NOAA/WDS Paleoclimatology - Monterey Bay Bamboo Coral phenylalanine d15N Data from 1977-2004 CE

RATIONALE: Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool fortracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the d15N value of phenylalanine (d15NPhe) represents an increasingly used proxy for source d15N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical. METHODS: A new offline approach has been developed for high-precision d15N measurements of amino acids (d15NAA), optimized for d15NPhe values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The d15N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). RESULTS: The combinedHPLC/EA-IRMSmethod separated most proteinAAs with sufficient resolution to obtain accurate d15N values, despite significant intra-peak isotopic fractionation. For d15NPhe values, the precision was ±0.16‰ for standards, 4Å~ better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a d15NPhe paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent d15NPhe values within error; however, the d15NPhe values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰±0.14‰ vs 0.60‰±0.20‰, respectively). CONCLUSIONS: These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for d15NAA measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. d15N paleoproxies).