Stress in paradise: Reconstructing late holocene hydroclimate to investigate the role of drought in the timing of human migration and colonisation in the tropical South Pacific

Abstract

The South Pacific was the final frontier of human colonisation on Earth. Human migration across the Pacific occurred in two waves, the first started 3000 yr BP and saw the Lapita civilisation colonise remote Oceania eastward to Samoa. After a long pause of 2000 years, the second occurred at approximately 1000 yr BP and saw the colonisation of eastern Polynesia out to the three corners of the Polynesian Triangle. Reasons for these migrations are contested, but climate is increasingly thought to have been a factor. Despite high levels of precipitation in this region, drought is prevalent and palaeoclimate evidence suggests the South Pacific has experienced shifts between dry and wet periods throughout the past three thousand years. Drought has significant repercussions for small Pacific islands, affecting both water and food resources. The aim of this thesis is to identify how the climate has changed over the Holocene in this region and to assess whether changing climate was a pivotal driver in the timing of human migration and colonisation. Using a sequence of cores from Emaotfer Swamp located on the island of Efate in Vanuatu and Lake Tiriara, located on the island of Mangaia in the Southern Cook Islands, this study utilises a multi-proxy approach, which includes stable isotopes (δ¹³C and δ¹⁵N from organic matter as well as δ¹⁸O and δ¹³C from carbonate) as well as Itrax µXRF profiles and diatoms to create new palaeo records. This thesis presents new 9,200 and 2,500-year environmental and climatic record from Emaotfer Swamp and Lake Tiriara respectively. The key findings from the palaeoenvironmental records show evidence for a shift towards dry conditions around 500-2300 ± 160 yr BP in the Emaotfer record and 1590-860 ± 126 cal yr BP and 650-470 ± 102 yr BP in the Tiriara record. This indicates that there was a notable change in hydroclimate conditions around the second wave of human migration into Eastern Polynesia and in the relatively early stages of colonisation. To unravel the connection between climate and impacts on island life, this thesis also presents a new socio-ecological model of prehistoric Pacific island life. The model identifies the impact of changing hydroclimate on agricultural carrying capacity and population dynamics to reinforce the evidence for a potential connection between the changing climate identified by palaeoenvironmental records and the dynamics of early Pacific island societies. The model outputs suggest that drought has a greater impact on population dynamics the closer to the absolute carrying capacity the population gets and that severity of the drought rather than the frequency is the key factor determining the impact of a drought on agricultural outputs and population dynamics. This thesis offers firstly, new insights into climatic and hydrological change during the Holocene, across the two waves of human migration in the tropical South Pacific - a region that is relatively under-represented in terms of palaeo-environmental research. Secondly, this thesis presents a new socio-ecological model of Pacific island life that provides insights into the pressure of environmental change on these early Polynesian societies following colonisation.

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Identifiers

ORCID for Charlotte Victoria Hipkiss: orcid.org/0000-0001-6379-079X

ORCID for David Sear: orcid.org/0000-0003-0191-6179

ORCID for Pete Langdon: orcid.org/0000-0003-2724-2643

ORCID for Justin Sheffield: orcid.org/0000-0003-2400-0630

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