--> Are Unbioturbated Mudstones Indicative of Anoxia?

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Are Unbioturbated Mudstones Indicative of Anoxia?

Abstract

X-radiographs of sediment box cores acquired from the western Gulf of Mexico reveal limited bioturbation in sediment deposited at water depths 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 intensity in the offshore direction. Cross-shore ichnological trends are compared to dissolved oxygen (DO) contents of bottom waters. Dissolved oxygen decreases by an average of 0.117 mg/l per one-meter increase in water depth, such that bottom waters in 100 m water depth contain an average of 4.55 mg/l oxygen. Above 35 m, DO content shows pronounced variability ranging from 100% O2 saturation through to hypoxia (DO < 2.0 mg/l), and reflect the periodic introduction of hypoxic waters during June-July ocean hypoxia events. Below bathymetries of 35 m, the DO contents of bottom waters are consistently 60–75% oxygen saturation of Gulf of Mexico seawater, and oxygen concentrations decrease 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) DO 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 — the low bioturbation intensities and diversities do not reflect hypoxia or anoxia. 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. Based on these results, we propose that unbioturbated and under-bioturbated marine mudstones and shales may simply reflect reduced DO concentrations of bottom water rather than anoxia in the paleoenvironment.