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Isotopic evidence for the early ending of the boring billion

THURSDAY June 11 - 4 PM CET (Webinar)

Ying Zhou
UCL Earth Sciences, University College London

ABSTRACT: The period from ca. 1.8 Ga to 0.8 Ga has conventionally been regarded as the “boring billion” years of Earth history.
The boring billion was characterised by relatively muted carbon isotope variability (
d13Ccarb in the range of 0 to 4 ‰), stable and unchanging environmental (climate and redox) conditions and limited biological diversification. Recent studies show, however, that the boring billion was actually not that boring. My talk will focus on the understudied closing stages to the boring billion, which was originally suggested to have ended around ~0.8 Ga with the “Bitter Springs” negative d13Ccarb anomaly. Here I will present C, U and Sr isotope data from thick carbonate successions on the North China Craton that cover the transition between the end of the Mesoproterozoic Era through the early Neoproterozoic Era (ca. 1.1 to 0.9 Ga). The data show that much larger d13Ccarb oscillations existed than previously envisaged, accompanied by changes in seawater 87Sr/86Sr that reflect the final stages of amalgamation of the supercontinent “Rodinia” and sporadic weathering of large igneous provinces. Negative carbon isotope excursions have been interpreted as ocean oxygenation events and our preliminary d238U results confirm fluctuations in seafloor anoxia during the first ~100 Myr of the Neoproterozoic Era. This study therefore suggests that if there was ever a “boring billion”, it possibly had already ended by the very earliest Tonian. This may also be seen in the improving fossil record, which is providing increasing evidence for eukaryotic diversification around this time. In this talk, I will also introduce a special type of early diagenetic carbonate component, “calcite microspar cement” (CMC), which formed within “Molar Tooth Structure” cavities. The study of CMC shows it to be a reliable proxy archive in Proterozoic palaeoenvironment reconstructions.

 

BIO: Ying Zhou's background is geochemistry and physical geography, where she uses isotopes, as well as geochemical and geological approaches more generally, to trace ocean composition, redox conditions and nutrient cycling in deep time. Her focus is on how life and its physical environment have co-evolved, especially before the Cambrian explosion of animal life such as the redox conditions and nutrient cycling in the run up to the Cambrian bioradiations. While at the same time also working on reconstructing the strontium, lithium and possibly molybdenum isotopic composition of early Neoproterozoic seawater to address how and when the mysterious ‘boring billion’ ended.

YZ