) and a narrow range of PI (production index) values (0.10–0.19), substantiated by mature fluorescing characteristics of telalginite and lamalginite and vitrinite reflectance values of 0.6–0.7%. Wider ranges of Tmax values (421–443°C, avg. 427°C), vitrinite reflectance (0.4–0.8%) and production indices (0.06–0.29) suggest more variable thermal maturation ranging from immature to almost peak oil generation at the LOZ-1 well, NL, although only some intervals indicate advanced thermal maturity there. In contrast, the Lower Toarcian marlstones from the Swabian and Franconian Alb of SW-Germany are clearly immature, similar to the samples from Luxembourg. In NW-Germany, the Posidonia Shale covers a wide spectrum of maturity ranging from immature to overmature.Source potential index (SPI) indicates the highest value in the LOZ-1, NL followed by Wickensen, NW-Germany and Esch-sur-Alzette, LU, mainly due to differences in thickness of the studied sections. The lowest value was observed in SW-Germany. The Lower Toarcian (Jet Rock) in Runswick Bay, UK, is assumed to have a high shale oil potential because of its advanced thermal maturity, although its SPI is not high due to the limited thickness of the section studied. The PS in SW-Germany has low shale oil potential due to low thermal maturity and limited thickness.

Geochemical characterisation and organic matter enrichment of Upper Cretaceous Gongila shales from Chad (Bornu) Basin, northeastern Nigeria: Bioproductivity versus anoxia conditions

Abstract The Upper Cretaceous Gongila Formation in the Chad (Bornu) Basin, northeastern Nigeria has always been considered as a transitional sequence between the underlying continental Bima Formation and the overlying marine Fika Formation. So far, little information has been available on this formation, which is regarded as one of the prospective source rocks in this Basin. Elemental and organic geochemical analyses were performed on the shales recovered from this formation in order to define the origin and type of organic matter within the sediments and also to examine the processes controlling organic matter enrichment. Based on their bulk geochemical characteristics and biomarker compositions, the selected Gongila shales were interpreted to be deposited in marine environment. Their biomarkers and chemical compositions also indicate enhanced bioproductivity within the photic zone of the water columns and a contribution by mixed aquatic algae/microorganisms and terrigenous organic matter input. The increased bioproductivity in the selected Gongila shales may be related to enhanced semi-arid/humid to humid–warm climate conditions. Low salinity stratification and suboxic to relatively anoxic bottom water conditions are also supported by Sr/Br, V/Ni, and Pr/Ph ratios. Therefore, a stratified water column with low salinity and suboxic to relatively anoxic bottom water conditions may have contributed to the preservation of organic matter in the sediments.

Characteristics and origin of lacustrine source rocks in the Lower Cretaceous, Erlian Basin, northern China

Four lacustrine rift sub-basins in the Erlian Basin, one of the most petroliferous basins in northern China, have been analyzed to characterize source rock potential, to reveal environmental and ecological changes and to construct depositional models for lacustrine source rocks in different tectonic settings. The first member of the Tengger Formation in these sub-basins is characterized by a wide range of total organic carbon contents, hydrogen indices and kerogen types. These sub-basins have dissimilar boundary fault activities and sedimentation rates. The differences in border fault activity and sedimentation rate are consistent with changes in productivity, redox conditions and clastic influx input. The carbon isotopic composition of carbonates in mudstone samples suggests that the productivity of Aer and Wuli sub-basins is higher than that of Naoer and Saihan sub-basins. Differences in tectonic conditions also result in different redox conditions of depositional environments. Total reduced sulfur and Pr/Ph ratios show an oxidizing environment in Aer and Wuli sub-basins; while a reducing environment was present in Naoer and Saihan sub-basins. With more active boundary faults and higher sedimentation rates, the terrigenous organic matter inputs to Aer and Wuli sub-basins are higher than Naoer and Saihan sub-basins. This can be confirmed by relative steranes abundances. Different tectonic conditions existed in different sub-basins, which lead to variations in ecological communities in the lake systems. These environmental and ecological changes enable us to construct models for the deposition of lacustrine source rocks under different tectonic backgrounds. During source rock deposition in Aer and Wuli sub-basins, the high influx of clastic sediments carrying high terrigenous organic matter and dissolved inorganic carbon and nitrate into the lake, might have sustained high productivity and oxidizing conditions. Due to high influx of clastic sediments and unstable water column stratification, the depositional environment of Aer and Wuli sub-basins are oxidizing. In a high productivity lake, there is still significant organic matter accumulation in oxidized environment after oxidative degradation. The high productivity is the dominant factor controlling the formation of organic rich sediments and the deposition of source rocks in Aer and Wuli sub-basins. In Naoer and Saihan sub-basins, the depositional environment during source rock formation is reducing due to low influx of clastic sediments with low productivity. In low productivity lakes, such as Naoer and Saihan sub-basins, which are low productivity lakes, redox condition substitutes for productivity and becomes the predominant factor controlling the formation of lacustrine source rocks. It is evident that the synergetic evolution of depositional environments and organisms in a lake system accounts for the formation of source rocks with different characteristics. The proposed models may have important implications for source rock prediction in other lacustrine rift basins.

