by Curt Stager
For professionals who deal directly with the geochemical effects of worldwide fossil carbon contamination on a regular basis, any doubt about its existence could only be found among the uninformed or the most hard-headed contrarians. Although its best-known attribute is its ability to change climate, the pollution itself isn’t necessarily all bad in this context; some aspects of it, as we’ll see in this chapter, are arguably somewhat positive. But its undeniable presence makes a powerful statement: the massive human influence on global carbon chemistry is real, measurable, and scientifically significant.
Chief among those for whom fossil fuel pollution is a decidedly here-and-now issue are three kinds of specialists: ecologists who monitor the levels of biological activity in ecosystems, forensic scientists who seek clues about when and where things have happened in the relatively recent past, and geoscientists who read deeper histories in ancient deposits of mud, ice, wood, and stone. These investigators use carbon atoms as tools in support of esoteric techniques that have revolutionized scientific understanding of the natural world, but their work is now strongly influenced by the same gases that are also changing climates. Fossil fuel carbon pollutes far more than air and water alone; it also contaminates the fundamental atomic structures of organisms and ecosystems, changing them in ways that most of us are unaware of.
Some brief introductions are in order here. In our daily lives we deal with chemical elements only as homogeneous clumps of unimaginably numerous particles. But not all carbon is alike, as the diagnostic 13C signal in PETM-age deposits makes clear. Scientists distinguish among three forms of it, separating them on the basis of weight, and each plays a unique role in the Anthropocene world.
These isotopes—or varieties—of carbon are like fraternal triplets, and apart from their weight they are similar enough to one another to appear in the same kinds of molecular compounds, from CO2 and methane to proteins and genes, and they participate in many of the same chemical processes. They differ mainly in the number of tightly clustered particles that make up their central nuclei.
Carbon-12 (or 12C) is what you might call the “normal” sibling, and 99 percent of all carbon atoms are of this sort. However, one in every hundred or so carries an extra neutron in its nucleus, which raises its atomic weight and changes its name to carbon-13 (13C). Beyond that the difference is minimal, rather like a slight paunch on a formerly slim, middle-aged person.
And then there’s carbon-14 (14C). One might say that it’s the bad apple in the bunch, a troublemaker with a reputation for making sudden outbursts. It is much rarer than 13C is, and one neutron heavier. Furthermore, it’s much younger than its siblings are. The two lighter isotopes were born long ago in the internal fusion reactors of distant stars, and they are essentially immortal, but 14C forms continuously in Earth’s upper atmosphere and it is doomed to die an early death. Its explosive personality stems from the physical stresses induced by that extra neutron, which doesn’t fit very well in the already-crowded nucleus; it’s almost as if its neighbor particles want to flick the superfluous neutron away. Because of this instability 14C is “radioactive,” meaning that sooner or later it spits out a nuclear chunk and undergoes a sudden conversion to a lighter, more stable state of existence (as a born-again nitrogen atom, to be precise).
A gram of carbon from coal, oil, or natural gas contains less 13C and 14C than a gram of carbon in atmospheric CO2 does. In part, this is because the long-ago plants and algae whose bodies formed those fossil fuels selected normal 12C over the heavier isotopes when building the molecular frameworks of their cells, just as their descendants do today. When a plant or alga inhales a CO2 molecule, it treats it like a seafood chef treats a hard shell clam. Out comes the shucking knife and off come one or both of the shells to expose the nutritious gob at the center; in the case of plants, the clamshells are twinned oxygen atoms and the gob is a carbon.
But like a finicky connoisseur, a leaf or algal cell is choosy about the molecules it consumes. Carbon dioxide molecules that contain heavy 13C and 14C atoms tend to be tossed aside in favor of the lighter, normal versions. Nevertheless, a very few oddball carbons do manage to slip into the mix and take up residence in the living tissues of plants.
