was less efficient because of the low oxygen concentration in the atmosphere and limited biological
weathering. The supply of ions to the ocean via rivers
was consequently less. On the other hand, seafloor
spreading was most probably faster with more seawater circulated through the spreading ridges. Isotope
studies (
87 Sr/
86 Sr) of seawater in early Precambrian
rocks indicate that at that time the composition of
seawater was more strongly controlled by circulation
through the basalt on the spreading ridges. We can say
that the chemical composition of the ocean was buffered by material from the spreading ridge, i.e. mantle
material (Veizer 1982).
3.10 Clastic Sedimentation in the
Oceans
Clastic sediments are produced chiefly on the
continents and are brought to ocean areas through
fluvial or aeolian transport. Island arcs associated
with volcanism may produce large amounts of sediment compared to their area because they are tectonically active, which leads to elevation and accelerated
erosion. Volcanic rocks are for the most part basic and
weather quickly, forming large quantities of sediments
around volcanic island groups, while fine-grained volcanic ash becomes spread over wide areas. Submarine
volcanism may also produce some sediment, for
example along the Mid-Atlantic Ridge, but this is
very limited.
The main supply of clastic sediment is fed into the
ocean through deltas, then transported along the coast
and down the continental slope to the abyssal plains.
Around Antarctica there is a significant amount of
deposition of clastic, glacial sediments. In areas in
the middle of the Atlantic Ocean, far from land, the
rate of sedimentation is as low as 1–10 mm/1,000
years. The Atlantic receives a relatively large supply
of clastic sediment, in particular from seven major
rivers: the St. Lawrence, Mississippi, Orinoco, Amazon, Congo, Niger and Rhine. Exceptionally high
Evaporation
............................................................
Anoxic
Pycnocline
Low salinity
Organic production
Freshwater
Addition of
nutrients
Positive water balance
Black Sea model
Threshold
Negative water balance
Mediterranean and Red Sea model
>
Supply of freshwater
Threshold
Oxic
Heavier
saltwater
Fig. 3.17 Circulation of water in basins with a surplus of
freshwater input compared to evaporation (Black Sea model).
This results in poor vertical circulation and anoxic (reducing)
bottom waters. The Mediterranean and Red Sea model
represents excess evaporation compared to freshwater input.
The surface water will then have the highest salinity and density
and sink to the bottom. This increases the vertical circulation
and helps to maintain oxic conditions
3 Sedimentary Geochemistry
115
weathering. The supply of ions to the ocean via rivers
was consequently less. On the other hand, seafloor
spreading was most probably faster with more seawater circulated through the spreading ridges. Isotope
studies (
87 Sr/
86 Sr) of seawater in early Precambrian
rocks indicate that at that time the composition of
seawater was more strongly controlled by circulation
through the basalt on the spreading ridges. We can say
that the chemical composition of the ocean was buffered by material from the spreading ridge, i.e. mantle
material (Veizer 1982).
3.10 Clastic Sedimentation in the
Oceans
Clastic sediments are produced chiefly on the
continents and are brought to ocean areas through
fluvial or aeolian transport. Island arcs associated
with volcanism may produce large amounts of sediment compared to their area because they are tectonically active, which leads to elevation and accelerated
erosion. Volcanic rocks are for the most part basic and
weather quickly, forming large quantities of sediments
around volcanic island groups, while fine-grained volcanic ash becomes spread over wide areas. Submarine
volcanism may also produce some sediment, for
example along the Mid-Atlantic Ridge, but this is
very limited.
The main supply of clastic sediment is fed into the
ocean through deltas, then transported along the coast
and down the continental slope to the abyssal plains.
Around Antarctica there is a significant amount of
deposition of clastic, glacial sediments. In areas in
the middle of the Atlantic Ocean, far from land, the
rate of sedimentation is as low as 1–10 mm/1,000
years. The Atlantic receives a relatively large supply
of clastic sediment, in particular from seven major
rivers: the St. Lawrence, Mississippi, Orinoco, Amazon, Congo, Niger and Rhine. Exceptionally high
Evaporation
............................................................
Anoxic
Pycnocline
Low salinity
Organic production
Freshwater
Addition of
nutrients
Positive water balance
Black Sea model
Threshold
Negative water balance
Mediterranean and Red Sea model
>
Supply of freshwater
Threshold
Oxic
Heavier
saltwater
Fig. 3.17 Circulation of water in basins with a surplus of
freshwater input compared to evaporation (Black Sea model).
This results in poor vertical circulation and anoxic (reducing)
bottom waters. The Mediterranean and Red Sea model
represents excess evaporation compared to freshwater input.
The surface water will then have the highest salinity and density
and sink to the bottom. This increases the vertical circulation
and helps to maintain oxic conditions
3 Sedimentary Geochemistry
115