An evolving atmosphere
About 1.7 billion years ago, banded iron formations — sedimentary rocks consisting of iron-rich layers alternating with iron-poor ones — stopped forming. By this time, photosynthesis had supplied enough oxygen to entirely deplete the oceans of their iron. With no more iron available to remove the oxygen, the gas slowly began to accumulate in the atmosphere, increasing to perhaps 2 percent near the end of the Precambrian (560 million years ago), and eventually to its present level of 20 percent.
Rocks the reveal conditions of early Earth
Clues to the conditions on the early Earth are found in rocks like the Huronian Supergroup, a set of sedimentary rock formations, up to 12 kilometers thick, laid down between 2.5 and 2.2 billion years ago. The rocks are exposed just north of Lake Huron, Canada. Many geologists believe that this sequence of rocks records a change from an oxygen-free to an oxygen-bearing atmosphere.
Topic: Earth Science
Subtopic: Climate/Climate Change
Keywords: Geology, Stratigraphic--Huronian, Huron, Lake (Mich. and Ont.), Photosynthesis, Oxygen, Atmosphere, Paleoclimatology, Life (Biology), Life--Origin, Sedimentary rocks
Clues to the conditions on the early Earth are found in rocks like the Huronian Supergroup, a set of sedimentary rock formations, up to 12 kilometers thick, laid down between 2.5 and 2.2 billion years ago.
This quartz-pebble conglomerate is from one of the oldest layers of the Huronian Super-group.
This quartzite formed in a riverbed, as indicated by the well-developed cross-bedding (sedimentary layers at angles to the main horizontal layers).
By observing other stars, we think that the Sun’s luminosity has increased by 25 percent since its formation.
The rocks in the upper part of the Huronian Supergroup are red-brown, like this quartzite specimen.