The origin, type and preservation of organic matter of the Barremian–Aptian organic-rich shales in the Muglad Basin, Southern Sudan, and their relation to paleoenvironmental and paleoclimate conditions

Barremian–Aptian organic-rich shales from Abu Gabra Formation in the Muglad Basin were analysed using geochemical and petrographic analyses. These analyses were used to define the origin, type of organic matters and the influencing factors of diagenesis, including organic matter input and preservation, and their relation to paleoenvironmental and paleoclimate conditions. The bulk geochemical characteristics indicated that the organic-rich shales were deposited in a lacustrine environment with seawater influence under suboxic conditions. Their pyrolysis hydrogen index (HI) data provide evidence for a major contribution by Type I/II kerogen with HI values of >400 mg HC/g TOC and a minor Type II/III contribution with HI values <400 mg HC/g TOC. This is confirmed by kerogen microscopy, whereby the kerogen is characterized by large amounts of structured algae (Botryococcus) and structureless (amorphous) with a minor terrigenous organic matter input. An enhanced biological productivity within the photic zone of the water columns is also detected. The increased biological productivity in the organic-rich shales may be related to enhanced semi-arid/humid to humid-warm climate conditions. Therefore, a high bio-productivity in combination with good organic matter preservation favoured by enhanced algae sizes are suggested as the OM enrichment mechanisms within the studied basin.

Geochemical characterization of source rocks and crude oils in the Upper Cretaceous Qingshankou Formation, Changling Sag, southern Songliao Basin

The Upper Cretaceous Qingshankou Formation is one of the main exploration target in the Changling Sag, southern Songliao Basin. However, the origin of source rock deposition and crude oils remains undiscovered. A comprehensive geochemical study, including Rock–Eval pyrolysis, gas chromatography-mass spectrometry and carbon isotope analysis on fifteen crude oils and twenty-two source rock samples from the Qingshankou Formation were performed in order to provide new insights into source organic matter, depositional environment and correlation between crude oils and source rocks.Bulk geochemical analysis on source rock samples indicates that the lower member of the Qingshankou Formation has more hydrocarbon generating potential than the middle member. These two potential source rock members were interpreted to be deposited in slightly different environments and have different sources of organic matter determined by biomarker assemblages. The lower member is organically rich (average TOC = 1.27 wt.%), oil-prone, mature and deposited under anoxic conditions with more contribution from sapropelic organic matter. The middle member is organically poor (average TOC = 0.78 wt.%), gas-prone, less mature and deposited under less anoxic conditions with more terrigenous organic matter input. Biomarker fingerprints and carbon isotope compositions suggest that all the crude oils were genetically related and could be classified into one oil family, which has significant contribution from the lower member source rocks and minor contribution from the middle member source rocks.

