  We report the detection of a spectral signature observed at 682 cm$^{-1}$ by
the Cassini Composite Infrared Spectrometer (CIRS) in nadir and limb geometry
observations of Titan's southern stratospheric polar region in the middle of
southern fall, while stratospheric temperatures are the coldest since the
beginning of the Cassini mission. The 682 cm$^{-1}$ signature, which is only
observed below an altitude of 300-km, is at least partly attributed to the
benzene (C$_6$H$_6$) ice $\nu_{4}$ C-H bending mode. While we first observed it
in CIRS nadir spectra of the southern polar region in early 2013, we focus here
on the study of nadir data acquired in May 2013, which have a more favorable
observation geometry. We derived the C$_6$H$_6$ ice mass mixing ratio in
5{\deg}S latitude bins from the south pole to 65{\deg}S and infer the
C$_6$H$_6$ cloud top altitude to be located deeper with increasing distance
from the pole. We additionally analyzed limb data acquired in March 2015, which
were the first limb dataset available after the May 2013 nadir observation, in
order to infer a vertical profile of its mass mixing ratio in the 0.1 - 1 mbar
region (250 - 170 km). We derive an upper limit of $\sim$1.5 $\mu$m for the
equivalent radius of pure C$_6$H$_6$ ice particles from the shape of the
observed emission band. Several other unidentified signatures are observed near
687 and 702 cm$^{-1}$ and possibly 695 cm$^{-1}$, which could also be due to
ice spectral signatures as they are observed in the deep stratosphere at
pressure levels similar to the C$_6$H$_6$ ice ones. We could not reproduce
these signatures with pure nitrile ice (HCN, HC$_3$N,CH$_3$CN, C$_2$H$_5$CN and
C$_2$N$_2$) spectra available in the literature except the 695 cm$^{-1}$
feature that could possibly be due to C$_2$H$_3$CN ice.
