Holocene atmospheric CO 2 increase as viewed from the seafloor

Three scenarios have been proposed to explain the 20‐ppm post‐8000 BP rise in atmospheric CO 2 content. Indermühle et al. [1999] call on a climate‐induced decrease in terrestrial biomass. W. F. Ruddiman (personal communication, 2002) calls on an anthropogenically induced decrease in terrestrial biomass. Broecker et al. [2001] suggest instead that this rise in CO 2 was a response to a CaCO 3 preservation event induced by an early Holocene increase in terrestrial biomass. The biomass decline hypothesis not only rests on shaky 13 C data, but also requires an unreasonably large decrease in biomass (195 ± 40 GtC). While evidence for a decrease in deep sea carbonate ion concentration over the last 8000 years reconstructed from CaCO 3 size index and foraminifera shell weight measurements appears to support the idea that the CO 2 rise was caused by a change in the inventory of terrestrial biomass, the decrease appears to be too large to be explained solely in this way. Regardless, the CO 3 = decline cannot be used to distinguish between the late Holocene biomass decrease and early Holocene biomass increase scenarios. Only when a convincing 13 C record for atmospheric CO 2 has been generated will it be possible to make this distinction.