  The Milky Way has undergone significant transformations in its early history,
characterised by violent mergers and the accretion of satellite galaxies. Among
these events, the infall of the satellite galaxy Gaia-Enceladus/Sausage is
recognised as the last major merger event, fundamentally altering the evolution
of the Milky Way and shaping its chemo-dynamical structure. However, recent
observational evidence suggests that the Milky Way remains undergone notable
events of star formation in the past 4 Gyr, which is thought to be triggered by
the perturbations from Sagittarius dwarf galaxy (Sgr). Here we report chemical
signatures of the Sgr accretion event in the past 4 Gyr, using the [Fe/H] and
[O/Fe] ratios in the thin disc, which is reported for the first time. It
reveals that the previously discovered V-shape structure of age-[Fe/H] relation
varies across different Galactic locations and has rich substructures.
Interestingly, we discover a discontinuous structure at z$_{\rm max}$ $<$ 0.3
kpc, interrupted by a recent burst of star formation from 4 Gyr to 2 Gyr ago.
In this episode, we find a significant rise in oxygen abundance leading to a
distinct [O/Fe] gradient, contributing to the formation of young O-rich stars.
Combined with the simulated star formation history and chemical abundance of
Sgr, we suggest that the Sgr is an important actor in the discontinuous
chemical evolution of the Milky Way disc.
