  We present VLT/UVES spectroscopy of the quasar Q0841+129, whose spectrum
shows a proximate damped Lyman-alpha (PDLA) absorber at z=2.47621 and a
proximate sub-DLA at z=2.50620, both lying close in redshift to the QSO itself
at z_em=2.49510+/-0.00003. This fortuitous arrangement, with the sub-DLA acting
as a filter that hardens the QSO's ionizing radiation field, allows us to model
the ionization level in the foreground PDLA, and provides an interesting
case-study on the origin of the high-ion absorption lines Si IV, C IV, and O VI
in DLAs. The high ions in the PDLA show at least five components spanning a
total velocity extent of ~160 km/s, whereas the low ions exist predominantly in
a single component spanning just 30 km/s. We examine various models for the
origin of the high ions. Both photoionization and turbulent mixing layer models
are fairly successful at reproducing the observed ionic ratios after correcting
for the non-solar relative abundance pattern, though neither model can explain
all five components. We show that the turbulent mixing layer model, in which
the high ions trace the interfaces between the cool PDLA gas and a hotter phase
of shock-heated plasma, can explain the average high-ion ratios measured in a
larger sample of 12 DLAs.
