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American Geophysical Union Chapman Conference

The Twilight Zone of the Marine Carbon Cycle and Climate Change Past and Future

Report by Paul Loubere with help from Andy Ridgwell (University of British Columbia), Heather Stoll (Williams College), Jelle Bijma (Alfred Wegener Institute, Bremerhaven), David Archer (University of Chicago), and Watson Gregg (NASA).

(Loubere: Dept. of Geology and Environmental Geosciences, Davis Hall, Northern Illinois University, DeKalb, Illinois 60115; paul@geol.niu.edu)

A beer hour with friends might, these days, include conversation about global warming and, in an unguarded moment, someone could suddenly ask “Hey, what controls the amount of carbon dioxide in the atmosphere anyway?” There, as an earth scientist, you are on the spot; you are supposed to know. Today, the ocean absorbs approximately one half of all CO2 emissions from fossil fuel burning and cement production, giving it a critical role in future ‘greenhouse warming’ and climate change. Also, we know that CO2 has varied in tandem with climate through the Ice Ages. Once again, the controlling processes are thought to be marine. So, a large part of the answer to our question must be hidden in the sea.

What the oceans do with CO2 has both physical and biological components. The physical part is, in principle, relatively well understood although how ocean circulation responds to climate change is an active area of research. The significance and mechanisms of the biological component are, to varying degrees, mysterious due in part to the complexity of marine ecosystems, the patchy and seasonal nature of biological behavior, and the sophistication of biological feedbacks. In July, an AGU Chapman conference was convened to explore fundamental mechanisms, modeling, some astonishing processes and some surprising gaps in our understanding of the marine system.

Moving that CO2:
Photosynthesis by marine phytoplankton captures CO2 into organic matter. A web of processes then transfer the organic matter into reservoirs in the sea. The most important of these, from a climate change point of view, is the deep ocean with its large volume and isolation from the atmosphere. Increased carbon storage in the deep ocean can lead to decreased CO2 in the atmosphere (Archer and Maier-Reimer, 1994).

Some of the plankton in the surface ocean not only make soft tissues but also precipitate carbonate to make shells. The presence of carbonate minerals makes the whole oceanic CO2 story more interesting, and tricky. The plankton in the surface ocean not only capture CO2 and convert it to organic matter, but also precipitate carbonate to make shells. Counter-intuitively, making calcite or aragonite, although it consumes bicarbonate dissolved in the water, ends up releasing CO2. This is because precipitation shifts the equilibrium among carbon species towards CO2 and away from CO3.

So, photosynthesis lowers the partial pressure of CO2 in seawater, while carbonate precipitation raises it. Carbon dioxide exchange between the ocean and atmosphere includes a balancing act whose net effect depends on the ratio of organic carbon to calcite production (Zeebe and Wolf-Gladrow, 2001). A shift in the ratio can therefore drive atmospheric CO2 in either direction, ameliorating or exacerbating future global change.

Ocean Pastures of Green and White:
Key questions are: What controls the production ratio and how variable is it; and what controls the organic matter to carbonate flux ratio (rain ratio) in the deep ocean, and how variable is that?

The answer to the first question depends on relative production among the various types of oceanic plankton. Of these, the ‘big three’ are diatoms (prolific generators of >50% of marine organic matter), Foraminifera (ocean going amoebas precipitating calcite shells) and Coccolithophores (algae precipitating calcite plates to cover their cells). See Figure 1.

In its simplest form, the organic carbon to calcite production ratio depends on diatom vs. coccolithophorid + foraminiferal production. The diatoms appear to have the greatest regional variability, dominating where silicate and Iron supplies are largest. They may play a key role in variations of the numerator in the production ratio (Sarmiento et al., 2004). Coccolithophores replace diatoms as the leading skeleton producing algae where stratification is well developed and nutrient supply is low. But, how will these different groups respond to climate change?

