Connections between nutrient-rich atmospheric dust and primary productivity in contemporary marine ecosystems are well established. Iron and other nutrients fertilize the ocean, supporting phytoplankton populations, which themselves form the basis of most marine food webs.
Considerably less is known about such connections in the distant past. Preliminary evidence suggests that large-scale dust fertilization may have occurred in the late Paleozoic, but results have been confined to the paleoequator region. A new study is the first to explore the effects of iron fertilization in the midlatitudes.
“It is fascinating that this mechanism that has been explored extensively in the current climate, and the glacial/interglacial cycles, could also be operating so far in the past,” Natalie Mahowald, Irving Porter Church Professor of Engineering in Cornell University’s Earth and Atmospheric Sciences Department, wrote in an email to Eos. Mahowald, who researches “atmospheric dust and iron deposition to the oceans, and how climate and humans have modified the atmospheric iron cycle,” wasn’t involved with the study.
The team analyzed dust contained in carbonate rock samples collected from the Alborz Mountains in northern Iran. First author Mehrdad Sardar Abadi had brought the samples to the United States when he began his position as a postdoctoral researcher at the University of Oklahoma.
The scientists valued the samples because they came from strata of an ancient shallow sea where researchers knew there wasn’t a nearby river. This provenance provided assurance that dust present in the samples wasn’t transported by water but instead “fell from the atmosphere,” said Lynn Soreghan, director of the University of Oklahoma’s Mewbourne College of Earth and Energy School of Geosciences and an author on the study.
Getting Down to Dust
Sardar Abadi and his team used coupled plasma–mass spectrometry and laser particle size analysis to extract 300-million-year-old dust from the samples. They discovered that most of the dust particles were less than 15 micrometers in size and had traveled long distances—possibly more than 1,000 kilometers—before deposition. They also found that samples with larger amounts of highly reactive iron tended to contain more small dust particles and to contain higher amounts of primary producers.
Together, the results suggest that “Earth’s atmosphere was much dustier during this ancient time,” according to a statement about the research. “While iron is not the only micronutrient potentially carried in dust, it is estimated that this ancient dust contained twice the bioavailable iron as the modern dust that fertilizes the Amazon Rainforest.”
Iron is a micronutrient necessary for photosynthesis in organisms like algae and cyanobacteria. Over millions of years, iron fertilization of the ocean may have supported the growth of these microbial communities, contributed to greater carbon sequestration in the ocean, and helped create a more oxygenated atmosphere.
“I think that what we see in the fossil record, very specifically in Mehrdad’s research, but more broadly through the fossil record of the late Paleozoic, is a remarkable algal dominance,” Soreghan said. “I noted the remarkable high-abundance and low-diversity record of algal and microbial activity in Mehrdad’s ‘high-dust’ interval.…Such a consequence generally means that the algae out-competed everything else, with repercussions in the ecosystem. It’s definitely an ecosystem disturbance.”
The study was published in Geology in December 2019.