coastal dead zone
May 19, 2014

Iron Helps Prevent Formation Of Coastal Dead Zones

Brett Smith for - Your Universe Online

Containing dissolved oxygen concentrations of less than 2 or 3 parts per million, hypoxic waters in estuaries and sections of coastline are essentially “dead zones” where life cannot exist.

According to a new study in the journal Nature Geoscience, iron discharged from continental sediments acts as a limiting switch that could prevent coastal waters from creating a runaway feedback loop that leads to less and less dissolved oxygen and the formation of a dead zone.

The German-American study team found that in high-oxygen conditions, almost all of the iron dissolved within the water precipitates – changing into rust-like iron oxide particles, which fall to the seafloor. The remains of dead plants and animals also sink to the seafloor and their rotting remains also use up oxygen dissolved in seawater. As oxygen decreases, a hypoxic dead zone may develop. If this takes place, iron oxides break down and may enter back into the water column where the iron becomes available to fertilize oxygen-generating plankton, the study researchers said.

"When this moderate hypoxic state occurs, the iron release fuels more biological productivity and the organic particles fall to the sea floor where they decay and consume more oxygen, making hypoxia worse," said study author Florian Scholz, a postdoctoral earth sciences researcher at Oregon State University. "That leads to this feedback loop of more iron release triggering more productivity, triggering more iron release.”

"But we found that when the oxygen approaches zero a new group of minerals, iron sulfides, are formed," Scholz added. "This is the key to the limit switch because when the iron gets locked up in sulfides, it is no longer dissolved and thus not available to the plankton. The runaway hypoxia stops and the hypoxic region is limited."

The team's finding were made possible by the advancement of indicators for sedimentary iron discharge during past intervals of ocean deoxygenation. These indicators were used to look at a sediment core from the upwelling area of Peru, where the water column has one of the lowest continuous oxygen levels on Earth.

The researchers said they were able to look at concentrations of minerals in ocean sediments going back 140,000 years. They were able to determine if sediments buried during previous ocean deoxygenation events had an iron deficiency, which would suggest that the iron was removed and potentially transferred out into iron-deficient waters. On the other hand, when the sediments held a great deal of iron, the metal likely was retained and so not available for plankton.

"Florian found that there are two states in which iron is locked up and unavailable to fuel plant growth," said study author Alan Mix, a geochemist at OSU. "When there is a lot of iron in the sediment, but no molybdenum, the iron is stored in oxide minerals.”

"This happens when oxygen is abundant," Mix continued. "But if there is iron and molybdenum, then the iron is stored in sulfide minerals like pyrite, meaning the system has little or no oxygen available.”

"These basic reactions have been known for a while," Mix said, "but documenting them in the real world on a large scale – and associating them with climate change – is quite significant and especially important given projections of growing hypoxia in a warming climate."