Lower Triassic limestones contain the largest ooids of the Phanerozoic, up to 10 mm in diameter, along with other precipitated carbonate textures more typical of Precambrian carbonate strata. These features appear to result from changes in seawater chemistry associated with or resulting from the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained beyond available proxies. To constrain these conditions, a physicochemical model of ooid formation is applied using new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state (Ω) and lower transport frequency (f) than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least 6 to 10 times supersaturated with respect to aragonite and that the f of ooids was about an order of magnitude lower than observed for modern, sand-size ooids. When combined with independent constraints on atmospheric pCO₂ and oceanic [Ca²⁺], these findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon (DIC) and alkalinity (ALK) and a pH near 7.5. Such conditions can be produced in the aftermath of a carbon injection event such as has been hypothesized to occur through Siberian Traps flood basalt eruptions and may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.

Giant Ooids