Mud libraries hold the story of the Earth’s climate past — and foretell its future

Tucked away in the rolling green hills of the New York Palisades, there’s an unusual library: the Lamont-Doherty Core Repository. Instead of shelves, it has more than 50,000 white, 8-foot-long trays. And instead of books, those trays hold chalky whitish half-cylinders of sediment.

“It’s a mud library,” says Nichole Anest, the lab’s curator and self-described “mud librarian.”

These sections of mud, known to scientists as marine sediment cores, are special because they contain Earth’s history, written in the language of minerals and microscopic shells.

Most critically, tubes of mud like these are “the backbone of climate science,” according to Anest. She sends around 4,000 samples from this library to researchers around the world every year. Those samples contain key information that helps scientists piece together the story of the Earth’s climate going back hundreds of thousands of years: how our planet’s changing position in space can change temperatures, and how shifts in greenhouse gasses affect climate.

“Within a human lifespan, you can’t get a big picture of what’s normal because we’re so fleeting,” Anest says. But by reading these unusual “books,” then, researchers can better understand what is — and isn’t — normal for our planet, on the scale of thousands, even millions of years. But looking back at Earth’s history also helps predict our future: It helps scientists build the computer models that predict what the world might look like in generations to come.

Mud libraries like this one are not complete, though, which limits scientists’ ability to predict the future. And while researchers are adding to them year by year — pulling up new “books” from the ocean’s depths — they’re about to lose one really excellent source of new cores due to funding cuts, leaving a lot of potential gaps in our understanding.

The mud at the bottom of oceans is made up of detritus from all around the world. Dust from land can blow over the waves before sinking down to the ocean’s depths. Ocean currents and even icebergs can carry sediments around, and then drop them to the floor below as they lose energy or melt. Tiny sea creatures called foraminifera float at the surface of the ocean, creating shells for themselves out of calcium, carbon, and oxygen. When these organisms die, they sink to the seafloor, mixing with the other material there.

All these sediments build up very slowly over time: usually around 2 centimeters every 1,000 years. But as conditions change on the surface of the ocean, or on land, the sediments change too. “Each layer is like a page out of the book of Earth’s history,” says Anest.

Similar layers also build up on land, too, but those make for a less reliable historical record. “On land, those layers have been all crumpled up,” Suzanne O’Connell, a geoscience professor at Wesleyan University who works with marine sediment cores, told me back in 2021. Layers of sediments are constantly being shuffled around by erosion and weather, or the formation of mountains. “In the ocean, nobody’s crunched them up yet.”

Which is why, for the last half-century or so, various ships have been deployed to travel all around the world, lowering drills and other equipment to extract these historical records from the seafloor. It’s a complicated process that involves lowering a drill through the ocean’s currents, down to specific spots on the seafloor. Rebecca Robinson, a professor of oceanography at the University of Rhode Island, says it’s like “stringing dental floss with a toothpick tied at the end off of the Empire State Building and trying to get it in a bucket.” Tricky work.

But researchers have done this delicate process many, many times. The Lamont-Doherty Core Repository alone holds close to 20,000 cores. Most stretch back hundreds of thousands of years. (The Lamont-Doherty Core Repository is just one library of many around the world. Some have samples that go back much further — on the scale of millions or tens of millions of years.)

Anest showed me how researchers “read” the history in these cores. First, she pulled out a core from the Northeastern Atlantic.

It’s full of little gouges. Like the dog-eared pages of a well-loved book, these gouges show how many times scientists have investigated its contents (each gouge is a sample a scientist has taken for study). Anest says this core has been heavily studied because it is a little easier to read than most. Every 4 centimeters of the core represents 1,000 years of history. (Usually, with other cores in this collection, it’s closer to 2 centimeters for 1,000 years.) That makes this core more like a high-definition picture. There’s more detail available. And so researchers have used it to understand the changes in climate in between ice ages, for example, and ice age cycles themselves.

How do they do that? If the layers in this mud are like the pages of a book, then the tiny foraminifera and other sediments in them are like the words that scientists read. There are thousands of species of forams, many with very specific niches. Some respond better to warmer or cooler temperatures, for example, so the species of forams in each layer can tell researchers something about the climate at the time.

The composition of forams’ calcium carbonate shells is also telling. They always form their shells from the same chemicals — calcium, carbon, and oxygen — but the nature of the chemicals available in the seawater around them can change over time. Oxygen, for example, can come in several different forms. When there are a lot more glaciers around, for instance, there tends to be a lot more of a heavier type of oxygen, also known as a heavier “isotope,” available in the water.

