Lithogenic flux and textural analysis of the sediment trap samples collected at 150 m depth in the Canary Basin, Spain.

Abstract

Saharan desert dust is the major source of lithogenic fluxes input on the Canary Basin. Throughout the atmospheric lifetime, the dust particles are subject to mixing and ageing. These actions bring about significantly changes in the molecular composition of the airborne particles before their removal from the atmosphere on the ocean surface. The “post-depositional” processes began in the ocean surface. This model assumed that once lithogenic particles encounter the micro-layer of the sea surface (made by organic compounds), it produces an important change in the composition of the lithogenic material. Physico-chemical changes occur during particle settling through the water column producing aggregates. The distribution of the lithogenic particles along the water column depends on their size, morphology, density and aggregation processes. Oceanic samples were collected and analyzed from mesopelagic layer (at 150 m depth), utilizing a P.P.S3/3-24 S time-series sediment trap, with a captation area of 0.125 m2, and sampling frequency of 12 hours. The sediment trap was deployed in a drifting system between LCF3 and LCF4 station, 18.5 km apart from Gran Canaria Island, by LUCIFER II project, from January to April of 2011. Oceanic samples in water suspension were filtered using Nucleopore©. These filters were treated with hydrogen peroxide (3%) previously heated at 50ºC in order to eliminate organic matter mass and particles. However, jelly masses (agglutinating particles) persisted after treatment. These masses remind to the Transparent Exopolymer Particles (TEPs). The lithogenic particles were characterized using the particle size (grain size) and shape descriptors (Circularity Index (IC), Aspect Radio (AS). The particle identification, size and these indexes have been undertaken through image treatment, with ImageJ Program. This treatment allowed to characterize the whole particles size range and to estimate the particles volume. All samples were analyzed and photographed with a Leika® MZ6 stereomicroscope equipped with a photographic camera and non-polarized natural light at an image resolution of 5.0 megapixels. It was selected a 8x zoom to take the photographs and to run the image treatment analysis. A grid of 4 mm was employed to scan through the whole filter. Similar light conditions were fixed for each picture to guarantee a standard process in each filter. Multiple light focuses were used in order to avoid the creation of shadows. To discriminate particles a filter Corel Paintshop Program was used. Saharan dust events were monitorized following the web portal of the Barcelona Super Computing Center. Airborne concentrations in the dust events during sampling days were up to 10 µg/m3 and a dust deposition up to 2 mg/m2day. Saharan dust before, during and after the sampling period, permitted to determine the terrigenous input into the Canary Basin and their lithogenic fluxes in Saharan dust events and the lithogenic fluxes at 150 m depth in the mesopelagic layer.