11:10 AM - 11:25 AM
[MIS19-07] Transport mechanism of microplastics in deep-sea sediments collected on the slopes of the Suruga Trough, Sagami Trough, and Izu-Bonin Trench
Keywords:Plastics in oceans, Fluorescence microscope, Shinkai 6500, Hyper Dorphin
We disclose the transportation route of Microplastics (hereafter MPs) to deep-sea beds.
The dive surveys were carried out by the cruise YK18-04 of the support mother ship "Yokosuka" and by the cruise KS-18-J02 of the Tohoku Marine Ecosystem Research Vessel "Shinseimaru". We have collected the sediment samples by means of Shinkai 6500 (hereinafter 6K) and Hyper-Dolphin (hereinafter HPD). The sampling points are the inner part of the Suruga Trough (HPD # 2041St.1; depth 1548m), the central part (6K # 1514 St. 7; depth 1641m), and the outer part (HPD # 2044St.3; depth 3595m and HPD # 2044St.3; Water depth 3585m), Sagami Trough eastern end (6K # 1511 St.1; Water depth 1236m and 6K # 1511 St.2; Water depth 1225m), Izu-Bonin Trench landward slope (6K # 1513 St.5; Water depth 6378m) There are a total of 7 points. Sediment samples are 20 to 30 cm long using an aluminum corer. They were treated to prevent contamination immediately. It was cut into pieces of 1 cm interval according to the depth of the deposit and sealed in a burned glass bottle.
The MPs were analyzed by the following procedure. First, the muddy part of the sample was removed by a 63 µm sieve. Second, heavy liquid separation and hydrogen peroxide treatment was performed. Finally, MPs in the sample were stained with Nile Red and observed with a fluorescence microscope. At the observation, 1) particles that fluoresce well and 2) particles with a smooth surface were selected. In addition, the fibrous ones and the debris-like ones were counted separately. Furthermore, in the process of sample processing, a simple clean bench was used as much as possible, plastic products were not used, and a cotton lab coat was worn. An evaporating dish filled with distilled water was placed during the treatment as a blank test.
The inner part of the Suruga Trough and the central part consisted entirely of silty clay, and the sample of the outer part consisted of silty clay and sand. It was concluded that the sandy part is turbidite together with the results of δ13C analysis of organic matter. The samples in the Sagami Trough area and the Izu-Bonin Trench were silty clay throughout the core sediments. It was concluded hemipelagic muds.
A total of 158 MPs could be detected in the sediment. Of these, 78 were fibrous. The results are shown by the number (pieces) and the number per 1g of dry sediment (pieces/g). In addition, there were no MPs mixed in from the environment during the extraction process. The maximum number of MPs detected was 14 (22.6/g) (HPD#2041 St.1) and 4 (3.1/g) (6K#1514 St.7), 1 (0.54/g) (HPD#2044 St.3) and 3 (1.89/g) (HPD#2044 St.4) in the mud area of the Suruga Trough in the inner part, the central part, and the outer part. In addition, the detected amount varied in a spike shape in the muddy part. In the sandy part of the outer part, the maximum number was 1 (0.03/g) (HPD#2044 St.3) and 6 (1.17/g) (HPD#2044 St.4).
In the Sagami Trough, the maximum values detected in the mud area are 4 (5.06 / g) (6K#1511 St.1), 4 (4.93/g) (6K#1511 St.2). In the Izu-Bonin Trench landward slope area, a maximum of 4 (3.57/g) (6K#1513 St.5) were detected.
In the mud area, more MPs are present in the waters near the land at the Suruga Trough site. This also indicates that the δ13C value is more derived from land plants in the waters near land. It is presumed that such mud of terrestrial origin is brought from the Fuji River by steady or flooding. For example, Nakajima et al. (2022) report a large amount of plastic drifting due to a typhoon. By depositing such a large amount of plastic in the mud layer while losing buoyancy, more MPs may be present in the waters near land.
MPs were shown in the muddy part of the landside slope of the Izu-Bonin Trench offshore, and this abundance did not change from the value of the muddy part of the Sagami Trough site. This may be related to the fact that this sampling site is located at the western end of the giant vortex. The plastic concentration due to the giant vortex is reported by Nakajima et al. (2021).
In this way, MPs are mixed in the mud part mainly by subsidence from the sea surface. And it is shown from the lead isotopes that such pelitic deposits are vertically mixed by biological disturbance. Due to this vertical mixing, the number of MPs does not seem to reach a constant value in the muddy part.
In the turbidite part found in the Suruga Trough site, many MPs were detected even if they were far from the land. This strongly suggests that it was transported from land by turbidity current, unlike contamination by sedimentation in the mud.
