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

In order to evaluate bioturbation in equatorial Pacific sediments, profiles of excess Th-234 were measured in cores collected during November and December 1992, at six abyssal sites along the JGOFS Equatorial Pacific 140 degrees W transect, from 5 degrees S to 9 degrees N. Excess Th-234 (half-life = 24.1 days) was detected at all sites. Mean inventories ranged from a low of 0.25 dpm cm(-2) at 9 degrees N, to a high of 2.6 dpm cm(-2) at 2 degrees N. Eddy-diffusive models of the excess Th-234 profiles suggest that, on 100-day timescales, particle mixing is slow at 5 degrees S and 2 degrees S (eddy diffusion coefficient, D-b, < 0.4 cm(2) year(-1)), faster at 0 degrees and 2 degrees N (D-b = 0.34-4.8 cm(2) year(-1)), and fastest at 5 degrees N (D-b = 4.7-11 cm(2) year(-1)). At 9 degrees N, Th-234 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 Th-234 profiles were measured in at least two core sets (i.e. 0 degrees, 2 degrees N, 5 degrees N and 9 degrees 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-h values at 5 degrees S, 2 degrees S and 9 degrees N are in rough agreement with published D-b values calculated from the profiles of intermediate half-life radioisotopes, such as Pb-210 and Cs-137, in open-ocean abyssal sediments. However, at 0 degrees, 2 degrees N and 5 degrees N, Th-234-D-b values are as much as an order of magnitude greater than published Pb-210-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 degrees S and 5 degrees N appears to be best described as a largely eddy diffusive process. However, in cores collected at 2 degrees S, 0 degrees, 2 degrees N and 9 degrees N, non-local mixing processes appear to affect 15-30% of the excess Th-234 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. Copyright (C) 1996 Elsevier Science Ltd.