A fabled cross-estuarine Oyster Reef (July 2021)

Skiffing along the thumping waves, Tony, Naomi and I set out for Cross Rocks, a fabled oyster reef that strikes across the Newport river. A week of wind and storms was now subsiding; it’s final blows slowing our approach. We relieved the engine as we neared the confluence of the Newport and Harlowe Creek, aware of a shallow sill. With a high tide floating us, we coasted over the sill and used a distant water tower as our north star. A thin brow of shells grew thicker, and soon we were atop the targeted reef.

We stepped overboard and secured an anchor far enough from the reef that the soon falling tide wouldn’t strand us. Scouting the area by foot, we found the spot which we sampled weeks before and across which we would extract a new transect of 15-foot oyster-reef cores. This was our familiarly termed “study area”, the raison d’etre for Naomi’s honor thesis. And for good reason.

Word has it that Cross Rocks was used as a natural footbridge by the Coree Native Americans, for whom the Core Sound and Core Banks complex was named. Now distinctly discontinuous, the several segments of oyster reef filter massive quantities of brackish water and redirect shallow flows. If the stories are true, the reef is at the very least several hundred years old, with the last known Coree’s perishing in the bloody 1711 Tuscarora War. We suspect it is much, much older. And that is where the Carbon-14 comes in.

By isolating the deepest articulated (intact) oysters at several locations along Crossrocks, Naomi aims to recreate the growth pattern of the trans-estuary reef. When exactly did it form? Did it build from the central thalweg of the river outwards? Or from the shore? And do the rates of growth represent past sea-levels, nutrient supplies, or salinity gradients? There’s no better way to answer these questions than by posing them to the sands of time.

The method of radio-isotope dating has become widely used in paleoclimatic and geologic reconstructions for its relative ease and quantitative accuracy. As organisms cease to metabolize (…die), they no longer incorporate Carbon-14. Knowing the half-life of this ever-decaying carbon isotope (5,730 years), the ratio of unstable Carbon-14 to the stable Carbon-12 can be used to determine years postmortem. Different decay chains found in different elemental isotopes have different half-lives, thus different applications. Rubidium-87, for example, has a half life of 49 billion years and as such is used to date igneous rocks and meteorites. In my own research, I leverage Lead-210’s half life of 22.6 years to reconstruct decadal trends in marsh accretion.

Last summer’s field work becomes this year’s lab work, and Naomi will spend many climate-controlled days sheltered from the beating North Carolina sun. Oysters, which can filter through some 50 gallons of water in their adult form, are researched intensively at the UNC Institute of Marine Sciences. Naomi hopes to add the perspective of geologic time, drawing us closer to understanding an oyster reef’s response to changing sea levels. Like the sediments some 15 feet down, we patiently await what may be uncovered.

In the video below, look for the towering sediment core being vibrated into the Oyster Reef subsurface.