Hydrogeochemical models locating sulfate-methane transition zone in marine sediments overlying black shales: A new tool to locate biogenic methane?

Marine and Petroleum Geology 59 (2015) 563e574

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Research paper

Hydrogeochemical models locating sulfate-methane transition zone in marine sediments overlying black shales: A new tool to locate biogenic methane? Esther T. Arning a, *, Eric C. Gaucher b, Wolfgang van Berk c, Hans-Martin Schulz a a b c

Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany Total, CSTJF, 64018 Pau Cedex, France Clausthal University of Technology, Department of Hydrogeology, D-38678 Clausthal-Zellerfeld, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 January 2014 Received in revised form 26 September 2014 Accepted 6 October 2014 Available online 15 October 2014

Precise hydrogeochemical modeling of early diagenesis is a key in the reconstruction of sedimentary basin models. This determines the mineralogical evolution of the sediment and consequently the porosity of the rock. During early diagenesis also part of the initial organic matter is converted into biogenic gas: CH4 CO2, and H2S. These processes are part of complex reaction chains during sedimentation, and biogeochemical reactions leave different signals that can be observed today. In this work, we reproduce the early diagenetic processes as integrated signals over geological times in sediments of the Demerara Rise by applying chemical thermodynamics using the PHREEQC (version 2) computer code. The investigated sediments are characterized by the presence of black shales in 410e490 mbsf and by a diagenetic barite layer above in 300e350 mbsf at depth of sulfate-methane transition (SMT). We determine the parameters that influence the location of diagenetic barite peaks in sediments overlying black shales by means of a novel modeling approach. Crucial parameters are the amount of bacterial organic matter mineralization, sedimentation rates and bottom water sulfate concentrations. All parameters are intertwining and influence the sulfate-methane cycle. They affect the location of the SMT visualized by diagenetic barite peaks. However, our model approach opens a wide field in exploring early diagenetic reactions, processes and products (such as biogenic methane) over geological times mirrored by diagenetic minerals and pore water concentration profiles that can be detected in present-day sediments. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Barite AOM Methane Sulfate reduction Organic PHREEQC Diagenesis Marine sediments

1. Introduction Early diagenesis in marine sediments is a process of various intertwining biogeochemical reactions, which are difficult to quantitatively investigate without the help of numerical tools. A precise quantification of these reactions is thus a necessary addition for general basin modeling. Indeed, this determines the mineralogical evolution of the sediment and consequently the porosity of the rock. During early diagenesis also part of the initial organic matter is consumed and partly converted to biogenic gas: CH4, CO2, and H2S. Sulfate reduction and methanogenesis are of special interest due to their dominant role in organic matter remineralization in anoxic marine sediments (D'Hondt et al., 2002; Jørgensen, 2005). Organic matter (simplified as CH2O) is primarily

* Corresponding author. Tel.: þ49 331 288 1488. E-mail address: [email protected] (E.T. Arning). http://dx.doi.org/10.1016/j.marpetgeo.2014.10.004 0264-8172/© 2014 Elsevier Ltd. All rights reserved.

oxidized by sulfate until sulfate depletion (Eq. (1)), and is furthermore converted by methanogenesis (simplified by the reaction: Eq. (2); Claypool and Kaplan, 1974; Whiticar et al., 1986).  2CH2 O þ SO2 4 /H2 S þ 2HCO3

(1)

  2CH2 O þ Ca2þ Mg2þ þ H2 O/CH4 þ ðCa; MgÞCO3 þ 2Hþ (2) The hotspot of microbial and diagenetic processes is within the sulfate-methane transition (SMT). The SMT and its development over time are difficult to characterize and to determine. However, recently, diagenetic barite has been found to be an indicator of this reactive zone (e.g. Arndt et al., 2006; Riedinger et al., 2006; Henkel et al., 2012; Kasten et al., 2012). In the cycle of barium and sulfate in seawater and sediment, primary biogenic barite particles are formed in the water column in

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association with the degradation of organic matter (Dehairs et al., 1980; Bishop, 1988; Dymond et al., 1992; Francois et al., 1995). SO4 is progressively reduced by bacteria in the sediment column until an almost total disappearance in the SMT. Primary barite gets undersaturated in sulfate-depleted pore waters below the SMT, and will be dissolved. Subsequently, barium ions are released in high concentrations (100e300 ppm) into the pore water, diffuse upwards into sulfate-enriched sediments, and re-precipitate as diagenetic barite (Fig. 1; Eq. (3)) in a distinct zone slightly above the SMT, where the presence of sulfate allows the precipitation of barite (e.g. Gingele and Dahmke, 1994; Torres et al., 1996a; Dickens, 2001; Aloisi et al., 2004; Arndt et al., 2006; Riedinger et al., 2006; Henkel et al., 2012; Kasten et al., 2012).

Ba2þ þ SO2 4 4BaSO4

(3)

The SMT represents an important redox-boundary in organic matter-rich sediments with respect to the diagenetic barium cycle and locates the beginning of the production biogenic methane (Borowski et al., 1999). Through advection and diffusion methane migrates upwards from deeper sediment layers while sulfate migrates downwards. At the SMT sulfate and methane are contemporaneously consumed through anaerobic oxidation of methane (AOM; e.g. Reeburgh, 1976; Iversen and Jørgensen, 1985; €hner et al., 1998; Hoehler et al., 1994; Boetius et al., 2000, Niewo Eq. (2)).   CH4 þ SO2 4 /HCO3 þ HS þ H2 O