Heavy oxygen dominates in the ocean during ice ages because water with the lighter form of oxygen evaporates more easily. When the world grows colder, that evaporated water then turns into snow and gets trapped in glaciers, leaving more of the heavier oxygen around in the water itself. When the glaciers then melt again, that lighter form of oxygen is released back into the oceans. And all these shifts are recorded in the shells of the foraminifera.

If the forams in one core give researchers clues about this one aspect of climate, then the clues in other cores help them put together other pieces, building towards a fuller picture. Animals and minerals in the mud have helped them to understand changes in ancient ocean currents that loop around the world, or to infer the presence of icebergs to study the periodic nature of ice ages.

“I love looking under the microscope at all the minerals that come up,” says Leila Tarabein, Anest’s research assistant, listing off a jewel box of options with enthusiasm: “Bright orange quartz, or lots of rose quartz, or your classic clear quartz.”

The size of these mineral grains is informative, especially if a core has some relatively large grains of minerals in it.

“Large usually meaning 1 millimeter,” clarifies Tarabein.

When researchers find “big” pebbles like this in and amongst the smaller ones, they know that they were probably dropped by melting, breaking bits of icebergs.

And mud can also contain clues about conditions on land. If a continent is covered in forests, for example, pollen from those trees can drift over the ocean and settle into a layer of sediment. If that continent is then covered in ice, the next layers of sediment record contain much less pollen, the changes in pollen — marking the absence of the disappearing trees. If atmospheric shifts make winds start to blow more aggressively on land, or drought makes soil dryer and dustier and easier for winds to transport, those changes, too, will be reflected in the layers of ocean mud. Anest showed me a mud core that shows that, approximately every 20,000 years, the Sahara turns from desert into grassland due to heavy rainfall.

As researchers study lots and lots of these cores, comparing their results, and piecing together clues, they build up a story about the Earth over time. “You’re not going to base anything off of just one tube of mud,” Anest says. But with many “you can start to piece together this bigger picture.”

If you’ve read articles about how changes to the large ocean currents changed the historical climate in Europe or heard about the periodic nature of ice ages, those narratives come, in part, from libraries of muddy sediment cores. And this record of past climate changes helps us predict how our climate could change in the future, feeding models.

Researchers are still missing some really critical pieces of this puzzle, though. They need more information, from cores they don’t yet have, cores that haven’t yet been added to our existing mud libraries.

Take the Pliocene, for example, a period of time around 3 million years ago, or the Miocene — over 10 million years ago. Scientists believe these time periods were very warm due to increased carbon dioxide in the atmosphere. But while they have some idea of the atmospheric conditions of those time periods, they still don’t know as much as they’d like about how those conditions affected icebergs or weather systems.

“If we want to know how the monsoons responded in these warm climates, we need to get into this hard-to-core material that is in the Bengal Fan,” Robinson explains, giving just one example. The Bengal Fan is an underwater delta in the Bay of Bengal — a large spread of sandy sediments that can be difficult to grab cores from. If researchers could core it, and learn how monsoons responded to extra carbon dioxide in our atmosphere in the past, we may get a better picture of how dangers like flooding will mount as our Earth warms today.

Only two ships are currently equipped to drill very high-quality cores that stretch back far enough in time to answer these questions. In March of this year, the National Science Foundation announced that it would retire one of them, the JOIDES Resolution, next year.

Robinson was recently the chair of the US Advisory Committee to Scientific Ocean Drilling, advising the NSF as it made its decision about the JOIDES Resolution. She explains that the $70 million or so budget for the ship proved difficult to fund. The ship is also over 40 years old and was set to come offline in 2028. Now there’s no replacement in sight.

There are alternative ways to potentially collect new material — hiring vessels that can drill at depths, but are not specifically set up to collect cores, for example — and she is hopeful that there might be a new core drilling ship built in the future, but that future is uncertain.

The loss of the JOIDES Resolution will make a dent in what we can know about our world, though, and how much of its history we can read.

“It’s limiting our ability to ask creative questions and come up with research designed to explore those questions,” Robinson says.

But there is still reading to do. Back at the core repository, there are still thousands of cores that have not yet been explored — chapters of Earth’s history we have not read.

Kimberly Mas contributed reporting.