U. Schacht

Safe storage and effective monitoring of CO2 in depleted gas fields

Carbon capture and storage (CCS) is vital to reduce CO2 emissions to the atmosphere, potentially providing 20% of the needed reductions in global emissions. Research and demonstration projects are important to increase scientific understanding of CCS, and making processes and results widely available helps to reduce public concerns, which may otherwise block this technology. The Otway Project has provided verification of the underlying science of CO2 storage in a depleted gas field, and shows that the support of all stakeholders can be earned and retained. Quantitative verification of long-term storage has been demonstrated. A direct measurement of storage efficiency has been made, confirming that CO2 storage in depleted gas fields can be safe and effective, and that these structures could store globally significant amounts of CO2.

Subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth: Evidence from biogeochemical experiments and satellite data

Volcanoes confront Earth scientists with new fundamental questions: Can airborne volcanic ash release nutrients on contact with seawater, thereby excite the marine primary productivity (MPP); and, most notably, can volcanoes through oceanic fertilization affect the global climate in a way that is so far poorly understood? Here we present results from biogeochemical experiments showing that 1) volcanic ash from subduction zone volcanoes rapidly release an array of nutrients (co-)limiting algal growth in vast oceanic areas, 2) at a speed much faster (minute-scale) than hitherto known and that marine phytoplankton from low-iron oceanic areas can swiftly, within days, utilize iron from volcanic sources. We further present satellite data possibly indicating an increase of the MPP due to the seaward deposition of volcanic particulate matter. Our study supports the hypothesis that oceanic (iron) fertilization with volcanic ash may play a vital role for the development of the global climate.

Surface ocean iron fertilization: The role of airborne volcanic ash from subduction zone and hot spot volcanoes and related iron fluxes into the Pacific Ocean

Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere-ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long-term (millennial-scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

Pacific offshore record of plinian arc volcanism in Central America: 1. Along-arc correlations

We collected 56 marine gravity cores from the Pacific seafloor offshore Central America which contain a total of 213 volcanic ash beds. Ash-layer correlations between cores and with their parental tephras on land use stratigraphic, lithologic, and compositional criteria. In particular, we make use of our newly built database of bulk-rock, mineral, and glass major and trace element compositions of plinian and similarly widespread tephras erupted since the Pleistocene along the Central American Volcanic Arc. We thus identify the distal ashes of 11 Nicaraguan, 8 El Salvadorian, 6 Guatemalan, and 1 Costa Rican eruptions. Relatively uniform pelagic sedimentation rates allow us to determine ages of 10 previously undated tephras by their relative position between ash layers of known age. Linking the marine and terrestrial records yields a tephrostratigraphic framework for the Central American volcanic arc from Costa Rica to Guatemala. This is a useful tool and prerequisite to understand the evolution of volcanism at a whole-arc scale.

