20TH AND 21ST CENTURY SEA LEVEL CHANGES IN WINYAH BAY, SC

Georgetown County, South Carolina, a historic area with many of its economic activities focused near the waterfront, is low-lying and vulnerable to various coastal impacts which become amplified by sea level rise. The county is located on Winyah Bay where the historic changes of tides and sea level are widely unknown and the anthropogenic influence on these oceanic components is not well understood.

A sea level history for the last 120 years was reconstructed using tide gauge records and intertidal marsh foraminifera assemblages in sediment cores from local marshes. Hand-written historic tide gauge data from the Georgetown Lighthouse were documented for the years 1899 to 1904. These tide gauge documents were recovered from the US National Archives and digitized. The original benchmark used as a vertical reference was located and the elevation was measured using RTK-GPS. A water-level logger was installed at the lighthouse dock in October 2018 to monitor the modern water level and its variations. The historic and the current gauge data were vertically related and compared to each other in detail in order to calculate the overall change in sea level over the past 120 years.

Three cores were taken in vicinity to the Winyah Bay mouth that were used to calculate sea-level index points. These cores were analyzed using foraminiferal assemblage identification and the indicative meaning approach to relate past and modern sea level. The modern vertical faunal zonation of the foraminifera along with the exact elevation measured with RTK-GPS was used to calibrate the fossil assemblages to their respective paleo-elevation. The cores were dated using a combination of ²¹⁰Pb,¹³⁷Cs and a regional trace metal stratigraphy.

The results describe how relative sea level has changed from 1899 until 2019 as well as compare tidal statistics and annual sea level variations between the historic and modern records. The preliminary analysis suggests a rate of sea level rise at least comparable if not exceeding that of Charleston, SC. Quantifying how sea level has changed in the recent past is fundamental to sustainable management decisions in Georgetown into the future. The data will also close an existing spatial gap in the regional vertical behavior of the US Atlantic coast.