This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or cu... more This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by, you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles): August 1, 2012 www.sciencemag.org (this information is current as of The following resources related to this article are available online at
In the Cretaceous of North America, environmental sensitivity and habitat specialization have bee... more In the Cretaceous of North America, environmental sensitivity and habitat specialization have been hypothesized to explain the surprisingly restricted geographic ranges of many large-bodied dinosaurs. Understanding the drivers behind this are key to determining broader trends of dinosaur species and community response to climate change under greenhouse conditions. However, previous studies of this question have commonly examined only small components of the paleo-ecosystem or operated without comparison to similar modern systems from which to constrain interpretations. Here we perform a high-resolution multi-taxic δ13C and δ18O study of a Cretaceous coastal floodplain ecosystem, focusing on species interactions and paleotemperature estimation, and compare with similar data from extant systems. Bioapatite δ13C preserves predator-prey offsets between tyrannosaurs and ornithischians (large herbivorous dinosaurs), and between aquatic reptiles and fish. Large ornithischians had broadly o...
Stable isotopes are powerful tools for elucidating ecological trends in extant vertebrate communi... more Stable isotopes are powerful tools for elucidating ecological trends in extant vertebrate communities, though their application to Mesozoic ecosystems is complicated by a lack of extant isotope data from comparable environments/ecosystems (e.g. coastal floodplain forest environments, lacking significant C 4 plant components). We sampled 20 taxa across a broad phylogenetic, body size, and physiological scope from the Atchafalaya River Basin of Louisiana as an environmental analogue to the Late Cretaceous coastal floodplains of North America. Samples were analysed for stable carbon, oxygen and nitrogen isotope compositions from bioapatite and keratin tissues to test the degree of ecological resolution that can be determined in a system with similar environmental conditions, and using similar constraints, as those in many Mesozoic assemblages. Isotopic results suggest a broad overlap in resource use among taxa and considerable terrestrial–aquatic interchange, highlighting the challenge...
ABSTRACT IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent... more ABSTRACT IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent margin in a slow, diffuse fluid flow environment. These transect sites are an ideal data set to compare variation in gas hydrate (GH) occurrence with distance from the deformation front. In this study, we quantify the variation with depth of GH saturation (fraction of pore space occupied by GH) from pore water chlorinity and downhole logs of porosity and electrical resistivity. At each site we determine a GH occurrence zone (GHOZ), defined as the depth interval where gas hydrates are actually observed. The GHOZ is not necessarily the same as the GH stability zone (GHSZ), which is the depth interval where gas hydrates are thermodynamically stable. In three of the Exp. 311 transect sites gas hydrates occur down to the base of GH stability, but the top GHOZ is not at the seafloor (the top of the GHSZ). The top GHOZ deepens moving away from the deformation front, from 47 mbsf at Site U1326 (5 km from the deformation front), to 73 mbsf at Site U1325 (11 km), and 111 mbsf at Site U1327 (21 km). Site U1329 (38 km from the deformation front) shows no clear evidence of GH. Fluid expulsion from the accretionary wedge has been proposed as the main process to advect methane and form GH at the Cascadia margin. Published studies have quantified fluid expulsion rates from the compaction of sediments that are scraped off the subducting plate and are incorporated into the accretionary wedge. These compaction models generally show fluid expulsion rates that decrease sharply moving inland from the deformation front. The Exp. 311 transect sites show a GHOZ that thins moving away from the deformation front and correlates with the decrease in fluid expulsion rates predicted by the compaction models. Our results are consistent with fluid expulsion being a primary control on the formation of GH in the Cascadia convergent margin.
