A study of recent laminated sediments, accumulated in the oxygen minimum zone of Mejillones Bay (Northern Chile), shows that the organic matter is autochthonous and deposited under varying oceanographic conditions. Sedimentary units dominated by light laminations have low values of total organic carbon, total nitrogen and sulphur, and Chaotoceros content, coupled with high values of yellow amorphous organic matter. These were deposited under conditions of low primary productivity, favoring the recycling of the major part of metabolizable organic matter in the water column, and the accumulation only of non-metabolizable organic matter in the bottom sediments. In contrast, a unit rich in dark laminations with high values of total organic carbon, total nitrogen and sulphur, shows high values of brown and black AOM, and Chaetoceros content. This sedimentary unit reflects increased flocculation of organic particles during a period of high productivity and fast transit of organic material through the column water, thus diminishing the recycling of metabolizable organic matter and its accumulation with non-metabolizable organic matter in the sediments. Finally, a unit characterized by presence of both light and dark laminations was formed by abrupt alternations of the two oceanographic states described previously. © 2004 Elsevier Ltd. All rights reserved.