Laboratory experiments have shown empirically the stoichiometric assumption that 1 tonne of olivine, when it dissolves, captures 1.25 tonnes of CO2.
While the extent of CO2 sequestration is already well established and the available evidence suggests that the olivine will dissolve and complete its sequestration within 2-5 years, the exact rate of CO2 sequestration on a real world beach remains a source of some uncertainty. The following approach will enable us to reduce that uncertainty as much as possible and establish a weathering rate model that can accurately predict the speed at which olivine weathers under varying natural conditions.
In the pilot project we are proposing, a relatively small embayment is used for deployment of ground olivine. As a control, another similar embayment in the vicinity is used, but without olivine addition. The experimental bay system is half-open, with limited seawater exchange with the open ocean. The considerable residence time of the water in the bay should enable us to measure accumulation of olivine dissolution products in the overlying water of the bay, on a timescale of days to months. Olivine dissolution rates will be quantified by open flux measurements. Over a period of days to weeks, the accumulation of various end-products of olivine dissolution (alkalinity (AT), dissolved silicate (DSi), dissolved iron (DFe), trace metals) will be traced by monitoring the overlying water chemistry at regular time intervals. In addition, benthic chambers (a kind of upside down aquarium) are used to close off a part of the seafloor with seawater standing on top. In this closed off volume of seawater, accumulation of olivine dissolution products can be measured on a timescale of hours. These results can be used to extrapolate to, and checked against the results for the entire embayment. Water refreshment rates can be calculated and determined from local and historical data. To enable a full picture of geochemical cycling, water samples will be taken for a range of analyses, including nutrients (DSi, NO3, NH4, PO4), anions (SO4, Cl–), metals (DNi, DFe, trace metals) and carbonate system parameters (see below).
CO2 sequestration will be determined by monitoring the carbonate chemistry in the overlying water at regular time intervals. When Total Alkalinity (AT) accumulates in the overlying water, there will be a transfer of CO2 from the atmosphere, which increases the Dissolved Inorganic Carbon (DIC) concentration and pH. The carbonate system will be determined by simultaneously measuring the AT, DIC and pH and verifying the consistency of the measurements using model simulations in the AquaEnv model framework (Hofmann et al., 2010).
Automated sensor platforms (temperature, salinity, pH, AT, O2, and chlorophyll a) will be placed in the water at deployment sites to follow up the water chemistry and carbon dynamics over timescales of days to months. Primary production and algal community composition will be monitored to determine any fertilization effect from the release of silica or iron from olivine.
The accumulation of olivine reaction products with depth in the pore water will provide insight into possible saturation effects on the olivine dissolution rate. The geochemical conditions within the sediment (i.e. the actual conditions at which olivine dissolves) will be quantified in detail through microsensor profiling (O2, H2S, pH), high-resolution pore water geochemistry (AT, DIC, NO3, NH4, PO4, SO4, DFe, DSi, trace metals) and solid phase analysis (organic C and N, CaCO3).
At regular time intervals, olivine particles will be recovered from the sediment to inspect the grain surfaces for dissolution features and secondary mineral precipitates by means of X-ray computed tomography combined with XRF. The resulting geochemical dataset (pore water depth profiles, fluxes) will be used in geochemical model simulations in order to accurately constrain fluxes and process rates, including organic matter mineralization and olivine dissolution (virtual seafloor environment)