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

Abstract. 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 ε_den. 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.