Cretaceous black shales as active bioreactors: A biogeochemical model for the deep biosphere encountered during ODP Leg 207 (Demerara Rise)

A transport-reaction model was designed to identify the combination and importance of biogeochemical processes operating in four sites drilled during ODP Leg 207 (Demerara Rise, Equatorial Atlantic). Almost 100 Ma after their deposition, deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths and control the biogeochemical reaction network of the whole sediment column. According to a model calibrated at the four drill sites through inverse modeling techniques, methanogenesis could be identified as a key process that dominates not only organic matter degradation but also sulfate availability through the anaerobic oxidation of methane above the black shales. A complete depletion of sulfate within the black shale sequences promotes the remobilization of biogenic barium that reprecipitates as authigenic barite at the top of the sulfate depletion zone. Temporal dynamics of degradation processes caused continuous shifts of the barite precipitation zone during burial, thus inhibiting the formation of an authigenic barite front or causing the dissolution of earlier formed fronts. Major deviations of pore water sulfate profiles from a linear gradient coincide with depths of decelerated or accelerated transport caused by local porosity minima or maxima. Model-determined reaction rates are by far lower than those found in shallower sediments due to the low metabolic activities that are characteristic for the Deep Biosphere. But even after almost 100 Ma, changing organic matter quality still influences the degradation within the black shale sequences, as it is indicated by model results.     

20 My of nitrogen fixation during deposition of mid-Cretaceous black shales on the Demerara Rise,equatorial Atlantic Ocean

Thick sequences of dark colored, organic carbon rich, finely laminated Santonian–Cenomanian claystones and homogeneous Albian siltstones were recovered from Ocean Drilling Program Sites 1257, 1258 and 1260 on the Demerara Rise in the western equatorial Atlantic Ocean. Total organic carbon (TOC) concentrations vary from 2 to over 20 wt% in the sequences of “black shales” that were deposited over a period of ～20 million years. Similarly long periods of elevated marine productivity implied by the high TOC concentrations are uncommon in the geological record and must have required unusual paleoceanographic conditions. The importance of nitrogen fixing bacteria to sustaining the amplified export production of organic matter is indicated by δ¹⁵N values that remain between ?4‰ and 0‰, a range that is notably less positive than the average of +5‰ for modern ocean sediments. Although containing mostly marine organic matter, the black shales have TOC/TN molar ratios between 20 and 40 that mimic those of land plant organic matter. The anomalously large TOC/TN ratios suggest selective organic matter degradation, probably associated with low oxygen conditions in the water column, that favored preservation of nitrogen poor forms of organic matter relative to nitrogen rich components. Deposition of black shales on the Demerara Rise was likely a consequence of the mid-Cretaceous warm and wet greenhouse climate that strengthened thermohaline stratification of this part of the Atlantic Ocean, which in turn encouraged bacterial nitrogen fixation, enhanced primary production, magnified organic matter export, and ultimately established anoxic conditions at the seafloor that improved preservation of organic matter for much of the 20 My period represented by these thick sequences.     

Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).     

Chlorins in mid-Cretaceous black shales of the Demerara Rise: The oldest known occurrence

Liquid chromatography, multi-stage mass spectrometry (LC-MSⁿ) of acetone extracts confirms the presence of mesochlorophyllone in the mid-Cretaceous black shales of Demerara Rise. This finding represents molecular confirmation of the oldest primary chlorins in the geologic record and is evidence for a chlorophyll a source for bicycloalkanoporphyrins in the Demerara Rise black shales.     

New indices and calibrations derived from the distribution of crenarchaeal isoprenoid tetraether lipids: Implications for past sea surface temperature reconstructions