Chapter 9 – Early Diagenesis of Deep-Sea Sediments

Abstract This review covers the early diagenesis of the various types of deep-sea sediments. It uses pore-water profiles as a sensitive tool to gain insight in diagenetic processes, to depict the mineral reactions associated with the changes in the pore-water chemistry and to group deep-sea sediments into a limited number of diagenetic classes. In doing so, a sequence is followed from the simplest diagenetic systems encountered in pelagic sediments on the ocean crust with pore-water profiles lacking chemical gradients to the most highly reactive organic-matter rich sediments on the continental margins at the oxygen-minimum zone. Pore-water profiles are characteristic of specific sediment types only if they are reaction-controlled. In advection- and/or diffusion-dominated profiles, fluid-chemistry related to the nature of the host sediment is overprinted by ions derived from external sources. Advection and diffusion-dominated profiles are treated at the beginning of the chapter. The simplest type of pelagic sediment in terms of diagenesis and the first to be covered is the brown abyssal clay on the deep ocean floor below the calcite compensation level. The review of the early diagenesis of biogenic siliceous oozes which follows covers a wide range of topics from pore-water chemistry to burial-diagenetic stages, characteristics of the solid silica phases, nature of the conversion mechanisms and their interpretation in terms of Ostwald processes, crystallographic structural changes of opal-CT and quartz during progressive burial, rate-controlling factors of the diagenetic reactions, the formation of bedded chert and the physical diagenesis of biogenic siliceous deep-sea sediments. The next topic is the early diagenesis of biogenic pelagic carbonates which is characterized by recrystallization involving (pressure) solution and reprecipitation leading to the formation of chalk. It is accompanied by a marked redistribution of strontium isotopes in the pore waters. Rhythmic limestone/marlstone alternations are generally attributed to Milankovitch palaeoclimate cycles but may also be accentuated or even originate during burial diagenesis. For hemipelagic sediments the oxidation of organic matter becomes the driving force of diagenetic reactions. During early diagenesis the organic-matter of the sediments is decomposed mostly by bacteria. Microbial organic-matter oxidation is accomplished by five different oxidant-specific populations of bacteria (aerobic bacteria, nitrogen reducers, sulphate reducers, carbonate reducers or methanogens, and fermenters) that follow one another during burial in the order of decreasing energy efficiency of their metabolic reactions and give rise to the vertical sequence of organic-matter decomposition zones comprising (1) the oxidation zone, (2) the nitrate-reduction zone, (3) the sulphate-reduction zone, (4) the carbonate-reduction zone, and (5) the fermentation zone. The latter is followed by (6) the thermocatalytic decarboxylation zone. The oxidation zone corresponds to what is called ‘oxic diagenesis’, the nitrate-reduction zone to ‘suboxic diagenesis’, and the top of the sulphate-reduction zone to the top of ‘anoxic diagenesis’. The brown abyssal clay which is almost free of organic matter essentially remains in the oxidation zone during burial. A typical representative of suboxic diagenesis are manganese nodules and crusts, as are glauconitic sediments. Phosphorites are diagenetic in origin and frequently occur together with glauconite. Anoxic diagenesis in organic-matter rich hemipelagic sediments leads to the formation of gas-hydrates, if the water depth (i.e. pressure) is sufficient. Euxinic sediments are hemipelagic sediments undergoing anoxic diagenesis in stagnant basins under oxygen-free bottom water. Evaporite dissolution in the subsurface gives rise to a specific pore-water type, as does hydrothermal activity and the intrusion of igneous dykes and sills above sediment-covered mid-ocean ridges. Active margins affected by advective lateral fluid flow show typical step-curves of their pore-water profiles. Highly reactive volcanogenic sediments produce their own pore-water anomalies. A major section is devoted to early diagenetic mineralization reactions in anoxic deep-water sediments covering sulphide precipitation, authigenic carbonates (calcite, siderite, organogenic dolomite, complex authigenic carbonates) and barite. The final section deals with early diagenetic clay mineral formation.

Pacific offshore record of plinian arc volcanism in Central America: 3. Application to forearc geology

[1] Sediment gravity cores collected on the Pacific slope and incoming plate offshore Central America reach up to 400 ka back in time and contain numerous ash layers from plinian eruptions at the Central American Volcanic Arc. The compositionally distinct widespread ash layers form a framework of marker horizons that allow us to stratigraphically correlate the sediment successions along and across the Middle America Trench. Moreover, ash layers correlated with 26 known eruptions on land provide absolute time lines through these successions. Having demonstrated the correlations in part 1, we here investigate implications for submarine sedimentary processes. Average accumulation rates of pelagic sediment packages constrained by bracketing tephras of known age range from ∼1–6 cm/ka on the incoming plate to 30–40 cm/ka on the continental slope. There are time intervals in which the apparent pelagic sedimentation rates significantly vary laterally both on the forearc and on the incoming plate where steady conditions are usually expected. A period of unsteadiness at 17–25 ka on the forearc coincides with a period of intense erosion on land probably triggered by tectonic processes. Unsteady conditions on the incoming plate are attributed to bend faulting across the outer rise triggering erosion and resedimentation. Extremely low apparent sedimentation rates at time intervals >50–80 ka suggest stronger tectonic activity than during younger times and indicate bend faulting is unsteady on a longer timescale. Submarine landslides are often associated with ash layers forming structurally weak zones used for detachment. Ash beds constrain ages of >60 ka, ∼19 ka, and <6 ka for three landslides offshore Nicaragua. Phases of intense fluid venting at mud mounds produce typical sediments around the mound that become covered by normal pelagic sediment during phases of weak or no activity. Using intercalated ash layers, we determine for the first time the durations (several hundred to 9000 years) of highly active periods in the multistage growth history of mud mounds offshore Central America, which is essential to understand general mud-mound dynamics.

