Gas composition, concentration and stable isotope ratios (Table 2, 3, 4)

The deployment of CCS (carbon capture and storage) at industrial scale implies the development of effective monitoring tools. Noble gases are tracers usually proposed to track CO2. This methodology, combined with the geochemistry of carbon isotopes, has been tested on available analogues. At first, gases from natural analogues were sampled in the Colorado Plateau and in the French carbogaseous provinces, in both well-confined and leaking-sites. Second, we performed a 2-years tracing experience on an underground natural gas storage, sampling gas each month during injection and withdrawal periods. In natural analogues, the geochemical fingerprints are dependent on the containment criterion and on the geological context, giving tools to detect a leakage of deep-CO2 toward surface. This study also provides information on the origin of CO2, as well as residence time of fluids within the crust and clues on the physico-chemical processes occurring during the geological story. The study on the industrial analogue demonstrates the feasibility of using noble gases as tracers of CO2. Withdrawn gases follow geochemical trends coherent with mixing processes between injected gas end-members. Physico-chemical processes revealed by the tracing occur at transient state. These two complementary studies proved the interest of geochemical monitoring to survey the CO2 behaviour, and gave information on its use.

(Table 4) Estimated surface water phosphate concentrations for the past 200 kyr at the location of sediment core GeoB1016-3

We compared ocean atlas values of surface water [PO4]3- and [CO2(aq)] against the carbon isotopic fractionation (ep) of alkenones obtained from surface sediments of the South Atlantic and the central Pacific (Pacific data are from Pagani et al. 2002, doi:10.1029/2002PA000756). We observed a positive correlation between ep and 1/[CO2(aq)], which is opposite of what would be expected if the concentration of CO2(aq) were the major factor controlling the carbon isotopic fractionation of C37:2 alkenones. Instead, we found inverse relationships between ep and [PO4]3- for the two ocean basins (for the Atlantic, ep = -4.6*[PO4]3- + 15.1, R = 0.76; for the Pacific, ep = -4.1*[PO4]3- + 13.7, R = 0.64), suggesting that ep is predominantly controlled by growth rate, which in turn is related to nutrient concentration. The similarity of the slopes implies that a general relationship between both parameters may exist. Using the relationship obtained from the South Atlantic, we estimated surface water nutrient concentrations for the past 200,000 years from a deep-sea sediment core recovered off Angola. Low ep values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C37 alkenones, low surface water temperatures, and decreased bulk d15N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed ep decrease.