The impact of depositional conditions on biogeochemical cycling of iron and stable iron signatures in sediments of the Argentina Continental Margin

2021 ◽  
Author(s):  
Anne-Christin Melcher ◽  
Susann Henkel ◽  
Thomas Pape ◽  
Anette Meixner ◽  
Simone A. Kasemann ◽  
...  
Keyword(s):  
Steady State ◽  
Sulfate Reduction ◽  
Pore Water ◽  
Continental Margin ◽  
Geochemical Processes ◽  
Fe Oxides ◽  
Diagenetic Alteration ◽  
Fe Isotopes ◽  
Oxic Zone ◽  
Fe Reduction

<p>The Argentina Continental Margin represents a unique geologic setting to study interactions between bottom currents and sediment deposition as well as their impact on (bio)geochemical processes, particularly the cycling of iron (Fe). Our aim was to determine (1) how different depositional conditions control post-depositional (bio)geochemical processes and (2) how stable Fe isotopes (δ<sup>56</sup>Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of δ<sup>56</sup>Fe of solid Fe pools as a proxy to trace past diagenetic alteration of Fe, which might be decoupled from current redox conditions. Sediments from two different depositional environments were sampled during RV SONNE expedition SO260: a site dominated by contouritic deposition on a terrace (Contourite Site) and the lower continental slope (Slope Site) dominated by hemipelagic sedimentation. Sequentially extracted sedimentary Fe [1] and δ<sup>56</sup>Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary δ<sup>56</sup>Fe dataset at the Argentina Continental Margin.</p><p>The depositional conditions differed between and within both sites as evidenced by variable grain sizes, organic carbon contents and sedimentation rates. At the Contourite Site, non-steady state pore-water conditions and diagenetic overprint occurs in the post-oxic zone and the sulfate-methane transition (SMT). In contrast, pore-water profiles at the Slope Site suggest that currently steady-state conditions prevail, leading to a strong diagenetic overprint of Fe oxides at the SMT. Pore-water δ<sup>56</sup>Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water δ<sup>56</sup>Fe values are mostly positive and range between -0.35‰ to 1.82‰. Positive δ<sup>56</sup>Fe values are related to high sulfate reduction rates that dominate over Fe reduction in the post-oxic zone. The HS<sup>- </sup>liberated during organoclastic sulfate reduction or sulfate-mediated anaerobic oxidation of methane (AOM) reacts with Fe<sup>2+</sup> to form Fe sulfides. Hereby, light Fe isotopes are preferentially removed from the dissolved pool. The isotopically light Fe sulfides drive the acetate-leached Fe pool towards negative values. Isotopic trends were absent in other extracted Fe pools, partly due to unintended dissolution of silicate Fe masking the composition of targeted Fe oxides. Significant amounts of reactive Fe phases are preserved below the SMT and are possibly available for reduction processes, such as Fe-mediated AOM [4]. Fe<sup>2+</sup> in the methanic zone is isotopically light at both sites, which is indicative for a microbial Fe reduction process.</p><p>Our results demonstrate that depositional conditions exert a significant control on geochemical conditions and dominant (bio)geochemical processes in the sediments of both contrasting sites. We conclude that the applicability of sedimentary δ<sup>56</sup>Fe signatures as a proxy to trace diagenetic Fe overprint is limited to distinct Fe pools. The development into a useful tool depends on the refining of extraction methods or other means to analyse δ<sup>56</sup>Fe in specific sedimentary Fe phases.</p><p> </p><p>References:</p><p>[1]Poulton and Canfield, 2005. Chemical Geology 214: 209-221.<br>[2]Henkel et al., 2016. Chemical Geology 421: 93-102.<br>[3]Homoky et al., 2013. Nature Communications 4: 1-10.<br>[4]Riedinger et al., 2014. Geobiology 12: 172-181.</p>

Significance of early-diagenetic water-rock interactions in a modern marine siliciclastic/evaporite environment: Salina Ometepec, Baja California

2002 ◽  
Vol 114 (9) ◽  
pp. 1055-1069 ◽  
Author(s):  
Anna M. Martini ◽  
Lynn M. Walter ◽  
Timothy W. Lyons ◽  
Victoria C. Hover ◽  
John Hansen
Keyword(s):  
Sulfate Reduction ◽  
Pore Water ◽  
Gulf Of California ◽  
Baja California ◽  
Mineral Dissolution ◽  
Pore Waters ◽  
Near Surface ◽  
Compositional Evolution ◽  
Diagenetic Alteration ◽  
Evaporative Concentration