Methane-carbon flow into the benthic food web at cold seeps--a case study from the Costa Rica subduction zone.

Cold seep ecosystems can support enormous biomasses of free-living and symbiotic chemoautotrophic organisms that get their energy from the oxidation of methane or sulfide. Most of this biomass derives from animals that are associated with bacterial symbionts, which are able to metabolize the chemical resources provided by the seeping fluids. Often these systems also harbor dense accumulations of non-symbiotic megafauna, which can be relevant in exporting chemosynthetically fixed carbon from seeps to the surrounding deep sea. Here we investigated the carbon sources of lithodid crabs (Paralomis sp.) feeding on thiotrophic bacterial mats at an active mud volcano at the Costa Rica subduction zone. To evaluate the dietary carbon source of the crabs, we compared the microbial community in stomach contents with surface sediments covered by microbial mats. The stomach content analyses revealed a dominance of epsilonproteobacterial 16S rRNA gene sequences related to the free-living and epibiotic sulfur oxidiser Sulfurovum sp. We also found Sulfurovum sp. as well as members of the genera Arcobacter and Sulfurimonas in mat-covered surface sediments where Epsilonproteobacteria were highly abundant constituting 10% of total cells. Furthermore, we detected substantial amounts of bacterial fatty acids such as i-C15∶0 and C17∶1ω6c with stable carbon isotope compositions as low as −53‰ in the stomach and muscle tissue. These results indicate that the white microbial mats at Mound 12 are comprised of Epsilonproteobacteria and that microbial mat-derived carbon provides an important contribution to the crabs nutrition. In addition, our lipid analyses also suggest that the crabs feed on other 13C-depleted organic matter sources, possibly symbiotic megafauna as well as on photosynthetic carbon sources such as sedimentary detritus.

Laboratory and Mathematical Modelling of Fines Production from CSG Interburden Rocks

Twelve clastic core samples from the Walloon Coal Measures, Surat Basin were tested for disintegration in artificially produced fluids varying in ionic strength. XRD data confirm the presence of smectite (water sensitive clay) in the samples. Flow-through rock disintegration experiments demonstrate that the higher the concentration of smectite and soluble plagioclase is, the quicker rock disintegrates in artificial low ionic strength fluid. Pre-soaking of rocks with high ionic strength fluid reduces rock disintegration rate in low ionic strength fluids. This is explained by very strong clay-clay and clay-sand attraction forces, evidenced through zeta-potential measurements, which inhibit rock degradation. For the studied samples it is clear that rock disintegration rate is proportional to fluid velocity. Experimental rock disintegration data are fitted by a power erosion model with two adjusted parameters: fluid ionic strength and Reynolds number. The experimental results satisfactorily agree with theoretical data. Rock disintegration rates are calculated as released particle volume per thickness of interburden layer per day at a fixed Reynolds number and low ionic strength. The laboratory work suggests that keeping wells under strong ionic fluid during shut-in times and a reduction of water production rate will preserve rock integrity for a longer period of time.

New Laboratory Method to Assess Formation Damage in Geothermal Wells

The new method to assess permeability damage in geothermal reservoirs and predict well productivity decline is presented. The laboratory methodology developed aims to determine permeability decline from mobilisation, migration and straining of natural reservoir fines. Laboratory coreflood testing with constant and stepwise decreasing ionic strength has been performed with measurements of the pressure drop along the core and accumulated effluent particle concentration. Stabilisation of rock permeability occurs after injection of numerous pore volumes, suggesting slow drift of mobilised particles if compared with the carrier water velocity. Low ionic strength water increases electrostatic repulsion forces between clay particles and sand grain surfaces, further mobilising particles and resulting in formation damage. Kaolinite and illite/chlorite mixed layer clay minerals are identified by SEM-EDAX analysis and are the minerals primarily responsible for the permeability damage. The competitive effects of decreasing water viscosity and weakening electrostatic attraction on the attached particle concentration during temperature increase have been observed. The micro-modeling of the fine particle mechanical equilibrium shows that the water viscosity effect on the fine particle attachment dominates. It results in decreased fines detachment and permeability decline at high temperatures.