Flowcam analysis of the ocean lithogenic particles during the 2021 volcanic eruption of La Palma, Canary Islands

Abstract

Lithogenic flux (mineral particles), plays a key role in the oceanic fluxes, as nutrient and its ballast effect. Lithogenic flux in the Canary basin is mainly Saharan dust. In September 2021, a volcanic eruption struck in La Palma Island, reaching the coast, which contributed substantially with a fresh volcanic ash oceanic flux input. Flow imaging microscopy uses digital images to measure the size and shape of each particle. FlowCAM® was utilized to measure large numbers of diatom valves in an automated manner. However, FlowCAM® has still not been applied to lithogenic particle studies, despite its great potential for image identification analysis. Thousands of particle images are captured per second by the camera which includes all the spectrum of organic and mineral particles, allowing a precise visualization of the lithogenic particles. Particle samples were obtained during an oceanographic campaign (PLOCAN/ULPGC campaign) close to the new lava delta (28º36’N-17º55’W) using Niskin bottles (at depths from 5 to 20 m, collected into glass bottles and preserved with formol acetic acid). Two pattern samples, one of Saharan dust and another of volcanic ash, from the recent eruption of La Palma, were analyzed previous to the particle evaluation for a more realistic identification. A mineral identification of La Palma ashes (particles composed of 75% vitreous and 25% pyroxene and olivine) and Saharan dust (45% quartz, 25% carbonates, 15% feldspars, and 10% Mn-Fe oxides) was incorporated to the study. A specific software was used to characterize the oceanic water particles into different classes based on grain size distribution as the main criterion. Particle concentration, transparency, roughness, and edge gradient particle parameters were obtained consequently as secondary criteria. This study demonstrates the effectiveness of FlowCAM® analysis for textural studies of oceanic lithogenic particles. Observed results showed a mix of both volcanic ash and Saharan dust grain size distribution patterns constituted the lithogenic flux particle samples, yet these tend to resemble more the Saharan dust pattern.