61st Annual Report on Research 2016

Shales have been traditionally used to deduce the role of continental weathering and sediment recycling through geologic time and to constrain the average chemical composition of the continental crust and for prospecting this reservoir for oil and gas. We have performed compilation of published analyses of oxygen isotope systematics of shales through time, topics which have been insufficiently studied so far, even though unaltered shales are preserved since ~3.5 Ga. During the Archean-Paleoproterozoic transition, several major environmental changes could have happened, including the appearance of oxygen in the atmosphere, increases in the continental landmass, decreases in CH₄ concentrations, Snowball Earth glaciations, and evolution of aerobic microbial life, all affecting continental weathering rates and attainment of isotopic fractionation. Furthermore, inferred variable d¹⁸O values of seawater could have left an imprint on these lithologies during diagenesis.

We have performed oxygen, hydrogen, and carbon isotope analyses, and total carbon concentration investigation, for 206 bulk shales samples that were collected from drillholes on all continents, using collection of Dr. Andrey Bekker, and have done fieldwork in northern Russia last summer. A paper summarizing our discoveries has been published in the Earth and Planetary Science Letters, the top journal in the field of Geosciences. The abstract of the paper, outlining our results, is attached below.

The grant supports work of graduate student David Zakharov and a postdoc Matt Loewen, positively contributes to maintaining analytical infrastructure of the University of Oregon Stable Isotope Laboratory. This grant also provides support for my transition to low-T geochemistry and petroleum science, topics that I have not been exposed before, through active research in these fields we are learning basics and contributing new fresh insights. We presented results of our investigation at the Annual Meeting of the American Geophysical Union in San Francisco, December 2014 and 2015. These abstracts are published in the conference proceedings volume.

Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event

Bindeman I.N.^1*, Bekker A.², Zakharov D.¹

¹Department of Geological Sciences, 1272 University of Oregon, Eugene OR 97403

²Department of Earth Sciences, University of California, Riverside, CA 92521

corresponding author: Bindeman@uoregon.edu

Earth and Planetary Science Letters vol. 437: pages: 101-113.

Abstract.

We present stable isotope and chemical data for 206 Precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0.5 to 3.5 Ga with a dense coverage for 2.5–2.2 Ga time interval when Earth experienced four Snowball Earth glaciations and the irreversible rise in atmospheric O2. We observe significant, downward shift of several ‰ and a smaller range of δ¹⁸O values (7 to 9‰) in shales that are associated with the Paleoproterozoic and, potentially, Neoproterozoic glaciations. The Paleoproterozoic samples consist of more than 50% mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering. Their pervasively low δ¹⁸O and δD (down to −85‰ SMOW) values provide strong evidence of alteration and diagenesis in contact with ultra-low δ18O glacial meltwaters in lacustrine, deltaic or periglacial lake (sikussak-type) environments associated with the Paleoproterozoic glaciations. The δDsilicate values for the rest of Precambrian shales range from −75 to −50‰ and are comparable to those for Phanerozoic and Archean shales. Likewise, these samples have similar ranges in δ₁₃Corg values (−23 to −33‰ PDB) and Corg content (0.0 to 10 wt%) to Phanerozoic shales. Precambrian shales have a large range of δ¹⁸O values comparable to that of the Phanerozoic shales in each age group and formation, suggesting similar variability in the provenance and intensity of chemical weathering, except for the earliest 3.3–3.5 Ga Archean shales, which have consistently lower δ¹⁸O values. Moreover, Paleoproterozoic shales that bracket in age the Great Oxidation Event (GOE) overlap in δ¹⁸O values. Absence of a step-wise increase in δ¹⁸O and δD values suggests that despite the first-order change in the composition of the atmosphere, weathering cycle was not dramatically affected by the GOE at ∼2.4–2.3 Ga. Shales do not show comparable δ¹⁸O rise in the early Phanerozoic as is observed in the coeval δ¹⁸O trends for cherts and carbonates. There is however a sharp increase in the average δ¹⁸O value from the Early Archean to the Late Archean followed by a progressively decelerating increase into the Phanerozoic. This decelerating increase with time likely reflects declining contribution of mantle-extracted, normal-δ18O crust and lends support to crustal maturation and increasing ¹⁸O sequestration into the crust and recycling of high-δ¹⁸O (and ⁸⁷Sr/⁸⁶Sr) sedimentary rocks. This secular increase in the δ¹⁸O