Having already passed through the filters of once-living bodies, the carbon stored in fossil fuels has been partially scrubbed of those heavy isotopes. Furthermore, any unstable radioactive 14C that once existed in those ancient deposits has long since broken down. As a result of these processes, fossil fuels contain only a tiny remnant of the isotopic carbon diversity that one might encounter in a random puff of wind. And so do the carbon-rich fumes that emerge when fossil fuels are burned. In an odd twist of chemical ecology, fossil fuel exhaust actually helps to “purify” the air of heavy isotopes by diluting it with lightweight 12C.
The human-driven deficit of 13C and 14C in the atmosphere is called the Suess effect, after Austrian American scientist Hans Suess who originally measured it (and who, to his irritation, was often the recipient of fan mail to his contemporary, “Dr. Seuss”). Because atmospheric carbon flows abundantly through the world’s food chains, moving from plant sap to rabbit muscle to fox DNA, the telltale signs of the Suess effect are intimately woven into the molecular tapestry of life on Earth. Normally, they go unnoticed, though we carry them in every lump and fiber of our bodies. But for many ecologists, the Suess effect is a threat to the accuracy of one’s data.
Imagine investigating the ecological history of a lake, as was done recently for Lake Erie. Phosphorus pollution from cities, lawns, sewer pipes, and farms fed nuisance algae living in its surface waters for many years, boosting nasty aquatic scum growth just as fertilizers boost crop yields on land. Each year, new layers of dead algae piled up on the lake floor, enriching the mud with their carbon-laden remains and preserving a nicely layered, if rather fetid sedimentary record of the lake’s pollution problems.
Like plants, algae preferentially remove lightweight 12C from solution as they absorb waterborne CO2 for photosynthesis, and the green, plankton-choked surface of Lake Erie began to show signs of that selectivity. Like a desktop candy dish full of tasty jelly beans plus a few pebbles, over time the water became more and more abundantly stocked with unwanted heavy CO2 molecules that the algae left behind. As the chance of accidentally plucking a pebble of heavy 13C rather than a desirable 12C increased, generations of green slime became more and more enriched with 13C as they lived, died, and sank to the mucky bottom of the lake.
In the mid-1980s, after strict water quality regulations were put into place, phosphorus inputs to Lake Erie decreased to a quarter of their earlier volume; the city of Detroit alone reduced its annual phosphorus outflow by nearly two-thirds. But was the strategy working?
University of Florida ecologists Claire Schelske and David Hodell decided to find out by probing the sediment archives under the lake. They knew that the 13C contents of bottom muds should have increased while the algae were abundant. They also knew that cleanup efforts should have shrunk the algal crops and therefore slowed the delivery of 13C to the bottom. By lowering weighted core pipes to the lake floor, they collected sedimentary records of the last century and measured the 13C contents of sequentially stacked layers of mud. Sure enough, the 13C values rose to a peak in strata that were deposited during the late 1960s, when the 13C-enriched algae were at their thickest. Then the trend reversed direction, dropping all the way back to pre-pollution conditions by the 1980s.
That seemed like good news at first, but something about it didn’t ring true. The landscapes around Lake Erie are now more heavily populated and developed than they were at the turn of the twentieth century, and yet the 13C records seemed to show a complete recovery. Were the pollution controls really so effective? Schelske and Hodell doubted it, and they had a good idea of what might cause such an illusion of exaggerated success.
Sure enough, it was the Suess effect at work. Carbon-13 concentrations in algae worldwide have been declining throughout the twen
tieth century, thanks to the burning of fossil fuels. When that factor was included in the sediment core analyses, the recovery of Lake Erie didn’t look nearly as complete as it had before. The hoped-for turnaround to cleaner waters did occur, but the Suess effect had exaggerated the subsequent decline of 13C concentrations in the algae-laden muds. In fact, the lake was still a fair bit slimier than it used to be in the early 1900s.
The newly corrected take-home message was still supportive of regulations; they had certainly saved Lake Erie from becoming a vat of green pea soup. But by recognizing the distorting effects of fossil carbon in the environmental record, Schelske and Hodell also showed that more cleanup work still remains to be done.
