Mid-late holocene accretional history of low-lying, coral-reef rim islets, South-Marutea Atoll, Tuamotu, central South Pacific: The key role of marine hazard events

Abstract

South-Marutea Atoll is located in the Gambier Island Group, at the south-eastern end of the Tuamotu Archipelago, French Polynesia, central South Pacific. Understanding the modalities of islet building from low-lying atolls over the mid to late Holocene, in relation to sea-level changes and cycles of marine hazard events, is a prerequisite for better anticipate future geomorphic changes to which the islets will likely be faced in the next decades under global climate warming. Herein is presented the depositional history of two selected atoll islets, based on chronostratigraphic analysis of sedimentary, coral-dominated sequences from six excavations. Identified as Motu Aramu and Motu Vainono, these islets are located respectively in north-north-east and due south of the atoll rim. Additional surficial sampling was conducted on modern ocean-facing shingle ridges, respectively on north-north-west (Motu Oire), west (Motu Aranui) and east (Motu Tekava) rim areas, in order to date the latest ridge-emplacement stages. Oire and Aranui sites, located along the leeward atoll sites, are protected from trade winds, Aramu, Vainono and Tekava are located on the windward sides, directly exposed to northeasterly and southeasterly storm swells respectively. A total of 88 coral clasts were collected to be U/Th dated. The excavated sequences range between 2.50 ​m and 0.90 ​m in thickness, from the outer islet sides lagoonwards. Five lithofacies, including two subfacies, were recognized based on texture and biological composition: a coral boulder-cobble-dominated, a coral pebble-dominated – pebble-supported and sand-supported subfacies – and a foraminifera-rich, sand-dominated and an organic-rich, pebble to sand facies. These facies tend to be distributed from ocean sides landwards according to a decreasing grain-size gradient. A model of atoll-island accretion emerges in relation with changes in frequency and intensity of marine hazard events. The islet foundations consist of conglomerate platforms, locally up to 1.0 ​m thick, deposited from about 5,000 to 1,000 ​calendar year (cal yr) BP. Islets began to accrete from 5,000 ​yr cal BP. While at Motu Vainono, islet building occurred continuously over the last 5,000 years, at Motu Aramu, there is an apparent non-depositional episode, from 4,000 to 2,000 ​yr cal BP, interpreted as caused by a marked decreased in enso-related cyclone activity. The outer shingle ridges in all studied sites were regularly reshaped during the last millennium. During the last 5,000 years, the major accretion-islet episodes occurred irrespective of the course of sea level, indicating that sea-level change was not a driver of islet accretion. Periodical, marine high-energy events clearly appear to be the key controls of islet shaping. Shifts of cyclone source areas further south and increasing cyclone intensity, but lower frequency, due to enhanced enso variability throughout the 21st century, is postulated to expose the Gambier island Group to stronger, but fewer disturbance events when compared to the last millennia.