Sulphate reduction associated with hardgrounds: Lithification afterburn!

Negative excursions in δ13C profiles from platform carbonates that coincide with pyritised hardgrounds are commonly linked to subaerial exposure events, but we show here that they can also result from subsurface bacterially-mediated early cementation in addition to the precipitation of syndepositional marine cements. A non-soil-derived origin for δ13C-depleted micrites offers an alternative origin and bathymetric interpretation for these surfaces.

The Lower Cretaceous Lekhwair Formation platform carbonate successions from offshore Abu Dhabi contain abundant hardgrounds that are important for both regional correlation and control of subsurface flow. The micrite from these hardgrounds have average δ13C values of + 0.7‰; 1.5‰ lower than non-hardground micrites that are similar to contemporary open ocean values. Hardground δ13C values are due to the addition of 13C-depleted carbonate, generated as a by-product of sulphate reduction, to the ‘normal’ marine calcite that caused hardground lithification. Pyritisation of the hardgrounds occurred before, during and after ‘normal’ calcite precipitation. The persistence of sulphate reduction after hardground lithification is shown by the presence of pyrite and low δ13C micrite in sediment up to a few mm above the hardgrounds