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| Titel |
Constraints on oceanic N balance/imbalance from sedimentary 15N records |
| VerfasserIn |
M. A. Altabet |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 4, no. 1 ; Nr. 4, no. 1 (2007-01-23), S.75-86 |
| Datensatznummer |
250001500
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| Publikation (Nr.) |
 copernicus.org/bg-4-75-2007.pdf |
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| Zusammenfassung |
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 N2 fixation consumed by water column
denitrification that sets average oceanic δ15N under
steady-state conditions. Several factors (conversion of organic N to
N2, 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 εden.
If so, the average oceanic δ15N of ~5‰
is consistent with a canonical contribution from water column
denitrification of 50% of the source flux from N2 fixation. If an
imbalance in oceanic N sources and sinks changes this proportion then a
transient in average oceanic δ15N would occur. Using a simple
model, changing water column denitrification by ±30% or N2
fixation by ±15% produces detectable (>1‰) changes in average
oceanic δ15N over one residence time period or more with
corresponding changes in oceanic N inventory. Changing sedimentary
denitrification produces no change in δ15N but does change N
inventory.
Sediment δ15N records from sites thought to be sensitive to
oceanic average δ15N 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 δ15N 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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