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

Abstract

Sediment accumulation rates and biological mixing intensities were determined at three sites on the North Carolina slope based on profiles of naturally occurring14C,210Pb and234Th. 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 Corg 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 Corg accumulation rate to Corg deposition rate times 100) was estimated to vary from 6.0% to 54% to 88% at Sites I, II and III, respectively. The14C 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 excess234Th (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 mean234Th 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 excess234Th activity reveal that the biological mixing intensity is greater at Site III (meanDb = 19 ± 11cm2year−1, n = 5) than at either Site I (meanDb = 6.0 ± 6.2cm2year−1, n = 6) or Site II (meanDb = 4.6 ± 5.2cm2year−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 (R2 = 0.99, p = 0.06) exists between macrofaunal abundance and the organic carbon deposition rate. An equally strong correlation (R2 = 0.99, P = 0.04) occurs between macrofaunal abundance and the234Th inventories (index of 100-day particle flux). Fine-sand size glass tracer beads were dispersed at these three sites by submersible and then the field plots were sampled ≈1 year later. The vertical distributions of beads at the three sites are consistent with a higher mixing intensity at Site III than at the other two sites, but the estimated mixing coefficients generally are lower than those determined from the profiles of234Th. The slower mixing of the glass beads may be the result of their low food value and/or their relatively large size (compared to the surrounding fine-grained sediments).