Why Does Calcium Carbonate Precipitate in the Ocean Crust?

Calcium carbonate (CaCO₃) precipitation in extant ocean crust may provide lessons for industrial carbon dioxide (CO₂) sequestration to mitigate against future climate change. The formation and alteration of the ocean crust during mid-ocean ridge spreading plays an integral role in the geological carbon (C) cycle. Magma-degassing during mid-ocean ridge volcanism releases large volumes of CO₂ to the oceans and atmosphere, whereas the precipitation of CaCO₃ minerals in rocks and veins during hydrothermal alteration of the ocean crust is a major mechanism of natural CO₂ sequestration. However, the physical and chemical conditions that control the nature, extent and rate of calcium carbonate precipitation in the ocean crust are not well known, and the magnitudes of the carbon fluxes associated with the formation and alteration of the ocean crust remain poorly quantified. Mesozoic ocean crust contains significantly more hydrothermal CaCO₃ than young ocean crust, but the past ocean conditions responsible for this enhanced C-uptake by the ocean crust are yet to be determined. The thermal, hydrogeologic, and chemical characteristics of active, low temperature seawater-basalt interaction have been investigated by a series of integrated studies in young ocean crust (<3.6 Ma) on the eastern flank of the Juan de Fuca ridge (JdFR). This has included sampling by scientific ocean drilling of CaCO₃ veins, their parent fluids, and the basalts that host the veins. This area provides a unique opportunity to investigate the conditions of in situ hydrothermal CaCO₃ precipitation on short geologic timescales. Fluid and vein analyses provide boundary conditions for geochemical modeling using 'Phreeqc' to determine why basement fluids are saturated with respect to CaCO₃. We evaluate the controlling effects of key parameters including fluid dissolved inorganic carbon (DIC), Mg and Ca concentrations, alkalinity, temperature, and pH on CaCO₃ precipitation. Our results provide insights into how differing past conditions (e.g. ocean temperature and pCO₂) affected the ocean crustal C-reservoir, and enable better quantification of the magnitude of this reservoir and how it has varied over geological time. This information is directly applicable to the development of industrial scale CO₂ sequestration using methods that mimic but accelerate natural CO₂-drawdown processes.