The bookmark of the stratum - the "golden nail"

The bookmark of the stratum - the "golden nail"

When we read books or comics, we can't always finish them in one go, so in order to make it easier to read next time or to savor some wonderful content repeatedly, we always leave some special marks on a certain page in different ways. For example, fold the corner of the page back or put a bookmark inside. Also, when we look up a dictionary, we can quickly locate the first letter of the pinyin by observing the small black block on the side of the dictionary to find the target content.

What is the purpose of setting bookmarks or designing such small black blocks? In fact, it is to quickly locate certain content, or to use it as a feature point to sort out the previous and next content.

In real work and life, we cannot do without such "bookmarks". Sometimes they are used to quickly find and locate, such as the ctrl+F function in Word; sometimes they are used to distinguish the past and the present, such as the historical event of "Wang Mang usurping the Han Dynasty" that divided the Han Dynasty into two; sometimes they can be used as a basis for judgment, such as judging the specific species of a butterfly by observing butterfly specimens. In our geological work, there is also such a concept similar to "bookmarks", which we call "golden nails".

1. What is the "Golden Nail"?

The so-called "golden nail" is actually the common name for the "Global Chronostratigraphic Unit Boundary Stratotype Section and Point (GSSP)".

In geological work, if the tectonic evolution of the earth for hundreds of millions of years is compared to a thick encyclopedia, then different strata are the "pages" that record the evolutionary stories of different tectonic periods. During the long tectonic movement, some "pages" were formed into mountains and rivers on land as the strata were lifted, some "pages" were buried deep in the sea as the plates moved and became seabed bedrock, some "pages" were melted by volcanic activities and then erupted, and re-"printed" on the surface; some "pages" were transformed beyond recognition by geological activities such as folding, overturning and fracture. But there are still some "lucky ones" who have preserved the continuous evolution process and recorded complete geological events. Our geological scientists have established this type of "lucky ones" as bookmarks of the book of strata and given it a more meaningful name, called "golden nails".

2. What is the meaning of the “golden nail”?

So what is the significance of setting up this golden nail?

Geological activity is an evolutionary process with millions of years as the time unit, just as our historians divide ancient Chinese history into different dynasties such as Tang, Song, Yuan, Ming, Qing, etc. Geologists have established a set of chronostratigraphic units according to the order of rock formation, which are called Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic respectively. Each epoch is further divided into the next level of chronostratigraphic (geological age) units, such as system, epoch and stage. This time system is used to describe our different geological periods and the geological activities that occurred there.

In addition, from a professional perspective, the introduction of the concept of "golden nails" avoids the problems of unequal top and bottom boundaries and repeated missing strata in "unit stratotypes" with both bottoms and tops. Simply put, the stratigraphic stratotype we originally thought of was that many "two slices of bread with a piece of meat - similar to a hamburger" structures were considered a "stratotype", and then many "hamburgers" piled up to form a stratigraphic whole. But when the "golden nail" was introduced, the bottom of the previous stratigraphic layer was automatically the top of the next stratigraphic layer, avoiding the previous confusion. In this way, from the perspective of geological activities, the time of stratigraphic formation is continuous, which is more in line with the actual situation.

3. How to establish the “golden nail”?

It is very demanding for a stratum to be established as a "golden nail". First of all, there must be a stratum outcrop with sufficient sediment thickness in the field and no disturbance from other factors. Then there must be abundant paleontological fossils in the key stratigraphic sections, because specific fossil species only live in specific geological periods, so specific paleontological fossils can indicate the signs of specific strata. Next, there must be sufficient evidence such as magnetic stratigraphy, sequence stratigraphy, isotope age stratification analysis, etc. as important theoretical basis support. The third is some objective requirements, such as being easy to open to the outside world, moderate research and practice costs, and having a permanent fixed mark, etc. In short, the production of any "golden nail" is extremely rigorous and complex.

4. Where are the “golden nails” around you?

The world's first "golden spike" was born in the Czech Republic in 1972, when the global Silurian/Devonian stratigraphic boundary was demarcated. my country's "golden spike" research was more than ten years behind the international level. It was not until 1997 that my country successfully established the "golden spike" of the Tongda Ruiwei Stage of the Middle Ordovician System in Huangnitang, Changshan, Zhejiang. As of November 2022, my country has established a total of 11 "golden spikes". The details are as follows:

5. What can the “golden nails” do?

The research and establishment of the "Golden Spike" not only provides reliable and realistic evidence for clarifying and defining strata in academia, but also provides an intuitive reference for the division of strata in regional geological surveys, and provides sufficient physical data for reflecting regional geological activities, identifying regional geological structures, and studying paleontological fossils. At the same time, the establishment of each "Golden Spike" also demonstrates the strong scientific research capabilities of a country or region in related research, and is a high honor in the field of international stratigraphic research.

Geological work is a very interesting job, which allows you to travel all over the mountains and rivers of the motherland and enjoy various different scenery. At the same time, geological work is also a very hard job, often crossing wilderness and dense forests, and often sleeping in the open air. The "Golden Nail" is the honor and spiritual heights that our geologists have condensed in the field for many years!

Authors: Xi’an Mineral Resources Survey Center, China Geological Survey, Wang Yanzhou, Liu Lin, Chen Long, Hu Jianhui, Zhao Zihao.

Image credit: Xi’an Mineral Resources Survey Center, China Geological Survey, Chen Long.