2026MNRAS.545f1551O

We study the formation of chemical sequences in stellar discs of Milky Way (MW)-mass galaxies in a full cosmological context with the AURIGA simulations, with implications for both the MW and external galaxies like M31. The analysis focuses on the conditions giving rise to bimodal $\alpha$-chemistry and the potential influence of mergers (e.g. Gaia-Enceladus, GSE). We find a wide diversity of chemical sequences, without correlation between the emergence of dichotomous $\alpha$-chemistry and GSE-like mergers. The transition between multiple $\alpha$-sequences is sequential, and is mediated by modulations in the star formation rate (SFR). In some cases, this can be caused by the starburst and subsequent quiescence induced by mergers. In others, by exhaustion or violent disruption of the gas disc. Realizations with singular sequences either lack significant modulations in their SFR, or form too late to have a significant high-$\alpha$ sequence. The metallicity overlap between the high- and low-$\alpha$ sequences (as seen in the Solar neighbourhood of the MW) arises from accretion of metal-poor gas from the circumgalactic medium. This depends on gas disc thickness, with thinner discs losing their metal-poor extremities. Gas donation from singular gas-rich merger events are incapable of driving long-lived metal dilution ($\Delta \rm {[Fe/H]} \gtrsim 0.3$), and we rule-out this scenario for the low-$\alpha$ sequence in the MW. Finally, the shape of $\alpha$-sequences in the [Fe/H] versus [Mg/Fe] plane is related to long-term SFR trends, with a downward slanted locus (as observed in the low-$\alpha$ sequence of the MW) owing to a sustained or declining SFR.