Optimization of the acetic acid method for microfossil extraction from lithified carbonate rocks: Examples from the Jurassic and Miocene limestones of Saudi Arabia

An optimization experiment with different acid concentrations was carried out to assess the use of acid to minimum sustainable limits for the extraction of microfossils from indurated limestones. Two different limestone formations of Jurassic and Miocene ages were tested. Different concentrations of acid ranging from 50 to 100% and processing times varying from 2 to 10 h were tested for optimal recoveries. The acid residue recoveries show a similar trend for both formations. The weight percentage of residue with particle size >1 mm decreased as the acid concentration increased, especially in the 50–80% acid concentration range. On the other hand, the weight percentage of the smallest size particles > 0.063 mm increased as acid concentration increased. This means that the higher concentrations of acid dissolve more of the unnecessary large particles while the foraminifera, which comprise the sand fraction size, are left in the residue. Although higher acid concentrations with longer reaction times yielded better recoveries than with less reaction time, we recommended a 60% concentration of acetic acid and a reaction time of 10 h for optimal recovery of micropaleontological samples in Saudi Arabian carbonate rocks. By lowering the recommended concentration, the consumption of acid is reduced without compromising the recovery of microfossils. • Acetic acid leaching method is applied on two different age limestone samples to extract foraminifera. • Different concentrations of acetic acid are tried and tested, and consensus is made on an optimum concentration of 60% for a submersion time of 10 h. • The sample recoveries are optimal while using this concentration for a time of 10 h.