  Trends in the enrichment of elements with stellar ages are a powerful avenue
to identify unexplained origins of the elements. We investigate the stellar
abundance trends of low to intermediate-mass stars using the GALAH DR3
high-resolution spectroscopic dataset of 6234 solar-type stars. Our study
explores the elemental abundance [X/Fe] of sodium (Na) with age. We find a
pronounced enrichment in [Na/Fe] at super solar metallicity (i.e., [Fe/H] above
0) in the old sequence of Milky Way disc stars, a trend demanding a deeper
understanding of the underlying source(s) responsible for the nucleosynthesis.
This progressive [Na/Fe] enrichment at the young end of the old sequence has
essential implications for Galactic archaeology. In this work, we propose a
novel selection technique for separating the Milky Way's thick and thin disc
stellar populations (i.e., old and young sequences) based on the observed
[Na/Fe] rise of roughly 0.1 dex for stars around 5 - 8 Gyr old. We also compare
our selection method to the conventional [Mg/Fe] vs [Fe/H] selection approach,
and we find that our new Na-based selection method better disentangles the
overlap between young- and old-sequence disc stars at these intermediate ages.
This is especially true at super solar [Fe/H], where [Mg/Fe] vs [Fe/H] or
[alpha/Fe] vs [Fe/H] separation approaches exhibit a lot of overlap. This new
selection method should help us better understand the Milky Way disc's
formation history.
