Carbon Isotopic Composition Of Organic Matter Research Articles

Biogeochemical processes captured by carbon isotopes in redox-stratified water columns: a comparative study of four modern stratified lakes along an alkalinity gradient

Abstract. Redox-stratified water columns are a prevalent feature of the Earth's history, and ongoing environmental changes tend to promote a resurgence of such settings. Studying modern redox-stratified environments has improved our understanding of biogeochemical processes and element cycling in such water columns. These settings are associated with peculiar carbon biogeochemical cycling, owing to a layered distribution of biological processes in relation to oxidant availability. Metabolisms from distinct biogeochemical layers are diverse and may differently imprint the sedimentological record. Paired carbon isotope compositions of organic matter and carbonates, which are commonly used to characterize these ecological dynamics, can thus vary from one stratified environment to another. Changes in the organic/inorganic carbon sources and mass balance can further complicate the isotopic message in stratified environments. Better understanding of these multifaceted carbon isotope signals requires further evaluation of how the processes occurring in redox-stratified water columns are transferred to the sediments. We therefore characterized and compared the isotopic signatures of dissolved inorganic carbon (DIC), carbonate, and organic matter reservoirs at different depths in the water column and upper sediments of four stratified Mexican lakes that follow a gradient of alkalinity and salinity. Comparing these systems shows strong diversity in the carbon isotope signals of the water column and sediments. Differences in inorganic carbon isotope signatures arise primarily from the size of the DIC reservoir, buffering the expression of redox-dependent biological processes as alkalinity increases. Combining this isotopic dataset with water column physicochemical parameters allows us to identify oxygenic photosynthesis and aerobic respiration in the four lakes studied, while anoxygenic photosynthesis is evidenced in only two of them. Sedimentary organic matter does not originate from the same water column layers in the four lakes, highlighting the ecological variability that can stem from different stratified water columns and how it is transferred or not to the sedimentary record. The least alkaline lake shows higher isotopic variability and signatures typical of methanogenesis in the sediment porewaters. This metabolism, however, does not leave diagnostic isotopic signatures in the sedimentary archives (organic matter and carbonates), underlining the fact that even when alkalinity does not strongly buffer the inorganic carbon reservoir, a comprehensive picture of the active biogeochemical carbon cycling is not necessarily transferred to the geological record.

Geochemistry and organic petrography of the Middle Permian Lucaogou alkaline lacustrine oil shale in the southern Junggar Basin, China: Implications for formation conditions and organic matter accumulation

Organic matter (OM) accumulation in terrestrial sediments shows not only a significant carbon sink in the Earth's carbon cycle but an important origin of fossil fuels, which is closely associated with the complex and diverse depositional environments and climate conditions. The Junggar Basin developed the most laterally extensive and thickest alkaline lacustrine sediments as well as the richest hydrocarbon source rock/oil shale interval of all the world during the deposition of the Lucaogou Formation and equivalent units. In this study, the sedimentary succession of 564 m with a mudstone and dolomite matrix of the Lucaogou oil shale from the Qi 1 well located in the southern Junggar Basin was investigated, which can be divided into two members based on lithology. This paper is a synthetic use of major element oxides, trace, and rare-earth elements as well as organic petrography data as proxies to evaluate the provenance, paleotectonic setting, paleoclimate and weathering conditions, paleoenvironment, and paleoproductivity as well as origin and accumulation of OM of the Lucaogou sediments.It is suggested that no recycling sediments in all the studied samples, based on parameters including the Th/Sc vs. Zr/Sc plot, the Chemical Index of Alteration (CIA) being linearly dependent on Weathering Index of Parker (WIP), and the high Index of Compositional Variability. The provenance mainly from intermediate-felsic volcanic rocks (e.g., granodiorite, andesite and dacite) is supported by the bivariate plots of Euanom vs. Th/Sc, La/Th vs. Hf, La/Sc vs. Co/Th, Cr/Th vs. Sc/Th, Y/Ni vs. Cr/V, and Nb/Y vs. Zr/TiO2 as well as triangular diagrams of mafic–felsic–weathering, M+–4Si–R2+ and Rb/V–Zr/Zn–Sc/Nb. The continental island arc is inferred from paleotectonic setting discrimination diagrams (i.e., the La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 ternary as well as Ti/Zr vs. La/Sc binary diagrams). The chemical weathering indices (i.e., the CIA, WIP, weathering index (W), Weathering Intensity Scale, Sodium Depletion Index, and Ga/Rb vs. K2O/Al2O3 binary diagram) indicate the paleoclimate conditions were cold/arid with weak chemical weathering in a warming and/or enhanced continental weathering episode. Besides, the Sr/Ba ratios and gallium concentrations indicate the paleosalinity was brackish to hypersaline and was higher in the lower member in relation to that in the upper member. However, the suboxic environment of the benthic water (inferred from the MoEF vs. UEF co-variations and Corg/P ratios as well as large sizes of framboidal pyrites) and the moderate paleoproductivity (evaluated via the Sibio, Babio, and Pbio values, as well as Ni/Al and Cu/Al ratios) caused by predominate algae and bacteria (inferred from the organic petrography) kept stable in the paleolake during this period. The OM accumulation was mainly controlled by the preservation conditions rather than the paleoproductivity and limited detrital inputs/low deposition rates, which was indicated by the plot of Co × Mn values vs. Cd/Mo ratios and the total organic carbon contents being negatively related to stable carbon isotope compositions of OM.

