HYDROGEN PEROXIDE GREATLY INCREASES BREAKDOWN OF FOSSILIFEROUS SEDIMENTS FROM UPPER CRETACEOUS MICROVERTEBRATE SITES IN THE WILLIAMS FORK FORMATION, NORTHWESTERN COLORADO

Screenwashing is a technique commonly used by paleontologists to reveal microvertebrate fossils by some combination of soaking and sieving fossiliferous sediment. We collected sediment from two sites in the Upper Cretaceous (Edmontonian) Williams Fork Formation (WFF) in northwestern Colorado– ReBecca’s Hollow (RH) and Super Charger Heaven (SCH). Rebecca’s Hollow yields osteichthyans (bony fish), amphibians, turtles, crocodiles, dinosaurs, and mammals. We have found similar fossils such as crocodilian teeth and gar scales from SCH and expect to find many more due to its close proximity to RH.

Traditional screenwashing processes of WFF sediments began with weighing ~600 grams of sediment, placing it in sieves, and soaking in gently agitated water for 24 hours. Afterwards, the sieves were removed, dried, and re-weighed to calculate the amount of sediment lost. This method averaged 56% breakdown at RH and 62% at SCH. Anecdotal reports suggest that the addition of 3% hydrogen peroxide (H2O2) improves sediment breakdown, apparently by assisting in breaking bonds between clay particles and breaking down organics (carbonaceous material). Greater sediment breakdown results in a higher concentration of fossils relative to sediment, improving picking efficiency. After verifying that H2O2 did not degrade fossils we incorporated it by submerging sediment in a separate container of 3% H2O2 ; letting it soak for 5-30 minutes. During this we observed off gassing as the clays were breaking down. The sieves were then relocated to the screenwashing station to replicate the traditional process. Using the H2O2 method increased average breakdown to 87% for both RH and SCH. This process therefore has reduced the concentrate for picking by ~70%, making this a more efficient method of screenwashing for the recovery of microvertebrate fossils. Hydrogen peroxide (3%) is not an especially hazardous chemical and has not shown any visible damage to the fossils, which include bones, scales, and teeth. Re-washing previously screenwashed concentrate from RH with H2O2 resulted in 42% additional breakdown, and fossils were immediately recovered from this concentrate that were not seen previously, supporting the efficacy of H2O2 in recovering microfossils.