ATOC 530 : Climate Dynamics I

ATOC 530 : Climate Dynamics I

Chemical Weathering

There are two main types of chemical weathering that are most common in the environment. We saw in class that there is Hydrolysis and Dissolution. These processes disintegrate minerals in rocks but they also help remove carbon dioxide from the atmosphere. In Both cases, the weathering process cannot proceed without the initial formation of acid. Carbon dioxide dissolves readily in rain water as follows, to produce the most commonly used acid, carbonic acid :

H2O + CO2 => H2CO3

Carbonic acid reacts with silicate(common to many rocks) to produce carbonate and silica in the hydrolysis reaction:

CaSiO3 + H2CO3 => CaC03 + SiO2+ H2O

Carbonation helps in dissolving limestone (calcite) to form clay in the dissolution process:

H2CO3 + CaCO3 => CaCO3 + H2O + CO2

Chemical weathering accounts for an exchange of 0.2 Gigatons of carbon between the atmosphere and the solid rock reservoir every year (the same amount that volcanoes put into it on average). It is however climate dependent. When the temperature increases, chemical weathering also increases by a factor of 2. When the moisture content of the atmosphere increases (ie. Precipitation) weathering also increases. Also there is 2-10 times more chemical weathering in areas of dense vegetation than on bare land. Chemical weathering works on long timescales (millions of years) and makes for a very slow negative feedback to a warming climate.

Geological Time Scales

The past has been divided into four different time scales. There are four different EONS:

Hadean, Archean (presence of unicellular life), Proterozoic (Snowball earth and multicellular life) and Phanerozoic “Mega Trees” (spans from 550Ma - present) The latter eon is divided into three ERAS:

Paleozoic (Break up of Pangaea occurred - 300Ma), Mesozoic (Dinosaurs and the formation of the Rockies - 100Ma) and Cenozoic (Mammal dominated). Each of them divided again into PERIODS, totaling 11.

The most recent two periods are also divided again into EPOCHS.

Only withing the last 10 Ma at the earliest did anything like the human walk the earth, this is only 0.2% of the earths history.

The take home message of the geological timescales is that the more recent the time the more divided and subdivided the categories of timescales become. This is due to the precise records we have of the recent past, and the more sparse data records we have to time long ago.

Sarah Dyck