In addition to the spatial variability in the fluid venting across Hydrate Ridge there may also be a temporal effect. The older stratigraphy uplifted and exposed at the seafloor at NHR has been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR. ODP drilling results at Sites 892 and 1244 (beneath the slope basin cap, Fig. F4) confirm the stratigraphy is well lithified and fractured, much more so than in the overlying younger strata at SHR (Westbrook, Carson, Musgrave, et al., 1994; Tréhu, Bohrmann, Rack, Torres, et al., 2003). A more intense and a longer period of deformation occurred in the uppermost strata exposed at NHR compared to the younger, less deformed and less dewatered shallowest strata at SHR. This suggests the extent of authigenic carbonates at NHR may result from a longer history of fluid flow, which is supported by U/Th ages from carbonates at NHR of 68,700 and 71,700 yr and SHR of 7,300 to 11,400 yr (Teichert et al., 2003). Without an abundance of fracture conduits and with an overlying cap of relatively impermeable sediments sealing their outlet toward the seafloor, stratigraphic conduits for fluid flow likely become important at SHR. The significance of stratigraphic conduits for fluid flow was observed during Leg 204 drilling at SHR, which revealed the major authigenic carbonate occurrence there is the result of fluid flow through a high-porosity and permeable ash-rich stratigraphic horizon (Tréhu et al., 2004a).