CHAPTER 4 – CHEMICAL WEATHERING
SECTION 1 – BIOGEOCHEMICAL CYCLES
The biogeochemical cycle
Water near the land surface can flow via different pathways picture 45:
The entering of water to the ground is by rainfall and throughfall (water falls on the
leave after which is falls on the ground).
The leaving of water is by surface runoff (water passing directly into the nearest
stream), evaporation, transpiration (evaporation from vegetation) and groundwater.
o Groundwater moves to below the water table; the zone were the pore spaces are
completely filled with water (saturated zone)
The level of the water table depends on the season; during summer there is more
evaporation and the water tables lowers with 10’s of meters.
o When the water table intersects with the land surface, the groundwater becomes
surface water to form springs, rivers and lakes.
The base flow is the contribution of groundwater to rivers between rain events.
The biogeochemical cycle is the interaction of the hydrologic cycle with the biological
cycle. The biological cycle is the reservoir of nutrient elements needed by living thing;
elements essential for nutrition of plants are mostly carbon, oxygen, hydrogen and
nitrogen. Each element provides a specific function picture 46.
The nutrients become available from internal and external sources, after which they will
be stored in trees, plants and other organic matter or lost in groundwater and streams.
picture 47.
The biological activity shows the ratio of stored living and dead mass to the annual
output from the system. A high biological active element has a high concentration and
is a essential biological element, like P, N en K. A low biological active element has a
low concentration and is a non-essential biological element, like Mg and Na.
Important for growth is a well balanced N and P ratio; only a supply of N will cost a lot
of extra water usage, which can make the ground dryer.
The rate of turnover of nutrients via the decomposition of litter varies with the type of
vegetation and climate. At high latitudes the turnover rate is around 353 years, while in
the tropical rain forest it is around 0.4 years.
Comparison with the residence time of soil organic matter, a few 1000 years, shows
the elements can be better stored in soils, like wetland.
Soil water and micro-organisms: acid production
The soil type is determined by contributions and formation factors: organisms,
topography, time, climate and parent material. The soils are produced by a combination
of physical and chemical weathering:
The physical weathering is the breaking of materials into smaller pieces due to the
effects of temperatures, jointing, root wedging, salt wedging, trampling
The chemical weathering is the dissolution of minerals in soils and rocks due to
mineral reactions under influence of acid attack and/or oxidation and reduction
o Primarily by acids: acid production aids weathering reactions
,!!! Physical and chemical weathering work together; physical weathering creates a bigger
surface for chemical weathering to have effect on.
For the transformation of rainwater into soil water several factors play a big rol:
Weathering
Evapotranspiration: evaporation and transpiration
Dry deposition of aerosols: the drying of aerosols cause them to react with minerals
Vegetation and micro flora: they have an enhanced aerosols uptake, they are an
acid generation and they participate in biological cycling.
o For example: the role of mycorrhiza (fungi) at the roots of plants: enhancement of
weathering, linking of plants, transfer of nutrients and carbon
!!! For bugdets include evapotranspiration, biogeochemical cycling and weather.
The chemical composition of soil water is affected by the inputs from throughfall, rainfall,
additions from rock weathering and inputs and removals due to biological activity. At the
same time, soil microorganisms (bacteria, fungi and algae) cause the production of soil
acids and organic acids.
The soil acids release hydrogen ions (H+) which replace cations (Ca2+, Mg2+, K+.
Na+, NH4+) on mineral surfaces. In this way, they are the principal agents of rock
weathering.
o Produced soil acids are carbonic acids (H2CO3) and sulfuric acids (H2SO4)
The organic acids are formed by a variety of processes; different kind of acids
o Organic acids with a low molecule mass, weak acids, are formic acid (HCOOH),
acetic acid (CH3COOH) and oxalic acid (COOH)2.
o Organic acids with a high molecule mass are humic acids and fulvic acids.
!!! Depth variation in soil composition picture 48 and 49. At the throughfall; high
concentrations of K and C, which shows that these elements are reused. At the depth of
30 cm; high concentration of Al due to the present of oxalic acids picture 50
The depth variations gives us different layers in the soils, called horizons. Each horizon
(O A B) has his own reactions; mostly organic acids which form complexes with otherwise
isoluble cations pictures 51, 52 and 53
Horizon O and horizon A are the upper humus rich surface layer, horizon B is the
zone of accumulation
Zone A and B work together to create a net reaction in which the dissolved products in
ratio are Na+ : HCO3- : H4SiO4 = 1:1:2
Soil formation
The soil is formed as a result of both physical and chemical weathering. Soil is a complex
system of air, water, decomposing organic matter, rock weathering, living plants and
animals organized into structural patterns by environmental conditions.
, SECTION 2 – CHEMICAL WEATHERING
Chemical Weathering
The chemical weathering is the dissolution of minerals in soils and rocks due to mineral
reactions under influence of acid attack and/or oxidation and reduction.
A primary mineral is a mineral that undergoes destruction by weathering; mainly
silicates and carbonates
A secondary mineral is a minerals that is formed by weathering; mainly oxides, clay
minerals, calcite and gypsum.
o Important oxides are hematite, goethite and gibbsite; important clay minerals are
kaolinite, smectite and vermiculite picture 54
Weathering reactions are classified according to the nature of the attacking substance
(soil acids, dissolved oxygen and water itself), and which of the reactions occurs picture 55
A congruent dissolution is simple dissolution; where the primary minerals undergo
complete dissolution and all the molecules are used. This weathering reaction is rare.
o Primary mineral + acid + water cations + dissolved silica (H4SiO4) + bicarbonates
(HCO3-)
o For example the weathering of Olivine; Mg2SiO4 + 4H2CO3 2Mg2+ + H4SiO4 + 4HCO3-
A incongruent dissolution is the weathering reaction where only parts of the mineral
undergo dissolution. This weathering reaction is common.
o Primary mineral + carbonic acid (H2CO3) + water (H2O) secondary mineral +
cations + dissolved silica (H4SiO4) + dissolved bicarbonates (HCO3-)
o !!! Which secondary mineral is formed is determined by the soil solution
composition (2) and degree of weathering (1)
o For example the weathering of albite into kaolinite: 2NaAlSi3O8 + 2H2CO3 + 9 H2O
Al2Si2O5(OH)4 + 2HCO3- + 2Na+ + 4H4SiO4
Degree of Weathering (1)
Minerals differ in the degree of resistance to weathering; the rate in which they weather.
The way the minerals weather is determined by the crystal structure; dissolution occurs
at outcrops of dislocations where there is excess energy. Bigger the density of
dislocations, bigger the dissolution percentage.
A dislocation is a row of atoms in a crystal that is slightly out of place and is therefore
more energetic picture 56. The place of this imperfections is where weathering will first
occur and etch pits are formed and further grown.
The weathering rates are determined by temperature, precipitation and mineralogy.
The temperature determines the rate of minerals dissolution; with higher temperature
comes a higher weathering rate picture 57.
The precipitation determines the amount of flushing of the soil
o The faster a rock is flushed with water, the shorter the time of contact with the primary
minerals and the lower the dissolved concentrations in the standing waters
The mineralogy determines the kind of mineral that is available for weathering, and
thus the density of dislocations
o picture 58 Quickest weathering of Mg- and Ca-rich, silica poor minerals
o !!! The role of fungi in weathering: preference for minerals that contain nutrient
elements. For example more weathering in potassium feldspar, than quartz picture
59.
