Rapid bioturbation in equatorial Pacific sediments: evidence from excess 234Th measurements

Abstract

In order to evaluate bioturbation in equatorial Pacific sediments, profiles of excess 234Th were measured in cores collected during November and December 1992, at six abyssal sites along the JGOFS Equatorial Pacific 140°W transect, from 5°S to 9°N. Excess 234Th (half-life=24.1 days) was detected at all sites. Mean inventories ranged from a low of 0.25 dpm cm −2 at 9°N, to a high of 2.6 dpm cm −2 at 2°N. Eddy-diffusive models of the excess 234Th profiles suggest that, on 100-day timescales, particle mixing is slow at 5°S and 2°S (eddy diffusion coefficient, D b , < 0.4 cm 2 year −1), faster at 0° and 2°N ( D b =0.34–4.8 cm 2 year −1), and fastest at 5°N ( D b =4.7−11 cm 2 year −1). At 9°N, 234Th is found almost entirely in the upper 5 mm of the seabed, and D b values are <0.1 cm 2 year −1. D b values generally increase as POC flux increases, although the linear relationship ( r 2=0.334, p=0.365) displays considerable scatter. However, if the only sites included in the regression are those where 234Th profiles were measured in at least two core sets (i.e. 0°, 2°N, 5°N and 9°N), the linear regression improves substantially ( r 2=0.754, p=0.0012). These results suggest that measurements on a single core may not fully characterize the sediment mixing regime in a particular region, and highlight the need for multiple samples from an individual site. Thorium-234- D b values at 5°S, 2°S and 9°N are in rough agreement with published D b values calculated from the profiles of intermediate half-life radioisotopes, such as 210Pb and 137Cs, in open-ocean abyssal sediments. However, at 0°, 2°N and 5°N, 234Th- D b values are as much as an order of magnitude greater than published 210Pb- D b values. This phenomenon has been observed in other marine environments and may result from age-dependent mixing, non-steady-state mixing or a combination of the two processes. Particle mixing in cores collected at 5°S and 5°N appears to be best described as a largely eddy diffusive process. However, in cores collected at 2°S, 0°, 2°N and 9°N, non-local mixing processes appear to affect 15–30% of the excess 234Th inventory. At these sites, non-local mixing appears to transport recently-deposited material at least 3–4 cm below the sediment-water interface within 100 days of its arrival at the seafloor. The combination of rapid biodiffusive mixing and non-local transport allows recently-deposited, reactive biogenic debris to be rapidly mixed into the seabed. This may affect a wide range of diagenetic processes, including oxygen consumption, dissolution of biogenic debris and organic carbon remineralization. Models of early diagenesis in deep-sea sediments may need to incorporate both non-diffusive transport terms and rapid biodiffusive mixing of recently-deposited particles.