DO MASS EXTINCTIONS HAVE DIAGENETIC CONSEQUENCES?

Mass extinction intervals are associated with a suite of biological, geochemical, and sedimentological effects, likely linked to unusual ocean geochemical conditions at the time. Here, we report unique early diagenetic features at the Triassic-Jurassic boundary and suggest that early diagenetic fabrics may be a heretofore untapped source of information about ocean and shallow porewater geochemistry across mass extinction intervals.

We have recognized early diagenetic calcium carbonate fans associated with the Triassic-Jurassic boundary section at Williston Lake, British Columbia. The fibrous CaCO₃ layers are all within the 2 m boundary interval, bounded by the last Triassic conodont and first Jurassic ammonoid. They were deposited in an outer shelf to slope setting, likely under oxygen-restricted conditions. A total of seven fibrous calcite layers occur over 80 cm, ranging from 0.8 to 10.5 cm in thickness and interbedded with very fine siliciclastics. Blunt crystal terminations and pseudohexagonal crystal cross-sections indicate that the fans were originally aragonite. δ¹³C transects up the fans shift negatively by 5‰ or more over 10 cm, suggesting that the fans grew as organic matter was remineralized, likely very early in the diagenetic succession at or just below the sediment water interface.

The boundary interval is devoid of carbonate, save for the fan layers, whereas the underlying and overlying units contain appreciable CaCO₃. Interestingly, a mass balance would suggest that a remobilization of carbonate, assuming total inorganic carbon values similar to underlying/overlying units, would account for the mass of CaCO₃ in the fan layers. Therefore, it is possible that these unique early diagenetic features are linked to ocean acidification, one of the hypothesized causes of the end-Triassic mass extinction, which affected early porewater diagenesis, remobilizing carbonate that was subsequently re-precipitated as the fan layers.