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Lower Triassic limestones contain giant ooids (>2 mm) along with other precipitated carbonate textures more typical of Precambrian strata. These features appear to have resulted from changes in seawater chemistry associated with the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained challenging. To constrain seawater carbonate saturation state, dissolved inorganic carbon, alkalinity, and pH, we applied a physicochemical model of ooid formation constrained by new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least seven times supersaturated with respect to aragonite and calcite. 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 and alkalinity and a pH near 7.6. Such conditions may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.

[Bucher et al.'s Comment (2007)][1] questioned the validity of our 247.2 Ma age estimate for the Early-Middle Triassic (Olenekian-Anisian, O-A) boundary based on a comparison of recent data from Lower Guandao section presented by us ([Lehrmann et al., 2006][2]) with preliminary data reported from

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

Stanley’s model (1988) proposed that a Permiantype reef fauna survived the Early Triassic reefless period in yet undiscovered refuges in the ancestral Pacific ocean and then reappeared as the builders of Middle Triassic reefs. Flugel (1994), in contrast, suggested that the Middle Triassic reefbuilding fauna are not holdovers from the Permian, but bear only a superficial resemblance to Permian fossils. Recent publications reflect a consensus that the Early Triassic lacked reefs and reef mounds although some authors acknowledged the possibility that reef-like structures may have existed in poorly explored areas such as south China and Russia (Senowbari-Daryan et al., 1993; Flugel, 1994). It is shown herein that Calcimicrobial framework mounds and biostromes existed in the Early Triassic in the Nanpanjiang basin of south China. Organic framework in this paper refers to a three dimensional, biologically-constructed network of calcified fossils that forms a rigid mass and supports an interconnected network of internal cavities that were open during accumulation. Although the calcimicrobial frameworks discussed herein are much smaller than frameworks of modern metazoan reefs, the term framestone is more appropriate for these fabrics than other reef-rock terms such as bindstone or bafflestone (Embry and Klovan, 1971). These calcimicrobial mounds and biostromes are important as the first organic framestones documented from the Early Triassic.