Nakajima R. et al. (2022) Front. Mar. Sci. 8:806952.
Nakajima R. et al. (2021) Marine Pollution Bulletin 112188-11218
The dive surveys were carried out by the cruise YK18-04 of the support mother ship "Yokosuka" and by the cruise KS-18-J02 of the Tohoku Marine Ecosystem Research Vessel "Shinseimaru". We have collected the sediment samples by means of Shinkai 6500 (hereinafter 6K) and Hyper-Dolphin (hereinafter HPD). The sampling points are the inner part of the Suruga Trough (HPD # 2041St.1; depth 1548m), the central part (6K # 1514 St. 7; depth 1641m), and the outer part (HPD # 2044St.3; depth 3595m and HPD # 2044St.3; Water depth 3585m), Sagami Trough eastern end (6K # 1511 St.1; Water depth 1236m and 6K # 1511 St.2; Water depth 1225m), Izu-Bonin Trench landward slope (6K # 1513 St.5; Water depth 6378m) There are a total of 7 points. Sediment samples are 20 to 30 cm long using an aluminum corer. They were treated to prevent contamination immediately. It was cut into pieces of 1 cm interval according to the depth of the deposit and sealed in a burned glass bottle.
The MPs were analyzed by the following procedure. First, the muddy part of the sample was removed by a 63 µm sieve. Second, heavy liquid separation and hydrogen peroxide treatment was performed. Finally, MPs in the sample were stained with Nile Red and observed with a fluorescence microscope. At the observation, 1) particles that fluoresce well and 2) particles with a smooth surface were selected. In addition, the fibrous ones and the debris-like ones were counted separately. Furthermore, in the process of sample processing, a simple clean bench was used as much as possible, plastic products were not used, and a cotton lab coat was worn. An evaporating dish filled with distilled water was placed during the treatment as a blank test.
The inner part of the Suruga Trough and the central part consisted entirely of silty clay, and the sample of the outer part consisted of silty clay and sand. It was concluded that the sandy part is turbidite together with the results of δ13C analysis of organic matter. The samples in the Sagami Trough area and the Izu-Bonin Trench were silty clay throughout the core sediments. It was concluded hemipelagic muds.
A total of 158 MPs could be detected in the sediment. Of these, 78 were fibrous. The results are shown by the number (pieces) and the number per 1g of dry sediment (pieces/g). In addition, there were no MPs mixed in from the environment during the extraction process. The maximum number of MPs detected was 14 (22.6/g) (HPD#2041 St.1) and 4 (3.1/g) (6K#1514 St.7), 1 (0.54/g) (HPD#2044 St.3) and 3 (1.89/g) (HPD#2044 St.4) in the mud area of the Suruga Trough in the inner part, the central part, and the outer part. In addition, the detected amount varied in a spike shape in the muddy part. In the sandy part of the outer part, the maximum number was 1 (0.03/g) (HPD#2044 St.3) and 6 (1.17/g) (HPD#2044 St.4).
In the Sagami Trough, the maximum values detected in the mud area are 4 (5.06 / g) (6K#1511 St.1), 4 (4.93/g) (6K#1511 St.2). In the Izu-Bonin Trench landward slope area, a maximum of 4 (3.57/g) (6K#1513 St.5) were detected.
In the mud area, more MPs are present in the waters near the land at the Suruga Trough site. This also indicates that the δ13C value is more derived from land plants in the waters near land. It is presumed that such mud of terrestrial origin is brought from the Fuji River by steady or flooding. For example, Nakajima et al. (2022) report a large amount of plastic drifting due to a typhoon. By depositing such a large amount of plastic in the mud layer while losing buoyancy, more MPs may be present in the waters near land.
MPs were shown in the muddy part of the landside slope of the Izu-Bonin Trench offshore, and this abundance did not change from the value of the muddy part of the Sagami Trough site. This may be related to the fact that this sampling site is located at the western end of the giant vortex. The plastic concentration due to the giant vortex is reported by Nakajima et al. (2021).
In this way, MPs are mixed in the mud part mainly by subsidence from the sea surface. And it is shown from the lead isotopes that such pelitic deposits are vertically mixed by biological disturbance. Due to this vertical mixing, the number of MPs does not seem to reach a constant value in the muddy part.
In the turbidite part found in the Suruga Trough site, many MPs were detected even if they were far from the land. This strongly suggests that it was transported from land by turbidity current, unlike contamination by sedimentation in the mud.
Nakajima R. et al. (2022) Front. Mar. Sci. 8:806952.
Nakajima R. et al. (2021) Marine Pollution Bulletin 112188-11218