Nitrogen isotope simulations show the importance of atmospheric iron deposition for nitrogen fixation across the Pacific Ocean

Abstract

Nitrogen (N) fixation by specialized microorganisms (diazotrophs) influences global plankton productivity because it provides the ocean with most of its bio-available N. However, its global rate and large-scale spatial distribution is still regarded with considerable uncertainty. Here we use a global ocean nitrogen isotope model, in comparison with δ 15 NO 3 - observations, to constrain the pattern of N 2 fixation across the Pacific Ocean. N 2 fixation introduces isotopically light atmospheric N 2 from to the ocean (δ 15 N = 0‰) relative to the oceanic average near 5‰, which makes nitrogen isotopes suitable to infer patterns of N 2 fixation. Including atmospheric iron limitation of diazotrophy in the model shifts the pattern of simulated N 2 fixation from the South Pacific to the North Pacific and from the East Pacific westward. These changes considerably improve the agreement with meridional transects of available δ 15 NO 3 - observations, as well as excess P (PO 4 3- - NO 3 - /16), suggesting that atmospheric iron deposition is indeed important for N fixation in the Pacific Ocean. This study highlights the potential for using δ 15 N observations and model simulations to constrain patterns and rates of N fixation in the ocean. © 2010 by the American Geophysical Union.