  We investigate the doping-dependence of the Seebeck coefficient, as
calculated from the Kelvin formula, for the Fermi Hubbard model using
determinantal quantum Monte Carlo simulations. Our key findings are: (1)
Besides the expected hole to electron-like behavior change around half filling,
we show that the additional sign change at an electronic density $n_s$ (and
correspondingly a hole density $p_s$) is controlled by the opening of a charge
gap in the thermodynamic density of states or compressibility and not by the
pseudogap scale in the single particle density of states. (2) We find that
$n_s(T,U)$ depends strongly on the interaction $U$ and shows an unusual
non-monotonic dependence on temperature with a maximum at a temperature
$T\approx t$, on the order of the hopping scale. (3) We identify local moment
formation close to half filling as the main driver for the anomalous behavior
of the thermoelectric transport coefficient.
