One of the fundamental aspects of Earth's ecological and climate systems is the way carbon moves through the biosphere. From land to air to water, through living organisms and even the plant's crust, carbon—the stuff of life—is always on the move. Scientists thought they had a pretty good understanding of how the carbon cycle works, until now. Recent work with strange, jellyfish like creatures called thaliaceans is causing scientists to re-evaluate the workings of the carbon cycle.
There are three orders of Thaliacea: Pyrosomida, Doliolida, and Salpida. An example of a thaliacean species is Thalia democratica, more commonly known as salps. Although salps appear similar to jellyfish because of the simple form of their bodies and their free-floating way of life, they are structurally most closely related to vertebrates, animals with true backbones. Salps are related to the pelagic tunicate groups, as well as to other bottom-living (benthic) tunicates.
Salps appear to have a form preliminary to vertebrates, and are used as a starting point in models of how vertebrates evolved. Scientists speculate that the tiny groups of nerves in salps are one of the first instances of a primitive nervous system, which eventually evolved into the more complex central nervous systems of vertebrates. Studies
on salp brains have been done by Thurston Lacalli and Linda Holland and published in Philosophical Transactions of the Royal Society of London, but not much is known about what happens to animals with gelatinous bodies, whether thaliaceans or jellyfish, after they die.
It has been known for some time that salps and other thaliaceans assimilate CO2 and, when they die, take that carbon with them as they drift towards the bottom of the oceans. Most marine biologists thought that the dead salps simply became nutriment for other creatures. Then, in 2006, Mario Lebrato and Daniel Jones of the National Oceanography Centre in Southampton, England, made a surprising discovery off the cost of Côte d’Ivoire in Africa—a thaliacean graveyard.
According to a report in the Economist, the researchers were using a remotely operated deep-sea vehicle to study the sea floor near an oil pipeline when they happened upon the graveyard, but they immediately recognized the possible importance of their discovery. The existence of such large deposits of thaliacean corpses calls into question accepted thinking about one important aspect of climate change: how much carbon from the atmosphere ends up at the bottom of the sea? If thaliaceans are falling to the bottom of the sea in large numbers, they might be taking a lot of carbon with them.
As we explained in The Resilient Earth
, there are two large pools of terrestrial carbon; geologic carbon stored in rock and fossil fuel deposits, and biologic carbon stored in living things or “in play” in the atmosphere and oceans. The source of both types of stored carbon is life. Over billions of years, uncountable billions of living things have collected carbon; growing, eating, breathing, reproducing and finally, dying. Most of this carbon came from Earth's primitive atmosphere in the form of carbon dioxide, which has steadily decreased over time.
Through a number of different biological and physical processes, carbon is circulated through Earth's biosphere and lithosphere. Vast amounts of carbon are now trapped in Earth's crust in the form of sedimentary rocks; limestone, dolomite, and chalk. This type of storage, or sink, accounts for the majority of carbon on Earth, 66,000,000 to 100,000,000 billion metric tons (gigatons or Gt).
SinkAmount in Billions of Metric Tons (Gt)
Atmosphere 578 (as of 1700) - 766 (as of 1999)
Ocean 38,000 to 40,000
Terrestrial Plants 540 to 610
Soil Organic Matter 1500 to 1600
Marine Sediments and Sedimentary Rocks66,000,000 to 100,000,000
Fossil Fuel Deposits 4000
Amount of carbon stored in sinks. Source Dr. Michael Pidwirny
As seen in the table above, fossil fuel deposits, the other major type of geologic carbon, accounts for a paltry 4,000 Gt. The important difference between geologic and biologic carbon sinks is that geologic carbon is out of short term circulation. It is only released by slow processes, such as volcanism, degassing, and rock weathering, which can take millions of years. At least that was true before Man started digging up fossil fuels and burning them.
The other carbon sinks shown—the oceans, soil, atmosphere, and plants—all participate in what is called the carbon cycle of life, shown in the diagram below. This carbon is involved with life over the short term. It is the build-up of this carbon, in the form of CO2 in the atmosphere, that is responsible for the current global warming scare.
