AN ANALYSIS OF CARBON ISOTOPES IN BIVALVE SHELLS AS A SALINITY PROXY (Invited Presentation)

More than 50 years of marine and freshwater research confirm that oxygen isotopes (δ¹⁸O_s) in bivalve shells are robust recorders of both water temperature and water oxygen isotopes (δ¹⁸O_w). However, water isotopes are rarely known hindering the use of δ¹⁸O_s values as a temperature proxy. This has driven the quest for either a salinity independent temperature proxy or a salinity proxy. Carbon isotopes (δ¹³C_s) in bivalve shells have been explored as a salinity proxy. Here we test this possibility on the hard calm Mercenaria mercenaria and the eastern oyster Crassostrea virginica from the Cape Lookout region of North Carolina USA. Waters were sampled for salinity, temperature, and δ¹⁸O_w over the growth period of the bivalves (August 2016 to April 2019). We also measured carbon isotopes of the dissolved inorganic carbon (δ¹³C_DIC) along salinity gradients during five cruises spaced over 2.5 years. Temperature and δ¹⁸O_w allowed predicted shell reconstruction, which allowed the measured δ¹⁸O_s values to be fit to a time domain. Shell δ¹³C_s values were then compared with salinity. Preliminary data from 14 months of M. mercenaria shell growth show δ¹³C_s values match fairly well between three shells that grew contemporaneously. Shell δ¹³C_s variations coincide with salinity changes during late summer and winter, but decouple during spring and early summer. There is a general inverse relationship between δ¹³C_s values and water temperature, but the relationship is not strong and reverses at times. There were strong relationships between δ¹³C_DIC and salinity, with R² values ranging from 0.96 (fall 2017, spring 2018, and spring 2019) to 0.87 (summer 2018). When all data were combined the R² value was 0.84. Using δ¹³C_DIC values calculated from salinity suggests that large variations in δ¹³C_DIC values were not recorded in the shells. These preliminary data illustrate the difficulty to use δ¹³C_s values as a proxy of salinity or δ¹³C_DIC values due to various effects such as metabolic carbon. However, in some species, carbon isotopes can be useful as qualitative salinity or δ¹³C_DIC proxy.