Sodium oceanic mantle

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

The controls on carbon dioxide would have been somewhat different. Today, carbon dioxide is stored in carbonate minerals in the ocean floor and on the continental shelf. Subduction, followed by volcanism, cycles the carbon dioxide to the mantle and then restores the CO2 to the air. Metamorphic decarbonation of the lower crust also returns carbon dioxide. The carbon dioxide is then cycled back to the water, some via rain, some dissolved via wave bubbles. Erosion provides calcium and magnesium, eventually to precipitate the carbonate. In the earliest Archean, parts of this cycle may have been inefficient. The continental supply of calcium may have been limited however, subseafloor hydrothermal systems would have been vigorous and abundant, exchanging sodium for calcium in spilitization reactions, and hence providing calcium for in situ precipitation in oceanic crust. 

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