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Special Session 3: Size matters or not, particles export in marine environments |
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Compound/position specific Nitrogen isotope analysis in amino acids and its application in paleoceanography
Tuesday 10th @ 1050-1110
Room 4
Lin Zhang* , Texas A&M University-Corpus Christi
Hao Yu, Texas A&M University-Corpus Christi
Mark Altabet, University of Massachusetts Dartmouth
Presenter Email: lin.zhang@tamucc.edu
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| The nitrogen isotopic composition (d15N) of bulk sedimentary N (d15Nbulk) has been used widely to study past water column nitrate variations in paleoceanographic records. Previous results suggests that natural fluctuations in the d15N of surface nutrient N were preserved in the d15N of exported planktonic biomass and in sedimentary d15Nbulk. However, bulk sediment N includes various N containing compounds, in which marine derived amino acids are major components. d15Nbulk records can be affected by changes in water column nutrient N usage, trophic effects, and diagenesis on amino acids. Compound/Position specific nitrogen isotope analyses of individual amino acids (d15N AA) are novel measurements with the potential to disentangle these variations and improve the understanding of paleoceanographic sediment N records. The goal of this study is to develop a more robust separation method to enables N compound specific isotope analysis (CSIA) and position specific isotope analysis (PSIA) on more individual amino acids than previous method (17 vs 13). Another goal is to eliminate the derivatization required by the GC-C-IRMS approach. d15N2O has been used as a final analyte to significantly reduce the required sample size by HPLC-EA-IRMS approach (20 nmoles vs 1umoles). We used an ion-exchange chromatography to separate amino acids and collect individual ones using a fraction collector. Collected amino acids were converted to nitrite and reduced to nitrous oxide for d15N2O analysis on a Purge and Trap (PT)-IRMS. This novel and robust method was applied to analyze amino acids in samples from phytoplankton tows, sediment traps and surface sediments from various preservation settings. d15Nbulk measured by EA-IRMS and amino-N d15NAA measured by this method were linearly correlated with a slope very close to 1. Our approach provided a robust approach for the determination of total d15N of amino-N in sediment samples with complex matrices and interferences. For our experimental convenience, 15–30mg sediment samples with N content of ~0.4% were hydrolyzed with a subsequent ten-fold dilution. In reality, only 0.1–1mg of these sediments would be needed to provide enough material for analysis with this method. If intact AA amino N retains the pre-diagenetic d15N signal, this lays out a promising approach for correcting diagenetic influence on sediment d15N paleo-records. |
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