Seasonal Oyster Collection by Gullah/Geechee Communities of the American Southeast through Laser Induced Breakdown Spectroscopy (LIBS)

Matthew Picarelli-Kombert is a third year PhD student at the Pennsylvania State University in State College, Pennsylvania. His research focuses on how novel social and economic conditions, by way of either the emergence or collapse of institutions or institutional arrangements, affect the organization of human-environment relationships and dynamics. To explore these relationships, he is investigating how the collapse of the U.S. Southern plantation system in the mid-nineteenth century influenced the subsistence practices of formerly enslaved African and African American communities. His research is centered on Ossabaw Island, Georgia, USA, where formerly enslaved communities lived, relying on coastal and estuarine resources. These communities are still present along the coastline today, and are a self-identifying Indigenous group known as the Gullah/Geechee.

The Gullah/Geechee developed across the Lowcountry of the U.S. Southeast (coastal Georgia and South Carolina). They trace their cultural origin to the enslavement and forced migration of West and Central African peoples to North America to grow rice and cotton (Opala 1987; Westmont and Clay 2022). These island geographies shaped Gullah/Geechee identity through spatial and intentional separation from mainland populations, which led to the development of distinct cultural practices that continue to persist today. Following Emancipation, social and economic changes increased this separation, and self-sufficiency became a hallmark of their identity (Jackson et al. 1991; Matory 2008; Ochiai 2004). A key component of Gullah/Geechee self-sufficiency was, and still is, reliance on coastal eastern oysters (Crassostrea virginica; Armstrong 1980; Lange and Handler 1985; Reidy 1993). This species is abundant along the U.S. Southeastern coastline and is prominent in shell midden deposits formed by these historic communities.

Matt’s project investigates the impacts that Emancipation had on Gullah/Geechee oyster harvesting practices, and the key role that oysters may have played in establishing self-reliance after the collapse of the plantation system. This work takes place on Ossabaw Island, Georgia, where plantations enslaved African and African American Gullah/Geechee communities until 1865. By the mid-1800s, four plantations had been established on Ossabaw Island, enslaving c. 300 ancestral Gullah/Geechee people (Dorsey 2010; Roberts Thompson 2020). After Emancipation, many of those freed continued to reside, or newly settled, on the island, likely reoccupying structures once used during enslavement. A group of formerly enslaved people also formed a community just north of one of the former plantations, Middle Place Plantation (1865-Early 1900s; Price 2007).

In 2024 and 2025, Matthew took part in excavations at the South End and Middle Place Plantations on Ossabaw Island, sampling two enslaved and two post-Emancipation shell middens respectively. Laser induced breakdown spectroscopy (LIBS) was conducted on a sample of archaeological eastern oyster collected from these middens (South End Plantation (enslaved), n=54; Middle Place (freed), n = 52). Left valves were bisected down the growth axis of the shell and scanned with the LIBS-System at the Leibniz-Zentrum für Archäologie (LEIZA) in Mainz, Germany.

Georgia coastal water temperatures shift across the year, with summer highs near 30–33°C and winter lows near 6–12°C (Dahlberg 1972; Lulewicz et al. 2018; Verity 2002). Eastern oyster growth slows as water temperatures increase towards summer temperature peaks, meaning that winter conditions better support shell growth. As oysters grow, they draw magnesium (Mg) and calcium (Ca) from surrounding water. Calcium stays stable throughout the year, while magnesium varies with changes in water temperature. Mg/Ca variation, therefore, reflects seasonal shifts in the coastal environment (Hausmann et al. 2019). LIBS records these Mg/Ca patterns, allowing us to evaluate seasonal changes, and can be used to identify the season of collection at the end of the growing edge.

Matthew’s preliminary analysis found that the enslaved and free communities collected oysters at similar times of the year: throughout winter and spring. These results align with previous historic and smaller-scale studies of oyster harvesting patterns on the Georgia coast that found that oysters are primarily collected in colder months (Blount and Kitner 2007; Forbes 2025). It shows that even during enslavement, when oyster collecting may have been opportunistic, and likely heavily supplemented rationed food, there were still key constraints on the timing of oyster harvesting, indicating that oysters were not exploited as a year-round supplement to the foodways of enslaved communities. Whatever cultural or ecological drivers shaped this pattern, they seemed to have continued to shape harvesting practices into the post-Emancipation-era. Historic and archaeological studies also note heightened preference for oysters in colder months due to better flavor, a potential reason for the similar collection patterns between two contexts that varied in their ability to allocate their own time. Alternatively, vibrio, a bacterium that can cause harmful health outcomes in humans, grows exponentially in the warm waters of the U.S. Southeastern summers, another potential reason the communities may have avoided harvesting in hotter months (Frank et al. 2021; Froelich and Daines 2020).

This application of LIBS provides the first historical comparison of oyster harvesting practices in relation to Emancipation’s influence on African American communities. The next phase of Matthew’s analysis includes a complete morphometric study of the thousands of oysters in these historic contexts and will also leverage stable oxygen isotope (δ¹⁸O) analysis, both of which will help to understand the specific habitats and collections strategies employed by enslaved and free Gullah/Geechee communities. Traditional oxygen isotopic analyses require 15–20 samples to be drilled per shell to make seasonality determinations. Through the use of LIBS, only one or two samples for oxygen isotopes are now needed per shell to determine their habitat of collection (salinity). This multi-method archaeomalacological approach will link seasonality, habitat, and salinity data, allowing for an integrative reconstruction of oyster harvesting practices and the impacts on these practices as Gullah/Geechee communities navigated their novel social and economic landscapes following Emancipation.

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