  Terrestrial planets orbiting M dwarfs may soon be observed with the James
Webb Space Telescope (JWST) to characterize their atmospheric composition and
search for signs of habitability or life. These planets may undergo significant
atmospheric and ocean loss due to the superluminous pre-main-sequence phase of
their host stars, which may leave behind abiotically-generated oxygen, a false
positive for the detection of life. Determining if ocean loss has occurred will
help assess potential habitability and whether or not any O2 detected is
biogenic. In the solar system, differences in isotopic abundances have been
used to infer the history of ocean loss and atmospheric escape (e.g. Venus,
Mars). We find that isotopologue measurements using transit transmission
spectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may be
possible with JWST, if the escape mechanisms and resulting isotopic
fractionation were similar to Venus. We present analyses of post-ocean-loss O2-
and CO2-dominated atmospheres, containing a range of trace gas abundances.
Isotopologue bands are likely detectable throughout the near-infrared (1-8 um),
especially 3-4 um, although not in CO2-dominated atmospheres. For Venus-like
D/H ratios 100 times that of Earth, TRAPPIST-1 b transit signals of up to 79
ppm are possible by observing HDO. Similarly, 18O/16O ratios 100 times that of
Earth produce signals at up to 94 ppm. Detection at S/N=5 may be attained on
these bands with as few as four to eleven transits, with optimal use of JWST
NIRSpec Prism. Consequently, H2O and CO2 isotopologues could be considered as
indicators of past ocean loss and atmospheric escape for JWST observations of
terrestrial planets around M dwarfs.