Deposition conditions of the Jurassic lacustrine source rocks in the East Fukang Sag, Junggar Basin, NW China: Evidence from major and trace elements

The Fukang Sag hosts prolific Jurassic lacustrine source rocks, which include the Badaowan (J1b), Sangonghe (J1s), and Xishanyao (J2x) formations in the Junggar Basin. Depositional conditions were reconstructed based on various major and trace element proxies for the East Fukang Sag during the Jurassic. The palaeowater depth in the sag was shallow and did not display a great change during the deposition of the J1b, J1s, and J2x formations, as indicated by lithological composition and the widespread ripple cross‐beddings. The oxic to dysoxic and fresh‐water conditions were postulated to prevail in the sag during the deposition of these source rocks, as revealed by concentrations of salinity‐ and redox‐sensitive trace elements [i.e., V/Cr, Ni/Co, (Cu+Mo)/Zn, Sr/Ba ratios, and Sr, Mn, Ga contents] derived from the source rocks. The seasonal salinity stratification superposed on preexisting thermal stratification led to the appearance of the chemocline in the J1b Formation. The palaeoclimate regime was humid and warm during the depositions of the J1b and the upper J1s formations, and warmer and drier during the deposition of the lower J1s Formation, but the palaeoclimate gradually became semi‐arid and semi‐cold during the deposition of the upper J2x Formation, as implied by various proxies (i.e., Sr/Cu ratio, C‐value, and CIA value). Despite the fact that the oxic to dysoxic conditions were not conducive for organic matter (OM) preservation, the significantly moderate‐high palaeoproductivity (suggested by, i.e., Ba content and Ba/Al ratio), which was promoted by a suitable supply of nutrient from terrestrial inputs in the shallow water was the controlling factorfor the OM accumulation on the margin of the sag, but the low OM concentration was controlled by the palaeoredox conditions in the central area, which was far from the OM input source [suggested by the relationship between total organic carbon content and stable carbon isotope composition of OM (δ13Corg)]. The analyses based on Rock‐Eval pyrolysis, maceral composition, C/N ratio of isolated kerogen, and δ13Corg value showing the OM was derived from terrestrial higher plants with C3 plants (type III kerogen) are consistent with the palaeoenvironmental analysis.

Petrographic, palynofacies and geochemical characteristics of organic matter in the Saouef Formation (NE Tunisia): Origin, paleoenvironment, and economic significance

Saouef lignite layers and carbonaceous shales are studied in order to reconstruct the origin and depositional environment of organic matter (OM), as well as to evaluate their economic potential. The study is based on organic compositions, bulk geochemical parameters, biomarker data, and carbon isotope composition of OM (δ13C). Lignite samples, collected next to the Saouef village, were macroscopically identified as stratified matrix coal or as humic coal. Rock Eval pyrolysis and the palynofacies analysis indicate the predominance of Type-III-IV kerogen (OM). The depositional environment fluctuated between terrestrial environments with a low water table to the limnic environment. The OM is derived from higher land plant remains (e.g. wood, root, bark) from a mixed vegetation including conifers and angiosperms, and emerged macrophytes. The dominance of huminite over liptinite and intertinite maceral groups, along with the phytoclasts, confirms the woody origin of the OM. A high bacterial activity is evidenced by the abundance of hopanoids in the extractable OM as well as by the amorphous OM content. The high measured TOC values exceeding 20wt% in some samples may give an economic value of the organic layers of Saouef Formation. However, a Tmax<435°C and Rr<0.5% induce the immaturity of the samples despite the presence of expelled bitumen in one sample.

