Friday September 29 CSL Meeting

Stable isotopes in proteinaceous black coral skeletons as proxies for ecosystem variability in the Gulf Stream over millennial timescales

Presenter: John Schiff
Time: Friday 29 September, 2:00-3:00 p.m.
Location: 805 O&M

Abstract: The Gulf Stream is one of the most important components of the global ocean circulation. While the current nutrient dynamics in the Gulf Stream continue to be well studied, changes to these dynamics over time remain poorly understood. The proteinacous skeletons of deep-sea corals provide an opportunity to investigate surface water processes over century to millennial time-scales as these corals feed on fresh, sinking particulate organic matter (POM) and are extremely long-lived. These skeletons grow outward via concentric growth rings, yielding high-resolution stable isotopic records across the lifespan of each coral specimen. The combined chronologies of three black corals (Leiopathes sp.) used in this study cover the last 3,000 years of surface water history in the Gulf Stream in the Southeastern United States. The bulk d15N record exhibits relative stability, with short term ~1‰ cyclicity. Gradual, long-term increases and decreases in d15N appear at multiple sections of the record, each with a timescale of several hundred years. These long-term cycles exhibit a ~2‰ change with an average bulk value of 8.5‰ The peak of this record occurs ~1,000 years before present after a gradual increase. In the youngest portion of the record, a ~2‰ change in d15N values occurs over a relatively short period of time that is atypical of the rest of the record. We can interpret d15N changes as changes in nitrogen source or trophic dynamics, as both of these can influence the bulk d15N signal. Therefore, we analyzed the d15N of individual amino acids comprising the proteinaceous skeleton in order to decouple nitrogen source changes from trophic dynamics within the water column. Aliquots from individual growth bands were chosen for compound-specific analysis and indicate that d15N for Phenylalanine (Phe) and the average d15N of all Source Amino Acids (Sr-AA) were approximately 4-6‰ higher in the older portion of the record compared to the more recent 500 years. This excursion in d15N-Phe and Sr-AA suggests a change in d15N at the base of the food web, independent of trophic effects. This change occurred over the past several hundred years and is atypical of historical trends. We interpret this change to represent a substantial alteration in nutrient source for phytoplankton within the Gulf Stream.

Bulk d15N values from two black coral specimens (4902bc1 and 4684bc1) plotted with time in years before present (BP), showing cyclical variability and long-term trends. On the secondary axis, the d15N values for Phenylalanine (Phe) from three black coral specimens (Bulk values for 4907bc1 not currently shown) are plotted with time. The Phe values for both corals before 500 years BP are higher than after that point by several permil. The Phe values from 4684bc1, while already being lower, continue to decline and diverge from the bulk values toward the end of the record.