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Paleoenvironmental reconstruction using geochemical and rock magnetic analyses for carbonates obtained from the Kingdom of Tonga, South Pacific

Paleoenvironmental reconstruction using geochemical and rock magnetic analyses for carbonates obtained from the Kingdom of Tonga, South Pacific
South Pacific islands are extremely vulnerable to environmental changes such as sea-level rise, storm surges, earthquakes, and volcanic eruptions. The changes increase the probability of forced migration in the future. On the other hand, archaeological studies suggest that the comparable situation had occurred in this region during the middle and late Holocene. Thus, there is an urgent need to understand the mechanisms, variability and impacts of the environmental changes. However, limited availability of environmental observations in this region before the mid-twentieth century means that paleoenvironmental reconstructions from natural archives, such as bivalves and speleothems, offer valuable tools to reconstruct past variability of the environmental changes. However, no quantitative paleoenvironmental studies using geochemical or geophysical methods have been reported. Here, I reconstructed paleoenvironment in the kingdom of Tonga (Tonga) using geochemical and rock magnetic analyses for bivalves and speleothems. Tonga is one of south pacific islands countries and was a source area for the migration to East Polynesia around 1000 years ago; thus it is suitable for studying paleoenvironmental reconstruction in the south Pacific and its relation to human migration.
First, I reconstructed the sea-level history of Tongatapu Island using radiocarbon measurements and glacio-hydro-isostatic adjustment (GIA) modelling. Our analyses reconstructing the evolution of the lagoon suggest that the average size of Gafrarium tumidum decreased synchronously with corresponding changes in the paleoenvironment. These changes also correspond to the increasing trend of the lagoon specific local marine reservoir ages (ΔRlagoon) 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 the 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 modelling 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.
Second, I employed scanning SQUID microscopy (SSM) to conduct paleomagnetic measurements on a stalagmite collected at Anahulu cave in Tonga. A stronger magnetic field was observed above the greyish surface layer, as compared to that of the white inner layer associated with the laminated structures of a speleothem at the submillimeter scale with the SSM. The magnetization of the speleothem sample calculated by an inversion of isothermal remanent magnetization (IRM) also suggests that magnetic mineral content in the surface layer is higher than the inner layer. This feature was further investigated by low-temperature magnetometry and was suggested that it contains magnetite, maghemite, and goethite. The first-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with different coercivities and domain states. The contribution from maghemite and goethite to the total magnetization of the greyish surface layer is much higher than the white inner layer. The speleothem retaining magnetically and visually two distinct layers indicates that the depositional environment was shifted when the surface layer was deposited and was likely changed to the oxidative environment.

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