Carbon isotopic composition in suspended organic matter and bottom sediments of the East Arctic seas

The samples of water and bottom sediments of the East Siberian and Chukchi Seas collected during the second Russian-American RUSALCA expedition were used to analyze patterns of the isotopic composition of carbon in the organic matter (OM) of suspended material (SOM) and bottom sediments (BOM). Similar to other marine environments, the SOM isotopic composition depended on the ratio between the terrigenous and planktonic OM, both in the water body as a whole and in its parts. Thus, in the East Siberian Sea the carbon of SOM was poorer in 13C (δ13C = −24.51‰) than the open part of the more productive Chukchi Sea (δ13C = −22.16‰). In the less productive coastal waters of the Chukchi Sea, the ratio of terrigenous OM increased, resulting in a δ13C shift to lower values (−23.40‰). Due to the influx of reduced products of anaerobic diagenesis of the sediments, elevated total number of microorganisms and dark CO2 fixation were found in the near-bottom water at the water-sediment biogeochemical barrier. The newly formed biomass of autotrophic microorganisms shifted the carbon isotopic composition of the near-bottom suspended material to more positive δ13C values, with the average values of −23.39 and −20.37‰ for the East Siberian and Chukchi Sea, respectively. Changes in the carbon isotopic composition of OM resulting from microbial activity continued in the upper sediment layers. When the rate of biomass synthesis increased that of biomass consumption, the 13C content increased further. At higher rates of OM mineralization, 12C accumulated in its remaining part.

Carbon-isotope stratigraphy and palynostratigraphy of the Triassic–Jurassic transition in the Tiefengraben section — Northern Calcareous Alps (Austria)

The end-Triassic is characterized by enhanced rates of biotic turnover in both the marine and terrestrial realms. Carbon-isotope anomalies have been reported worldwide that may indicate coeval global disturbances in the biogeochemical cycles. Here we present the results of an integrated study of the palynomorph assemblages and the carbon-isotope composition of organic matter in the Tiefengraben section near St Wolfgang (Austria). Carbon-isotope values from bulk organic matter show significant fluctuations through the Triassic–Jurassic (T–J) boundary interval with two prominent negative δ 13C org excursions. An initial negative carbon-isotope excursion occurs at the base of the Kendlbach Formation, Tiefengraben Member (= Pre-Planorbis Beds). Although Triassic ammonoids and conodonts have their highest occurrence at the top of the underlying Kössen Formation, the palynomorph and calcareous microfossil assemblages indicate still a Rhaetian age for the lower part of the Tiefengraben Member. High abundance of prasinophytes during the interval of the initial carbon-isotope excursion may represent a bloom of green algae that flourished as a result of the disturbance of the marine ecosystems. Increased spore abundance together with the disappearance of distinctive Late Triassic pollen types (e.g. Ovalipollis, Rhaetipollis) indicate vegetation changes on land that are contemporaneous with the marine biotic crisis. A second negative carbon-isotope excursion occurs in the middle part of the Tiefengraben Member. Based on palynological evidence, the T–J boundary may be drawn at about this level by the lowest occurrence of Cerebropollenites thiergartii. The established carbon-isotope curve, as well as palynostratigraphic and other biostratigraphic events, can be correlated with St Audrie's Bay (England).

Late Quaternary environmental and climatic changes in central Europe as inferred from the composition of organic matter in annually laminated maar lake sediments

Geochemical (elemental analysis, bulk δ13CTOC, thermal degradation techniques, molecular analysis of n‐alkanes) and microscopic investigations (maceral analysis, vitrinite reflectance) were performed (1) to characterize the organic matter (OM) in sediments from two maar lakes in the Westeifel volcanic field, Germany, and (2) to discuss paleoenvironmental processes which have controlled the accumulation and preservation of OM. The annually laminated sediments show a wide range of organic carbon contents between 0.3 and 21.4% TOC and Hydrogen Index values between 80 and 501 mg HC/g TOC. Well‐developed laminations mirror suboxic to anoxic bottom waters. Diagnostic compounds in the pyrolysates and microscopic analysis permit discrimination between the principal OM sources, and their variations over time provide important evidence for the reconstruction of the environmental histories of the lakes and their watershed. The significance of terrigenous OM is reflected by the occurrence of methoxyphenols in the insoluble as well as mid‐ and long‐chain n‐alkanes in the soluble OM which are indicative of land plant or macrophyte input to the lakes. The relative abundance of certain homologues together with the amount of terrigenous macerals can be used to show differences in higher plant input and vegetation changes through time. During the Pleniglacial and Younger Dryas period, high‐reflecting reworked OM was introduced by soil erosion in an environment with relatively open vegetation. The alkylphenols in the pyrolysates originate partly from the microbial reworking of proteinaceous tyrosine moieties and/or reflect a direct contribution from cyanobacteria. We assume that variations in the carbon isotopic composition of OM are mainly due to changes in the lake's primary productivity with the exception of the Pleniglacial. Various forms of land use are reflected by increasing amounts of terrigenous particles as well as chemical plant markers from 500 varve years B.P. until present. The organic geochemical and microscopic data correlate well with other proxy information about climate‐dependent fluctuations in the ecosystem of the studied sites.

