FIGURE 7.6 The sulfur cycle. Most sulfur exists as rocks. As these rocks are weathered over time, they release sulfate ions (SO₄^2–) that producers can take up and assimilate. This assimilated sulfur then passes through the food web. Volcanoes, the burning of fossil fuels, and the mining of copper put sulfur dioxide (SO₂) into the atmosphere. In the atmosphere, sulfur dioxide combines with water to form sulfuric acid (H₂SO₄). This sulfuric acid is carried back to Earth when it rains or snows.