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Evidence: Sediment Cores

Every year, billions of tons of dead plankton and other marine organisms, dust blown from far-off lands, and river sediment settle on the ocean floor on top of materials from previous years. Scientists collect long sediment cores like this one (right) and examine the materials trapped within to reconstruct past ocean conditions. The varieties and concentration of certain microorganisms record past changes in ocean temperature and composition.

A Sediment Story

Sediments cored from the ocean bottom serve as a timeline of events: each year's sediments are stacked on top of the ones from the year before. The deeper the sediment, the older it is. This core (right) is modeled after one taken from the seafloor almost 5 kilometers (3 miles) underwater in the southeastern Atlantic.

These sediments accumulated about 55 million years ago over a span of about 300,000 years and give clear evidence of a major shift in ocean composition.

Light-colored sediments = Lots of shells

Dark-colored sediments = Few shells

Shells Slowly Return

Here, the sediments become gradually lighter toward the top of the core, indicating that shelled ocean organisms slowly became more abundant over about 100,000 years.

Shells Suddenly Disappear

Younger dark-brown sediments, which do not contain any carbonate shells, lie on top of older light-brown sediments, which contain many tiny shells. This sharp boundary indicates that many shelled organisms suddenly disappeared from the ocean about 55 million years ago.

Lots of Shells

The sediments below the sharp boundary are light brown because they contain plentiful carbonate shells from many marine organisms.

Ancient Acidification

The sharp boundary between light- and dark-brown sediments in this core is also visible in similarly aged cores from around the world.

What happened?

About 55 million years ago, an enormous amount of CO2 entered the atmosphere in perhaps only 1,000 years, causing global temperature to rise. Much of the CO2 was absorbed by the ocean, quickly changing its composition. Under these new conditions, many organisms disappeared from the ocean. Some survived in the new conditions, but when they died their shells dissolved as they sunk into deeper water, leaving no trace in the sediments on the seafloor.

Where did all that CO2 come from?

One theory is that gradual warming of the ocean melted ices containing methane that were part of shallow ocean sediments. The released methane, in turn, reacted with oxygen in the atmosphere to form CO2.

How does this compare to current climate change?

The amount of CO2 that was injected into the atmosphere 55 million years ago is comparable to the amount that humans will put into the atmosphere over the next century if we do not reduce CO2 emissions. So that event of the distant past could have important lessons for today.

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