migration of the SMT relative to the sediment surface (Kasten et al., 2003). In turn, the location of the diagenetic barite front is dependent on the SMT location and the period of time the SMT is fixed at a defined sediment depth (e.g. Dickens, 2001; Riedinger et al., 2006; Snyder et al., 2007). It has been applied successfully to determine upward fluxes of methane from gas hydrate systems of the Blake Ridge (Dickens, 2001; Snyder et al., 2007) and of the northern Congo Fan (Kasten et al., 2012), from Cretaceous black shales in deposits of the Demerara Rise (Arndt et al., 2009), and in sediments in the Benguela upwelling system (Riedinger et al., 2006). Diagenetic barite fronts may serve as indicators for past and present location of redox fronts due to changing methane and sulfate fluxes, and for hiatuses in sedimentation and changes in he ret and Brumsack, sedimentation rates (van Os et al., 1991; Bre 2000; Dickens, 2001). However, authigenic minerals are sensitive against diagenetic alteration. For instance, diagenetically formed barite fronts may dissolve and re-precipitate under changing redox-conditions over time. The evaluation of measured data about authigenic minerals in sediments may thus be difficult and may lead to misinterpretations. Regarding to this, geochemical

(4)

The location of the SMT is determined by the sulfate-methane interplay. Changes in, for example, methane fluxes induce a

Figure 1. Scheme of important parameters in diagenetic barite (BaSO4) formation in marine sediments overlying black shales. SMT: sulfate-methane transition.

Figure 2. Concept of the PHREEQC model. The model describes the development of geochemical conditions in a growing sediment column with time. Each shift corresponds to a time step of 1,307,692 years. Diffusion regulates the exchange of dissolved species within the sediments and at the sedimentewater interface. Equilibrium reactions between aqueous solution, solids, and gaseous phases occur within the cells and determine aqueous solution, mineral, and gas composition in each cell at each time step. The right column describes the recent situation within the modeled sediments after calculations of the last time step (shift 30). At each time step, equilibrium reactions are calculated “cell by cell” followed by remineralization of defined amount of organic carbon and calculation of species distribution. The remineralization of organic matter induces irreversible redox reactions. The time step is ended with calculations of new equilibrium conditions resulting from diffusion and sediment burial. The next time step begins with “cell by cell” equilibrium calculations (modified from Arning et al., 2011).

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modeling provides an effective tool to enhance our qualitative and quantitative understanding of diagenetic processes. Due to the dynamic nature these processes, appropriate modeling approaches must describe a growing sediment column (Fig. 2). Such reactive mass transport models can help to decipher the geochemical evolution of sediments causing observed present-day signals. Arndt et al. (2006, 2009) have shown by means of reactiontransport models that the barite front in deeply buried black shales at the Demerara Rise (Ocean Drilling Program (ODP) Leg 207) can be used to trace migration of the SMT due to changing sedimentation rate, changing sulfate concentrations, and changing rates of methanogenesis over time. In our study, we will generally define influencing factors for barite peak and, consequently, SMT locations in sediments overlying black shales. In order to identify and constrain these parameters, we apply generic hydrogeochemical reactive transport model scenarios that are based on the detailed data set of the Demerara Rise, ODP Leg 207, Site 1258 (Erbacher et al., 2004; Shipboard Scientific Party, 2004; Arndt et al., 2009). Our modeling is focused on the controlling factors and not explicitly on the development of the SMT at Demerara Rise compared to the studies by Arndt et al. (2006, 2009). Our results will enable to assess the response of the SMT to changing environmental and geochemical conditions in geological times, and may be used to evaluate the local and shallow biogenic gas potential.

2. Material and methods 2.1. Geological setting and geochemical characterization ODP Leg 207, Site 1258 at Demerara Rise (Shipboard Scientific Party, 2004) has been chosen as example site for the identification of influencing factors to explain diagenetic barite peak formation and location in marine sediments, overlying black shale units. The Demerara Rise is located off the coasts of Surinam and French Guyana. The rise expands approximately 380 km along the coast and reaches around 220 km in width. Sediments drilled at the northern flank of the Demerara Rise during ODP Leg 207 are located within a water depth between 3000 and 4000 m and are of lower Albian to Neogen origin. The Cenomanian to Santonian sequence consists of laminated calcareous black shales sometimes intercalated with layers of pure limestone and chert. Several sediments of oceanic anoxic events (OAE) 2 and 3 were recovered (Erbacher et al., 2004). In the late Oligocene to early Miocene a prominent submarine channel system and erosional surface developed. The channel system was short lived and most of the Neogene sediments are only a few meters thick or absent from the distal part of the plateau. All drilled sediment sequences at ODP Leg 207 are characterized by the presence of a Cretaceous black shale in 150e500 meters below seafloor (mbsf). These black shales still reveal high total organic carbon contents (TOC, up to 30 wt.%) in contrast to overlying Campanian to Pleistocene sediments (around 4 wt.%; Fig. 3). Pore water sulfate concentrations of Site 1258 decrease linearly from the top of the sediments to the top of the black shale unit. Instead, pore water methane concentrations are high in the black shale layer (up to 3 mM) and decrease upwards (Fig. 3). In these sediments, sulfate-reduction and methanogenesis are the major pathways of organic matter decomposition. Further, the pore water depth profiles indicate that the deeply buried Cretaceous black shales still display an active bioreactor (almost 100 Myrs after their deposition) and control the biogeochemical reactions in these sediments (Erbacher et al., 2004).

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At site 1258, barium concentrations are high in the black shale layer (up to 80 mM) and decrease upwards within the sediment sequences (Shipboard Scientific Party, 2004, Fig. 4). The remobilized barium diffuses out of the black shale layer and reprecipitates as authigenic barite (BaSO4) in the sulfate methane transition zone where saturation of pore water with respect to barite is reached due to the presence of sulfate (Arndt et al., 2006, 2009, Fig. 4). Peak contents of barite are up to 8 mol m3. 2.2. Conceptual model The model aims to reproduce the observed barite depth distribution and the present-day pore water depth profiles to retrace the evolution of sediments in terms of past conditions and influencing factors of diagenetic barite peak formation. The model approach is a backward calculation, similar to an inverse modeling procedure. Problems that may arise in such a modeling are i) free model parameters that may not be fulfilled by available data, and ii) measured datasets maybe explained by several model set ups. However, previous modeling on the Demerara Rise sediments has shown that the long-term redox processes below the shallow subsurface are mainly triggered by the microbial degradation of organic matter (Arndt et al., 2006, 2009). Therefore, our model approach is based on and adjusted with the amount of organic matter that has been degraded. The ODP data set (Shipboard Scientific Party, 2004) constraints the possible model scenarios, and enables calibration of our model and definition of well-defined input parameters. A major concern relates to the presented, measured methane gas concentrations which have been determined on core material which was not recovered by pressure core sampling (PCS). Such data as well as, e.g., methane/carbon dioxide ratios, are highly dependent on core sampling. PCS has been

Figure 3. Amounts (wt.%) and rates (mmol m3 yr1) of organic matter remineralized in each representative volume (RV) of Demerara Rise, ODP Leg 207, Site 1258, as well as measured total organic carbon (TOC) content (wt.%). Measured data are taken from Shipboard Scientific Party (2004).

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demonstrated to deliver more realistic as higher methane gas contents (Dickens et al., 2003; Paull et al., 2000). 2.2.1. Model approach Numeric hydrogeochemical 1-dimensional reactive transport models can reflect the complex web of early diagenetic processes in marine sediments. The model approach and description have been adopted and modified from Arning et al. (2011). The modeled sediment column is composed of 30 cells (“generic reactors”). The generic 1.7 L reactor is filled with an aqueous solution of present-day chemical seawater composition (Nordstrom et al., 1979), and with sediments of defined mineral composition according to sediments from ODP Leg 207, Sites 1258 (Shipboard Scientific Party, 2004). Equilibrium species distribution and coupled mass transfer that results from reactions in the modeling reactor are calculated by using the PHREEQC (version 2) program (Parkhurst and Appelo, 1999). Calculations are based on mass action laws that include all species of Ca, Mg, Na, K, Al, Fe, Ba, Si, Cl, C, S, N, P, H2O, and their corresponding equilibrium constants. Species, mass-action equations, and equilibrium constants are listed in the thermodynamic database “wateq4f.dat” (Parkhurst and Appelo, 1999) and Table 1. Conceptually, equilibrium species distribution is triggered by the irreversible redox conversion of organic matter, which proceeds in preassigned amounts. The accumulation of the organic matter metabolites leads to the development of new inorganic equilibrium conditions in the system. The modeled sediment column is 510 m thick (representing the zone most crucial for early diagenetic reactions), and is subdivided into thirty 17 m thick cells (Table 1). Each cell is an original reactor (expressed as representative volume (RV); 1.7 L ¼ 17 m$0.01 m$0.01 m) and contains 1 L of pore water (V(aq)) and 1.89 kg of solid sediment. Hence, the average porosity (ɸ) is 0.59 and the average specific weight of solids (r(s)) is 2.7 kg L1. The model design displays a growing sediment column (Fig. 2). Initially, equilibrium calculations are performed on one sediment cell that is overlain by five cells containing seawater. At a second time step, the first cell is instantaneously buried to a depth of 17 m. The time step for each cell (1,307,692 yrs; Table 1) is calculated from the sedimentation rate (Table 1; Shipboard Scientific Party, 2004). The total time modeled is 39.19 Myrs due to a hiatus at the top of the sediment column (the last 42 Myrs, Shipboard Scientific Party, 2004) that is not resolved in our model. Therefore, the oldest model layer started burial 81.19 Myrs ago (39.19 Myrs þ 42 Myrs) in the Upper Cretaceous. Freshly deposited sediments at the sedimentewater interface form the second cell in the growing sediment column. After 30 time steps the modeled sediment column consists of 50 cells and represents the upper 510 m of the investigated sites. Once all calculations have been completed, the mineral assemblage and pore waters of the first cell (from the first time step) shift from the sedimentewater interface to the base of the modeled sediment column at a depth of 493e510 mbsf. The active transportation processes considered in this model are 1) one-dimensional molecular diffusion of aqueous species, and 2) burial of solids (including methane hydrate) and aqueous species, according to the observed sedimentation rate, which also represents advection. Porosity changes that occur within the upper few meters and towards greater sediment depths (Shipboard Scientific Party, 2004) cannot be resolved by the model, hence, compaction flow and advection are not incorporated. Diffusion enables the transportation of dissolved species between all cells during sediment deposition and burial. A mean diffusion coefficient (0.86$109 m2 s1) for all aqueous species is calculated from diffusion coefficients (corrected for tortuosity) according to

Giambalvo et al. (2002) who investigated similar sediments from the South Atlantic. The upper boundary of the model is defined as a constant concentration boundary within the topmost cell of the seawater column. The lower boundary is defined as a boundary closed for diffusion. Diffusive exchange between seawater and pore water takes place at the sedimentewater interface, between each newly deposited sediment cell and the lowermost cell containing seawater. Temperature increases linearly throughout the modeled sediment column consistent with the geothermal gradient (Table 1; Shipboard Scientific Party, 2004). A constant pressure (344.7 atm; Table 1, calculated as hydrostatic pressure according to water depth and 255 m of sediments) was selected for calculations of gas solubility because of software limitations, as the effect of varying the total pressure on aqueous species distribution and associated solid phases is minor (cf. Arning et al., 2011). Secondary minerals and gas phases (Tables 1 and 2) are not present at the beginning of the equilibrium calculations but are by default allowed to form and react during the model run. Saturation indices (SI; SI ¼ log(IAP/K) with IAP the ion activity product and K the equilibrium constant) with respect to the mineral phases of interest are initially set to 0 except for calcite and siderite. Supersaturation (saturation index of 0.85) for these carbonates is assumed due to phosphate adsorption onto calcium carbonate, in accordance with observations by Raiswell and Fisher (2004). Equilibrium reactions and their thermodynamic constants for

