USING STABLE ISOTOPES TO RECONSTRUCT GROWTH OF THE HARD CLAM MERCENARIA MERCENARIA

Bivalve mollusks shells archive environmental conditions in the form of biogeochemical variations and periodic growth lines and increments. When calibrated with observed environmental records, these archives become valuable sources of (paleo)ecological information. Here we present new information of the growth of Mercenaria mercenaria from Jarrett Bay, in the Cape Lookout region of North Carolina, USA. Environmental records were collected between August, 2016 and June, 2018. Hourly water temperatures were recorded with multiple in situ loggers (±0.2°C). Weekly water samples were collected to determine water oxygen isotope (δ¹⁸O_W) variation (analytical uncertainty ±0.07‰). Daily maximum and minimum temperatures, together with interpolated daily δ¹⁸O_w values, were used to calculate the predicted carbonate oxygen isotope (δ¹⁸O_C) envelope (i.e., the daily range of potential δ¹⁸O_C values.) Multiple specimens of M. mercenariawere grown under natural conditions at the same site throughout 2016-2018. Clams were stained with calcein, which fluoresces under UV light, on three dates: 8/13/16; 4/8/17; and, 7/13/17. Shells were then collected on 10/21/17. Carbonate samples were collected from thick-sections using a computer-controlled XYZ micromill. Analytical uncertainty of measured δ¹⁸O_C samples was ±0.07‰. Measured δ¹⁸O_C samples were fit within the oxygen isotope envelope using the stained samples and known collection date to calibrate shell growth. Additional constraints on the timing of shell deposition assumes growth between 9 and 31°C. Our results suggest M. mercenaria in Jarrett Bay grow essentially throughout the year. Surprisingly, the dark annual bands, thought to form between May and October, were deposited between early April and October. These bands likely reflect heat stress and may reflect earlier warming of Jarrett Bay waters in the spring. Placement of the measured δ¹⁸O_C samples in the oxygen isotope envelope suggests these specimens grew preferentially during the warm hours of the day. Finally, our results suggest these specimens stopped growing during the extreme precipitation event, Tropical Storm Julia (September, 2016). If these storm-induced growth cessations can be recognized it may provide a new tool for reconstruction of prehistoric tropical storms.