Answering that question by direct shipboard observation is extremely difficult given the variability and granularity of the oceans. However, recent advances in combining satellite observations with modeling, and in mesocosm experiments, are yielding insights. Nevertheless, estimating carbonate production, and our ratio denominator, has been problematic, especially since is done by both plants, coccolithophores (10 - 80%), and animals, Foraminifera (20-90%).

So, how does surface production translate into fluxes and rain ratio? Those fluxes from the shallow production zone depend on mass and reprocessing. Mass is why the hard skeleton producing plankton are important. That skeletal matter serves as ballast in the sinking process. Curiously, the shell mass produced by the diatoms and the coccolithophores depends on both the species present and the biogeochemical conditions. For the coccolithophorids, mass depends on the species and on the pCO2 of the water. If the latter is raised, certain species make thinner shells. The same effect is seen with Foraminifera (and corals). So, increasing pCO2 decreases shell mass, and for carbonate, the amount of CO2 released to the upper ocean due to calcification. That translates to less CO2 released to the atmosphere and a lower flux than one might expect of organic carbon into the deeper ocean (less ballasting). Thus, we see the hallmarks of a negative feedback (buffered) system (Ridgwell, 2003).

Still, the negative feedback is not simple. Some coccolithophore species make thin shells while others make them thick, and some coccolithophore shell thicknesses respond to pCO2, while others do not. In particular, certain ‘heavy’ taxa like Coccolithus pelagicus have little pCO2 response (Fig. 1). In the end, the most important factor in carbonate mass production may be distribution of particular taxa of both coccolithophores and foraminifera. An illustration of this point is that in spite of having equivalent overall production, the western equatorial Pacific appears to have a coccolithophore calcite flux that is ½ that of the North Atlantic. The difference is due to the species present in each basin.

The 5% solution (4 to 7% actually):
The surface production has to reach the deep ocean to have an effect on climate. How does it get there? Most ocean production is either light as with organic matter (fluffy marine ‘snow’; Fig. 1) or tiny (coccolithophore plates). Rapid sinking can only be accomplished by amalgamation of fine material and incorporation of ballast (skeletal matter). How does reactive organic carbon get into the deep sea? Assembly, packaging and speed of transport are what count. The trip down for organic matter involves agglomeration with other particles, accumulating ballast, and sinking using an anchor that is dissolving (calcite or opal) while being dissipated by bacteria and grazers in the process.

Marine snow contains 1 - 5% solid matter glued together with transparent exopolymers (TEP) secreted by plankton. The TEP appears to be climatically sensitive and its abundance seems to increase at higher pCO2. Particles aggragate in the upper 100m of the water column, become more dense, but maintain an organic carbon content of about 4%. The compounds present have not been well characterized but vary with region.

The Foraminifera have their own flux story since they have an unusual means of reproduction. Adult Forams subdivide their protoplasm into tens of thousands of gametes that swim off, abandoning the shell which settles rapidly to the ocean floor. Thus, we have carbonate ‘bombs’ delivered to the depths containing relatively little organic carbon. The gametes develop into little foraminifera, most of which die before maturity with their shells sinking slowly as debris, or being incorporated in some way with other fine material. Much of this gets dissolved in the upper 700m of the water column. Large foraminiferal shells could dominate the calcite flux to the deep sea but might have nothing to do with the organic carbon flux!

Remarkably, at the end of all this, there is a fairly constant 5% organic matter in sediments trapped below 2000m water depth. At first pass, the highly complex leads to a consistent, simple result.

Showers in the Deeps:
What falls appears linked to the material serving as ballast. Calcite may be the most effective anchor, with opal and dust following. So, in the deeper ocean, there is evidence that calcite and organic carbon fluxes are linked by ballasting. If so, then the rain ratio won’t be highly variable.

There is a caveat however, this picture applies largely to open ocean settings outside the high productivity, high CO2 flux regions, of the eastern tropical oceans. These areas are very dynamic, and not well sampled in time and space, but are critical to the transfer of CO2 to the atmosphere and carbon to the deep sea. Here, the complexities should come into full play. Diatom production is important, and variable. Calcite flux will increasingly include not only finer particles included in organic carbon bearing aggragates, but also the rain of larger foraminifera which left their organic material in the upper ocean as living zooids. Hence, decoupling of organic carbon and calcite rain is possible and the variability in the rain ratio is not well assessed.

