Shelled pteropods are at risk from ocean acidification, with known effects on their shell durability and calcification. Their shells are composed of aragonite, which typically dissolves when the aragonite saturation state of seawater is <1: a process known as aragonite undersaturation. Pteropods typically form their shells at specific depth ranges known as the ‘calcification depth’, which varies depending on species and habitats. Some Southern Ocean regions are already experiencing aragonite undersaturation and this could negatively affect shelled pteropods. However, the calcification depths of the pteropods have not been determined nor used to infer consequences of changing seawater carbonate chemistry in the Southern Ocean. In this study, we analysed the stable oxygen isotope of Limacina helicina antarctica shells collected by sediment traps in the sub-Antarctic zone to estimate their calcification depth. Shallow calcification depths (12.5-126 m) were detected in summer-autumn. Conversely, L. helicina antarctica calcify their shells deeper in the water column during winter-spring, with the maximum average calcification depth of 525.2 m. Recent shoaling of the aragonite saturation horizon is likely to affect pteropods that calcify their shells in deep water. This rapid change will limit their ability to form their shells, which will have significant impacts on their populations.