Several studies have shown that there is a strong relationship between the distribution of crenarchaeotal isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sea surface temperature (SST). Based on this, a ratio of certain GDGTs, called TEX₈₆ (TetraEther indeX of tetraethers consisting of 86 carbon atoms), was developed as a SST proxy. In this study, we determined the distribution of crenarchaeotal isoprenoid GDGTs in 116 core-top sediments mostly from (sub)polar oceans and combined these data with previously published core-top data. Using this extended global core-top dataset (n = 426), we re-assessed the relationship of crenarchaeal isoprenoid GDGTs with SST. We excluded data from the Red Sea from the global core-top dataset to define new indices and calibration models, as the Red Sea with its elevated salinity appeared to behave differently compared to other parts of the oceans. We tested our new indices and calibration models on three different paleo datasets, representing different temperature ranges. Our results indicate that the crenarchaeol regio-isomer plays a more important role for temperature adaptation in (sub)tropical oceans than in (sub)polar oceans, suggesting that there may be differences in membrane adaptation of the resident crenarchaeotal communities at different temperatures. We, therefore, suggest to apply two different calibration models. For the whole calibration temperature range (−3 to 30 °C), a modified version of TEX₈₆ with a logarithmic function which does not include the crenarchaeol regio-isomer, called , is shown to correlate best with SST: (r² = 0.86, n=396, p <0.0001). Application of on sediments from the subpolar Southern Ocean results in realistic absolute SST estimates and a similar SST trend compared to a diatom SST record from the same core. , which is defined as the logarithmic function of TEX₈₆, yields the best correlation with SST, when the data from the (sub)polar oceans are removed: (r² = 00.87, n = 255, p < 0.0001). Furthermore, gives the best correlation for mescosm data with temperatures ranging between 10 and 46 °C. For Quaternary sediments from the tropical Arabian Sea, both and yield similar trends and SST estimates. However, the extrapolation of calibration on a sediment record from a greenhouse world ocean predicts more reliable absolute SST estimates and relative SST changes in agreement with estimates based on the δ¹⁸O of planktonic foraminifera. Based on the comparison of and derived SSTs using the core top data, we recommend applying above 15 °C and below 15 °C. In cases where paleorecords encompass temperatures both below and above 15 °C, we suggest to use .     

Kinetics of organic matter degradation, microbial methane generation, and gas hydrate formation in anoxic marine sediments

Seven sediment cores were taken in the Sea of Okhotsk in a south-north transect along the slope of Sakhalin Island. The retrieved anoxic sediments and pore fluids were analyzed for particulate organic carbon (POC), total nitrogen, total sulfur, dissolved sulfate, sulfide, methane, ammonium, iodide, bromide, calcium, and total alkalinity. A novel method was developed to derive sedimentation rates from a steady-state nitrogen mass balance. Rates of organic matter degradation, sulfate reduction, methane turnover, and carbonate precipitation were derived from the data applying a steady-state transport-reaction model. A good fit to the data set was obtained using the following new rate law for organic matter degradation in anoxic sediments:

     

Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin,USA)

Late Devonian (Famennian) marine successions globally are typified by organic-rich black shales deposited in anoxic and euxinic waters and the cessation of shelf carbonate sedimentation. This global ‘carbonate crisis’, known as the Hangenberg Event, coincides with a major extinction of reef-building metazoans and perturbations to the global carbon cycle, evidenced by positive carbon-isotope excursions of up to 4‰. It has been suggested that authigenic carbonate, formed as cements in sedimentary pore spaces during early burial diagenesis, is a significant mass fraction of the total global carbon burial flux, particularly during periods of low oxygen concentration. Because some authigenic carbonate could have originated from remineralization of organic carbon in sediments, it is possible for this reservoir to be isotopically depleted and thereby drive changes in the carbon isotopic composition of seawater. This study presents bulk isotopic and elemental analyses from fine-grained siliciclastics of the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA) to assess the volume and isotopic composition of carbonates in these sediments. Carbonate in the Bakken black shales occurs primarily as microscopic disseminated dolomite rhombs and calcite cements that, together, comprise a significant mass-fraction (ca 9%). The elemental composition of the shales is indicative of a dynamic anoxic to sulphidic palaeoenvironment, likely supported by a fluctuating chemocline. Despite forming in an environment favourable to remineralization of organic matter and the precipitation of isotopically depleted authigenic carbonates, the majority of carbon isotope measurements of disseminated carbonate fall between −3‰ and +3‰, with systematically more depleted carbonates in the deeper-water portions of the basin. Thus, although there is evidence for a significant total mass-fraction of carbonate with contribution from remineralized organic matter, Bakken authigenic carbonates suggest that Famennian black shales are unlikely to be sufficiently ¹³C-depleted relative to water column dissolved inorganic carbon to serve as a major lever on seawater isotopic composition.     

