| Processing Steps |
- Parameter or Variable: microplastic concentration (measured); Units: pieces per g d.w.; Observation Category: in situ; Sampling Instrument: Megacorer; Sampling and Analyzing Method: In this study, microplastics were examined in deep-sea sediments collected from the Rockall Trough, which is situated to the west of Scotland, United Kingdom. The monitoring site ‘Gage Station M' is located in the Rockall Trough (57.300°N, −10.383° W) at a depth of 2200 m. During the 2017 research cruise DY78–79 on-board R.R.S. Discovery, three Megacorer deployments were carried out within the locality of Gage Station M. Megacorers are designed to sample without creating a bow-wave and thus ensure sediments are not disturbed. An OSIL megacorer was utilized to obtain sediment cores from three sites around Gage Station M. The megacorer was rigged with six 60 cm long x 10 cm wide internal diameter core tubes to allow for a 50% redundancy. The megacorer was deployed at a rate of 0.8 ms-1 until 50 m from the seabed, upon where the winch rate was slowed to 0.3 ms-1. The megacorer remained on the seabed for 5 minutes before recovery to allow the core tubes to sink into the sediment. Hauling rate was limited to 0.2 ms-1 for the first 50 m from the seabed then increased to a rate of 0.6 ms-1 to the surface. Once on deck the core tubes were carefully removed from the megacorer frame and sealed with pre-cleaned rubber bungs at each end, before being transferred to wooden stands. The supernatant water from each of the sediment cores was carefully siphoned off so as not to disturb any of the underlying sediment. Each core (A – C) was sliced using a stainless-steel cutter at discrete depth intervals; 0.5 cm sections were taken for the uppermost 5 cm of sediment, 1 cm intervals between the depths of 5–10 cm, and 5 cm sections were taken thereafter until the end of the core. The extrusion of cores can lead to smearing effects at the outer edges, thus a few millimeters from the edges of each core section were removed with a stainless-steel spatula to avoid mixing artefacts caused by this process. Sediment horizons were transferred to labelled clean polyethylene zip lock bags, sealed immediately and frozen at −20 °C for later analysis of microplastic concentrations. The entire mass of sediment obtained from each depth horizon from the cores A and C from each of the three megacorer deployments (MG1697, MG1678, MG1699) were assessed for microplastics. Cores were processed in a random order to prevent bias in this extraction phase. Sediments were freeze dried and the weight of each horizon was recorded prior to processing. Microplastics were extracted from the sediment using the oil extraction protocol, with slight modifications to account for the larger sediment masses and smaller grain sizes analyzed here than in the original method. Dry sediment ranged in weight from 9 g to 79.6 g; samples with a large mass (> 40 g) were divided and two separate extractions were carried out. These samples (maximum 40 g dry weight (d.w.)) were put into separate pre-cleaned 250 ml conical flasks and double the volume of deionized water was added. For subsamples>25 g d.w., 7 ml of canola oil was added to this, while 5 ml of canola oil was added to sediment (sub)samples weighing<25 g d.w. which was indicated from preliminary experiments using spiked sediment samples. An aluminum foil lid was placed onto the conical flask and the contents were swirled for 30 s. This was then transferred to a 100 ml borosilicate glass separating funnel. The conical flask was rinsed twice with 25 ml of deionized water to ensure no particles remained on the internal walls and this was decanted into the separating funnel. The separating funnel was mixed vigorously for 30 s and then left to settle for 30 min. Following the settling period, the sediment and aqueous layers were emptied from the separating funnel into a waste beaker. A further 30 ml of deionized water was added to the separating funnel and it again was shaken vigorously for 30 s. This was then left for a second settling period of 30 min before the aqueous layer containing remaining sediment grains were emptied from the funnel into the waste beaker. The oil layer was retained and vacuum filtered through a 52 μm mesh size disc of transparent nylon gauze. The separating funnel was rinsed twice with 20 ml of 4% non-foaming detergent (Alcojet, Sigma-Aldrich) to remove any oil and remaining particles, this was then emptied through the gauze filter. The gauze was transferred to a lidded glass petri dish and was examined thoroughly three times under a dissecting microscope (Wild M5). Potential microplastics were transferred to a 30 mm petri dish containing a disc of filter paper (Whatman No. 1). To remove the oil residue from the surface of the microplastics a mixture of 99% ethanol and 99% isopropanol in a 1:1 ratio was used. Microplastics specific to each sample, were transferred to a glass cavity block in which a 5 ml volume of the ethanol: propanol mixture had been added. This was covered and incubated for 15 min, before the microplastics were recovered from the solvent mixture and returned to their specific 30 mm petri dish and sealed for further analysis. Putative microplastics were analyzed with a Perkin-Elmer One Fourier Transformation infrared (FTIR) microscope in transmission mode. Infrared radiation in the wavenumbers 600–4000 cm−1 were used and each spectra produced was the average from 16 co-added scans and was corrected against a background scan carried out prior to each sample. A variable aperture size was used and the spectral resolution was 4 cm−1. Data were visualized in OMNIC 9 (Thermo Fisher Scientific Inc.) with use of the inbuilt Hummel polymer library and the Alfred Wegener Institute ‘AWI’ (Primpke et al., 2018) library to facilitate polymeric identification; additionally, the characteristic functional group signals from each spectra were manually examined.; Data Quality Method: QA/QC was implemented wherever possible while on board the research cruise. Core tubes and rubber bungs were rinsed with deionized water prior to their use. The core collar, stainless steel cutter and spatula used to section the sediment cores were also washed thoroughly between use to prevent sample contamination or cross-contamination. Personnel wore nitrile gloves and where possible wore cotton coveralls, however due to weather limitations sometimes waterproof outwear was required. Samples of putative contaminants, such as the ropes used on the megacorer, tubes and bungs as well as clothing were taken to be analyzed alongside the microplastics samples. Within the laboratory, thorough cleaning of work benches and laboratory equipment was undertaken prior to any work. Glass and metal laboratory equipment were preferentially used over plastic and consumables were used directly from sterile packaging. Equipment was kept covered with clean aluminum foil when not in use and the samples were covered as much as possible to minimize exposure risk. Tapelift screening and atmospheric controls to monitor background contamination were implemented..
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