Question

When did oxygen levels start to decline from 35 percent down to today's levels during what period

Answer 1

about 95 million years ago, during the Carboniferous Period

Answer 2

During Carboniferous period, part of the late Paleozoic era.

Step-by-step explanation:

In the geological story oxygen levels have been relatively stable for the las 200 million of years, but 300 million ago there was a dramatic spike in Oxygen levels until 35% in the Carboniferous , next it plunged in 15 percent in the Triassic, 100 million years later. The growth of the forests removed huge amounts of carbon dioxide from the atmosphere, leading to a surplus of oxygen. The higher oxygen concentration is related with the presence of larger bodies in reptile and insects.