Abstract:Reactive iron (Fe) oxides and sheet silicatebound Fe in reservoir rocks may affect the subsurface storage of CO 2 through several processes by changing the capacity to buffer the acidification by CO 2 and the permeability of the reservoir rock: (1) the reduction of threevalent Fe in anoxic environments can lead to an increase in pH, (2) under sulphidic conditions, Fe may drive sulphur cycling and lead to the formation of pyrite, and (3) the leaching of Fe from sheet silicates may affect silicate diagenesis. In… Show more
“…As shown by leaching experiments (Kasina et al . 27 ) fraction III includes reactive sheet silicates, such as smectites, from which Fe could potentially react with sulphide.Figure 2Abundance of pyrite-bound iron relative to different solid-phase iron fractions in sediment of ODP Site 1229. ( A ) Degree of pyritization with respect to total Fe (orange triangles), extracted Fe (light-blue squares), and extracted Fe without the poorly reactive silicate fraction (black diamonds) plotted vs. depth.…”
Microbial life below the seafloor has changed over geological time, but these changes are often not obvious, as they are not recorded in the sediment. Sulphur (S) isotope values in pyrite extracted from a Plio- to Holocene sequence of the Peru Margin (Ocean Drilling Program, ODP, Site 1229) show a down-core pattern that correlates with the pattern of carbon (C) isotopes in diagenetic dolomite. Early formation of the pyrite is indicated by the mineralogical composition of iron, showing a high degree of pyritization throughout the sedimentary sequence. Hence, the S-record could not have been substantially overprinted by later pyrite formation. The S- and C-isotope profiles show, thus, evidence for two episodes of enhanced microbial methane production with a very shallow sulphate-methane transition zone. The events of high activity are correlated with zones of elevated organic C content in the stratigraphic sequence. Our results demonstrate how isotopic signatures preserved in diagenetic mineral phases provide information on changes of past biogeochemical activity in a dynamic sub-seafloor biosphere.
“…As shown by leaching experiments (Kasina et al . 27 ) fraction III includes reactive sheet silicates, such as smectites, from which Fe could potentially react with sulphide.Figure 2Abundance of pyrite-bound iron relative to different solid-phase iron fractions in sediment of ODP Site 1229. ( A ) Degree of pyritization with respect to total Fe (orange triangles), extracted Fe (light-blue squares), and extracted Fe without the poorly reactive silicate fraction (black diamonds) plotted vs. depth.…”
Microbial life below the seafloor has changed over geological time, but these changes are often not obvious, as they are not recorded in the sediment. Sulphur (S) isotope values in pyrite extracted from a Plio- to Holocene sequence of the Peru Margin (Ocean Drilling Program, ODP, Site 1229) show a down-core pattern that correlates with the pattern of carbon (C) isotopes in diagenetic dolomite. Early formation of the pyrite is indicated by the mineralogical composition of iron, showing a high degree of pyritization throughout the sedimentary sequence. Hence, the S-record could not have been substantially overprinted by later pyrite formation. The S- and C-isotope profiles show, thus, evidence for two episodes of enhanced microbial methane production with a very shallow sulphate-methane transition zone. The events of high activity are correlated with zones of elevated organic C content in the stratigraphic sequence. Our results demonstrate how isotopic signatures preserved in diagenetic mineral phases provide information on changes of past biogeochemical activity in a dynamic sub-seafloor biosphere.