It’s all there in the Sediments
Where present knowledge fails, past experience may fill in. Does the ocean’s paleo-record reveal variations in the rain ratio that correlate with past changes in atmospheric CO2? The first problem here is reliable proxies for each of the fluxes (reactive organic carbon and calcite), and for production by the various plankton groups in the upper ocean. All of these are in development and preliminary applications have been tantalizing (Loubere et al., 2004), especially in the critical regions of the eastern tropical oceans. But, definitive results await more work.

Certain extreme events may reveal basic mechanisms, and a fascinating example may be the Paleocene-Eocene Thermal Maximum, when apparently vast quantities of CH4 and/or CO2 was released to the atmosphere, leading to strong greenhouse warming. Recovery from that event would depend on processes partitioning CO2 between ocean and atmosphere. There is developing evidence that oceanic carbonate flux was impacted and provided feedback in the early Eocene. If so, then a similar mechanism could be important in the centuries to come.

What’s the bottom line:
So, there are aspects of the biogenic flux system that appear self-regulating. But there are snakes in the grass. Potential triggers to change lurk in the calcite flux, transport to depth and with the mix of taxon groups in the phytoplankton.

Calcite flux has two separate components: large forams hurtling to the seabed with little organic carbon in train, and fine grained calcite (coccoliths) aggragated as ballast in marine snow, with a certain amount of organic matter. These do not covary since their response to bioproductivity is generally opposite. That means that organic carbon flux need not be closely tied to calcite flux. This is even more probable since calcite ballasting is not a constant. The mass of coccoliths is species dependent, so change the species and change the ballasting equation. Couple that with the complexity of the processes operating on sinking organic matter and it is reasonable to wonder: Is that constant 5% rule universal, or serendipitous?

What controls the amount of CO2 in the atmosphere? Certainly, the physical conditions of the surface ocean and circulation. But we should not forget that the fluxes of organic carbon and carbonate to the deep sea maintain a CO2 gradient across the surface ocean. If this were to disappear (“Strangelove Ocean”) pCO2 would increase markedly within a millennium. This system appears to have the classic features of a buffered process that has switching points we haven’t resolved very well to date. It includes a number of semi-independent components which, aligned properly by the right environmental circumstances, would amplify developing carbon system changes, instigating climate variation through shifts in atmospheric CO2 concentrations and greenhouse warming.

Acknowledgments:

The overview presented here was developed from presentations made at the American Geophysical Union Chapman Conference “Organic Carbon and Calcite Fluxes in the Open Ocean Driving Climate Change, Past and Future” which was held at Woods Hole Oceanographic Institution on July 24-27, 2005. Credit for the insights presented here goes to the more than 60 scientists who attended. The conference was supported by grants from the National Science Foundation, The Woods Hole Oceanographic and Climate Change Institute and the Analytical Center for Climate and Environmental Change at Northern Illinois University.

References:

Archer, D., and Maier-Reimer, E. (1994), Effect of deep-sea sedimentary calcite preservation on atmospheric CO2 concentration, Nature, 367, 260-264.

Loubere, P., Mekik, F., Francois, R., and Pichat, S. (2004), Export fluxes of calcite in the eastern equatorial Pacific from the Last Glacial Maximum to the Present, Paleoceanography, 19, doi 10.1029/2003PA000986.

Ridgwell, A. (2003), An end to the ‘rain ratio’ reign?, Geochem. Geophys. Geosys., 4, doi: 10.1029/2003GC000512.

Sarmiento, S., Gruber, N., Brzezinski, M., and Dunne, J. (2004), High latitude controls of thermocline nutrients and low latitude biological productivity, Nature, 427, 56-60.

Zeebe, R., and Wolf-Gladrow, D. (2001), CO2 in Seawater: Equilibrium, Kinetics, Isotopes, Elsevier Oceanography Series, 346p., Elsevier, Amsterdam