Changes in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur are ... more Changes in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur are the dominant control on atmospheric oxygen levels, and the partitioning itself depends to a large degree on microbial redox processes remineralizing organic matter (OM). However, the controls of organic matter preservation in marine sediments are one of the most complex and controversial issues in contemporary
ABSTRACT Estimates of gas hydrate saturation S_h (percentage of pore volume occupied by gas hydra... more ABSTRACT Estimates of gas hydrate saturation S_h (percentage of pore volume occupied by gas hydrate) are needed to map gas hydrate accumulations and to quantify the global amount of gas hydrate stored in marine sediments. Two widely used methods to quantify S_h use pore water chlorinity (which decreases when gas hydrate dissociates and releases fresh water) or well log measurements of electrical resistivity (which is anomalously high in sediments that contain gas hydrate). These two techniques are complementary: whereas the determination of S_h from chlorinity can only be carried out on a few samples, S_h can be estimated continuously from well logs. However, the empirical relationship between S_h and measured resistivity (Archie's equation) needs to be calibrated in gas hydrate environments. We used the chlorinity data at Site U1325 to calibrate the well log analysis and obtain a continuous estimate of S_h that is consistent with all the measurements. We also took into account uncertainties in the depth of the core samples, in the measurements, and in the parameters of the equations that relate S_h to the data. By propagating these uncertainties, we compute the typical uncertainty of the estimated S_h as ± 1.5% for S_h from chlorinity data, and ± 5% for S_h from well log data (one standard deviation). The highest values of S_h at Site U1325 are found in the interval 180-250 m below seafloor (mbsf), where the average S_h is 7.8 ± 1.5 %. Superimposed on this broad high in S_h, there are discrete sand layers where gas hydrate is preferentially formed. The highest value of S_h ≈ 55 % corresponds to a 9 cm-thick sand layer at about 210 mbsf. With the exception of a few thin (1-3 cm) sand layers where chlorinity data document the presence of gas hydrate, there is no significant S_h above 180 mbsf. The average S_h in the gas hydrate stability zone (0-250 mbsf) is 3 ± 0.7 %.
ABSTRACT Microbially mediated sulfate reduction affects the isotopic composition of dissolved and... more ABSTRACT Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Although several details of the fractionation process remain controversial, the overall process is well understood and can be described as the sum of several mass dependent fractionations during the stepwise reduction of sulfate to sulfide. Experiments and field data show that the 18O/16O of sulfate is also modified in the presence of sulfate-reducing microorganisms. Here we use a reaction transport model to analyze these processes and to constrain the rates of organotrophic versus methanotrophic sulfate reduction. Our results show that even in cases where sulfate concentrations decline in a linear fashion, up to 50% of all sulfate is consumed by organotrophic sulfate reduction.
ABSTRACT An outstanding question in carbon cycling models, particularly those that involve gas hy... more ABSTRACT An outstanding question in carbon cycling models, particularly those that involve gas hydrate formation in accretionary margins, is the rate of methane generation in anoxic sediments. This rate is a key parameter in numerical models involving gas hydrate formation, magnitude of the deposits and recharge of the gas hydrate reservoir following a destabilization event. Pore fluid samples recovered from ~100 to 200 mbsf along a transect of sites drilled in the Cascadia convergent margin during ODP Legs 146 and 204 and IODP Leg 311 show a distinct enrichment in the d13C values of the residual dissolved CO2 with progression from a reference site drilled west of the deformation front (Site 888, d13C: -5 ppt) to Site U1329 (d13C: +32 ppt) located ~65 km from the shore, at the eastern limit of gas hydrate occurrence in the Cascadia margin. Assuming that methane production proceeds primarily by carbonate reduction at relatively rapid rates in a closed system (Rayleigh distillation) with a fractionation factor of 1.07, the measured d13C-DIC at Site U1329 corresponds to ~50% removal of the available dissolved carbonate pool that is assumed to be characterized by 10 mM ?CO2, having a d13C of -20 ppt. Although it is hard to know the available DIC pool, 10 mM represents an average value based on alkalinity measurements at these sites. These estimates correspond to an in situ production of 5 mmol/l methane. Sediments from Site U1329 have been dated as late Miocene, ~5.3 Myr old, thus based on the assumption that the methane was generated over this period, in sediments with porosity of 50%, the first order approximation of the calculated rate of methane production is 0.4 mmol/m3yr. Our estimated methane production rate compares well with the published rates for Leg 204, that vary from 10 mmol/m3yr (upper 100 mbsf) to 0.1 mmol/m3yr (deeper than 100 mbsf), based on an alkalinity model. However, estimates using methane production rates for starved methanogens and the numbers of methanogens in the sediments for most of the Leg 204 samples, yielded values of ~0.005 mmol/m3yr, although 25% of these samples yielded estimates that were higher than this value.
