Distribution and paleoecology of molluscan skeletal remains along an upwelling tract: Benguela system, Namibian shelf

Abstract

Abstract This study describes and categorizes the preservable, and thus predominantly molluscan, component of the macrofauna death assemblages of sub-environments within the northern Benguela upwelling system. This is especially important for assembling actualistic data that can later serve for identifying similar settings in the geological record. A total of 16 samples were examined. Contrary to stereotypes, benthic communities are present but highly variable across the high-productivity tract and are characterized by distinctive community structures. Benguela system document a decrease in body size of chemosymbiotic bivalves (lucinids) and percent of chemosymbionts and deposit-feeders, and an overall increase in species richness and epifaunal-infaunal ratio. This trend is in accordance with the upwelling facies-gradient from diatomaceous ooze in areas of maximum upwelling intensity outward to the aerated but food-abundant carbonate oozes and spatially coexisting phosphorite concentrations. Differences in seafloor oxygen concentration along with variable food availability played major roles in structuring the death assemblages under the Benguela upwelling system. The ecologic data provide ways to distinguish between facies of low and high shell production along an increasing oxygen gradient, and also between assemblages with moderate and higher species richness associated with time-averaging. The data support the existence of a wider range of time averaging in the formation of molluscan sand and gravel samples in the northern Benguela than previously appreciated. Given the scarcity of information on macrobenthic ecology from high-productivity settings, these death assemblages can represent a modern analog.