“Pollen-ology”: what microfossils can tell us about sea level rise

Reference: Yao, Quiang, 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

Aerial photo of a site used for study in Shark River Estuary at south end of the Everglades.

The everglades, which sit at the bottom of the Floridian Peninsula, is the largest subtropical wetland ecosystem in north America. Sometimes called “the river of grass”, it is an incredibly delicate complex of wetlands and prairies the size of Delaware. In salty areas of the everglades the wetlands are mainly made up of mangrove forests.

You may have seen mangroves before: sort of squat trees and extensive root systems with pneumatophores that grow above the water and soil. Coastal mangrove forests are an important ecosystem because they provide shelter for animals and protect our coastlines from high winds or storms. They are also known to store A LOT of carbon and prevents it from entering the atmosphere and contributing to the greenhouse effect.

The south Florida mangrove system has been dynamic throughout its history and continues to adjust rapidly in the face of changing climate conditions. Because mangrove forests are so prolific in the southern everglades it’s hard for us to imagine that they were not there in the geologic past. But how can we study the history of these environments?

Pollen-ology ?
Microscope photos of mangrove pollen of various species. From: Pandey et al. 2010

Palynology is one method used to reconstruct plant communities that may have been present in a place thousands of years ago. Researchers examine samples from cores of sediment or rock under microscopes; they find and identify tiny fossilized pollen, spores or even small plankton.

By comparing the abundance of different types of pollen and finding the age of the rock or soil in which they were found, palynologists can estimate what an environment may have looked like and when it looked like that.

We know that mangrove forest distribution in the past in South Florida was mainly driven by sea level, either rising or falling. Mangroves are totally marine, which means they thrive in salty wetlands. If sea level rises, introducing salt water into coastal areas, mangrove forests can over take other environments.

Before using palynology, researchers have been able to only track changes in mangroves and associated brackish (part salt and part freshwater) marshes for the past few decades. With the help of fossilized pollen and geochemistry, researchers are now able to go back several millennia!

Mangrove pollen in the Shark River Estuary
Photos of soil and rock cores used in pollen study and of freshwater snail fossils found within the cores.

A group from Louisiana State University put together the most comprehensive history of the mangrove forests in the Shark River Estuary near to the southern tip of Florida. They chose sites along a gradient of very salty to very fresh environments so they could track coastal plant changes over space and time.

The group collected soil and rock cores and found 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 these sites in the past.

The results showed that 5700 years ago, there was not even wetland plant pollen present at most of the sites in this estuary. Sea level was very low and wet conditions had not yet arrived. As time moves forward over the next 3000 years, sea level rose at a rate of 2.3 millimeters per year.

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 increases in the cores. Finally, once the sites become totally marine, mangrove pollen and marine plankton species began to dominate in almost every core. It wasn’t until 800 years ago that the mangrove forest was finally as extensive as it is today.

These illustrations demonstrate the changes in plant communities over time. Starts at 5700 years before present, ends 800 years before present. Transitions from prairie, to freshwater marsh to brackish marsh and ends on mangrove swamp.
The future of mangroves

Palynological data revealed that the mangrove forest in Shark River Estuary migrated at a rate of 4 miles per millennium. As these areas transformed into new environments, they began to provide different functions for the ecosystem and coastline like new types of habitats or a different way of storing carbon.

Today, it is hard to look around us and comprehend that what we see hasn’t always been. We tend to think about the world on a human timescale, when really the earth has adapted and changed for thousands and even millions of years.

When we look into a future with changing climate and threats of rising sea level, we can examine the past to help us predict what may be coming. With a better understanding of how the natural world adapts, we can learn how to adjust our societies and make way for change.

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