The biomarkers are the proxy for the paleoenvironment interpretation. Biomarkers are named as a biomarker which is related to the geological study. When an organic matter discomposed, by some diagenetic processes, then some more stable organic matter is produced and they are called the biomarker or the geo-biomarker, or the molecular fossils or geochemical fossils. Biomarkers are generally derived from the lipid of organic matter. Suppose, Pristane, a molecular fossil that indicates the redox environments and widely recognized as a redox proxy.
How Biomarkers/molecular fossils are formed?
We can give an example of the production of the Pristane in the sediments (Tetramethylepenta decane). Pristane is the branched acyclic isoprenoid containing 19 carbon atom. A prominent peak of the Pristane elutes just after the C17 n-alkane peak in the Gas chromatography-mass spectrometry (GCSS) measurements. The original source for the Pristane (Pr) is from the phytol. Phytol is derived from Phytyle, a major chain off of the chlorophyll-a molecule from the tree leaf. If phytol undergoes diagenesis and categenesis then pristane and phytane (another related biomarker) is produced.
Pristane produces with the oxidation and by the decarboxylation of the phytol. The phytane has the carbon 20 atom, and produce by the dehydration of the phytol and remain in the sediments as they are most stable.
Some important biomarkers in the paleoenvironments study and petroleum industry.
Moretane and hopane are the most important biomarker in geochemistry for identifying the maturity of rocks and paleoenvironments of the past.
The list of the important biomarker are;
Pristane (Pr), Phytane (Ph), Phenanthrene (Phe), Dibenzothiophene (DBF), Cadalane, Retane, DBF/Phe, Cadalane/Phe, Bonzo(e)pyrene, n-alkane, C33ACH (C33 n-alkane cyclohexane), Metane, Hopane, Sterane, Simonelite, coronene, Isorenairatane, 2-Methyl hopane, etc.
How do we know the soil erosion event occurred in the past?
The high value of the Pr/Ph ratio indicates the oxic condition in the soil or marine depositional environments. Hence, d13 Corg positive shift indicates the relation between the soil erosion event and deposited sediments in the ocean bottom as the high value of the Pr/Ph in the ocean sediments indicate the oxic environments as for the incorporation of the more soil form the land.
On the other hand, DBF/Phe high value also indicates the soil erosion event. So it must be seen the high value if there is a soil erosion event occur. The value of the Cadalane also very important to make a correlation among them.
Because the cadalane is usually derived from the higher plants of terrestrial environments. Cadalane, retane, simonelite are all derived from the higher plants, so, in the soil erosion events, the high value of cadalane, retane, simonelite will be prominent in the rock samples. Another, evidence of the soil erosion event is the dominance of the higher n-alkane like, C25 n-alkane to C35 n-alkane. Because these higher chain n-alkane is usually derived from the higher plants or tree which contain higher cellulose. Hence, higher n-alkane is another proxy for the identification of soil erosion events.
The total organic carbon from that sample will be high due to the incorporation of high organic matter during the soil erosion events. Soil carries lots of organic matter along with the soil grain and deposited in the ocean bottom. Hence, the TOC value will be higher in the rock samples. Soil erosion event may occur in many ways, like, huge volcanic eruption, forest fire, meteorite impact, etc, The main cause of the soil erosion event can be identified based on some important biomarker proxy and among them, coronene index is one of them.
Coronene index can differentiate the forest fire and meteorite impact event.
Coronene index indicates the event whether it was a forest fire, volcanic eruption or massive meteorite impact based on the value of the coronene index?
The coronene index=coronene/coronene+benzo (a) pyrene + benzo (g,h,i) pyrelene.
This coronene index value increases if any extraterrestrial body hits the planet earth where the higher amount of organic-rich sediment exists. The combustion of coal or petroleum beneath the ground will produce a higher amount of coronene. The value of the coronene index higher than 0.8 indicates the meteorite impacts, less than 0.8-0.5 is for volcanic eruption and bellow 0.5 is for the forest fire event.
So, based on the observation of the coronene index value geologist can interpret the paleo events like soil erosion event due to a forest fire or due to volcanic eruption or due to meteorite impacts. The evidence is the real fact because the coronene production in the rock sample depends on the heat used during the soil burning. If more than 4000 degrees Celcius is applied, the relatively higher amount of coronene will be produced and if less heat applied then less amount.
So, the coronene production in a rock sample is directly proportional to the heat applied. As the meteorite impact imposes huge heat in the soil hence high amount of coronene will produce.