Modern Sediments Rich in Organic Matter

Modern Sediments Rich in Organic Matter

Normally, Organic Matter within clastic sediments: 0.5% and in Carbonate rocks: 0.3%.

There are various deduction for organic rich sediments:

Bitterli (1963), transgression/regression sequence, best for organic rich sediments and, however, associated with fine grained sediments.

Hunt (1963), confined the fact that organic sediments tends to form in small size sediments.

Skimkus and Trimomis (1974), high concentration of organic matter do not coincides with high production of organic matter.

> However, there is a good correlation between organic matter and CaCo3 concentration: high in low clastic input and vice-versa and is about 30%.

> Meade et al. (1975) and Milliman et al. (1975) recognized that suspended matter was found to be rich in organic matter in offshore area rather than near shore about 40-80% to 10-25%, suggesting low density for organic matter, thus staying in longer water and ultimately destroyed by oxygenation or bacterial activity.

Accumulation and Preservation of Organic Matter

Generally:

The accumulation of organic matter in sediments is controlled by a number of condition and restricted to sediments deposited in aquatic environment, may be autochthonous or allochthonous.

Organic matter can be supplied in the form of POC or DOC

If energy high: erosion rather than deposition and coarse grained doesn’t retain the organic matter, due to circulation of O2.

> If energy low: too little sediments supply (no appreciable organic matter).

DOC, POC and its role

Menzel and Ryther (1970); states that DOC and POC homogeneous below the depth of 200-300m in conc. POC: 3-10microgram and DOC: 0.35-0.7microgram

They also states that above this depth the concentration are variable; such that POC directly proportional to primary production and DOC inversely proportion to primary production.

They concluded that oxygen occurs within 200-300m depth, below this depth there is no cycling of organic matter. 

Clay mineral, when settle through water column get coated with DOC (polar organic compounds: Sugar, Phenolic, amino acids etc) and settle down as POC, inversely related to grain size, thus contribute to organic matter.

Gormly and Sackett, 1977: Extraction of Organic matter from sea water by process of adsorption of DOC is very much controlled by suspended mineral particles.

Accumulation/Preservation Mechanism

Preservation rate 0.1%

There is a relationship between; quantity of OM supply and Concentration of OM: OM high in high production areas, land areas (margins or upwelling areas)

However, supply is modified by those processes responsible for conversion and concentration of organic matter on one hand, and those responsible for destruction and dilution on the other.

On earth surface all OM is unstable.

Conservation of OM is aided by number of factors:

High productivity of OM, creates low Oxygen concentration along with production of H2S (by sulphate reducing bacteria) creates minimum Oxygen at the bottom of the euphotic zone of seawater, which decrease the decay rate of OM

The adsorption of DOC or POC on surface of mineral particles enhance the stability in two ways:

protected by biological consumption

settle more rapidly from water column (residence time less in oxygen containing water)

Therefore, in shallow water there are favorable condition of OM to accumulate.

A similar effect from stratification in deep water, by thermocline that limits the oxygen within ocean bottom and prevents the consumption of OM.

Rate of Sedimentary particle plays a key role, preservation and concentration of OM:

a. assume that a constant supply of OM: OM concentration will be inversely related to rate of deposition of mineral particles. The resulting dilution effect.

b. the relationship is not that much simple because it is modified by other factors: i. grain size distribution of mineral particle, ii. Length of water column, iii. Residence time in it and iv. Consumption of OM by organisms.

Empirical observation indicate that, with increasing rate of deposition, the concentration of OM passes through maximum and only at higher rates of deposition dilution is effective.

Lipidfraction, insoluble then non lipidfraction, in addition many highly resistant parts of organism, such as membrane, cuticles, waxes, etc, thus it survive in POC rather than DOC, therefore, it is deduced that POC is more important than DOC in potential source rock

Favorable condition for deposition of sediment rich in OM: continental shelf, quite waters (lagoons, estuaries and deep basin) and continental slope...