Effects of an oceanic anoxic event on the stable carbon isotopic composition of early Toarcian carbon

The Early Toarcian transgression is marked by the occurrence of organic carbon-rich shales in large parts of western Europe and in other parts of the world as well. Based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum Zone in several of the Tethyan sections, the widespread occurrence of the Early Toarcian shales was explained by an Oceanic Anoxic Event (OAE) (Jenkyns, 1988; Jenkyns and Clayton, 1997). The rapid burial of large amounts of organic carbon, which is rich in 12 C, would have led to a relative enrichment in 13 C of the global carbon reservoir and hence to an increase in delta 13 C of the limestones. In Southwest Germany (and a number of other locations) both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum Zone and the lower part of the falciferum Zone synchronous to black shale deposition and predating the positive isotopic excursion (Kuspert, 1982, 1983). It is presently unclear what the exact cause for this widespread negative isotopic excursion is (Jenkyns and Clayton, 1997). The objective of our study was to examine if the negative carbon isotope excursion of organic matter (OM) in the Southwest German Toarcian shales can be attributed to compositional changes of the OM, or if the excursion is related to variations in delta 13 C (sub [CO2]aq) in the photic zone. To this end, we analyzed the molecular and stable carbon isotope composition of OM in ten samples spanning the Early Toarcian (TOC 2.5-10.5 percent; HI 350-700). Biomarker distributions and kerogen pyrolysates differ only slightly among the samples, and delta 13 C values of primary production markers follow closely those of associated carbonate and kerogen (delta 13 C (sub CARB/TOC) ). This strongly suggests that differences in delta 13 C TOC are related to differences in delta 13 C of [CO 2 ] aq in the photic zone supporting Kuspert9s model of advection of isotopically light, organic matter-derived CO 2 from anoxic bottom waters. Evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids, such as isorenieratane, from anoxygenic phototrophic sulfur bacteria. Their concentrations are extremely low in black shales from the tenuicostatum Zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum Subzone suggesting that photic zone euxinia (PZE) was a common feature of the Early Toarcian sea in Southwest Germany. Our data, therefore, support the Kuspert model of advection of isotopically light, organic matter-derived CO 2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike. The enrichment in 13 C in the middle of the falciferum Zone, which followed the negative spike, may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition.

δ13C composition of sediments from the Sanaga River, Cameroon

The Sanaga River basin in Cameroon represents an ideal system for investigating the relationship between the carbon-isotope composition of organic matter in river sediments, and the type of vegetation in the river catchment. The headwaters of the river and its tributaries are located on a savannah-covered plateau in central Cameroon, with abundant Ca grasses (813 C = _ 12°/oo). The river then flows through transitional forest /savannah regions and finally through tropical rainforest (comprising only C3 vegetation with 813 C = _ 29%o) before discharging into the Gulf of Guinea. 8t3C values in the headwaters of the river are as high as 19%0 consistent with the presence of abundant C4 carbon derived from extensive savannah and montane regions. As the Sanaga River descends into the zone of transitional vegetation and into the forested regions, the 8~3C value of carbon in the samples decreases progressively to -26.9%0 due to the addit ion of forest-derived C 3 carbon. Below its confluence with the Mbam River, the ~13C value of the sediments increase to ~ 24%o, due to the addition of organic carbon with a ~3C value of -23%o derived from the Mbam River, which has extensive savanna and agricultural regions in its catchment. The 8~3C value again decreases downstream of the confluence to around -25%o at the mouth of the river. The trends in the ~3C data can be duplicated using a simple box model which assumes that the observed ~3C values are the result of mixing of carbon derived from C3and C4dominated vegetation types within the catchment. This suggests that the carbon-isotope composit ion of riverine organic carbon can provide information on catchment vegetation, and that ancient sediments can provide information on changes in catchment vegetation in the past.