Biological mixing intensity and rates of organic carbon accumulation in North Carolina slope sediments

Sediment accumulation rates and biological mixing intensities were determined at three sites on the North Carolina slope based on profiles of naturally occurring 14C, 210Pb and 234Th. The three sites all were at a water depth of 850 m with a spacing of 150–180 km between sites. Sediment accumulation rates increase from south to north from values of 7 cm ky −1 at Site I, to 160 cm ky −1 at Site II, to 1100 cm ky −1 at Site III. The organic carbon burial rate at these sites also increases in the northward direction from 0.65 (Site I) to 20 (Site II) to ⪕150 g C org m −2 year −1 (Site III). These data indicate that continental margin environment can exhibit highly variable carbon fluxes over relatively small distances on the seafloor. The rate of organic carbon accumulation at Site III is one of the highest values reported for the marine environment. Based on these accumulation rates and dissolved inorganic carbon flux estimates from each site, the seabed organic carbon preservation efficiency (i.e. the ratio of C org accumulation rate to C org deposition rate times 100) was estimated to vary from 6.0% to 54% to 88% at Sites I, II and III, respectively. The 14C age of organic matter in surface sediments was older at Site III (1800 years BP) than at Sites I and II (800 years BP), indicating that Site III receives a greater proportion of old sediment from either up-slope areas or from terrigenous sources. Inventories of excess 234Th (half-life of 24 days) were used as a tracer for particle flux covering the 100 days prior to the October 1989, July–August 1990 and August 1991 cruises. The mean 234Th inventories at the three sites were 4.7 ± 1.9, 8.4 ± 6.3and23.1 ± 7.3dpm cm −2 for Sites I, II and III, respectively. Profiles of excess 234Th activity reveal that the biological mixing intensity is greater at Site III (mean D b = 19 ± 11cm 2year −1, n = 5) than at either Site I (mean D b = 6.0 ± 6.2cm 2year −1, n = 6) or Site II (mean D b = 4.6 ± 5.2cm 2year −1, n = 9). In addition to the trend in mixing coefficients, the depth of particle mixing on a 100-day time scale generally is greater at Site III than at the other two sites. These observations of particle mixing intensity are consistent with the northward increase in the mean abundance of macrofauna (300microns) from mean values of 9400 m −2 at Site I, to 21,400 m −2 at Site II, to 55,500 m −2 at Site III. For the three study sites off North Carolina, a strong correlation ( R 2 = 0.99, p = 0.06) exists between