GSA Connects 2021 in Portland, Oregon

Paper No. 225-14
Presentation Time: 9:00 AM-1:00 PM


SULLIVAN, Colleen and KEENAN, Sarah, Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 East St. Joseph Street, Rapid City, SD 57701

Fossils are periodically left in the field, either in-situ or as float, for numerous reasons (i.e., funding constraints, time constraints, lack of storage space, etc.). However, studies regarding the longevity of fossils subject to erosional processes, such as acidic precipitation, are absent in the literature. The goal of this study is to determine vertebrate fossil dissolution rates at varying pH to aid paleontologists in making better informed field collection decisions.

Three fossil vertebrae were selected and cut into thirds to conduct the experiment in triplicate. The fossils were completely submerged in a tap water solution; 0.1N hydrochloric acid (HCl) was used to adjust the solution to the desired pH of 4, 5, and 6. The fossils were submerged for 21 days in a closed system, and the pH was maintained manually (+/- 1.72 for pH 4, +/- 0.90 for pH 5, and +/- 0.56 for pH 6) by adding HCl every two hours from 08:00 to 18:00. Fossil dissolution was quantified by mass loss and by differences in pre- and post-dissolution water chemistry analyzed with inductively coupled plasma mass spectrometry (ICP-MS). The fossils exhibited an average mass loss of 650.5 mg in the pH 4 solution, 548.7 mg in the pH 5 solution, and 360.8 mg in the pH 6 solution. At a constant rate of dissolution, this would be a loss of 31.0 mg/day for pH 4, 26.1 mg/day for pH 5, and 17.2 mg/day for pH 6. Solutions exhibited an increase in Ca, P, Na, Mg, Al, K, Mn, Fe, Sr, and Ba concentrations, likely reflecting fossil dissolution.

This study provides an estimate of dissolution that can be refined by examining how surface area, fossil shape, fossil element type, taxon variation, temperature, salinity, or sediment type can affect dissolution rates. These results indicate fossils exposed at the surface will be eroded within a few years (based on extrapolating the experimentally determined daily dissolution rate) depending on the initial size of the fossil and amount of precipitation within the region. These results provide a new understanding of fossil longevity at the surface and give insights relevant to fossil conservation efforts.