This post belongs to a special series of posts written by students in Dr. Simon Engelhart’s Coastal Geologic Hazards course at the University of Rhode Island. In this course students learn about coastal processes, including storm surges and sea level rise, and how these impact people and the environment.
It’s no secret that sea level is rising. All across the world, we see evidence of water levels creeping higher and more alarmingly towards existing cities and towns. An imperative question for coastal cities is the rate, or speed of sea-level rise. And while no one knows quite for certain how sea-level rise rates will change in the future, the answers to past sea-level rise lie right under our feet. This summary highlights some of the most innovative sea-level rise research from the 20th century.
Geoscientists have long studied sediments to reveal the history of local earth processes. At the core of this work, pun intended, is radiocarbon dating. Sediment cores, samples of sediment extracted from cores in the ground, possess a chronological record of events that shaped our environments today. In the late 1940s, Willard Libby developed methods for radiocarbon dating.1 Radiocarbon dating determines the age of organic matter that is preserved in a sediment core, which helps researchers to construct a timeline of when sediment was deposited into its environment. Libby’s ability to determine the ages of sediment cores greatly advanced the science of tracking historic sea levels. In 1962 Alfred Redfield and Meyer Rubin used radiocarbon dating of sediment cores to estimate the rate of high marsh peat (partially decayed organic matter) accretion in Barnstable, Massachusetts. In their paper, “The Age of Salt Marsh Peat and its Relation to Recent Changes in Sea Level at Barnstable, Massachusetts”, researchers used radiocarbon dating to create a timeline of how the marshes in Barnstable developed.2 From this information, Redfield and Rubin were able to determine the elevation of the sea in relation to land over the past 3,700 years. Just over twenty years later, improvements in stratigraphy (the study of sediment layering) and micropaleontology (the study of microfossils in rocks and soils) could be correlated with radiocarbon data to better estimate historic sea levels.
In 1986, Ian Shennan (Durham University, Durham, United Kingdom) tracked sea level movements in the Fenlands of eastern England for the past 6,300 years. By combining radiocarbon dating with improved stratigraphic and micropaleontologic data, Shennan determined the boundaries between seven periods of sea level rise and six periods of sea level fall. His paper, “Flandrian Sea-Level Changes in the Fenland. II: Tendencies of Sea Level Movement, Altitudinal Changes, and Local and Regional Factors”, goes into great detail regarding boundary differentiation and how they compare to previous studies.3 Shennan’s boundary distinctions did come with levels of uncertainty. Good radiocarbon data is dependent upon well preserved macrofossils such as plant rhizomes (horizontal underground plant stems) in the sediment column. Furthermore, unclear boundaries between sediment layers makes it difficult to interpret distinct depositional events.
Over the years, it became apparent that using radiocarbon dating in conjunction with macrofossils wasn’t a perfect indicator of sea level history. In order to use radiocarbon dating, the sediment had to have been deposited before the twentieth century, and there needed to be preserved organic matter in the sediment record. This was problematic for environments with little vegetation. Alternatively, a plant’s ability to thrive in a wide range of environments means that it is difficult to infer centimeter- to decimeter-scale sea level trends, let alone date them. These predicaments drove geoscientists to search for other clues within the sediment record.
A promising discovery was microfossils of sediment dwelling organisms could help determine habitat. Organisms such as diatoms and foraminifera are well preserved in the sediment record, and their narrow habitat tolerances reveals a lot about the past environments that these fossils are preserved in. Foraminifera are single celled protists that live in marine and estuarine environments. Each individual foraminifera has specific habitat ranges within a salt marsh. Preferred habitats of distinct foraminifera species are well documented and greatly vary in terms including desired salinity, inundation, plant life, and sediment type. These species-specific preferences can be used to infer physical properties of past habitats preserved in the sediment record. An abundance of empirical data on foraminifera habitat preference enabled Benjamin Horton (Nanyang Technological University, Singapore) and Robin Edwards (Trinity College, Dublin, Ireland) to develop a mathematical equation to use foraminifera location in coastal environment as a proxy for sea level.4 By developing this mathematical model, the foraminifera assemblage at a certain depth in a core can tell you at what elevation that assemblage was forming in the past.
Each of these researcher’s findings are just a small piece of the puzzle. But combining these analyses strengthens our understanding of historic coastal habitats and sea level rise rates. Being able to date a sediment core with several methods helps build the credibility of reconstructing sea level history. Combining radiocarbon dates with microfossil and geological stratigraphic data reconstructs the history of past environments, or paleoenvironments. Such efforts have been utilized by coastal hazard researchers across the world, including URI Professor Simon Engelhart. The tools and techniques of scientists will continue to advance. And if the past is any indication- so will sea level.
Michaela Cashman is an ORISE fellowship participant at the US EPA Atlantic Ecology Division in Narragansett, Rhode Island. She is concurrently working on a Ph.D. in microplastic detection and isolation in marine sediments. Michaela is interested in emerging contaminants, microplastics, remediation technologies, and hydrogeology. Her free time is spent constructing stained glass windows and advocating for her graduate student union, URI GAU.
Casey Dannhauser is currently working towards a Masters of Oceanography at the University of Rhode Island. When she isn’t focusing on her schoolwork she can be found working to improve the coastal water quality in Cotuit, Massachusetts for the Barnstable Clean Water Coalition.
Feature image: Graduate Student Casey Dannhauser looks through a sediment core in the field. Photo by Simon Engelhart.
Libby, W. F., et al. “Age Determination by Radiocarbon Content: World-Wide Assay of Natural Radiocarbon.” Science, vol. 109, no. 2827, 1949, pp. 227–228., doi:10.1126/science.109.2827.227.
Redfield, A. C., and M. Rubin. “The Age Of Salt Marsh Peat And Its Relation To Recent Changes In Sea Level At Barnstable, Massachusetts.” Proceedings of the National Academy of Sciences, vol. 48, no. 10, 1962, pp. 1728–1735., doi:10.1073/pnas.48.10.1728.
Shennan, Ian. “Flandrian Sea-Level Changes in the Fenland. II: Tendencies of Sea-Level Movement, Altitudinal Changes, and Local and Regional Factors.” Journal of Quaternary Science, vol. 1, no. 2, 1986, pp. 155–179., doi:10.1002/jqs.3390010205.
Edwards, B. P., and R. J. Edwards. “Quantifying Holocene Sea Level Change Using Intertidal Foraminifera: Lessons from the British Isles.” Cushman Foundation for Foraminiferal Research, Special Publication, vol. 40. 2006. 97pp.