Are Phosphorites Reliable Indicators of Upwelling ?

Most models of phosphorite genesis involve upwelling as an essential element, if only to provide a mechanism for continuous nutrient supply to ocean surface water and hence sustain a high flux of phosphorus to the sediment via organic matter. Although recent studies have confirmed a close link between upwelling and phosphorite formation on the continental margins of Namibia and Peru-Chile, the simplified assumption that phosphorites in the sedimentary record invariably are indicative of upwelling can be challenged on several grounds: (a) some major phosphorite deposits in the geologic record cannot be adequately explained in terms of reasonable upwelling models; (b) formation of phosphorites in modern times, as demonstrated by uranium-series age determinations and other methods, is not confined to major upwelling centers, but also occurs on the continental margin of Eastern Australia, an area of only moderate seasonal upwelling, and quite limited organic productivity. Apatite in phosphorites off Eastern Australia is most likely formed during post-mortem alteration of organic phosphorus originally present in bacterial cells. Populations of transiently motile, facultatively chemolithotropic bacteria may proliferate within the upper continental slope sediments during times when the environment became quite restrictive, such as when nutrient supply was drastically limited. Additional clues for differentiating phosphorites might be found in the rare earth element pattern of the “Eastern Boundary Current”-type; i.e., absence of the Ce-anomaly. Phosphorites or phosphatic sediments lacking these features do not reliably indicate upwelling and should not be used indiscriminately in the reconstruction of former upwelling centers.