Being a scientist and trying to understand our planet is a bit like being a detective. We rarely see the full picture. Instead, we collect small pieces of evidence that, when put together, tell us something about how the Earth and life on it function.
The missing “person”? Carbon from deep within our Earth, that is emitted into the ocean at hot spots (so-called hydrothermal vents). Here, hot material from the inside of our planet comes in close contact with the ocean floor. As seawater penetrates the oceanic crust and travels deeper, it gets heated and becomes buoyant. As it rises back to the ocean floor, it interacts with the surrounding rock, changing its composition; it becomes a ‘fluid’. The carbon in the fluids originates from the Earth's interior, either from deeply buried limestone or from the Earth’s mantle, and has been isolated from the atmosphere for thousands to millions of years. When it is released into the ocean through hydrothermal vents, it suddenly re-enters a world dominated by modern carbon. But what happens next? Does it gas out, mix into the ocean, and disappear, and therefore becomes relevant to our climate? Or is it retained close to its source and becomes part of living systems?
This question is almost impossible to answer on a global scale. Therefore, we focused on one system first: a shallow-water hydrothermal vent field off Kueishantao (Taiwan). Here, we had sample material from two venting sites: one extreme vent at 8 meters water depth (‘Yellow vent’) and one more moderate vent at 13 meters water depth (‘White vent’).
Following the first clues
Hydrothermal vents are natural gateways between the Earth’s interior and the ocean. They release fluids that contain carbon with a specific isotopic signal: it is 14C-free. This radioactive isotope is created in the Earth's upper atmosphere by cosmic radiation. The resulting 14C then enters the natural carbon cycle as carbon dioxide and is absorbed by plants, microorganisms, and, ultimately, animals. As long as an organism is alive, the proportion of 14C remains almost constant. However, once an organism dies and is no longer exchanging carbon with the atmosphere, the 14C gradually decays, and after several tens of thousands of years, it becomes virtually undetectable. Carbon from the Earth's interior is extremely old and has been separated from the atmosphere for a very long time, and therefore no longer contains 14C.
But how do we follow the incorporation of 14C free carbon from the hydrothermal system into the ocean?
In the environment, we can theoretically measure anything with enough carbon for its 14C content. However, which material would be most useful to us? Within this question, I turned to lipids, molecules that form the membranes of cells. Different microorganisms produce different lipids, and these molecules carry chemical signatures that can tell us how the organisms lived and what carbon sources they used. Lipids can even be individually measured for their Δ14C content, which could tell us if the microorganisms build their biomass from modern or ancient carbon, or a mix of these two. In addition, the idea was to measure filtered material for the water column for its 14C content to follow the uptake.
Now, we had a plan, but this is also when the investigation gets a bit complicated. Like all good detective stories, this one did not go smoothly. But who would want to read a murder mystery book if they knew from the first page on who the killer was?
The main problem was that hydrothermal samples often contain very little biomass – in my case, well below 0.2%. That means only tiny amounts of lipids were available for analysis. Radiocarbon measurements, however, require a minimum amount of material, and getting there turned out to be one of the biggest challenges of the project. My first attempt used a Soxhlet extraction, a method that involves heating samples for a long time to extract lipids. It is a standard technique used to recover lipids for compound-specific 14C. The problem was that this method is used for plant waxes (longer molecules that are not as prone to degradation). In my case, it failed. The shorter bacterial lipids, which are especially important for studying microbes, did not survive the process. They were effectively lost. I then switched to a gentler method, the Bligh and Dyer extraction, which is normally used for microbial lipids and is better at preserving these fragile compounds. In hindsight, I could have known that this was the better option, but all detectives learn over time. This meant re-extracting everything from scratch (thank God I still had sample material!). After that came preparative gas chromatography (prep-GC), in which individual lipid compounds are separated and collected, one by one, into small glass traps. Traps that are very prone to breakage. During all this time, there is always the same question in the back of your mind: will there be enough material at the end?
But eventually, after a lot of careful work (and a bit of luck), we had enough material. We could measure the radiocarbon content of specific lipid molecules. The case could move forward.
Solving the case: where does the carbon go?
With the data in hand, we could finally return to the original question: what happens to ancient carbon once it enters the ocean?
The answer was at once obvious and surprising: Microorganisms in hydrothermal systems build their biomass using ancient carbon from Earth's interior, making them appear older than they really are (at least in 14C measurements). They incorporate approximately 30% of aged carbon into their cell membranes. We could follow the trace of this uptake in the sediments and in the water column – in the water column, we only used 14C measurements of filtered material (here, no time-consuming preparation was necessary). Especially at less extreme vents, the uptake was clearly visible meters beyond the vent source. In other words, ancient carbon quickly becomes part of living organisms. Not only microorganisms make use of this ancient carbon source, but they can be part of an entire ecosystem: larger organisms, such as crabs (‘Xenograpsus testudinatus’), feed on the microorganisms that previously built their biomass from ‘aged’ carbon sources. Meaning, while the crabs themselves are very much alive and modern, part of the carbon that makes up their bodies is not. In a sense, they are older than they should be, similar to the microorganisms at these venting sites.
You are what you eat
This is where a simple phrase becomes scientifically meaningful: you are what you eat. This phrase is originally attributed to Jean Anthelme Brillat-Savarin, who wrote "Dis-moi ce que tu manges, je te dirai ce que tu es" ("Tell me what you eat, and I will tell you what you are", 1826). In those times, it was about social standing and class distinction. In my case, the organisms are not only affected by their environment, but they are built from it. Carbon that has been locked away deep inside the Earth becomes part of microbial life and is then transferred through the food web. What began as a “missing person’s case” turned into a story of connection: between geology and biology and between the deep Earth and living ecosystems. Of course, this is only one hydrothermal system. There are many more across the planet, and each may behave differently. But this study provides a first step toward understanding how ancient carbon moves through these environments.
And, like any good detective story, solving one case often raises new questions. How much of this ancient carbon enters the global ocean? And how important is this pathway for the marine carbon cycle?
For now, we at least know one thing: The missing carbon was never really missing. We just had to know where and, especially, how to look.
And like any good detective, I could not have solved this case alone. This story was only possible thanks to the people who helped uncover the clues along the way: my co-authors, M. Elvert, H. Grotheer, Y.S. Lin, G. Mollenhauer, I.T. Lin, S. Bühring, and E. Schefuß.
This study builds on earlier work on 13C in Kueishantao, explained in the video, with the full paper available here.