The diagram shows a simplified box-and-arrow model representing the reservoirs and flows of oxygen (O2O_2O2) during the early history of the Earth.
Despite the evolution of oxygenic photosynthesis by cyanobacteria, atmospheric oxygen levels did not rise significantly for hundreds of millions of years. Which of the following biogeochemical processes is primarily responsible for the massive oxygen sink labeled Sink X in the diagram?
The chemical weathering of continental silicate rocks, which directly binds free oxygen to form carbonate minerals in the lithosphere.
The oxidation of dissolved ferrous iron (Fe2+Fe^{2+}Fe2+) in the anoxic oceans to ferric iron (Fe3+Fe^{3+}Fe3+), precipitating insoluble iron oxides.
The microbial consumption of oxygen during the aerobic respiration of early eukaryotic multicellular decomposers.
The thermal decomposition of atmospheric carbon dioxide (CO2CO_2CO2) by solar UV radiation, forming carbon monoxide (COCOCO) that consumes oxygen.