We report the first data of nitrogen downhole variation in a 2-km-thick section of the ocean crust at DSDP/ODP Site 504. N concentrations in the volcanic section (275–856 mbsf) of Hole 504B are 0.73±0.13 ppm, similar to the N contents of average MORB. The lower sheeted dike complex (SDC) is characterized by significantly lower N contents of 0.29±0.17 ppm. We interpret this loss of N as a result of leaching processes at high temperatures. N data display a similar downhole trend as Zn, which is also depleted in the lower SDC. Therefore the lower SDC may be not only a source for base metals but also for nitrogen. Maximal estimates of the nitrogen flux through hydrothermal leaching, on the basis of the novel N data presented here, are 1.2 × 108 mol N2/yr. This is less than 8% of the estimated present N2 flux at mid-ocean ridges, indicating that hydrothermal leaching plays a minor, though not negligible, role in the geochemical cycle of nitrogen. The effect of addition of NH4+ through oxidative alteration of the upper volcanic sequences needs to be evaluated in our ongoing investigations.
Date of initial receipt: 23 August 1994
Date of acceptance: 14 March 1995
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