Table 1 PHREEQC model setup and input parameters of ODP Leg 207, Site 1258 for the calibrated model. RV: representative volume, SI: saturation index, CEC: cation exchange capacity. Model setup parameters Number of RVs Height RV Vtot RV Vaq RV Time per shifta Total time modeledb Total sediment depthb Diffusion boundary conditions Database

Physical parameters Bottom water temperatured Geothermal gradientd Sedimentation rated Water depthd Pressure (constant)

a

Geochemical parameters

30 Starting solutions 17 m Primary phasesd 1.7 L Montmorillonite 1.0 L Illite 1,307,692 yrs Chlorite14A 39,230,769 yrs Kaolinite 510 m Goethite

Seawatere mol/RV 0.20e0.90 0.19e0.85 0.08e0.43 0.23e0.99 0.34

Constant (top) Closed (bottom) wateq4f.datc

3e12 0.001e0.007

3 C 30  C km1 2.8 cm kyr1 3192 m 344.7 atm

Calcite Baritef Secondary phases Calcite Siderite Barite Greigite

mol/RV 0 0 0 0

Struvite Vivianite Sepiolite (d) CH4Hydrate Gas phases CECg Transport Diffusion coefficienth

0 0 0 0 CO2(g), N2(g), H2S(g) 1.2 mol/RV Diffusion 0.86E09 m2s1

Calculated from sedimentation rate and height of RVs. Calculated from sedimentation rate, number and height of RVs. Parkhurst and Appelo (1999). d From Shipboard Scientific Party (2004). e After Nordstrom et al. (1979). f Primary barite content is 0.007 mol/RV in the organic matter-rich Black shale layer and 0.001 mol/RV background content in the other sediments. g Cation exchange capacity, depends on the clay mineral content, estimated according to Jasmund and Lagaly (1993). h Mean value for all dissolved species, calculated according to diffusion coefficients given in Giambalvo et al. (2002). b

c

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Table 2 Equilibrium phases, mass-action equations and equilibrium constants (log K, at 25  C and 1 bar pressure). Equilibrium constants are from the “wateq4f.dat” data set (Parkhurst and Appelo, 1999). Equilibrium phase

Equilibrium reaction

log K

Barite Calcite Chlorite14A CH4Hydrate Goethite Greigite Illite Kaolinite Montmorillonite Sepiolite(d) Siderite Struvite Vivianite CO2 (g) CH4 (g) N2 (g) H2S(g)

BaSO4 ¼ Ba2þ þ SO2 4 CaCO3 ¼ Ca2þ þ CO2 3 Mg5Al2Si3O10(OH)8 þ 16Hþ ¼ 5Mg2þ þ 2Al3þ þ 3H4SiO4 þ 6H2O CH4:6H2O ¼ CH4 þ 6H2O FeOOH þ 3Hþ ¼ Fe3þ þ 2H2O Fe3S4 þ 4Hþ ¼ 2Fe3þ þ Fe2þ þ 4HS þ K0.6Mg0.25Al2.3Si3.5O10(OH)2 þ 11.2H2O ¼ 0.6Kþ þ 0.25Mg2þ þ 2.3Al(OH) 4 þ 3.5H4SiO4 þ 1.2H Al2Si2O5(OH)4 þ 6Hþ ¼ 2Al3þ þ 2H4SiO4 þ H2O Ca0.01Na0.31Al2.33Si3.67O10(OH)2 þ 12H2O ¼ 0.01Ca2þ þ 0.31Naþ þ 2.33Al(OH)4 þ 3.67H4SiO4 þ 2Hþ Mg2Si3O7.5OH:3H2O þ 0.5H2O þ 4Hþ ¼ 2Mg2þ þ 3H4SiO4 FeCO3 ¼ Fe2þ þ CO2 3 3 MgNH4PO4:6H2O ¼ Mg2þ þ NHþ 4 þ PO4 þ 6H2O Fe3(PO4)2:8H2O ¼ 3Fe2þ þ 2PO3 4 þ 8H2O CO2 ¼ CO2 CH4 ¼ CH4 N2 ¼ N2 H2S ¼ Hþ þ HS

9.97 8.48 68.38 1.073a 1.0 45.035 40.267 7.435 45.027 18.66 10.75 13.26b 36.0 1.468 2.86 3.26 6.994

a b

Calculated from Gibb's free energy (DG ¼ 5.736 kJ mol1), pressure (P ¼ 10 MPa), and temperature (T ¼ 279.55 K) after Lu et al. (2008). Defined after Ronteltap et al. (2007).

methane hydrate and struvite (Table 2), which are lacking in the wateq4f.dat database, are defined in the modeling input files; the database itself remains unmodified to ensure the consistency of data. Given in-situ pressure and temperature conditions, the sediments are within the gas hydrate stability zone (e.g. Sloan, 1998 and references therein). Therefore, methane hydrate is defined as an additional equilibrium phase in these model scenarios. The equilibrium constant for methane hydrate is calculated from Gibb's free energy (DG ¼ 5.736 kJ mol1) of the methane dissolution reaction: CH4$6H2O(s) ¼ CH4(aq) þ 6H2O, pressure (P ¼ 10 MPa), and temperature (T ¼ 279.55 K) after Lu et al. (2008). Input parameters that define primary and secondary equilibrium phases and physical boundary conditions are given in Table 2. A detailed description of the physical and chemical considerations and the calculation scheme of the applied model are given in Arning et al. (2011).

