Variability in Particle Mixing Rates in Sediments with Polymetallic Nodules in the Equatorial Eastern Pacific as Determined from Measurements of Excess Pb-210

Abstract

Radionuclide activities of Pb-210 and Ra-226 were measured to determine bioturbation coefficients (D-b) in seven sediment cores from the Korean licensed block for polymetallic nodules in the Clarion-Clipperton Fracture Zone. Variability in D-b is considered in the context of the sedimentological, geochemical, and geotechnical properties of the sediments. D-b values in the studied cores were estimated using a steady-state diffusion model and varied over a wide range from 1.1 to 293 cm(2)/yr with corresponding mixing depths (L) of 26 to 144 cm. When excepting for spurious results obtained from cores where diffusive mixing does not apply, D-b values range from 1.1 to 9.0 cm(2)/yr with corresponding mixing depths (L) of 26 to 63 cm. Such wide variability in D-b and L values is exceptional in sites with water depths of similar to 5000 m and is attributed in this study to an uneven distribution of sediment layers with different shear strengths and total organic carbon (TOC) contents, caused by erosion events. The studied cores can be grouped into two categories based on lithologic associations: layers with high maximum shear strength (MSS) and low TOC content, showing a narrow range of D-b values (1.1-9.0 cm(2)/yr); and layers with low MSS and high TOC content, yielding much higher D-b values of over 30 cm(2)/yr. The distribution of different lithologies, and the resultant spatial variability in MSS and labile organic matter content, controls the presence and maximum burrowing depth of infauna by affecting their mobility and the availability of food. This study provides a unique case showing that shear strength, which relates to the degree of sediment consolidation, might be an important factor in controlling rates of bioturbation and sediment mixing depths.