Effects of microbial activity and silicate alteration on porefluid alkalinity and carbonate diagenesis in a deep methanogenic zone at the Peru Margin (ODP Site 1230)

Die CO32--Sättigung im Porenwasser mariner Sedimente bestimmt maßgeblich die Möglichkeit der authigenen Karbonatfällung. Starke mikrobielle Aktivität, insbesondere Sulphatreduktion und anaerobe Oxidation von Methan, produziert Alkalinität und begünstigt die Karbonatbildung, während Methanogenese den pH-Wert des Porenwassers, durch einen Anstieg an gelöstem CO2 (DIC) verringert. Ohne Puffer-Effekte führt diese „Versauerung“ zu einer Auflösung von Karbonaten. Die Verwitterung von Silikaten während der frühen Diagenese scheint einen Einfluss auf die Alkalinität des Porenwassers zu haben und es wird angenommen, dass zusätzliche Alkalinität durch die Lösung untersättigter Tonminerale produziert wird. Diese Prozesse scheinen einen großen Einfluss auf die Karbonatdiagenese zu haben und könnten sogar in stark methanogenen Zonen zur Karbonatfällung führen. Diese Studie quantifiziert den Anteil mikrobiell produzierter Alkalinität an der gemessenen Gesamtalkalinität im Porenwasser der ODP, Leg 201, Site 1230 (Ocean Drilling Project) mittels eines Diffusions-Modells. Des Weiteren wurde die Möglichkeit eines Anstieges der Alkalinität durch Gleichgewichtsreaktionen von Tonmineralen simuliert. Obwohl das Modell DIC-Konzentrationen von bis zu 240 mmol/l ergibt, bewirken 115 mmol/l mikrobiell produzierter Alkalinität, dass Calzit bis zu einer Profiltiefe von 140 mbsf leicht übersättigt ist. Die XRD-Analyse von Sedimenten der ODP Site 1230 ergab einen Calzitnachweis zwischen 60 und 140 mbsf. Für die Berechnungen der Tonmineralgleichgewichte wurde der PHREEQC-Code, bereitgestellt vom United State Geological Survey, verwendet. 3 von 48 thermodynamisch möglichen Mineralkombinationen von K-Feldspat, Illit, Kaolinit, Montmorillonit, Chlorit, Calzit und Opal A (SiO2) reproduzierten die im ODP Site 1230 Profil gemessenen [Ca2+]-, [Mg2+]-, DIC-Konzentrationen und Alkalinitätswerte am besten. Die Ergebnisse zeigen, dass Tonminerale in Tiefseesedimenten weit weg von einem Gleichgewicht und mögliche Reaktionen kinetisch gehemmt sind. Die Überwindung dieser kinetischen Barrieren wäre unerlässlich für die ersten Konzepte von CO2-Speicherreservoiren in geeigneten geologischen Einheiten (Fischer et al., 2010; Kasina et al., 2014). Die Ausfällung von Karbonaten könnte dazu führen, dass CO2 in Form von Karbonatgestein dauerhaft gespeichert wird. Hierbei ist die geochemische Modellierung eine wichtige Methode, um komplexe Probleme in diesem Zusammenhang zu verstehen (zB Boudreau, 1997).The formation of early diagenetic carbonates is of interest as it affects the geological record and preserves and/or alters geochemical signatures indicative of past environments and life. The factors affecting carbonate diagenesis are diverse, but largely depend on processes increasing carbonate (CO32-) concentration and therefore the saturation state of the porefluid with respect to different carbonate minerals. An important factor for carbonate precipitation is alkalinity production by microbial activity, such as sulphate reduction (SR) and anaerobic oxidation of methane (AOM). In contrast, production of CO2 by fermentative processes, such as methanogenesis, cause acidification of the porefluid and counteract the formation of carbonates. Partial dissolution (weathering) of silicates during early diagenesis seems to have an influence on fluid alkalinity during burial diagenesis, and it has been suggested that the additional alkalinity from clay mineral alteration may drive carbonate precipitation even in strongly methanogenic zones. These processes seem to have great influence on carbonate diagenesis but are not fully understood yet. In this study, the share of alkalinity produced by microbial activity to 150 mmol/l measured alkalinity at ODP Site 1230 is quantified with a diffusion model and the possibility of additional porewater alkalinity increase through (clay) mineral equilibrium calculations is evaluated. With up to 115 mmol/l alkalinity in porewater (calculated by fitting the results of a reaction transport model with measured ammonium concentrations) exlusively produced through microbial activity, calcite is slightly oversaturated between 30 and 140 mbsf at ODP Site 1230, despite a contribution of up to 240 mmol/l of dissolved CO2. Apart from previously described shallow dolomite layers (Meister et al., 2011), bulk XRD shows the occurrence of calcite between 60 and 140 mbsf which is deep in the zone of methanogenesis between 60 and 140 mbsf: this confirms the prediction of the model. For calculations of mineral reactions the computer program PHREEQC, available from the United State Geological Survey, was used. Three out of 48 thermodynamically possible combinations of minerals, K-feldspar, illite, kaolinite, montmorillonite, chlorite, calcite and SiO2 (opal A) with input-parameter from the diffusion-model, reproduce measured [Ca2+], [Mg2+], DIC and alkalinity of the porewater of ODP Site 1230 best. Single calcite + SiO2(a) equilibrium (scenario 6) with initial solution leads to a decrease of alkalinity but fits well [Ca2+] of the profile. Chlorite + SiO2(a) (scenario 5) reaction increases alkalinity by ≈ 45 mmol/l but does not include any carbonate phase. The combination of both reactions (scenario 21), respectively the equilibrium of chlorite, calcite and SiO2(a) with the initial solution, comes closest measured [Ca2+], [Mg2+], DIC and alkalinity of the porewater of ODP Site 1230. Due to simulating mineral reaction through congruent dissolution, chlorite may stand vicarious for any reactive clay mineral. One insight gained from this study is that incongruent dissolution of clay mineral has a major capacity to drive carbonate diagenesis. However, the processes are largely inhibited or slow. Results demonstrate that clay mineral alteration processes are far away from equilibrium. Overcoming these kinetic barriers would be essential for first concepts of CO2 storage reservoirs in eligible geological units (Fischer et al., 2010; Kasina et al., 2014). Carbonate precipitation could be induced to permanently trap and store CO2 in the form of carbonate rock. Regarding questions of chemical equilibria, geochemical modelling is an adequate and important method to get an idea of very complex problems (e.g Boudreau, 1997)

