Isotopic Records Across Two Phosphorite Giant Episodes Compared: The Precambrian-Cambrian and the Late Cretaceous-Recent
Published:January 01, 2000
Graham Shields, Peter Stille, Martin D. Brasier, 2000. "Isotopic Records Across Two Phosphorite Giant Episodes Compared: The Precambrian-Cambrian and the Late Cretaceous-Recent", Marine Authigenesis: From Global to Microbial, Craig R. Glenn, Liliane Prévôt-Lucas, Jacques Lucas
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Sedimentary phosphorite formation has occurred episodically over geologic time. Substantia] phosphorite deposits were formed during P-giant episodes of which the most widespread and long-lasting were the Precambrian-Cambrian and the Cretaceous-recent. The aim of this review is to reevaluate these two episodes in the light of sedimentary Sr, Nd, S, and C isotopic records. Some, although not all of these phases of phosphorite formation were accompanied by increases in seawater 87Sr/s6Sr unrelated to changes in seafloor spreading, which supports a possible link between orogeny and global weathering rates, P-input, and P-giant formation. Assessing the relationship between δ13C and phases of widespread phosphogenesis is more complicated due to the counterbalancing effects on seawater δ13C of productivity/organic matter deposition and subsequent phosphogenesis/early diagenetic carbon oxidation. It is recommended that proposed links between phosphogenesis and global changes of δ13C or P- input continue to be reexamined on a case-to-case basis. The application of secular trends in Nd isotopic ratios to unravelling the often vital role of paleocurrents in phosphorite formation is also discussed. In addition, recent isotopic research is outlined that has led to significant improvements in stratigraphic resolution around the Precambrian-Cambrian boundary. A temporal and causal connection is put forward between metazoan evolution, that is the introduction of bioturbation, fecal pellets, biomineralization, and filter feeders, which would have helped to concentrate mineral phosphate in sediments, and widespread phosphogenesis at this time.
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Marine Authigenesis: From Global to Microbial
This volume is a collection of 33 state-of-the-art papers focusing on various aspects of authigenic and diagenetic marine minerals and related global elemental cycling. The commingling of the various studies of authigenic minerals in this volume, including the most recent advances in knowledge concerning the occurrence and origins of phosphorites, glauconites, dolomites, siderites, manganese-iron associations, barites, ironstones, and other marine chemical sediments/sedimentary rocks of early authigenic/diagenetic origin, is partly the result of the increasing awareness that there are many overlaps, even direct co-associations, between different authigenic minerals, both in time, space, and genesis. Taken together, this compilation represents a holistic approach towards marine authigenesis that considers the integrated whole more than the simple sum of its parts.