Variations in bioturbation across the oxygen minimum zone in the northwestArabian Sea

Oxygen minimum zones are expected to alter substantially the nature, rates and depths of bioturbation along continental margins, yet these effects rem ain poorly studied. Using excess Pb-210 profiles, sediment X-radiography an d box-core samples for macrofauna, we examined bioturbation processes at si x stations (400, 700, 850, 1000, 1250 and 3400 m deep) along a transect acr oss the oxygen minimum zone (OMZ) on the Oman margin. Bottom-water oxygen c oncentrations ranged from similar to 0.13 mi l(-1) at 400 m to similar to 2 .99 mi l(-1) at 3400 m. Pb-210 mixed-layer depth and bioturbation intensity (D-b) exhibited high within-station variance, and means did not differ sig nificantly among stations. However, the mean mixed-layer depth (4.6 cm) for pooled OMZ stations (400-1000 m depths, 0.13-0.27 mi l(-1) bottom-water ox ygen) was half that for stations from similar water depths along well-oxyge nated Atlantic and Pacific slopes (11.1 cm), suggesting that oxygen stress reduced Pb-210 mixing depth on the Oman margin. Modal burrow diameter and t he diversity of burrow types at a station were highly correlated with botto m-water oxygen concentration from the edge to the core of the Oman OMZ (Spe arman's rho greater than or equal to 0.89, p less than or equal to 0.02), s uggesting that these parameters are useful proxies for bottom-water oxygen concentrations under dysaerobic conditions. In contrast, neither the maximu m diameter and nor the maximum penetration depth of open burrows exhibited oxygen-related patterns along the transect. Reduced Pb-210 mixing depth wit hin the Oman-margin OMZ appeared to result from a predominance of surface-d eposit feeders and tube builders within this zone, rather than from simple changes in horizontal or vertical distributions of macrofaunal abundance or biomass. The number of burrow types per station was highly correlated with macrofaunal species diversity, suggesting that burrow diversity may be a g ood proxy for species diversity in paleo-dysaerobic assemblages. We conclud e that bottom-water oxygen concentrations of 0.13-0.27 mi l(-1) substantial ly alter a number of bioturbation parameters of importance to diagenetic an d biofacies models for continental margins. (C) 1999 Elsevier Science Ltd. All rights reserved.