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Geochemistry of iron-rich sediments on the outer continental shelf off northern New South Wales — Comment
Marine Geology, 59 (1984) 319--322 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
319
Discussion GEOCHEMISTRY OF IRON-RICH SEDIMENTS ON THE OUTER CONTINENTAL SHELF OFF NORTHERN NEW SOUTH WALES -- COMMENT
G.W. O'BRIEN* School of Earth Sciences, The Flinders University of South Australia, Bedford Park, S.A. 5042 (Australia) (Accepted for publication November 29, 1983}
In his recent paper, Marshall (1983) has described the geochemistry of iron-rich sediments on the outer continental shelf off northern New South Wales, Australia. These sediments, which were recovered at water depths ranging from 197 to 274 m, had Fe203 (total iron expressed as Fe203) concentrations which varied between 10 and 36%. Marshall noted a marginal enrichment of P, As, and Mn in these iron-rich sediments, and suggested anionic adsorption onto the surface of iron oxides as the likely enrichment mechanism. Furthermore, he proposed that during intervals of negative Eh, the adsorbed phosphate in the iron-rich sediments is released, thereby elevating the phosphate concentrations in the interstitial waters. Marshall contends that, considering the absence of significant coastal upwelling and the lack of associated enhanced biological productivities on the East Australian continental margin, it is this "phosphate release mechanism" that provides the source of the phosphorus for the formation of the Quaternary phosphorite nodules that have been reported from the region (O'Brien and Veeh, 1980; Kress and Veeh, 1980; O'Brien et al., 1981). Moreover, he proposes that such a mechanism may also explain the origin of other "East C o a s t " - - t y p e phosphorite deposits which formed in areas of little or no coastal upwelling. We wish to point out in this discussion that Marshall's model cannot be used to explain the genesis of phosphorite nodules on the East Australian continental margin. Firstly, the Quaternary phosphorite nodules are restricted to a narrow depth interval between 360 and 450 m on the upper continental slope (O'Brien and Veeh, 1980; O'Brien et al., 1981), and do not occur on the outer shelf, as suggested by Marshall. They thus occur at water depths approximately 150--250 m deeper than Marshall's iron-rich sediments. Secondly, the Quaternary nodules are associated with iron-poor glauconitic, foraminiferal sands (O'Brien and Veeh, 1980; O'Brien et al., 1981; O'Brien, 1982). These upper-slope sediments typically contain between 3 *Present address: Western Mining Corporation Limited, 168 Greenhill Road, Parkside, S.A. 5063, Australia. 0025-3227/84/$03.00
© 1984 Elsevier Science Publiahers B.V.
320 and 5% Fe:O3 (see table 2 of O'Brien et al., 1981); this Fe203 is contained within the glauconite and clay minerals in the sediments and is n o t present as an iron staining. Furthermore, the Quaternary nodules form by the cementation of these iron-poor upper slope sediments with bacterial apatite (O'Brien et al., 1981), and consequently these nodules have Fe203 concentrations which are similar to those of the associated sediments (usually in the 4--5% range; see table 1 in O'Brien and Veeh, 1980, and table 2 in O'Brien et al., 1981). Clearly, the upper-slope sediments associated with the Quaternary nodules have quite different mineralogical and geochemical compositions to the iron-rich outer shelf sediments described by Marshall. Thirdly, uranium-series studies have shown that, whereas the iron-poor upper continental slope phosphorite nodules usually have Late Pleistocene or Holocene ages, iron-rich phosphatic nodules from the outer shelf are invariably relict, having ages in excess of 800,000 years (O'Brien and Veeh, 1980; O'Brien et al., 1981). An example of such a relict, iron-rich phosphatic nodule is sample 1516-A; compositional and radiochemical data for this nodule are presented in tables 1 and 2 of O'Brien and Veeh (1980). Significantly, Marshall also presents an analysis of a phosphatic sample from location 1516 in his paper (see table 1, Marshall, 1983). This sample is even more iron-rich than the sample analysed in O'Brien and Veeh (1980) and is therefore also almost certainly relict. In summary, it should be reiterated that Marshall's paper deals exclusively with iron-rich outer shelf sediments, most, and probably all of w h i c h a r e relict. These iron-rich sediments have markedly different mineralogical anti. geochemical compositions to the iron-poor Quaternary phosphorite nodules and sediments on the upper continental slope. Consequently, Marshall's "phosphate release mechanism" cannot be used to explain the origin of phosphorite nodules on the East Australian continental margin. REFERENCES Kress, A.G. and Veeh, H.H., 1980. Geochemistry and radiometric ages of phosphatic nodules from the continental margin of northern N e w South Wales, Australia. Mar. Geol., 36: 153--157. Marshall, J.F., 1983. Geochemistry of iron-rich sediments on the outer continental shelf off northern N e w South Wales. Mar. Geol., 51: 163--175. O'Brien, G.W., 1982. "Non-upwelling" phosphorites from the East Australian continental margin. Abstr. IGCP-156 5th Int. Field Workshop and Seminar on Phosphorites, Kunming, China. O'Brien, G.W. and Veeh, H.H., 1980. Holocene phosphorite on the East Australian continental margin. Nature, 288: 690--692.
O'Brien, G.W., Harris, J.R., Milnes, A.R. and Veeh, H.H., 1981. Bacterial origin of East Australian continental margin phosphorites. Nature, 294: 442--444.
