In situ tracer experiments investigated short-term sediment mixing processe
s at two Carolina continental margin sites (water depth = 850 m) characteri
zed by different organic C fluxes, Th-234 mixing coefficients (D-b) and ben
thic assemblages. Phytoplankton, slope sediment, and sand-sized glass beads
tagged with Pb-210, Sn-113, and Th-228, respectively, were placed via subm
ersible at the sediment-water interface at both field sites (Site I off Cap
e Fear, and Site m off Cape Hatteras). Experimental plots were sampled at 0
, 1.5 days, and 90 days after tracer emplacement to examine short-term, ver
tical transport. Both sites are initially dominated by nonlocal mixing. Tra
nsport to the bottom of the surface mixed layer at both sites occurs more r
apidly than Th-234-based D-b values predict; after 1.5 days, tagged particl
es were observed 5 cm below the sediment-water interface at Site I and 12 c
m below at Site III. Impulse tracer profiles after 90 days at Site m exhibi
t primarily diffusive distributions, most likely due to a large number of r
andom, nonlocal mixing events. The D-b values determined from 90-day partic
le tagging experiments are comparable to those obtained from naturally occu
rring Th-234 profiles (similar to 100-day time scales) from nearby location
s. The agreement between impulse tracer mixing coefficients and steady-stat
e natural tracer mixing coefficients suggests that the diffusive analogue f
or bioturbation on monthly time scales is a realistic and useful approach.
Tracer profiles from both sites exhibit some degree of particle selective m
ixing, but the preferential transport of the more labile carbon containing
particles only occurred 30% of the time. Consequently, variations in the ex
tent to which age-dependent mixing occurs in marine sediments may depend on
factors such as faunal assemblage and organic carbon flux.