Carbon cycle in reefs and warm-water carbonate platforms:
global assessment and modeling
 

PD Dr. Adam Vecsei



Importance of carbon cycle research

Through the massive buring of fossil fuels and other impacts resulting in the emission of greenhouse gases like carbon dioxide (CO2) to the atmosphere, mankind is performing a global geophysical experiment with unknown but likely detrimental outcome. The ongoing and strengthening global climate change, including atmospheric warming, shifting climate zones, floods, droughts, and sealevel rise, threaten many modern societies. The robust prediction of future changes must be based on the understanding of the present dynamics and past changes in the climate system. Assessing their causes and mechanisms is a major scientific challenge.Earth's orbital parameters and the small insolation changes they are causing are known to be the pacemaker of the alternating Quaternary glacial and interglacial periods. The atmospheric CO2 content, with strong glacial to interglacial shifts paralleling those of the atmospheric temperature, is likely to be the major amplifier that translates the small insolation changes into great climate changes. The dynamics of sedimentary carbonates are resulting in important albeit poorly understood effects on the global carbon cycle. The background is that the precipitation of carbonates concomitantly results in the acidification of the upper ocean, which on its turn releases CO2 to the atmosphere or reduces the CO2 uptake of the oceans. I am contributing to the understanding of these effects through studies of the world's reefs and shallow-water platforms, the globally most important sites where innumerable plants and animals precipitate huge amounts of carbonate. 

The coral reef project

The world's coral reefs are likely the most important sites of carbonate precipitation by bottom-living organisms. Estimates of reefal carbonate production are based on their global area and production rates. Only since 1997 do we have good knowledge of the area and precise distribution of the world's reefs. The production rates are well established for typical reef-flats and uppermost fore-reefs, where the chemical changes in the overflowing waters have been used to determine the fluxes. The deeper fore-reefs with their lush cover of corals and other biota have remained enigmatic, because such methods could not be applied there. I have developed a new procedure to estimate fore-reef carbonate production from biometric and census data in typical reef transects. Although these estimates are very crude as yet, they suggest that up to half of the reefal carbonate production takes place in the fore-reefs. The results will be used to improve global estimates of reef production and its climatic effects.  


 

 
The carbonate platform project

The world's carbonate platfoms are the second most important sites of carbonate production. Surprisingly neither their area nor number, depth or sediment types have been known until recently, and these are still largely unknown for many platforms, despite the excellent documentation of a few structures like the Bahamas Banks. During the last few years I have determined the areal and depth distribution of the world's carbonate banks, isolated from the continents by deep seaways, in the tropical and subtropical zones. The data have allowed the estimation of carbonate production on these banks, and of its effect on global warming during the flooding period since the last glacial maximum, around 20'000 years ago, when sealevel was ca. 120 m lower than today. In another study I have shown that the flow of nutrient-laden waters is a major control on the growth and production of many platforms. A detailed estimate of modern carbonate production on the isolated banks is underway. As a corollary to this research, I contribute to the understanding of the geological origin of the world's platforms.


 
 

 
Climatic impact

I have made a very cautious estimate of the amount of carbon emitted to the atmosphere as CO2 as a result of the precipiatation of shallow-water carbonates since the last glacial maximum ca. 20 Thousand years (k.y.) ago. The estimate suggests a total emission of at least ~203 Gt (billion tons) of carbon. Remarkably this corresponds to a ~97 parts per million (p.p.m.) rise in atmospheric CO2 concentration, about the amount of total atmospheric CO2 rise since the last glacial maximum determined from air bubbles trapped in Antarctic ice cores. Further analysis of the carbonate precipitation suggests its greatest contribution has come from reefs since ~14 k.y. before present, with a maximum during the early Holocene ~8 to 6 k.y. years ago. The contribution of the isolated banks has steadily increased since ~14 k.y. years ago, but its total is an order of magnitude smaller than that of the reefs.