Deep-sea sedimentation was long modeled as a relatively uniform process — a slow rain of particles settling through thousands of meters of water, accumulating on the abyssal plain at millimeters per millennium. The variation was assumed to be in composition, not timing.
Year-long continuous monitoring inside a submarine canyon reveals that internal tidal bores — subsurface waves driven by tides interacting with seafloor topography — change character seasonally. Their strengthening and weakening follows a seasonal rhythm, and this rhythm selectively transports different grain sizes of sediment at different times of year.
In Monterey Canyon, internal tidal flows reach speeds of 20-80 cm/s — an order of magnitude larger than open-ocean tidal currents. These enhanced flows move suspended sediment and organic carbon along the canyon axis, with velocities increasing up-canyon and intensifying near the seafloor. The seasonal variation in flow strength determines which particles get transported and when.
The deep ocean floor receives a temporally sorted shipment schedule, not a random rain. Coarser particles move during stronger flow seasons; finer particles during weaker ones. This has implications for interpreting sediment cores used in paleoclimate reconstruction — what appears as a compositional change in the record may actually be a change in the delivery schedule, driven by seasonal tidal dynamics rather than by changes in the material's source.
Even the deep ocean floor has seasons. They are written in grain size instead of temperature.