Don't Bones Decay?

Image of a decaying bone.

Credit: needpix.com

I'll just answer the question up front. Bones do decay. They do turn into dust and disappear one day. Ok, that's the end - Bye! But before you decide to go on to the next article or watch YouTube or even wonder why did I write such an intro, I'd just like you to contemplate on just one thing, or two things for the matter. If bones decay,  how is it that dinosaur bones are preserved for millions of years? How do scientists determine the age of Dinosaur bones, which range from 65 million to an impressive 230 million years? Don't need to contemplate further, this article shall answer your questions in a swift and detailed manner! 

Is there a possibility that organic matter from dinosaur bones remains preserved?

Image of a decaying bone.

Credit: Paleontology World

Yes, it is possible. However, for that to happen, these bones would have to been stored in an environment where there is minimal to none decay-causing bacteria present. Conditions which result in the natural preservation of organic matter are as posited below.

Encasement in Amber

Small organisms like insects and arachnids could become trapped in tree resin which toughens to form amber, over time. This amber provides an anaerobic environment for the carcass and shields it from decay-causing bacteria. Consequently, the organism is preserved in stunning detail. This type of preservation is typically rarer for large organisms. 

Preservation in Ice

Extremely cold temperatures prevent bacteria from digesting organisms entrapped in ice like permafrost or glaciers. As a result, the decomposition of such organisms slows down. 

Preservation in Peat Bogs

Peat bogs are acidic, waterlogged, and oxygen-poor. These conditions make these bogs ideal for inhibiting bacterial growth, thereby slowing down the decomposition of organic matter entrapped in it.

Desiccation (Drying)

This condition occurs in arid environments. Over time, the dead organism could lose moisture through evaporation. The lack of moisture can inhibit the growth of decomposing bacteria, thereby preserving the dead organic matter.

Anaerobic environments

Refers to environments devoid of oxygen. The lack of oxygen can prevent the growth of decay-causing bacteria and prevent such bacteria from decomposing dead organic matter, thereby preserving said matter.

Fossilisation (mineralisation)

Refers to a process where buried organic material is replaced by surrounding minerals, over time. For this process to occur, the dead organism would have to be rapidly buried by sediment. For the organism to be rapidly buried, it would have to be in a depression on Earth's surface (Depositional environment). The burial shields the organism from oxygen and scavengers, thereby slowing decomposition of organic matter. 

Over a long period of time, the organic matter from the organism would be decomposed by anaerobic bacteria, leaving bones with inorganic content. It is important to note that bones are partially inorganic as it is made up of minerals such as calcium phosphate. 

Mineral-rich groundwater seeps into the pores of the bones. Due to factors such as temperature, pH, pressure, etc. the minerals precipitate from the groundwater, thereby filling the space left by the decomposed organic matter and preserving the bone structure. The structure is preserved as the filled pores maintain the integrity of the bone structure as, over time, hollow pores may allow for the bone to crush under the pressure from the above sediment.  

Some inorganic minerals from the bone could dissolve in the groundwater, especially if the groundwater is acidic, leading to the precipitated mineral replacing the dissolved bit. The rest of the inorganic minerals from the bone could remain in the bone for millions of years! 

Which method mainly preserves dinosaur bones?

Dinosaur skeleton in the museum.

Credit: Stunningdino.com

While conditions like amber and ice can preserve small organisms, fossilization, which frequently takes place in anaerobic environments, is the most common method of preserving dinosaur bones. 

As the organic matter in the bones decays, it is replaced by minerals from groundwater. This process, known as mineralization, ensures that the bone's structure is preserved, often leaving behind a 'stone replica' of the original bone. It is very rare for the organic matter of the bones to be preserved, especially over millions of years, with exposure to anaerobic bacteria.

However, a question arises, presuming that scientists uncovered a dinosaur bone that is completely made of inorganic minerals, how could carbon dating be conducted on this bone to find out its age? It would not contain carbon in the first place. Moreover, carbon dating is effective for objects less than 50,000 years of age as the half-life of carbon-14 is only 5,730 years so carbon dating dinosaur bones is a long stretch. 

Then, how do scientists determine the age of dinosaur bones?

Methods of estimating the age of dinosaur bones

Relative dating

Refers to where the age of sediments that surround the fossilized bones and fossil records are compared to the fossilized bones to determine their age. 

Relative dating can tell us the age range of a fossil by comparing the layers of sediment in which it is found, but it doesn't provide a precise age. This method is often combined with radiometric dating for more accurate results.

Stratigraphy

This method assumes that sediments in the bottom layer are older than those in the top layer. Scientists then determine and designate the age range for each layer of sediment, from the top to the bottom. Stratigraphy is used to determine the relative age of rock layers on not their age ranges. Moreover, the layers are categorized based on the physical layering of rock and soil deposits.  

Biostratigraphy

Biostratigraphy involves categorising and dating sediment rock layers based on the types of fossilized creatures found in each layer. The age range of these fossils is determined using methods like radiometric dating. Thereafter, the age of the sediment rock layer is compared to that of the dinosaur bones to estimate their age. 

Magnetostratigraphy

Magnetostratigraphy consists of classifying and dating sediment rock layers based on the magnetic field orientation of minerals found in each layer. The magnetic poles of Earth have reversed many times throughout history and minerals align their magnetic poles in accordance to Earth's magnetic poles at the time of their formation. Therefore, the orientation of the magnetic fields of these old minerals could be used to reconstruct the past direction of Earth's magnetic field. 

This reconstruction would be compared to the Geomagnetic Polarity Time Scale (where specific magnetic fields are designated to specific periods of time) to match an age range for these minerals and thus the sediment rock layer in which these minerals are situated. This information can then be used to help date dinosaur bones found within or associated with that particular sediment layer.

