Sedimentary phosphate formation in warm shallow waters: new insights into the palaeoceanography of the Permian Phosphoria Sea from analysis of phosphate oxygen isotopes

✍ Scribed by Eric E. Hiatt; David A. Budd

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 410 KB
• Volume: 145
• Category: Article
• ISSN: 0037-0738
• DOI: 10.1016/s0037-0738(01)00127-0

✦ Synopsis

Oxygen isotopic analyses of the structural phosphate (d 18 O P ) in sedimentary phosphorites of the Upper Permian Phosphoria Formation reveal that phosphogenesis occurred across a broad range of palaeoenvironments. The d 18 O P values from wellpreserved hand-picked phosphate-grain separates range from 20.2 to 13.7 (^0.5)½ SMOW, with the lowest values in the most landward shallow-water deposits and the highest values in the most basinward deeper water deposits. The relationship between the maximum burial depth and d 18 O P is opposite to that predicted if burial diagenesis was responsible for the isotopic signature of these phosphate grains. This suggests that the regional trend in oxygen isotopic data is not due to burial diagenesis. The range in oxygen isotopic values does correspond to the potential range of palaeotemperatures in the shallow and broad epicontinental Phosphoria Sea. The range is 14±428C (^3.58C) assuming a seawater d 18 O of 22.5 ½ SMOW. Palaeotemperatures along an offshore to onshore transect indicate that upwelling involved temperate (14±268C; mean of 218C) but not cold waters, and that warming across the palaeoshelf resulted in mean shallow-water temperatures of 34±378C. The latter values approximate those predicted by climate and ocean circulation models. These warm palaeotemperatures provide further evidence that phosphogenesis in the epicontinental Phosphoria Sea occurred under palaeoceanographic conditions that were different from modern phosphorite depositional systems. This suggests that a purely actualistic approach to the interpretation of the Phosphoria phosphorites is not valid.