Productivity of pre-vascular continental biota inferred from the Fe(CO₃)OH content of goethite

Abstract

CHEMICAL weathering of rocks by CO₂—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 weathering¹. 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 recently⁵ that the CO₂ partial pressure in a Late Ordovician soil was 5.6 times that in the atmosphere; Keller and Wood⁶ 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.