Coring for Clues

How do we know what the climate was like thousands of years ago, before there were instruments to monitor it? Research geologist Dave Wahl finds answers at the bottom of bodies of water. Wahl received a PhD from UC Berkeley in paleoclimatology, and now works on the Pacific Holocene Climate Project for the US Geologic Survey in Menlo Park.

Your career has been distinguished by your affiliation with a particular tool: the sediment corer. Tell me about it.
The main method that I use is looking at various components of sediment that is in lakes, bogs, marshes, or the bottom of the ocean, and I use a sediment corer to get them out. The sediment at the bottom of a lake accumulates with time, so the farther you go down into that sediment column, the further back in time you are going. You can get into that sediment and get snapshots of time going back thousands of years.

What is the advantage of sampling sediment from the bottom of a body of water?
At the bottom of a lake, there is very little oxygen, so anything that makes it down there is going to be preserved because of this anoxic condition—there’s no microbial breakdown or oxidization happening. You can compare one level of sediment to the next, look at frequency changes, and talk about real changes.

You work for the USGS, but you also contract yourself out to clients who want you to get core samples for their research.
There are a ton of scientists out there that work on cores that other people have taken, but they don’t have the ability or wherewithal to get these sediment cores themselves. I have this mechanical side of my brain, where I like working with my hands, so I have designed, built, and refined all these coring devices to push them in different directions. And that has led to a lot of cool trips to various places, like a frozen lake in Banff, Canada or my recent trip to Palmyra, an atoll in the middle of the Pacific Ocean.

I’ve been getting into these deep lakes with this hand-operated equipment. I am being contracted out basically for the labor of getting the core. But generally, people who are interested in getting the core have one objective. [For example,] they may be interested in looking at charcoal to reconstruct fire history. And I can have the rest of the sediment to look at geochemistry, stable isotopes, pollen, and pursue my own research interests.

Are clients asking you to look into climate change?
Not necessarily. I did my dissertation and lots of research down in Guatemala, and there was definitely a climate component, but I was also looking at human impacts on the environment. There are so many things you can look at when you are looking at lakes. Lakes are a depositional basin—the trash can of the environment. Everything that happens in a watershed will show up in the sediment of a lake. It washes down and eventually gets preserved. I can look at erosion, deforestation, the human burning of vegetation. I can look at prehistoric Native American activity. For this island core [from Palmyra], the funding is coming from National Geographic,
and the questions [the client] is asking is when the coconut palm arrived on this island, and did the Polynesians bring it as part of their big oceanic voyages, or did the seed arrive here on its own?

The client knows that I am interested in paleoclimatology studies, so she’s going to need one cubic centimeter of sediment from forty samples that size. But there are hundreds and hundreds of cubic centimeters of sediment left for me to dive into and look at precipitation records, sea surface temperature records, El Nino… I can get all this data out of that from this killer spot right in the middle of the Pacific Ocean.

In your USGS work, is there a push to study climate change?
Right now, there is a big push toward climate, so I found myself situated nicely coming out of school with this degree in paleoclimatology. Even if you look at the most recent IPCC [Intergovernmental Panel on Climate Change] report, we have only been taking temperature and precipitation records for a maximum of 150 to 200 years in some places. The statistically reliable data shows this trend toward warming. But we have to know what the baseline is. And that’s what I do.

We’ve gotten the modern instrumental record, but we need to know, over the last 10,000 years, what has climate, precipitation, what has temperature done on its own so that we can really talk about what we are doing to the planet. USGS is interested in that for predictive reasons.

Climate studies can be broken into three components: the paleo studies (which is what I do), the modern monitoring, and the modeling. Basically, you take the modern data and tie it in with the paleo data that goes back thousands of years, and that is what makes a robust model. You can’t model what the future is going to do just using the instrumental data. You have to understand the baseline data in order to judge whether the data from modern monitoring is within the range of natural climate fluctuation. Data from the sediment cores provides the backdrop against which you can judge the changes that have occurred in the recent past.

What are the other tools that scientists use to get climate data from the paleo records?
People take cores out of ice sheets, like the ice drilling that has happened in Greenland and Antarctica. They are very valuable. They go back hundreds of thousands of years. In Al Gore’s An Inconvenient Truth, his giant graph showing the CO2 curve is based on ice core data. Ice cores are based on the same principle—it is a depositional environment that grows with time, so the farther you go down into the core, the farther back in time you are going.

People also look at tree rings. All of the various methods have strengths and weakness. Ice has little bubbles of air trapped in it that are snapshots of what the air was like at that time. They go in and extract that air and quantify the CO2 in it. You don’t get that in the sediment, but the beauty of the sediment is that there is no oxygen, so you get nicely preserved microfossil samples. All these approaches have a suite of analyses that they are ideal for. The ideal is to pull them all together and to synthesize these big records.

How do you actually get the core sample?
I use a gravity corer. I drop it, and it will stick into the mud. It is weighted and has an empty barrel at the bottom. However much weight I have attached to it will dictate how deep it will sink into the mud. I can adjust that back and forth to maximize the mud I’m getting. You can typically go a meter and a half or two meters into the mud.

In Banff, with a hand-operated design, working on ice, I was able to put the corer on a raft, which no one else has done before, and get this thing dialed in. The lagoon in the middle of the Palmyra atoll was fifty meters deep. I have a tripod on my raft, and there is a weight on a guide tube that you lower it in and let it do the gravity thing. I have a rope attached to that weight that goes up into a pulley on my tripod, and I pull it up five feet, then drop it. Then pull it up again. Basically, it’s a percussion. It goes in about a centimeter each time.

Doesn’t it muck up all the layers?
Amazingly, it doesn’t. There’s the sediment-water interface, which is a little bit sloshy, but once you get below that, the best description of most sediment is that it is the consistency of toothpaste.

What’s your favorite thing to look at once you’ve got your core sample back to the lab?
Pollen. That’s my main thing. It’s just amazing. It’s beautiful. It’s everywhere. There are probably 100,000 pollen grains that fell on us today. It’s a complete representation for the plants that are growing anywhere. It’s got a very cool morphology, and I like the process of it. You can count charcoal fragments. You go centimeter by centimeter through a core and get a complete fire history of an area. You can tie that in with the vegetation and look at fuel loads versus fires and how all these dynamics work together.

What has been one of your more surprising finds?
Apart from the dead body I found in the lake in Mexico? I have worked on a number of cores from Guatemala, where I did my dissertation, and the collapse of the Mayan civilization shows up visually in the view of the core. There was such a dramatic shift in the environment when the area was depopulated around a thousand years ago. I look at pollen and all this other stuff and that tells me when exactly that happened. But when I go back and look at the core itself, I see the line that goes from a clay-rich carbonate mud to almost pure organic mud above it, indicating that transition. And it happened on a dime. What caused the collapse of the Mayan civilization is the million-dollar question. I think it was a combination of environmental degradation and climate shift. There was an incredibly long history of deforestation and erosion, and then the climate shifted right when they were really overextended. It’s becoming clear that it was a double whammy.

What’s your personal take on climate change?
It’s a moral issue. I’ve puzzled over our role in climate change. There’s a lot of fear-mongering around climate change. Some populations are going to be severely affected, others are going to be just fine. Some species will go extinct, others won’t. That’s basically the way the earth has been moving along for four billion years. So what it boils down to is, do we have a moral obligation to not push all these other species to the brink? Because we have consciousness and an awareness of what we are doing; because we have the ability to make a decision; therefore we have the responsibility.

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