Geochemical characterisation of Early Cretaceous lacustrine sediments of Bima Formation, Yola Sub-basin, Northern Benue Trough, NE Nigeria: Organic matter input, preservation, paleoenvironment and palaeoclimatic conditions

The Bima Formation is the oldest lithogenetic unit occupying the base of the Cretaceous successions in the Yola Sub-basin of the Northern Benue Trough. It is differentiated into Lower (B1), Middle (B2) and Upper (B3) Bima Members. Integrated organic and inorganic geochemical studies were applied on the lacustrine sediments of the Bima Formation with the aim of reconstructing the palaeodepositional environment and organic matter input in response to climate and tectonism. The analysed sediments were deposited in a freshwater lacustrine environment with a low-salinity stratified water column and suboxic to relatively anoxic conditions, as indicated by the bulk geochemical parameters. The biomarkers provide evidence for a contribution of aquatic algae and microorganisms, with a significant amount of terrigenous organic matter input. The preservation of the organic matter is possibly attributed to the stratified water column with low salinity and suboxic to relatively anoxic conditions. Based on the geochemistry of major and trace elements, the sediments were deposited during semi-arid climatic conditions within passive continental margin setting. This agrees with the tectonic events in the West and Central African Rift System (WCARS) during the Early Cretaceous period.

Pregnanes as molecular indicators for depositional environments of sediments and petroleum source rocks

Steroids with unconventional side chains have increasingly been applied as diagnostic markers for geological source and age assessments. However, one of the most distinctive characteristics, the abnormal abundance of pregnane and homopregnane in ancient sediments and petroleum, remains unresolved. Higher pregnane and homopregnane, as well as C23–C26 20-n-alkylpregnanes, relative to the regular steranes were observed in samples collected from different petroleum basins in China. These included Precambrian marine carbonate-derived petroleum (NW Sichuan Basin), Lower Paleozoic marine marl derived crude oils (Tarim Basin), and Eocene hypersaline lacustrine carbonate source rocks and associated petroleum (Bohai Bay Basin). However, all of the samples have many common biomarker characteristics, such as pristane/phytane ratios<1, low amounts of diasteranes and high C29/C30 hopane (∼0.6–1), C35/C34 hopane (mostly⩾1) and dibenzothiophene/phenanthrene (DBT/PHEN, mostly 0.5–1) ratios revealing a contribution from anoxic carbonate/marl source rocks deposited in restricted, clastic-starved settings. We suggest that 5α,l4β,l7β-pregnane and homopregnane, as well as their higher C23–C26 homologues, are geological products derived from steroids bound to the kerogen by a sulfurized side chain. Carbon or carbonate minerals are considered to be natural catalysts for this cracking reaction via preferential cleavage of the bond between C-20 and C-22. Similar distributions occur in the short chain analogues of 4-methylsterane, triaromatic steroid and methyltriaromatic steroid hydrocarbons, providing circumstantial evidence for this proposal. The ratio of pregnane and homopregnane to the total regular steranes and the ratio of C27 diasteranes to cholestanes can be sensitive indicators of sedimentary environments and facies. In general, high diasteranes and low pregnanes (with homologues) indicate an oxic water column or significant input of terrigenous organic matter in clay rich source rocks and some organic lean carbonate rocks. Low diasteranes with high pregnanes implies restricted, sulfur rich conditions, typical of anoxic carbonate source rocks. Furthermore, the two ratios may be useful to assess the variation of mineralogy and openness of source rock depositional settings.

Origin and occurrence of crude oils in the Zhu1 sub-basin, Pearl River Mouth Basin, China

