| Summary: | The CO2content of the atmosphere has increased during the past two centuries as a result of the combustion of fossil fuels for energy1and the reduction of forest and soil carbon reservoirs on land2. The amount of CO2added to the atmosphere from fossil-fuel burning is known from historical records1(±10%), but the contribution from reduction of the terrestrial biosphere is far less certain. Several authors have estimated the relative contributions from the two sources by measuring the change in the13C/12C ratio in atmospheric CO2as revealed in tree rings3-8(CO2derived from these two sources is depleted in13C with respect to that in the atmosphere). Using trees in the Northern Hemisphere, recent estimates of the integrated CO2release from the terrestrial biosphere since AD 1800 ranged from 70% (ref. 5) to 90% (ref. 9) of that released from fossil fuels. Here we present surface ocean δ13C and δ18O records measured in the skeleton of a living sclerosponge (Ceratoporella nicholsoni), which accretes aragonite in isotopic equilibrium with the surrounding sea water/dissolved inorganic carbon (DIC) system. The δ13C record reveals a decrease of 0.50‰ from 1820 to 1972. Using a model of the world carbon cycle and a deconvolution of our δ13C data, we estimate that the amount of excess CO2derived from the terrestrial biosphere is ∼38% of that from fossil-fuel sources. Our model calculations support a preindustrial CO2concentration in the atmosphere of 280 p.p.m.v. (parts per million by volume), in agreement with direct measurements of air occluded in Antarctic ice cores10. © 1986 Nature Publishing Group.
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