Chemosymbiont-dominated seafloor communities in modern and Cretaceous upwelling systems support a new, high-productivity variant of standard low-oxygen models

Faunal analysis of modern (Benguela upwelling system, Namibia) and ancient (upper Cretaceous, Israel) sedimentary records rich in organic matter, biogenic silica, carbonate, and phosphate indicates that, contrary to stereotypes of upwelling systems as dead zones, macrobenthic communities are present but highly variable. Biofacies vary with distance from the upwelling core and as a function of both the supply of food to the seafloor and the oxygen demand it creates there: under a high organic supply, large-bodied chemosymbiotic bivalves dominate exaerobic and lower dysaerobic seafloors, and deposit feeders are abundant in upper dysaerobic and aerobic zones. Macrobenthic biofacies under upwelling thus contrast strongly with the overwhelmingly filter feeder–dominated biofacies encountered in the dark siliciclastic shales on which standard oxygen-restricted biofacies models have been based, and argue that, mechanistically, the low-oxygen conditions characterizing those shales reflect ordinary levels of water-column productivity and arise largely from water-column stratification. Our biofacies model requires testing in other upwelling records. However, we expect it to be robust and useful given the long evolutionary histories of the chemosymbiotic and deposit-feeding guilds, providing a new means for discriminating the relative roles of high organic flux and low ventilation in creating low-oxygen conditions at the seafloor. INTRODUCTION Factors governing the deposition of silica-, organic carbon–, and phosphate-rich sediments in highly productive upwelling systems are complex, such that many fundamental questions about the variability and drivers of bottom-water oxygen levels remain unclear (Parrish, 1998; Levin, 2003). This situation has been exacerbated by the diagenetic lability of such sediments (Soudry et al., 2006; Hatcher et al., 2014), which complicates geochemical inference. In other natural settings, paleo-oxygen gradients are reconstructed from trace-fossil and bodyfossil evidence with considerable confidence, building on the work of Rhoads and Morse (1971): with decreasing oxygen, seafloors are characterized by a less diverse, less calcified, smaller bodied, and more exclusively epifaunal macrobenthic fauna and by lower bioturbation indices (e.g., Savrda et al., 1984; Savrda and Bottjer, 1991). Ecological and paleoecological studies show that the taxonomic composition of shelly macrobenthos can be diagnostic and that oxygen levels are temporally variable (Rhoads et al., 1991; Wignall, 1994; Boyer and Droser, 2007, 2009; Gooday et al., 2009). The seafloors of upwelling systems also typically exhibit oxygen gradients, with anoxia developing in core areas where cells of upwelling nutrient-rich water are most persistent on an annual to interannual basis and organic matter flux to the seafloor is highest. We can thus expect that macrobenthic abundance, richness, community structure, and body size will vary along oxygen gradients in upwelling systems, but possibly in different ways than under the lower levels of primary productivity that characterize most shelves and inland seas. Understanding biofacies variation in high-productivity settings is important for stratigraphic analysis in order to discriminate (1) oxygen depletion associated with upwelling and other oceanic eutrophication from (2) depletion that arises largely from water-column stratification alone (e.g., oxygen minimum zones and oceanic anoxic events; Rabalais et al., 2010). Here we present the first quantitative evaluation of shelled macrobenthic invertebrate assemblages under upwelling conditions, using both modern and ancient settings, and propose a new dedicated low-oxygen model. We stress diversity and functional groups rather than taxonomy in order to identify patterns that are robust across geologic time. METHODS The modern Benguela upwelling system (BUS) offshore of Namibia is one of the major eastern boundary upwelling systems (Shannon et al., 2006; Hutchings et al., 2009) (Fig. 1). Sediments on this broad continental shelf contain the highest organic matter contents known in any modern upwelling areas (as high as 25 wt%), with siliceous and carbonate oozes and patches rich in authigenic phosphorite (>20 wt%; Bremner, 1978; Rogers and Bremner, 1991; Compton et al., 2004). Highest productivity and lowest bottom-water dissolved oxygen levels occur close to shore (Bailey, 1991). These conditions contrast with other perennial systems such as Peru, where upwelling is most intense over deep seafloors (~700 m) due to the steepness of the shelf, and northwest Africa, where water energy prevents anoxic conditions and ooze accumulation on the shelf (Filipsson et al., 2011). The upper Cretaceous (Campanian) Mishash Formation (MIS) of Israel provides an ancient counterpart from the southern coast of the Tethys, another passive margin setting. The MIS comprises an ~9 m.y. record of organic-rich carbonate, porcelanite, chert, and phosphorite from one of the most extensive high-productivity complexes known in the stratigraphic record (Soudry et al., 2006; Edelman-Furstenberg, 2009). Molluscan data for the modern BUS were sieved from 17 box cores of organic-rich diatomaceous (opal) and biogenic carbonate (micrite) oozes and phosphate-rich shell gravels collected GEOLOGY, November 2015; v. 43; no. 11; p. 975–978 | Data Repository item 2015329 | doi:10.1130/G37017.1 | Published online 1 October 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. AM BIA Kunene River