Marine sediments represent the main sites of OM degradation and burial in the ocean, hosting microbes in densities up to 1000 times higher than found in the water column. The DOC concentrations in sediments are often an order of magnitude higher than in the overlying water column. This concentration difference results in a continued diffusive flux and suggests that sediments are a major DOC source releasing 350 Tg C yr, which is comparable to the input of DOC from rivers. This estimate is based on calculated diffusive fluxes and does not include resuspension events which also releases DOC  and therefore the estimate could be conservative. Also, some studies have shown that geothermal systems and petroleum seepage contribute with pre-aged DOC to the deep ocean basins, but consistent global estimates of the overall input are currently lacking. Globally, groundwaters account for an unknown part of the freshwater DOC flux to the oceans. The DOC in groundwater is a mixture of terrestrial, infiltrated marine, and in situ microbially produced material. This flux of DOC to coastal waters could be important, as concentrations in groundwater are generally higher than in coastal seawater, but reliable global estimates are also currently lacking.