INTRODUCTION

The organic matter contents of marine sediments are influenced by a variety of processes. Concentrations of organic matter reflect delivery rates from marine and continental sources, sediment accumulation rates, and post-depositional degradation rates. The character of sedimentary organic matter is controlled by its biotic sources, transport routes, and degree of preservation. Because the amount and type of organic matter are affected by these various oceanographic processes, much information about oceanic conditions of the past can be inferred from the organic matter contents of sediment cores. Reliable determination of the concentration of sedimentary organic matter is therefore a fundamental prerequisite for many paleoceanographic studies.

Different analytical procedures, however, potentially lead to different values for measurements of organic carbon concentration (Arthur et al., 1987). It is therefore not wise to use results from different procedures uncritically when comparing organic matter characterizations from various settings. The method used onboard the JOIDES Resolution during Leg 149 estimates organic carbon concentrations indirectly as the difference between total and carbonate carbon concentrations. Small errors in either the total or the carbonate carbon measurement can become magnified in the organic carbon estimate, particularly where organic matter levels are low. We have investigated how important this potential source of analytical error might be in estimating the concentrations of organic matter by comparing shipboard results with those of direct measurements of organic carbon in the same samples. In addition, we have employed organic ¹³C determinations to confirm preliminary identifications of organic matter sources based on organic C/N ratios. The results, from a Pliocene-Pleistocene turbiditic unit, are discussed in their paleoceanographic context.