Category: Earth Sciences
Determining the Geologic Age of Offshore Sand Resources
October 13, 2021 Written by Adam Thomas | Photo Courtesy of John Wehmiller
When hurricanes and large coastal storms hit coastal communities, the beaches and dunes are the first line of defense for many areas, providing a natural barrier between the ocean and inland communities, ecosystems and resources.
Because of this, it is important to replenish those beaches to keep that defense bolstered in case of a major storm. Beach replenishment, or beach nourishment, is one way to keep or expand the amount of sand on the foundation of the coast, and this sand can be supplied by onshore or offshore sand resources.
In 2014, the Bureau of Ocean Energy Management, in response to Hurricane Sandy, organized a program to assess the offshore sand resources for the mid-Atlantic coast. The Delaware Geological Survey coordinated this project and the efforts of many colleagues in Virginia, Maryland, Delaware, and New Jersey.
One of the goals of this assessment has been the enhanced understanding of the geologic framework of the coastal plain and inner continental shelf, including the structure, ages and physical properties of the offshore sediments. The geology of this coastal system has evolved over several million years, particularly during multiple large-scale (~100 m) changes in sea level during ice-age climate cycles of the past one million years. Understanding this geology can assist the assessment of a variety of offshore resources, including not only sediment supplies but also sites appropriate for offshore wind structures. Understanding the onshore framework is particularly useful for identifying ground water resources.
In order to properly identify onshore and offshore sand resources to help with beach replenishment, it is important to know the geologic age of the resources that are going to be used: if the sand is younger, it is likely to be replenished rapidly by natural processes, making it an ideal candidate to be used for beach replenishment. If the sand is older, however, it indicates that the resource takes longer to replenish and should not be harvested.
A new study from the University of Delaware published in the Quaternary Geochronology scientific journalanalyzed more than 600 offshore core samples from 93 sites between northeast North Carolina and the central New Jersey shelf. They dated shell fragments from the cores and compared them with results from onshore sites, putting the offshore work and the onshore work together into the bigger picture of the evolution of the Delmarva Peninsula and the adjacent offshore strata.
John Wehmiller, professor emeritus in the UD College of Earth, Ocean and Environment’s Department of Earth Sciences, served as the lead author on the paper and said that it would not have been possible if it had not been for the prior work of former UD graduate students: Daniel Belknap, June Mirecki, Linda York, Brian Boutin, Marguerite Toscano, David Pate and Steven Rahaim.
Those students attended UD in the 1970s, ’80s and ’90s and all conducted work that formed the basis for this most recent study, as they provided some of the onshore and offshore samples that were re-analyzed in this study.
“This paper built upon the work of a lot of former students,” said Wehmiller. “We were able to go back and revisit the students’ work and put it in the context of this more recent work. In most cases, what we were thinking back then, much of which was doubted because of the methods we were using way back 30-40 years ago, has pretty much been supported by the newer work. That, for me, has been very satisfying: to have been around long enough to see that come to fruition.”
He also credited co-authors Kelvin Ramsey, who received his PhD from UD in 1987 and works as a scientist and geologist at the Delaware Geological Survey, for coming up with the funding for the study, and Laura Brothers, a research geologist with the United States Geological Survey (USGS), whose work was what Wehmiller said allowed the team to tie the offshore and onshore geology together.
What the Age of Core Samples Can Tell Researchers
To estimate the age of the core samples, the researchers used amino acid racemization (AAR), a technique that measures the change of “left-handed” amino acids to “right-handed” amino acids within the organic matter preserved in fossil mollusks. This method, which can reach back in time to nearly one million years, was supplemented with carbon-14 dating, which is useful for samples roughly 40,000 years old and younger. Mollusk samples from onshore outcrops, cores, beaches, barrier island and inner shelf vibracores were used in the study.
Wehmiller said that one of the interesting findings of the study has to do with barrier islands in Virginia—specifically, Smith Island and Wreck Island, uninhabited islands located off of the southern tip of the peninsula.
The study showed that those islands are perched on some underlying, older sedimentary material, being supported in a position that eventually, because of sea level rise, is going to go away.
“When that sea level rises a little bit, they are going to migrate off of that platform that they are sitting on,” said Wehmiller.
In addition, there are two common shells of mollusk species found on the islands—Spisula and Mercenaria—and the two kinds turned out to be vastly different from one another in age.
Analyzing 15 shells of each species from both of the islands, the Spisula were found to all be Pleistocene in age—which means they are about 50,000 years or older in age—while all of the Mercenaria were less than 5,000 years old.
“This is a very strange combination,” said Wehmiller. “Literally, all of these were collected within 10 meters of each other, and so here’s this enigma where some shells are coming from some source that is underneath those islands, and then other shells are much younger and part of the dynamics of the modern, coastal environment.”
Another interesting aspect between the two shells is that even though they are separated in age by thousands of years, there are negligible differences in the ways the shells look.
The influence of the underlying geology on the beach samples is further seen by results from islands to the north of Smith and Wreck islands. The occurrence of the older shells decreases and becomes negligible as one proceeds to Paramore, Cedar, and Wallops islands, demonstrating different sources of sediment under the islands or immediately offshore.
“There are places on the Atlantic coast where there is a huge concentration of fossil material, and that says something about the units that are offshore that are supplying sediment to that area,” said Wehmiller. “It’s not as if there’s a whole lot of active sand out there on the continental shelf that is feeding the beaches. So it’s a big picture view that identifies the variable sources of sand that get to the beaches.”
All in all, it is important to know what sources are contributing to the coastal beaches because, as governments looks to offshore resources to replenish their beaches, they need to make sure that they are collecting sand from areas that are younger.
“If you find an area with shells that were all a million years old, it would mean that that feature is what’s referred to as a relic feature, that it’s no longer active,” said Wehmiller. “So if the state of New Jersey or the state of Delaware is looking for places to find sand to put on the beaches, they would prefer to work with the young stuff because it’s likely to be naturally replenished.”
This project was funded through a cooperative agreement with the Bureau of Ocean Energy Management of the U.S. Department of the Interior, Offshore Sand Resources for Coastal Resilience and Restoration Planning.