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 N-2 fixation consumed by water column denitrification that sets average oceanic delta N-15 under steady-state conditions. Several factors ( conversion of organic N to N2, Rayleigh closed and open system effects) likely reduce the effective fractionation factor (epsilon) for water column denitrification to about half the inherent microbial value for epsilon(den). If so, the average oceanic delta N-15 of similar to 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 delta N-15 would occur. Using a simple model, changing water column denitrification by +/- 30% or N-2 fixation by +/- 15% produces detectable (> 1%) changes in average oceanic delta N-15 over one residence time period or more with corresponding changes in oceanic N inventory. Changing sedimentary denitrification produces no change in delta N-15 but does change N inventory. Sediment delta N-15 records from sites thought to be sensitive to oceanic average delta N-15 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 N-15 records from the literature covering the period since the last glacial maximum show excursions of up to several parts per thousand 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.