2.2.2. Model calibration and organic matter remineralization Organic matter remineralized in the models (Redfield-CH2O) was assumed to be of dominantly marine origin with minor terrestrial contributions and is defined with an approximate Redfield stoichiometry of C:N:P equal to 100:15:1 (Redfield, 1958). In our model, the organic matter is considered as a whole, and reaction rates are included by converting different amounts of Redfield-CH2O in each time step. It has to be noted that the stoichiometry of the remineralized organic matter is an approximation as the Redfield ratio of organic matter depositing on the seafloor is not necessarily conserved. Results from different studies show that the organic matter in ancient, deeply buried black shales still provides a suitable substrate for persistent microbial degradation that is dominated by methanogens (Krumholz et al., 1997; Coolen et al., 2002; Krumholz et al., 2002; Moodley et al., 2005; Arndt et al., 2006, 2009).

Figure 4. PHREEQC model results of the Demerara rise. Measured pore water concentration data are taken form Shipboard Scientific Party (2004) and measured barite data are taken from Arndt et al. (2009). Gray bar: organic carbon rich black shale, SMT: sulfate-methane transition.

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Measured pore water alkalinity profiles are used for calibration. The release of CO2 from the conversion of organic matter and its subsequent dissolution influences pore water alkalinity. To calibrate the model, the amount of organic matter (simplified as (CH2O)x(NH3)y(H3PO4)z) that can be converted in each representative volume at each time step (cf. Fig. 3) is readjusted until the modeled alkalinity profiles of the investigated site nearly match the general trend of measured alkalinity profile (cf. Fig. 4). Cells 6 to 2 (present day sediment depth 416.5e484.5 mbsf) represent the organic carbon-rich black shale layer. According to this, the amount of the converted organic matter is higher in these cells compared to other sediment depths Fig. 3. The parameters alkalinity (except for the lower part of the model column), dissolved methane (non-PCS data), barium, and barite are reproduced very well with the model (Fig. 4). However, we were not able to reproduce exactly the sulfate profile but the modeled point of exhaustion fits the measured one (Fig. 4). 2.2.3. Incorporation of barite in the model Barite precipitation and barite dissolution is allowed to take place in the whole modeled sediment column ðBa2þ ðaqÞ þ SO2 4 ðaqÞ⇔ВaSΟ4ðSÞ Þ. Barite (BaSO4) is given as primary phase in all cells of the model column. A background content of 0.001 mol cell1 is given in cells 1 and 7e30. Together with the black shale layer (cells 2e6) a higher content of primary barite (0.007 mol cell1) is given in the model calculations. Our assumption of higher primary barite input together with the sedimentation of organic carbon-rich sediments is based on the theory that the formation of barite particles (biogenic barite) in the water column is associated with the decay of organic matter. Biogenic barite is thought to form within microenvironments of decaying organic matter (Bishop, 1988; Dehairs et al., 1980; Dymond et al., 1992; Ganeshram et al., 2003; Gingele and Dahmke, 1994; Paytan et al., 1993). Following, barite fluxes to the seafloor and primary (biogenic) barite accumulation is highest in areas of high productivity and sediments underlying these areas are often enriched in barium phases (e.g. Goldberg and Arrhenius, 1958; Dymond et al., 1992; Francois et al., 1995; Paytan et al., 1996; Schenau et al., 2001). The biogenic barite is discussed to be labile, which is subject to remobilization due to sulfate depletion in pore waters (e.g. McManus et al., 1994; Torres et al., 1996a,b; McManus et al., 1998). 3. Results and discussion 3.1. Environmental factors influencing diagenetic barite peak location The applied geochemical model unravels such environmental parameters that have influence on diagenetic barite peak formation. Important in this regard are i) the amount of organic matter degradation, ii) sedimentation rate, iii) bottom water sulfate concentration, and iv) biogenic barite input. Generic model scenarios highlight the consequences of varying environmental conditions on diagenetic barite peak location relative to the sediment surface and an organic matter-rich black shale layers (Fig. 5). In the following

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chapters, we will discuss the impact of the influencing factors in detail. 3.1.1. Organic matter degradation and diagenetic barite peaks One important, if not the most important, parameter is the amount of organic matter that is degraded in the total sediment column. In this regard, two mechanisms have to be considered. On the one hand, the bacterial sulfate reduction (Eq. (2)) taking place in the upper sulfate containing sediments, and, on the other hand, methanogenesis (Eq. (3)) taking place in sulfate-depleted sediments, especially in the organic matter-rich black shale layer. Both processes influence fluxes of sulfate and methane and, consequently, the location of the SMT and the anaerobic oxidation of methane (AOM) zone. The penetration depth of sulfate is controlled, besides sulfate reduction and bottom water sulfate concentration that will be discussed later (cf. 3.1.2 and 3.1.3), by bacterial sulfate reduction and AOM. In turn, the location of the zone of AOM is controlled by the methane flux from below that derived from microbial methanogenesis in the present scenarios. The microbial degradation rates are dependent on the accumulation rate of organic matter and the reactivity of organic matter. In general, the reactivity of organic matter is a result of several interacting factors, such as organic carbon nature, physical protection, geopolymerization, metabolite inhibition or redox zonation of the depositional environment (e.g. Canfield, 1994; Burdige, 2007; LaRowe and van Cappellen, 2011). The present-day total organic carbon content is high in the black shale horizon (Fig. 3; Shipboard Scientific Party, 2004) and these sediments still display a hotspot for microbial activity. Porosities are between 40 and 50% in sediment depth between 300 and 500 mbsf (Shipboard Scientific Party, 2004) and do not limit microbial activities. As such they provide a substrate for enhanced rates of organic carbon mineralization (Fig. 3; Erbacher et al., 2004; Arndt et al., 2006). Such a microbial hotspot is necessary in the model calibration, otherwise no fit in measured and modeled alkalinity profiles could be achieved and no peak in methane concentration within the black shale layer would have been developed (Fig. 4). With increasing organic matter conversion, the diagenetic barite front migrates upwards within the sediment column in association with the SMT, and the peak height decreases (Fig. 5a). Increased organic matter conversion leads to early sulfate depletion within the sediment column (Fig. 5a). Sulfate gets more intensely consumed due to bacterial sulfate reduction. Furthermore, elevated rates of methanogenesis produce higher methane concentrations (Fig. 5a) and induce elevated upward methane fluxes. Consequently, an upward shift of the SMT and an early onset of AOM established that additionally reduces sulfate. A doubling of the organic matter conversion in all cells over the modeled sediment column (within the sulfate reduction as well as in the methanogenic zone) results in an upward shift of the barite peak by ca. 300 m due to significant higher methane production and rapid sulfate consumption. Similar results were obtained by Arndt et al. (2009) which show that a doubling of the degradation rate constant of methanogenesis within the black shale layer results in an upward migration of the SMT by ca. 100 m. At some point of increasing organic matter conversion (here: more than double of the calibrated amounts, cf. Figs. 3 and 5a), no

