Unbioturbated sediments on a muddy shelf: Hypoxia or simply reduced oxygen saturation? Shahin E. Dashtgard a, ⁎, John W. Snedden b , James A. MacEachern a a Applied Research in Ichnology and Sedimentology (ARISE) Group, Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada b Institute for Geophysics, The University of Texas at Austin, 10100 Burnet Road, Austin, TX 78558-4445, United States abstract article info Article history: Received 11 September 2014 Received in revised form 11 February 2015 Accepted 24 February 2015 Available online 3 March 2015 Keywords: Hypoxia Anoxia Shale Bioturbation Ichnology Shelf 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 biotur- bated with averaged bioturbation indices (for all beds in a core) between 2.1 and 5.6, and trace diversities be- tween 2 and 9 distinct burrow forms. Below 35 m water depth, box cores exhibit trace diversitiesof 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, dis- solved oxygen (DO) contents show pronounced variability, ranging from 100% DO saturation through to hypoxia (b 2.0 mg l -1 ), and reflect the periodic introduction of oxygen-depleted waters into otherwise fully oxygen- saturated 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 in- tensities and diversities to hypoxia (b 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Fine-grained sediments deposited near or below storm-wave base are commonly manifested as black mudstones and shales that appear unbioturbated. The paucity of bioturbation in these deposits has been regularly attributed to either hypoxia/anoxia (e.g., Bromley and Ekdale, 1984; Wetzel, 1991; Pemberton, 1992; Uchman and Wetzel, 2011) or to a lack of lithologic contrast that masks pervasive bioturba- tion (e.g., MacEachern et al., 1999; Gingras et al., 2011; Shank and Plint, 2013). These two contrasting scenarios for explaining the pres- ence of unbioturbated or “under-bioturbated” mudstones deposited in marine environments rely solely upon interpretations of ancient sedi- mentary strata, and have not been vetted against modern examples. Re-examination of data collected from the muddy, storm-dominated western Gulf of Mexico (GOM) provides an opportunity to evaluate the neoichnological character of a mud-dominated shelf and to compare bioturbation trends to infaunal population distributions and seawater oxygenation. Through this comparison, the dominant control on biotur- bation in marine muds is proposed and these data are presented as a possible analog for interpreting unburrowed marine mudstones and shales in the rock record. Ocean hypoxia occurs when the DO content of seawater drops below either 2.0 mg l -1 (Tyson and Pearson, 1991) or 3.0 mg l -1 (Ritter and Montagna, 1999), and an ocean basin is considered anoxic when the DO content of seawater drops to 0 mg l -1 (Tyson and Pearson, 1991). Once hypoxia is established, benthic communities experience elevated mortality rates (Diaz and Rosenberg, 2008), with the degree and dura- tion of oxygen depletion affecting the magnitude of mortality in the faunal community. In oxic waters, Ritter and Montagna (1999) demon- strated that the abundance and diversity of benthic communities increased exponentially as O 2 concentrations increased from 3.0 to 6.0 mg l -1 , and healthy (unstressed) infaunal communities can exist when DO is 80% saturation or higher. Many black shales appear to be unbioturbated or exhibit bioturba- tion indices (BI) of 1–2. In more proximal positions, such shales com- monly pass into muddy sandstones and sandy mudstones and exhibit burrowing intensities of BI 5–6 (e.g., MacEachern and Pemberton, 1992, 1994; MacEachern et al., 1999; Dashtgard et al., 2008). This Palaeogeography, Palaeoclimatology, Palaeoecology 425 (2015) 128–138 ⁎ Corresponding author. E-mail addresses: sdashtgard@gmail.com (S.E. Dashtgard), jsnedden@ig.utexas.edu (J.W. Snedden), jmaceach@sfu.ca (J.A. MacEachern). http://dx.doi.org/10.1016/j.palaeo.2015.02.033 0031-0182/© 2015 Elsevier B.V. All rights reserved. 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