Oyster reefs are often the only natural hard substrate in estuaries and are even labeled “oyster rock” on many old nautical charts. Oyster reefs have the potential to grow extremely rapidly (10 cm/year); in comparison, coral-reef growth, is measured in mm/year. Oyster reefs are not only composed of oyster shells, they have an abundance of mud and organic carbon filling pore spaces between shells. A core through an oyster reef samples compositional changes through time, but extracting that record is tedious. In this time-lapse video, Rachel Quindlen, Molly Bost, and Carson Miller are subsampling an oyster-reef core. The constituents of every 5-cm long subsample are separated using a sieve and later by combusting organic matter and measuring particle size with a laser. It’s time-consuming, but worth it.
Oyster reefs are similar to other carbonate depositional environment such as coral reefs. The largest difference is that oyster reefs are located in estuaries and sediment loading from external sources, like rivers and shoreline erosion, is much higher than what corals experience. Being filter feeders, oysters are designed for the estuarine environment and take advantage of the plentiful suspended matter in the water column. However, too much of a good thing can be deathly. Oyster reefs need to grow rapid enough so that they don’t become buried in sediment. In most estuaries, sediment accretion matches the rate of sea-level rise (SLR), so if oyster reefs cannot grow more than 3 or 4 mm/year (the rate of SLR) they will cease to exist. Above, is a seismic line from Nueces Bay, Texas. Highlighted in blue is a large oyster reef, which kept pace with SLR and sediment accumulation (highlighted in those other colors), until it gave up and was buried by that most recent sedimentary unit shown in gray. Why did the reef fall behind and give up all of a sudden? Perhaps it became diseased or maybe sediment loading in the water column was just too much for the reef to handle? Important questions to address if we want to improve conservation and restoration efforts.
During large storms, barrier islands are temporarily underwater because of storm surge and high waves. This is called overwash, and during overwash sand is moved from the beach and deposited in back-barrier environments. The sandy deposit that forms as a result of overwash is called a washover. Below is an animation that shows the evolution of a washover (Site 2) on Onslow Beach, NC during a 4-year period. Some of the most dramatic morphologic changes occurred after Hurricane Sandy and during a persistent nor’easter in October, 2015. We are still working with these data, so stay tuned.
June: Collecting overhead photos with a toy drone.
A bit of what we have been up to the past six months in photos. We also worked inside many days, teaching classes and writing papers and proposals. It hasn’t been all fun and games, though. A horrible summer in terms of funding, with all of our grant proposals being rejected (insert clip art here of man with pockets turned inside out). On a positive note, Charlie Deaton joined the lab over the summer. He is in many of the photos below.
June: Outreach activities.
June: Waiting for the interview to begin.
June: Vibracoring old washovers at Onslow Beach.
July: Dune vegetation surveys with Barrier Island Geology and Ecology class.
July: Oyster reef research in Shallotte, NC.
August: Lab hiking trip. We started after work and ended after dark…unforgettable, in a good way.
October: Testing the BlueView sonar 3D scanner in the neighborhood pool. Best place to learn how to use new gear.
September: Visiting oyster sills in the White Oak River Estuary, NC.
August: Selecting a site to monitor marsh-shoreline movement.
August: Lots of samples were incinerated to measure loss on ignition.
October: Donuts go well with field work. These were especially delicious.
October: Documenting modifications to a washover fan.
We’re contemplating how to sample the barrier with all of that shrubbery in the way.
For the past few years we have been working on compiling a data set of the age and landward extent of ancient washover fans on a Barrier Island in North Carolina. An ancient washover fan is identified as a sand bed sandwiched between two saltmarsh units. We had one last washover fan to map and decided to use an Edelman Auger instead of vibracoring because we knew that this part of the beach was difficult to access. The vibracoring rig is heavy, bulky, and requires a lot of space to use so we thought a hand auger would be the way to go; big mistake.
We first tried it on the backshore, and everything was going well until we hit the water table and the hole kept filling in. We are supposed to be able to sample below the water table with this tool, but it is very difficult to pull the tool out of the ground after the hole collapses. One thing I learned is that it’s time for Ethan and me to hit the gym.
Each pile represents a 25-cm sample interval. We sampled to a depth of 2 m. Wish we had brought the vibracore.