In like manner, fossil carbon pollution is also complicating studies of climatic change and aquatic ecology elsewhere around the world. In 2003, I worked in the lakeside town of Kigoma, Tanzania, as an instructor for the Nyanza Project, an undergraduate research training program supported by the National Science Foundation and the University of Arizona. Shortly after my arrival, several of my coinstructors and colleagues published papers showing that Tanganyika, the lake upon which our field studies were based, is warming.
Lake Tanganyika is huge, roughly 400 miles long (670 km) and almost a mile deep (1,470 m), making it the world’s second deepest lake, after Siberia’s Baikal. Most of it is devoid of animal life because the deeper portions contain no oxygen; only the upper 300 feet (100 m) of surface water has enough photosynthetic algae and churning wave action to oxygenate it. Nonetheless, hundreds of species of colorful cichlid fish live in that relatively thin upper zone and nowhere else on Earth. The lake’s beautifully clear waters also support delicately sculpted snails, freshwater crabs and jellyfish, and even aquatic cobras, all unique to Lake Tanganyika.
The two sets of authors, headed by Vassar College’s Catherine O’Reilly and Piet Verburg from the University of Waterloo, Ontario, compiled different assortments of data and observations, but they independently reached the same conclusion. The waters of Lake Tanganyika have warmed by about 2°F (1°C) during the last century, a pattern that resembles similar trends in neighboring Lakes Victoria and Malawi, as well.
The O’Reilly team went another step further, though, by linking that warming to the main protein source of many local Tanzanians: fish. By studying Tanganyika sediment cores, they found that the younger layers contained less 13C than the older layers did, which could mean that algae were declining as they did during the cleanup process in Lake Erie. But nobody has been controlling phosphorus pollution in this lake, so something else must be at work.
Students and staff of the Nyanza Project preparing to collect a sediment core from Lake Tanganyika in 2003. Curt Stager
O’Reilly’s group proposed that warming had stabilized the lake’s surface by making it less dense and therefore more buoyant, rather like capping it with a thick layer of oil. Planktonic algae can find it difficult to stay afloat in the increasingly isolated sunlit zone, and they begin to sink into deeper, darker waters where they die of light deprivation, as plants will blanch when locked away in a darkened room. Such a planktonic population decline, in turn, could weaken the entire food web and leave the sardinelike fishes of the open lake with less to eat. According to O’Reilly’s calculations, this might cause large declines in annual catches, quite sobering news in an economically depressed region where about a third of all dietary protein comes from Tanganyika fish.
However, the case may not be as neatly closed as it first appeared to be. In a later reevaluation of the evidence for algal declines, Verburg applied a standard Suess effect correction to the sedimentary records of 13C, the O’Reilly team’s measure of plankton productivity. When he included the skewing effects of fossil carbon in the analysis, he found that 13C concentrations (that is, algal growth) may actually have increased where agriculture and other human activities pollute the near-shore waters with nutrient-laced runoff. Although the lake is indeed warming, it is now unclear what effect, if any, that trend is having on the resident plankton or fish.
These and a growing number of similar stories show that fossil carbon pollution contaminates ecosystems worldwide. Scientists usually measure the Suess effect in units that reflect the abundance of heavy carbon in relation to regular carbon-12, and the values of “delta-13C” (or 13C divided by 12C) have been falling faster and faster as we pump more and more lightweight fossil carbon into the atmosphere. During the eighteenth century, a typical sample of CO2 from the air would yield a delta-13C value of—6.3 parts per thousand (ppt); today, after two centuries of dilution with ancient carbon fumes, it is down closer to—8 ppt.
Though few of us have noticed it, we’re already living through a global 13C decline comparable to the one that the PETM super-greenhouse caused 55 million years ago, and most of it has happened during the last century. The Suess effect now lowers global delta-13C values by nearly a fifth of a unit per decade. At that rate, our Anthropocene isotope excursion could reach that of the PETM by the time airborne CO2 concentrations peak within the next few centuries. If a team of oceanographers drives a core pipe into sediments of the deep sea at some point in the distant future, they’ll find signals from our carbon-enriched times that resemble those left by the PETM in the reddened layers of Lowell Stott’s marine cores. The physical effects of these isotopic changes on our daily lives are easily missed, but to scientists who work closely with them their historical significance is breathtaking. Without even knowing it, we’re writing our own environmental epitaphs in indelible carbon isotope ink.