319
Discussion GEOCHEMISTRY OF IRON-RICH SEDIMENTS ON THE OUTER CONTINENTAL SHELF OFF NORTHERN NEW SOUTH WALES -- COMMENT
G.W. O'BRIEN* School of Earth Sciences, The Flinders University of South Australia, Bedford Park, S.A. 5042 (Australia) (Accepted for publication November 29, 1983}
In his recent paper, Marshall (1983) has described the geochemistry of iron-rich sediments on the outer continental shelf off northern New South Wales, Australia. These sediments, which were recovered at water depths ranging from 197 to 274 m, had Fe203 (total iron expressed as Fe203) concentrations which varied between 10 and 36%. Marshall noted a marginal enrichment of P, As, and Mn in these iron-rich sediments, and suggested anionic adsorption onto the surface of iron oxides as the likely enrichment mechanism. Furthermore, he proposed that during intervals of negative Eh, the adsorbed phosphate in the iron-rich sediments is released, thereby elevating the phosphate concentrations in the interstitial waters. Marshall contends that, considering the absence of significant coastal upwelling and the lack of associated enhanced biological productivities on the East Australian continental margin, it is this "phosphate release mechanism" that provides the source of the phosphorus for the formation of the Quaternary phosphorite nodules that have been reported from the region (O'Brien and Veeh, 1980; Kress and Veeh, 1980; O'Brien et al., 1981). Moreover, he proposes that such a mechanism may also explain the origin of other "East C o a s t " - - t y p e phosphorite deposits which formed in areas of little or no coastal upwelling. We wish to point out in this discussion that Marshall's model cannot be used to explain the genesis of phosphorite nodules on the East Australian continental margin. Firstly, the Quaternary phosphorite nodules are restricted to a narrow depth interval between 360 and 450 m on the upper continental slope (O'Brien and Veeh, 1980; O'Brien et al., 1981), and do not occur on the outer shelf, as suggested by Marshall. They thus occur at water depths approximately 150--250 m deeper than Marshall's iron-rich sediments. Secondly, the Quaternary nodules are associated with iron-poor glauconitic, foraminiferal sands (O'Brien and Veeh, 1980; O'Brien et al., 1981; O'Brien, 1982). These upper-slope sediments typically contain between 3 *Present address: Western Mining Corporation Limited, 168 Greenhill Road, Parkside, S.A. 5063, Australia. 0025-3227/84/$03.00
© 1984 Elsevier Science Publiahers B.V.
320 and 5% Fe:O3 (see table 2 of O'Brien et al., 1981); this Fe203 is contained within the glauconite and clay minerals in the sediments and is n o t present as an iron staining. Furthermore, the Quaternary nodules form by the cementation of these iron-poor upper slope sediments with bacterial apatite (O'Brien et al., 1981), and consequently these nodules have Fe203 concentrations which are similar to those of the associated sediments (usually in the 4--5% range; see table 1 in O'Brien and Veeh, 1980, and table 2 in O'Brien et al., 1981). Clearly, the upper-slope sediments associated with the Quaternary nodules have quite different mineralogical and geochemical compositions to the iron-rich outer shelf sediments described by Marshall. Thirdly, uranium-series studies have shown that, whereas the iron-poor upper continental slope phosphorite nodules usually have Late Pleistocene or Holocene ages, iron-rich phosphatic nodules from the outer shelf are invariably relict, having ages in excess of 800,000 years (O'Brien and Veeh, 1980; O'Brien et al., 1981). An example of such a relict, iron-rich phosphatic nodule is sample 1516-A; compositional and radiochemical data for this nodule are presented in tables 1 and 2 of O'Brien and Veeh (1980). Significantly, Marshall also presents an analysis of a phosphatic sample from location 1516 in his paper (see table 1, Marshall, 1983). This sample is even more iron-rich than the sample analysed in O'Brien and Veeh (1980) and is therefore also almost certainly relict. In summary, it should be reiterated that Marshall's paper deals exclusively with iron-rich outer shelf sediments, most, and probably all of w h i c h a r e relict. These iron-rich sediments have markedly different mineralogical anti. geochemical compositions to the iron-poor Quaternary phosphorite nodules and sediments on the upper continental slope. Consequently, Marshall's "phosphate release mechanism" cannot be used to explain the origin of phosphorite nodules on the East Australian continental margin. REFERENCES Kress, A.G. and Veeh, H.H., 1980. Geochemistry and radiometric ages of phosphatic nodules from the continental margin of northern N e w South Wales, Australia. Mar. Geol., 36: 153--157. Marshall, J.F., 1983. Geochemistry of iron-rich sediments on the outer continental shelf off northern N e w South Wales. Mar. Geol., 51: 163--175. O'Brien, G.W., 1982. "Non-upwelling" phosphorites from the East Australian continental margin. Abstr. IGCP-156 5th Int. Field Workshop and Seminar on Phosphorites, Kunming, China. O'Brien, G.W. and Veeh, H.H., 1980. Holocene phosphorite on the East Australian continental margin. Nature, 288: 690--692.
O'Brien, G.W., Harris, J.R., Milnes, A.R. and Veeh, H.H., 1981. Bacterial origin of East Australian continental margin phosphorites. Nature, 294: 442--444.