LONG-TERM CONTROL OF ATMOSPHERIC CARBON-DIOXIDE - LOW-TEMPERATURE SEA-FLOOR ALTERATION OR TERRESTRIAL SILICATE-ROCK WEATHERING

Accumulation of authigenic carbonate in the basaltic ocean basement during low-temperature alteration has been proposed as a major sink for carbon in the oceans and atmosphere, Francois and Walker (1992) suggested that seafloor alteration, and not the terrestrial silicate-rock weathering feedback, is the most important factor determining long-term steady-state atmospheric COP concentrations, They suggested that basalt dissolution and carbonate accumulation in the ocean basement is proportional to the deep-ocean hydrogen-ion and total carbon concentrations; however, they recognized that their parameterization was a first estimate that should be improved, In this work, I develop an improved parameterization and incorporate it in a global carbon-cycle model, Silicate-mineral dissolution rates have been found to be nearly independent of pH in the pH range 5 to 8 in a wide variety of experiments, Because deep-ocean PH is > 7.5, low-temperature sea-floor basalt alteration is not likely to be an effective feedback on atmospheric CO2, If proposed terrestrial silicate-rock weathering formulations are at least approximately correct, then CO2 consumption by terrestrial weathering Is several orders of magnitude more sensitive to changes in atmosphere and ocean CO2 concentration than is CO2 consumption by low-temperature seafloor alteration, This suggests that silicate-rock weathering on land, and not low-temperature seafloor basalt alteration, has been the primary control on long-term atmospheric CO2 content, Nevertheless, ocean basement carbonate accumulation should be included in global carbon budgets and may impact atmospheric CO2 content by affecting the degassing of CO2 during subduction zone metamorphism.