22.3 – Formation of the Atmosphere and Oceans

Formation of the Atmosphere Atmosphere began to form as Earth formed Objects that collided with Earth probably contained water that would have vaporized on impact forming a haze around the planet

Formation of the Atmosphere Atmosphere changed with addition of volcanic gases Volcanic eruptions release large quantities of gasses Lots of volcanic activity during the Precambrian

Outgassing Present-day volcanoes release large amounts of water vapor, carbon dioxide, and trace amounts of nitrogen and other gases in a process called outgassing Earth’s early atmosphere probably contained the same gases that vent from volcanoes today

Oxygen in Atmosphere Only negligible amounts of free oxygen in Earth’s atmosphere until the early Proterozoic

First Oxygen Producers Fossils preserved in rocks that are about 3.5 billion years old have traces of tiny, threadlike organisms called cyanobacteria Ancient cyanobacteria used photosynthesis and produced the nutrients they needed to survive Oxygen gas is a waste product of the process of photosynthesis

Cyanobacteria Could have produced enough oxygen to change the composition of the atmosphere that existed on Earth during the Archean By the early Proterozoic, large, coral reef-like mounds of cyanobacteria called stromatolites dominated the shallow oceans that covered most of Earth’s continents

Evidence in Rocks Late Archean rocks do not contain iron oxides This indicates that no free oxygen gas was in the atmosphere at that time Cyanobacteria increased oxygen gas levels enough for iron oxides to form

Iron Oxides Formed in localized areas called banded-iron formation Consist of alternating bands of iron oxide and chert (an iron-poor sedimentary rock) Red beds in sedimentary rock deposits provide evidence that free oxygen existed in the atmosphere during the Proterozoic.

Importance of Oxygen Most animals require it for respiration Also provides protection from harmful ultraviolet radiation (UV) from the Sun

Ozone As oxygen accumulated in Earth’s atmosphere, an ozone layer began to develop Ozone filtered out much of the UV radiation, providing an environment where new life-forms could develop

Formation of the Oceans Water in oceans probably originated from two sources Volcanic outgassing Objects bombarding Earth As Earth cooled, the water vapor in the atmosphere condensed to form liquid water

Rain As liquid water formed, rain began to fall It filled the low-lying basins and eventually formed the oceans

Water and Life Precambrian began with an environment not suitable for life It ended with much of Earth was covered with oceans that were teeming with tiny cyanobacteria and other life-forms. Life as it exists on Earth today cannot survive without liquid water The search for life elsewhere in the solar system and universe today is centered on the search for water

In-class Assignment/Homework