Earth’s Ancient Atmosphere – Understanding how anaerobic and aerobic bacteria evolve teaches us how modern Earth ecology works. Oxygen is the key turning point in Earth’s bacterial history. The GOE is one of the turning points in Earth’s creation of life. Below is a pictorial summary when free oxygen appeares in Earth’s atmosphere. The rest of the post takes you through most of Earth’s atmospheric history to the present.
The first atmosphere consists of gases in the solar nebula, primarily hydrogen. In addition there are simple hydrides such as are now found in gas-giant planets like Jupiter and Saturn, notably water vapor, methane and ammonia. As the solar nebula dissipated, these gases escape, partly driven off by the solar wind.
Earth’s early atmosphere contained only small amounts of free oxygen, probably produced entirely by the reaction of sunlight with water vapor from volcanoes. The oxygen-rich atmosphere that evolved later, and upon which oxygen-breathing life now depends, was a result of the origin of photosynthesis. During the Precambrian, vast numbers of single-celled algae and cyanobacteria living in the seas eventually released enough oxygen to transform the environment. Cyanobacteria use water, carbon dioxide, and sunlight to create their food. A layer of mucus often forms over mats of cyanobacteria cells. In modern microbial mats, debris from the surrounding habitat can become trapped within the mucus, which can be cemented together by the calcium carbonate to grow thin laminations of limestone. These colonies of cyanobacteria create mats after dying called Stromatolite. Pictured below is a modern landscape of Stromatolite located in Western Australia.
The oldest evidence of cyanobacteria dates to 2.7 billion years ago, although oxygen did not begin to build up in the environment until about 2.3 billion years ago. Below are two pictures of cyanobacteria in modern Stromatolites. When Stromatolites die, they leave a hard shell making the black stumps.
Anaerobic bacteria appears on the scene around 3.8 Billion years ago. They are single cell bacteria belonging to the prokaryotes. It’s characterized by the absence of a nuclear membrane and by DNA that is not organized into chromosomes. Anaerobic means without oxygen. At this time, very little oxygen is present in the atmosphere. The atmosphere mainly consisted of hydrogen (about 60%), methane and some nitrogen.
Pictured below is a prokaryote.
Pictured below is a Bradyrhizobium; a rod prokaryotic nitrogen-fixing soil Bacteria that forms a symbiotic relationship with nodulated plant root systems, better known as legumes.
The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen is first produced by photosynthetic organisms (prokaryotic, then eukaryotic) which emits oxygen as a waste product. These organisms lived long before the GOE,(about as early as 3,500 million years ago). The oxygen they produce is quickly removed from the atmosphere by the weathering of reduced minerals, most notably iron. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a draw-down, oxygen could accumulate very rapidly.
Free oxygen is toxic to anaerobic organisms and the rising concentrations may have wiped out most of the Earth’s anaerobic inhabitants at the time. During the transition from oxygen-poor to oxygen-rich atmosphere, the first banded iron formations may have formed. Banded iron formations are silica-rich rocks that show alternating thin layers of dark and red iron-rich rock. The silica probably was dissolved from volcanic ash and rock, and the iron came from sea floor vents or the weathering of iron-rich volcanic rocks. In the absence of free oxygen, iron dissolves in water. This must have occurred throughout the Archean, resulting in ocean waters that contained a great deal of dissolved iron. In the Proterozoic, however, the dissolved iron bonded with oxygen released into ocean water by photosynthesizing cyanobacteria to form magnetite (Fe3O4). This magnetite was then deposited on the ocean floor. The alternating layers in banded iron formations are thought to reflect the alternation of oxygen-rich and oxygen-poor conditions on the sea floor. A vast amount of iron dissolved in the oceans was available to react chemically with oxygen, which kept oxygen from accumulating in the ocean and atmosphere. Once all of the dissolved iron was used up, the oxygen released by photosynthetic organisms could escape directly into the atmosphere. As gaseous oxygen built up, the atmosphere began to change from one that was chemically reducing to one that was oxidizing (i.e., rust-forming), like today’s. Iron weathered from basaltic volcanoes was oxidized on land before it reached the oceans. This resulted in the formation of red beds. The red color of these rocks comes from the particular variety of iron mineral precipitated on land, mostly hematite (Fe2O3).Thus, the history of Earth’s early crust also tells the story of its early atmosphere. Banded iron formations were precipitated from about 3.1 to about 2 billion years ago—most (92%) during the Proterozoic between 2.5 and 2 billion years ago. Until all the available iron had been deposited in banded iron formations, oxygen could not build up in the atmosphere. Red beds appeared only after free oxygen was released into the atmosphere, beginning about 2.0 to 1.8 billion years ago. They are still being formed today. Additionally the free oxygen reacted with the atmospheric methane triggering the Huronian glaciation, possibly the longest snowball Earth episode.
There are a great many differences between Eukaryotic cells and Prokaryotic cells in size, complexity, internal compartments. The main difference is eukaryotic cells have a nucleus.
However, there is a curious similarity between prokaryotic cells and the organelles of eukaryotic cells. Some of these similarities were first noted in the 1880s, but were largely ignored for almost a century!
Aerobic bacteria appeared on the scene around 2.5 Billion years ago. Aerobic means with oxygen.
In the meantime around 1.5 billion years ago, Bangiomorpha pubescens appears. It is a red algae. It is the first known sexually reproducing organism. A multicellular fossil of Bangiomorpha pubescens was recovered from Arctic Canada that strongly resembles the modern red algae Bangia (freshwater) despite occurring in rocks dating to 1,200 million years ago. Pictured below are two fossilized red algae strands.
The next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases, was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids. A major rainfall led to the buildup of a vast ocean. A major part of carbon dioxide exhalations were soon dissolved in water and built up carbonate sediments.
Water-related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the then stable “second atmosphere”. An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago. The fact that this is not perfectly in line with the 30% lower solar radiance (compared to today) of the early Sun has been described as the “faint young Sun paradox”.
In the late Archaean eon an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotropy (isotope ratio proportions) is very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.
Below is a graph of the change of oxygen in the atmosphere over the past 1 Billion years.
The latest supercontinent on Earth is Pangea. As modern continents split up, oxygen is still rich enough to encourage higher forms of life. Dinosaurs existed between 300 million to 65 million years ago. Below is an illustration how the last supercontinent split into today’s smaller continents.
|Nitrogen (N2)||780,840 ppmv (78.084%)|
|Oxygen (O2)||209,460 ppmv (20.946%)|
|Argon (Ar)||9,340 ppmv (0.9340%)|
|Carbon dioxide (CO2)||394.45 ppmv (0.039445%)|
|Neon (Ne)||18.18 ppmv (0.001818%)|
|Helium (He)||5.24 ppmv (0.000524%)|
|Methane (CH4)||1.79 ppmv (0.000179%)|
|Krypton (Kr)||1.14 ppmv (0.000114%)|
|Hydrogen (H2)||0.55 ppmv (0.000055%)|
|Nitrous oxide (N2O)||0.325 ppmv (0.0000325%)|
|Carbon monoxide (CO)||0.1 ppmv (0.00001%)|
|Xenon (Xe)||0.09 ppmv (9×10−6%) (0.000009%)|
|Ozone (O3)||0.0 to 0.07 ppmv (0 to 7×10−6%)|
|Nitrogen dioxide (NO2)||0.02 ppmv (2×10−6%) (0.000002%)|
|Iodine (I2)||0.01 ppmv (1×10−6%) (0.000001%)|
|Not included in above dry atmosphere:|
|Water vapor (H2O)|