The origin of the seventeen major oil fields in the Zhu1 sub-basin, Pearl River Mouth Basin (PRMB) was studied based on the results of Rock–Eval pyrolysis on more than 370 samples and biomarker analysis on 31 source rock samples and 63 oil samples. The two possible source rock intervals have different biomarker assemblages and were deposited in different environments. The Wenchang Formation (E2w, 56.5–32Ma) is characterized mainly by low C19/C23 tricyclic terpane (<2.0), low C24 tetracyclic terpane/αβC30 hopane (<0.06), low bicadinane-T/αβC30 hopane (<2.0) and high 4-methyl steranes/∑C29 steranes (most >0.4) ratios, and were deposited in anoxic to suboxic environments with important contribution from Pediastrum and Dinoflagellates. The Enping Formation (E3e, 32–30Ma) has high C19/C23 tricyclic terpane, high C24 tetracyclic terpane/αβC30 hopane, widely variable yet overally high bicadinane-T/αβC30 hopane and low 4-methyl steranes/∑C29 steranes ratios, and were deposited in freshwater lacustrine to swamp conditions with significant terrigenous organic matter input. According to oil-source correlation, three oil classes can be identified in the Zhu1 sub-basin. Class 1 oil is E2w-derived and occurs widely. Class 2 oil is E3e-derived and refers to oils from F field in the north of the Huizhou depression. Class 3 oil is a mixture of oils generated from E2w and E3e, only distributed in the Huizhou depression and on its southern margin. The petroleum distribution pattern is mainly controlled by the distribution of source rocks and the migration pathways of oils. This research has important implications for future exploration.

Novel tetracyclic terpanes in crude oils and source rock extracts in pearl river mouth basin and their geological significance

Novel tetracyclic terpanes X and Y namely C24-des-A-oleanane and C27 tetracyclic terpane were detected in crude oils and source rock extracts in Zhu 1 depression in Pearl River Mouth Basin by GC-MS analysis technology. These compounds are similar to oleanane in the structure, and their relative abundance in m/z 191 partial mass chromatogram has a good correlation with oleanane. Here compounds X and Y are considered to be derived from the des-A degradation of oleanoid precursor. The ratio of X/(X+C24) and Y/(Y+C24) increase with the increasing inputs of terrigenous organic matter in crude oils. Wenchang Formation middle-deep lacustrine source rocks with planktonic algae organic matter inputs and oil generating from it contain low abundance of compounds X and Y, while Enping Formation coal measures source rocks with terrigenous higher plants organic matter inputs and oil generating from it usually have higher compounds X and Y. In general when two terrigenous compounds C19 tricyclic terpane and bicadinane-T are high in crude oils, there are also a corresponding high abundant compounds X and Y. Relative abundance of compounds X and Y is closely related to the source of organic matter and it can indicate the input of terrigenous organic matter.

Organic facies variability in the Posidonia Black Shale from Luxembourg: Implications for thermal maturation and depositional environment

Posidonia Shale (Lias ε, Lower Toarcian, “Schiste bitumineux”) samples were obtained from a well in southern Luxembourg (Esch-sur-Alzette). Elemental composition and organic facies were studied along the cores, and accordingly the thermal maturity and depositional environment were evaluated. The organic matter content is high with an average of 7.2%, ranging between 2.8% and 13.5%. The average vitrinite reflectance value is ca. 0.55%, indicating an early stage of oil generation. Several biomarker based maturity parameters support the low thermal maturity including OEP values and C29 5α, 14α, 17α (H)-steranes 20S/(20S+20R).Rock-Eval data and microscopy observations reveal that the organic matter is composed of hydrogen-rich type II kerogen derived from alginite. Small algal bodies derived from nannoplankton predominate, but large telalginite (Tasmanales) also occurs. Pyrites are abundant and fish bones are frequently observed, characterizing a marine depositional environment where strong sulfate reduction occurred. Small amount of autochthonous vitrinite observed microscopically, combined with predominance of short-chain n-alkanes over long-chain n-alkanes, as well as a low terrigenous/aquatic ratio (TAR), indicate a low input of terrigenous organic matter. Sterane distribution is in good agreement with an origin from a marine carbonate depositional environment which corresponds to the general marlstone lithology. Additionally, C30-24-propyl-14α(H), 17α(H)-cholestane (20R) was detected, supporting the marine origin.The depositional environment was strongly oxygen-depleted in bottom water, as indicated by low concentrations of C35 homohopane, fair to low Pr/Ph ratios (slightly less than 1.0) and high sulfur content (ca. 3wt.% on average) and sulfur/organic carbon ratios, with only few interruptions of dysoxic to suboxic conditions during deposition of the Posidonia Shale in Luxembourg. Gammacerane detected in low abundance reveals that a stratified water column existed and water salinity was slightly enhanced in the paleoenvironment. The highest C27/C29 sterane ratio due to more algal input indicates a maximum flooding during the upper falciferum zone of the Lower Toarcian in Luxembourg, which fits well to the global sea level curve and might correspond to a productivity-oceanic anoxic event (P-OAE). Furthermore, Toarcian greenhouse climate led to generally low oxygen concentration in bottom water of the shallow sea.

