Resolving the Snowball Earth conundrum: the role of marine dissolved organic carbon pool.

The Cryogenian Sturtian (717-660 Ma) and Marinoan glacial deposits (∼650-635 Ma), typically consisting of alternating layers of glaciogenic diamictite and clast-free lithofacies, indicate dynamic glaciers or glacial-interglacial cycles during the global glaciations. This may result from ice sublimation in tropics under a Snowball Earth condition. However, this model fails to explain the deposition patterns observed in mid-latitude continents. We propose the presence of unfrozen oceans while the continents are covered, i.e., the icy-continents. The open-ocean condition requires low atmospheric pCO₂ level. We argued that the mantle CO₂ degassing could be counterbalanced by a growing pool of dissolved organic carbon (DOC) in the deep ocean, maintaining a small marine dissolved inorganic carbon (DIC) pool and a low atmospheric pCO₂ level. The persistent marine productivity in the open ocean would support the expanded DOC pool due to reduced ocean ventilation and limited terrestrial inputs of oxidants. However, the global glaciation with open oceans was climatically unstable. The fluctuation of the DOC pool on a local or regional scale likely contributed to the frequent glacial-interglacial oscillations recorded in the rock records. Additionally, the expansion of the DOC pool removed seawater nutrients, e.g., phosphorus (P), and insufficient nutrient supply prevented the transfer of mantle-degassing carbon as DOC, ultimately leading to the termination of global glaciation. The turnover of the DOC pool, caused by deep ocean ventilation in the deglacial period, significantly increased the atmospheric pCO₂ level. This event was followed by intense continental weathering, increased seawater pH, recovery of primary productivity, cap carbonate precipitation, and eventually, the emergence of new life forms and innovations in the biosphere.