Nitrogen Isotopic Composition of Modern and Plio-Pleistocene Fossil Shells from the Two Sides of the Isthmus of Panama

Abstract:

The isotopic composition of organic nitrogen preserved in fossil shells (shell-bound δ¹⁵N) bears information about the past ocean. We have analyzed shell-bound δ¹⁵N in modern and fossil bivalve and conus shells using a sensitive method adapted from coral- and foraminifera-bound δ¹⁵N measurements. This method allows for analyses of fossil shells that are at least millions of years old, yielding a precision of 0.2-0.3‰ on less than 20 mg of shell powder. Shell-bound δ¹⁵N variability on single shells along the growth directions are 1-3‰, which might reflect changes in environmental nitrogen cycling on seasonal to annual timescales. To begin to investigate environmental signals of interest that might be recorded by control shell-bound δ¹⁵N, we focused on shells from Central America, where a large oceanographic δ¹⁵N gradient (>5‰) exists between the Caribbean and Pacific side of the isthmus of Panama. The modern shell-bound δ¹⁵N from the Pacific side of Panama is high (>8‰ vs. air on average), apparently reflecting the isotopic impact of water-column denitrification in the Eastern Tropical North Pacific. In contrast, the modern shell samples from the Caribbean side of Panama have a δ¹⁵N 4-5‰ lower than in the Pacific-side samples. The Caribbean-side data are consistent with the low δ¹⁵N of thermocline nitrate in the tropical Atlantic Ocean, a region signal of N₂ fixation. Preliminary results on fossil bivalves suggest that the shell-bound δ¹⁵N on the Caribbean side of Panama has remained low since at least 3.5 Ma, while the Pacific side’s shell-bound δ¹⁵N has remained high since 0.4 Ma. More samples will be analyzed to confirm and expand on these initial observations.