The Oligocene Maikop series of Lahich (eastern Azerbaijan): Paleoenvironment and oil–source rock correlation

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene, represents a prolific source unit for the oils found within the southern Gobustan area and the Kura Depression. Based on biomarker characteristics, the oils are believed to have originated from a clastic source rock deposited in an anoxic to suboxic, transitional marine environment with low to moderate input of terrigenous organic matter. However, due to the thermal immaturity of accessible Maikop rocks, oil–source rock correlation based on biomarker fingerprints is hampered.The results from a sedimentary succession sampled in the Shemakha–Gobustan area near the village of Lahich revealed the presence of marginally mature (Rr=0.6–0.7%) Maikop rocks. Based on lithology, geochemical data and paleontological analyses, the sampled section contains Upper Eocene and Lower Oligocene sediments. Palynological analyses of dinoflagellate cysts indicate an early Rupelian to early Chattian (Oligocene) age for the upper part of the investigated Maikop succession. However, the stratigraphic position of the middle and lower part of the succession remains unclear.Maceral composition, as well as biomarker and stable isotope (C, N) data indicate a major contribution of aquatic organisms and variable inputs from macrophytes and land plants to organic matter deposition during the Oligocene. Microbial communities most probably included heterotrophic bacteria and cyanobacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the lower part of the succession. Increased remineralisation of organic matter in the water column is suggested to result in low TOC and hydrocarbon contents, as well as 15N enriched total nitrogen of the Eocene to earliest Oligocene sediments. Organic carbon accumulation during the Oligocene occurred in a stratified, episodically euxinic environment. This is indicated by low TOC/S ratios and higher contents of aryl isoprenoids in several samples from the upper units. A moderate to good oil–source rock correlation with oils from the Kura Depression is indicated by sterane and triterpane fingerprints as well as carbon isotopic composition of hydrocarbon fractions.

Geochemical characterisation of Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria: Implications for depositional environment and tectonic setting

Late Cretaceous shales of the Fika Formation in the Chad (Bornu) Basin, northeastern Nigeria, were analysed to define paleoenvironment and source of the organic matter, and their relation to tectonic setting. The organic carbon and sulphur contents of Fika shale samples are in the range of 0.51–2.13 and 0.31–1.65wt.%, respectively, pointing that these shales were deposited in suboxic-anoxic marine conditions. The biomarker and chemical compositions provide evidence for a major contribution of aquatic algae and microorganisms with minor terrigenous organic matter input. Moderate salinity stratification and relatively anoxic-suboxic bottom water conditions are also likely in the Fika shales. Therefore, stratified water column with moderate salinity and relatively anoxic-suboxic bottom water conditions have contributed to organic matter (OM) preservation in the Fika shale layer. Fika shale samples are rich in SiO2 (54.80wt.%), followed by Al2O3 (23.75wt.%) and Fe2O3 (10.19wt.%). Compared with average shale, the analysed shale samples are obviously enriched in Al2O3 (23.75wt.%), TiO2 (1.34wt.%), and P2O5 (0.30wt.%), indicating that these sediments are rich in clay minerals and represent a good possibility for enhanced organic matter production and enrichment.Plots of Fika shale on bivariate discriminant function diagram suggest an active continental margin setting for the provenance. The inferred tectonic setting for the late Cretaceous shales of the Fika Formation of the Chad (Bornu) Basin is in agreement with the tectonic evolutionary history of the west and central Africa during the Cretaceous period.

