Impact of hydro‐isostatic holocene sea‐level change on the geologic context of Island archaeological sites, Northern Ha'apai group, Kingdom of Tonga

Abstract

Archaeological sites in the northern Ha'apai Group of central Tonga occur on small islands within the uplifted forearc belt of the Tonga-Kermadec arc-trench system. The present inland positions of occupation sites that probably once occupied coastal settings imply significant expansion of some island shorelines during late Holocene time (ca. 3250 B.P. to present). Geologic processes leading potentially to enlargement ofthe islands include continuing forearc uplift, eustatic or glacio-hydro-isostatic fall in sea level following a mid-Holocene highstand, and progressive accretion of beach ridges to island coasts, with or without changes in relative sea level. Radiometric dates for uplifted coral terraces in Tonga indicate that forearc uplift has been negligible during Holocene time. By contrast, theoretical considerations, regional analysis of shoreline indicators throughout the South Pacific, and limited empirical data from Tonga itself all imply that regional sea level has declined locally by 1-2 m since a mid-Holocene highstand (ca. 6000-3000 B.P.), which was a hydro-isostatic response to transfer of water mass from Pleistocene ice caps to the ocean basins. Emergence of originally coastal sites is thus expected since initial settlement of the islands by Lapita peoples. Accretionary coastal flats composed of multiple beach ridges are 250-500 m wide on favorable leeward shores and the flanks of sand cays, but some presently unknown proportion of this incremental island growth may have occurred prior to the post-mid-Holocene decline in relative sea level. Ash falls from tephra eruptions a t Tongan volcanoes also modified island environments through Quaternary time. Evidence for significant change in the configuration and morphology of islands in Ha'apai during the period of human settlement highlights the need for systematic interdisciplinary archaeological and geological research in the study of Pacific prehistory.

Rapid prehistoric extinction of iguanas and birds in Polynesia

“…1). Preliminary assessments of the stratigraphy, chronology, and cultural context of site Li7 were based on small excavations in 1984 and 1992 (11)(12)(13)(14)(15). More extensive excavations (11 m 2 ) in 1995 and 1997, directed primarily by D.V.B., have yielded a much larger sample of materials from site Li7, including abundant prehistoric artifacts (pottery, shell beads and bracelets, and bone awls and needles) and associated nonhuman bones that represent indigenous species of marine fish (Osteichthyes, Chondrichthyes), sea turtles (Cheloniidae), small lizards (Gekkonidae, Scincidae), iguanas (Brachylophus undescribed sp.…”

“…in 1997, with the goal of obtaining bones from precise stratigraphic contexts for AMS radiocarbon ( 14 C) dating. AMS 14 C dates can be done on individual identified bones of small species because the method requires much less bone (as little as 15 mg, if well preserved chemically) than the 100 -1,000 g needed for conventional ␤-decay 14 C dating (20,21). Thus, AMS 14 C dating eliminates the uncertainty of Stratigraphic profile of the east wall of excavation unit 10, Tongoleleka (site Li7), Lifuka, Tonga.…”

“…AMS 14 C dating also has no problem with timeaveraging, as long as each 14 C date is based on a single bone rather than multiple bones (22).…”

Altered Ecologies: Fire, climate and human influence on terrestrial landscapes: Terra Australis 32

“…Habitat change in Pacific tropical ecosystems may be due to human factors, including fire, habitat destruction (deforestation) and the introduction of exotic animals or plants (Southern 1986;Stevenson 1998;Haberle et al 2001;. Natural factors such as tropical cyclones ), sea-level variation (Dickinson et al 1994;Dickinson 2001Dickinson , 2003, or the effects of ENSO (Figure 1). Tonga is formed by a north-south alignment of islands that includes the three main uplifted limestone island groups of Vava'u, Ha'apai and Tongatapu.…”

Oceanic Explorations (TA26) : Lapita and Western Pacific Settlement

“…Geologist William Dickinson was invited to address these issues, and his findings became essential for site discovery as well as interpretation of site location on island landscapes (Dickinson et al 1994. Relative sea level in Tonga at the time of first Lapita occupation was as much as 2 m higher than present, resulting from a mid-Holocene hydro-isostatic highstand throughout the central South Pacific region.…”