Constraints on oceanic N balance/imbalance from sedimentary 15N records
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 colu...
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Published in | Biogeosciences Vol. 4; no. 1; pp. 75 - 86 |
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Main Author | |
Format | Journal Article |
Language | English |
Published |
European Geosciences Union
23.01.2007
Copernicus Publications |
Subjects | |
Online Access | Get full text |
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Summary: | 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 ?15N under steady-state conditions. Several factors (conversion of organic N to N2, Rayleigh closed and open system effects) likely reduce the effective fractionation factor (?) for water column denitrification to about half the inherent microbial value for ?den. If so, the average oceanic ?15N 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 ?15N would occur. Using a simple model, changing water column denitrification by ±30% or N2 fixation by ±15% produces detectable (>1?) changes in average oceanic ?15N over one residence time period or more with corresponding changes in oceanic N inventory. Changing sedimentary denitrification produces no change in ?15N but does change N inventory. Sediment ?15N records from sites thought to be sensitive to oceanic average ?15N 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 ?15N 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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ISSN: | 1726-4170 1726-4189 |