A possible terrigenous origin for perylene based on a sedimentary record of a pond (Lorraine, France)

The origin of perylene, a five-ring polycyclic aromatic hydrocarbon (PAH), in sediments has been a matter of constant debate and its formation mechanisms remain unclear. A perylene record in sediment was described and could support a clear link between perylene and terrigenous organic matter (OM) input. The distributions of PAHs, the variations of organic proxies such as the ratio of terrigenous to aquatic n-alkanes (TAR(HC)) and 5α(H)-stanols/Δ5-sterols ratio in sediments have been investigated. Sediments were sampled from a pond artificially created during the Middle Ages from a swampy area. In the period prior to the pond creation, for which high contributions of terrigenous OM were evidenced, perylene was found to be the predominant PAH. Furthermore, perylene content was shown to increase in response to the establishment of more reducing conditions. This result supports the common idea that the main source of perylene is natural and derives from biogenic precursors under reducing conditions. The creation of the pond in this wetland is marked by the deposition of a wood rich level characterized both by more oxygenated conditions and higher concentrations of perylene. These high concentrations could result from the introduction of high concentrations of biogenic precursors of perylene under oxidative conditions. Subsequently, the progressive burial of the woody level could have allowed the establishment of oxygen depleted conditions and the formation of perylene. These results validate the use of perylene as a paleoenvironmental marker of terrigenous sources but it must be carefully used as a marker for syn- or post-depositional oxygen depleted conditions in lacustrine environments.

Sources of δ15N variability in sinking particulate nitrogen in the Cariaco Basin, Venezuela

Ten years of monthly observations of the δ15N of sinking particulate nitrogen (δ15N–PN (in ‰ versus atmospheric N2)=[(15N/14N)sample/(15N/14N)standard)−1]1000) in the Cariaco Basin, Venezuela, confirm that the basin's bottom sediments store information about nitrogen dynamics related to seasonal and interannual variability in regional surface ocean processes. During the upwelling period of the southern Caribbean Sea (February–April), the δ15N–PN is similar to that of the thermocline nitrate (∼3.5‰). This nitrate is imported into the Cariaco Basin with Subtropical Underwater (SUW), which wells up near the coast. Thus, particles generated by phytoplankton photosynthesis during this productive period bear a sub-tropical North Atlantic isotopic imprint of N2 fixation (low compared to the global average of nitrate δ15N≈5‰). During the non-upwelling period when surface waters are stratified (September–November), the δ15N–PN is also 3.5–4.0‰, and reflects a mixture of local N2 fixation within the mixed layer, inputs of terrigenous organic matter and SUW nitrate consumption by phytoplankton below the mixed layer, which most likely exerts the strongest control on the δ15N–PN signal during this time. In the transition periods of May–July and December–January, the δ15N–PN increases to 4.5–6.5‰. This coincides with maxima of continental material fluxes (terrestrial PON δ15N is >6‰) into the Cariaco Basin. The δ15N signal in the sediments of the Cariaco Basin thus provides information about the relative strength of the local coastal upwelling, the relative input of continental material via river runoff, and local N2 fixation. The findings contribute to interpretations of the basin's paleoclimatic nitrogen cycle variations based on observations of the sedimentary δ15N record at this location.

Organofacies and paleoenvironment of the Oligocene Maikop series of Angeharan (eastern Azerbaijan)

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene times, contains prolific source rocks with primarily Type II organic matter. Paleontological analyses of dinoflagellate cysts revealed a Lower to Upper Oligocene age for the investigated succession near Angeharan. A major contribution of aquatic organisms (diatoms, green algae, dinoflagellates, chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids and δ13C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the Upper Oligocene units of the Maikop Formation from Angeharan mountains. The terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the Shamakhy–Gobustan area.High primary bioproductivity resulted in a stratified water column and the accumulation of organic matter rich sediments in the Lower Oligocene units of the Maikop Formation. Organic carbon accumulation during this period occurred in a permanently (salinity-) stratified, mesohaline environment with free H2S in the water column. This is indicated by low pristane/phytane ratios of all sediments (varying from 0.37–0.69), lower methylated-(trimethyltridecyl)chromans ratio in the lower units and their higher contents of aryl isoprenoids and highly branched isoprenoid thiophenes. Subsequently, the depositional environment changed to normal marine conditions with oxygen deficient bottom water. The retreat of the chemocline towards the sediment–water interface and enhanced oxic respiration of OM during deposition of the Upper Oligocene Maikop sediments is proposed.Parallel depth trends in δ13C of total OM, n-alkanes, isoprenoids and steranes argue for changes in the regional carbon cycle, associated with the changing environmental conditions. Increased remineralisation of OM in a more oxygenated water column is suggested to result in low TOC and hydrocarbon contents, as well as 15N enriched total nitrogen of the Upper Oligocene units.

