Composition of organic matter in subducted and unsubducted sediments off the Nicoya peninsula, Costa Rica (ODP Leg 170, Sites 1039 and 1040)

Investigation of sediment samples from Sites 1039 and 1040 (ODP Leg 170) dr illed off the Nicoya peninsula (Costa Rica) by organic geochemical and orga nic petrological methods has revealed that subduction has only little influ ence on the composition of the sedimentary organic matter. Organic carbon c ontents reached 1.5% in the Pleistocene samples but Miocene and Pliocene se diments had an average organic carbon content of less than 0.5%. Organic ca rbon/sulfur ratios are generally below 2.8, reflecting an intense sulfate r eduction in the uppermost sediments which was enhanced by sulfate supply bo th from sea water and deeper strata, Microscopical examinations indicate th at the organic matter is mainly derived from marine sources. A small amount of terrigenous organic matter is, however, present as well according to n- alkane and Fatty acid distributions. The alkenone unsaturation index U-37(k ') shows only a slight decrease during the Miocene and Pliocene, and strong er variations in the Pleistocene, probably indicating more stable sea surfa ce temperatures during the Miocene and Pliocene. Variations in the Pleistoc ene can possibly be related to glacial/interglacial changes. (C) 2000 Elsev ier Science Ltd. All rights reserved.