25th International Meeting on Organic Geochemistry IMOG 2011

O-52
Nitrogen isotopes of amino acids: first results in sediments and
gorgonian corals of a new tool to reconstruct organic nitrogen
source and cycling from paleoarchives
Matthew McCarthy 1 , Owen Sherwood 2 , Fabian Batista 1 , Moritz Lehmann 3 , Christina
Ravelo 1 , Carsten Schubert 4
1 University of California, Santa Cruz, Santa Cruz, CA, United States of America, 2 Memorial University of
Newfoundland, Memorial University of Newfoundland, Canada, 3 Universität Basil, Basil, Switzerland, 4 Swiss
Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland (corresponding
author:mccarthy@pmc.ucsc.edu)
Most paleoceanographic studies of the N cycle
use the bulk � 15 N values (� 15 Nbulk) in sediments or
other paleoarchives as a proxy for � 15 N of surface
water nitrate, which assumes that � 15 Nbulk represents
integrated exported organic nitrogen. However,
� 15 Nbulk can be impacted by multiple factors, including
the addition of microbial biomass, trophic transfers,
and microbial reworking during degradation and early
diagenesis, all potentially varying based on local and
regional oceanic and depositional environments.
Here we describe first results in the development
of a fundamentally new approach toward
understanding � 15 N records, as well as nitrogen
source and transformation in organic matter paleoreservoirs:
compound-specific isotopic analyses of
amino acids (δ 15 N-AA). We hypothesize that aspects
of δ 15 N-AA patterns can be combined to deconvolute
the effects of trophic transfer, heterotrophic microbial
reworking, and autotrophic versus heterotrophic
sources on δ 15 N values, and potentially reconstruct
the average δ 15 N of exported primary production. We
present an overview of first δ 15 N-AA results for two
paleoarchives, ocean sediments and deep-sea
gorgonian corals.
The � 15 N-AA measurements in gorgonian corals
off the Nova Scotia shelf represents the first
application of this approach in a paleoarchive, and
demonstrates the potential to clarify previously
ambiguous bulk � 15 N records. In this region multiple
archives indicate a decline in � 15 Nbulk of exported
production over approximately the past century.
However, the � 15 Nbulk record inherently reflects both
source nitrate � 15 N, and also δ 15 N change due to
zooplankton trophic transfers. Therefore, at least two
different interpretations of existing � 15 Nbulk data are
possible: 1) a small shift in average trophic structure
(possibly driven by ecosystem shifts or climate
change), or 2) a shift in nitrate δ 15 N values,
associated with a shift in local current regimes. The
� 15 N-AA derived parameters ΣV and �Tr allowed us
for the first time to decouple these two factors. The
low degree of bacterial degradation indicated by the
ΣV parameter gives strong confidence in the integrity
of the δ 15 N-AA record in the coral protein skeleton.
The constant trophic structure indicated by the �Tr
parameter over the centennial record then indicates
that bulk δ 15 N trends likely are tied to physical shifts
in local current regimes (with their associated nitrate
� 15 N offsets), with timing suggestive of recent climate
change linkage.
We also recently completed the first sedimentary
� 15 N-AA measurements, from surface sediments in
the suboxic Santa Barbara Basin (SBB). We
analysed fluff layer and shallow intervals, to focus on
the transition from surface sources to sediments, in
the context of overlying plankton and sinking POM
samples
The basic � 15 N-AA patterns, as well as the
diagnostic Phe � 15 N values, appear preserved
essentially unchanged from plankton, through POM,
and into accumulating sediments. This indicates that
despite perhaps two orders of magnitude attenuation
in ON flux (expected from primary production to
surface sediment burial), the sedimentary � 15 N-AA
patterns show little change in individual isotopic
values, attributable to bacterial resynthesis. Certainly
bacterial degradation has been occurring, but the
molecular level data (and ΣV values) suggest that
preserved proteinaceous material in these samples is
composed of AA isotopically similar to their original
planktonic sources. The data ultimately suggests that
microbially resynthesis of AA (the process which
would alter � 15 N-AA patterns) has been minor. We
hypothesize finding similar preservation of � 15 N-AA
patterns with deeper burial.
Together, these first data from two paleoarchives
suggests that δ 15 N-AA can represent a powerful new
set of tools for understanding δ 15 Nbulk records of
organic matter in diverse paleoarchives.
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