  Using particle-scale simulations of non-suspended sediment transport for a
large range of Newtonian fluids driving transport, including air and water, we
determine the bulk transport cessation threshold $\Theta^r_t$ by extrapolating
the transport load as a function of the dimensionless fluid shear stress
(`Shields number') $\Theta$ to the vanishing transport limit. In this limit,
the simulated steady states of continuous transport can be described by simple
analytical model equations relating the average transport layer properties to
the law of the wall flow velocity profile. We use this model to calculate
$\Theta^r_t$ for arbitrary environments and derive a general Shields-like
threshold diagram in which a Stokes-like number replaces the particle Reynolds
number. Despite the simplicity of our hydrodynamic description, the predicted
cessation threshold, both from the simulations and analytical model,
quantitatively agrees with measurements for transport in air and viscous and
turbulent liquids despite not being fitted to these measurements. We interpret
the analytical model as a description of a continuous rebound motion of
transported particles and thus $\Theta^r_t$ as the minimal fluid shear stress
needed to compensate the average energy loss of transported particles during an
average rebound at the bed surface. This interpretation, supported by
simulations near $\Theta^r_t$, implies that entrainment mechanisms are needed
to sustain transport above $\Theta^r_t$. While entrainment by turbulent events
sustains intermittent transport, entrainment by particle-bed impacts sustains
continuous transport. Combining our interpretations with the critical energy
criterion for incipient motion by Valyrakis and coworkers, we put forward a new
conceptual picture of sediment transport intermittency.
