Survival strategies of Antarctic vegetation during extensive glacial expansion across the Oligocene/Miocene Transition

Abstract. Antarctica’s terrestrial ecosystems are at risk from a rapidly changing climate. Investigating how Antarctica’s vascular plants responded to major climatic variations in the geological past, especially under atmospheric CO₂ values similar to modern and future projections, may provide insight into how organisms could migrate across the continent as conditions change. Here, we investigate vegetation trends across the Oligocene/Miocene Transition (OMT, ~23 Myr), one of the largest transient glaciations of the Cenozoic. Despite extensive ice sheet expansion, Antarctic vegetation survived throughout this glacial episode. We use compound specific isotope trends (δ¹³C and δ²H) of plant waxes in an Antarctic proximal sediment core from the Ross Sea (Deep Sea Drilling Project site 270) to investigate the response and survival mechanisms of Antarctic vegetation during this event. We detect the first observation of a marked negative n-alkane δ¹³C excursion over the OMT, coupled with a shift to more positive n-alkane δ²H. We interpret this as plants sacrificing water use efficiency to maintain photosynthesis and carbon uptake during increasing glacial conditions, as atmospheric CO₂ decreased and orbital configurations favoured shorter, colder growing seasons with lower light intensity. We consider further drivers of these isotopic trends to be enhanced aridity, and a shift to a stunted, low elevation vegetation. These findings establish the adaptability of ancient Antarctic vegetation under atmospheric CO₂ conditions comparable to modern, and mechanisms that allowed vegetation to keep a foothold on the continent despite prolonged hostile conditions.