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The answer is in rocks and mud !
There are organisms living at the surface of the ocean. These organisms record the ocean, environmental and climate conditions during their lives, as their shells are growing recording the current conditions in which they are living in. It can be temperature, ocean circulation, oxygen, or even light presence. At their death, these organisms plus some dust and sand accumulates on the sea floor.
These sediments accumulate over time on the seafloor, and over years to million years, it is meters to hundreds of metres of sediments that are accumulated, the most recent sediments over the older (it’s a similar process for the ice on poles and glaciers). At the surface, it is mud, and the deeper, the most compact it is, until it becomes rock. This is exactly how the great limestone cliffs were formed, and you can even find these rocks, formed a very long time ago at the bottom of the ocean, at the top of some mountains!
Schematic representation of how the seafloor is drilled
We drill the seafloor at wisely chosen location (several holes per site), and obtain sediment cores, from meters to hundreds of meters.
Each core is cut in smaller sections, and all of them are labelled, with the name of the expedition, the site, the hole, the core, the type and de depth. These cores are opened up in 2 halves in the long side, one to work on and the second one for archive, for the future generations of researchers.
We observe the “events” occurring along the sediment, it can be magnetic inversion, volcanic eruption, an occurrence or last appearance of specimens belonging to species that we know to appear/disappear at a precise date. These events are related to a specific depth, and knowing this, we reconstruct the age model, attributing an age to each depth of the sediment core. This step is essential, because for any data we find, we have to know WHEN it does occur, in order to reconstruct the past climate and ocean conditions.
We select a batch of samples to work on, depending on the study we are doing. We analyse them, it can be for the taxonomy of the organisms in the sediment, the physical or the chemical properties of the sediment. We can identify the different species, count their abundances but also look at the physics and chemistry of their shells. We analyse the results, link them to the associate depth using the age model and make a comparison with other studies from other research groups.
They are then interpreted, discussed and published in peer reviewed journals in order to make them available to the research community for the advancement of science.
September 2024
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