<p>The Palaeocene-Eocene Thermal Maximum ca. 55.93 Ma is considered one of the best geological analogues for worst-case, near-future climate states. During this warming interval, calcareous nannoplankton—critical marine primary producers and important components of the biological pump—experienced significant turnover in community composition, which likely had a major impact on ecosystem function. However, determining the extent of this relationship is complicated by recognized geographic variability in the pattern and magnitude of palaeoecological change, as well as a sampling bias towards lower latitude oceanic sites. In this study, we present new calcareous nannofossil assemblage data from the climatically-sensitive, high latitude south-west Pacific Ocean. Critically, our record shows that major changes in nannoplankton community composition—coincident with a 0.5 ‰ decrease in bulk sediment δ¹³C—preceded the onset of the Palaeocene-Eocene Thermal Maximum by 200 kyr. We suggest this precursor event led to the establishment of a new palaeoecological baseline that caused a relatively muted response of nannofossil assemblages to the main warming event, especially when compared to other high latitude sites. Therefore, our results highlight the importance of examining the influence of local and regional processes on palaeoecological background states when considering geographically heterogenous responses to major global palaeoclimatic events.</p>