SALINITY AND WATER OXYGEN ISOTOPE RECORDS OF HYDROLOGIC VARIABILITY IN JARRETT BAY, NORTH CAROLINA, USA

Salinity and water oxygen isotope (δ¹⁸O_w) records from Jarrett Bay, in the Cape Lookout Region of North Carolina, USA, record regional spatial and temporal hydrologic variability. Transects collected in April 2017; October 2017; and, June 2018 document spatial variability in the spring, summer, and autumn seasons, respectively. All transects began in Smyrna Creek, at the head of Jarrett Bay, and extend to Core Sound. The July and October transects extend farther to Cape Lookout and the Atlantic Ocean. The spring and summer data sets have similar salinity and δ¹⁸O_w ranges (April: 3.42—33.64 PSU and -3.00—1.00 ‰, n = 30; June: 1.17—37.08 PSU and -4.39—1.01 ‰, n = 51). Furthermore, their best-fit lines are similar (April: δ¹⁸O_w = 0.132 * salinity - 3.314, r = 0.997; June: δ¹⁸O_w = 0.136 * salinity - 3.922, r = 0.971). That these transects have similar low salinity, δ¹⁸O_w values and best-fit intercepts likely reflects the influence of isotopically light groundwater (local well water, approximately -4.2 ‰, n = 3) in the upper reaches of Jarrett Bay. The low salinity/δ¹⁸O_w values in spring and summer may also reflect lower spring evaporation rates and/or elevated precipitation in the summer months. The October transect had a much narrower range of salinity and δ¹⁸O_w values (24.96—32.52 PSU; -0.03—0.57 ‰, n = 26). Its best-fit line has a lower slope and higher intercept than either the other transects (δ¹⁸O_w = 0.010* salinity - 0.017, r = 0.134). The lack of low salinity/δ¹⁸O_w samples in the October transect may reflect the combination of reduced groundwater input, decreased precipitation, and/or elevated rates of evaporation in the later summer and autumn months. Weekly water samples, collected from a single site between August 2016 and June 2018 (n = 89), reflect temporal variation in Jarrett Bay hydrology. These data range between 10.69—36.69 PSU and -3.8—1.86 ‰. The line of best-fit has a higher slope and lower intercept than those from the seasonal transects (δ¹⁸O_w = 0.182 * salinity - 5.052, r = 0.965). Taken together, these data are consistent with region precipitation records and calculated evaporation rates and suggest salinity and δ¹⁸O_w variation in Jarrett Bay reflect regional environmental factors.