Provenance geochemical detection of soil deposits from archaeological limestone caves in the Bubing Basin, tropical China

Abstract

Today, the limestone karst cave systems in southern China are extensive and well developed. Due to the slow uplift of the crust since the Neogene and extensive river downcutting that made these caves accessible, the sediments in these karst caves preserve a plethora of Quaternary vertebrate fossils (including hominins) and occasionally stone artifacts. These remains provide important clues for studying the paleontology, archaeology and paleoenvironment in the region. However, in these caves it is difficult to understand how these fossils were deposited. In fact, it is often extremely difficult to determine the provenance and transport history of homogeneous clastic sediments, which may carry these fossils and artifacts into these caves, because they often do not have distinct sedimentological features. Here we present a provenance analysis of the Pleistocene clastic sediments from Chuifeng, Mohui, Ganxian and Luna Caves, four caves that may be considered representative of the basin, and compared with the sediment samples from the local fluvial terraces and the surface of limestone hills in Bubing Basin, Guangxi, southern China, utilizing their major and trace element composition as a provenance marker. The major, trace element ratios, and rare earth element distribution data all indicate that the clastic sediments of these four cave sites are directly derived from the weathered residual sediments found on the limestone hills where these caves are located. In all cases, the cave deposits are clearly different from the clastic sediments from the four local fluvial terraces in the basin. Our results provide a new set of data that complement and further expands the common view that these clastic sediments and encased mammal fossils are transported into these cave sites by fluvial action. The clastic sediments on these limestone hills are primarily transported into the caves through cave entrances and/or pipeline or fractures in the hills by sheet flow. As such, it is likely that biological agents (e.g., porcupine and/or carnivore/hominin transport) and not fluvial activity may have played an even greater role in the accumulation of vertebrate fossils in cave sites than traditionally thought. This study should have broader implications in investigations into the relationship between clastic sediments and fossils in cave sites across time and space.