High-Resolution Image

This figure shows an interesting and unusual phenomenon at the far end of South America. The figure on the left shows a dust cloud originating in the north end of the Patagonian desert in Argentina which extends over the southwestern Atlantic Ocean. The figures on the right display a computer simulation of the same event illustrating the area where the dust deposited within the first 24 hours (top right) and 48 hours (bottom right). By sheer serendipity, this event may help scientists understand one of the most hotly debates issues on the effect of dust on marine biology.

The Indo-German iron fertilization experiment LOHAFEX (LOHA is Hindi for iron, FEX stands for Fertilization EXperiment) is being carried out from the German research vessel Polarstern in the southwest Atlantic from January 7 to March 17, 2009. One of the chief objectives of the mission is to test the iron-fertilization hypothesis, where iron (an essential nutrient) is intentionally introduced (in the form of iron sulfate) to the upper ocean to stimulate phytoplankton growth resulting in increased sequestration of carbon dioxide from the atmosphere. Fertilization supports the growth of marine phytoplankton blooms by physically distributing microscopic iron particles in nutrient-rich, but iron-deficient, ocean waters. Fertilization occurs naturally when upwellings bring nutrient-rich deep-water up to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world's largest marine habitats. In addition, fertilization may occur when wind carries dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers, rivers and icebergs. Recently, there has been interest in artificially seeding the ocean with iron as a geoengineering technique to tackle global warming.
 
The Southern Ocean encircling Antarctica is rich in the nutrients nitrate, phosphate and silicon but phytoplankton growth is limited by the supply of iron. It is in this region where the LOHAFEX campaign has been setup. On the evening of January 27, 2009, after an initial controversy, the Polarsten was cleared to initiate the seeding phase of the experiment in the central south Atlantic (approximately 48S, 17W). Coincidentally, an unusual dry season in the southern end of South America resulted in several dust events in the Patagonian desert and in other areas that have become active dust sources in this season such as the north end of Patagonia, an area known for its fertile pastures. Swept by the strong westerlies, dust emitted in this region can travel long distances. On January 23, 2009 a strong event was captured by MODIS (Moderate Resolution Imaging Spectroradiometer) which is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. The image on the left illustrates the length (~1100km) and the extent of the plume over the South Atlantic. The dust cloud advected towards the center of the South Atlantic. Could this dust cloud reach the area being sampled by the Polarstern?

Indeed it appears to be a possibility. The event was simulated using the aerosol transport model HYSPLIT. The top figure shows the first 24-hours average of the deposited dust. The bottom image shows the region with the distribution of the deposited dust (white line contour) and the total concentration in the column (blue line contour) two days after the event. The spatial distribution of the concentration indicates how far the dust travels whereas the location of the deposited material shows the areas where phytoplankton might be potentially impacted. In addition, the image shows the location of the Polarstern (red thick line) during the same period and it appears that dust deposited in the west end of the course. Because the intensity and the amount of dust emitted are unknown, this simulation cannot tell us the actual amount of dust deposited over the ocean. It may well be that dilution of the dust cloud is so large that the quantity deposited is too minute to have any impact. However, this simulation does illustrate that the meteorological conditions were favorable for putting the dust over an area known to be iron deficient. It also raises the interesting question whether the blooms that are being sampled (and perturbed) by the Polarstern are reacting to the injected iron sulfate and/or to the naturally deposited dust.

(submitted by Santiago Gassó, UMBC/GEST). The image on the left was created by Louis Gonzalez at the University of Lille and the image of the
right was created by the HYSPLIT model developed by the Air Resources Laboratory at NOAA.