Biogeosciences
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2007
10.5194/bg-4-75-2007
http://www.biogeosciences.net/4/75/2007/
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http://www.biogeosciences.net/4/75/2007/bg-4-75-2007.pdf
75
86
2007-01-23
Constraints on oceanic N balance/imbalance from sedimentary <sup>15</sup>N records
M. A. Altabet
School for Marine Science and Technology, U. Massachusetts Dartmouth, New Bedford, USA
According to current best estimates, the modern ocean's N cycle is in severe
deficit. N isotope budgeting provides an independent geochemical constraint
in this regard as well as the only means for past reconstruction. Overall,
it is the relative proportion of N<sub>2</sub> fixation consumed by water column
denitrification that sets average oceanic δ<sup>15</sup>N under
steady-state conditions. Several factors (conversion of organic N to
N<sub>2</sub>, Rayleigh closed and open system effects) likely reduce the
effective fractionation factor (ε) for water column
denitrification to about half the inherent microbial value for ε<sub>den</sub>.
If so, the average oceanic δ<sup>15</sup>N of ~5‰
is consistent with a canonical contribution from water column
denitrification of 50% of the source flux from N<sub>2</sub> fixation. If an
imbalance in oceanic N sources and sinks changes this proportion then a
transient in average oceanic δ<sup>15</sup>N would occur. Using a simple
model, changing water column denitrification by ±30% or N<sub>2</sub>
fixation by ±15% produces detectable (>1‰) changes in average
oceanic δ<sup>15</sup>N over one residence time period or more with
corresponding changes in oceanic N inventory. Changing sedimentary
denitrification produces no change in δ<sup>15</sup>N but does change N
inventory.
<br><br>
Sediment δ<sup>15</sup>N records from sites thought to be sensitive to
oceanic average δ<sup>15</sup>N all show no detectible change over the
last 3 kyr or so implying a balanced marine N budget over the latest
Holocene. A mismatch in time scales is the most likely meaningful
interpretation of the apparent conflict with modern flux estimates. Decadal
to centennial scale oscillations between net N deficit and net surplus may
occur but on the N residence timescale of several thousand years, net
balance is achieved in sum. However, sediment δ<sup>15</sup>N records from
the literature covering the period since the last glacial maximum show
excursions of up to several ‰ that are consistent with sustained N deficit
during the deglaciation followed by readjustment and establishment of
balance in the early Holocene. Since imbalance was sustained for one N
residence time period or longer, excursions in ocean N inventory of 10 to
30% likely occurred. The climatic and oceanographic changes that occurred
over this period evidently overcame, for a time, the capacity of ocean
biogeochemistry to maintain N balance.
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