Cryptic Biogeochemical Cycles in Methanic Marine Sediments

Marine sediment biogeochemistry has long been interpreted through the framework of vertical redox zonation, in which organic matter remineralization proceeds through a thermodynamically ordered sequence of terminal electron acceptors. Evidence accumulated over the past two decades has fundamentally challenged this concept. In many marine sediments, multiple metabolic pathways operate simultaneously, linked through rapid recycling of reactive intermediates and sustaining microbial activity without significant expression in net concentration profiles, referred to as cryptic cycling. These cryptic cycles are also prevalent in the deep methanic zone, which has been considered generally inactive. Here, we review cryptic biogeochemical cycles in methanic marine sediments. We discuss methodological approaches for detecting these hidden processes and synthesize evidence from both the base of the sulfate–methane transition zone and the deeper methanic zone. These cycles demonstrate that methanic sediments are far more metabolically active than has been traditionally assumed, with important implications for methane fluxes, nutrient cycling, and mineral diagenesis.