Bioturbation in shelf/slope sediments off Cape Hatteras, North Carolina: the use of 234Th, Chl-a, and Br− to evaluate rates of particle and solute transport

Biogenic particle reworking (234Th, Chl-a), chloropigment distributions, and pore-water irrigation rates (Br− tracer) were examined in the continental shelf-break/upper-slope region off the North Carolina, Cape Hatteras coastline. Sediment cores were obtained along three primary east–west transects (water depth ∼75–800 m; 36°20′N, 35°50′N, 35°25′N), at additional shallow stations along 35°40′N, and at slope stations within a region of complex topography known as the Manteo Lease Block. Samples were collected during August 1994, July 1996, and August 1996, and were recovered using two shipboard techniques (Haps Corer and Box Corer) as well as by the deep submersible, Johnson Sea-Link. Natural and experimental tracer distributions demonstrate that with few exceptions surface deposits throughout this region are rapidly reworked and irrigated by abundant infaunal benthos. Excess 234Th (t1/2=24.1 days) was present at all stations, with surface activities (0–0.5 cm) ranging from ∼2 to 62 dpm cm−3 (5–54 dpm g−1), average inventories of ∼28±21 (median∼24) dpm cm−2, and typical penetration depths of 5–7 cm. Steady-state particle mixing coefficients (Db) estimated using excess 234Th ranged from ∼1 to 200 cm2 yr−1. Although the highest mixing intensities were found between ∼300 and 500 m water depths, rates were locally variable, and there was little or no evidence for any consistent attenuation with bathymetric depth in either 234Th inventories or mixing intensity. Estimates of Db made using Chl-a distributions are similar to those estimated using 234Th, ranging from ∼36 to 110 cm2 yr−1. Added Br− tracer penetrated >7 cm in ∼24 h periods in shipboard-incubated sediment cores, representing rates ranging from 1.5 to 38X molecular diffusion (mean=13.1±13.0; median ∼11X). Sedimentary Chl-a and phaeophytin-a distributions below the photic zone are indicative of high input of fresh planktonic debris and rapid remineralization. These inputs presumably fuel the abundant benthic fauna, which rework sediments on the Hatteras slope at some of the highest mixing rates yet reported.