· The process leading up to the mystery of iodoethane is explained in detail below.
In that case, please click the link below.
The mystery explaining the leading role of iodoethane in the seabed sediment!
· The illustration below shows the bio-geochemical cycle of iodine on the earth's surface. “org-I” refers to the organic iodine. The organic iodine that is volatilized from the ocean to the atmosphere is organic iodine gas . Emissions of organic iodine gas from the ocean surface plays an important role in supplying iodine atoms to the atmosphere. Therefore, organic iodine gas in the atmosphere and surface of the ocean has long been investigated.
The aforementioned illustration also includes org-I (gas) and org-I (solid) in the marine sediment layer. It is thought that the settlement of iodine-rich organic matter derived from marine plants on the seafloor results in the accumulation of iodine in the marine sediment. High concentrations of iodine are accumulated, as observed during the observation of methane gas layers deep underground in the Pacific coastal area of Chiba Prefecture. The large quantity of marine plants containing iodine that settled long ago is thought to be a reason for this accumulation. Some of this is thought to be organic iodine (org-I) because this iodine is present in sediments rich with organic matter. Therefore, we showed org-I (solid) as well, based on these assumptions.
Our laboratory has firstly measured the organic iodine gas (org-I (gas)) in the sediment (began in 2018). We have been finding a lot mysteries about org-I gas in the sediment.
Oceanographic observations were conducted in the Chukchi Sea, which is located in the Pacific sector of Arctic Sea, over the continental shelf with the JAMSTEC research vessel “Mirai” in September 2012. In our laboratory, seawater was vertically sampled (from the sea surface to just above the seafloor on the continental shelf) at the observation stations shown below, where we measured the concentrations of organic iodine gas (org-I) in seawater.
Biogeosciences, 13, 133–145, 2016, www.biogeosciences.net/13/133/2016/doi:10.5194/bg-13-133-2016, © Author(s) 2016. CC Attribution 3.0 license
The vertical profiles of the concentration of organic gas species from the continental shelf area to the basin area (towards the North Pole). The vertical axis is the water depth, and the horizontal axis is the concentration of each component. Concentrations were low near the surface, but high concentrations could be seen at around depths of 50-100 m, close to the seafloor depth over the continental shelf.
Therefore, we began to think, “are organic iodine gases being produced in marine sediment?”.
During the JAMSTEC “Mirai” voyage in 2012, we speculated that the marine sediments may be the cause of the generation of organic iodine gas species. Hence, we continued making observations at Funka Bay in Hokkaido during the voyages of the training ships from the Hokkaido University School of Fisheries Sciences in order to confirm these hypotheses (sites in the figure below).
I am the principal author of J Atmos Chem, Shimizu et al, DOI 10.1007/s10874-016-9352-6 (Ooki), so I will reuse the figure as the copyright holder.
The seasonal changes in the vertical distribution of iodoethane (left in the bottom figure) and phosphate (right in the bottom figure) in Funka Bay, Hokkaido (seabed depth of 96 m) are shown. A diatom bloom is observed in spring (March) in Funka Bay, but afterward, we observed a sudden increase in iodoethane near the seabed in May-June.
Ocean sediments in the coastal area were collected at the Arctic Ocean (Chukchi Sea) and Funka Bay, and interstitial water (pore water) on the surface of the sediments (layers in contact with seawater) was collected. The organic iodine gas species concentrations in the pore water were then measured with GC-MS system. We have firstly known the concentrations of organic iodine gas components in the atmosphere, seawater, and sediments. Their averages are summarized in the table below.
Iodomethane （CH3I） is a major component (component shown on left in the table below) of the atmosphere. This is because it exists in a relatively stable state in the atmosphere. It can also be a major component of seawater. Diiodo-methane (CH2I2) can also become a major component in seawater (the reason remains a mystery). Iodoethane is always a minor component in both the atmosphere and seawater (second component on the left in the table below). We discovered that iodoethane was always a predominantly major component in the sediment. This is a mystery of the iodine cycle. We are currently trying to determine the underlying reason by conducting experiments in which organic matter deposited on the sediment layers is cultured. in order to solve this mystery.
We collected phytoplankton using a NORPAC net during the diatom bloom in Funka Bay.
We are confirming iodoethane generation by decomposing and culturing diatom organic matter. We observed that the amount suddenly increased approximately three days later.
（Second-year master’s student Meng is conducting experiments）
We have begun observations of collecting marine sediments with the voyages of the Hokkaido University School of Fisheries Sciences Training Ships Oshoro-Maru and Ushio-Maru. We have copied links to videos taken during these observations.
This is a video taken when seabed sediments were collected during multiple-core observations on the Oshoro-Maru. Please take a look.