Effect of a controlled sub-seabed release of CO 2 on the biogeochemistry of shallow marine sediments, their pore waters, and the overlying water column

Abstract

Please cite this article in press as: Lichtschlag, A., et al., Effect of a controlled sub-seabed release of CO 2 on the biogeochemistry of shallow marine sediments, their pore waters, and the overlying water column. Int. J. Greenhouse Gas Control (2014), http://dx.Article history: Available online xxx Keywords: Carbon capture and storage CO2 leakage Sediment biogeochemistry Release of metals Environmental impact a b s t r a c tThe potential for leakage of CO 2 from a storage reservoir into the overlying marine sediments and into the water column and the impacts on benthic ecosystems are major challenges associated with carbon capture and storage (CCS) in subseafloor reservoirs. We have conducted a field-scale controlled CO 2 release experiment in shallow, unconsolidated marine sediments, and documented the changes to the chemical composition of the sediments, their pore waters and overlying water column before, during and up to 1 year after the 37-day long CO 2 release. Increased levels of dissolved inorganic carbon (DIC) were detected in the pore waters close to the sediment-seawater interface in sediments sampled closest to the subsurface injection point within 5 weeks of the start of the CO 2 release. Highest DIC concentrations (28.8 mmol L −1 , compared to background levels of 2.4 mmol L −1 ) were observed 6 days after the injection had stopped. The high DIC pore waters have high total alkalinity, and low ı 13 C DIC values (−20‰, compared to a background value of −2‰), due to the dissolution of the injected CO 2 (ı 13 C = −26.6‰). The high DIC pore waters have enhanced concentrations of metals (including Ca, Fe, Mn) and dissolved silicon, relative to non-DIC enriched pore waters, indicating that dissolution of injected CO 2 promotes dissolution of carbonate and silicate minerals. However, in this experiment, the pore water metal concentrations did not exceed levels considered to be harmful to the environment. The spatial extent of the impact of the injected CO 2 in the sediments and pore waters was restricted to an area within 25 m of the injection point, and no impact was observed in the overlying water column. Concentrations of all pore water constituents returned to background values within 18 days after the CO 2 injection was stopped.

Impact of sub-seabed CO 2 leakage on macrobenthic community structure and diversity

“…This area was termed Zone 1 and measured approximately 20 m across . No evidence of CO 2 transport or of impact on sediment or water carbonate chemistry was observed outside of this area, although sampling of water columns parameters may not have been optimal (see Lichtschlag et al, 2014). After the injection of CO 2 was stopped (23rd June 2012) the release of bubbles from the seafloor stopped immediately.…”

“…The gas migrated quickly up through the sediment overburden and into the overlying water (Blackford et al, 2014;Cevatoglu et al, 2014). As the gas passed through the sediment, it reacted with the resident pore waters, significantly altering the carbonate chemistry within the sediment (see Section 2.2 below and Lichtschlag et al, 2014). However, the impacts of CO 2 leakage (bubbles and carbonate chemistry disruption) were restricted to a small area located 10 m to the west of the porous diffuser.…”

“…These changes are summarised here, but a detailed description of the physical and chemical impacts is presented in Blackford et al (2014), Lichtschlag et al (2014), and Cevatoglu et al (2014). Within a few hours of starting CO 2 injection, small gas bubble plumes were observed at the seafloor.…”

Mineralogical and geochemical analysis of Fe-phases in drill-cores from the Triassic Stuttgart Formation at Ketzin CO2 storage site before CO2 arrival

“…Fe release may be accelerated in the presence of sulphide leading to the precipitation of pyrite. Also more Fe could be leached through silicate alteration due to acidification by CO 2 (Lichtschlag et al 2015). In situ reduction of Fe in clay minerals under reducing conditions has been demonstrated (Kostka and Luther III 1994;Lee et al 2006;Ribeiro et al 2009;Stucki and Kostka 2006), and this process can be catalysed by microbes.…”

The response of seagrass (<i>Posidonia oceanica</i>) meadow metabolism to CO<sub>2</sub> levels and hydrodynamic exchange determined with aquatic eddy covariance

“…-, PO4 + ) were determined with a QuAAttro nutrient analyzer according to the methods employed by Lichtschlag et al, (2015). Samples for both alkalinity and DIC were collected by divers at the beginning and end of each deployment.…”