A Pliocene-Pleistocene turbidite sequence sampled at four drill sites in a 75-km-long transect on the landward edge of the Iberia Abyssal Plain illustrates the effects of non-steady-state sedimentation on postdepositional geochemical processes. Organic carbon concentrations average 0.7% in sediments of this sequence at Sites 897 and 898 and 0.4% at Sites 899 and 900. Headspace concentrations of interstitial methane exceed 100,000 ppm in sediments from Sites 897 and 898 but are essentially zero in those from Sites 899 and 900. Methane concentrations do not rise until interstitial sulfate concentrations are virtually depleted, suggesting the presence of in situ methanogenic bacterial activity to sub-bottom depths of 300 m at Site 897 and 170 m at Site 898 and its absence at Sites 899 and 900. The turbidity flows that created the turbidite sequence on the Iberia Abyssal Plain evidently influenced postdepositional diagenesis at these sites. The principal factor is that the thicker turbidite sequences at Sites 897 and 898 protected organic matter from oxic, early degradation and thereby permitted anoxic, later degradation to proceed. In contrast, organic matter in the thinner turbidite sequences at Sites 899 and 900 was oxidized soon after deposition and was therefore not suitable for later microbial utilization. The difference in organic matter preservation between the sites has impacted microbial sulfate reduction and methane production in the turbidite sequences.