Paleoenvironment and the oxygen isotope geochemistry of ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA

Details Paleoenvironment and the oxygen isotope geochemistry of ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA Paleoenvironment and the oxygen isotope geochemistry of ironstone of the Upper Ordovician Neda Formation, Wisconsin, USA

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.2319y&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 10, pp.2319-2327

Mathematics

Logic

7

Scientific paper

Geological evidence suggests that the goethite-dominated, oolitic ironstone of the Upper Ordovician Neda Formation in the north central United States was subaerially weathered in the Late Ordovician. The oxygen isotope ratios of the goethites in the "hard" and "soft" ores of this deposit are rather uniform with a 18 O value of about -1.0%.. This 18 O value indicates the presence of meteoric water at the time of goethite formation and supports the idea that the goethites in hard and soft ores were formed in the same chemical weathering environment 440 million years ago. The 18 O value of accessory phosphate in some soft ore goethite-rich ooids was measured and combined with the goethite 18 O value to calculate a temperature of formation of about 23°C. The corresponding 18 O value of the ancient water was calculated to have been -7.3 . The relatively "cool" temperature calculated for the low-altitude, tropical site occupied by the Neda ironstone in the Late Ordovician is consistent with published evidence from fossil marine invertebrates for global cooling at that time. The 18 O value of -7.3%. for the ancient tropical precipitation could indicate intense rainfall of a seasonal (perhaps monsoonal) character.

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