The total amount of organic matter that can be
produced in the ocean is dependent on the nutrient
supply from rivers, but river water does not only
carry inorganic nutrients. It also contains significant
amounts of organic matter, in particular humic acid
compounds, lignin and similar substances formed by
the breakdown of plant material which are weakly
soluble in cold water. When the river water enters
the sea, there is precipitation due to the increased pH
and lower surface temperature in the ocean.
Other plant materials, like waxes and resins, are
more chemically resistant to breakdown and are insoluble in water. Such organic particles tend to attach
themselves to mineral grains and accompany sediment
out into the ocean.
Most of the oil reservoirs which have been formed
since the Palaeozoic have been uplifted and eroded,
and over time vast quantities of oil have flowed
(seeped) out onto the land or into the sea. In this
sense, oil pollution is a natural process. Only a small
proportion of the petroleum that has been formed in
source rocks has actually become trapped in a reservoir. One might expect this seepage to have provided a
source of recycled petroleum in younger sediments,
but petroleum breaks down extremely rapidly when
subjected to weathering, oxidising to CO 2 , and the
nutrients (P, N) that were required to form the organic
matter are released and may act like a fertilizer.
On land, evaporation will remove the lighter
components while bacteria will degrade the heavier
components. Fossil asphalt lakes consist of heavy
substances which neither evaporate nor can be easily
broken down by bacteria. In the ocean, the lighter
components will dissolve quite rapidly, while the
heavier asphalt fraction will sink to the bottom and
be degraded and recycled.
In uplifted sedimentary basins like the Ventura
Basin and the Los Angeles Basin in Southern
California there are abundant natural oils seeps both
onshore and offshore.
On the beaches from Santa Barbara towards Los
Angeles there are many natural oil seeps.
1.3
Breakdown of Organic Matter
Almost all (>99%) of the organic matter which is
produced on land and in the oceans is broken down
through direct oxidation or by means of
microbiological processes. If oxygen is present,
organic matter will be broken down in the following
manner:
CH 2 O þ O 2 ! CO 2 þ H 2 O
Where oxygen is available, organic matter is
oxidised relatively rapidly both on land and in the
sea. As organisms die, organic material suspended in
seawater sinks through the water column consuming
oxygen. If water circulation is restricted due to density
stratification of the water column, the oxygen supply
will be exhausted. Instead, the bound oxygen in
sulphates or nitrates is used by sulphate-reducing and
denitrifying bacteria which decompose organic material in an anoxic environment. The first few
centimetres below the seabed are usually oxidised,
while reducing conditions prevail 5–30 cm below the
sea floor. Below this redox boundary where there is no
free oxygen, sulphate-reducing bacteria react with
organic matter as indicated below:
2CH 2 O þ 2H
þ
þ SO
ÀÀ
4
! H 2 S þ 2CO 2 þ 2H 2 O
NH 3 þ H
þ
þ SO 2 ! NO 3 þ H 2 S þ H 2 O
H 2 S is liberated, giving stagnant water and mud a
strong smell. Through denitrification we get
5CH 2 O þ 4H
þ
þ 4NO
À
3 ! 2N 2 þ 5CO 2 þ 7H 2 O
When the rate of accumulation of organic matter
exceeds the rate of oxygen supply the redox boundary
will be in the water column, separating the oxidising
surface water from the reducing bottom water.
This is typical of basins separated from the deep
ocean by a shallow sill, like the Black Sea and some of
the deep Norwegian fjords. Fresh or brackish surface
water floating on more saline water also helps to
maintain a stable water stratification with little vertical
mixing. Lakes may have good water stratification
8
K. Bjørlykke
produced in the ocean is dependent on the nutrient
supply from rivers, but river water does not only
carry inorganic nutrients. It also contains significant
amounts of organic matter, in particular humic acid
compounds, lignin and similar substances formed by
the breakdown of plant material which are weakly
soluble in cold water. When the river water enters
the sea, there is precipitation due to the increased pH
and lower surface temperature in the ocean.
Other plant materials, like waxes and resins, are
more chemically resistant to breakdown and are insoluble in water. Such organic particles tend to attach
themselves to mineral grains and accompany sediment
out into the ocean.
Most of the oil reservoirs which have been formed
since the Palaeozoic have been uplifted and eroded,
and over time vast quantities of oil have flowed
(seeped) out onto the land or into the sea. In this
sense, oil pollution is a natural process. Only a small
proportion of the petroleum that has been formed in
source rocks has actually become trapped in a reservoir. One might expect this seepage to have provided a
source of recycled petroleum in younger sediments,
but petroleum breaks down extremely rapidly when
subjected to weathering, oxidising to CO 2 , and the
nutrients (P, N) that were required to form the organic
matter are released and may act like a fertilizer.
On land, evaporation will remove the lighter
components while bacteria will degrade the heavier
components. Fossil asphalt lakes consist of heavy
substances which neither evaporate nor can be easily
broken down by bacteria. In the ocean, the lighter
components will dissolve quite rapidly, while the
heavier asphalt fraction will sink to the bottom and
be degraded and recycled.
In uplifted sedimentary basins like the Ventura
Basin and the Los Angeles Basin in Southern
California there are abundant natural oils seeps both
onshore and offshore.
On the beaches from Santa Barbara towards Los
Angeles there are many natural oil seeps.
1.3
Breakdown of Organic Matter
Almost all (>99%) of the organic matter which is
produced on land and in the oceans is broken down
through direct oxidation or by means of
microbiological processes. If oxygen is present,
organic matter will be broken down in the following
manner:
CH 2 O þ O 2 ! CO 2 þ H 2 O
Where oxygen is available, organic matter is
oxidised relatively rapidly both on land and in the
sea. As organisms die, organic material suspended in
seawater sinks through the water column consuming
oxygen. If water circulation is restricted due to density
stratification of the water column, the oxygen supply
will be exhausted. Instead, the bound oxygen in
sulphates or nitrates is used by sulphate-reducing and
denitrifying bacteria which decompose organic material in an anoxic environment. The first few
centimetres below the seabed are usually oxidised,
while reducing conditions prevail 5–30 cm below the
sea floor. Below this redox boundary where there is no
free oxygen, sulphate-reducing bacteria react with
organic matter as indicated below:
2CH 2 O þ 2H
þ
þ SO
ÀÀ
4
! H 2 S þ 2CO 2 þ 2H 2 O
NH 3 þ H
þ
þ SO 2 ! NO 3 þ H 2 S þ H 2 O
H 2 S is liberated, giving stagnant water and mud a
strong smell. Through denitrification we get
5CH 2 O þ 4H
þ
þ 4NO
À
3 ! 2N 2 þ 5CO 2 þ 7H 2 O
When the rate of accumulation of organic matter
exceeds the rate of oxygen supply the redox boundary
will be in the water column, separating the oxidising
surface water from the reducing bottom water.
This is typical of basins separated from the deep
ocean by a shallow sill, like the Black Sea and some of
the deep Norwegian fjords. Fresh or brackish surface
water floating on more saline water also helps to
maintain a stable water stratification with little vertical
mixing. Lakes may have good water stratification
8
K. Bjørlykke