Carbon Mineralization Pathways Research Articles

Carbon mineralization and abiotic methane synthesis within fluid inclusions in mafic minerals from postcollisional pyroxenite

Carbon mineralization in mafic-ultramafic rocks is not only a natural process that can form significant carbon sinks in the lithosphere, but also a promising procedure for artificial CO2 storage. In this study, we report unique records of in situ carbon mineralization within fluid inclusions in postcollisional pyroxenite from the Dabie orogen, central China. Olivine, orthopyroxene, clinopyroxene, and amphibole in the pyroxenite trapped CO2-rich magmatic fluids as fluid inclusions, which were subjected to internal carbon mineralization due to interaction between trapped fluids and host minerals during cooling of the pyroxenite. In fluid inclusions, carbonation of olivine produced magnesite, talc, magnetite, and CH4; carbonation of orthopyroxene generated magnesite, talc, cristobalite, and CH4; carbonation of clinopyroxene formed calcite, dolomite, actinolite, talc, cristobalite/quartz, and CH4; carbonation of amphibole created calcite, dolomite, chlorite, cristobalite/quartz, talc, mica, a TiO2 phase, a NaAlSi3O8 phase, and CH4. Carbonation reactions within the fluid inclusions have general implications for reaction pathways of carbon mineralization in mafic-ultramafic rocks. Moreover, it is shown that iron oxidation is thermodynamically favored during high-extents carbonation of diverse mafic minerals in the presence of CO2-rich fluids, facilitating abiotic synthesis of CH4 that is a crucial gas in both natural carbon cycling and engineered carbon storage.

Organic carbon mineralization pathways in the muddy sediments of the South Yellow Sea: Insights from steady-state modeling of porewater

In marine sediments, microbial mineralization of organic carbon (OC) proceeds through a cascade of pathways using different electron acceptors. Rates and relative importance of individual pathways can provide quantitative constraints on biogeochemical cycling of a number of elements, such as carbon, nitrogen, sulfur, iron, and manganese. However, such information is hitherto very scarce in the China marginal seas. To fill the gap, a multicomponent-coupled reaction-transport model CANDI was applied to quantify the rates and relative contributions of individual pathways of OC mineralization at three sites in the South Yellow Sea (SYS) sediments, based on which OC budgets were constructed. Model results at steady state generally well-reproduced the profiles of measured porewater solutes. The fast degrading OC pools G1 become exhausted over the upper 4 cm at the studied sites with the degradation rate constants k1 ranging between 0.6 and 1.65 yr−1, while the slow degrading OC pool G2 remains almost undegradable on the timescales of the studied depth (∼25 cm) due to the low degradation rate constants k2 (10−5‒10−4 yr−1) The depth-integrated rates of OC mineralization at the studied sites are in the range of 5.1–18.3 mmol C m−2 d−1, and aerobic respiration stands out as the dominant pathway, with its average contribution of 62%, followed by sulfate reduction (22.6%), dissimilatory manganese/iron reduction (8.4%), and denitrification (6.1%). Methanogenesis is a negligible process of OC mineralization within the studied depths. The contributions of aerobic respiration in the SYS sediments are much higher than the global average for marginal seas, but the contributions of sulfate reduction and iron dissimilatory reduction are much lower than their global averages, which could be ascribed to the low availability of degradable OC for anaerobic respiration. Budget of OC indicates that the mineralization and burial of OC in the muddy sediments account for 7.3 and 1.7%, respectively, of the total OC supplied to the sea, with the net OC accumulation efficiency of ∼19%.

Importance of nitrate reduction in benthic carbon mineralization in two eutrophic estuaries: Modeling, observations and laboratory experiments

