Nitrogen Isotopic Composition of Proteinaceous Coral Skeletal Amino Acids Records Change in Source Nitrate to the Euphotic Zone in the Western Tropical Pacific

Abstract:

Instrumental and proxy data and global climate model experiments indicate a multi-decadal shoaling of the western tropical Pacific (WTP) thermocline potentially related to a shift in ENSO frequency. In the WTP, the nutricline coincides with the thermocline, and a shoaling of the nutricline brings more nitrate-rich seawater higher in the water column and within the sunlit euphotic zone. In the nutrient-poor WTP, this incursion of nitrate-rich water at the bottom of the euphotic zone may stimulate productivity in the water column. However, there is a general paucity of measurements below the surface with which to investigate recent changes in seawater chemistry. Nitrogen isotope (δ¹⁵N) measurements of particulate organic matter (POM) can elucidate the source of nitrogen to the WTP and related trophic dynamics. This POM is the food source to the long-lived proteinaceous corals, and drives the nitrogen isotopic composition of their skeleton. Here, we report time series δ¹⁵N values from the banded skeletons of proteinaceous corals from offshore Palau in the WTP that provide proxy information about past changes in euphotic zone nitrogen dynamics. Bulk skeletal δ¹⁵N values declined between 1977 and 2010 suggesting a progressively increasing contribution of deep water with isotopically-light nitrate to the euphotic zone and/or a shortening of the planktonic food web. Since only some amino acids are enriched in δ¹⁵N with each trophic transfer in a food web, we measured the δ¹⁵N composition of seven individual amino acids in the same coral skeleton. The δ¹⁵N time series of the individual amino acids also declined over time, mirroring the bulk values. These new data indicate that the changes in the source nitrogen to the base of the euphotic zone drives a decline in coral skeletal δ¹⁵N values, consistent with the shoaling nutricline, with no coinciding alteration of the trophic structure in the WTP.