Figure 5. Effect of A) amount of converted organic matter (a: calibrated model (1x organic matter), b: 0.5x organic matter, c: 1.5x organic matter, d: 2x organic matter), B) sedimentation rate (a: calibrated model (1.3 cm kyr1), b: 0.65 cm kyr1, c: 1.625 cm kyr1, d: 1.95 cm kyr1, d: 2.6 cm kyr1), C) bottom water sulfate concentration (a: calibrated model (28 mM), b: 10 mM, c: 19 mM, d: 24 mM, e: 32 mM), and D) amount of primary barite (a: calibrated model (0.007 mol cell1 within the black shale layer and 0.002 mol cell1 as background content), b: 0.0035 mol cell1 within the black shale layer and 0.001 mol cell1 as background content, c: 0.014 mol cell1 within the black shale layer and 0.004 mol cell1 as background content) on diagenetic barite formation, methane and sulfate concentrations; please note that alkalinity, sulfate and dissolved methane concentrations are equal in all model scenarios of varying primary barite contents. Measured pore water concentration data are taken form Shipboard Scientific Party (2004) and measured barite data are taken from Arndt et al. (2009) Gray bar: organic carbon rich black shale, SMT: sulfate-methane transition, SR: sedimentation rate, OM: organic matter.

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diagenetic barite peak would be present in the sediments, anymore. Sulfate would be exhausted at the sedimentewater interface, methane diffuses out of the sediments. Such a situation could be comparable with diagenetic barite formation at cold methane seeps or submarine mud volcanoes (Torres et al., 1996a, 2003; Snyder et al., 2007; Kasten et al., 2012; Vanneste et al., 2013). Here, barite crusts formed on top of the sediments and methane is sourced either by the decomposition of gas hydrates or derived from a thermogenic source. The upper location of SMT enables a higher diffusive flux of barium into the overlying seawater, and consequently the amount of diagenetic barite and the height of the barite peak diminish. A further explanation for the absence of a diagenetic barite front is low organic matter conversion (here: less than half of the calibrated amounts, cf. Figs. 3 and 5a). Sulfate gets not depleted in the sediments (Fig. 5a) due to minor bacterial sulfate reduction, low rates of methanogenesis and the retarded process of AOM. Pore waters remain saturated with respect to barite and primary biogenic barite will not dissolve. At a first glance, it seems difficult to distinguish both scenarios in which the formation of a diagenetic barite peak is inhibited. However, very high rates of organic matter conversion would not leave barite at all within a sediment column. Instead, a barite crust would form on top of the sediments. In contrast, low rates of organic matter conversion would leave the primary biogenic barite in the black shale layer. For conclusions from barite cycling on organic matter degradation rates, a whole sediment sequence from the top to the base of a black shale unit has to be measured. 3.1.2. Sedimentation rate and diagenetic barite peaks Modeling results reveal that the location of the barite peak is essentially influenced by the sedimentation rate (Figs. 5b and 6). Simulation results by Arndt et al. (2009) reveal that long-term fluctuations in sedimentation rate trigger considerable shifts in redox-zonation of sediments at the Demerara Rise. The rapid burial during the late Paleoceneemiddle Eocene period induces a prominent upcore shift of the SMT (Arndt et al., 2009). This result from reactive transport modeling fits our more generic simulation results. Increasing sedimentation rates lead to a more and more rapid burial of the black shale layer (Fig. 6). Resulting increasing diffusion lengths and ongoing sulfate reduction induces an upcore migration of the SMT and the addicted diagenetic barite peak (Figs. 5b and 6). A 1.25 increase in sedimentation rate (from 1.3 cm kyr1 to 1.625 cm kyr1) leads to an upward shift of the SMT by 245 m. The SMT is situated approximately 330 m above the black shale layer in model simulations with the higher sedimentation rate, while in the

initial model the SMT is located 85 m above the black shales (Figs. 5b and 6). Running the model with a doubled sedimentation rate (2.6 cm kyr1) leads to a completely sulfate-depleted sediment column. Consequently, high sedimentation rates may lead to the absence of diagenetic barite fronts, even though barium has been remobilized form biogenic barite in organic matter-rich sediment layers. On the other hand, model calculations with halve of the initial sedimentation rate (0.65 cm kyr1) inhibit biogenic barite dissolution and diagenetic barite re-precipitation (Figs. 5b and 6). Sulfate concentrations remain high in the entire sediment column, comparable to the model situation of very low organic matter conversion (Fig. 5a) and no redox zonation establishes in the sediments. The slow burial of the black shale layer (Fig. 6) enables continuous diffusion of sulfate into the depth of the black shales. The absence of diagenetic barite can be caused by two oppositional environmental conditions, either by high sedimentation rates or by very low sedimentation rates. However, comparable to the organic matter conversion as influencing factors: In a situation with low sedimentation rate, primary biogenic barite should be detected in an organic matter-rich layer, whereas primary biogenic barite is dissolved in scenarios with high sedimentation rates, at all.

