Sea Sediments

Scientists hunt for records of past climate in many places — icy glaciers, rock layers, even the bottom of the ocean. How scientists deduce past climates from long tubes of ocean bottom sediment?

Tiny creatures — called plankton — live in the ocean. When they die, they drift to the bottom of the sea. There they form layer after layer of miniature fossils. Analyses of these fossil layers help reveal past climate.

That’s because some plankton are known to prefer warm water — while others prefer cold. Maureen Raymo is a geologist at Boston University. She studies ancient climate by taking sea sediment cores.

Maureen Raymo: It’s essentially from the deck of a large drilling ship, we put a large pipe down, like sticking a straw in the bottom of the ocean, and then we put our finger over the top of the straw and draw it up. And so we bring up a long tube of the sediment … that goes back through time. As we go down the core we’re getting time slices of the ocean at times in the past, and the animals and plants that lived in the ocean at that time.

Sediment cores have revealed a cooler climate for Earth over the last 40 million years. In 1988, Raymo proposed that this cooling was due to the rising of the Himalayan mountains. As the sides of the mountains eroded, chemical reactions might have removed greenhouse gases from our atmosphere — resulting in a cooler Earth.

Cores taken from the Dead Sea. Courtesy of the Minerva Dead Sea Research Center.

Sometimes, it can take days to drill a 1000-meter core of ocean sediment and lift it out of the ocean floor. By contrast, it takes thousands of years for just a few centimeters of sediment to form from these bodies.

Scientists studying ice cores have found evidence for abrupt climate changes in Earth’s past. For example, during the last ice age, some places warmed as much as 15 degrees Fahrenheit — or 8 degrees Celsius — in just 10 years. According to Maureen Raymo, “Ice core records have HIGHER resolution than ocean records — they just don’t go back very far in time.” So ocean cores are needed to extend the climate record.

In the early 1980s, Raymo took her first ocean sediment core. Again, about five years ago, Raymo led an expedition to the North Atlantic Ocean to recover more samples. She’s still in the process of synthesizing the information from her last sediment cores.

Raymo won’t be going out on more coring adventures in the near future. She has small children at home who, as she puts it, don’t take too well to mom disappearing for several months. She spends most of her time trolling the journal literature, trying to keep up to date on various research projects and hunt for new avenues of research. When I asked her if she does labwork any more, she said no not really — its the grad students that do all the plankton counting and such work. She does the background research and the synthesizing. Pretty standard for someone in her position, actually.

Excerpts from an a Interview with Maureen Raymo:

Just like trees or animals, there are different species that like different temperature environments. There’s cold-water species, there’s warm water species. So we can look at the relative abundance of different species, and infer sea surface temperature patterns.

It’s essentially from the deck of a large drilling ship, we put a large pipe down, like sticking a straw in the bottom of the ocean, and then we put our finger over the top of the straw and draw it up. And so we bring up a long tube of the sediment … that goes back through time. As we go down the core we’re getting time slices of the ocean at times in the past, and the animals and plants that lived in the time.

They really are giving a record of climate that goes back millions of years, a record you could never get in an ice core, which only goes back about 100,000 years.

When did you get interested in science?

I had always wanted to be a scientist. I can remember being six and wanting to be scientist. I remember by the time I was seven, Jacques Cousteau, was my hero, and I wanted to be an oceanographer. Specifically a marine biologist. By the time I got to college, mountains and geology had really grasped my interest. So when I found out in college there was a way to combine geology with oceanography, I thought I’d really stumbled into my perfect field. Someone who had always wanted to be some kind of scientist who worked in the field, finding the perfect combination of science disciplines for them.

It’s interesting, there are many many people in my field, in my generation, who were inspired by Jacques Cousteau. He sent many people to the ocean.

What other projects are you working on?

I’m working on this project with a grad student. One thing we know about how earth’s climate changes through time is that it’s controlled by our distance from the sun, which varies through time, because the orbit is not perfectly circular. However we don’t know exactly how those subtle changes in orbit are translated into the climate changes. One example of this, for most of the Pliocene and early Pleistocene period, which is the last few million years, the ice sheets in the N. Hemisphere grew and retreated at a periodicity on a cycle of 40,000 years. And yet if you look at the insolation, and look at the sunlight received at the high latitudes, it varies with a cycle of 23,000 years. So there’s this mismatch between the dominant solar cycle, and the dominant ice age cycle.

So we’re trying to use data and climate models to show how this occurred. There has to be something about the physics of the climate that we are not understanding. And we think it has something to do with the gradients in heating between low and high latitudes. [18] The idea is that it’s not so much directly the amount of sunlight received at high latitudes that controls the ice volume, as it is the amount of moisture and heat transported poleward from low latitudes. So when there’s times when there’s much more moisture transported from low to high latitudes, by stronger latitudinal temperature gradients that might really promote the growth of ice sheets at that time despite what this sunlight at high latitudes is doing.

What you have in your mind?