Why should anyone care about the age of carbon dioxide coming out of the Southern Ocean?

First a bit of background:

The Southern Ocean is one of the main places where CO₂ escapes from the deep ocean and it functions as a release valve that fundamentally regulates how much CO₂ is in the atmosphere. But first CO₂ has to get into the deep ocean. It starts with photosynthesis in the surface ocean that  takes CO₂ that has dissolved into the ocean from the atmosphere turning it into organic matter. When the plants die their bodies sink into the deep ocean - sequestering that carbon in the deep ocean. This mechanism is so effective at pulling CO₂ out of the atmosphere, it's called the "biological pump". Once that carbon sinks it can be eaten, or metabolized by organisms in the deep ocean that turn it back into CO₂. This CO₂ then remains trapped in the deep ocean because the warm surface layer serves as a cap-kind of the way a cap on a carbonated drink keeps the fizz (which is CO₂ in the drink) dissolved in the water. Take the cap off, and the CO₂ escapes. In the Southern Ocean the deep, cold CO₂ rich waters rise to the surface and release this CO₂ that the deep ocean has been storing for hundreds of years (or more) to the atmosphere again.

How long this CO₂ stays stored in the deep ocean and more importantly how much stays stored can change. If somehow the effectiveness of the biological pump increases and puts CO₂ in the ocean faster, then the amount of CO₂ in the atmosphere will drop. Another way to reduce the amount of CO₂ in the atmosphere is increase the amount of time the deep water in the ocean stays deep and sequester, or holds that CO₂ in the deep ocean. The ocean holds 50 times more CO₂ than the atmosphere, so it is easy for this to happen. Changes in the deep circulation can make a big difference to the atmosphere's CO₂ content. We know that in the last ice age (or glacial period), there is lots of evidence that there was a change in ocean circulation that pulled CO₂ out of the atmosphere into the ocean. We also know that the CO₂ in the atmosphere in the last ice age was one third lower than it was pre industrial - this was an important factor in making that glacial period - or ice age - colder than today- an important climate feedback.

What we are doing:

What we have been studying is how and where that CO₂ got in and out of the ocean. Because... although there is evidence that the "missing CO₂" in the glacial atmosphere was in the deep ocean, there was no proof for where or how it came back out at the end of the ice age and helped warm the planet up.

We can track where that carbon dioxide thousands of years ago using radiocarbon to determine how old the water was (that is how long it was held in the deep ocean). Radiocarbon (¹⁴C) is the carbon isotope that is used to date, or determine the age of carbon containing items. Once the carbon is pumped into the deep ocean, the water begins to "age" and we can use the ¹⁴C content of that water to estimate how long it was sequestered. AND when that old water mixes up in the Southern Ocean and degasses to the atmosphere, that old or "dead" signal from the deep ocean is going to show up and leave a trace in the atmosphere. If we add a lot of old dead carbon to the atmosphere it's going to look older. The evidence from ice cores and tree rings show that is almost certainly what happened at the end of the last ice age. At the end of the last glaciation, the ocean circulation changed so that the CO₂ that had been in the ocean began coming back out-FAST. We know that the CO₂ release came out of the deep ocean because about the same time that atmospheric CO₂ increased rapidly, the radiocarbon content of the CO₂ decreased.

We can trace the past ¹⁴C content of ocean CO₂ thousands of years ago by measuring the radiocarbon in microfossils that grew in that water, that had been sampled from a deep sea core. By measuring microfossils from deep core cores in the Southern Ocean and around New Zealand we showed that the deep water was old. Now we are working to determine how and where all that CO₂ got in and out of the ocean. In this study we went back and measured microfossils from shallower that are close to where deep waters rise to the surface in the Southern Ocean. This location means that the microfossils there recorded the of age of the waters just after they had finished mixing and releasing their CO₂ to the atmosphere. That means we had a record not of the deep water that rose out of the ocean, but the signal of the mixing process itself. We were able to show that the deep waters rose to the surface and degassed fast and mixed very well. This gives us a better understanding of how the global circulation and circulation in the Southern Ocean in particular regulate carbon dioxide exchange between the ocean and atmosphere.

We care about how CO₂ came out of the Southern Ocean 17,000 years ago because we are putting a lot of CO₂ into the ocean today and we need to understand how the ocean exchanges CO₂ with the atmosphere as climate changes. The important changes in sequestration and exchange of CO₂ through the Southern Ocean that we studied occurred as global ocean circulation changed in response to the major global warming of that time. We can use this understanding to better appreciate how the ocean might respond to the increased CO₂ and potential changes in circulation that we are undergoing today in response to global warming.

About Liz Sikes

Liz Sikes began her career in oceanography as a masters student in the UNC Marine Science program studying deep coral reefs in the Straits of Florida. From there she moved on to do her PhD in the MIT Woods Hole Joint Program in Oceanography, specializing in paleoceanography. Her interest in climate change lead her to conduct her thesis research on refining a new sea surface temperature estimation technique. This technique ( U^k'₃₇ ), based on organic compounds, that is now widely used to determine past sea surface temperatures and quantify how much ocean temperatures changed in the past when during glacial to interglacial climate swings. In addition, she also studied past deep ocean circulation changes using isotopes. When she moved to Australia for a post-doctoral fellowship after finishing her PhD, Liz began to focus on the Southern Ocean’s influence on climate change – going on 6 cruises in the Southern Ocean to collect samples for her work, before she moved to New Zealand to work at the University of Auckland and then moved to Rutgers where she is now an Associate Professor.

After moving to Rutgers, Liz has continued to investigate the Southern Ocean’s influence on past climate and global circulation, traveling frequently to New Zealand to join research vessels voyaging to the Southern Ocean to obtain samples. In 2005, she was the chief scientist on the cruise that obtained cores that her team has been using to examine past temperature and deep ocean circulation over the last 30,000 years. When she’s not at sea (which is most of the time) Liz runs an organic geochemistry lab, where in addition to investigating past climate change, the group investigating what controls the biological production of the temperature marker compounds in and modern carbon cycling and carbon transport in the coastal ocean. The thing she finds most interesting about being an oceanographer is the fact it truly is an interdisciplinary science where she can use her training in geology, chemistry and biology to investigate the exciting questions in marine science today.