Flow and nutrient dynamics in a subterranean estuary (Waquoit Bay, MA, USA): Field data and reactive transport modeling

A two-dimensional (2D) reactive transport model is used to investigate the controls on nutrient (, , PO₄) dynamics in a coastal aquifer. The model couples density-dependent flow to a reaction network which includes oxic degradation of organic matter, denitrification, iron oxide reduction, nitrification, Fe²⁺ oxidation and sorption of PO₄ onto iron oxides. Porewater measurements from a well transect at Waquoit Bay, MA, USA indicate the presence of a reducing plume with high Fe²⁺, , DOC (dissolved organic carbon) and PO₄ concentrations overlying a more oxidizing -rich plume. These two plumes travel nearly conservatively until they start to overlap in the intertidal coastal sediments prior to discharge into the bay. In this zone, the aeration of the surface beach sediments drives nitrification and allows the precipitation of iron oxide, which leads to the removal of PO₄ through sorption. Model simulations suggest that removal of through denitrification is inhibited by the limited overlap between the two freshwater plumes, as well as by the refractory nature of terrestrial DOC. Submarine groundwater discharge is a significant source of to the bay.     

白果园黑色岩系型银（钒）矿床沉积古环境与银（钒）初始富集

白果园银（钒）矿产于震旦系陡山沱组黑色色岩系中。矿床的地球化学和有机地球化学研究表明，黑色页岩含丰富以低等海生生物为主的腐泥型有机质，黑色页岩形成于局限的滞留海盆。     

Arsenic, iron and sulfur co-diagenesis in lake sediments

Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (ΣS^−II), total zero-valent sulfur (ΣS⁰), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As (). The model defines depth intervals where As is either released to (positive ) or removed from (negative ) the porewaters.At two of the sites, whose bottom water were oxygenated at sampling time, a production zone ( = 12 × 10⁻¹⁸ mol cm⁻³ s⁻¹-71 × 10⁻¹⁸ mol cm⁻³ s⁻¹) is inferred a few cm below the sediment-water interface, coincident with sharp porewater As and Fe peaks that indicate an intense coupled recycling of As and Fe. This process is confirmed by solid As and Fe maxima just below the sediment surface. In these two lakes a zone of As consumption ( = −5 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −53 × 10⁻¹⁸ mol cm⁻³ s⁻¹), attributed to the slow adsorption of As to authigenic Fe oxyhydroxides, occurs just above the production zone. A second-order rate constant of 0.12 ± 0.03 cm³ mol⁻¹ s⁻¹ is estimated for this adsorption reaction.Such features in the porewater and solid profiles were absent from the two other lakes that develop a seasonally anoxic hypolimnion. Thermodynamic calculations indicate that the porewaters of the four lakes, when sulfidic (i.e., ΣS^−II ? 0.1 μM), were undersaturated with respect to all known solid As sulfides; the calculation also predicts the presence of As^V oxythioanions in the sulfidic waters, as suggested by a recent study. In the sulfidic waters, the removal of As ( = −1 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −23 × 10⁻¹⁸ mol cm⁻³ s⁻¹) consistently occurred when saturation, with respect to FeS_(s), was reached and when As^V oxythioanions were predicted to be significant components of total dissolved As. This finding has potential implications for As transport in other anoxic waters and should be tested in a wider variety of natural environments.     

Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: Implications for oceanic isotope cycles and the sedimentary record

