  Finding and characterizing extrasolar Earth analogs will rely on
interpretation of the planetary system's environmental context. The total
budget and fractionation between C-H-O species sensitively affect the climatic
and geodynamic state of terrestrial worlds, but their main delivery channels
are poorly constrained. We connect numerical models of volatile chemistry and
pebble coagulation in the circumstellar disk with the internal compositional
evolution of planetesimals during the primary accretion phase. Our simulations
demonstrate that disk chemistry and degassing from planetesimals operate on
comparable timescales and can fractionate the relative abundances of major
water and carbon carriers by orders of magnitude. As a result, individual
planetary systems with significant planetesimal processing display increased
correlation in the volatile budget of planetary building blocks relative to no
internal heating. Planetesimal processing in a subset of systems increases the
variance of volatile contents across planetary systems. Our simulations thus
suggest that exoplanetary atmospheric compositions may provide constraints on
$when$ a specific planet formed.
