One of the most elegant mechanisms forwarded for late Quaternary atmospheric CO2 variability concerns the sensitivity of calcium carbonate preservation in deep ocean sediments to the relative delivery rates of calcium carbonate and particulate organic carbon (the CaCO3:POC rain ratio). It was implicitly assumed that any change in the CaCO3:POC rain ratio of biogenic material produced in the surface ocean will be communicated directly to the sediments. This would allow relatively subtle shifts in ecosystem composition to affect sedimentary CaCO3 preservation (and thus atmospheric CO2). However, recent research into the controls on the transport of POC to depth suggests that the rain ratio seen by the sediments may instead be buffered against any perturbation occurring at the surface. This casts doubt on the viability of hypotheses envisaging ecological changes as a means of accounting for the observed glacial-interglacial CO2 signal.