Friday, December 9, 2011

Saharan Dust

Saharan dust storm in 2003. We are just south of the frame of this picture
Now that we are nearing the coast of Africa, we have been seeing lots of Saharan dust coming from the continent. Why would anyone care about this dust, you might be asking? The Sahara Desert is the second largest desert in the world (Antarctica is considered the first), and is a vast dry area full of sand and dust. Strong winds often sweep over the continent from Europe, and can carry this dust thousands of miles-it can be seen across the Atlantic Ocean and into the United States. These large dust storms can carry important nutrients, as well as harmful things into the ocean and onto land, and is one of the important sources of nutrients to the ocean that is being studied on this cruise.
Aerosol samplers on the flying bridge with lots of dust!

Some scientists on board from University of Alaska, Fairbanks and from Florida State University, are studying these dust storms and other "aerosols" in the marine environment. Aerosols are tiny particles that are generally made up of some kind of dust or sea salt, and water. They are called aerosols because they are a mixture between a liquid and a solid, and are found floating in the air. Aerosols are comprised of many things, and oceanographers often distinguish between anthropogenic aerosols (aerosols that are human-derived, often forms of smoke/smog and other pollution) and natural aerosols (like those from the Sahara). By measuring the sizes of the aerosols, and the different metals and nutrients in the aerosols, scientists can often distinguish where these particles originated from and how far they have traveled. This is just what scientists on our ship have been doing everyday of the cruise since we left. They have large aerosol samplers on the "flying bridge" of the ship, which is the highest desk on the ship above everything else so that the samples don't get contaminated by any pollution coming from the ship itself.
Red/orange Sahara dust in the medium size range
Black/gray anthropogenic dust in the small size fraction
The dust from the Sahara is visibly orange/red and is generally greater than 1 micron in size. Knowing the size of the dust is helpful, because both the color and size can help scientists distinguish it from other types of dust, like anthropogenic dust, which is generally gray/black in color and in the small size range (0.45-0.95 micron). Each type of dust contains different levels of metals, and when it lands on the ocean it may have different effects on the resident phytoplankton communities.
Sahara dust on the aerosol filters

For example, dust from the Sahara usually has high levels of iron, an important nutrient for phytoplankton, so a dust storm could mean an increase in nutrients that the phytoplankton need. On the other hand, anthropogenic dust from industrialized areas has high levels of toxins such as lead and sometimes mercury, which can be potentially harmful to phytoplankton and other organisms.

No comments:

Post a Comment