Unbioturbated sediments on a muddy shelf: Hypoxia or simply reduced oxygen saturation?
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| Authors: | Dashtgard, SE; Snedden, JW; MacEachern, JA |
| Year: | 2015 |
| Journal: | Paleogeogr. Paleoclimatol. Paleoecol. 425: 128-138 Article Link (DOI) |
| Title: | Unbioturbated sediments on a muddy shelf: Hypoxia or simply reduced oxygen saturation? |
| Abstract: | X-radiographs of sediment box cores acquired from the western Gulf of Mexico 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. Above 35 m, dissolved oxygen (DO) contents show pronounced variability, ranging from 100% DO saturation through to hypoxia (<2.0 mg l(-1)), and reflect the periodic introduction of oxygen-depleted waters into otherwise fully oxygensaturated seawater. Below 35 m, DO content of bottom waters is consistently at 60-75% saturation. DO 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) DO. The data reveal a direct correlation between: a) the density of infauna and the diversity and density of burrows, and b) DO of bottom water. The degree of bioturbation is significantly reduced in waters that are oxic, but below 80% DO saturation. Based on these observations, we suggest that it is inappropriate to link low bioturbation intensities and diversities to hypoxia (<2.0 mg l(-1)), and by extension, anoxia (0 mg l(-1)). Instead, reduced oxygen contents (4.3-5.3 mg l(-1)) that lie well above hypoxic levels have a dramatic impact on the health of infaunal communities, and this is reflected by severe reductions in the ichnological character of sediments. (C) 2015 Elsevier B.V. All rights reserved. |
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