Estuaries are important nutrient filters between rivers and coastal zones. However, the quantification of the nutrient mitigation capacity related to benthic diagenesis is still poorly quantified. In this paper, we investigated carbon mineralization and the contribution of benthic nitrate reduction in two macrotidal eutrophic estuaries (Elorn and Aulne, Brittany, France) during winter and spring. These anthropized estuaries exhibited large variations of bottom water nitrate concentrations from very high values in upstream waters (up to 500 μM) to low values downstream (andlt; 10 μM). Bottom water oxygen concentrations presented small gradients compared to nitrate concentrations gradients resulting in large variation in nitrate to oxygen ratios NOx −/O2 between downstream and upstream (0.03–1.6). We combined the use of diagenesis modeling with field data (porosity, organic carbon and nitrogen, pore water profiles of dissolved oxygen, nitrogen, iron, manganese and sulfide concentrations, published in Khalil et al., 2013) and experimentally-determined nitrate reduction rates, in order to investigate the different organic carbon mineralization pathways in these estuarine sediments including denitrification and the contribution of benthic nitrate reduction to the estuarine N budget. Overall a good agreement between pore water data (organic carbon, oxygen, nitrate and ammonium) and model simulations was observed. The modeled organic matter mineralization rates were high in the upstream estuary and low in the saline estuary for the two estuaries Aulne and Elorn. This decrease may be related to the dilution and the trapping of allochtonous organic matter in estuarine sediments and its subsequent recycling in the upper estuary. Organic carbon mineralization rates were higher in the Elorn than in the Aulne estuary, which is most likely related to the labile character of the organic matter from urban origin exported from the Elorn watershed. The contribution of nitrate reduction to the total mineralization was generally high in upstream sediments (15–35%) of both Aulne and Elorn estuaries and decreased consistently downstream to 5–10%. The relative large contribution of nitrate reduction to organic matter degradation was to a large extent related to high bottom water nitrate concentrations that fueled 37–78% of total nitrate reduction in the upstream part of the estuaries. Overall, the reduction of bottom water nitrate by denitrification to N2 represented 3–13% of the river flux of nitrate in these estuaries in winter and spring, and could reach ~ 50% during summer. These results highlight that nitrate reduction in the sediment attenuate the high river nitrate flux, despite the huge quantities of anthropogenic nitrate discharged by the two rivers to the estuaries.

Impacts of increasing salinity and inundation on rates and pathways of organic carbon mineralization in tidal wetlands: a review

To improve our understanding of the carbon cycling response to imminent sea-level rise and saltwater intrusions, we review the existing literature on the likely effects of the increasing salinity and inundation on organic carbon mineralization in tidal wetlands. Enhanced salinity and inundation will reduce the pool of the organic carbon substrate, but may expand that of microbes with strong capacities for carbon metabolism. Sulfate availability increases with the increasing salinity, while availability of other electron acceptors, e.g., oxygen, nitrate, ferric oxides, and carbon dioxide, could transiently increase but would ultimately fall with the increasing salinity and inundation. The changing electron acceptor pattern may result in microbial sulfate reduction predominating over other carbon mineralization pathways. Data derived from natural salinity and inundation gradients suggest clear negative effects of salinity and inundation on production rates or emission fluxes of carbon dioxide and methane. However, results for brackish wetlands are conflicting, probably due to their unique geographic location. Salinity and inundation exert their influence on organic carbon mineralization through sulfate enrichment, elevating ionic and osmotic stress and decreasing oxygen concentrations and redox conditions, among other biogeochemical changes. Future studies should address the combined effects of salinity and inundation on carbon biogeochemistry in low-level salinity tidal wetlands.

