Organic matter diagenesis in sediments on the continental shelf and slope of the Eastern Tropical and temperate North Pacific

Abstract

Carbon and nitrogen mineralization were determined in sediments from about 100 to 1000 m depth on the continental shelf and slope off the Pacific coast of Mexico and Washington State. Vertical down core profiles of carbon mineralization were measured both as sulfate reduction using short-term 35 S incubations and as net TCO 2 production using long-term anaerobic incubations (`jar’ technique). Nitrogen mineralization was measured as net NH + 4 production using only the latter technique. The down core attenuation of jar rates was described adequately by a double exponential decay model. By subtraction of 35 S rates from jar rates in the upper 3 cm of the sediment, oxic plus suboxic respiration were roughly estimated to account for at least 5–28% (Mexico) and 10–19% (Washington) of the 0–30 cm integrated carbon mineralization. The agreement between measured porewater profiles of TCO 2 and NH + 4 and those estimated by diagenetic modelling using rates obtained from jar incubations in the oxygen deficient zone off Mexico indicated no infaunal irrigation. At all other stations the benthic fauna appeared responsible for considerably lower measured porewater concentrations than predicted by a simple diffusion-based diagenetic model. Total carbon mineralization in sediments off Mexico was reduced 40% with water depth from 100–200 m to 1000 m, while the reduction in the same depth interval off Washington was about 80%. Water depth attenuation of total sediment nitrogen mineralization at the two locations, on the other hand, was almost similar with a reduction of 70–80%. The overall molar C : N ratio of mineralization products was generally higher off Washington than in the oxygen deficient zone off Mexico. It is suggested that the difference in sediment mineralization patterns are associated with changes in water column processes related to the different strength of the oxygen minimum at the two locations.