Strontium-isotope evidence for marine or freshwater origin of fossil dipnoans and arthrodires

Abstract

STRONTIUM has a uniform isotopic composition in well mixed regions of the sea1,2 because of its long residence time. Though the strontium isotope composition of sea-water has fluctuated through the Phanerozoic 3, it apparently has been relatively homogeneous during particular periods of time. Marine organisms that utilize calcium (and strontium) in their shells or skeletons are known to incorporate strontium of marine isotopic composition. Strontium in fresh or brackish water may diverge isotopically from the marine value, depending principally on the age and Rb/Sr ratio of source rocks that contribute soluble strontium4. Thus the isotopic composition of strontium in fossil aquatic organisms may rule out a marine origin for the organisms, provided that the age of the organisms is known, and the isotopic composition of the fossil has not been perturbed by strontium addition or exchange; on the other hand, the isotopic data may be consistent with (but not prove) a marine origin. Non-equilibrium of strontium in phosphate and strontium in carbonate (or seawater) during diagenesis may be inferred from a lack of microscopically detectable alteration, and by analogy with the non-equilibrium of certain clay-carbonate systems5. © 1970 Nature Publishing Group.