Phytane facts for kids

Phytane is a type of chemical compound called an alkane. It's a special kind of alkane known as an isoprenoid. Phytane is formed from a substance called phytol, which is a key part of chlorophyll. Chlorophyll is the green stuff in plants that helps them make food using sunlight. When phytol loses a specific part (its hydroxyl group), it turns into phytane. If phytol loses one carbon atom, it becomes another similar compound called pristane.

Both pristane and phytane are often found in petroleum (crude oil). Scientists use them to understand how oil was formed and where it came from. They can also help track oil spills in the environment.

What is Phytane Like?

Phytane is a clear, colorless liquid at room temperature. It doesn't have any smell. It's a non-polar compound, which means it doesn't mix well with water, but it mixes with other oily substances. Its chemical formula is C₂₀H₄₂, meaning it has 20 carbon atoms and 42 hydrogen atoms.

Phytane has many different forms that have the same chemical formula but different structures. These are called structural isomers. One example is crocetane, which is very similar to phytane.

The part of a molecule that comes from phytane is called a phytanyl group. You can find phytanyl groups in the outer membranes of tiny living things called archaea, especially those that produce methane or live in very salty places.

Phytene is like phytane but has a double bond in its structure, making it slightly different. The alcohol form of phytene is phytol. Phytene is also found in important biological molecules like chlorophyll, tocopherol (which is Vitamin E), and phylloquinone (which is Vitamin K₁).

Where Does Phytane Come From?

The main source of phytane and pristane is believed to be chlorophyll. Chlorophyll is the most important green pigment in plants, algae, and some bacteria that perform photosynthesis.

When chlorophyll breaks down in marine sediments or when animals eat plants, it releases phytol. This phytol then changes into phytane or pristane.

Another way phytane and pristane can form is from special fats found in archaea. Studies show that when these archaea are heated deep underground, they can produce pristane and phytane.

How Phytane is Preserved

In the right conditions, molecules like chlorophyll can be preserved as biomarkers. These are like chemical "fingerprints" that tell us about past environments. When chlorophyll breaks down over time, it often loses parts like double bonds or hydroxyl groups.

Scientists believe that pristane and phytane form from phytol under different conditions.

• Pristane tends to form when there's a lot of oxygen (oxic conditions).
• Phytane tends to form when there's very little or no oxygen (anoxic conditions).

However, the exact ways these compounds form can be more complex and depend on many factors, not just oxygen levels.

Geochemical Clues

Scientists use the amounts of pristane and phytane to learn about Earth's history.

Pristane/Phytane Ratio

The ratio of pristane to phytane (Pr/Ph) is a common tool. It helps scientists guess how much oxygen was present when the rocks or oil formed.

• If the Pr/Ph ratio is low (less than 0.8), it often means the environment was very salty or had little oxygen.
• If the Pr/Ph ratio is high (above 3), it usually means the environment had more oxygen, like in rivers or deltas.

This ratio is easy to measure using a method called gas chromatography.

However, this ratio should be used carefully. Sometimes, pristane and phytane might come from different sources. Also, the ratio can change as oil matures deeper underground.

Other Ratios

Scientists also use other ratios, like pristane to n-heptadecane (Pr/nC₁₇) and phytane to n-octadecane (Ph/C₁₈). These ratios can help link oil to its original source rock. For example, oil from open oceans might have low Pr/nC₁₇, while oil from swampy areas might have higher ratios.

These ratios can also be affected by how much the oil has been heated or broken down by bacteria. For instance, bacteria tend to eat up the simpler alkanes before they touch pristane and phytane, which changes the ratios.

How Resistant They Are to Breakdown

Pristane and phytane are more resistant to biodegradation (breakdown by living things) than simpler alkanes. This means they stick around longer in oil. However, they are less resistant than some other biomarkers like steranes and hopanes.

Looking at Carbon and Hydrogen Atoms

Scientists can also study the types of carbon and hydrogen atoms in pristane and phytane.

Carbon Atoms

The types of carbon atoms in pristane and phytane can tell us about how photosynthesis happened in the past. For example, the carbon in phytane found in marine sediments has been used to figure out how much carbon dioxide was in the Earth's atmosphere millions of years ago.

If pristane and phytane come from the same original source, their carbon atom types should be very similar.

Hydrogen Atoms

The types of hydrogen atoms in phytol (the precursor to phytane) from tiny ocean plants are usually very light. As the oil matures underground, the heavier hydrogen atoms are released, making the pristane and phytane heavier.

Important Note: Using Pr/Ph Ratio

It's important to remember that the Pr/Ph ratio alone might not always tell the whole story about oxygen levels in ancient environments. Scientists usually look at other clues too, like the amount of sulfur in the rocks or other chemical markers.

For example, a low Pr/Ph ratio might mean high salt levels instead of low oxygen levels. This shows why it's important to use many different types of evidence when studying ancient Earth.

See also

• Phytol
• Pristane
• Biomarker
• Crocetane
• Archaeol
• Tocopherols
• Sterane
• Hopane

Images for kids

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  Chemical structure of an archaeol, with two phytanyl groups.

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  Chemical structure of α-tocopherol.

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  Chemical structure of trimethyl 2-methyl-2-(4,8,12-trimethyltridecyl)chroman, a type of MTTCs.

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  Structure of chlorophyll a, with a side chain containing a phytyl group.

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  Pristane and phytane are formed by diagenesis of phytol under oxic and anoxic conditions, respectively.