We tried to access the middle of the island, but the vegetation was too dense. We crawled into the thicket, and in addition to the lack of space, it smelled like something had died nearby and was in the middle of decomposing. We ended up hiking around the back of the thicket to collect our samples and managed to penetrate down to about 3 m, which was deep enough to sample the entire island. The Edelman Auger might work well in other environments, but it is not the right tool for sampling a barrier island, unless you are only interested in the upper 1 m of stratigraphy.
Ethan confirming the smell of death and the lack of room to collect samples in the thicket.
Collecting samples on the back side of the thicket in the phragmites marsh. We spent all day basically digging holes.
The Ocean 180 video challenge is to create a video abstract, which is a short (180 second) piece summarizing the results of a recently-published, peer-reviewed study on any ocean-science related topic. Justin Ridge entered and chose a recent GRL paper on sea-level anomalies and beach erosion as the topic. Who better to star in the video than the author of the paper, Ethan Theuerkauf? The target audience is middle school students. Justin did a great job, but unfortunately didn’t progress to the next round. Take a look at his video, then watch the Ocean 180 Video Challenge finalists. Justin’s going to try again next year and focus on a different topic, possibly some of his work on oyster reefs.
Justin Ridge found out last week that he is one of six winners, nation wide, for excellence in coastal and marine graduate study. “Justin Ridge, a Ph.D. student at the University of North Carolina at Chapel Hill, is using innovative techniques to assist in restoration efforts of oyster reef communities. He also co-founded a unique educational program for K-12 teachers, where fellow students and faculty share their research with the teachers, who in turn, bring that information into their classrooms.” Check out information on all of the winners at the NOAA website, and more information on North Carolina winners at NC Sea Grant. We are all very proud of Justin at UNC-IMS! Keep up the good work.
Collecting a vibracore from Onslow Beach, NC.
There are many reasons to collect cores from a beach. One of the most interesting is to look back in time and see what environments used to be where the beach is today. The core is like a time machine, or better yet, a history book. You just have to learn the language geology. The cores we took today tell us that there used to be a saltmarsh where the beach is, because below the beach sand we sampled old marsh plants. Before the saltmarsh, an estuary occupied the area because we sampled gray mud with an oyster reef below the marsh sediment. This stacking pattern of different environments is evidence that sea level has been rising in the area of Onslow Beach, NC. Earth’s history is beneath our feet and collecting cores is one way of exposing it.
Justin, Tony and Matt ready to get into the water at Onslow Beach.
The southwestern half of Onslow Beach, NC is starved of sand. Using a side-scan sonar, we imaged peat and organic-rich sediment at the seafloor just seaward from where the waves start to break. Offshore from that, Miocene rock is imaged at the seafloor. It is difficult, at least for us, to get a true sense of what the seafloor looks like from these geophysical data (scale, rock type, relief, etc.). To gain perspective, we decided to SCUBA dive and collect a video of the seafloor from the shoreline to about 300 m offshore. Visibility nearshore was very low because that organic-rich mud was eroding and being suspended in the water column. Once we reached about 200 m from shore, visibility improved and we could clearly see rock outcropping at the seafloor (Belgrade Formation). That rock is shown in the video below, taken at a depth of about 7 m (23 feet).
Onslow Beach, NC during a sea-level anomaly in 2009.
Sea-level anomalies are periods greater than 2 weeks when the water level at the beach is high. They are not necessarily related to storm surge or sea-level rise, rather they are forced by changes in ocean currents. On the US East Coast, slowing of the Gulf Stream or meteorological phenomena, like northeasterly winds or pressure changes, can pile water up against the shore and cause a sea-level anomaly. They impact large stretches of coastline (e.g. Massachusetts to Florida) and occur every year, but some years they are more frequent. Ethan Theuerkauf recently published a paper in Geophysical Research Letters that presents the first direct measures of the effects of sea-level anomalies on beaches. He shows that a year with frequent sea-level anomalies can cause as much beach erosion as a year with a hurricane. Compare Onslow Beach, NC during a sea-level anomaly, above, with Hurricane Arthur (July 3, 2014), below. The hurricane made landfall at night, but you can still make out overwash (the camera is pointed landward across a washover fan).