The mobility of deep sediments was measured by Parkes et al. 2000.  By removing a central plug from a sediment core, irradiating it, re-inserting it into the core, and doing cell counts at various places in the sediment plug.  The bacterial movement rate was about 18 mm/year, which is fast enough for the surficial bacteria populations to prevent burial.  Also, it means that the bacteria in the top layers of sediment could move fast enough to move into the newly deposited layers. 

            Bacterial movement should be closely tied to activity and growth (Parkes et al. 2000).  Growth rates in this experiment matched other near surface ODP cores.  However, because growth and activity decrease by orders of magnitude with depth, it is likely that some portion of deeper bacterial populations become buried.  If the bacteria are buried below the zone where their energy source is available, they can be preserved in sediment records as dead cells, their organic material can be broken down by other bacteria, or, if they are buried deep enough, may contribute to the carbon tied up in thermogenically produced hydrocarbons.  Also, impermeable layers may be deposited on the seafloor forcing the burial of bacteria populations.  An example of this occurring is in Mediterranean sapropels (Parkes et al. 2000).

            Using the established logarithmic decrease of bacteria numbers with increasing depth below the sea floor, bacteria populations could exist, though in greatly reduced numbers, in the deepest sediments, usually taken to be 10 to 20 kilometers thick (eg. Parkes et al. 1994).  Though bacteria have been found to survive in hydrothermal events when temperatures are elevated above 113 degrees Celsius, it is often assumed that these bacteria are the result of hydrothermal circulation.  These bacteria have survived long enough to appear as intact and active bacteria (Parkes et al. 2000).  The possibility does exist, however, for yet to be discovered bacteria that exist at these elevated temperatures and into even deeper sediments.

The timescale for the length of these communities was determined in the following manner:  Assuming an average oceanic sedimentation rate of 1.5 cm/1000years and a known depth of 800 m, bacteria should have been living in the sediments for on the order of 60 Ma.  As discussed above, at some point, the bacteria are trapped and become buried, meaning the communities are of approximately the same age as the sediment they are found, below some critical burial depth.