Constraints on oceanic N balance/imbalance from sedimentary <sup>15</sup>N records

Abstract

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 N 2 fixation consumed by water column denitrification that sets average oceanic δ 15 N under steady-state conditions. Several factors (conversion of organic N to N 2 , 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 δ 15 N of ∼5‰ is consistent with a canonical contribution from water column denitrification of 50% of the source flux from N 2 fixation. If an imbalance in oceanic N sources and sinks changes this proportion then a transient in average oceanic δ 15 N 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 δ 15 N over one residence time period or more with corresponding changes in oceanic N inventory. Changing sedimentary denitrification produces no change in δ 15 N but does change N inventory. Sediment δ 15 N records from sites thought to be sensitive to oceanic average δ 15 N 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 δ 15 N 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.Correspondence to: M. A. Altabet (maltabet@umassd.edu) 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.

The evolution of Earth's biogeochemical nitrogen cycle

“…Over the last 70 million years nitrogen isotope values in marine sediments have been relatively stable (+3.8 ± 1.9‰, Algeo et al, 2014), apart from short-term perturbations after the last glacial maximum in the Pleistocene (Altabet, 2007). These may have been caused by a large input of nitrogen from land during the continental glaciation (McElroy, 1983) and the oceans may still be recovering from this process (Christensen et al, 1987).…”

“…These long-term Phanerozoic nitrogen isotope trends are usually ascribed to climatic rather than redox fluctuations (Algeo et al, 2014;Altabet, 2007). According to this model, lower isotopic values (-2‰ to +2‰) occurred in greenhouse climates when sea-level was higher, such that the dominant locus of denitrification was ocean sediments rather than the water column.…”

Global rates of water-column denitrification derived from nitrogen gas measurements

“…Denitrification also occurs in suboxic pore waters of sediments throughout the ocean, but the rates are spatially heterogeneous and have been measured at only a handful of sites 15 , precluding a direct global estimate. However, the global rate of sedimentary N loss (primarily denitrification, but also a small burial of organic N) is constrained by the isotopic ratio of mean ocean nitrate to be one to four times as large as the rate of denitrification in the suboxic water column 6,16,17 . Estimates of the total rate of N loss in the ocean therefore inherit and amplify the uncertainty in the rate of denitrification in suboxic waters, yielding a combined rate of about 270 Tg N yr −1 (ref.…”

Recent advances in the biogeochemistry of nitrogen in the ocean

“…However, Deutsch et al (2004) pointed out that since the oceanic waters are not well mixed and denitrification and N 2 -fixation are dominant in well-defined and often different geographical areas, the dilution effect will lower this ratio to <3. Altabet (2007) further argues that due to other factors (conversion of organic N to N 2 , Rayleigh closed and open system effects) the effective discrimination factor for denitrification in the water column is only about half of its inherent microbial value, and so the sedimentary to water column ratio should be even lower. The application of this ratio in constraining the total oceanic denitrification thus seems to have limitations.…”

“…However, if the large imbalance between the losses and inputs estimated by Tg N yr −1 ) persists, it should show up in highresolution sedimentary δ 15 N records. Altabet (2007) uses a simple model to demonstrate that changing water column denitrification by ±30% or N 2 fixation by ±15% produces >1‰ shifts in average oceanic δ 15 N on the time scale of nitrogen turnover in the ocean. Altabet also examines high resolution δ 15 N records from several sites considered to be sensitive to oceanic average δ 15 N and finds no detectable change over the last 3000 years, implying a balanced marine nitrogen budget through the latest Holocene.…”