  Interpreting the spectra of brown dwarfs is key to determining the
fundamental physical and chemical processes occurring in their atmospheres.
Powerful Bayesian atmospheric retrieval tools have recently been applied to
both exoplanet and brown dwarf spectra to tease out the thermal structures and
molecular abundances to understand those processes. In this manuscript we
develop a significantly upgraded retrieval method and apply it to the SpeX
spectral library data of two benchmark late T-dwarfs, Gl570D and HD3651B, to
establish the validity of our upgraded forward model parameterization and
Bayesian estimator. Our retrieved metallicities, gravities, and effective
temperature are consistent with the metallicity and presumed ages of the
systems. We add the carbon-to-oxygen ratio as a new dimension to benchmark
systems and find good agreement between carbon-to-oxygens ratio derived in the
brown dwarfs and the host stars. Furthermore, we have for the first time
unambiguously determined the presence of ammonia in the low-resolution spectra
of these two late T-dwarfs. We also show that the retrieved results are not
significantly impacted by the possible presence of clouds, though some
quantities are significantly impacted by uncertainties in photometry. This
investigation represents a watershed study in establishing the utility of
atmospheric retrieval approaches on brown dwarf spectra.
