To estimate vertical thermal gradients and paleo–water depths to marine platforms we present a new method based on the difference between δ18O values of contemporaneous brachiopod carbonate and fish phosphate. Present-day marine fauna of well-known ecology from the surface to the sea floor record isotopic temperatures that agree with measured temperatures. We predict distributions of isotopic data that result from sampling strategy, basin morphology, and fauna ecology and discuss limitations. Application of the method to the Jurassic Paris-London basin gives vertical thermal variations of up to 14 °C associated with depths varying from a few meters to 170 ± 30 m. The estimated depths are minimum values and are greater than those estimated from sedimentological criteria for this ancient epicontinental sea.

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