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:

CH2O + O2 → CO2 + H2O

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:

2CH2O + 2H+ + SO– – 4 → H2S + 2CO2 + 2H2O NH3 + H+ + SO2 → NO3 + H2S + H2O H2S is liberated, giving stagnant water and mud a strong smell. Through denitrification we get

5CH2O + 4H + + 4NO– 3 → 2N2 + 5CO2 + 7H2O

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 because warm surface water is less dense than the colder bottom water. Black mud deposited at the bottom of lakes may produce good source rocks.

In cold climates, however, the water in the lakes overturns in the winter because the maximum water density is at 4◦C, preventing the stable stratification required to form source rocks.