Sedimentary biomarkers of human presence and taro cultivation reveal early horticulture in Remote Oceania

Remote Oceania, encompassing islands from the eastern Solomons to Polynesia, represents one of the last regions settled by humans. The precise timing of initial settlements and the early adoption of horticulture remain subjects of debate. Existing archaeological evidence and radiocarbon dating often suffer from uncertainties due to post-depositional disturbances, reservoir effects, and limitations in dating techniques. While archaeological evidence suggests agricultural practices in Vanuatu, the precise timing and extent of early horticulture have been difficult to establish due to uncertainties in stratigraphy, dating, and biological proxies. This study aims to address these uncertainties by employing a multiproxy approach using lipid biomarkers from Emaotfer swamp sediments on Efate, Vanuatu, located near the Teouma archaeological site, the oldest known cemetery in Remote Oceania. The researchers will use coprostanol and epicoprostanol as indicators of human presence, palmitone as a biomarker for taro cultivation, and leaf wax hydrogen isotope composition to reconstruct past hydroclimate. This integrated approach will provide a refined chronology of human arrival and contextualize the role of climate in early settlement and horticultural practices.

Previous research on the settlement of Remote Oceania has highlighted the role of seafaring horticulturalists from Southeast Asia and Papua. The Lapita Cultural Complex, characterized by dentate-stamped ceramics, is associated with the earliest settlers. The influence of climate on migration patterns has been suggested, with climate windows potentially favoring certain routes and adverse conditions leading to land abandonment or migration. However, the resolution and chronologies of existing climate records in Remote Oceania have hindered the precise determination of climate's influence on settlement patterns. While archaeological research in Vanuatu has revealed evidence of Lapita settlement and agricultural practices, including terraced gardens and introduced crops, dating of early sites is often complicated, and identifying the precise timing and extent of early horticultural practices has proven challenging due to uncertainties in dating and biological proxies. Studies have also focused on the Lapita diet using stable isotope analysis of bone collagen, often revealing a broad spectrum of marine and terrestrial resources but potentially underrepresenting plant foods.

This study utilized a sediment core retrieved from Emaotfer swamp, located near the Teouma archaeological site on Efate, Vanuatu. The core, spanning the last 5000 years, was analyzed using multiple proxies. A high-resolution radiocarbon chronology was established using 40 radiocarbon-dated samples (primarily plant remains), creating an age-depth model. Bulk geochemical analyses were conducted to determine total carbon, total nitrogen, total inorganic carbon, total organic carbon, and bulk sediment δ15N. Elemental counts were measured using X-ray fluorescence (XRF) core scanning. Lipid biomarker analysis was performed to quantify coprostanol, epicoprostanol (indicators of human presence), and palmitone (a biomarker for taro). Compound-specific hydrogen isotopic composition (δ2H) of leaf wax long-chain n-alkanoic acids was measured to reconstruct past hydroclimate. Data analysis and visualization were performed using R statistical software. The age-depth model was created using the rbacon package. Careful consideration was given to potential age reversals, root intrusions, and hard-water effects in radiocarbon dating. The robustness of the δ2H signal as a hydroclimate proxy was carefully evaluated, considering the potential role of changing vegetation and the amount effect. Statistical methods used included Bayesian chronological modeling and error propagation.

The sediment core revealed three main periods of human occupation. The first period, dated to 2739–2879 cal BP, coincides with the earliest Lapita settlement at Teouma. This period shows a simultaneous increase in faecal biomarkers (coprostanol and epicoprostanol) and palmitone, indicating the introduction and cultivation of taro by the first settlers. This first human activity coincides with a shift towards wetter climatic conditions, likely linked to a southward shift of the Intertropical Convergence Zone (ITCZ) and expansion of the South Pacific Convergence Zone (SPCZ). A second period of human occupation, correlated with the Erueti phase (archaeologically identified by a midden deposit covering the Lapita cemetery), is evident between 2400–2602 BP and 2298–2477 BP. This phase also shows increased palmitone and faecal biomarkers, but with more climate variability, potentially linked to El Niño-Southern Oscillation (ENSO) effects. A third period shows continued human occupation and taro cultivation from 2000 BP onwards. The hydrogen isotopic composition (δ2H) of leaf wax n-alkanoic acids reveals a general trend towards wetter conditions between 3600 BP and 2200 BP, punctuated by drier periods. The initial settlement at 2800 BP occurred amidst a shift towards significantly wetter conditions. The changes in the δ2H values are consistent with those observed in other parts of the Pacific and suggest a southward migration of the ITCZ. The high-resolution age model from the Emaotfer sediment core refines previous estimates of earliest human occupation at the Teouma site and provides a precise timeline for the introduction of taro cultivation.

The findings demonstrate the simultaneous arrival of humans and the introduction of taro cultivation in Remote Oceania around 2800 years ago, a period characterized by a shift to wetter climatic conditions. The hydrological changes observed in Emaotfer Swamp could be attributed to a combination of tectonic uplift, the expansion of the SPCZ, or human activities. The study highlights the interplay between climate, human migration, and the adoption of horticultural practices. The wetter conditions during the Lapita period likely facilitated the successful establishment of taro cultivation, whereas the more variable climate during the Erueti period might have contributed to a decline in human presence or a shift in agricultural practices. The continued high levels of faecal biomarkers and palmitone after 2000 BP suggest sustained human occupation and taro cultivation in the region, despite the absence of archaeological evidence at the Teouma site. This might reflect the incorporation of material from a wider catchment area in the swamp sediments.

This study provides strong molecular evidence for the early introduction of horticulture in Remote Oceania, specifically the cultivation of taro by the first Lapita settlers around 2800 years ago. This event occurred within a period of increasing wetness, highlighting the influence of climate on human settlement and agricultural practices. The findings refine our understanding of the chronology of human settlement in the Pacific and underscore the complex interplay between climate and human adaptation. Further research could explore the specific impact of ENSO variability on agricultural productivity and settlement patterns in the region, and examine the relationship between ITCZ and SPCZ dynamics in influencing precipitation patterns during this period.

The study is focused on a single location (Emaotfer swamp), limiting the generalization of findings across all of Remote Oceania. While efforts were made to address dating uncertainties and potential biases in the proxies, some uncertainties remain, particularly concerning the relative contributions of tectonic uplift, climate change, and human activities to hydrological changes in the swamp. The interpretation of leaf wax δ2H values as solely reflecting precipitation changes relies on assumptions about the minimal influence of other factors, such as changes in plant communities and post-rainfall evaporation.