Mass Flux Calculations Show Strong Allochthonous Support of Freshwater Zooplankton Production Is Unlikely

Many studies have concluded terrestrial carbon inputs contribute 20–70% of the carbon supporting zooplankton and fish production in lakes. Conversely, it is also known that terrestrial carbon inputs are of very low nutritional quality and phytoplankton are strongly preferentially utilized by zooplankton. Because of its low quality, substantial terrestrial support of zooplankton production in lakes is only conceivable when terrigenous organic matter inputs are much larger than algal production. We conducted a quantitative analysis of terrestrial carbon mass influx and algal primary production estimates for oligo/mesotrophic lakes (i.e., TP≤20 µg L−1). In keeping with the principle of mass conservation, only the flux of terrestrial carbon retained within lakes can be utilized by zooplankton. Our field data compilation showed the median (inter-quartile range) terrestrial particulate organic carbon (t-POC), available dissolved organic carbon (t-DOC) inputs, and in-lake bacterial and algal production were 11 (8–17), 34 (11–78), 74 (37–165), and 253 (115–546) mg C m−2 d−1, respectively. Despite the widespread view that terrestrial inputs dominate the carbon flux of many lakes, our analysis indicates algal production is a factor 4–7 greater than the available flux of allochthonous basal resources in low productivity lakes. Lakes with high loading of t-DOC also have high hydraulic flushing rates. Because t-DOC is processed, i.e., mineralized or lost to the sediments, in lakes at ≈0.1% d−1, in systems with the highest t-DOC inputs (i.e., 1000 mg m−2 d−1) a median of 98% of the t-DOC flux is advected and therefore is not available to support zooplankton production. Further, advection is the primary fate of t-DOC in lakes with hydraulic retention times <3 years. When taking into account the availability and quality of terrestrial and autochthonous fluxes, this analysis indicates ≈95–99% of aquatic herbivore production is supported by in-lake primary production.

Climatic warming and overgrazing induced the high concentration of organic matter in Lake Hulun, a large shallow eutrophic steppe lake in northern China

An abnormally high concentration of organic matter (OM) in Lake Hulun, a large shallow eutrophic lake located in the sparsely populated Hulun Buir Steppe, was observed in a field investigation. Little was known about the origin of the OM. To identify the source of the OM in Lake Hulun, the carbon/nitrogen (C/N) ratio, natural abundance of stable isotope and three dimensional excitation emission matrix (3DEEM) fluorescence spectroscopy techniques were employed. Furthermore, a cyanobacterial incubation and degradation experiment was conducted in the laboratory to quantify the contribution of algae to dissolved organic matter (DOM) in Lake Hulun. C/N, the stable carbon isotope (δ13C) values typical of C3 plant debris in particulate organic matter (POM) and the fluorescence indices of DOM indicate that most of the OM in Lake Hulun is of terrigenous origin. It was deduced that only about 10.2% and 7.3% of DOM were contributed by algae in September and January, respectively, according to the linear correlation between the concentrations of algae-derived DOM and the fluorescence intensities of tyrosine-like matter. According to the stockbreeding development and climate change in Hunlun Buir Steppe, we deduced that the destruction of the grassland ecosystem by overgrazing in specific locations and trends in climatic warming and drying were the main factors causing the increase of OM and nutrient concentrations in Lake Hulun. This result highlights the need to pay more attention to the inputs of terrigenous organic matter to the lakes in northern China.

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.