  Using large-scale dynamical cluster quantum Monte Carlo simulations, we
explore the unconventional superconductivity in the hole-doped Hubbard model on
the triangular lattice. Due to the interplay of electronic correlations,
geometric frustration, and Fermi surface topology, we find a doubly degenerate
singlet pairing state at an interaction strength close to the bare bandwidth.
Such an unconventional superconducting state is mediated by antiferromagnetic
spin fluctuations along the $\Gamma$-$K$ direction, where the Fermi surface is
nested. An exact decomposition of the irreducible particle-particle vertex
further confirms the dominant component of the effective pairing interaction
comes from the spin channel. Our findings provide support for chiral $d +i d$
superconductivity in water-intercalated sodium cobaltates Na$_{x}$CoO$_{2}
\cdot y$H$_{2}$O, as well as insight into the superconducting phases of the
organic compounds $\kappa$-(ET)$_{2}$X and Pd(dmit)$_{2}$.
