Lida Ajer and Ngalau Gupin are karstic caves situated in the Padang Highlands, western Sumatra, Indonesia. Lida Ajer is best known for yielding fossil evidence that places the arrival of Homo sapiens in Southeast Asia during Marine Isotope Stage 4, one of the earliest records for the region. Ngalau Gupin recently produced the first record of hippopotamid Hexaprotodon on the island, representing the only globally extinct taxon in Pleistocene deposits from Sumatra. Microstratigraphic (micromorphological) analyses were applied to unconsolidated fossil-bearing cave sediments from these two sites. We use micromorphology as part of a micro-contextualised taphonomic approach to identify the diagenetic processes affecting fossils and sediments within these caves, through phases of their depositional history. The fossil-bearing sediments in Lida Ajer have been subjected to a suite of natural sedimentation processes ranging from water action to carnivore occupation, which would indicate the fossils underwent significant reworking prior to lithification of the deposit. The results demonstrate that the base of the unconsolidated fossil-bearing sediments in Ngalau Gupin were derived from the interior of the cave, where the matrix was partially phosphatized as a result of guano-driven diagenesis. These observations can be used to test hypotheses about the integrity of incorporated vertebrate remains and to aid in local palaeoenvironmental reconstructions. The methods employed in this research have not previously been applied to cave sediments from sites in the Padang Highlands and provide key new insights into the palaeontological and natural history of the western region of Sumatra.

Unguja Ukuu, located on the Zanzibar Archipelago, eastern Africa, was an active Indian Ocean trading settlement from the mid-first millennium until the early second millennium AD. As part of recent archaeological excavations aimed at understanding the site’s transregional trade networks, geoarchaeological analyses were undertaken to document the geomorphic context of the ancient settlement. Here, we outline the results of these field and laboratory studies to discuss patterns of anthropogenic sediment deposition. Unguja Ukuu’s deep coastal stratigraphy appears to record progradation of an inhabited back-reef shore from the mid-seventh to the nineth centuries AD, perhaps in the wake of an earlier middle to late Holocene marine transgression. Excavations on the back-beach show that deposits associated with the ancient settlement include interlayered middens, paleofloors, and backshore sands and, in later phases, a peri-urban dump with dark-earth-type anthrosols developed on these deposits. Coastal progradation appears to have been driven in part by the accumulation of anthropogenic detritus and compaction of ancient surfaces. We hypothesize how the inherited, submerged relic Late Pleistocene geomorphology of the intertidal zone and later Holocene sediment supply from the hinterland may have supported the emergence of Unguja Ukuu as a trading locale, and possibly contributed to its decline in the early second millennium AD.

North Vietnam is situated on a major route of Pleistocene hominin dispersal in East Asia, and the area's karstic caves preserve many prehistoric shell middens. Fossil and genomic evidence suggest a complex human history in this region and more widely across Southeast Asia and southern China, but related archaeological investigations are hampered by challenging site stratigraphies. Recent investigations of shell middens in other geographical settings have revealed the microstratigraphic complexity of these anthropogenic deposits. But caves promote distinctive site formation processes, while tropical climates may catalyse geomorphic and diagenetic changes. These environmental factors complicate the interpretation of North Vietnam's shell middens and constraining their effects upon the formation, preservation and destruction of these sites is critical to understanding the archaeology of this region. We examine two archaeological cave sites, dated to the Late Pleistocene and located in the limestone uplands surrounding the Hanoi Basin. Each contains multiple shell midden layers associated with prehistoric occupation and burials. Using thin-section micromorphology (microstratigraphy), we reconstruct the depositional and post-depositional histories of these sites, presenting a geoarchaeological framework of interpretation that is applicable to shell middens in mainland Southeast Asia and tropical zones more widely. This work represents a further step towards improving our understanding of prehistoric human dispersals and adaptations in this region. This article is part of the theme issue ‘Tropical forests in the deep human past’.

Caves in tropical regions form a key resource for those reconstructing human evolution and dispersals. However, the complex sedimentary records they contain remain under-interpreted because of the poorly constrained effects of humid tropical climates upon archaeological site formation processes. Guano-driven phosphatic diagenesis impacts archaeological preservation in caves, but atypical features observed in guano layers in prominent Southeast Asian archaeological sites suggested that hot and humid conditions had promoted the formation of distinctive sedimentary environments with unknown effects on assemblage taphonomy. Few reference data exist that are relevant to geomorphic processes in tropical caves and this laboratory-based modern analogue study was designed to conclusively relate analytical observations to sedimentary palaeoenvironments and explore their effects on assemblage taphonomy. Stratigraphic models were kept under simulated tropical conditions, each containing an identical range of faunal remains, organic materials and minerals, which was buried in wet bat guano. These were excavated, one per month over 24 months, while nine further stratigraphic models were used to control for environmental variables. Sedimentary environments were investigated using sedimentological characterisations of excavated sediments, while post-depositional alterations to buried materials were investigated through optical and scanning electron microscopic analysis of thin-section samples combined with chemical characterisations of excavated samples through Fourier-transform infrared spectroscopy (FTIR). Experimental conditions prevented the sediment acidification that is typically associated with guano-driven diagenesis, but buried materials were severely, and distinctively, diagenetically altered. Organic decomposition in guano, bamboo and charcoal was associated with the activities of sulphate-reducing bacteria, but diagenetic alterations affecting charcoal remain poorly resolved due to variation in the experimental assemblage. Bone was discoloured and affected by dissolution, recrystallisation and alteration to secondary minerals, including gypsum, while carbonates reprecipitated as calcium phosphates and non-stochiometric mineral species. Diagenesis of clay minerals was observed, but related diagenetic trajectories remain poorly resolved. These alterations provide micromorphological and mineralogical indicators of sedimentary palaeoenvironments in the earliest stages of burial and this experiment provides a framework within which to understand the taphonomy of archaeological materials in tropical caves.

This paper presents the results of geoarchaeological and geochronological investigations at Con Moong Cave, North Vietnam. Beneath the published, terminal Pleistocene sequence, recent excavations have uncovered a similar to 5m stratigraphic sequence containing flaked stone artifacts and sedimentary features that indicate extensive post-depositional change. As the effects of tropical conditions on Pleistocene cave sediments are poorly resolved, a range of complementary techniques was selected to reconstruct the nature of on-site sedimentation and occupation, while assessing the taphonomy of archaeological and palaeoecological materials. Our approach incorporates microstratigraphic, geochemical and sedimentological analyses, using optically stimulated luminescence (OSL) dating to estimate the time of sediment deposition in the cave. This case study has broad application to tropical zones worldwide. Sedimentation began in early Marine Isotope Stage (MIS) 4, while micromorphologically observed human occupation commenced before 42 thousand years ago (ka). By placing our results within the context of published, high-resolution regional records of climate, we demonstrate that on-site rhythms of Pleistocene occupation correlated with environmental changes in the region. During MIS 3, episodic abandonment of the site coincided with periods of drier conditions, while rapid climate fluctuations in MIS 2 corresponded with short-lived occupation events and a switch to predominantly geogenic deposition in the cave.