Dating the records of our times won’t be easy, though. The same Suess effect that dilutes 13C in the environment today also dilutes 14C, the radioactive tool of choice for dating old historical objects. Carbon-14 is not a perfect guide to the past; its shelf life (more properly termed “half-life”) is short in comparison with those of radioactive marathon runners such as uranium-238, which is used for dating rocks that are billions of years old. It only works on things that contain carbon, such as wood, bone, shell, peat, or aquatic muds, and only on those that are also less than about 50,000 years old. Even so, the time-tracking ability of radiocarbon dating has revolutionized our understanding of human and environmental history. Unfortunately, it won’t be quite as useful for future scientists, thanks to the Suess effect.
It’s easy to imagine some of the questions that scholars might want to ask when they look back on the story of our times. When did the last ice sheets melt away? At what point did the oceans finish acidifying? How rapidly were national boundaries reshuffled in response to sea-level rise? Some of those questions will be answered by written documents, but not necessarily as many as we might think.
Much of today’s recorded history will eventually be lost simply because so many of its documents are electronic. The devices and codes that create and decipher them change so often in the interest of big business that they become useless within decades or less, not to mention centuries. The effects of this are already apparent in my own home. I’m still saving some old-fashioned floppy discs that used to feed data into my TRS-80 computer back in the 1980s, even though I’ll never be able to read them again. They’re so full of hard-won information that I just can’t bear to throw them away. And don’t even mention my old eight-track tapes, or the memory chips from my first digital camera that the latest card readers won’t read.
Fortunately, the geological archives of ice, coral, tree ring, stalagmite, and sediment layers around the world are still depositing their annual environmental updates as they have for millions of years. Even deposits in what are now thinly inhabited regions contain indelible signs of our presence on the planet, and carbon isotope glitches or buildups of airborne lead and other pollutants will identify them as such. Those signals will speak as clearly as written text to knowledgeable people in later phases of the Anthropocene, long after today’s information technology moguls and their ephemeral media are forgotten.
Or rather, all but one of those traditional g
eochemical signals will continue to come through clearly. As a direct result of our fossil fuel emissions, the code with which we read 14C records has now been scrambled. Before I can explain exactly why this is so, you’ll need to understand how 14C dating works. Let me begin by informing you, dear reader, that you are radioactive.
Because the carbon-based food molecules that we build our skin, flesh, and bones from contain small amounts of 14C, we are all slightly radioactive. And that’s because the photosynthetic plants and microbes that support Earth’s ecosystems absorb 14C atoms with the CO2 that they inhale day after day. Radioactive carbons in your average lungful of air are quite rare, forming less than one in a trillion CO2 molecules, but that slight dose of natural radioactivity infects all of us through the global network of food webs. As much as one in every four cells in your body contains a 14C atom in its DNA or in the histone proteins that surround it, and a gram of carbon purified from any given body part contains enough 14C to trigger a little more than a dozen telltale clicks per minute on a Geiger counter, a frequency roughly equivalent to that at which an average person draws breath. According to one estimate, a typical adult human experiences about 300 internal 14C explosions every second.
How did the air become radioactive in the first place? Cosmic rays. They come at us from all directions, in waves of subatomic debris that blast through space from distant stars and galaxies. After traveling many billions of miles on any one of countless possible paths, some strike the upper reaches of our atmosphere and collide with air molecules there. The impact of a fast-moving cosmic neutron striking a nitrogen atom, the most common component of air, can kick a tiny proton pellet out of the atom’s inner kernel, the nucleus. By the rules of atomic nomenclature, it is now no longer a nitrogen atom; it has become a radioactive carbon-14 atom.