Biogeochemistry of carbon in continental slope sediments: The North Carolina211 margin

摘要

The responses of the continental slope benthos to organic detritus deposition211u001ewere studied with a multiple trace approach. Study sites were offshore of Cape 211u001eFear (I) and Cape Hatteras (III), N.C. (both 850 m water depth) and were 211u001echaracterized by different organic C deposition rates, macrofaunal densities and 211u001etaxa. Natural abundances of (sup 13)C and (sup 12)C in particulate organic carbon 211u001e(POC), dissolved inorganic carbon (DIC) and macrofauna indicate that the reactive 211u001eorganic detritus is marine in origin. Natural abundance levels of (sup 14)C and 211u001euptake of (sup 13)C-labeled diatoms by benthic animals indicate that they 211u001eincorporate a relatively young component of carbon into their biomass. (sup 13)C-211u001elabeled diatoms (Thalassiorsira pseudonana) tagged with (sup 210)Pb, slope 211u001esediment tagged with (sup 113)Sn and (sup 228)Th-labeled glass beads were 211u001eemplaced in plots on the seafloor at both locations and the plots were sampled 211u001eafter 30 min., 1-1.5 d and 14 mo. At Site I, tracer diatom was intercepted at the 211u001esurface primarily by protozoans and surface-feeding annelids. Little of the 211u001ediatom C penetrated below 2 cm even after 14 months. Oxidation of organic carbon 211u001eappeared to be largely aerobic. At Site III, annelids were primarily responsible 211u001efor the initial uptake of tracer. On the time scale of days, diatom C was 211u001etransported to a depth of 12 cm and was found in animals collected between 5-10 211u001ecm. The hoeing of tracer from the surface by the maldanid Praxillela sp. may have 211u001ebeen responsible for some of the rapid nonlocal transport. Oxidation of the 211u001ediatom organic carbon was evident to at least 10 cm depth. Anaerobic breakdown of 211u001eorganic matter is more important at Site III. Horizontal transport, which was 211u001eprobably biologically mediated, was an order of magnitude more rapid than 211u001evertical displacement over a year time scale. If the horizontal transport was 211u001eassociated with biochemical transformations of the organic matter, it may 211u001erepresent an important but nearly invisible diagenetic process.