Abstract
Field data show that cell counts decrease with sediment depth. Suggested limiting factors include age, high temperature, low porosity and a lack of water, nutrients or energy. This letter shows that pore size distribution, rather than porosity, can properly account for the reduction in population with depth. Clay mineralogy controls the constitutive parameters that define pore size; therefore, the sediment mineralogy must be properly considered in the analysis of bioactivity in sediments.