Abstract
An underwater volcanic eruption off the Vava’u island group in Tonga on 7 August 2019 resulted in the creation of floating pumice on the ocean’s surface extending over an area of 150 km2. The pumice’s far-reaching effects from its origin in the Tonga region to Fiji and the methods of automatic detection using satellite imagery are described, making it possible to track the westward drift of the pumice raft over 43 days. Level 2 Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Radiometer Suite (VIIRS), Sentinel-3 Ocean and Land Color Instrument (OLCI), and Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) imagery of sea surface temperature, chlorophyll-a concentration, quasi-surface (i.e., Rayleigh-corrected) reflectance, and remote sensing reflectance were used to distinguish consolidated and fragmented rafts as well as discolored and mesotrophic waters. The rafts were detected by a 1 to 3.5 °C enhancement in the MODIS-derived “sea surface temperature” due to the emissivity difference of the raft material. Large plumes of discolored waters, characterized by higher satellite reflectance/backscattering of particles in the blue than surrounding waters (and corresponding to either submersed pumice or associated white minerals), were associated with the rafts. The discolored waters had relatively lower chlorophyll-a concentration, but this was artificial, resulting from the higher blue/red reflectance ratio caused by the reflective pumice particles. Mesotrophic waters were scarce in the region of the pumice rafts, presumably due to the absence of phytoplanktonic response to a silicium-rich pumice environment in these tropical oligotrophic environments. As beach accumulations around Pacific islands surrounded by coral shoals are a recurrent phenomenon that finds its origin far east in the ocean along the Tongan trench, monitoring the events from space, as demonstrated for the 7 August 2019 eruption, might help mitigate their potential economic impacts.
Highlights
The question of detecting floating pumice is interesting for islands in the Pacific.Pumice is an important agent for rafting that effectively can increase the dispersal of marine biodiversity and link isolated shallow coastal and marine ecosystems [1]
We investigate the diversity of floating material that can be automatically detected following the August 2019 submarine volcanic eruption in Tonga and examine optical and biological influences on regional waters
Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Radiometer Suite (VIIRS), Sea and Land Surface Temperature Radiometer (SLSTR) data was critical in making observations for detecting pumice rafts, chlorophyll-a
Summary
Pumice is an important agent for rafting that effectively can increase the dispersal of marine biodiversity and link isolated shallow coastal and marine ecosystems [1]. Understood, this natural phenomenon creates dynamic interactions between volcanism and marine biology [1]. If clasts end up completely populating beaches it could lead to their closure, potentially affecting tourism [2]. This is crucial for the Pacific island country of Fiji, which predominantly relies on ocean tourism for revenue generation and job creation
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