17. DATA REPORT: NITROGEN ISOTOPIC COMPOSITION OF PORE-WATER AMMONIUM,
BLAKE RIDGE, SITE 997¹

Walter S. Borowski^{2, 3} and Charles K. Paull^{2, 4}

INTRODUCTION

Ammonium (NH₄⁺) concentration profiles in piston-core sediments of the Carolina Rise and Blake Ridge generally have linear concentration profiles within the sulfate reduction zone (Borowski, 1998). Deep Sea Drilling Project (DSDP) Site 533, located on the Blake Ridge, also displayed a linear ammonium concentration profile through the sulfate reduction zone and the profile linearity continues into the upper methanogenic zone to a depth of ~200 meters below seafloor (mbsf), where the first methane gas hydrates probably occur (Jenden and Gieskes, 1983; Kvenvolden and Barnard, 1983). Sediments from the Ocean Drilling Program (ODP) Leg 164 deep holes (Sites 994, 995, and 997) also exhibit linear ammonium profiles above the top of the gas hydrate zone (~200 mbsf) (Paull, Matsumoto, Wallace, et al., 1996).

We hypothesized that a possible cause of linear ammonium profiles was diffusion of ammonium from a concentrated ammonium source at depth. We further reasoned that if this ammonium were produced by microbial fermentation reactions at depth, that a comparison of the nitrogen isotopic composition of sedimentary organic nitrogen and the nitrogen with pore-water ammonium would test this hypothesis. Convergence with depth of ¹⁵N values of the nitrogen source (sedimentary organic matter) and the nitrogen product (dissolved NH₄⁺) would strongly suggest that ammonium was produced within a particular depth zone by microbial fermentation reactions.

Here, we report ¹⁵N values of pore-water ammonium from selected interstitial water (IW) samples from Site 997, sampled during ODP Leg 164.

¹Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
²Geology Department, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3315, U.S.A.
³Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-47778, U.S.A. borowski@lcc.net
⁴Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, U.S.A.

Date of initial receipt: 21 April 1998
Date of acceptance: 13 January 1999
Ms 164SR-213