seacarbx is a seacarb extension written in R, which enables to use seacarb (Gattuso et al., 2019) for deep-time carbonate system calculations or for culturing studies carried out in seawater with modified [Mg²⁺] and [Ca²⁺]. For this, the functions carb, K0, K1, K2, Ks, Kw, Kb, Kspa, and Kspc were modfied to account for the effect of seawater [Mg²⁺] and [Ca²⁺] on the dissociation constants of carbonic and boric acid. In addition to the modified functions, seacarbx contains MyAMI that are tabulated parameters defining the temperature and salinity dependencies of the conditional equilibrium constants for [Mg²⁺] and [Ca²⁺] in the range 1–60 mM (from Hain et al., 2015, 2018), as well as a function for bilinear interpolation.