Post-Depositional Alterations in Dolerite Neolithic Stone Tools from Garo Hills, Northeast India: A Case Study in Indo-Pacific Geoclimatic Contexts

Rohit* and Sukanya Sharma

This study examines post-depositional alterations in Dolerite-made Neolithic stone tools from the Garo Hills region of Northeast India and situates these alterations within broader Indo-Pacific geoclimatic contexts. Using XRD, FTIR, ICPMS, Light microscopy, and SEM, we analysed physicochemical and morphological changes in archaeological stone tools in buried conditions. The results reveal site-specific degradation patterns influenced by both surrounding soil environment and artefact's own composition. At the Misimagre Neolithic site in the Garo Hills, dolerite stone tools exhibited noticeable surface weathering under the influence of acidic soils, resulting in a mealy clayey outer layer enriched in secondary sesquioxides and clays. This layer detaches through powdering, contributing to inorganic soil fractions during prolonged pedogenesis. When these archaeological findings were studied from a geological standpoint and compared with geoclimatic contexts across various regions of the Indo-Pacific on a macro scale, it was found that the alterations in these stone tools are not unique but part of a larger geological phenomenon. Although these alterations are region-specific, for example, in the Australian and Tasmanian landscapes, Dolerite rocks experience chemical weathering & physical degradation, which is responsible for formation of red-brown loamy and clayey soils. However, in Antarctica, freezing desert conditions prevent chemical change, resulting in slower surface alterations. Correlating Dolerite alterations in Garo Hills with other Indo-Pacific case studies revealed that these alterations are linked to long-term pedogenic processes that form Dolerite-derived soils. While detected alterations reflect changes in material intrinsic properties, undetected modifications may persist beyond diagnostic resolution of methods applied in this study.