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Chemistry

Dust may feed more life in a warmer ocean

Reviewing: Félix‐Bermúdez, Armando, et al. “Does sea surface temperature affect solubility of iron in mineral dust? The Gulf of California as a case study.” Journal of Geophysical Research: Oceans: e2019JC015999.

 

Hematite is a type of iron-rich mineral in the Earth’s crust that is mined from all over the world for iron. (Image from flickr)

Iron is a mineral we all need to live a healthy life, specifically to make cells that transport oxygen to our hearts and muscles. And just as we need iron in our diet, ocean microbes also need iron to grow. While we get all the iron we need from food, ocean microbes need to look for iron that is dissolved in the surrounding seawater. A major source of iron in the ocean is dust from land. The Earth’s crust is made of iron-rich minerals, and mineral dust from land can travel all the way to the ocean by wind. But when dust falls on the ocean surface, only a part of it dissolves into seawater to release iron, while the rest sinks to the bottom of the ocean.

 

 

The “solubility” of dust (or the ability of dust to dissolve) in seawater can vary greatly: sometimes less than a hundredth of dust will dissolve in seawater, or at other times up to 95% of dust will dissolve. This wide range of dust solubility results from a series of factors, such as dust minerals, dust grain size, amount of dust, and acidity and temperature of surface seawater. A group of scientists focused on understanding how sea surface temperature (SST) controls dust solubility in the Gulf of California.

The Gulf of California gets lots of wind-blown dust from surrounding areas such as the Baja California peninsula and southwest Arizona. The surface waters in the Gulf of California also experience large seasonal changes in temperature, with a range of 12ºC (or 22ºF) between summer and winter. In this study, the research team collected seawater from Gulf of California, kept half of the seawater at “summer” temperature (28.9ºC) and the other half at “winter” temperature (16.8ºC), then added dust from Baja California soils and observed how much dust dissolved in seawater over time for a week.

Dust storm from Mexico and the Baja California peninsula blowing into the Gulf of California. (NASA image courtesy Jeff Schmaltz)

Sea surface temperature in July 2019 (left) and January 2019 (right). Images were obtained from NASA Ocean Color

 

The research team found that dust dissolves up to 24 times more in the warmer “summer” seawater. Over the course of the week, they found that dust solubility continued to increase over time in the warm waters (with up to 5% of dust added to seawater dissolving and releasing iron), while it decreased over time in the cold waters.

The team also added different amount of dust into waters to find that the solubility of iron is up to 13 times higher when a small amount of dust is added to seawater. This may seem counterintuitive at first, as one may expect to find more dissolved iron in seawater when more dust (and iron-containing minerals) is added to seawater, but this happens due to a process called “scavenging”. This term refers to a process during which chemicals dissolved in ocean (e.g. metals like iron, copper and aluminum) stick to the surface of particles in the ocean surface and sink to the deep ocean. When a higher amount of dust enters the surface ocean, there are more particles iron can stick to. As these particles get heavier and sink, dissolved iron gets removed from the surface, leaving less available dissolved iron in surface waters for microbes to use.

While this study specifically focused on the Gulf of California, the 12ºC difference in summer and winter sea surface temperatures is also observed in other parts of the ocean, like the Mediterranean Sea, Red Sea and the Persian Gulf. The research team suggests that future studies in different areas of the global ocean would be useful, to find out whether the relationship between dust solubility and seawater temperature applies to the global ocean. If this relationship turns out to be a global trend, a future warming ocean may face changes in dust and iron solubility, affect growth of ocean microbes that depend on iron supply from dust, and eventually all marine life that feeds on the microbes.

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