  We report the results of a Monte Carlo global QCD analysis of unpolarized
parton distribution functions (PDFs), including for the first time constraints
from ratios of $^3$He to $^3$H structure functions recently obtained by the
MARATHON experiment at Jefferson Lab. Our simultaneous analysis of nucleon PDFs
and nuclear effects in $A=2$ and $A=3$ nuclei reveals the first indication for
an isovector nuclear EMC effect in light nuclei. We find that while the
MARATHON data yield relatively weak constraints on the $F_2^n/F_2^p$ neutron to
proton structure function ratio and the $d/u$ PDF ratio, they suggest a
strongly enhanced nuclear effect on the $d$-quark PDF in the bound proton.