Reaction-transport modeling of biological mediated processes is a commonly used technique to anal... more Reaction-transport modeling of biological mediated processes is a commonly used technique to analyze the subsurface distribution of dissolved species. Coupled with isotope geochemical data, it becomes a powerful tool to investigate metabolic processes. However, numerical instabilities limits its usefulness to cases without complete substrate consumption. This is a consequence of the reaction term dependence on total substrate concentration and isotopic species concentration. Formulating a correct boundary condition is furthermore complicated by the fact that the isotopic ratios for infinitesimal small concentrations are usually not know a priory, and that in many cases, the depth of complete substrate consumption is an unknown as well. Here we present a scheme which allows for complete substrate consumption and computes isotopic ratios for infinitesimal small concentrations if the upper boundary concentrations are known. We first resolve the floating lower boundary condition problem...
Biologically mediated sulfate reduction is an important process linking the global cycles of carb... more Biologically mediated sulfate reduction is an important process linking the global cycles of carbon, oxygen and sulfur, but often difficult to quantify by direct measurements. Inverse reaction transport modeling allows to constrain these processes from interstitial water data, but is typically only available from sediment cores. Here we provide a mathematical framework which allows us to model the isotopic evolution of sedimentary sulfides as well. This enhances our ability to use delta^{34}S from sedimentary sulfides as proxy of the conditions under which they formed. Unlike previous pyrite isotope models, we consider sulfate reduction/sulfide production, active iron supply, non-equilibrium iron monosulfide formation and ultimately pyrite formation, all of which are modeled using 1-D reaction transport model based on standard diagenetic equations. Our current model assumes that the precipitation of the solid phases occurs in isotopic equilibrium with the dissolved phase. Comparing ...
This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or cu... more This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by, you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles): August 1, 2012 www.sciencemag.org (this information is current as of The following resources related to this article are available online at
In the Cretaceous of North America, environmental sensitivity and habitat specialization have bee... more In the Cretaceous of North America, environmental sensitivity and habitat specialization have been hypothesized to explain the surprisingly restricted geographic ranges of many large-bodied dinosaurs. Understanding the drivers behind this are key to determining broader trends of dinosaur species and community response to climate change under greenhouse conditions. However, previous studies of this question have commonly examined only small components of the paleo-ecosystem or operated without comparison to similar modern systems from which to constrain interpretations. Here we perform a high-resolution multi-taxic δ13C and δ18O study of a Cretaceous coastal floodplain ecosystem, focusing on species interactions and paleotemperature estimation, and compare with similar data from extant systems. Bioapatite δ13C preserves predator-prey offsets between tyrannosaurs and ornithischians (large herbivorous dinosaurs), and between aquatic reptiles and fish. Large ornithischians had broadly o...
Stable isotopes are powerful tools for elucidating ecological trends in extant vertebrate communi... more Stable isotopes are powerful tools for elucidating ecological trends in extant vertebrate communities, though their application to Mesozoic ecosystems is complicated by a lack of extant isotope data from comparable environments/ecosystems (e.g. coastal floodplain forest environments, lacking significant C 4 plant components). We sampled 20 taxa across a broad phylogenetic, body size, and physiological scope from the Atchafalaya River Basin of Louisiana as an environmental analogue to the Late Cretaceous coastal floodplains of North America. Samples were analysed for stable carbon, oxygen and nitrogen isotope compositions from bioapatite and keratin tissues to test the degree of ecological resolution that can be determined in a system with similar environmental conditions, and using similar constraints, as those in many Mesozoic assemblages. Isotopic results suggest a broad overlap in resource use among taxa and considerable terrestrial–aquatic interchange, highlighting the challenge...
