Zhou, Y; (2017) Reconstructing the strontium isotopic composition of Neoproterozoic seawater. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The Tonian Period (1000 - c.720 Ma) followed a long interval of relative stasis, in terms of climate, carbon isotopes and biological evolution, and led into the Cryogenian Period of environmental extremes and instability. Despite its pivotal situation, the Tonian Period is still relatively understudied, and this is partly due to the lack of robust age constraints in key Proterozoic successions around the world. The fossiliferous Neoproterozoic strata of the North China craton were until recently thought to be of Ediacaran age. However, fossil evidence and new geochronological constraints combine to show that most of the ‘Qingbaikou’ System, which reaches a great thickness in some areas, was deposited between c. 980 and c. 920 Ma. The isotopic signature of these strata confirms their Tonian age, showing typical moderately high 13C values together with low 87Sr/87Sr ratios, <0.7065. Another characteristically Tonian feature is the unusually widespread abundance of early diagenetic ‘molar-tooth’ low-Mg calcite microspar. In this study, I compare MT samples with their surrounding ‘bulk’ matrix in correlative successions on the North China craton in order to 1) demonstrate their propensity to preserve a primary seawater isotopic signature; and 2) reconstruct, together with published data, the strontium (and carbon) isotopic evolution of early Tonian seawater. Second-order fluctuations of less than ~0.001 are superimposed on a general 87Sr/87Sr rise from ~0.7052 to ~0.7063 by c. 920 Ma, accompanied by a profoundly negative carbon isotope excursion. Increased chemical weathering has been linked with both climatic and carbon isotope instability, and this study indicates an earlier beginning to such carbon cycle perturbations, which coincide with early stages of supercontinent rifting, as evidenced by the newly dated Dashigou igneous province of North China. These and other new and published data are used to reconstruct and reinterpret the strontium isotopic evolution of Neoproterozoic seawater.

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