3.1.3. Bottom water sulfate concentration and diagenetic barite peaks Bottom sea-water sulfate concentrations have changed over geological times (Horita et al., 2002). Therefore, we generated model scenarios with concentrations higher and lower compared to present-day concentrations (28 mM). Simulations reveal an upward shift of the SMT and the barite front as response to the decrease in bottom water sulfate concentration (Fig. 5c). This is the logical consequence of fewer sulfate that is available for bacterial sulfate reduction and AOM. Methanogenesis is favored and upward methane fluxes increase and further drive sulfate consumption due to AOM. A reduction of bottom water sulfate concentration by 4 mM causes an upward migration of the SMT by more than 100 m (Fig. 5c). This response of diagenetic barite fronts on bottom water sulfate concentration is of importance, as for example, during the deposition of the sediments of the Demerara Rise, where bottom water sulfate concentrations increased over the geological time (19 mM at 36.5 Myrs before present to 28 mM at present-day; Horita et al., 2002). Arndt et al. (2009) have shown that simulated barite depth profiles display differences in the height, width, and location of the diagenetic barite peaks if different bottom water sulfate concentrations are applied that were present at distinct geological times (Fig. 8).

Figure 6. Formation of diagenetic barite according to sedimentation rate. Model time decreases with increasing sedimentation rate (cm kyr1). Gray bar: organic carbon rich Black shale. SR: sedimentation rate.

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In turn, higher bottom water sulfate concentrations in the applied model scenarios lead to a deepening of the SMT. Similar observations have been made by Henkel et al. (2012) with simulations for anoxic Black Sea sediments. Penetration depth of sulfate increases with increasing bottom water sulfate concentrations. Following, higher amounts of the reactive organic matter are consumed by bacterial sulfate reduction till greater sediment depth. Methanogenesis is hindered and insufficient to produce methane concentrations high enough to compensate for the increased downward flux of sulfate (Fig. 5c). Calculations of a generic scenario with bottom water sulfate concentrations of 32 mM result in only minor dissolution of biogenic barite and in the absence of a diagenetic barite peak. 3.1.4. Biogenic barite input and diagenetic barite peaks The exact amount of biogenic barite that precipitated together with organic matter during events, such as the Cretaceous OAEs, is not well constrained. Therefore, we run three model scenarios with varying amounts of biogenic barite within the black shale layer and background sediments (calibrated, half of the calibrated scenario, and double of the calibrated scenario; Fig. 5d) It is assumed that high amounts of biogenic barite accumulated together with high amounts of organic matter during times of enhanced paleoproductivity (e.g. Goldberg and Arrhenius, 1958; Dymond et al., 1992; Francois et al., 1995; Paytan et al., 1996; Schenau et al., 2001). Model results reveal that the amount of biogenic barite impacts the height and shape of the diagenetic barite peak, whereas the location remains unaffected (Figs. 5d and 7). A doubling of the biogenic barite content increases modeled diagenetic barite contents by 3 mol m3. The other way round, bisection of biogenic barite results in a much lower but broader peak (7.5 mol m3). These changes can be attributed to changing amounts of barium that are remobilized from the black shale layer due to biogenic barite dissolution. Barium diffuses upwards and sulfate at the SMT is sufficient to capture all remobilized barium, even if high concentrations enter this zone. It is still a matter of debate, how barite in sediments could be used as tracers for paleoproductivity (e.g. von Breymann et al., 1992; Gingele and Dahmke, 1994; Gingele et al., 1999; Kasten et al., 2001; Pfeifer et al., 2001; Plewa et al., 2006; Henkel et al., 2012). As mentioned before, the amount of biogenic barite seems to be coupled to times of enhanced paleoproductivity and, consequently, enhanced organic matter burial. However, modeling results of this study and of other studies (e.g. von Breymann et al., 1992; McManus et al., 1998; Gingele et al., 1999; Arndt et al., 2006; Riedinger et al., 2006; Arndt et al., 2009) show that the amount of barite peaks in sediments are often not dominated by biogenic barite input. Many other factors strongly affect dissolution of biogenic barite and the location, height, and shape of diagenetic barite peaks. This limits the use of barite as paleoproductivity proxy. Usability is only given if the biogenic barite has not undergone any dissolution. The generic model scenarios suggest that this could be the case in settings controlled by low organic matter conversion and low sedimentation rate. Furthermore, high bottom water sulfate concentrations would favor biogenic barite preservation under these conditions (Figs. 5 and 7).

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evolution of biogeochemical transformation rates trigger prominent shifts in the redox-zonation of sediments on the Demerara Rise. Our generic model should describe the evolution of a SMT and an associated diagenetic barite peak in relation to an organic matter-rich layer over a time period of about 80 Myrs (Fig. 8). It is based on data of the Demerara Rise, however it is simplified by using a constant sedimentation rate and constant present-day bottom water sulfate concentration (28 mM) over the whole simulation time. The model starts 79.0 Myrs ago during Upper Cretaceous and approach present-day conditions (Fig. 8). On basis of our results, we suggest four stages that determine the evolution of the SMT and the distribution of barite within such sediments. In the first step only background contents of primary barite reach the seafloor. High primary barite contents were deposited together with the organic matter-rich black shale layer and buried (stage 1). When sulfate in the pore water got exhausted, the primary barite dissolved (stage 2). New diagenetic barite formed at the SMT above the organic matter-rich layer at suitable sulfate concentrations (stage 3). The diagenetic barite peak got bigger and sharper with time until it is fixed in the present-day sediment depth between 300 and 350 mbsf (stage 4). The distance between the SMT and the black shale layer increases over time with increasing burial of the black shale. The burial importantly influences the biogeochemical reactions within the sediments. Sulfate diffusion into the black shale layer is hindered and the diminished sulfate concentration cannot drive bacterial sulfate reduction within the black shale, anymore. This promotes the onset of methanogenesis, and the increasing production of biogenic methane leads to an upward migration of the SMT. Biogenic methane drives the additional consumption of sulfate by AOM. These coupled processes result in the formation of a distinct diagenetic barite peak that is associated with the SMT at stage 4. 4. Model discussion Geochemical reactive transport models are an important tool to understand diagenetic processes that have created observed