ABSTRACT IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent... more ABSTRACT IODP Exp. 311 drilled a transect of four sites crossing the northern Cascadia convergent margin in a slow, diffuse fluid flow environment. These transect sites are an ideal data set to compare variation in gas hydrate (GH) occurrence with distance from the deformation front. In this study, we quantify the variation with depth of GH saturation (fraction of pore space occupied by GH) from pore water chlorinity and downhole logs of porosity and electrical resistivity. At each site we determine a GH occurrence zone (GHOZ), defined as the depth interval where gas hydrates are actually observed. The GHOZ is not necessarily the same as the GH stability zone (GHSZ), which is the depth interval where gas hydrates are thermodynamically stable. In three of the Exp. 311 transect sites gas hydrates occur down to the base of GH stability, but the top GHOZ is not at the seafloor (the top of the GHSZ). The top GHOZ deepens moving away from the deformation front, from 47 mbsf at Site U1326 (5 km from the deformation front), to 73 mbsf at Site U1325 (11 km), and 111 mbsf at Site U1327 (21 km). Site U1329 (38 km from the deformation front) shows no clear evidence of GH. Fluid expulsion from the accretionary wedge has been proposed as the main process to advect methane and form GH at the Cascadia margin. Published studies have quantified fluid expulsion rates from the compaction of sediments that are scraped off the subducting plate and are incorporated into the accretionary wedge. These compaction models generally show fluid expulsion rates that decrease sharply moving inland from the deformation front. The Exp. 311 transect sites show a GHOZ that thins moving away from the deformation front and correlates with the decrease in fluid expulsion rates predicted by the compaction models. Our results are consistent with fluid expulsion being a primary control on the formation of GH in the Cascadia convergent margin.
Changes in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur are ... more Changes in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur are the dominant control on atmospheric oxygen levels, and the partitioning itself depends to a large degree on microbial redox processes remineralizing organic matter (OM). However, the controls of organic matter preservation in marine sediments are one of the most complex and controversial issues in contemporary
ABSTRACT Estimates of gas hydrate saturation S_h (percentage of pore volume occupied by gas hydra... more ABSTRACT Estimates of gas hydrate saturation S_h (percentage of pore volume occupied by gas hydrate) are needed to map gas hydrate accumulations and to quantify the global amount of gas hydrate stored in marine sediments. Two widely used methods to quantify S_h use pore water chlorinity (which decreases when gas hydrate dissociates and releases fresh water) or well log measurements of electrical resistivity (which is anomalously high in sediments that contain gas hydrate). These two techniques are complementary: whereas the determination of S_h from chlorinity can only be carried out on a few samples, S_h can be estimated continuously from well logs. However, the empirical relationship between S_h and measured resistivity (Archie's equation) needs to be calibrated in gas hydrate environments. We used the chlorinity data at Site U1325 to calibrate the well log analysis and obtain a continuous estimate of S_h that is consistent with all the measurements. We also took into account uncertainties in the depth of the core samples, in the measurements, and in the parameters of the equations that relate S_h to the data. By propagating these uncertainties, we compute the typical uncertainty of the estimated S_h as ± 1.5% for S_h from chlorinity data, and ± 5% for S_h from well log data (one standard deviation). The highest values of S_h at Site U1325 are found in the interval 180-250 m below seafloor (mbsf), where the average S_h is 7.8 ± 1.5 %. Superimposed on this broad high in S_h, there are discrete sand layers where gas hydrate is preferentially formed. The highest value of S_h ≈ 55 % corresponds to a 9 cm-thick sand layer at about 210 mbsf. With the exception of a few thin (1-3 cm) sand layers where chlorinity data document the presence of gas hydrate, there is no significant S_h above 180 mbsf. The average S_h in the gas hydrate stability zone (0-250 mbsf) is 3 ± 0.7 %.
ABSTRACT Microbially mediated sulfate reduction affects the isotopic composition of dissolved and... more ABSTRACT Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Although several details of the fractionation process remain controversial, the overall process is well understood and can be described as the sum of several mass dependent fractionations during the stepwise reduction of sulfate to sulfide. Experiments and field data show that the 18O/16O of sulfate is also modified in the presence of sulfate-reducing microorganisms. Here we use a reaction transport model to analyze these processes and to constrain the rates of organotrophic versus methanotrophic sulfate reduction. Our results show that even in cases where sulfate concentrations decline in a linear fashion, up to 50% of all sulfate is consumed by organotrophic sulfate reduction.
