Fine scale distributions of porosity and particulate excess ²¹⁰Pb, 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, excess ²¹⁰Pb 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, excess ²¹⁰Pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates. However, extremely high excess ²¹⁰Pb 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, excess ²¹⁰Pb-enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms.