Investigating the use of microfossils to reconstruct past sea levels from salt-marsh environments in northern Georgia, USA

Lay summary authored by Huixian Chen. Read the full paper here: http://doi.org/10.5334/oq.80

Changes in sea level vary geographically and with time and in response to a variety of processes. These include the melting of land-based ice from ice sheets (Antarctica and Greenland) and glaciers, thermal expansion of the oceans, vertical motions of land (e.g. earthquakes) and by local processes such as the compaction of sediments. Understanding past sea levels provides important background information that is useful for determining how sea levels may change in the future. Our understanding of past sea levels stems from a variety of geological reconstructions that utilize evidence from natural environments (e.g. salt marshes, mangroves and corals). Along the Atlantic coast of North America, salt marshes have provided abundant information of how and why sea levels have changed during the past ~10000 years.

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Salt-marsh environment at Georgetown, northern Georgia, U.S. Atlantic coast. Credit: H. Chen.

The use of salt marshes in providing such information stems from their relationship with tidal levels. Salt-marsh environments typically exist between mean tidal level up to the highest astronomical tide. Further to this, microfossils such as foramnifera living on the surface of the salt marsh may be used to further subdivide the environment into narrower vertical ranges. This is due to different foraminiferal species preference to living conditions including level of salinity and duration of exposure as the tides rise and fall. If we understand and document where these different species live today, we can apply this information to fossil counterparts in sediment cores of known age through methods such as radiocarbon dating and reconstruct how sea levels have changed with greater accuracy.

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Scanning electron microscope images of the most abundant foraminiferal species observed in the salt marshes at Thunderbolt and Georgetown. From left to right: Haplophragmoides wilberti, Miliammina fusca, Ammoastuta inepta. Haplophragmoides wilberti is typically found at higher elevations in the salt marshes. Credit: H. Chen.

In this study, we investigated the surface (top 1cm) distributions of foraminifera from two salt marsh sites, Thunderbolt and Georgetown, in northern Georgia to document where different species live in relation to tidal levels. To do this, we set up multiple surface transects and collected surface sediment for analysis under a microscope. We show that certain species are more prevalent in different areas of the salt-marsh environment and thus have different relationships with tidal levels. We also sampled short (50 cm) sediment cores and analyzed the depth to which living (at the time of collection) foraminifera penetrate down into the subsurface to assess their potential implications for when reconstructing sea level from these environments.

Full paper: Chen, H., Shaw, T.A., Wang, J., Engelhart, S., Nikitina, D., Pilarczyk, J.E., Walker, J., García-Artola, A. and Horton, B.P., 2020. Salt-Marsh Foraminiferal Distributions from Mainland Northern Georgia, USA: An Assessment of Their Viability for Sea-Level Studies. Open Quaternary, 6(1), p.6. DOI: http://doi.org/10.5334/oq.80

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