Kelps are highly productive macroalgae that form habitats along one-quarter of the worlds’ coastlines. Emerging evidence suggests that kelps have the potential to sequester carbon through the export of detritus to deep marine sinks, yet how much of this detrital carbon is remineralized through grazing and microbial decomposition before it reaches these sinks remains a critical knowledge gap. We measured decay ofLaminaria hyperboreadetritus in shallow kelp forests (10 m) and adjacent deep fjords (300 m), and experimentally tested the effect of temperature and oxygen conditions similar to those at these habitats inex situexperiments. Initial decay rate (k) was high (-0.107 to -0.183 d^-1) with 40-60% of the original carbon biomass being lost within few weeks, after which decay rates slowed down (k= -0.009 to -0.038 d^-1). Temperature had little effect on the rate and extent of decomposition within the temperature range tested (4-10°C). Blade detritus decomposed almost completely in 300 d under aerobic conditions. Anaerobic decay of both blade and stipe detritus ceased, in contrast, after 150-200 d, leaving 20-30% of the initial biomass to decompose extremely slowly or not at all. Decomposition was followed by changes in chemical composition; C:N ratios increased substantially, while mannitol and phenolics disappeared almost completely from the detritus matrix. Slow and incomplete anaerobic decomposition suggest that the potential for long-term burial and sequestration of kelp carbon will be enhanced if detritus is exported to nearby deep areas with permanent or periodic hypoxia near the bottom.