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

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion EGU 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 5 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 10 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 denitrifi-cation ±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 15 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 20 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 25 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 …