Photosynthesising organisms in the surface water
use up CO 2 and produce oxygen and organic matter:
CO 2 þ H 2 O þ nutrients ¼ CH 2 O þ O 2
This helps to keep the pH high so that carbonates
are stable or dissolve slowly. At depth in the absence
of sunlight, respiration and oxidation prevail, releasing
CO 2 and thus lowering the pH and increasing the
solubility of carbonate.
The depth at which the solubility of carbonate
increases relatively rapidly is called the lysocline.
The depth where the rate of solution is greater than
the rate of carbonate sedimentation is called the carbonate compensation depth (CCD). Near the equator
the CCD may be at 4–5 km, becoming shallower at
higher latitudes. Carbonate can therefore still accumulate on the seafloor even when the water column is
undersaturated, if the rate of supply exceeds the rate of
dissolution. This is analagous to snow accumulating
when the temperature is above freezing so long as it
falls quicker than it melts This solution of organic
material liberates nutrients which can then be returned
to the surface through upward flow. In this manner
they are repeatedly recycled. The annual biological
production in the ocean is therefore many times
greater than the supply of nutrients from the land.
That fraction of the organic production which has
been removed from the ocean by preservation in seabed sediments must be replaced with nutrients, mostly
from land. They cannot be returned to the ocean before
the sediments are elevated and subjected to erosion
and weathering. The amount of organic matter which
is deposited in sediments is a function of the rate of
production minus the rate of solution.
The growth of authigenic (newly formed) minerals
on the seabed (G in Fig. 3.16) is also an important
process in removing elements from seawater. The
most significant are the zeolites, which can develop
where sediments on the seabed have a high silicate or
aluminium content, particularly from volcanic material (glass). They may remove Na
+
, K
+ and Ca
2+ from
seawater, but growth can also proceed very much at
the expense of elements already present in the sediment (e.g. in the Pacific Ocean). This applies particularly to phillipsite, heulandite, clinoptilolite and
analcime.
Apart from this there is little direct chemical precipitation from seawater with normal salinity. This is
because biological precipitation is more efficient in
many cases and prevents the build-up of sufficient
concentrations of the elements needed for chemical
precipitation. Sulphate-reducing bacteria are active in
the uppermost few centimetres of the sediment, however, removing sulphur from seawater in the form of
sulphate and reducing it to sulphides which are then
precipitated (e.g. iron sulphides, FeS and FeS 2 ).
In areas that are almost cut off from the open ocean,
where evaporation exceeds the freshwater rainfall and
supply by rivers, we find that even very soluble salts
are being precipitated. Under these conditions, there is
enrichment of elements which are otherwise
precipitated only to a limited degree through
biological or chemical processes, such as Na, Cl, S,
Mg and trace elements such as B and Br. The amount
of salt thus precipitated in evaporites has probably
varied very markedly throughout the geological ages.
Biological and chemical precipitation, as described
above, are not sufficient alone to account for the geochemical balance of the ocean. Important geochemical
reactions are taking place in the spreading ridges in the
oceans (F in Fig. 3.16). Heat from the basalt that is
flowing up along the spreading ridge, drives convection cells which cause ocean water to flow through the
basalts and up along the ridge. The seawater reacts
with hot basalt (basic rock melt) and disolves minerals
containing iron and other metals. Since seawater
contains sulphur (as S0 4
À ), this leads to precipitation
of sulphides, for example iron sulphides and copper
sulphides. When hot water flows up pipe-like
chimneys (black smokers) in the ocean floor near
spreading ridges, it mixes with the seawater and
again sulphides are precipitated. When water is
oxidised, iron oxides and manganese oxides are
precipitated around the spreading ridges.
3.8.1 Residence Periods for Different
Elements in the Ocean
How long does an element spend in the sea after being
delivered by rivers, before it is chemically or biologically precipitated? This residence period is an expression of how rapidly an element is removed compared
to its concentration in the ocean. For example, sodium
has the longest period of residence (about 55 million
years) because little sodium is removed through
biological or chemical precipitation except in
3 Sedimentary Geochemistry
113
use up CO 2 and produce oxygen and organic matter:
CO 2 þ H 2 O þ nutrients ¼ CH 2 O þ O 2
This helps to keep the pH high so that carbonates
are stable or dissolve slowly. At depth in the absence
of sunlight, respiration and oxidation prevail, releasing
CO 2 and thus lowering the pH and increasing the
solubility of carbonate.
The depth at which the solubility of carbonate
increases relatively rapidly is called the lysocline.
The depth where the rate of solution is greater than
the rate of carbonate sedimentation is called the carbonate compensation depth (CCD). Near the equator
the CCD may be at 4–5 km, becoming shallower at
higher latitudes. Carbonate can therefore still accumulate on the seafloor even when the water column is
undersaturated, if the rate of supply exceeds the rate of
dissolution. This is analagous to snow accumulating
when the temperature is above freezing so long as it
falls quicker than it melts This solution of organic
material liberates nutrients which can then be returned
to the surface through upward flow. In this manner
they are repeatedly recycled. The annual biological
production in the ocean is therefore many times
greater than the supply of nutrients from the land.
That fraction of the organic production which has
been removed from the ocean by preservation in seabed sediments must be replaced with nutrients, mostly
from land. They cannot be returned to the ocean before
the sediments are elevated and subjected to erosion
and weathering. The amount of organic matter which
is deposited in sediments is a function of the rate of
production minus the rate of solution.
The growth of authigenic (newly formed) minerals
on the seabed (G in Fig. 3.16) is also an important
process in removing elements from seawater. The
most significant are the zeolites, which can develop
where sediments on the seabed have a high silicate or
aluminium content, particularly from volcanic material (glass). They may remove Na
+
, K
+ and Ca
2+ from
seawater, but growth can also proceed very much at
the expense of elements already present in the sediment (e.g. in the Pacific Ocean). This applies particularly to phillipsite, heulandite, clinoptilolite and
analcime.
Apart from this there is little direct chemical precipitation from seawater with normal salinity. This is
because biological precipitation is more efficient in
many cases and prevents the build-up of sufficient
concentrations of the elements needed for chemical
precipitation. Sulphate-reducing bacteria are active in
the uppermost few centimetres of the sediment, however, removing sulphur from seawater in the form of
sulphate and reducing it to sulphides which are then
precipitated (e.g. iron sulphides, FeS and FeS 2 ).
In areas that are almost cut off from the open ocean,
where evaporation exceeds the freshwater rainfall and
supply by rivers, we find that even very soluble salts
are being precipitated. Under these conditions, there is
enrichment of elements which are otherwise
precipitated only to a limited degree through
biological or chemical processes, such as Na, Cl, S,
Mg and trace elements such as B and Br. The amount
of salt thus precipitated in evaporites has probably
varied very markedly throughout the geological ages.
Biological and chemical precipitation, as described
above, are not sufficient alone to account for the geochemical balance of the ocean. Important geochemical
reactions are taking place in the spreading ridges in the
oceans (F in Fig. 3.16). Heat from the basalt that is
flowing up along the spreading ridge, drives convection cells which cause ocean water to flow through the
basalts and up along the ridge. The seawater reacts
with hot basalt (basic rock melt) and disolves minerals
containing iron and other metals. Since seawater
contains sulphur (as S0 4
À ), this leads to precipitation
of sulphides, for example iron sulphides and copper
sulphides. When hot water flows up pipe-like
chimneys (black smokers) in the ocean floor near
spreading ridges, it mixes with the seawater and
again sulphides are precipitated. When water is
oxidised, iron oxides and manganese oxides are
precipitated around the spreading ridges.
3.8.1 Residence Periods for Different
Elements in the Ocean
How long does an element spend in the sea after being
delivered by rivers, before it is chemically or biologically precipitated? This residence period is an expression of how rapidly an element is removed compared
to its concentration in the ocean. For example, sodium
has the longest period of residence (about 55 million
years) because little sodium is removed through
biological or chemical precipitation except in
3 Sedimentary Geochemistry
113