A new study suggests that Saharan dust blown thousands of kilometers across the Atlantic Ocean, becomes increasingly nutritious for marine microbes as it travels. Chemical reactions in the atmosphere break down iron minerals in the dust, making them more water-soluble and creating an essential nutrient source for iron-deficient marine environments, according to researchers reporting in Frontiers in Marine Science on September 20.
Dust from the Sahara plays a significant role in marine ecosystems by supplying iron to iron-starved parts of the Atlantic, triggering phytoplankton blooms. These tiny marine organisms are vital for the ocean food web as they convert carbon dioxide into sugars via photosynthesis, explains Timothy Lyons, a biogeochemist at the University of California, Riverside. “Iron is incredibly important for life,” he emphasizes.
Each year, more than 240 million metric tons of Saharan dust cross the Atlantic, settling on islands like Bermuda and the Bahamas and turning their soils red. However, much of this dust lands in the ocean, acting as a major iron source for regions too distant to receive it from rivers. In these areas, dust-derived iron helps fuel marine food chains, supporting not just phytoplankton but also small fish, plankton-grazers, and their predators.
Dust’s Journey and Transformation
To explore how Saharan dust has evolved over time, Lyons and marine geologist Jeremy Owens, then both at UC Riverside, examined seafloor sediment cores from the eastern and western Atlantic. Their findings revealed that dust minerals closer to Africa contained more reactive iron — about 18% — compared to only 9% in dust collected farther west, near North America. This surprising result prompted the team to investigate the transformations occurring during the dust’s transatlantic journey.
As Saharan dust travels across the ocean over several days, it undergoes photochemical transformations due to ultraviolet light and acidic conditions in the atmosphere. These processes make iron more soluble and accessible to marine life, allowing phytoplankton to absorb the nutrients when the dust finally settles into the ocean. According to the study, the longer the dust travels, the less reactive iron reaches the seafloor, having already been consumed by marine organisms.
Marine Impacts
The iron from Saharan dust helps trigger phytoplankton blooms, supporting entire marine ecosystems. Small fish, like those consumed by larger predators, benefit from the plankton, and even commercial species such as Atlantic skipjack tuna are attracted to areas where Saharan dust has enriched the waters.
While the study primarily focused on Saharan dust, other potential sources of iron, such as smoke from North American wildfires over the past 120,000 years, were also considered. Still, the researchers used specific ratios of iron and aluminum to identify the Sahara as the primary source of the dust in their samples.
Further studies analyzing sediment from additional sites in the Atlantic could provide a clearer understanding of how atmospheric processes alter dust over long distances, offering insights into how marine ecosystems are sustained by this seemingly distant source of nutrients.
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