QUANTITATIVE ASSESSMENT OF FOSSIL BONE DISSOLUTION UNDER ACIDIC CONDITIONS—IMPLICATIONS FOR GUIDING FIELD COLLECTION EFFORTS

Vertebrate taphonomy culminates in the exposure of fossilized bone to environmental conditions. Interactions with precipitation, which is typically acidic, has the potential to negatively impact fossils through dissolution. However, the impacts of precipitation-fossil interactions and the implications for fossil stability are understudied. Additionally, quantitative values of fossil longevity exposed to present-day environmental conditions are limited, leaving our understanding of the long-term stability of fossils exposed at the surface uncertain. The objective of this study was to experimentally determine vertebrate fossil dissolution rates in various pH solutions, one of the most important controls on mineral stability. It was hypothesized that fossils would dissolve when exposed to acidic solutions and that dissolution rate would increase with decreasing solution pH. Experiments were conducted in triplicate on three fossil vertebrae submerged for 21 days in closed reaction vessels with pH 4, 5, and 6 solutions adjusted using 0.1N hydrochloric acid. Fossil dissolution was quantified by changes to: fossil mass, fossil and water elemental chemistry with inductively coupled plasma mass spectrometry (ICP-MS), minerology with X-ray diffraction (XRD), and histologic structure degradation with thin section analyses. All fossils exhibited mass loss. The greatest mass loss was at pH 4 (477 to 803 mg), and mass loss decreased with increasing pH (285 to 459 mg loss at pH 6). ICP-MS indicated an increase of both calcium (315 ppm maximum increase) and phosphorus (19 ppm maximum increase) in solutions and a loss of those same elements from the fossils (maximum loss:10 ppm Ca, 6 ppm P). The results of ICP-MS and XRD viewed together suggest dissolution of secondary phases, including calcite and gypsum, and primary phases, including apatite the main mineralogical component of bone. Thin section analysis showed degradation of trabeculae in all post-dissolution samples, demonstrating physical changes to the fossils as a result of the acidic treatments. These findings provide a quantitative estimate of fossil dissolution, emphasize a potential bias in the vertebrate fossil record, and suggest that fossil bone dissolution rates may be a useful guide to prioritize collection efforts in the field.