Three scenarios have been proposed to explain the 20-ppm post-8000 BP rise in atmospheric CO2 content. Inderm¿hle 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 CO2 was a response to a CaCO3 preservation event induced by an early Holocene increase in terrestrial biomass. The biomass decline hypothesis not only rests on shaky 13C 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 CaCO3 size index and foraminifera shell weight measurements appears to support the idea that the CO2 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 CO3= decline cannot be used to distinguish between the late Holocene biomass decrease and early Holocene biomass increase scenarios. Only when a convincing 13C record for atmospheric CO2 has been generated will it be possible to make this distinction.