[M-GI25] Holocene paleoenvironment, paleoclimate, and paleohazards in the Pacific Islands

[MGI25-P02] Sea Level and Paleoenvironmental Reconstruction of the Last 3000 Years in the Kingdom of Tonga, South Pacific

*Naoto Fukuyo¹, Yusuke Yokoyama², Geoffrey Clark³ (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Atmosphere and Ocean Research Institute, The University of Tokyo, 3.College of Asia and the Pacific, The Australian National University)

Keywords:South Pacific, Radiocarbon, Sea level, Holocene

Reconstructing the history of Holocene relative sea levels around Tonga provides essential constraints on the recent geological evolution of this region and paleoenvironmental context for archaeological studies. However, few sea level records are currently available in the region, and no quantitative paleoenvironmental studies using geochemical or geophysical methods have been reported. Here, we reconstruct the sea level history of Tongatapu Island using radiocarbon measurements and glacio-hydro-isostatic adjustment (GIA) modeling. Our analyses reconstructing the evolution of the lagoon suggest that the average size of Gafrarium tumidum decreased synchronously with corresponding changes in the paleo- environment. These changes also correspond to the increasing trend of the lagoon specific local marine reservoir ages (ΔR_lagoon) between ~2.6 and 0.4 ka. Sea surface salinity (SSS) decline within Fanga ’Uta lagoon was also synchronous with these changes caused by a gradual decrease in the exchange of water in and out of the lagoon. Estimated SSS from shell was somewhat higher ca. 2.6 cal kyr B.P. than the present, suggesting that the lagoon was relatively open to the ocean at that time. Our GIA modeling predicts mid-Holocene sea level highstand (HHS) was less than 1 m above the present sea level in Tongatapu, suggesting that previously reported observations of an HHS require additional contributions, perhaps from crustal uplift. Furthermore, recent GNSS observations of vertical uplift rates at Tongatapu are an order of magnitude higher than the long-term uplift rate obtained from Holocene sea level data.