Abstract Although marine brines are a significant component of pore waters in sedimentary basins, there are few geochemical studies of modern analogues of such systems, especially in siliciclastic settings. For these reasons, we chose the evaporite-associated siliciclastic sediments deposited in the salt flats of the Salina Ometepec, Baja California, for an integrated investigation of sediment, pore-water, and overlying brine geochemistry. Here, the detrital components include quartz, K-feldspar, plagioclase, chlorite, biotite, and smectite, and authigenic minerals are dominated by halite, gypsum, and K-rich magnesium smectite. Thermal and saline stresses on the sediments of the Salina Ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments. Sediment pore waters exhibit little microbial sulfate reduction, and dissolved inorganic C contents are also very low. As a result, we did not observe carbonate and sulfide mineral authigenesis in the Salina Ometepec sediments. Instead, pore-water geochemical evolution is largely controlled by evaporative concentration of seawater, evaporite-mineral dissolution and recrystallization, and diagenetic alteration of detrital aluminosilicates. Evaporite-mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment. Specifically, Na+ and Cl− concentrations are increased owing to halite dissolution. We see significant Br− enrichment relative to expected seawater evaporation trends in near-surface pore water, secondary to dissolution of K- and Mg salts. Because bacterial sulfate reduction is inhibited in the Salina Ometepec sediments, sulfate concentrations are more accurate indicators of the degree of evaporation than Br−, a usually conservative element during geochemical reactions. Pore waters exhibit down-core increases in dissolved Mg2+, K+, and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(H_{4}SiO_{4}^{0}\) \end{document} over the upper 1 m. Authigenic K-rich Mg-smectite formation is promoted by the concurrent processes of brine concentration, selective dissolution of K- and Mg-bearing salts, and dissolution of detrital aluminosilicates. Pore waters at a depth of 1 m have 87Sr/86Sr ratios that require input of Sr that is less radiogenic than that of Gulf of California seawater. This Sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources. These results show that significant chemical interactions among marine brines, evaporite minerals, and detrital aluminosilicates can occur relatively soon after sedimentation.

Process network modelling of the geochemical reactions responsible for acid mine drainage emanating from the Witwatersrand tailings facilities

2020 ◽  
Vol 123 (3) ◽  
pp. 357-368
Author(s):  
R.N. Hansen
Keyword(s):  
Steady State ◽  
Acid Mine Drainage ◽  
Pore Water ◽  
Negative Feedback ◽  
Positive Feedback ◽  
Feedback Loop ◽  
Mine Drainage ◽  
Geochemical Processes ◽  
Network Modelling ◽  
O2 Diffusion

ABSTRACT Acid mine drainage (AMD) and associated metal(loid) and SO42- pollution of soil, surface water and groundwater is ubiquitously associated with tailings material generated by Au mining in the Witwatersrand Basin in South Africa. The individual geochemical processes responsible for the AMD generation in this tailings material are relatively well understood. What is less clear are how these different processes interact as a network within the tailings system. Process network modelling (PNM) is a tool that can be used to study such interactive and complex networks of geochemical processes, especially when stochastic methods, e.g. Monte Carlo simulation, are included in the model development. Secondary mineral phase supersaturation requirements from classical nucleation theory are also built into the model. A PNM was developed for a tailings facility in the Witwatersrand gold basin focussing on pH, Fe(total) and SO42- concentrations in the tailings pore water and the relationship of these parameters to the dissolution of pyrite, O2 diffusion into the tailings, oxidation of Fe2+ and the precipitation of secondary minerals, specifically goethite and jarosite. The model indicated that AMD conditions develop fairly rapidly after the sulphidic material is exposed to the Earth’s oxygenated atmosphere. The concentration of H+, and hence the pH, in the tailings pore water is controlled by a number of feedbacks. The positive feedback, implying addition of H+, is the dissolution of pyrite and the precipitation of the secondary Fe3+-bearing minerals goethite and jarosite. Jarosite precipitation was shown to increase the median H+ addition rate by ~2%. The negative feedback, i.e. decrease in H+, is the oxidation of Fe2+ to Fe3+. This feedback loop produces a net excess of H+. Together with the buffer effect of goethite and jarosite precipitation, system steady-state conditions are eventually achieved with respect to pH. The pore water SO42- concentration is controlled by the positive and negative feedback of pyrite dissolution and jarosite precipitation. This feedback loop produces a large excess of SO42- and steady state conditions can only be achieved if SO42- is physically removed from the tailings system, e.g. seepage to groundwater. Oxidation of the Fe2+ produced by pyrite dissolution to Fe3+ is the only positive feedback for tailings pore water Fe3+ concentrations. The negative feedbacks are precipitation of goethite and jarosite and the oxidation of pyrite by Fe3+. The former effect is delayed as these phases first have to achieve a certain level of supersaturation in the tailings pore water solution before they can form. The precipitation of jarosite and goethite, by removing Fe3+ from solution, decreases the effect of Fe3+ pyrite oxidation causing O2 to remain the dominant oxidation mechanism. This feedback loop produces a small excess of Fe3+ over time, however, the model is very sensitive to other factors, e.g. O2 diffusion deeper into the tailings facility.

