First results from a controlled deep sea CO2 perturbation experiment: Evidence for rapid equilibration of the oceanic CO2 system at depth
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Abstract:
We have carried out series of remotely operated vehicle–controlled oceanic CO 2 system perturbation experiments off the coast of California at depths down to 1000 m to observe reaction rates and pathways with both HCl and HCO 3 − addition. The work was done to evaluate possible barriers to carrying out future Free Ocean CO 2 Enrichment experiments to simulate the chemistry of the emerging high CO 2 –lower pH ocean. A looped 460 mL flow cell with a pH sensor was used to monitor the time to equilibrium for 900 μL additions of 0.008 N HCl and for small slugs of HCO 3 − enriched seawater. The results were compared to equivalent experiments at the same temperature and 1 atm pressure. In each case the experiments at depth showed significantly faster time to equilibrium than those at 1 atm. These results are consistent with the low partial molal volume of CO 2 in seawater, favoring the hydration reaction rate. The results imply, but do not prove, a significant effect of pressure on the rate constants. The relatively rapid equilibration times observed in seawater of 4°C and at 10 MPa indicates that there are no fundamental physical chemistry limits for carrying out small‐scale free‐ocean CO 2 enrichment experiments.Keywords:
Molality
An equation of state of solute silica in NaCl brines at 500 to 900°C and 4 to 15 kbar is formulated by making use of two experimentally determined properties of quartz solubility: the silica molality decreases in direct proportion to the logarithm of the NaCl mole fraction (X(NaCl)) at pressures approaching 10 kbar, and the relative silica molality (molality at a given NaCl mole fraction, mx, divided by the molality in pure H2O at the same P and T, mo) is independent of temperature in the evaluated range. These two properties are expressed in the relation: log(mx/mo)∗ = A + BX(NaCI), where log(mx/mo)∗ denotes the logarithm of the ideal molality ratio, and A and B are functions of pressure, but not temperature or salinity, such that B = −1.730 − 1.431 × 10−3P + 5.923 × 10−4P2 −9.243 × lO−5P3, and A = 0 at P>10 kbar, whereas A = 0.6131 − 0.1256P + 6.431 × 10−3P2 at P≤10 kbar, as derived from fits to experimental data (Newton and Manning, 1999). The parameter A decreases from 0.214 to 0 from 4 to 9.5 kbar, and remains zero to 15 kbar; B decreases from −1.373 to −1.571 from 4 to 15 kbar. With the above relationship defining a variable X(NaCl)-T-P standard-state of solute silica, the activity of SiO2 can be replaced by its molality for calculations of mineral-fluid equilibria over most of the conditions for metasomatism in the deep crust and upper mantle. Significant departures from ideality occur only at the lowest pressures, and low salinities. Calculations on peridotite mineral stability in the simple system CaO-MgO-SiO2-H2O-NaCl at high T and P show that antigorite, brucite, and diopside are stable at 500°C and pressures of 5 to 15 kbar in the presence of concentrated NaCl solutions at low SiO2 activities. At 700°C, anthophyllite is stable over a wide range of salinities at 5 kbar with tremolite but not with diopside. The presence of anthophyllite buffers silica solubility at a high, salinity-independent value close to quartz saturation. At 10 and 15 kbar and 700°C, talc replaces anthophyllite as the stable hydrate, and talc-trem-olite assemblages buffer SiO2 fluid concentrations at high values nearly independent of salinity. At 900°C hydrates are unstable and diopside again becomes stable and coexists with enstatite in peridotites. These stability calculations correspond well to the observed progressive metamorphic sequence in peridotite bodies in the Central Alps. This method of analysis may be useful in interpretation of metamorphosed ultramafic bodies in general, including the basal portions of obducted ophiolitic mantle lithosphere and the mantle wedge above subduction zones. More detailed calculations, including rocks containing feldspars, must take into account the more soluble major components of rocks, especially alkalis, as these will affect the activity coefficient of SiO2 in NaCl solutions. The solubility of silica in the presence of minerals containing these components must be determined by additional measurements.
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A densimetric method is used for determining the partial molal volumes, V *( i ), of NaCl, KCl, CaCl 2 , MgCl 2 , Na 2 SO 4 , and MgSO 4 in seawater and thermal expansibilities of V *( i ) of these salts are deduced. A semiempirical relation is proposed for the partial molal volumes of salts, in the range of temperatures and salinities of the world ocean. The partial molal volumes of these ions are estimated on the assumption that the partial molal volume at infinite dilution of proton, V * ° (H*), is exact and that the volume of transfer of Cl − from pure water to seawater does not vary with temperature. A convenient equation of partial molal volume of ions as a function of temperature and salinity is given.
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Maximum flow problem
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