The occurrence of methane in recent deltaic sediments and its effect on soil stability

Abstract

River-mouth depositional pattern are modified by sediment-deforming processes of sufficient magnitude to severely endanger bottom-supported structures. Several types of deformations are present, including (a) peripheral slumping, (b) differential weighting and diapirism, (c) radial tensional faulting, (d) mass wasting and flowage induced by wave motion and degassing, and (e) deepseated clay flowage. The processes of bacterial methane production and the resulting effects on sediment deformation have been investigated in four cores taken in the Recent deltaic sediments of the Mississippi River. Dissolved methane in the interstitial waters ranged in concentration from 2 × 10−3 to 1.7 ml/l, whereas total CH4 (dissolved plus bubble phase) ranged from 5 × 10−3 to over 300 ml 1. High concentrations of methane corresponded to zones of low shear strength and were observed where dissolved sulfate was depleted. Calculations of maximum in situ methane concentrations, based on chemical reduction of excess total CO2, indicate that methane could be present above saturation levels (bubble phase). Classical anaerobic geochemical gradients were observed in sediment profiles where no movement had previously occurred. The pore water geochemistry of sediment profiles within peripheral mudflows suggest that coexistence of methane and sulfate indicate convective mixing of sediments and bottom seawater. Calculations using the equilibrium slopes of wave-induced mudflows indicate that shear strengths during movement must be less than the values measured before or after the flow. An improved model of mass movement is presented which relates the physical and geochemical properties of unstable sediments.