Diapycnal mixing in the ocean interior is largely fueled by internal
tides. Mixing schemes that represent the breaking of internal tides are
now routinely included in ocean and earth system models applied to the
modern and future. However, this is more rarely the case in climate
simulations of deep-time intervals of the Earth, for which estimates of
the energy dissipated by the tides are not always available. Here, we
present and analyze two IPSL-CM5A2 earth system model simulations of the
Early Eocene made under the framework of DeepMIP. One simulation
includes mixing by locally dissipating internal tides, while the other
does not. We show how the inclusion of tidal mixing alters the shape of
the deep ocean circulation, and thereby of large-scale biogeochemical
patterns, in particular dioxygen distributions. In our simulations, the
absence of tidal mixing leads to a deep North Atlantic basin mostly
disconnected from the global ocean circulation, which promotes the
development of a basin-scale pool of oxygen-deficient waters, at the
limit of complete anoxia. The absence of large-scale anoxic records in
the deep ocean posterior to the Cretaceous anoxic events suggests that
such an ocean state most likely did not occur at any time across the
Paleogene. This highlights how crucial it is for climate models applied
to the deep-time to integrate the spatial variability of tidally-driven
mixing as well as the potential of using biogeochemical models to
exclude aberrant dynamical model states for which direct proxies do not
exist.