(Table 1) Stable nitrogen isotopes of ammonium in interstitial waters from ODP Site 164-997

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, doi:10.2973/dsdp.proc.76.114.1983; Kvenvolden and Barnard, 1983, doi:10.2973/dsdp.proc.76.106.1983). 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, doi:10.2973/odp.proc.ir.164.1996).
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 d15N 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 d15N values of pore-water ammonium from selected interstitial water (IW) samples from Site 997, sampled during ODP Leg 164.

卡罗莱纳海隆（Carolina Rise）与布雷克海岭（Blake Ridge）的活塞岩芯沉积物中，铵离子（NH₄⁺）浓度剖面在硫酸盐还原带内通常呈线性分布（Borowski, 1998）。位于布雷克海岭的深海钻探计划（Deep Sea Drilling Project, DSDP）533站位，其硫酸盐还原带内的铵离子浓度剖面同样呈线性，且该线性特征延伸至产甲烷带上部，直至海底以下约200米（mbsf）处——该深度大概率为首个甲烷水合物赋存位置（Jenden & Gieskes, 1983, doi:10.2973/dsdp.proc.76.114.1983; Kvenvolden & Barnard, 1983, doi:10.2973/dsdp.proc.76.106.1983）。大洋钻探计划（Ocean Drilling Program, ODP）第164航次深孔（站位994、995、997）的沉积物，在天然气水合物带顶部（约200 mbsf）以上同样呈现线性铵离子浓度剖面（Paull, Matsumoto, Wallace et al., 1996, doi:10.2973/odp.proc.ir.164.1996）。

我们提出假说：铵离子浓度剖面呈线性的可能成因，是铵离子从深部富铵源发生扩散。我们进一步推断，若该铵离子由深部微生物发酵反应生成，则可通过对比沉积有机氮的氮同位素组成与孔隙水中铵离子的氮同位素组成，验证这一假说。若作为氮源的沉积有机质与作为氮产物的溶解态铵离子的δ¹⁵N值随深度趋同，则可有力证明铵离子是在特定深度带内由微生物发酵反应生成的。本文报道了大洋钻探计划第164航次期间采集的997站位部分间隙水（interstitial water, IW）样品中孔隙水铵离子的δ¹⁵N值。