Sedimentary S cycling is usually conceptualized and interpreted within the context of steadily accreting (1-D) transport-reaction regimes. Unsteady processes, however, are common in many sedimentary systems and can result in dramatically different S reaction balances and diagenetic products than steady conditions. Globally important common examples include tropical deltaic topset and inner shelf muds such as those extending from the Amazon River ∼1600 km along the Guianas coast of South America. These deposits are characterized by episodic reworking of the surface seabed over vertical depths of ∼0.1-3 m. Reworked surface sediments act as unsteady, suboxic batch reactors, unconformably overlying relict anoxic, often methanic deposits, and have diagenetic properties largely decoupled from net accumulation of sediment. Despite well-oxygenated water and an abundant reactive organic matter supply, physical disturbance inhibits macrofauna, and benthic communities are dominated by microbial biomass across immense areas. In the surficial suboxic layer, molecular biological analyses, tracer experiments, sediment C/S/Fe compositions, and δ³⁴S, δ¹⁸O of pore water indicate close coupling of anaerobic C, S, and Fe cycles. δ¹⁸O- can increase by 2-3‰ during anaerobic recycling without net change in δ³⁴S-, demonstrating reduction coupled to complete anaerobic reoxidation to and a δ¹⁸O- reduction + reoxidation fractionation factor?12‰ (summed magnitudes). S reoxidation must be coupled to Fe-oxide reduction, contributing to high dissolved Fe²⁺ (∼1 mM) and Fe mobilization-export. The reworking of Amazon-Guianas shelf muds alone may isotopically alter δ¹⁸O- equivalent in mass to?25% of the annual riverine delivery of to the global ocean. Unsteady conditions result in preservation of unusually heavy δ³⁴S isotopic compositions of residual Cr reducible S, ranging from 0‰ to >30‰ in physically reworked deposits. In contrast, bioturbated facies adjacent to physically reworked regions accumulate isotopically light S (δ³⁴S to −20‰) in otherwise similar decomposition regimes. The isotopic patterns of both physically and biologically reworked regions can be simulated with simple diagenetic models. Heavy S isotopic signatures are largely a consequence of unsteady diffusion and progressive anaerobic burndown into underlying deposits, whereas isotopically depleted bioturbated deposits predominantly reflect biogenic diffusive scaling and isotopic distillation/diffusive pumping associated with reoxidation in burrow walls immediately adjacent to reduced zones. The S isotopic transition from unsteady physically controlled regions of the Amazon delta moving laterally into bioturbated facies mimics the transition of S isotopic patterns temporally in the geologic record during the rise of bioturbation. No special role for S disproportionation is required to explain these differences. The potential role of unsteady, suboxic diagenesis and dynamic reworking of sediments has been largely ignored in models of the evolution of surficial elemental cycling and interpretations of the geologic record.     

Formation of todorokite from vernadite in Ni-rich hemipelagic sediments

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg²⁺ cations. The average structural formula of 10Å-vernadite is if the layers contain vacancy sites, or alternately , if they contain Mn³⁺. The average formula of todorokite is .A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg²⁺ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg²⁺ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg²⁺ (i.e., high layer charge), or Mn³⁺ and Mn⁴⁺ cations ordered following the Mn³⁺-Mn⁴⁺-Mn⁴⁺ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.     

Microbial ammonia oxidation and enhanced nitrogen cycling in the Endeavour hydrothermal plume

Ammonium was injected from the subseafloor hydrothermal system at the Endeavour Segment, Juan de Fuca Ridge, into the deep-sea water column resulting in an -rich (?177 nM) neutrally buoyant hydrothermal plume. This was quickly removed by both autotrophic ammonia oxidation and assimilation. The former accounted for at least 93% of total net removal, with its maximum rate in the neutrally buoyant plume (?53 nM d⁻¹) up to 10-fold that in background deep water. Ammonia oxidation in this plume potentially added 26-130 mg into the deep-sea water column. This oxidation process was heavily influenced by the presence of organic-rich particles, with which ammonia-oxidizing bacteria (AOB) were often associated (40-68%). AOB contributed up to 10.8% of the total microbial communities within the plume, and might constitute a novel lineage of β-proteobacterial AOB based on 16S rRNA and amoA phylogenetic analyses. Meanwhile, assimilation rates were also substantially enhanced within the neutrally buoyant plume (?26.4 nM d⁻¹) and accounted for at least 47% of total net removal rates. The combined oxidation and assimilation rates always exceeded total net removal rates, suggesting active in situregeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone. Ammonia oxidation is responsible for turnover of 0.7-13 days and is probably the predominant in situ organic carbon production process (0.6-13 mg C m⁻² d⁻¹) at early stages of Endeavour neutrally buoyant plumes.     

