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Abstract:
RATIONALE The analysis of the nitrogen (N) isotopic composition of organic matter bound to30
fossil biomineral structures (BB-δ 15 N) using the oxidation-denitrifier (O-D) method provides a
novel tool to study past changes in N cycling processes.
METHODS We report a set of methodological improvements to the O-D method, including: (i) a
method for sealing the reaction vials in which oxidation of organic N to NO3- takes place; (ii) a
recipe for bypassing the pH adjustment step before the bacterial conversion of NO3- to N2 O; and35
(iii) a storage method of recrystallized dipotassium peroxodisulphate (K2 S 2 O8 ) under Ar
atmosphere.
RESULTS The new method eliminates the occasional contamination and vial breakage, and
increases sample throughput. The protocol for bypassing of pH adjustment does not affect BB-
δ 15 N, and it significantly reduces processing time. Storage of K2 S2 O8 reagent under Ar atmosphere40
produces stable oxidation blanks over more than two years. We report analytical blanks, accuracy
and precision for this methodology from eight users over the course of ~ 3.5 years of analyses at
the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by
low N content (0.30 ± 0.13 nmol N, 1σ, n=204) and stable δ 15 N (– 2.45 ± 3.50 ‰, n=204). The
analysis of reference amino acid standards USGS40 and USGS65 indicates an overall accuracy of45
– 0.23 ± 0.35 ‰ (1σ, n=891). The analysis of in-house fossil standards gives similar analytical
precision (1σ) across a range of BB-δ 15 N values and biominerals: zooxanthellate coral standard
PO-1 (6.08 ± 0.21 ‰, n=267), azooxanthellate coral standard LO-1 (10.20 ± 0.28 ‰, n=258),
foraminifera standard MF-1 (5.92 ± 0.28 ‰, n=243) and tooth enamel AG-Lox (4.06 ± 0.49 ‰,
n=78).50
CONCLUSIONS The proposed analytical improvements significantly increase sample
throughput without compromising analytical precision or accuracy down to 1 nmol of N.
