What Microfossils Can Tell Us About Sea Level Rise
By Alina Spera
Louisiana State University’s scientists are studying sea level rise from multiple perspectives, using geological data, and current environmental events, to predict how sea level rise will impact coastal regions. Experts from the College of the Coast and Environment (CC&E) are recording what will be the most comprehensive history of mangrove forests in Florida’s Shark River Estuary to date. This is the first study from coastal Florida to show that coastal ecosystems evolved and shifted landward in response to late-Holocene sea level rise. And, Kam-biu Liu, oceanography and coastal sciences professor and department chair, believes the geological record is invaluable in demonstrating potential future consequences of sea level rise in coastal Louisiana and South Florida.
Coastal mangrove forests are a critical ecosystem, providing shelter for animals and coastline protection from high winds and storms. These squat trees and extensive root systems that grow above the water and soil are also known to store large amounts of carbon, preventing it from entering the atmosphere and contributing to the greenhouse effect.
Liu and Qiang Yao, LSU CC&E post-doctoral researcher, are studying mangrove microfossils in the Shark River Estuary, via palynological procedures, as a means to collect and analyze historical environmental data.
Palynology, or the study of fossil pollen, is one method used to reconstruct plant communities that may have been present in a place thousands of years ago. Liu and Yao examine samples from cores of sediment or rock under microscopes in order to find and identify tiny fossilized pollen, spores or even small plankton. By comparing the abundance of different types of pollen and learning the age of the rock or soil in which they were found, the researchers can estimate what the environment might have looked like at a certain point in time.
In the past, mangrove forest distribution in South Florida was mainly determined by sea level rising or falling. Mangroves are totally seashore plants, which means they thrive in salty wetlands. So, if sea level rises and introduces salt water into coastal areas, mangrove forests can take over other plant communities.
Before the advent of palynology, researchers were only able to track changes in mangroves and associated brackish (part salt and part freshwater) marshes over the past few decades. Now, with the help of fossilized pollen and geochemistry, researchers like Liu and Yao are able to go back six millennia.
Mangrove Pollen in the Shark River Estuary
Liu and Yao chose sites along a gradient of very-salty-to-very-freshwater environments so they could track coastal plant changes over space and time. They collected soil and rock cores and determined the age of the different layers. After taking subsamples of the cores, among other pieces of geochemical evidence, they identified pollen from species of marsh grasses, mangrove trees, and even some marine plankton that may have been present at those sites in the past.
First, freshwater was introduced, which created extensive marshes with low lying grasses. Next, as brackish and then salt water pushed farther inland, mangrove pollen slowly increased in the cores. Finally, once the sites became totally saline, mangrove pollen and marine plankton species began to dominate in almost every core. It was not until 800 years ago that the mangrove forest finally became as extensive as it is today.“The results showed that 5,700 years ago, wetland plant pollen was not yet present at most of the sites in this estuary. Sea level was very low and wet conditions had not yet arrived. Over the next 3,000 years, sea level rose at a rate of 2.3 millimeters per year,” said Liu.
Liu’s and Yao’s palynological data revealed that the mangrove forest in Shark River Estuary migrated inland at a rate of four miles per millennium, or close to 23 feet (seven meters) per year. As these areas transformed into new environments, they provided different functions for the ecosystem and coastline, such as new types of habitats or a different way of storing carbon.
With the threat of accelerating sea level rise in the future, coastal managers and environmental planners must develop sound policy for adapting our coastal communities to environmental change using all available resources.
According to Liu, because there is no similar published palynological record for coastal Louisiana, “this kind of paleoecological dataset—and the long-term perspectives that can be derived from it—is desperately needed for developing sound plans for coastal protection and restoration in southern Louisiana.”
Yao, Qiang, and Kam-biu Liu. Dynamics of marsh-mangrove ecotone since the mid-Holocene: A palynological study of mangrove encroachment and sea level rise in the Shark River Estuary, Florida. PloS One. 2017. https://doi.org/10.1371/journal.pone.0173670