25th International Meeting on Organic Geochemistry IMOG 2011

O-04
Insights about the marine nitrogen cycle from nitrogen isotopes
of sedimentary porphyrins
Meytal B. Higgins 1,2 , Ann Pearson 1
1 Harvard University, Cambridge, United States of America, 2 Princeton University, Princeton, United States of
America (corresponding author:pearson@eps.harvard.edu)
Episodes of deposition of organic-rich sediments
in the mid-Cretaceous known as Oceanic Anoxic
Events (OAEs) are attributed to high productivity
resulting from increases in nutrient supply, or to
enhanced organic matter preservation resulting from
decreases in the ventilation of deep waters.
Sediments from the largest of these events, the
Cenomanian-Turonian Oceanic Anoxic Event (OAE2)
are characterized by low � 15 N values not seen in
modern marine settings. It has remained a challenge
to describe a nitrogen cycle that could achieve such
isotopic depletion. Here we use � 15 N values of
porphyrins to propose a conceptual model for the
nitrogen cycle in anoxic oceans.
The offset between � 15 N values of sedimentary N
and coeval porphyrins is known as �por. Differences in
the 15 N offset between chlorophyll and biomass in
cyanobacteria and eukaryotes results in systematic
differences in �por that depend on the taxonomic
source of organic matter in sediments. Using the
respective endmember values for �por, we show that
eukaryotes contributed the quantitative majority of
export production throughout OAE2 (Fig. 1). The
relative export of cyanobacteria increased during the
OAE event but on average did not contribute more
than ~20% of the export flux of nitrogen.
Such data require that any explanation for the
OAE nitrogen cycle and its isotopic values also be
consistent with a eukaryote-dominated ecosystem.
Our results agree with models suggesting that OAEs
were supported by upwelling of nutrient-rich waters,
which in anoxic basins primarily would have contained
reduced N species (i.e., NH4 + ). We propose that new
production primarily was driven by direct NH4 +
assimilation supplemented by diazotrophy, while
chemocline denitrification and anammox quantitatively
consumed downwelling NO3 - and NO2 - . We present a
simple isotope balance model using known kinetic
isotope effects, which shows that the NH4 + reservoir
can be depleted in 15 N when NH4 + assimilation
exceeds nitrification.
In this model, only a small fraction of net
production may be fueled by cyanobacterial N
fixation. Concurrent with this interpretation, N fixation
alone cannot generate biomass with � 15 N values as
depleted as are seen in many Mesozoic OAE
sections. Instead, our data suggest only a modest
increase in cyanobacterial production. A small deficit
in fixed N during OAEs is not surprising, as anoxia
promotes denitrification. What may be surprising is
that the deficit was not larger. We suggest that
counter-intuitively, rates of denitrification may
decrease under conditions of extreme basin-wide
anoxia. Denitrification and anammox both depend on
sufficient availability of NO3 - and NO2 - . Because these
oxidized N species are produced aerobically, extreme
oxygen limitation in the water column may decrease
their rate of formation, leaving a greater fraction of
remineralized organic nitrogen to enter the photic
zone as NH4 + . This in turn would limit the need for
compensating N-fixation. Evidence for photic-zone
sulfide oxidation during OAEs shows that NO3 - indeed
was completely absent beneath the photic zone, at
least episodically (1, 2). We suggest that this negative
feedback on the N cycle limits both the extent of
denitrification and the expansion of cyanobacteria.
(1) Kuypers MMM et al. (2002) Paleoceanogr. 17, 13PP. (2)
Pancost RD et al. (2004) J. Geol. Soc. 161, 353-364.
62