Fibrous sediments that originated fromold pulp and paper industry emissions are recognized as a potential threat to
the aquatic environment because they are highly contaminated. In addition, biogenic degradation of the organicmaterial
from so-called “fiberbanks” has a high potential to produce greenhouse gases (GHG). In this study, X-ray tomography,
optical sensors and gas analyzers were used to identify and quantify the gas produced and released
from samples of two different fiberbanks. We show that a finer fibrous structure allows the formation of larger
gas bubbles and higher gas production rates compared to coarsermaterial composed of wood pieces. High contents
of methane (average 56% to 65%) and carbon dioxide (average 18% to 20%) weremeasured in the gas emitted from
both types of fiberbank. Measured methane production rates from the fiberbanks samples are one to three orders of
magnitude higher than previously reported rates from sediments within the studied temperature range (between
0.03 and 0.51 μm CH4/h/g dw over 4.7 to 20 °C). The potential for methane and carbon dioxide production in the
fiberbank volume likely present in Sweden is estimated to correspond to 7% of Sweden's total knownGHGemissions
for 2019. These findings show that fiberbanks have the potential to be a significant emitter of GHG.