scholarly journals Benthic phosphorus and iron budgets for three NW African slope sediments: a balance approach

2010 ◽  
Vol 7 (2) ◽  
pp. 469-480 ◽  
Author(s):  
K. Küster-Heins ◽  
G. J. de Lange ◽  
M. Zabel
Keyword(s):  
Steady State ◽  
Pore Water ◽  
Analytical Data ◽  
General Distribution ◽  
Data Set ◽  
Transport Reaction ◽  
Particle Mixing ◽  
Open Questions ◽  
Close Relationship ◽  
Key Issues

Abstract. Despite intensive research on the different domains of the marine phosphorus (P) cycle during the last decades, frequently discussed open questions still exist especially on controlling factors for the benthic behaviour of P and its general distribution in sediment-pore water systems. Steady state or the internal balance of all relevant physical and (bio)geochemical processes are amongst the key issues. In this study we present and discuss an extended data set from surface sediments recovered from three locations on the NW African continental slope. Pore water data and results from sequential sediment extractions give clear evidence to the well-known close relationship between the benthic cycles of P and iron. Accordingly, most of the dissolved phosphate must have been released by microbially catalyzed reductive dissolution of iron (oxhydr)oxides. However, rates of release and association of P and iron, respectively, are not directly represented in profiles of element specific sediment compositions. Results from steady-state based transport-reaction modelling suggest that particle mixing due to active bioturbation, or rather a physical net downward transport of P associated to iron (oxyhydr)oxides, is an essential process for the balance of the inspected benthic cycles. This study emphasizes the importance of balancing analytical data for a comprehensive understanding of all processes involved in biogeochemical cycles.

scholarly journals Sulfate reduction in surface sediments of the southeast Atlantic continental margin between 15°38'S and 27°57'S (Angola and Namibia)

1999 ◽  
Vol 44 (3) ◽  
pp. 650-661 ◽  
Author(s):  
Timothy G. Ferdelman ◽  
Henrik Fossing ◽  
Kirsten Neumann ◽  
Horst D. Schulz
Keyword(s):  
Sulfate Reduction ◽  
Continental Margin ◽  
Surface Sediments

Segregation potential – pressure – salinity relationships near thermal steady state for a clayey silt

1990 ◽  
Vol 27 (2) ◽  
pp. 203-215 ◽  
Author(s):  
J.-M. Konrad
Keyword(s):  
Steady State ◽  
Pore Water ◽  
Moisture Transfer ◽  
Frost Heave ◽  
Water Salinity ◽  
Saline Soils ◽  
Clayey Silt ◽  
Segregation Potential ◽  
Pore Water Salinity ◽  
Thermal Steady State

Laboratory freezing tests were performed on a saturated clayey silt at various pore-water salinities and applied pressures to establish the relationships between pore-water salinity, overburden, and the amount of moisture transfer during freezing near thermal steady state conditions. The experimental data confirmed that the concept of segregation potential established for salt-free soils can be extended to saline soils. The segregation potential at the onset of the final ice lens in step-freezing tests (or near thermal steady state) should be related to the average salinity of the frozen fringe associated with the final ice lens. This pore-water salinity is different from the initial pore-water salinity as a result of solute exclusion at the ice lenses causing an enrichment as freezing proceeds. Unfortunately, it is very difficult to determine the average salinity in the frozen fringe owing to its small size. Instead, it is proposed to use the initial water salinity to develop the constitutive equations in freezing saline soils. A procedure for frost heave predictions in the field is outlined. Key words: freezing tests, clayey silt, saline water, frost heave.

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