Abstract
Iron-rich sediments on the outer continental shelf off northern New South Wales have formed as a result of authigenesis, mainly within skeletal chambers. These sediments are significantly enriched in Fe and marginally enriched in P, As and Mn when compared to other shelf sediments from the same region. Mineralogical data and microprobe analyses of the authigenic phases indicate that berthierine (chamosite), rather than glauconite, was the primary mineral, but this has subsequently broken down to poorly crystalline phases that are intermediate between berthierine and goethite. During this breakdown the foraminiferal infillings and pellets have become enriched in Fe, and both P and As have been incorporated by anionic adsorption. The geochemical data for the sediments and associated phosphatic nodules suggest that the source of phosphorus for the nodules may be the surrounding iron-rich sediments. The phosphatic nodules are relatively enriched in Fe and As and low in P 2O 5 compared to modern phosphorites associated with upwelling regions. It is postulated that reduction of the iron-rich sediments produces significant quantities of inorganic phosphate within the interstitial waters that are ultimately precipitated as carbonate-fluorapatite. This could explain the formation of modern (an ancient) phosphorites in east-coast situations that are not related to upwelling.
Published Version
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