Nothing lasts forever in the ocean. Organisms die and sink to the seafloor. Dust particles enter at the surface and fall to the bottom. Salts evaporate or stick to particles and are removed from the sea. Even the water itself cycles out of the ocean and through the atmosphere, continents, even the interior of Earth.
The final resting place for salts and particles in the ocean is the seafloor. As material accumulates at the ocean bottom, it buries what was previously sitting upon its surface. The newest arrivals always lie above what was there before. This means that the seafloor is layered with a predictable timescale: the deeper you go, the older the mud.
Human-induced climate change is causing temperatures to rise faster than they have in at least the last 65 million years of Earth’s history. Atmospheric carbon dioxide (CO2) levels, currently at 400 parts per million (ppm), haven’t been this high since the warm, humid Pliocene 2.5 to 5 million years ago.
How could we possibly know what temperature and CO2 were like millions of years ago? The seafloor’s layers hold the key to our knowledge of how Earth has changed through time.
Some of the particles that once fell through the ocean and got buried at the seafloor have special properties. Foraminifera, single-celled grazers that live both in the surface ocean and at the seafloor, make protective shells of calcium carbonate. When they build their shells, trace elements (like Paulina’s!) and rare isotopes are incorporated into their shells. The amount present in the shells is a function of key environmental parameters like temperature or CO2 concentration. By measuring these tracers in calcium carbonate shells from sediment samples deep below the seafloor, scientists can reconstruct how warm, how salty, or how CO2-rich Earth’s oceans were in the past.
Cibicidoides wuellerstorfi, a seafloor-dwelling foraminifera. Picture courtesy of Laura Haynes.
Why do we care what Earth was like in the past? Predicting the future is difficult—especially when it comes to Earth. Scientists use computer models of the earth’s atmosphere and oceans to predict how rising atmospheric CO2 levels will change the climate we live in. Measurements of Earth’s climate in the past are critical to our faith in the accuracy of these models for the future. To confirm that the models do their jobs correctly, scientists make sure the models can replicate the environmental changes that have been observed in the past—from our trusty chemical measurements in calcium carbonate from sediment deep below the seafloor.
A key time period for models to replicate is the last ice age, which reached its coldest around 20,000 years ago and ended for good about 10,000 years later. Atmospheric CO2 levels were nearly two times lower during the ice age than they were afterwards, causing the world to be much colder. The more we know about what the earth was like during that time, the more we can train and improve our climate models for predicting the future.
The Southern Ocean greatly affects Earth’s climate. It is the only place in the world where the deepest waters in the ocean are lifted to the surface and allowed to communicate with the atmosphere, like two friends catching up again after many years of separation. These waters, cold and rich with nutrients, supply food to the plankton living at the top of the ocean. However, the fertility of these deep waters comes with a drawback: it causes them to have high levels of CO2, which they release to the atmosphere upon arriving at the surface.
I am here on the RV/IB N.B. Palmer as part of a team from Lamont-Doherty Earth Observatory in New York investigating how this release of CO2 from the Southern Ocean to the atmosphere has changed from the last ice age until today. One of the leading theories for why there was an ice age 20,000 years ago is that the CO2-rich deep waters were trapped beneath the surface, perhaps due to increased sea ice coverage or changes in wind patterns, unable to communicate and exchange their carbon with the atmosphere. A large component of the atmosphere’s CO2 was thus sequestered away in the deep ocean, causing Earth’s climate to cool and kilometer-thick ice sheets to extend all the way down to the northern United States.
Frankie and Marty assist the gravity core’s passage into the ocean, while marine technician Joee communicates with the winch operator. Credit: Christina Riesellman
We have been collecting sediment cores 10-30 feet long to ensure we capture layers from the last ice age. Once we have shipped them home, we will pick out tens of thousands of calcium carbonate shells (one by one, under a microscope, with a tiny paintbrush!) in order to make precise chemical measurements of 14C, a radioactive form of carbon. From our analyses, we will learn whether deep waters could reach the surface and exchange their carbon with the atmosphere during the last ice age.
A photomicrograph of foraminifera shells we have collected from sediment. Credit: Christina Riesellman
One of the exciting parts of doing science is that you never know what the answers to your questions will be. Every time you test a hypothesis or conduct an experiment, you learn something new—something no one has ever discovered before. What will the radiocarbon content of calcium carbonate shells teach us about the Southern Ocean during the last ice age?
I don’t know yet, but I can’t wait to find out.
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