Trace element partitioning between ilmenite, armalcolite and anhydrous silicate melt: Implications for the formation of lunar high-Ti mare basalts

We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite (Ilm), armalcolite (Arm) and anhydrous lunar silicate melt, to constrain geochemical models of the formation of titanium-rich melts in the Moon. Experiments were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3 GPa and 1300-1400 °C using a synthetic Ti-enriched Apollo ‘black glass’ composition in the CaO-FeO-MgO-Al₂O₃-TiO₂-SiO₂ system. Ilmenite-melt and armalcolite-melt partition coefficients (D) show highly incompatible values for the rare earth elements (REE) with the light REE more incompatible compared to the heavy REE ( 0.0020 ± 0.0010 to 0.069 ± 0.010 for ilmenite; 0.0048 ± 0.0023 to 0.041 ± 0.008 for armalcolite). D values for the high field strength elements vary from highly incompatible for Th, U and to a lesser extent W (for ilmenite: 0.0013 ± 0.0008, 0.0035 ± 0.0015 and 0.039 ± 0.005, and for armalcolite 0.008 ± 0.003, 0.0048 ± 0.0022 and 0.062 ± 0.03), to mildly incompatible for Nb, Ta, Zr, and Hf (e.g. 0.28 ± 0.05 and : 0.76 ± 0.07). Both minerals fractionate the high field strength elements with D_Ta/D_Nb and D_Hf/D_Zr between 1.3 and 1.6 for ilmenite and 1.3 and 1.4 for armalcolite. Armalcolite is slightly more efficient at fractionating Hf from W during lunar magma ocean crystallisation, with D_Hf/D_W = 12-13 compared to 6.7-7.5 for ilmenite. The transition metals vary from mildly incompatible to compatible, with the highest compatibilities for Cr in ilmenite (D ∼ 7.5) and V in armalcolite (D ∼ 8.1). D values show no clear variation with pressure in the small range covered.Crystal lattice strain modelling of D values for di-, tri- and tetravalent trace elements shows that in ilmenite, divalent elements prefer to substitute for Fe while armalcolite data suggest REE replacing Mg. Tetravalent cations appear to preferentially substitute for Ti in both minerals, with the exception of Th and U that likely substitute for the larger Fe or Mg cations. Crystal lattice strain modelling is also used to identify and correct for very small (∼0.3 wt.%) melt contamination of trace element concentration determinations in crystals.Our results are used to model the Lu-Hf-Ti concentrations of lunar high-Ti mare basalts. The combination of their subchondritic Lu/Hf ratios and high TiO₂ contents requires preferential dissolution of ilmenite or armalcolite from late-stage, lunar magma ocean cumulates into low-Ti partial melts of deeper pyroxene-rich cumulates.     

Experiments on δS mixing between organic and inorganic sulfur species during thermal maturation

Reduced sulfur species were studied to constrain isotopic exchange-mixing with synthetic polysulfide cross-linked macromolecules (PCLM), model sulfur containing molecules and natural sulfur-rich kerogen, asphalt and oil of the Dead Sea area. PCLM represents protokerogens that are rich in sulfur and thermally unstable. Mixing rates of PCLM with (added as (NH₄)₂S_(aq)) at low to moderate temperatures (50-200 °C) are rapid. Elemental sulfur and H₂S_(gas) fully mix isotopes with PCLM during pyrolysis conditions at 200 °C. During these reactions significant structural changes of the PCLM occur to form polysulfide dimers, thiolanes and thiophenes. As pyrolysis temperatures or reaction times increase, the PCLM thermal products are transformed to more aromatic sulfur compounds. Isotopic mixing rates increase with increasing pyrolysis temperature and time. Polysulfide bonds (S-S) in the PCLM are responsible for most of these structural and isotopic changes because of their low stability. Conversely, sulfur isotope mixing does not occur between dibenzothiophene (aromatic S) or hexadecanthiol (C-SH) and at 200 °C after 48 h. This shows that rates of sulfur isotope mixing are strongly dependent on the functionality of the sulfur in the organic matter. The order of isotopic mixing rates for organic matter is kerogen > asphalt > oil, which is inverse to their sulfur thermal stability. Asphalt and oil with more refractory sulfur show significantly lower isotopes mixing rates than the kerogen with more labile sulfur. Based on the findings of the present study we suggest that sulfur isotopes mixing can occur from early diagenesis into catagenesis and result in isotopic homogenization of the inorganic and organic reduced sulfur pools.