Amino acid δ13C records from deep-sea corals of the NW Atlantic: Reconstructing plankton community composition in a rapidly changing ocean

Deep-sea corals of the species Primnoa resedaeformis secrete annually-banded, two-part skeletons of calcite and a diagenetically-resistant, fibrillar protein. The protein component is derived from sinking particles and thus records the geochemistry of export productivity over the century scale lifespans of individual colonies. Previous work on bulk and amino acid-specific δ¹⁵N records from Primnoa colonies from the Northeast Channel, offshore Nova Scotia, documented an anomalous 20th century decrease in δ¹⁵N that was attributed to increasing northward advection of Gulf Stream-associated waters along the continental slope (Sherwood O.A., et al., Proc. Nat. Acad. Sci. 108(3), 1011-1015, 2011). Here we examine δ¹³C records spanning from the year 1935 to present from the same original suite of Primnoa colonies and additional colonies collected more recently. Records of bulk δ¹³C from different colonies show remarkable reproducibility over interannual timescales, with an amplitude of 2 ‰ after correcting for the Suess effect. The δ¹³C of essential amino acids (δ¹³C_EAA) was more variable than that of bulk δ¹³C, up 5‰ for phenylalanine. Isotope mixing models [McMahon K.W., et al., Science 350(6267), 1530-1533, 2015) reveal interannual variability in δ¹³C_EAA signatures that suggest changes in plankton community composition through time, but also highlight the need for improved characterization of plankton end-member δ¹³C_EAA signatures in the NW Atlantic. Overall, these results further demonstrate the use of δ¹³C_EAA signatures in marine bio-archives as a proxy for primary producer functional diversity through time.