3.2. SMT development with time The dependency of diagenetic barite fronts on the SMT enables to draw conclusions on the past changes of this zone, determined by upward fluxes of methane and downward fluxes of sulfate he ret and Brumsack, 2000; Dickens, 2001; Paytan et al., 2004; (Bre Riedinger et al., 2006; Kasten et al., 2012). Arndt et al. (2009) show that long-term fluctuations in sedimentation rate and the past

Figure 7. General summary of the influencing factors (amount of organic matter (OM) conversion, sedimentation rate, bottom water sulfate concentration, and amount of primary/biogenic barite input) on location and shape of diagenetic barite fronts. In this modeling example, the OM conversion, the sedimentation rat, and the amount of primary barite have changed by a factor of 4; bottom water sulfate concentration have changed by a factor of 3.2.

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authigenic mineral and pore water composition of marine sediments. Geochemical models support to quantify and retrace biogeochemical reactions over geological timescales, and to reconstruct paleo-environments. However, regardless of which exact model approach is used, all are limited by the amount of data and the knowledge of paleoboundary conditions. In the presented case and in our model approach, we only have a rough estimate of the sedimentation rates and can conclude on initial sediment deposition only from presentday measured data. Furthermore, the evolution of seawater sulfate concentrations over the last 100 Myrs is not well constrained and the model cannot resolve short-term changes in redox-conditions, such as eventual or periodic oxic bottom water conditions. In this

respect, we calculated different generic model scenarios. Our bestfit model is based on data of the Demerara Rise Leg 207, Site 1258 sediments, and retraces the observed diagenetic signals (diagenetic minerals and pore water concentration profiles). However, it is limited to exactly represent the depositional conditions, as an average sedimentation rate is used, knowing that hiatuses and changes appeared over time. The same is true for bottom water sulfate concentrations. In this respect, different generic model scenarios have been calculated to present the consequences of different environmental and boundary conditions on the diagenetic evolution of sediments and the biogeochemical reactions over time. Potential uncertainties also derive from the closed lower boundary conditions. Possible methane fluxes from underlying

Figure 8. Snapshots of modeled depth profiles showing the evolution of A) the diagenetic barite peak, B) dissolved barium, and C) dissolved methane and sulfate in sediments of the Demerara Rise over time. Gray bar: organic carbon-rich black shale, SMT: sulfate-methane transition.

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sediments are not allowed and, consequently, by comparison with literature data, methane production from the black shale layer maybe overestimated as measured may be too high. In other settings, methane could derive by the decomposition of gas hydrates or from a thermogenic source, as well. Thus, the presented model approach is only usable in settings of a dominant biogenic methane production within the modeled sediment column or if biogenic methane can be distinguished from methane of other sources by, for example, isotope data. Despite all the uncertainties and limitations, the presented geochemical model approach and other reactive transport models are essential in studying biogeochemical processes of sediments. They are useful to support experimental data and observations. 5. Conclusions Simulation results show that several environmental factors influence the diagenetic barite cycle, especially in settings prominent for organic matter-rich sediment layers. The different generic model scenarios on example of sediments of the Demerara Rise (ODP Leg 207, Site 1258) highlight three factors to be most important in the diagenetic barite cycle (besides biogenic barite input) and the redox-zonation of studied and similar sediments: 1) The amount of the converted organic matter of the total sediment column, 2) The sedimentation rate, and 3) The bottom water sulfate concentration. Changes in these parameters will affect sulfate penetration within the sediments, the bacterial sulfate reduction, and, consequently, methane production and occurrence. This reaction chain will lead to a shift in the redox-zonation within the sediments and determines dissolution and re-precipitation of barite. Model results reveal the complex interaction of the influencing factors on the location of the sulfate methane transition (SMT) and the development of an interrelated diagenetic barite peak. The SMT as well as a diagenetic barite peak serve as indicators for biogenic methane production. Barite peaks in marine sediments may serve as indicators for underlying organic matter-rich layers and may be used as a proxy parameter to constrain changes in methane and sulfate fluxes over geological times. On a first glance, this looks straight forward. However, present-day sedimentary barium/barite analyses have to be interpreted with caution. Conclusions on location and presence of an organic matter-rich black shale layers, the accumulation of organic matter and microbial mineralization rates are not straight forward. In this respect, the absence of a barite peak does not automatically point to the absence of a black shale layer. The lack of diagenetic barite in sediments overlying a black shale could also be caused by high organic matter conversion, high sedimentation rates, low sulfate concentrations in bottom water or a combination off all. For better constrained conclusions on migration of the SMT over time, microbial organic matter degradation, and the diagenetic history of sediments the interrelation between various parameters have to be considered. In this respect, geochemical modeling is clearly useful. We suggest using barite as a trace mineral to locate sulfatemethane transition zones in marine sediments and to conclude on redox conditions and reactions crucial in such sediments. Further, our approach opens wide research fields on modeling sediment evolution. Not only barite, also other mineral minerals could be important in exploring biogeochemical reactions in sediments. Regarding to such research, geochemical modeling does and will be a strong and important tool.

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