They contain a record of
atmospheric CO2 levels
(Aka sclerosponges) A type of sponge that builds itself a limestone pedestal that resembles coral (hence the name). They have a long life span (e.g., 500 - 1,000 years), so when they are analyzed cleverly, they become a long-term record of CO2 levels.
Their role in the AGW debate
They are important to the AGW debate because they provide one of several proofs human fossil-fuel is propelling the increase in atmospheric CO2. Here's the reasoning:
(1) CO2 is generated whenever organic matter, wood, coal, petrol or gas are burned.
(2) During the past 150 years of industrialization we have burned lots of it.
(3) There are 3 different kinds of carbon atoms (i.e., isotopes). The most common is carbon-12; carbon-13 is much rarer. Organic matter is strongly depleted in carbon-13.
(4) When fossil fuels (i.e., former organic matter) are burned, the resulting CO2 molecules are also depleted in carbon-13.
(5) Atmospheric measurments show that during the past 150 years, Carbon-13 based CO2 molecules have become less and less common in the atmosphere.
(6) Because atmospheric CO2 dissolves in the ocean waters and is used by Coralline sponges to build their limestone pedestals, the isotopic composition of the sponge pedestals can be analyzed to reconstruct the isotopic history of atmospheric CO2.
(7) The Kiel sponge group did this for several sponges from the Caribbean, and their results (displayed in the diagram below), show a close resemblance to the atmospheric CO2 record.
Hence, human fossil-fuel burning is the cause of the recent CO2 increases. QED.
Upper Panel: Atmospheric CO2 increase.
Data from Etheridge et al. (1996, Journ. Geophys. Res. 101, 4115-4128) and from air measurements in Antarctica and Hawaii by the working group of Charles David Keeling.
Lower Panel: Carbon isotope ratios measured in carbonate produced by two Caribbean sponges living in a reef cave north of Jamaica.