Marine phytoplankton grow via photosynthesis by fixing carbon from CO2 in the atmosphere. Carbon (C) in these tiny organisms serves as the base of the food chain. The balance between the C returned to the atmosphere upon death and decay of these algae (and their consumers) and that sequestered and buried in sediments controls much of the Earth’s C-cycle. Disruptions of this complex C balance may have triggered changes in the planet’s climate, glaciation, and sea level that can be linked with mass extinctions. Carbon isotope compositions have been an essential tool in evaluating the balance of C stored in atmosphere/ocean and sequestered in sediments. Yet, the inability to measure C isotopes of individual species of the tiny, marine, organic-walled plankton has obscured a full understanding of this complex cycle. For much of the Palaeozoic, organic-walled microfossils (palynomorphs) are our only record of these vital processes at the base of the food web. Previous analytical limitations necessitated analyses of “bulk samples” that are complex organic mixtures. We are developing revolutionary new analytical tools, capable of routinely and accurately measuring the C isotopes of single planktic palynomorph specimens, which will provide a more complete view of the C-cycle in deep time. We propose applying this new tool to the Hirnantian glaciation/extinction event (444 Ma ago), to unravel the processes that led to this dramatic, but poorly understood, perturbation of the C-cycle.