When did the oceans begin to have chemical characteristics similar to those of modern oceans?

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Map graphic showing the boundaries of the five oceans-Pacific, Atlantic, Indian, Arctic, and Southern-using color-coded regions. Four different map projections are included: North Pole view, South Pole view, International Date Line view, and Greenwich Meridian view, each displaying the oceans' extents around the continents. — ocean Earth's oceans and their marginal seas cover nearly 71 percent of the planet's surface.

The oceans are estimated to have achieved their modern chemical characteristics between 2 billion and 1.5 billion years ago.12

Evidence for this transition is supported by the composition of sedimentary rocks from this period, which show mineralogical and chemical similarities to those from the Paleozoic Era. By this time, acidic sulfur gases had been neutralized into sulfate, and the presence of chemically precipitated ferric oxides in late Precambrian rocks indicates the availability of free oxygen.13

Since this period, the oceans have functioned as a steady-state chemical system, where the rates of material inflow from continental drainage and volcanic activity are balanced by removal processes, maintaining a relatively fixed chemical composition.13