Constraints on oceanic N balance/imbalance from sedimentary ¹⁵N records

• Main Author: M. A. Altabet
• Format: Article
• Language: English
• Published: Copernicus Publications 2007-01-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/4/75/2007/bg-4-75-2007.pdf
• ISSN: 1726-4170; 1726-4189

According to current best estimates, the modern ocean&apos;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 &delta;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 (ε) for water column denitrification to about half the inherent microbial value for &epsilon;_den. If so, the average oceanic &delta;¹⁵N of ~5&permil; 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 &delta;¹⁵N would occur. Using a simple model, changing water column denitrification by &plusmn;30% or N₂ fixation by &plusmn;15% produces detectable (&gt;1&permil;) changes in average oceanic &delta;¹⁵N over one residence time period or more with corresponding changes in oceanic N inventory. Changing sedimentary denitrification produces no change in &delta;¹⁵N but does change N inventory. <br><br> Sediment &delta;¹⁵N records from sites thought to be sensitive to oceanic average &delta;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 &delta;¹⁵N records from the literature covering the period since the last glacial maximum show excursions of up to several &permil; 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.