MIGHT OCEAN ACIDIFICATION LEAVE A DIAGENETIC RECORD?

One way to grasp the potential effects of modern ocean acidification is to study acidification events in the past. Unfortunately, acidification may dissolve or preclude the formation of the carbonate rock record, hindering recognition of past acidification events. Here, we suggest that the diagenetic record of the Triassic-Jurassic boundary may preserve a record of ocean acidification, possibly related to the T-J mass extinction.

The T-J boundary interval at Williston Lake, Canada contains fibrous calcite layers, previously assumed to have formed via late-stage diagenesis. However, our analyses suggest an early diagenetic origin. Thin sections show the fibrous layers have a fan-shaped morphology and were likely originally aragonite (based on the presence of blunt crystal terminae and a fibrous habit). The fans display unidirectional upward growth, precluding the possibility that they are void/crack infill. Vertical δ¹³C transects through the fans shift negatively by 5‰ or more over a few cm (from -3‰ to -8‰). Carbonate-associated sulfate decreases up a fan transect, suggesting the fans grew as sulfate was consumed in the pore waters, within the sulfate reduction zone. Taken together, the petrography, δ¹³C, and CAS results suggest the fans grew within the sediment, likely not far beneath the sediment-water interface.

Intriguingly, the beds hosting the fans are nearly devoid of carbonate, while the overall section averages ~30 wt% carbonate. The lack of carbonate in the host beds may be due to ocean acidification, where carbonate was remobilized from the boundary interval and partially reprecipitated as early diagenetic features. This scenario is internally consistent- an isotopic-mass balance shows that if sulfate reduction in pore waters caused the decrease in CAS values, this can adequately account for the magnitude of the δ¹³C shift given the low seawater sulfate concentrations at the time.

Similar fibrous calcite layers are found near the T-J boundary at St. Audrie’s Bay, UK, suggesting they may represent a global event. The coincidence of unique early diagenetic features near one (and perhaps multiple) T-J boundaries suggests that early diagenesis may be an untapped source of information about ocean and shallow pore water geochemistry across mass extinction or other acidification intervals.