Mineralized. For all four sediment samples, the mass balance on PEG-added 13 C was complete, as CNon-Mineralized and CMineralized added up to between 96 ± 1 % and 98 ± 2 % of the total PEG- C initially added into the incubation flasks (Figure ). Closing the mass balance is a unique feature of using 13 C-labelled polymers.