Peru upwelling region sediments near 15°S. 1. Remineralization and accumulation of organic matter1

Abstract

Six cores were obtained along a 70‐km transect perpendicular to the Peru coast in the highly productive upwelling region near 15°S, at depths ranging from 90 to 5,300 m. All of the sediments sampled were diatomaceous oozes. Three cores overlain by poorly oxygenated water had Thioploca‐like filamentous bacteria in surface sediments. Total organic carbon and total nitrogen contents of the sediments ranged from 0.8 to 10% and from 0.1 to 1% (dry wt). For the 90‐m and 268‐m cores, which had 210Pb surface sediment accumulation rates of about 0.6 and 1.1 cm · yr‒1, surface sediment organic carbon accumulation rates were 40 and 70 g C·m‒2·yr‒1. The organic carbon and total nitrogen distributions in the three cores from the oxygen minimum zone indicate that there have been variations in the rate of accumulation of organic matter over time. These variations may be related in part to the frequency and intensity of El Niño events.Porewater ammonium, nitrate, nitrite, total carbon dioxide, sulfate, and sulfide concentrations were measured. Remineralization rates in sediments calculated from the dissolved carbon dioxide profiles range from 0.6 to 20 g C·m‒2·yr‒1. Total carbon dioxide concentrations in porewaters of two oxygen‐minimum‐zone sediments were modeled using both steady state and nonsteady state distributions of metabolizable organic matter.