What is not clearly shown in the carbon cycle diagram are the geologic carbon sinks that store carbon for long periods of time. Carbon in these sinks take millions of years to cycle back into the biosphere. As mentioned, there is a tremendous amount of CO2 dissolved in the oceans. Though some of the CO2 in seawater remains as dissolved gas, a large portion is converted into other chemical compounds.
Among the compounds that are formed are carbonate (CO3) and bicarbonate (HCO3). Many forms of sea life (labeled Aquatic Biomass) have the ability to modify bicarbonate by adding calcium (Ca), producing calcium carbonate (CaCO3). Calcium carbonate is used by these organisms to build shells and other body parts. The illustration below shows how carbon cycles through the oceans and sedimentary rock deposits.
The ocean gets a disproportionate share of the carbon dioxide available to the ocean-atmosphere system. For every molecule of CO2 in the atmosphere there are about 50 CO2 molecules in the ocean. Why is this so? The main reason is that carbon dioxide readily reacts with water to make soluble ions, rather than simply diffusing gas molecules among the water molecules. There are a number of physical and biological mechanisms that “pump” CO2 about the ocean-atmosphere system. One is the physical pump: cold water holds more carbon dioxide in solution than warm water and because cold water is denser than warm water, this cold, carbon dioxide-rich water is pumped down by vertical mixing to lower depths.
Other reasons for the ocean’s big share of carbon are its “biological pumps.” The biological pumps, based on the life-cycles of various marine creatures, remove carbon dioxide from the surface water of the ocean, changing it into living matter and carrying it to the deeper water layers. When the ocean shares carbon dioxide with the atmosphere, it does so by not only simply taking on carbon dioxide into solution but also by incorporating the carbon dioxide into living organisms.
Carbon is constantly cycling between the atmosphere, the oceans, and living matter. Thaliaceans, jellyfish and other gelatinous animals roll in this process has largely been ignored by researchers because their bodies were thought to consist mostly of water with relatively little carbon content. However, as Lebrato and Jones report in Limnology and Oceanography, their analysis of thaliacean tissues revealed that the creatures were one-third carbon by weight—much more than expected. Jellyfish, by comparison, are 10% carbon, and single-celled algae around 20%. The high carbon content explains why thaliaceans are so dense and why they sink so quickly after they die. The key question regarding the dead thaliaceans is, when they get to the bottom does their carbon stay there? If so, thaliaceans may form a previously unsuspected CO2 pump—a “jelly pump.”
One way carbon stays on the seabed is in the form of calcium carbonate, the main ingredient of most animal shells. Uncountable numbers of tiny diatoms, raining down on the ocean floor over millions of years have created the vast deposits of chalk and limestone found within Earth's strata. But thaliaceans have no shells and no one is sure what a deep bed of dead salps turns into. Some thaliaceans do get buried before they have completely decomposed, and researchers have found evidence that dead jellyfish can also accumulate in trenches without much decomposition.
Why all this fuss about mostly transparent sea creatures that seem rather insubstantial? Because they gather around the world in feeding swarms, billions strong, feasting on algae, the amount of carbon thaliaceans are potentially taking to the bottom of the sea is by no means trivial. According to Lebrato, it is difficult to make accurate estimations because the research is still in its infancy. But he estimates that the “jelly pump” sinks almost twice as much carbon as algae do.
Even if the carbon is not permanently buried, the lack of mixing between deep and shallow water layers in the ocean means that it is likely to stay out of circulation for quite some time. Of course, some of the recent revelations
about the deep ocean return flows may change that conclusion. Either way, these are new factors that will have to be added to the GCM computer models used to predict future climate changes. Just like the thermohaline circulation, the carbon cycle we thought we knew so well has just changed in some fundamental ways. As a consequence, Earth's climate system has become even more complicated and environmental models previously concocted are shown to be wanting.
That is the way science works, new discoveries are made every day and what we thought we knew yesterday doesn't look so certain today. Every generation of scientists has its own long held, but erroneous theories to correct: Ptolemy's model of the solar system, fixed continents, any number of theories regarding the nature of matter, all have been accepted dogma and eventually discarded. New research is constantly changing the ideas that buttressed anthropogenic global warming, eroding its foundations like the tide excavates sand around a piling. How much longer before we move on to a more correct view of Earth's climate system? Look around, the change is happening right now.
Be safe, enjoy the interglacial and stay skeptical.