“…The presence of high concentrations of elements able to Minerals 2021, 11, 523 2 of 17 be included in the clay structure affects the reaction rates, promoting an increase of clay transformation speed [3]. In sedimentary and diagenetic systems, clay reactions can be influenced by elevated water salinity and oxygen-depleted conditions that favor reducing transformations (e.g., of Fe) mediated by microorganisms [4][5][6]. High concentration of cations and biological activity are frequently due to specific sediment composition, hydrology, and climatic conditions, such as those that can be generated in saline lakes or mangrove soils [6][7][8][9].…”
In this investigation, we showed that high salinity promoted by hydrothermal inputs, reducing conditions of sediments with high content in organic matter, and the occurrence of an appropriate clay mineral precursor provide a suitable framework for low-temperature illitization processes. We studied the sedimentary illitization process that occurs in carbonaceous sediments from a lake with saline waters (Sochagota Lake, Colombia) located at a tropical latitude. Water isotopic composition suggests that high salinity was produced by hydrothermal contribution. Materials accumulated in the Sochagota Lake’s southern entrance are organic matter-poor sediments that contain detrital kaolinite and quartz. On the other hand, materials formed at the central segment and near the lake exit (north portion) are enriched in organic matter and characterized by the crystallization of Fe-sulfides. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectrometry (EDX) data allowed for the identification of illite and illite-dioctahedral vermiculite mixed layers (I-DV), which are absent in the southern sediments. High humidity and temperate climate caused the formation of small-sized metastable intermediates of I-DV particles by the weathering of the source rocks in the Sochagota Lake Basin. These particles were deposited in the low-energy lake environments (middle and north part). The interaction of these sediments enriched in organic matter with the saline waters of the lake enriched in hydrothermal K caused a reducing environment that favored Fe mobilization processes and its incorporation to I-DV mixed layers that acted as mineral precursor for fast low temperature illitization, revealing that in geothermal areas clays in lakes favor a hydrothermal K uptake.
“…Typically, in suboxic sediments, where the dissimilatory rates are moderate, and the redox zonation accordingly expanded, δ 13 C values fall into this intermediate range, as observed in Ca-rich rhodochrosite occurring within mottled and bioturbated sediments of the Eastern Equatorial Pacific (Meister et al, 2009) [96]. Thereby Fe-and Mn-reduction may contribute to carbonate supersaturation (Kasina et al, 2017, and references therein) [97]. Intermediate values may also occur in shallow sulphate-reduction zones, e.g., in bituminous sediments, where laminae of authigenic carbonate form just below the sediment/water interface and, hence, early with respect to burial along the mixing gradient (cf.…”
Section: Controls Of δ 13 C Composition Of Diagenetic Carbonatesmentioning
Sub-seafloor microbial environments exhibit large carbon-isotope fractionation effects as a result of microbial enzymatic reactions. Isotopically light, dissolved inorganic carbon (DIC) derived from organic carbon is commonly released into the interstitial water due to microbial dissimilatory processes prevailing in the sub-surface biosphere. Much stronger carbon-isotope fractionation occurs, however, during methanogenesis, whereby methane is depleted in 13C and, by mass balance, DIC is enriched in 13C, such that isotopic distributions are predominantly influenced by microbial metabolisms involving methane. Methane metabolisms are essentially mediated through a single enzymatic pathway in both Archaea and Bacteria, the Wood–Ljungdahl (WL) pathway, but it remains unclear where in the pathway carbon-isotope fractionation occurs. While it is generally assumed that fractionation arises from kinetic effects of enzymatic reactions, it has recently been suggested that partial carbon-isotope equilibration occurs within the pathway of anaerobic methane oxidation. Equilibrium fractionation might also occur during methanogenesis, as the isotopic difference between DIC and methane is commonly on the order of 75‰, which is near the thermodynamic equilibrium. The isotopic signature in DIC and methane highly varies in marine porewaters, reflecting the distribution of different microbial metabolisms contributing to DIC. If carbon isotopes are preserved in diagenetic carbonates, they may provide a powerful biosignature for the conditions in the deep biosphere, specifically in proximity to the sulphate–methane transition zone. Large variations in isotopic signatures in diagenetic archives have been found that document dramatic changes in sub-seafloor biosphere activity over geological time scales. We present a brief overview on carbon isotopes, including microbial fractionation mechanisms, transport effects, preservation in diagenetic carbonate archives, and their implications for the past sub-seafloor biosphere and its role in the global carbon cycle. We discuss open questions and future potentials of carbon isotopes as archives to trace the deep biosphere through time.
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