Biogenic N2 and δ15 N-N2 As Proxies for N-Loss in the Eastern Tropical North Pacific: A Lagrangian Float Experiment.

Abstract:

A large portion of the ocean’s bioavailable N, a macronutrient limiting primary productivity, is lost in oxygen minimum zones (OMZs). Mesoscale processes (e.g. eddies, meandering currents), can transport highly productive waters from the coasts, increasing the downward flux of organic material, a substrate for N-loss, and thus can act as N-loss hotspots in OMZs. However, due to their transient nature, these mesoscale events are difficult to monitor using traditional shipboard observations. We deployed biogeochemical Lagrangian floats in the eastern tropical North Pacific during a research cruise in May/June 2014, where transport of high chlorophyll waters from the coast were inferred from satellite imagery. These Lagrangian floats are automonous platforms with the ability to follow isopycnals and were equipped with a suite of gas tension devices and other sensors to measure N₂(g), O₂, NO₃^- and NO₂^- concentrations. We concurrently collected discrete samples to calibrate and complement float measurements. We present here biogenic N₂, i.e. N₂ produced by local N-loss processes and derived from measured N₂/Ar and δ¹⁵N-N₂ anomaly, i.e. the difference between δ¹⁵N-N₂ observed and at equilibrium for in-situ temperature and salinity, during a ~4 weeks Lagrangian experiment. During N-loss, the product (N₂) is depleted in ¹⁵N because of kinetic isotope fractionation. While biogenic N₂ only reached up to ~10 µmol/kg, the δ¹⁵N-N₂ anomalies were relatively low (down to ~-0.4‰). The δ¹⁵N-N₂ anomalies are low compared to values always >-0.1‰) for equivalent biogenic N₂ in the OMZ of the eastern tropical South Pacific. We will discuss the implication of these results for the global oceanic N budget.