Uptake Of CO2 In The Upper Ocean Crust

Carbonates from the upper oceanic crust have been studied in order to determine the size of the CO2 reservoir, and to assess the conditions governing CO2 uptake. Carbonate is most abundant in volcanic sequences, where it generally marks the last phase of low temperature alteration. The CO2 inventory for eight new drill sites from Atlantic (396B, 543A, 556, 562) and Pacific (573, 595B, 597C, 1224F) ocean basins, spanning an age of 6.8 to 140 Ma, was calculated using whole rock CO2 contents, and the abundance of void filling carbonate. Our results, combined with published CO2 data, identify two age populations for the CO2 content of volcanic sequences, from 6.8 to 74 Ma (0.2-0.9 wt%\) and 110 to 140 Ma (1.95 to 4.1 wt%\). In order to explore what controls the uptake of carbonate, we have determined the O, C and Sr isotopic ratios and trace element contents for carbonate from each site. Stable isotopic data for carbonates indicates inorganic precipitation with a variation of d13C (PDB) between -2.321 and 3.412 per mil. d18O (SMOW) varies from 25.5 to 35.4 per mil, yielding temperatures of 1° to 31 °C, considering initial seawater value of d18O = 0 per mil. Preliminary Mg and Sr data, combined with published data, show these elements behave similarly during carbonate precipitation at most sites. Mg and Sr concentrations show a slight increase with decreasing of temperature, indicating that although temperature is an important parameter it does not control Sr and Mg contents. There is no clear relationship between Sr-isotopic values, Mg and Sr contents, and temperature. We will explore the relative importance of crust age, seawater chemistry and environmental conditions in controlling carbonate formation.