CHEMICAL weathering of rocks bysCO(2)-an important process in the carbon cycle-may have been affected by the appearance of vascular plants during the Silurian period (438-408 Myr ago), because of the enhanced efficacy of these plants in promoting weathering(1). The magnitude of this effect is uncertain, however(2-4). One key issue is the concentration of CO2 in soils, generated by microbial respiration; this is itself in part a function of soil productivity. We showed recently(5) that the CO2 partial pressure in a Late Ordovician soil was 5.6 times that in the atmosphere; Keller and Wood(6) have shown, however, that soil CO2 concentrations can be high even for low levels of microbial respiration, as a result of slow diffusion of respired CO2 out of the Soil. Here we present an analysis of the Fe(CO3)OH component of goethite (FeOOH) in an Upper Ordovician oolitic ironstone which underwent tropical chemical weathering 440 Myr ago(7,8). These data allow us to extract an estimate of the CO2 flux from the soil profile. The high rate that we obtain implies that the productivity of pre-vascular biota was similar to that in modern soils. Thus, attempts to model atmospheric CO2 concentrations over Phanerozoic(9-12) time need not assume that pronounced productivity changes accompanied the development of vascular plants.