WHY ARE REEF TOP ISLANDS (MOTU) ONLY ON THE NORTH SIDE OF THE SOCIETY ISLAND CHAIN? RAPID LATE-HOLOCENE CARBONATE LANDSCAPE EVOLUTION IN POLYNESIA

We model the formation and distribution of reef-top islands (Motu) in the Society Island Chain, and confirm that model using radiocarbon dating, carbonate petrography and wave climate data. The succession of events involves: 1) emergence of indurated reef crest following well documented 1-2 meter late-Holocene sea-level drop in the region; 2) over-top storm surge transport of fore-reef corals across this emergent crest to form unconsolidated reef-top islands; and 3) breaching of this paleo-reef crest, then leads to wave-driven dissection of these Motu by channels (Hoa); 4) this erosional process ultimately eliminates Motu. We confirm this succession of events by: a) carbon dating of the components of the system; showing that they formed in the proposed succession; b) carbonate petrographic evidence of meteoric diagenesis documents the previous burial under motu (which have ground water) of now exposed paleoreef features; c) the asymmetric distribution of the Motu is documented using satellite imagery; and, d) is shown to be a product of the wave energy impinging on the north relative to the south coasts of the Society Islands. Long period waves from Southern-ocean storms lead to wave energies on south coasts that are orders of magnitude greater than that experienced by north facing shores leading to almost complete loss of Motu. Less energetic northern-hemisphere waves that are diminished by the intervening Tuamotu Archipelago. We note that this entire landscapes evolution, involving both the formation and destruction of islands, is extremely rapid having transpired over the last 2.5ka. In addition, in this scenario tropical storm surge played a constructive role in island formation whereas waves from extratropical storms are the primary erosive force. Although many tropical reef-systems experienced similar late-Holocene sea-level drops, this phenomenon of motu formation and asymmetry is of only modest generality pertaining prmarily to, fast growing tropical reef systems with limited tidal range that experience differential wave energy, such as the Society Islands and the southeastern Tuamotu. The rapidity of landscape evolution documented here has significant implications for the archeological reconstruction of Polynesian history and the biotic history of the Polynesian Islands and lagoons.