Iron dynamics in a subtropical estuarine tidal marsh: effect of season and vegetation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 577:1-15 (2017) - DOI: https://doi.org/10.3354/meps12268 FEATURE ARTICLE Iron dynamics in a subtropical estuarine tidal marsh: effect of season and vegetation Min Luo1,2,3, Jiafang Huang3, Chuan Tong1,3,*, Yuxiu Liu3, Xun Duan2, Yin Hu2 1Postdoctoral Research Station for Ecology, Fujian Normal University, Fuzhou 350007, PR China 2School of Environment and Resources, Fuzhou University, Fuzhou 350116, PR China 3Key Laboratory of Humid Subtropical Eco-Geographical Processes, Ministry of Education, Fujian Normal University, Fuzhou 350007, PR China *Corresponding author: tongch@fjnu.edu.cn ABSTRACT: To better understand how seasonal change and the presence of plants affect biogeochemical iron (Fe) cycling in tidal marsh sediments, we examined the seasonal dynamics of microbial Fe reduction (FeR) and related Fe speciation from 2012 to 2014 using in situ vegetated and unvegetated mesocosms created with macrobenthos-proof enclosures. The pools of Fe(III) oxides, Fe sulfides, porewater Fe2+, and rates of microbial sulfate reduction (SR) peaked in summer (hot and wet), whereas rates of FeR and non-sulfidic Fe(II) levels peaked in winter (mild and dry). Sedge presence greatly increased the FeR rate, organic matter pools, and Fe(III) oxide levels, but decreased carbon:nitrogen ratios and sulfide abundances. FeR rates were mainly affected by the SR level, and temperature and plant primary productivity had comparable effects on SR rates. The dominant organic carbon mineralization pathway changed from SR in summer to FeR in winter in both vegetated and unvegetated mesocosms. FeR was more important in the vegetated mesocosms than in the unvegetated mesocosms, and seasonal change contributed more to the total variability of each anaerobic pathway than the presence of sedge. Our study reveals that temperature and plants both mediate variations in the organic carbon mineralization pathways of a subtropical estuarine tidal marsh. KEY WORDS: Seasonality · Sedge · Microbial iron reduction · Microbial sulfate reduction · Organic carbon mineralization · Tidal marsh · Min River estuary Full text in pdf format Information about this Feature Article NextCite this article as: Luo M, Huang J, Tong C, Liu Y, Duan X, Hu Y (2017) Iron dynamics in a subtropical estuarine tidal marsh: effect of season and vegetation. Mar Ecol Prog Ser 577:1-15. https://doi.org/10.3354/meps12268 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 577. Online publication date: August 18, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Carbon mineralization pathways and bioturbation in coastal Brazilian sediments.

Carbon mineralization processes and their dependence on environmental conditions (e.g. through macrobenthic bioturbation) have been widely studied in temperate coastal sediments, but almost nothing is known about these processes in subtropical coastal sediments. This study investigated pathways of organic carbon mineralization and associated effects of macrobenthic bioturbation in winter and summer (September 2012 and February 2014) at the SE Brazilian coast. Iron reduction (FeR) was responsible for 73–81% of total microbial carbon mineralization in September 2012 and 32–61% in February 2014. Similar high rates of FeR have only been documented a few times in coastal sediments and can be sustained by the presence of large bioturbators. Denitrification accounted for 5–27% of total microbial carbon mineralization while no SO42− reduction was detected in any season. Redox profiles suggested that conditions were less reduced in February 2014 than in September 2012, probably associated with low reactivity of the organic matter, higher rates of aerobic respiration and bioirrigation by the higher density of small-macrofauna. Bioturbation by small macrofauna may maintain the sediment oxidized in summer, while large-sized species stimulate the reoxidation of reduced compounds throughout the year. Therefore, bioturbation seems to have an important role modulating the pathways of carbon mineralization in the area.

Manganese, iron, and sulfur cycling in Louisiana continental shelf sediments

Sulfate reduction is considered the primary pathway for organic carbon remineralization on the northern Gulf of Mexico Louisiana continental shelf (LCS) where bottom waters are seasonally hypoxic, yet limited information is available on the importance of iron and manganese cycling in the region. Sedimentary manganese, iron, and sulfur cycling were investigated on the LCS using a combined chemical analysis and sediment diagenesis modeling approach. Three stations situated 320km across the LCS along the 20m isobath were sampled up to five times between the spring of 2006 and summer of 2007. Bottom water oxygen levels at the stations ranged from 203mmolm−3 in spring to 2.5mmolm−3 in summer. Porewater Mn and Fe2+ concentrations (up to 275 and 300μmolL−1, respectively), sulfate reduction rates (1.0–8.4mmolm−2d−1), and the fraction of total oxalate extracted iron obtained as Fe(II) (0.25–0.52) differed between station and season. Sediments at station Z02 on the eastern LCS, south of Terrebonne Bay, had higher organic matter content and sulfate reduction rates than sediments at Z03, 160km further west. Sulfate reduction rates were higher in summer than spring at station Z02 but not at Z03 where porewater Mn and Fe concentrations were highest in summer. Porewater Fe2+ concentrations, solid phase oxalate-extractable Fe concentrations, and sediment incubation experiments suggested iron reduction at Z03 may account for 20% or more of organic carbon remineralization. LCS Fe(III) concentrations decreased and sulfate reduction rates increased in model simulations by lowering interfacial dissolved oxygen levels and increasing the rates of organic matter deposited on the sediment surface. Results from this study demonstrate that LCS sedimentary metal oxide cycling may be more important in organic carbon mineralization pathways than previously recognized.

