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

Abstract

The distribution of authigenic phosphates in the sedimentary sequence of prehistoric Den-isova Cave (Altai, South Siberia) has important archeological implications. The sampled Late Pleis-tocene–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 path-ways, with acidic leaching and gradual neutralization of P-rich solutions. The depth profile of au-thigenic 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 leuco-phosphite corresponding to acidic environments; whitlockite, brushite, and hydroxylapatite, which are stable under slightly acidic and neutral conditions; and hydroxylapatite in coexistence with cal-cite 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 Den-isova 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”.