Marine sediments are one of the largest reservoirs of organic carbon (OC); biochemical processes happening therein, such as organic matter degradation, are a vital part of the global carbon cycle. Many studies have focused on how degradation occurs and on parameters relevant to the process; fewer have characterised how proxies for OC source, such as stable isotopes (SI), change in relation to burial duration (and degradation), or how can such information be used to quantify source-specific contribution of OC to marine sediments. Our study tests whether known combinations of organic matter inputs can be successfully reconstructed after periods of incubation extending up to two years using quantitative measurements of SI composition and mixing models. We conducted a long-term laboratory experiment in which known quantities of sediment and three macrophytes (red and brown macroalgae and seagrass) were incubated for various durations (4, 10, 21, 45, 90, 180, 365, and 730 days) under different treatments (control, single species, and multi-species) and without additional input of organic matter. We will present preliminary data (OC content and SI signals) of experimental samples and discuss how proxies are affected by degradation and how they can be used to predict/validate macrophyte contribution to sedimentary OC.