The diagram below outlines a key geochemical cycle representing the interaction between different Earth systems (the atmosphere, hydrosphere, and lithosphere).
Which of the following descriptions represents the primary geochemical pathway through which carbon dioxide was continuously removed from Earth’s early, volcanic atmosphere and sequestered long-term into the lithosphere, prior to the widespread expansion of photosynthetic life?
Photo-dissociation of CO2\text{CO}_2CO2 by solar UV radiation into solid carbon, which precipitated directly onto ocean beds to form early organic shales.
Dissolution of CO2\text{CO}_2CO2 in rainwater to form carbonic acid, chemical weathering of continental silicate rocks to release metal cations, and the subsequent precipitation of carbonate minerals on the seafloor.
Direct geochemical absorption of CO2\text{CO}_2CO2 by molten oceanic basalts during hydrothermal circulation, converting iron oxides into iron carbonates (siderite) within the deep mantle.
Biological fixation of CO2\text{CO}_2CO2 by anaerobic methanogens in the deep crust, converting carbon dioxide into methane which was then permanently sequestered as clathrates in the seafloor.