『Abstract
　Pore water chemistry and ²³⁴U/²³⁸U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the ²³⁴U/²³⁸U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m²/g based on both grain geometry and on the depletion of ²³⁴U/²³⁸U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water ²³⁴U/²³⁸U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10-20％ of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 10² to 10⁵ times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (10⁵), less substantial with the Al-inhibition formulation (10³), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (10²). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surfaces\ characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.』

1. Introduction
2. Site description
3. Measurements and data sources
　3.1. Pore water and sediment data
4. Multicomponent reactive transport model
　4.1. Solid and fluid burial
　4.2. Diffusive transport
　4.3. Speciation of the CO2 system in seawater
　4.4. Biogeochemical reactions
　4.5. Minerals and associated reactions
　　4.5.1. Fe(OH)3 and H2S(aq)
　　4.5.2. Silicate minerals
　4.6. Kinetic rate law formulations
　　4.6.1. Close-to-equilibrium rate law
　　4.6.2. Aluminum inhibition model
　4.7. Uranium-series disequilibrium model
　　4.7.1. Bulk dissolution rates from ²³⁴U/²³⁸U in fluid and solids; theory
　　4.7.2. Physical surface area from ²³⁴U/²³⁸U depletion in solids
　　4.7.3. Co-precipitation and recrystallization of uranium in calcite
　　4.7.4. U-series model architecture
　4.8. Model conditions and parameters
5. Results
　5.1. Organic matter degradation
　5.2. Major element, pH, and alkalinity profiles
　5.3. U isotopic and concentration profiles
　5.4. Alternative rate law formulations
　　5.4.1. Parallel rate law formulation
　　5.4.2. Aluminum inhibition
6. Discussion
　6.1. Interpretation of major element profiles and dissolution rates
　6.2. Reactive surface area vs. physical surface area
　6.3. Aluminum inhibition
　6.4. Effect of clay minerals on dissolution rate
　　6.4.1. Slow clay model
　　6.4.2. More soluble clay model
　6.5. Origin of the discrepancy between laboratory and field rates
7. Conclusion
Acknowledgments
Appendix 1. Derivation of rate law for Al-inhibition
Appendix 2. Predicted pore water aluminum concentrations
Appendix 3. Evaluation of model uncertainty
　3.1. Uncertainty in the U-series model architecture
　3.2. Uncertainty in major element model architecture
References