The end-Ordovician (Hirnantian) glaciation was causally linked with the first of five Phanerozoic mass extinction events. An enigma is how the short, sudden but severe glaciation happened at high atmospheric pCO₂ up to 8 ~ 12 times higher than present levels. Massive organic carbon burial has been proposed as a feasible resolution based on positive carbon isotope excursion, but it can be masked by the change in flux of weathered carbonate and/or terrestrial organic matter. Zinc isotopes (expressed as d⁶⁶Zn) are a novel and promising proxy for the change in organic carbon fluxes in past oceans. Here we present the first zinc isotope study on both carbonate and shale successions that span the Ordovician–Silurian boundary interval in South China. Two positive shifts in d⁶⁶Zn are observed during two main periods of glacial maxima, indicative of two pulses of sinking organic carbon. The enhanced organic carbon burial during glacial maxima intervals can be causatively linked to cooling-induced elevation in organic carbon transfer efficiency. This demonstrates that the storage of oceanic organic carbon storage was a large carbon reservoir to regulate the atmospheric pCO₂ and cause the Hirnantian glaciation. The organic carbon burial efficiency was amplified at relatively low pO₂ conditions of early-Paleozoic and produced the pattern of glacial-to-deglacial change sensitive to the temperature effect.
 

Zn isotopes,organic carbon burial