SILICON ISOTOPES IN PRECAMBRIAN CHERTS : A RECORD OF THE OCEANIC TEMPERATURE ?

The most continuous record of seawater temperature in the Precambrian comes from the variations of the 18O/16O isotopic ratio (expressed in d18O units) of siliceous sediments of different age [1, 2]. However, the general cooling of the oceans in the Precambrian, inferred from the increase of chert d18O values with age, is questioned because of possible post-deposition O isotopic exchanges with hydrothermal fluids [3] or change in the sea water d18O [4, 5]. In Precambrian cherts, Si isotopic compositions (expressed in d30Si units) show systematic variations with age that are correlated with d18O in a manner excluding post-depositional exchanges as the major source of the isotopic variations. However, some samples have constant d30Si associated with low and variable d18O, likely illustrating the interaction with high temperature fluids (Si being insoluble in fluids, the water/rock ratios for the two isotopic systems are quite different). The d18O-d30Si correlation demonstrates that cherts have preserved their isotopic records acquired at the time of their formation. Since a relation between d30Si and d18O is not an implicit consequence of the models describing the possible change in d18O of the oceans [4] the oceanic temperature is the most likely parameter able to link the two isotope systems. Accordingly, a Si model cycle shows seawater temperature fluctuations from ≈70±15°C 3.5 Ga ago to ≈ 30±10°C 0.8 Ga ago. This interpretation is now being tested by measuring d30Si and d18O at a micrometer (petrographic) scale with ionmicroprobe.

References

[1] Knauth, P.L., & Epstein, S. Geochim. Cosmochim. Acta, 40, 1095-1108 (1976). [2] Knauth, P.L. & Lowe, D.R.. Earth Planet. Sci. Lett. 41, 209-222 (1978). [3] Degens, E.T. & Epstein, S., Bull. Am. Assoc. Pet. Geol., 46, 534-542 (1962). [4] Jaffrès J.B.D., Shields G.A. & Wallmann K. Earth Science Reviews 83 83-122 (2007). [5] Shields G.A & Kasting, Nature 447, E1 (2007).