Solve this riddle." i am the first on earth, the second in heaven, i appear two times in a week, you can only see me once in a year although i am in the middle of the sea, what am i?

Why is the huronian supergroup rock formation particularly interesting to scientists? because it looks like a cross-section of a giant, stone encyclopedia because it formed during the period when oxygen began to accumulate in the atmosphere because it contains unusually large amounts of oxygen and sulfur because it dramatically altered the planet's development when it first formed