Fig. 5: Cartoon representing four stages of our conceptual model for renewal of North Pond bottom waters.

The model involves slow heating of bottom waters by the geothermal flux and diapycnal mixing, setting the stage for cold water intrusions inferred to be from the leading edge of diluted AABW flowing over the deepest sill on the southwest side of North Pond. Plots labeled Θ and ρ represent idealized profiles of Conservative Temperature and density at each stage. Their scales and structures are schematic and intended only to illustrate relative changes in properties with depth. As shown in the bottom key, horizontal two-sided arrows represent relative horizontal tidal velocity variations, red upward arrows represent geothermal heating, red squiggly arrows represent heat input by diapyncal mixing, and dashed blue lines represent inferred low-temperature advective flow within permeable basement. a Heating of bottom waters below sill depth by the geothermal flux through the seafloor and by diapycnal mixing in the water column. The curvatures in the profiles in the deepest geothermally heated boundary layer are drawn to be consistent with comparable bottom boundary layers documented in Cascadia Basin⁴. b Preconditioned stage reached when the vertical extent of the heated bottom waters reaches sill depth, or slightly shallower. c Gravity current inflow stage, when the horizontal tidal velocities over the sill are weak, and colder, denser diluted AABW replaces the deepest bottom water in the basin. d Reset stage after the intrusion of diluted AABW and vertical mixing have reestablished near uniform water temperature to sill depth.