The Early Earth Atmosphere and Early Life Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 3

came from oxygen-rich molecules, like CO 2 , that were broken apart by the Sun's
ultraviolet radiation. Then came the dioxygen (O 2 ) released as a by-product of plant
life activity. Early plant life in the oceans removed most of the remaining CO 2 by
converting it into dioxygen by photosynthetic processes. As O 2 , in the triplet
groundstate, is a thermodynamically highly reactive molecule, the early atmospheric
O 2 combined quickly with many elements on the Earth's surface, notably metals, to
form metallic oxides such as iron oxides or silicon oxides. About 2 10 9 years ago,

after all of the surface minerals that could combine with O 2 had done so, the
atmosphere began to fill with this gas, and it became the nitrogen-oxygen mixture
that we breathe today [ 1 ].
The early Earth atmosphere was a mildly reducing chemical mixture, whereas
the present atmosphere is strongly oxidizing. It has been estimated that the current
level of O 2 in the atmosphere was achieved about 400 millions years ago [ 3 ].

1.1 Earth's Internal Structure

Seismic studies reveal that the Earth's interior is divided into three main layers
differentiated by its rheological properties: the core, the mantle, and the crust.
The core is made of material of the highest density. It consists primarily of nickel
and iron alloys. The Earth's core has two distinct regions, the solid inner core and
the molten outer core. The core's density varies from 13.1 to 9.9 g/cm 3 . The mantle
is formed by rocky material of moderate density, mostly silicate minerals rich in
silicon, magnesium, and oxygen. Its density varies from 5.6 to 3.4 g/cm 3 .
The mantle surrounds the core and makes up most of the Earth's volume. The
uppermost layer consist of the lowest-density rocks such as granite and basalt
whose density value can vary from 2.9 to 2.2 g/cm 3 , and forms the thin crust that
is the Earth's outer skin [ 4 ].
In terms of rock strength, geologists define the Earth's outer layer as a relatively
cool and rigid rock that floats on warmer, softer rock. This layer is called lithosphere,
and encompasses the crust and the upper part of the mantle. The crust occupies less
than 1 % of the Earth's volume and can be differentiated into the oceanic crust
and the continental crust. The oceanic crust is made of high-density igneous rocks
such as basalt, diabase (also called dolorite), and gabbro. It is only 5-10 kilometers
thick and radiometric dating shows that it is quite young, with an average age of about
70 million years. Even the oldest seafloor crust is less than 200 million years old [ 5 ].
The continental crust thickness varies from 30 to 50 kilometers and is mostly
composed of less dense rocks such as granite. The oldest continental crustal rocks
on Earth have ages in the range of 3.7 to 4.28 10 9 years. The continental crust has

an average composition similar to that of andesite. It is enriched in elements such as
oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium,
principally found as oxides [ 5 , 6 ].