T139 Celebrating Dr. John W. Valley's Contributions to Isotope Geochemistry and Beyond, from the Hadean to the Holocene II

Monday, 23 October 2017
1:30 PM - 5:30 PM
Skagit 2

SHIFT IN THE HYDROLOGIC CYCLE AND RAPID GROWTH OF SUBAERIAL CONTINENTAL CRUST AT ~2.5 GA BASED ON TRIPLE OXYGEN ISOTOPE SYSTEMATICS OF SHALES

Paper number: T139-4
Program: Technical Sessions
Time: 2:10 PM – 2:25 PM

Authors

Abstract

The rates of growth of continental crust through time remains a matter of significant disagreement. Crustal growth models based on radiogenic isotope model ages, elemental cycles, 18O sequestration, and the detrital zircon record range widely from gradually increasing to decelerating to stepwise increasing. However, most of these studies do not distinguish between submarine and subaerial crust and the evolution of the extent of subaerially exposed crust is an issue seldom addressed in the literature. We will present precise (±0.01‰) triple oxygen isotope compositions of shales through time based on our newly built airlock sample chamber integrated through a continuous flow, gas chromatography line to a MAT253 gas-source mass-spectrometer. We measured the Δ17O values of 50 shales that include individual and formation-averaged composite samples, ranging in age from 3.5 to 0 Ga. Chemical index of alteration is invariable through time and δ18O values show a gradual increase from 3.5 to 0 Ga, consistent with our recently published larger dataset. Our results show a stepwise decrease in Δ17O from -0.12 to -0.2‰ near the Archean-Proterozoic boundary at 2.5 Ga. We interpret this new observation as reflecting a significant increase in the area of emerged continental crust by 2.5 Ga. This increase in the subaerial surface area enhanced continental weathering and might be linked to the contemporaneous oxygenation of Earth’s surface. The rapid emergence of land-masses could have contributed to the initiation of the Snowball Earth Events and the Great Oxygenation Event (GOE) in the early Paleoproterozoic via changes in albedo and weathering cycle. Alternatively, the GOE itself and the appearance of oxygen at ca. 2.4 Ga should have triggered the modern photolytic production of high-Δ17O stratospheric ozone, generating oxygen and carbon dioxide reservoirs with low-Δ17O values in the lower part of the atmosphere that imprinted their signal into waters and mudrock record through weathering reactions.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-294858

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