Impact of the invasive polychaete Marenzelleria viridis on the biogeochemistry of sandy marine sediments

This study investigated the effects of the invasive polychaete Marenzelleria viridis (Marenzelleria) on the biogeochemistry of sandy marine sediments. A 27-day microcosm experiment was conducted to evaluate net microbial reactions, fluxes and porewater profiles of key solutes under the presence of Marenzelleria in two sediment types; one containing autochthonous organic matter (0.4 %) and another slightly amended with seagrass detritus (Ruppia maritima; 0.5 %). Fluxes of TCO2 and O2 were two- to three-fold increased in the presence of Marenzelleria and were higher in amended (256 mmol m−2 d−1) than in non-amended sediments (117 mmol m−2 d−1). Marenzelleria stimulated carbon degradation and sulfate reduction in both sediment treatments by influencing porewater chemistry and DOC availability. Marenzelleria flushed out inhibitory porewater metabolites (i.e. TCO2, TH2S and NH4 +) while replenishing SO4 2−. Furthermore, sulfate consumption was significantly higher in Marenzelleria sediment (79–87 mmol m−2 d−1) than in defaunated controls (36–50 mmol m−2 d−1), which indicated a 75–92 % lower DOC availability in the former. DOC excreted by Marenzelleria may have sustained 17–32 % of SO4 2− reduction. Lower C:S ratios (1.6–1.7) in Marenzelleria compared to defaunated sediments (2.4–2.8) suggest basic differences in carbon mineralization pathways, with e.g. H2 as a possible substrate for sulfate reduction in the former treatment and higher fermentative TCO2 formation in the latter. No significant stimulatory effect of Marenzelleria was evident for microbial NH4 + production. Total budgets revealed that Marenzelleria increased C mineralization by 50 % in amended sediment, compared to only 9 % in non-amended sediment. This study suggests that the presence of Marenzelleria may lead to a marked stimulation of sulfate reduction and degradation of refractory organic matter. Marenzelleria invasion may therefore have widespread consequences for microbial pathways and organic matter processing in sandy marine sediments.

Variations and Controls of Sulfate Reduction in the Continental Slope and Rise of the Ulleung Basin off the Southeast Korean Upwelling System in the East Sea

Despite its importance in carbon mineralization pathways, relatively little is known about sulfate reduction along a continental slope and rise. We investigated the temporal and spatial variations and controls of sulfate reduction rates (SRRs) in the continental margin sediment of the Ulleung basin (UB), off the southeast Korean upwelling system in the East Sea. SRRs ranged from 1.22 to 8.07 mmol m−2 d−1 at the slope sites and from 0.69 to 3.18 mmol m−2 d−1 at the basin sites. These values were exceptionally higher than those observed within other marginal seas and were comparable to SRR values at the same depth range in the Peruvian and Chilean upwelling systems. The high sulfate reduction in the UB was attributable to enhanced primary production in the water column associated with coastal upwelling and a high export flux of large organic particles into the basin via the Ulleung warm eddy. The depth integrated SRR was approximately 4 times higher in the highly productive spring (4.91 ± 2.55 mmol m−2 d−1) than during the summer, which exhibited a stratified water column (1.28 ± 0.48 mmol m−2 d−1). In the meantime, despite the high organic carbon content (> 2.5% dry wt.) in the UB, the SRR was consistently suppressed in the Mn oxide-enriched (174 μ mol cm−3) surface sediments of the continental rise. Overall, these results indicate that the production of organic carbon in the water column and its subsequent deposition on the seafloor is the primary source controlling the temporal variability of sulfate reduction, whereas the Mn oxides that were enriched in the basin are responsible for the spatial variability of the SRR in the UB. Carbon oxidation in the UB accounted for approximately 30% of primary production and approximately 60% of the export flux, indicating that the continental slope and rise of the UB is a region of rapid organic carbon turnover and nutrient regeneration.

Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization

Solid phase and pore water chemical data collected in a sediment of the Haringvliet Lake are interpreted using a multi-component reactive transport model. This freshwater lake, which was formed as the result of a river impoundment along the southwestern coast of the Netherlands, is currently targeted for restoration of estuarine conditions. The model is used to assess the present-day biogeochemical dynamics in the sediment, and to forecast possible changes in organic carbon mineralization pathways and associated redox reactions upon salinization of the bottom waters. Model results indicate that oxic degradation (55%), denitrification (21%), and sulfate reduction (17%) are currently the main organic carbon degradation pathways in the upper 30 cm of sediment. Unlike in many other freshwater sediments, methanogenesis is a relatively minor carbon mineralization pathway (5%), because of significant supply of soluble electron acceptors from the well-mixed bottom waters. Although ascorbate-reducible Fe(III) mineral phases are present throughout the upper 30 cm of sediment, the contribution of dissimilatory iron reduction to overall sediment metabolism is negligible. Sensitivity analyses show that bioirrigation and bioturbation are important processes controlling the distribution of organic carbon degradation over the different pathways. Model simulations indicate that sulfate reduction would rapidly suppress methanogenesis upon seawater intrusion in the Haringvliet, and could lead to significant changes in the sediment’s solid-state iron speciation. The changes in Fe speciation would take place on time-scales of 20–100 years.

Rates and pathways of carbon oxidation in permanently cold Arctic sediments

We report here a comprehensive study of the rates and pathways of carbon mineralization in Arctic sediments. Four sites were studied at 115 to 329 m water depth in fjords on Svalbard and in coastal Norway. The Svalbard coastal region is characterized by permanently cold bottom water temperatures of -1.7 to 2.6°C. Carbon oxidation (avg = 20 to 400 nmol cm -3 d -1 ) and sulfate reduction rates (avg = 10 to 350 nmol cm -3 d -1 ) were measured at high resolution to 10 cm depth in sediment incubations. The distribution of oxidants available for microbial respiration was determined through porewater and solid phase geochemistry. By comparing the distribution of potential oxidants to the depth-integrated mineralization rates, the importance of various respiratory pathways to the oxidation of organic C could be quantified. Integrated C oxidation rates measured in sediment incubations (11 to 24 mmol m -2 d -1 ) were comparable to within a factor of 2 to dissolved inorganic carbon (DIC) fluxes measured in situ using a benthic lander. Sulfate reduction was the dominant microbial respiration pathway (58 to 92 % of total C oxidation) followed by Fe(III) reduction (10 to 26%), oxygen (5 to 14 %), and nitrate respiration (2 to 3%). At sediment depths where sulfate reduction was dominant, C oxidation equivalents, calculated from independently measured sulfate reduction rates, matched DIC production rates in incubations. Sediment geochemistry revealed that the same vertical sequence of oxidants is reduced/respired in these Arctic sediments as in temperate continental shelf sediments of equivalent water depths. Microbial communities in permanently cold Arctic sediments exhibited mineralization rates and pathways comparable to temperate nearshore environments. This study completely partitioned C oxidation pathways, showing a predominance of sulfate respiration and a substantial contribution of Fe(III) reduction to organic matter mineralization in Arctic sediments for the first time. Microbial communities in cold sediments exposed to relatively high C deposition appear to respond to the input or availability of organic matter rather than to temperature.

Dynamic response of deep‐sea sediments to seasonal variations: A model

We present a dynamic, numerical model of early diagenetic processes that can be used to examine the response of different organic carbon mineralization pathways, concentration vs. depth profiles, and the resultant fluxes to seasonally varying carbon deposition. We show that there can be substantial temporal variability in sediment‐water fluxes as well as in the relative contribution of different organic carbon mineralization pathways and oxygen consumption processes in deep‐sea sediments. The timing and amplitude of the sediment response are most sensitive to the reactivity of the organic matter. We show that the fluxes of dissolved substances can be approximated by assuming that these constituents are at steady state with the carbon profiles.The model is used to reconcile a time series of community oxygen consumption rates, dissolved oxygen, nitrate, ammonia, and manganese profiles, and solid‐phase organic carbon profiles at a deep‐sea station in the Pacific. The organic carbon that is deposited is very reactive (7.2 yr‒1), which causes a fast response of the sediment community to seasonal pulses in carbon flux and significant temporal variation in oxygen and nutrient fluxes. The model predicts large seasonal variations in the relative importance of oxic (48–71%), denitrification (12–17%), and anoxic (16–37%) pathways. There is also substantial variation in the pore‐water profiles of oxygen, nitrate, ammonia, and other reduced substances over the seasonal cycle, indicating that seasonal variability should be taken into account when extrapolating from a single observation to yearly averaged values.