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Sea-level Rise

Salt Marshes and Regional Sea Level

Salt Marshes and Regional Sea Level

Citation: Stearns, R.B., Engelhart, S.E., Kemp, A.C., Hill, T.D., Brain, M.J., and Corbett, D.R., 2023, Within-region replication of late Holocene relative sea-level change: An example from southern New England, United States: Quaternary Science Reviews 300: 107868. doi:

In order to accurately predict future changes, it is important to understand how sea level in a particular area has changed in the past. These local sea-level changes are the result of a variety of influences that can be global, regional, or site-specific in scale. By reconstructing and comparing sea-level records from several sites in close proximity to each other, it is possible to better constrain which of these factors are affecting sea level in the area.

Sea-Level Reconstructions

Sea level is a complex concept that is affected by a combination of different factors. As the name would imply, global factors, such as thermal expansion of seawater as temperatures rise and increases in the amount of land-ice that melts into the sea, influence sea level around the world. On the other hand, regional or local factors, such as vertical land movements from tectonic activity or changes in local tides, currents, or precipitation amounts, only affect sea level in that specific area. As a result of this variety of influences, different parts of the world experience different sea levels.

In order to try and isolate the importance of local or regional factors, it is necessary to analyze multiple sea-level records within a region. While there are several different ways to reconstruct sea-level changes, some of the most commonly utilized proxies, especially in the northeastern United States, are tide-gauge measurements and salt-marsh sediments. As salt marshes and the organisms, such as single-celled foraminifera, and plants that live on salt-marsh surfaces have a well-established relationship with sea level and tidal elevation, they can be used to interpret earlier sea levels as well. By comparing records spanning across the same time periods from multiple sites, it is possible to identify the spatial extents of sea-level trends or lack thereof across a region – thus isolating which trends are local or regional rather than global in scale.

Fox Hill Marsh, Rhode Island and Regional Sea Level

In their study, Stearns et al. (2023) assessed several sea-level records from the Canadian Maritimes down to southern New Jersey in order to identify common trends in sea-level changes over the last ~3000 years. The authors used a combination of tide-gauge records and salt-marsh sediment sea-level proxies, including observations of changes in foraminifera assemblages and in the presence of plants that live within salt marsh floral zones or above high tide.

Map showing study site locations in Rhode Island
Map showing the locations of study sites from the Canadian Maritimes down to southern New Jersey, including the location of Fox Hill Marsh in southern Rhode Island. Photo: Stearns et al., 2023.

This study added a sea-level reconstruction for the site of Fox Hill Marsh, Rhode Island to the regional record, where the authors determined that sea level rose by ~3.7 meters since 1200 BCE. In addition, by comparing multiple sites within the region, the authors were also able to better isolate the influence of regional and local changes on sea level. They determined that the fastest rate of sea-level rise (1.71 +/- 0.84 mm per year) in the region during the past 3000 years occurred in 2020. They also found that a suspected rapid rise in sea level during the 18th century was not visible in all of the sea-level records, meaning that when the increased rate was observed it was likely site-specific. 

So Why Do We Care?

Sea level is influenced by a variety of factors that are active at several scales (global, regional, and local). These variations mean that sea level does not change at the same rate or in the same way everywhere around the world. By reconstructing and comparing sea-level records regionally, it is possible to better ascertain which factors are active in that area. This improved understanding of regional sea-level changes makes it possible to more accurately predict future changes. With most of the world’s population living within coastal areas, these improved predictions could help to avoid unnecessary loss of infrastructure, property, and possibly of life.

Featured image from Beyond My Ken on Wikimedia Commons.

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