A ~125,000 YEAR OLD RECORD OF SUBMARINE GROUNDWATER DISCHARGE IN BERMUDA

Submarine groundwater discharge (SGD) is studied today in many coastal and island settings by direct observation of salinity, nutrient contents, and radiogenic isotopes. Records of past SGD behavior are typically erased with the mixing of ocean waters. Here, we show that δ¹⁸O_w can be used as a tracer for SGD in Bermuda, where outflowing precipitation-derived freshwater has a distinct isotopic composition (~-3.8‰) compared to surrounding seawater (+1 to +1.3‰). We present seawater, marsh, well, rain, and tap water samples from multiple locations around the islands of Bermuda. In the near-shore environment, we track changes on the order of ~2‰ in δ¹⁸O_w across 7 South Shore sites over the period of April 2019 to December 2020. We find that δ¹⁸O_w increases (decreases) as sea level increases (decreases), indicating reduced (increased) SGD and an isotopic composition of coastal waters more similar to the seawater (freshwater) endmember. Gastropods like Cittarium pica that live in tidal zones on the rocky shoreline record these changes in coastal δ¹⁸O_w in the isotopic composition of their shells (δ¹⁸O, Δ₄₇), along with information about seawater temperature. By analyzing fossil shells dating to the Last Interglacial period (~125 ka), we identify past variations in δ¹⁸O_w, and, through the relationship established using modern water samples, variations in SGD. We find highly variable reconstructed δ¹⁸O_w in C. pica shells from the south shore, indicating similar SGD behavior during the Last Interglacial. In total, we demonstrate previously undocumented past and modern variability in δ¹⁸O_w around the island of Bermuda, with implications for paleoclimate reconstruction in other coastal settings. We also show how clumped isotope paleothermometry can shed light on past submarine groundwater discharge behavior.