UNBIOTURBATED SEDIMENTS ON A MODERN MUD-DOMINATED SHELF: EVIDENCE OF HYPOXIA?

X-radiographs of sediment box cores acquired from the western Gulf of Mexico (GOM) reveal limited bioturbation in sediment deposited in bathymetries greater than 35 m. Between 15 and 35 m, sediments are thoroughly bioturbated, with averaged bioturbation indices (for all beds in a core) between 2.1 and 5.6, and trace diversities between 2 and 9 distinct burrow forms. Below 35 m water depth, box cores exhibit trace diversities of 1–3 and core-averaged bioturbation indices range between 0.3 and 3.6. There is an overall decrease in trace diversity and bioturbation indices in the offshore direction.

Cross-shore ichnological trends are compared to dissolved oxygen contents of bottom waters. Dissolved oxygen contents of bottom water decreases by an average of 0.117 mg l^-1 per one-meter increase in water depth, such that bottom waters in 100 m water depth contain an average of 4.55 mg l^-1. Above 35 m, oxygen contents show pronounced variability ranging from 100% O₂ saturation through to hypoxia (< 2.0 mg l^-1), and reflect the periodic introduction of hypoxic waters during June-July ocean hypoxia events. Below bathymetries of 35 m, the oxygen content of bottom waters is consistently at 60-75% oxygen saturation of GOM seawater, and decreases slightly offshore.

Although the present dataset is limited, there is a direct correlation between: a) the density of infauna and the diversity and density of burrows, and b) dissolved oxygen concentrations of bottom water. These trends indicate that the degree of bioturbation is significantly reduced in waters that are oxic but below 80% O₂ saturation. Based on these observations, we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia. Instead, reduced oxygen contents, but well above hypoxia, have a dramatic impact on the health of infaunal communities, which is reflected by severe reductions in the ichnological character of the sediments.