Impact of phosphate mineralization of sediments on the preservation of stone artifacts (based on evidence from Denisova cave)

Geochemical analysis of loose sediments in the East Chamber of Denisova cave has shown that the degradation products of bat guano caused authigenic phosphate mineralization of sediments. The most intensive changes in the substrate were observed in layer 9. This layer was the nearest to the source of reagents and contained the finds attributed to the Middle Upper Paleolithic. Not only loose sediments and fragments of bedrock limestone, but also stone artifacts and bone remains deposited in phosphatic areas underwent geochemical transformation. The most clearly observable changes in stone artifacts from layer 9 are referred to as “model” changes for describing the transformation of silicate rocks in the process ofpost-depositional phosphatization as a whole. A detailed analysis was conducted based on a sample of lithic artifacts which includes cores, flakes, and tools. Stone artifacts recovered from phosphate mineralization areas had thick white reaction rim. The mineral composition of the original rock and new formations in the rim were determined through visual analysis ofpetrographic thin sections and X-ray diffraction analysis. The presence of organic matter and englishite - a phosphate mineral - in the whitened outer zone of the artifacts indicates decomposition of silicate rocks and particularly feldspars, used for manufacturing these artifacts, resulting from the impact of strong organic and phosphoric acids. The resulting amorphous silicic acid was partly carried away, resulting in porosity of the modified zone up to the emergence of loose white rim. Development of alterations on silicate rocks used for producing the artifacts indicates that the reagents which caused these changes were very aggressive and reactive. It is evident that their impact on bones buried with the artifacts should be even greater.