Fine scale distributions of porosity and particulate excessPb, organic carbon and CaCO3 in surface sediments of the deep equatorial Pacific

Sediment samples were recovered from the central equatorial Pacific Ocean, sectioned at 1-mm intervals, and analyzed for porosity, organic carbon, excess²¹⁰Pb and CaCO₃. Steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface. Similarly, particulate organic carbon contents are highest at the sediment surface, decrease sharply in the upper 1 cm, and are relatively constant between 1 and 5 cm. CaCO₃ values, on the other hand, are lowest at the sediment surface and increase to a constant value below 5–10 mm depth. At the carbonate ooze sites, excess²¹⁰Pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates. However, extremely high excess²¹⁰Pb activities (> 100 dpm/g) are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing. This apparent contradiction along with the major features of the CaCO₃ and particulate organic carbon profiles can be explained by a particle-selective feeding mechanism in which organic carbon, excess²¹⁰Pb-enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms.