African dust once fertilized the Everglades

Numerous water lilies and other aquatic flowers once dotted the grass carpets of Florida’s Everglades thanks to nutrient-bearing dust from Africa.

Windblown sediment from the Sahara Desert landed in Florida around 4,600 years ago and enriched its nutrient-poor wetlands, scientists report October 7 in the Proceedings of the National Academy of Sciences. But an abrupt shift in winds around 2,800 years ago downsized the dust dump and stifled the nutrient flow, the authors suggest.

The discovery of airborne fertilizer revises scientists’ understanding of the history and workings of the Everglades. Previously, most scientists thought the Everglades had been undernourished until nutrient-rich runoff from human development and agriculture began to seep in over the last century. Lead author and earth scientist Paul Glaser of the University of Minnesota in Minneapolis says the finding highlights the importance of traveling dust in distant nutrient cycles.

Glaser and colleagues pieced together the Everglades’ history by dating layers of a 99-centimeter-long mud core from a swamp in the northeastern Everglades and then correlating the findings with climate records.

“The biggest surprise was when I started to look at the cores in the lab, and I started to see these large quartz grains,” Glaser says. By studying the unfamiliar grains, Glaser and his colleagues determined that the quartz didn’t originate in the region or even on the continent. Instead, the quartz closely resembled windblown sand from the Sahara.

The sediment core’s quartz-rich sand, which Glaser and his colleagues say started landing in Florida around 4,600 years ago, contained relatively high levels of phosphorus and nitrogen. The sand also showed up in the core at the same time as pollen from aquatic plants, such as water lilies and carnivorous bladderworts.

At the time, Florida was much wetter — a boon for aquatic plants — and Africa was undergoing a dry spell, making it perfect for dust dispersal.

Joseph Prospero, an atmospheric chemist at the University of Miami in Florida, agrees that the quartz data match climate records. Based solely on climate records, you could almost expect the burst of dust that the researchers discovered, he says.

Sediment layers younger than 2,800 years old lacked African dust and its nutrients. The timing coincides with when climate scientists say changing wind patterns reduced Africa’s dust stream. Pollen in the core also changed at this time, indicating a shift from aquatic flowers to sawgrass, the plants that now dominate the Everglades’ nutrient-poor marshes. When looking at all of the evidence, Glaser says, “everything seemed to fit together.”