ABSTRACT An outstanding question in carbon cycling models, particularly those that involve gas hy... more ABSTRACT An outstanding question in carbon cycling models, particularly those that involve gas hydrate formation in accretionary margins, is the rate of methane generation in anoxic sediments. This rate is a key parameter in numerical models involving gas hydrate formation, magnitude of the deposits and recharge of the gas hydrate reservoir following a destabilization event. Pore fluid samples recovered from ~100 to 200 mbsf along a transect of sites drilled in the Cascadia convergent margin during ODP Legs 146 and 204 and IODP Leg 311 show a distinct enrichment in the d13C values of the residual dissolved CO2 with progression from a reference site drilled west of the deformation front (Site 888, d13C: -5 ppt) to Site U1329 (d13C: +32 ppt) located ~65 km from the shore, at the eastern limit of gas hydrate occurrence in the Cascadia margin. Assuming that methane production proceeds primarily by carbonate reduction at relatively rapid rates in a closed system (Rayleigh distillation) with a fractionation factor of 1.07, the measured d13C-DIC at Site U1329 corresponds to ~50% removal of the available dissolved carbonate pool that is assumed to be characterized by 10 mM ?CO2, having a d13C of -20 ppt. Although it is hard to know the available DIC pool, 10 mM represents an average value based on alkalinity measurements at these sites. These estimates correspond to an in situ production of 5 mmol/l methane. Sediments from Site U1329 have been dated as late Miocene, ~5.3 Myr old, thus based on the assumption that the methane was generated over this period, in sediments with porosity of 50%, the first order approximation of the calculated rate of methane production is 0.4 mmol/m3yr. Our estimated methane production rate compares well with the published rates for Leg 204, that vary from 10 mmol/m3yr (upper 100 mbsf) to 0.1 mmol/m3yr (deeper than 100 mbsf), based on an alkalinity model. However, estimates using methane production rates for starved methanogens and the numbers of methanogens in the sediments for most of the Leg 204 samples, yielded values of ~0.005 mmol/m3yr, although 25% of these samples yielded estimates that were higher than this value.
Reaction-transport modeling of biological mediated processes is a commonly used technique to anal... more Reaction-transport modeling of biological mediated processes is a commonly used technique to analyze the subsurface distribution of dissolved species. Coupled with isotope geochemical data, it becomes a powerful tool to investigate metabolic processes. However, numerical instabilities limits its usefulness to cases without complete substrate consumption. This is a consequence of the reaction term dependence on total substrate concentration and isotopic species concentration. Formulating a correct boundary condition is furthermore complicated by the fact that the isotopic ratios for infinitesimal small concentrations are usually not know a priory, and that in many cases, the depth of complete substrate consumption is an unknown as well. Here we present a scheme which allows for complete substrate consumption and computes isotopic ratios for infinitesimal small concentrations if the upper boundary concentrations are known. We first resolve the floating lower boundary condition problem...
Biologically mediated sulfate reduction is an important process linking the global cycles of carb... more Biologically mediated sulfate reduction is an important process linking the global cycles of carbon, oxygen and sulfur, but often difficult to quantify by direct measurements. Inverse reaction transport modeling allows to constrain these processes from interstitial water data, but is typically only available from sediment cores. Here we provide a mathematical framework which allows us to model the isotopic evolution of sedimentary sulfides as well. This enhances our ability to use delta^{34}S from sedimentary sulfides as proxy of the conditions under which they formed. Unlike previous pyrite isotope models, we consider sulfate reduction/sulfide production, active iron supply, non-equilibrium iron monosulfide formation and ultimately pyrite formation, all of which are modeled using 1-D reaction transport model based on standard diagenetic equations. Our current model assumes that the precipitation of the solid phases occurs in isotopic equilibrium with the dissolved phase. Comparing ...
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