Absolute dating (Radiometric dating)

Involves comparing the proportion of 2 types of atoms with consecutive atomic numbers from minerals in igneous rocks (like volcanic ash) near the fossilized dinosaur bones. It is assumed that the atoms with a higher atomic number have undergone decay to give the atoms of a lower atomic number. The ratio of the 2 types of atoms, would be compared to the half-life of the atom with the larger atomic number to determine how much of time has passed for the atoms with the bigger atomic number to have decayed, thereby ascertaining the date of rapid burial. This method assumes that the type of atoms with the smaller atomic number purely originated from the half-life decay of the atoms with the bigger atomic number. 

Half-life is the time it takes for a group of the same types of atoms to decay until the remaining same type of atoms becomes half of the amount that originally existed.

For absolute dating, the exact age of the dinosaur bones themselves cannot always be directly determined, as the minerals in the surrounding igneous rocks may be older than the fossilized bones. Instead, the age of the surrounding rock layers is used to estimate the age of the bones, providing an age range rather than a precise date.

Potassium-Argon dating

This dating is tested on the igneous rock which is above or below the sediment layer possesses the dinosaur bones. Igneous rocks are often formed during volcanic activity and have a high likelihood of being deposited around the same time as the surrounding sediment layers. When found above or below sedimentary layers containing dinosaur fossils, they are suitable for radiometric dating, as their age closely aligns with the time of the fossils' deposition, particularly in cases of rapid burial. 

Potassium (19)(⁴⁰K) has a higher atomic number than Argon (18)(⁴⁰Ar), so in this case, Potassium decays into Argon. Potassium has a long half-life of 1.25 billion years, making potassium-argon suitable to date the sediments.  Argon is assumed to not have been originally deposited in the igneous rocks during deposition as it is a non-reactive gas and thus does not react with minerals in the rocks even though they are very hot when deposited. Hence, supposedly, all Argon would have escaped during rapid burial. 

The ratio of potassium to argon in the igneous rocks would be measured and then compared to the half-life of potassium to ascertain how old the igneous rocks are. The formula used to calculate the age of the rocks is as follows below.

Formula to calculate time elapsed for Potassium-Argon decay.
Credit: ChatGPT

Where:

• t = age of the rock
• $\lambda$ = decay constant of  ⁴⁰K = $5.81\times \mathrm{10⁻¹¹}$
• $\ln$ = natural logarithm

Thereafter, the age of the rocks would be compared to that of the dinosaur bones to estimate the bones' age.

Uranium-Lead dating

There are two decay chains for Uranium

$²³⁸U\to ²⁰⁶Pb$ (half-life: 4.47 billion years)

$\mathrm{²³⁵U}{\to }^{}\mathrm{²⁰⁷P}\mathrm{b }$(half-life: 703 million years)

Since Uranium has a very long half-life, it would be suitable for ascertaining the age of minerals which surround dinosaur bones. This dating is usually used on Zircon crystals (ZrSiO4) found near the dinosaur bones as the crystals includes Uranium atoms but excludes lead in its structure during formation. Hence, lead found in these crystals is implied to be a product of Uranium decay. During rapid burial, these crystals are most commonly found in igneous rocks. 

Uranium-lead dating is highly accurate and is often used to date the oldest rocks on Earth, sometimes billions of years old. It provides a precise age for zircon crystals, which can help date the surrounding rock layers and indirectly date dinosaur fossils.

The ratio of Uranium to lead in these Zircon crystals would be measured and then compared to the half-life of Uranium to ascertain how old the crystals are. The formula used to calculate the age of the crystal is as follows below.

The key equations for U-Pb dating are:

For ²³⁸U→²⁰⁶Pb:

Formula to calculate time elapsed for ²³⁸U-²⁰⁶Pb decay.
Credit: ChatGPT

Where:

• ²⁰⁶Pb is the daughter isotope,
• ²³⁸U is the parent isotope,
• ${\mathrm{\lambda ₂₃₈ }}_{}$is the decay constant for $²³⁸U$(half-life: 4.47 billion years),
• $t$ is the time elapsed since the rock or mineral crystallized.

For ²³⁵U→²⁰⁷Pb :

ormula to calculate time elapsed for ²³⁵U-²⁰⁷Pb decay.
Credit: ChatGPT

Where:

• ²⁰⁷Pb is the daughter isotope,
• ²³⁵U is the parent isotope,
• λ₂₃₅ is the decay constant for ²³⁵U (half-life: 703 million years),
• t is the time elapsed since the rock or mineral crystallized.

Thereafter, the age of the crystal would be compared to that of the dinosaur bones to estimate the bones' age. 

For radiometric dating, it is important to note that the exact age of the dinosaur bones cannot be determined as the minerals from the surrounding igneous rocks may be older than the fossilized bones itself. Hence an age range could be ascertained using this.

While absolute dating provides a more precise age, it is not always possible due to the absence of igneous rocks near the bones. In such cases, relative dating is used to estimate the age range by comparing the fossil's position in sediment layers.

Well, now you know how dinosaur bones are preserved and how their age are estimated! Please feel free to state your opinions or share more about what you know of this topic in the comments below. Thanks a lot for reading!

References

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3. Faure, G., & Mensing, T. M. (2005). Isotopes: Principles and applications. Wiley.

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5. Harington, C. R. (2003). The ice age mammals of Canada. Canadian Museum of Nature.

6. Hedges, R. E. M. (2002). Fossilization and preservation of organic matter. Geobiology, 1(1), 57-66.

7. Poinar, G. O. (1992). Life in amber. Stanford University Press.

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9. Gose, W. A., & Krol, M. M. (2002). Magnetostratigraphy in geological methods in the study of the Earth (pp. 149-171). Springer.