GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 89-6
Presentation Time: 9:15 AM


GILLIKIN, David P.1, GOODWIN, David H.2, JORN, Eva2, O'HORA, Heidi1 and WANAMAKER Jr., Alan D.3, (1)Geology Department, Union College, Schenectady, NY 12308, (2)Department of Geosciences, Denison University, FW Olin Science Hall, 100 Sunset Hill Drive, Granville, OH 43023, (3)Department of Geological and Atmospheric Sciences, Iowa State University, 253 Science I, Ames, IA 50011

More than 50 years of marine and freshwater research confirm that oxygen isotopes (δ18Os) in bivalve shells are robust recorders of both water temperature and water oxygen isotopes (δ18Ow). However, water isotopes are rarely known hindering the use of δ18Os values as a temperature proxy. This has driven the quest for either a salinity independent temperature proxy or a salinity proxy. Carbon isotopes (δ13Cs) 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 δ18Ow over the growth period of the bivalves (August 2016 to April 2019). We also measured carbon isotopes of the dissolved inorganic carbon (δ13CDIC) along salinity gradients during five cruises spaced over 2.5 years. Temperature and δ18Ow allowed predicted shell reconstruction, which allowed the measured δ18Os values to be fit to a time domain. Shell δ13Cs values were then compared with salinity. Preliminary data from 14 months of M. mercenaria shell growth show δ13Cs values match fairly well between three shells that grew contemporaneously. Shell δ13Cs variations coincide with salinity changes during late summer and winter, but decouple during spring and early summer. There is a general inverse relationship between δ13Cs values and water temperature, but the relationship is not strong and reverses at times. There were strong relationships between δ13CDIC 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 δ13CDIC values calculated from salinity suggests that large variations in δ13CDIC values were not recorded in the shells. These preliminary data illustrate the difficulty to use δ13Cs values as a proxy of salinity or δ13CDIC values due to various effects such as metabolic carbon. However, in some species, carbon isotopes can be useful as qualitative salinity or δ13CDIC proxy.