Microbial Alkalinity Production and Silicate Alteration in Methane Charged Marine Sediments: Implications for Porewater Chemistry and Diagenetic Carbonate Formation

A numerical reaction-transport model was developed to simulate the effects of microbial activity and mineral reactions on the composition of porewater in a 230-m-thick Pleistocene interval drilled in the Peru-Chile Trench (Ocean Drilling Program, Site 1230). This site has porewater profiles similar to those along many continental margins, where intense methanogenesis occurs and alkalinity surpasses 100 mmol/L. Simulations show that microbial sulphate reduction, anaerobic oxidation of methane, and ammonium release from organic matter degradation only account for parts of total alkalinity, and excess CO2 produced during methanogenesis leads to acidification of porewater. Additional alkalinity is produced by slow alteration of primary aluminosilicate minerals to kaolinite and SiO2. Overall, alkalinity production in the methanogenic zone is sufficient to prevent dissolution of carbonate minerals; indeed, it contributes to the formation of cemented carbonate layers at a supersaturation front near the sulphate-methane transition zone. Within the methanogenic zone, carbonate formation is largely inhibited by cation diffusion but occurs rapidly if cations are transported into the zone via fluid conduits, such as faults. The simulation presented here provides fundamental insight into the diagenetic effects of the deep biosphere and may also be applicable for the long-term prediction of the stability and safety of deep CO2 storage reservoirs