Phosphate Record in Pleistocene-Holocene Sediments from Denisova Cave: Formation Mechanisms and Archaeological Implications

The distribution of authigenic phosphates in the sedimentary sequence of prehistoric Denisova Cave (Altai, South Siberia) has important archeological implications. The sampled Late Pleistocene-Early Holocene sedimentary sequence in the East Chamber of the cave consists of argilo-sandy-phosphatic sediments intercalated with guano layers of insectivorous bats. The sediments bear partially degraded N-rich organic matter (OM); chitin fragments enriched in S, P, Zn, and Cu; and a set of phosphates. The guano layers record at least three prolonged episodes of cave occupation by colonies of insectivorous bats between 10 kyr and 5 kyr BP, after people had left the cave or visited it rarely in small groups. The formation of phosphates follows the OM biodegradation pathways, with acidic leaching and gradual neutralization of P-rich solutions. The depth profile of authigenic phosphates shows a suite of mineral assemblages that mark a trend from acidic to slightly alkaline pH conditions of guano degradation (from top to bottom): ardealite, taranakite, and leucophosphite corresponding to acidic environments; whitlockite, brushite, and hydroxylapatite, which are stable under slightly acidic and neutral conditions; and hydroxylapatite in coexistence with calcite and stable at the bottom of the leaching profile under alkaline conditions. Authigenic phosphates can be used as reliable indicators of human non-occupation (abandonment) periods of Denisova Cave. Acidic leaching is responsible for disturbance and/or elimination of archaeological and paleontological materials in Late Pleistocene-Early Holocene sediments that were exposed to at least three "acidic waves".